OCS Study

MMS 2005-032

Understanding the Processes

that Maintain the Oxygen Levels

in the Deep Gulf of Mexico

Synthesis Report

U.S. Department of the Interior

Minerals Management Service

Gulf of Mexico OCS Region
U.S. Department of the Interior

Minerals Management Service

Gulf of Mexico OCS Region

OCS Study

MMS 2005-032

Understanding the Processes

that Maintain the Oxygen Levels 

in the Deep Gulf of Mexico

Synthesis Report

Authors

Ann E. Jochens

Leslie C. Bender

Steven F. DiMarco

John W. Morse

Mahlon C. Kennicutt II

Matthew K. Howard

Worth D. Nowlin, Jr.

Prepared under MMS Contract

1435-01-02-CT-85080

by

Texas A&M University

Department of Oceanography

College Station, Texas 77843-3146

Published by

New Orleans

June 2005
iii

DISCLAIMER

This report was prepared under contract between the Minerals Management Service (MMS) and

the Texas A&M Research Foundation. This report has been technically reviewed by the MMS,

and it has been approved for publication. Approval does not signify that the contents necessarily

reflect the views and policies of the MMS, nor does mention of trade names or commercial

products constitute endorsement or recommendation for use. It is, however, exempt from review

and compliance with the MMS editorial standards.

REPORT AVAILABILITY

Extra copies of the report may be obtained from the Public Information Office (Mail Stop 5034)

at the following address:

U.S. Department of the Interior

Minerals Management Service

Gulf of Mexico OCS Region

Public Information Office (MS 5034)

1201 Elmwood Park Boulevard

New Orleans, Louisiana 70123-2394

Telephone: (504) 736-2519 or

1-800-200-GULF

CITATION

Suggested citation:

Jochens, A. E., L. C. Bender, S. F. DiMarco, J. W. Morse, M. C. Kennicutt II, M. K. Howard,

and W. D. Nowlin, Jr. 2005. Understanding the Processes that Maintain the Oxygen Levels

in the Deep Gulf of Mexico: Synthesis Report. U.S. Dept. of the Interior, Minerals

Management Service, Gulf of Mexico OCS Region, New Orleans, LA. OCS Study MMS

2005-032. 142 pp.

ABOUT THE COVER

The cover art depicts a vertical section of dissolved oxygen in the eastern Gulf of Mexico from

the Mississippi River Delta to Havana, Cuba, from data collected on the research vessel Hidalgo

cruise 58-H-4 during 25-28 June 1958. It was adapted from a drawing in McLellan (1960),

which was one of the early investigations of physical oceanography in the Gulf. The basic

vertical structure shown is typical for most transects across the Gulf. Dissolved oxygen values

are relatively high in the upper 100-200 m, showing the influence of the air-sea exchanges of O2

and photosynthetic inputs of oxygen. They then decrease to a minimum at depths ranging from

~350 m to 500 m in different parts of the section due to the circulation. This oxygen minimum is

found throughout the Gulf and is caused both by the local oxidation of organic material within

the Gulf and the transport of the Tropical Atlantic Central Water into the Gulf from the

Caribbean Sea. Below the minimum, the O2 concentrations increase with depth. No water masses

are formed in the Gulf to ventilate these deep waters. Instead, the source of the ventilation of the

deep Gulf waters is the transport of oxygen-rich water masses from the Caribbean Sea into the

Gulf interior. The distribution also shows the depressed (raised) isolines of O2 in the upper 800-

1000 m that are associated with anticyclonic (cyclonic) circulation features.
v

ACKNOWLEDGMENTS

This report would not have been possible without the contributions of a large number of people.

Each principal investigator (PI) contributed ideas to or authorship of portions of the text. The

principal investigators, their affiliations, and their tasks are:

Ann E. Jochens TAMU Program Manager, PI for Tasks 1 and 3

Leslie C. Bender GERG/TAMU PI for Task 2

Steven F. DiMarco TAMU Co-PI for Task 1

Matthew K. Howard TAMU Data Manager

M. C. Kennicutt II GERG/TAMU Co-PI for Tasks 2 and 3

John W. Morse TAMU Co-PI for Task 2

Worth D. Nowlin, Jr. TAMU Senior Scientist

The assistance of Leila Belabbassi in preparation of this report is appreciated. We thank Dr. Piers

Chapman, Louisiana State University, for his thoughtful comments on the document.

We appreciate the insightful comments of the Science Review Board members: Dr. Larry

Atkinson of Old Dominion University, Dr. Robert Key of Princeton University, and Dr. John

Morrison of the North Carolina State University. Their comments helped to improve this report.

The enthusiastic and timely support of Dr. Mary Boatman, the MMS Contracting Officer's

Technical Representative, and Dr. Alexis Lugo-Fernández has been invaluable and is greatly

appreciated.

Ann E. Jochens

Program Manager
vii

TABLE OF CONTENTS

PAGE

List of Figures............................................................................................................... ix

List of Tables................................................................................................................. xi

Acronyms...................................................................................................................... xiii

1. Executive Summary .......................................................................................... 1

1.1 Introduction ........................................................................................... 1

1.2 Data Sets................................................................................................ 1

1.3 Sources and Sinks................................................................................. 1

1.4 Box Model............................................................................................. 2

1.5 Major Results........................................................................................ 3

1.6 Information Gaps and Recommendations.............................................. 4

2. Introduction ...................................................................................................... 5

2.1 Background ........................................................................................... 5

2.2 Study Objectives and Description.......................................................... 5

2.3 Science Review Group........................................................................... 7

2.4 Report Organization............................................................................... 8

3. Data Sets ................................................................................................... 9

3.1 Data Assembly....................................................................................... 9

3.2 Data Quality Assurance and Quality Control......................................... 19

3.3 Box Model Initialization Data Sets........................................................ 31

4. Dissolved Oxygen in the Deepwater Gulf of Mexico ........................................ 35

4.1 Physical Oceanographic Background .................................................... 36

4.1.1 Brief Review of Physical Oceanography of the Deepwater Gulf

of Mexico .................................................................................. 36

4.1.2 Description of Water Masses in the Gulf of Mexico................. 40

4.1.3 Flushing Time............................................................................ 43

4.2 Sources and Sinks of Dissolved Oxygen............................................... 44

4.2.1 Physical Processes in the Gulf of Mexico ................................. 46

4.2.2 Biogeochemical Processes in the Gulf of Mexico...................... 48

4.3 Data Reanalysis..................................................................................... 51

4.3.1 Basic Statistics........................................................................... 52

4.3.2 Patterns of Dissolved Oxygen Distribution ............................... 62

4.4 Effects of Oil and Gas Inputs on Dissolved Oxygen Concentrations.... 75

5. Box Model ................................................................................................... 87

5.1 Introduction ........................................................................................... 87

5.2 The Model Setup ................................................................................... 88

5.2.1 Definition of the Boxes.............................................................. 88

5.2.2 Mass Transport.......................................................................... 93

5.2.3 Sources and Sinks of Dissolved Oxygen................................... 94

5.3 The Model Equations............................................................................. 97

5.3.1 Governing Equations................................................................. 97

5.3.2 Specification of Knowns and Unknowns................................... 100

5.3.3 Additional Details...................................................................... 101

5.3.4 Solution Strategy ....................................................................... 103
viii

TABLE OF CONTENTS

(continued)

PAGE

5.4 Baseline Model Run .............................................................................. 104

5.5 Scenarios............................................................................................... 106

5.6 Conclusions........................................................................................... 110

6. Discussion, Conclusions, and Recommendation................................................ 113

6.1 Discussion............................................................................................. 113

6.2 Conclusions........................................................................................... 116

6.3 Knowledge and Data Gaps.................................................................... 119

6.4 Recommendation ................................................................................... 120

7. Literature Cited .................................................................................................. 121
ix

LIST OF FIGURES

FIGURE PAGE

2.1 Study area for the reanalysis of the historical oxygen data. ............................ 6

3.1 Locations of bottle oxygen stations for the deepwater Gulf of Mexico........... 16

3.2 Locations of stations with dissolved oxygen sensor data in the deepwater

Gulf of Mexico............................................................................................... 19

3.3 Example of a data file header for the dissolved oxygen bottle data.................. 21

3.4 Histogram of bottle oxygen data count in 50-m bin increments. .................... 23

3.5 Oxygen statistics by depth bin. ...................................................................... 25

3.6 Locations of stations and oxygen data for R/V Alaminos cruise, 67-A-04,

on 11 June - 1 July 1967. .............................................................................. 26

3.7 Locations of stations and oxygen data for R/V Hidalgo cruise, 62-H-03,

on 14 February - 31 March 1962. .................................................................. 27

3.8 Locations of stations and oxygen data for R/V Gyre cruise, 00-G-05, on

4 May - 17 June 2000. ................................................................................... 28

3.9 Locations of stations and oxygen data for the Virgilio Uribe cruise, C12,

on 31 October - 13 November 1970. ............................................................. 29

3.10 Locations of stations and oxygen data for the Tursiops cruise, T7015, on

2 - 11 May 1970. ........................................................................................... 30

4.1 Dissolved O2 in the North Atlantic Ocean (based on Millero 1996, Fig.

6.9). ................................................................................................................ 35

4.2 Near-surface drifter velocity estimates. .......................................................... 37

4.3 Record-length mean current vectors at the first sigma level above the

bottom............................................................................................................. 38

4.4 Sea surface height fields in the Gulf of Mexico. ............................................ 39

4.5 Dissolved oxygen profiles for DGOMB cruise 1 in May/June 2000. ........... 43

4.6 Schematic of vertical zones in the Gulf of Mexico. ........................................ 47

4.7 Locations of 2790 stations with at least one good oxygen datum in the

Gulf of Mexico. ............................................................................................. 51

4.8 Mean and standard error of bottle oxygen data by depth bin for the whole

Gulf, Yucatan Channel, and Bay of Campeche. .............................................. 53

4.9 Mean and standard error of bottle oxygen data by depth bin for the

southeast, northeast, northwest, and southwest quadrants of the Gulf of

Mexico. .......................................................................................................... 56

4.10 Mean oxygen concentrations by bins for the Bay of Campeche and the

northwest Gulf of Mexico. ............................................................................. 58

4.11 Locations of 461 stations with at least one good oxygen datum in depths

≥1500 m in the Gulf of Mexico. .................................................................... 60

4.12 Station locations for Hidalgo cruises 58-H-1 and 58-H-4. ............................ 63

4.13 Vertical distribution of dissolved oxygen across the Yucatan Channel,

Cruise 58-H-4, 18 May 1958. ........................................................................ 64

4.14 Vertical distribution of dissolved oxygen in the Eastern Gulf, from the

Mississippi River Delta to Havana, Cruise 58-H-4, 25-28 June 1958. ........... 65

4.15 Vertical distribution of dissolved oxygen in the Western Gulf, along

94.5oW longitude, Cruise 58-H-1, 23-30 March 1958. .................................. 66

4.16 Sea surface height for TOPEX/ERS2 analysis for 13 May 2000 and

10 June 2001. ................................................................................................ 68

4.17 Dissolved oxygen on the western-most transect of DGOMB cruise DG1, 

4 May - 17 June 2000. ................................................................................... 69

4.18 Dissolved oxygen on DGOMB cruise DG2, 2-18 June 2001. ...................... 70

4.19 Dissolved oxygen on density surfaces for the eastern and western Gulf of

Mexico. .......................................................................................................... 71
x

LIST OF FIGURES

(continued)

FIGURE PAGE

4.20 Mean dissolved oxygen in the Gulf of Mexico by decade. ............................ 73

4.21 Mean dissolved oxygen in 50-m depth bins for selected total water depths

in the northern Gulf of Mexico. ..................................................................... 74

4.22 Percent of annual average hydrocarbon discharge by natural and

anthropogenic sources. .................................................................................. 76

4.23 Percent of annual average hydrocarbon discharge by natural and

anthropogenic sources compared to the discharge from the IXTOC I oil

spill. ............................................................................................................... 77

5.1 Dissolved oxygen versus depth for the available data in the Yucatan

Channel. ......................................................................................................... 89

5.2 Cumulative volume by depth of the deepwater Gulf of Mexico and the

vertical delineation between layers. ................................................................. 90

5.3 Box model configuration and notation used for the dissolved oxygen mass

balance. .......................................................................................................... 91

5.4 Velocity profiles of the Yucatan Channel inflow. ........................................... 95
xi

LIST OF TABLES

TABLE PAGE

2.1 Scientific Personnel for the Oxygen Study ....................................................... 7

2.2 Members of the Oxygen Study Science Review Group .................................... 8

3.1 Cruises with Bottle Oxygen Data ...................................................................... 11

3.2 Cruises with Oxygen Sensor Data .................................................................... 15

3.3 Bin Statistics for Bottle Oxygen Data in the Gulf of Mexico Used in

QA/QC ............................................................................................................. 24

3.4 Number of Stations With Bottle Dissolved Oxygen by Month and Decade...... 31

3.5 Yucatan Channel Canonical Profiles for Temperature, Salinity, and

Dissolved Oxygen ............................................................................................ 32

3.6 Deep Gulf Basin Canonical Profiles for Temperature, Salinity, and

Dissolved Oxygen ............................................................................................ 32

4.1 Water Masses of the Caribbean Sea and Gulf of Mexico ................................. 42

4.2 Sources and Sinks of Dissolved Oxygen in the Deepwater Gulf of Mexico ..... 45

4.3 Estimated Rate of Dissolved Oxygen Consumption in the Atlantic Ocean

Between 45°N and 54°N ................................................................................... 50

4.4 Dissolved Oxygen Mean and Standard Deviation in mL·L-1 by Bin for the

Whole Gulf of Mexico and the Yucatan Channel, Bay of Campeche, and

Florida Straits Sub-regions ............................................................................... 54

4.5 Dissolved Oxygen Mean and Standard Deviation in mL·L-1 by Bin for the

Southeast, Northeast, Northwest, and Southwest Sub-regions of the Gulf of

Mexico .............................................................................................................. 57

4.6 Historical Statistics for Dissolved Oxygen At or Below 1500 m Depth in the

Gulf of Mexico ................................................................................................. 59

4.7 Statistics for Dissolved Oxygen At or Below 1500 m Depth in the Gulf of

Mexico from Data in the Oxygen Archive ........................................................ 61

4.8 Reservoirs and Their Dissolved Oxygen Content per km2 ................................ 82

4.9 Number of Barrels of Petroleum per km2 to Produce a Given Change in

Different Reservoirs .......................................................................................... 83

4.10 Dissolved Oxygen Consumed by Total Oxidation of Hydrocarbon Sources .... 85

4.11 Dissolved Oxygen Content by Reservoir........................................................... 86

5.1 Volume, Thickness, and Sediment Surface Areas for Each Layer in the

Basin-wide Box Model of the Gulf of Mexico ................................................. 92

5.2 Fluxes of the Yucatan Channel Inflow .............................................................. 96

5.3 Values of Temperature, Salinity, and Dissolved Oxygen Used for the

Yucatan Channel Inflow and Gulf Interior ........................................................ 101

5.4 Box Model Output from Baseline Runs............................................................ 105

5.5 Model Estimated Dissolved Oxygen Concentrations by Box ........................... 107

5.6 Role of Transport in Establishing Dissolved Oxygen Concentrations .............. 108

5.7 Scenario of No Oxygen Production in the Surface Layer ................................. 109

5.8 Role of Sediment-water Reactions in Maintaining Dissolved Oxygen

Concentrations .................................................................................................. 109

5.9 Role of Carbon Doubling in Maintaining Dissolved Oxygen Concentrations... 109

5.10 Time Necessary for Dissolved Oxygen Concentration to Decrease by 50%

If There Is No Transport ................................................................................... 110
xiii

ACRONYMS

AAIW Antarctic Intermediate Water

ADCP Acoustic Doppler current profiler

AOU apparent oxygen utilization

BOD benthic oxygen demand

COH NEGOM Chemical Oceanography and Hydrography Study

Co-PI Co-Principal Investigator

CTD conductivity-temperature-depth sensor

DBDB2 Digital Bathymetric Data Base 2-min

DGOMB Deep Gulf of Mexico Benthic Study (Northern Bulf of Mexico Continental Slope

Habitats and Benthic Ecology Study)

DOC dissolved organic carbon

FS Florida Straits

ID identifier for cruises

LATEX A Louisiana-Texas Shelf Physical Oceanography Program; Study A

LC Loop Current

LCE Loop Current Eddy

MMS Minerals Management Service, U.S. Department of the Interior

NADW North Atlantic Deep Water (UNADW = Upper NADW)

NEGOM Northeastern Gulf of Mexico Physical Oceanography Program

NESDIS National Environmental Satellite, Data, and Information Service

NMFS National Marine Fisheries Service

NOAA National Oceanic and Atmospheric Administration

NODC National Oceanographic Data Center

NRC National Research Council

NPTS number of data points

OCL Ocean Climate Laboratory

OCS Outer Continental Shelf

PI Principal Investigator

POC particulate organic carbon

QA/QC quality assurance/quality control

R/V research vessel

SAIC Science Applications International Corporation

SEAMAP Southeast Area Monitoring and Assessment Program

SRG Science Review Group

SOOP Ship of Opportunity Program

STD salinity-temperature-depth sensor

SUW Subtropical Underwater

TACW Tropical Atlantic Central Water

TAMU Texas A&M University

TIGER Texas Institutions Gulf Ecosystem Research

2.3σ 2.3 sigma = 2.3 standard deviations from the mean

WOD World Ocean Database

XBT expendable bathythermograph probe

YC Yucatan Channel
1

1 EXECUTIVE SUMMARY

1.1 Introduction

The Minerals Management Service (MMS) of the U. S. Department of the Interior awarded the

contract for the "Deepwater Program: Understanding the Processes that Maintain the Oxygen

Levels in the Deep Gulf of Mexico" to the Texas A&M Research Foundation in July 2002.

Scientists at Texas A&M University (TAMU) conducted the research. The study area was the

deepwater Gulf of Mexico, defined as that part of the Gulf with water depths of 400 m or more.

Vertically the study area extended from sea surface to sea floor. Study results will assist MMS in

its management of the nation's Outer Continental Shelf Leasing Program, including oil and gas

leasing in federal waters of the northern Gulf of Mexico.

The objective of this study was to understand (1) the types and rates of processes occurring in the

deep Gulf of Mexico that affect the levels of oxygen in the deepwater and (2) the balance that

maintains this level. The objective was addressed in five tasks. First, data search, synthesis, and

reanalysis of dissolved oxygen data collected from water depths greater than 400 meters

throughout the entire Gulf of Mexico basin were conducted. Second, a simple box model that

described the sources and sinks of oxygen in the deep Gulf was developed. Third, the effect on

dissolved oxygen concentrations of the inputs from oil and gas activities were evaluated using

parameters and constants determined from the data assembled. Fourth, data gaps were identified,

including an evaluation of the quality of the data and how representative it is of the Gulf’s basin

environment. Finally, methods for filling the data gaps, including field work and modeling

studies were identified.

1.2 Data Sets

Dissolved oxygen data were acquired from MMS, other state and federal agencies, national

laboratories, universities, Mexican institutions, and the private sector. Two types of data were

acquired: titrated measurements of water-bottle samples and measurements from oxygen sensor

probes. Dissolved oxygen data from bottle samples were obtained at nearly 4000 stations from

approximately 270 cruises in the Gulf of Mexico. Data from oxygen probes were obtained for

approximately 1800 stations. It is not clear whether the sensor data were properly calibrated, so

these data were not used in the analyses.

The dissolved oxygen data were collected from 1922 through 2001, when this study ceased data

assembly. Many of the early cruises collected bottle samples throughout the full water column,

while more recent cruises sampled mainly in the upper 1000 to 1500 m. Waters at or below 1500

m are not well sampled, with less than 650 stations sampling from only a few bottles at those

depths. Only the decades of the 1960s and 1970s had station coverage throughout the Gulf basin.

Data quality is variable, even within cruises. This likely is due to different analysis techniques

used in the older data sets and differences in the analysts' visual identification of the end points

of titration, as well as poor sampling and/or laboratory analysis. All data were retained in the

archive, but suspect data points were flagged as such.

1.3 Sources and Sinks

The Gulf of Mexico is a semi-enclosed sea with two channels opening into other parts of the

world's ocean. The major inflow is at the Yucatan Channel, which has a sill depth that is deep

enough (~2000 m) to allow the transport of oxygen rich deep source waters from the Atlantic

Ocean into the Gulf from the Caribbean Sea. The Yucatan Current, which becomes the Loop

Current in the Gulf, brings these waters in. The major outflow at the Florida Straits is shallow
2

enough (~800 m) that the deeper, oxygen rich waters can mix into the interior of the Gulf, rather

than flowing directly out with the Loop Current.

The sources of dissolved oxygen in the upper waters (~100-200 m) in the Gulf of Mexico are the

atmosphere and photosynthesis. Wind and wave action control the air-sea gas exchange.

Photosynthesis is a local process that occurs in the upper layers and depends on light levels and

nutrient supplies. The source of dissolved oxygen to the deep waters is the transport and mixing

of oxygen-rich water masses into the Gulf of Mexico from the Caribbean Sea through the

Yucatan Channel. There is no water mass formation in the Gulf of Mexico to replenish the deep

oxygen concentrations. So, the deep circulation of the Gulf and its associated mixing are the

mechanisms that replenish the deep oxygen.

The major sink of oxygen in the Gulf, as elsewhere in the world's oceans, is oxidation of organic

matter. The organic matter consists of living organisms, detritus from living organisms (fecal

pellets, secretions, dead organisms, etc.), natural hydrocarbon seeps that are prevalent in the

Gulf, continental detritus washed into the ocean through river runoff, and anthropogenic inputs

(water-borne or air-borne organic pollutants as well as inputs from hydrocarbon extraction,

transportation, and consumption).

The rate of oxygen consumption in the water column by organic material has not been measured

in the Gulf of Mexico, and is not easily measured in the world's ocean. However, these rates are

known to decrease exponentially with increasing depth. Rates have been estimated for the

Atlantic, and these can be used to estimate rates in Gulf waters. However, because the coldest

deep Atlantic waters cannot enter the Gulf across the sill at the Yucatan Channel, rates of

consumption used for Gulf waters that are deeper than about 2000 m cannot be based on depth,

but rather should be taken at comparable temperature, which in the Gulf is approximately 4°C

below ~1500 m.

The oxygen minimum zone, which generally is found in the Gulf between 300-700 m, is derived

from two mechanisms. First, the Tropical Atlantic Central Water with the oxygen minimum at its

core brings in waters that are depleted in dissolved oxygen from processes occurring outside the

Gulf. Second, the decay of organic matter that occurs in the Gulf itself further decreases the

oxygen concentrations by the same processes. However, the productivity of the Gulf is not high

enough to create extreme oxygen minimum zones that are seen in other parts of the world's

ocean, such as in the Arabian Sea.

1.4 Box Model

Despite taking a simple approach to a complex problem, the box model provided useful insights

into how the distribution of dissolved oxygen in the deepwater Gulf might be established and

maintained. Foremost is the finding that the distribution of dissolved oxygen is primarily

controlled by extra-basin effects, i.e., the source of dissolved oxygen to the deep waters is the

transport and mixing of oxygen-rich water masses into the Gulf of Mexico through the Yucatan

Channel. Therefore, the success of any additional modeling efforts will be critically dependent

on obtaining accurate estimates of the transport and, more importantly, its distribution with

depth. Based on numerous model scenarios, the modeled flow through the Yucatan Channel at

depth is insufficient to maintain the observed oxygen profiles in the deepwater Gulf. The model

suggests that approximately 30% of the Yucatan flow is diverted into the Gulf, although this may

be a model artifact. Unfortunately the model is unable to provide an independent estimate of the

flux necessary to ventilate the deep Gulf waters below the 2000-m sill depth of the Yucatan

Channel. This vertical mass flux is critical to maintaining the deep oxygen profiles.

Within the basin, the model indicates there are differing secondary controls depending on the

depth. In the surface layer (0-200 m), the model-determined net production of oxygen is
3

consistent with the rates reported for air-sea exchanges and photosynthesis at the latitude

associated with the Gulf of Mexico. In the upper 800 m, the model indicates that the oxidation of

carbon in the water column plays a secondary, but important, role to that of transport in

maintaining oxygen profiles. This is evidenced by the model-determined oxidation rates that

would cause the oxygen concentrations within the Gulf to decay to half of their present value in

ten years if the Gulf was isolated from any transport. Below 1500 m, the model suggests that the

oxidation in the water column is not necessary to maintain oxygen levels, but that the oxidation

at the sediment interface plays a minor, but necessary, role.

1.5 Major Results

Spatial and Temporal Changes in Oxygen Levels: Early studies, using known high quality data

sets, found that below approximately 1500 m there was "no clearly discernable horizontal

variation in dissolved oxygen in these waters" and "only slight vertical oxygen gradients"

(Nowlin et al. 1969). Analysis of the additional data sets used in this study suggest the deep Gulf

waters may have slightly different oxygen values in three different regions: the southeastern Gulf

with mean dissolved oxygen concentrations of 4.99 mL·L-1, the northern Gulf with a mean of

4.95 mL·L-1, and the southwestern Gulf with a mean of 4.87 mL·L-1. However, the quality of the

data sets is variable enough that this finding is not definitive.

Comparison of data sets from the 1970s with those from 2000/2001 indicates there has not been

any discernible change in the vertical or horizontal distribution of dissolved oxygen in the Gulf

of Mexico. This suggests that the transport mechanisms that replenish the oxygen are adequate to

balance the oxygen consumption from decay of organic matter, including that from oil seeps and

anthropogenic sources.

Influences of Shelf Processes on Deepwater Dissolved Oxygen Concentrations: Data sets

available are not adequate to study the details of the effects of shelf-deep ocean exchanges on the

oxygen levels. However, they do show no water mass of consequence is formed on the shelves.

There are no shelf waters dense enough to sink to depth into the deep ocean of the Gulf and so to

provide a source of ventilation for the deep waters. Any shelf-deep ocean exchanges will impact

only the upper waters. Transport of organic matter off the shelf, particularly in the region of the

Mississippi River Delta, could have local effects on the oxygen concentrations in the deep Gulf if

the material sinks to depth before decaying, but evidence of this effect is marginal.

Effects of Natural Hydrocarbons on Dissolved Oxygen Concentrations: NRC (2003) reports an

estimated 140,000 ± 60,000 tonnes of hydrocarbons are introduced into the Gulf of Mexico from

natural seeps each year. This seepage begins at the sea floor and rises through the water column

to the surface, so it potentially can affect the total water column. Although this natural leakage of

hydrocarbons into the Gulf has been occurring for millions of years, the values of dissolved

oxygen throughout the water column have been stable over the period during which

measurements are available and no significant large-scale perturbations in the dissolved oxygen

content of the deep Gulf waters have been observed to result from this input. However, localized

low oxygen conditions are reported to occur within a few centimeters of the sediments and,

hence, it is likely that oxygen concentrations of the waters at the immediate sediment-water

interface may be depleted at the seeps or associated brine pools. Chemosynthetic communities

are active at the seep sites. Because they require access to oxygenated waters, their presence is

additional evidence of ventilation of the deepest Gulf waters.

Effects of Oil and Gas Activities on Dissolved Oxygen Concentrations: NRC (2003) reports

anthropogenic sources of hydrocarbons to the Gulf of Mexico are estimated at approximately

2000 tonnes from extraction (1700 tonnes of which are from produced waters), 1600 tonnes from

transportation, and 6800 tonnes from consumption. These inputs are widespread throughout the

Gulf. Thus, their impact on dissolved oxygen concentrations is negligible over the large scale in
4

the deepwater Gulf of Mexico. The localized effects, however, might be measurable and, in some

cases, substantial. Most of these discharges occur at or near the sea surface, so there would be

essentially no effect on the dissolved oxygen concentrations of the deep waters. An exception

might be any resulting increase in the quantity of decaying organic material, such as from growth

of organisms that utilize the hydrocarbons, that sinks through the water column.

Catastrophic oil spills can introduce hydrocarbons at 2-3 times the rate of the natural seeps. The

effect of such spill on the dissolved oxygen concentrations would depend on many factors,

including chemical characteristics of the spilled material, location in the water column and nature

of the spill, residence time in the water column of labile components of the hydrocarbon as the

spilled material rises toward the sea surface, and environmental conditions at the spill site,

including sites to which the material may be transported. In the large scale, the impact is

expected to be minimal. Effects on dissolved oxygen concentrations in the deepwater Gulf from

anthropogenic hydrocarbon discharges likely would be most substantial at the sediment-water

interface at a discharge site or at the sea surface reached by the plume, not within the water

column itself. Effects on dissolved oxygen concentrations are expected to be local, but

potentially could be severe for short periods. Further study is needed to examine such effects

because they are localized and complex.

Determination of local effects is a priority. However, knowledge of many factors, such as (1) the

environmental conditions (e.g., currents at all affected depths, wave action, water temperatures,

local biological characteristics of the ecosystems that are exposed) at the discharge location, (2)

the type and rate of the discharge, and (3) the chemical nature of the material discharged, is

necessary to make a thorough assessment of the effects of various oil and gas activities on

dissolved oxygen concentrations in the Gulf of Mexico. This is a highly complex problem that

requires a more sophisticated approach that can be accomplished by the simple box model and

the data reanalysis in this study.

1.6 Information Gaps and Recommendations

A number of data gaps have been identified. These include: sparse dissolved oxygen data from

depths ≥ 1500 m; lack of observations on circulation, particularly below 1500 m, to determine

horizontal advection and vertical fluxes and associated mechanisms; no measurements of the rate

of consumption of dissolved oxygen in the deep waters; lack of data on the age of the water

masses; and lack of information on the carbon cycle in the deep water column. These gaps could

be addressed by a combination of a high quality, World Ocean Circulation Experiment (WOCE

1991) type of hydrographic survey, including the full suite of tracers, chemical measurements,

and shipboard current observations, with measurements of currents (moorings and/or floats) and

detailed modeling studies.

A higher priority would be to determine whether anthropogenic activities are locally affecting

dissolved oxygen levels. This will require local monitoring of dissolved oxygen concentrations at

discharge sites. A study designed to assess the nature and extent of the effects on dissolved

oxygen concentrations of discharges at selected sites of oil and gas exploration and production

operations in the deepwater Gulf should be undertaken. Monitoring of discharges should include

measurements to determine the fate of drill cuttings, drilling fluids if discharged, produced

waters, and similar discharges. Local environmental conditions, such as currents, waves,

temperature, salinity, nutrients, carbon, particulate matter, and other chemical properties should

be monitored at the selected locations to allow determination of the importance of the various

complex of factors involved and to provide information that would allow development of an oil

spill model that could assess possible effects on dissolved oxygen of a hypothetical subsurface

blowout. Only if substantive effects on the dissolved oxygen concentrations are shown to occur

should detailed modeling, process, or other studies be undertaken.
5

2 INTRODUCTION

2.1 Background

With the increase of oil and gas activities in the deepwater Gulf of Mexico, the Minerals

Management Service (MMS), U. S. Department of the Interior, seeks a greater understanding of

the environment in Gulf of Mexico waters deeper than 400 m to aid in making decisions about

the protection of the marine environment. The MMS awarded the contract for the "Deepwater

Program: Understanding the Processes that Maintain the Oxygen Levels In the Deep Gulf of

Mexico" to the Texas A&M Research Foundation in July 2002. Under the contract, scientists at

Texas A&M University (TAMU) conducted the study. Because discharges of oil or gas can

consume oxygen during degradation, this study considers what is known about the distribution of

oxygen and processes that control it, what the information gaps might be, and how those gaps

might be filled.

The study area is the region of the Gulf of Mexico basin with water depths of 400 m or more

(Figure 2.1). The data sets and analyses used in this study, however, were extended shoreward to

the 200-m isobath, which is the approximate location of the shelf edge. The study area contains

the slopes off the United States and Mexico, and the Sigsbee and Florida abyssal plains of the

Gulf of Mexico basin. It includes the Yucatan Channel and Florida Straits. Waters of interest to

the study extend from the sea surface to the bottom, which can reach over 3700 m.

2.2 Study Objectives and Description

The objective of this study is to understand the types and rates of processes occurring in the deep

Gulf of Mexico that affect the levels of oxygen in the deepwater and the balance that maintains

this level. The objective will be addressed by four sub-objectives:

(1) conducting a data search, synthesis, and reanalysis of available historical oxygen data

collected from water depths greater than 400 meters throughout the entire Gulf of Mexico basin;

(2) developing a simple box model that describes the sources and sinks of oxygen in the deep

Gulf and that can be used to evaluate inputs from oil and gas activities, using parameters and

constants determined from item 1;

(3) identifying data gaps including an evaluation of the quality of the data and how

representative it is of the Gulf’s basin environment; and

(4) proposing methods for filling the data gaps which could include field work.

To accomplish these objectives, three tasks were set out for the Oxygen Study. Each task had a

Principal Investigator (PI) and one or more Co-PIs (Table 2.1). They were responsible directly to

the Program Manager, Ann E. Jochens, for successful completion of their task. All members

participated in the synthesis and interpretation work and in preparation of this synthesis report.

Matthew K. Howard was the Data Manager, who provided data administration and data archival

per the terms of the contract.

The three tasks were:

Task 1. Data Search, Synthesis, and Reanalysis: Historical dissolved oxygen data that have been

collected within the Gulf of Mexico basin were assembled and synthesized for this report. Along

with the data, the methodology for collecting the data was identified where possible. All data
6

Figure 2.1. Study area for the reanalysis of the historical oxygen data. The study area is

within the boundary denoted by the blue line. This boundary is the 200-m isobath

in the Gulf and the entrances of the Yucatan Channel and Florida Straits.

Bathymetric contours are 200, 400, 1000, 2000, and 3000 m.

were checked for quality, including assessment of sample collection and analysis methodology,

and flagged if determined to be in question. Trends in the data, such as differences between

eastern and western basins, a statistical analysis of the data, and flushing rates of the Gulf of

Mexico were evaluated (see Section 4.3 below).

Task 2. Development of a Simple Box Model: A simple box model was constructed to represent

the inputs to and outputs from the deep Gulf of Mexico basin. It was developed to address, as

well as such a model can, the complexity of the distribution of oxygen. It included the oxygen

minimum zone. An early version considered the eastern and western basins of the Gulf of

Mexico, but this was found to be too complex a system given the data sets available (see Section

5). East-west differences, however, were addressed as part of the data reanalysis work under

Task 1 (Section 4.3). The model was used to help estimate the flushing time of the Gulf of

Mexico and the potential impacts from oil and gas activities on dissolved oxygen in the water

column and in surficial sediments. These issues also were addressed by additional calculations

(see Sections 4.1.3 and 4.4).
7

Table 2.1

Scientific Personnel for the Oxygen Study

(The Principal Investigator (PI) is identified for each task.)

Description Personnel

Program Management Ann E. Jochens, Program Manager

Data and Information Management Matthew K. Howard, Data Manager

Interpretation and Synthesis Team Ann Jochens, Team Leader

Les Bender

Steve DiMarco

Matt Howard

Mahlon Kennicutt

John Morse

Worth Nowlin

Task 1 (Data Assembly and QA/QC) Ann Jochens, PI

Steve DiMarco, Co-PI

Matt Howard, Data Manager

Worth Nowlin

assistance from Task 2 team

Task 2 (Box Model) Les Bender, PI

Mahlon Kennicutt, Co-PI

John Morse, Co-PI

assistance from Task 1 team

Task 3 (Gaps Identification) Ann Jochens, PI

Mahlon Kennicutt, Co-PI

Task 1 members

Task 2 members

Task 3. Identify Information Gaps: From the box model and data synthesis, information gaps

were identified. These included identification of gaps as seen by the inventory of data and

metadata and of gaps in knowledge related to physical and/or biogeochemical processes and

biogeochemical rates from evaluation of the box model results. Methods for filling the gaps, such

as field work, modeling, or a combination, were developed. Resulting recommendations are

described in Section 6 of this report.

2.3 Science Review Group

The Science Review Group (SRG) for the Oxygen Study is composed of three members from the

oceanographic community. Table 2.2 shows the members and their affiliations. The purpose of

the SRG is to review the progress and scientific value of the study, recommend improvements,

and review and comment on the draft Synthesis Report. Quarterly Reports including a Technical

Summary were provided to the SRG for review and comment during the Study.
8

Table 2.2

Members of the Oxygen Study Science Review Group

Member Affiliation

Dr. Larry P. Atkinson Old Dominion University

Dr. Robert M. Key Princeton University

Dr. John M. Morrison North Carolina State University

2.4 Report Organization

This report presents the synthesis of the data assembled and the scientific results of the Oxygen

Study. The Executive Summary, Section 1, provides a brief review of the findings, results, and

significance of this study. Descriptions of the data base assembled and the quality control

processing methods used are given in Section 3. Background information on the physical

oceanography and water masses of the Gulf of Mexico, description of physical and

biogeochemical sources and sinks of dissolved oxygen, and the results of the data reanalysis are

presented in Section 4. The simple box model formulated to aid in the interpretation of the data

distributions is given in Section 5. Information gaps and recommendations for future studies are

presented in Section 6. References are provided in Section 7.
9

3 DATA SETS

The objective of this study is to understand the types and rates of processes occurring in the deep

Gulf of Mexico that affect the levels of oxygen in the deepwater and the balance that maintains

this level. The objective was met first by conducting a data search, synthesis, and reanalysis of

all oxygen data collected from water depths greater than 200 meters throughout the entire Gulf of

Mexico basin. This chapter presents the results of the data search, assembly, and quality control

processing.

3.1 Data Assembly

Dissolved oxygen data sets in the deepwater Gulf of Mexico have been identified through search

of the Texas A&M University (TAMU) Deepwater Physical Oceanography data archive and

review of the literature that identifies cruises on which oxygen data were collected. The TAMU

Deepwater Physical Oceanography data archive was initially developed for the MMS-funded

Deepwater Reanalysis and Synthesis of Historical Data Study (Deepwater; Nowlin et al. 2001).

This archive contains the then available physical oceanographic data, including dissolved oxygen

concentrations, from the Gulf of Mexico in water column depths greater than 200 m (DiMarco et

al. 2001). Data were acquired from MMS, other state and federal agencies, national laboratories,

universities, Mexican institutions, and the private sector, especially the oil and gas industry. The

TAMU Deepwater Physical Oceanographic archive was searched, and stations with oxygen data

were extracted into a separate database (Oxygen Archive) for this study.

A brief survey was made of literature that contained discussion of dissolved oxygen in the

deepwater Gulf of Mexico. The review included peer-reviewed journals as well as readily

available technical and data reports, manuscripts, and other documents. The results of the

literature review were cross-matched with the Deepwater inventory to assess whether referenced

data sets were accounted for in the archive. Data sets identified as not being in the archive were

sought by contacting either the individual or institution responsible for their collection and/or by

contacting authors of publications or reports in which the data were discussed. Data obtained

were added to the Oxygen Archive. Some missing data sets were deemed as lost or inaccessible.

The temporal range of hydrographic data (which includes temperature, salinity, and dissolved

oxygen observations) currently in the Oxygen Archive covers the range from 1922 through 2001.

The data sources from which this database was assembled are:

• CD-ROM NODC-02 "Global Ocean Temperature and Salinity Profiles Vol. 1 - Atlantic,

Indian, and Polar Oceans." U.S. Dept. of Commerce. NOAA/NESDIS/NODC. 1991. This CDROM contains data collected by various researchers and institutions between 1900 and 1990 and

includes oceanographic station (i.e., bottle) data and CTD/STD data.

• CD-ROM NODC-72 "Gulf of Mexico Hydrographic Data. CTD, XBT, and Bottle Data

1987-1995 and GulfCET Program CTD, XBT, and Marine Mammals Database, 1992-1994."

December 1996. U.S. Dept. of Commerce, NODC. A collection of station data from the Texas

A&M University Ship of Opportunity (TIGER/SOOP) and MMS GulfCET programs.

• CD-ROM NODC-126 to NODC-130 "WOD98 World Ocean Data Base 1998 Version 2."

Jan 2000. U.S. Dept. of Commerce. NOAA/ NESDIS/NODC/OCL. Data sets includes bottle and

CTD data from 1900 to 1995. This is a revised version of World Ocean Atlas 1994. Some

duplicate stations were removed and many new stations added from data archaeology and data

rescue programs. A 5-CD-ROM set.
10

• CD-ROM NODC-92 "Texas-Louisiana Shelf Circulation and Transport Processes Study.

Hydrography, Drifters, ADCP, and Miscellaneous Sensors Data and Reports 1992-1994."

September 1998. U.S. Minerals Management Service and U.S. Dept. of Commerce.

NOAA/NODC. A 5-CD-ROM set. This archive contains bottle and CTD data. Most data are for

the waters of the continental shelf but some samples were taken in water as deep as 1000 m.

• CD-ROM NODC 148 "World Ocean Database 2001, Ocean Station and Surface Data,

Observed Depth Data" – an updated version of the WOD98. This is part of an 8-CD-ROM set

prepared by the Ocean Climate Laboratory for the National Oceanographic Data Center, Silver

Spring, MD 20910. Data are also available online through various NOAA portals.

• CD-ROM "Northeastern Gulf of Mexico Physical Oceanography Program. Chemical

Oceanography and Hydrography Study. Data and Reports." 2002. Dept. of Oceanography, Texas

A&M University and U.S. Dept. of the Interior. Minerals Management Service. This contains

hydrographic data, including dissolved oxygen data, collected from the northeastern Gulf shelves

out to 1000-m water depth between 1997 and 2000.

• CD-ROM "Deepwater Physical Oceanography Reanalysis and Synthesis of Historical

Data. Deepwater CD-ROM 2 Hydrography and expendable instruments data." April 2001. U.S.

Dept. of the Interior. Minerals Management Service. A 3-CD-ROM set. This contains available

physical oceanographic data for the deep (greater than 200 m) Gulf of Mexico. These data were

assembled from the above CD-ROM sources and from previously unreleased, non-proprietary oil

industry surveys.

• Originator data files from the cruises in May/June 2000 and June 2001 aboard the R/V

Gyre, conducted as part of the MMS-funded "Deepwater Program: Northern Gulf of Mexico

Continental Slope Habitats and Benthic Ecology" Study (DGOMB). Oxygen data were collected

on the June 2002 DGOMB cruise, but were discarded by the originator because the analysis

instrumentation was faulty and all analyses were bad.

• Data sets typed in and checked from several TAMU Oceanography Technical Reports

and TAMU Data Reports for various cruises of the R/V Jakkula, Hidalgo, Alaminos, and Gyre.

Data from these cruises were unavailable through NODC and were not digitally available.

• Some data were found in NODC’s Ocean Archive System. This online resource serves

data sets in the originator’s format together with the originator’s metadata. Sixty-one data sets

spanning 1969-1995 were downloaded and scanned for oxygen data. 1339 profiles were

extracted, reformatted and examined. Of these 230 were from deep Gulf stations, the rest were

from the shelf or just outside the study region.

• National Marine Fisheries Service (NMFS): The Southeast Area Monitoring and

Assessment Program (SEAMAP) is a cooperative program between NMFS and the five Gulf

States fishery management agencies. Mark Mcduff, a data manager for the program, kindly

provided the entire SEAMAP environmental database containing 16383 profiles. Most were

from stations over the shelf but 319 stations occurred in the deepwater study area.

• Various individuals who kindly provided data sets missing from the NODC archive.

The number of stations with discrete oxygen data is approximately 3600 from over 200 cruises.

There are nearly 35,000 discrete data points that were collected between 1922 and 2003.

Additionally, there are over 1800 CTD casts with oxygen sensor data. Table 3.1 gives the cruises

on which bottle oxygen data were identified as having been collected, information on the cruise

dates, an indicator of whether the data are in the database, and any references consulted for the

particular data set. Table 3.2 gives the cruises on which oxygen sensor data were collected.
11

Table 3.1

Cruises with Bottle Oxygen Data

(The data sources are the data originator, if known, or the data repository, usually NODC, from

which the data were obtained. Dates give the range of sampling dates. The number of stations in

the deepwater data base are given. References to literature and/or reports are shown if known.)

Vessel Cruise ID Dates

(mm/dd/yyyy)

No.

Stns

Data

Source

References

Dana II* 02/01/1922-02/06/1922 10 CCF

Atlantis I* 05/04/1933-05/05/1933 5 WHOI Wennekens 1959

Atlantis I* 03/01/1934-03/041934 10 WHOI

Atlantis I* 02/16/1935-04/13/1935 66 WHOI Wennekens 1959; Le Bureau Du

Conseil Service Hydrographique

1936; Nowlin et al. 1969;

Nowlin 1972

San Pablo* 7 12/02/1950-12/09/1950 14 NOO

Rehoboth* 7 12/03/1950-12/09/1950 13 NOO

A. A. Jakkula 54-09 07/08/1954-07/17/1954 35 TAMU Leipper 1956

A. A. Jakkula 54-10 08/14/1954-09/07/1954 40 TAMU Leipper 1956; Austin 1955;

Wennekens 1959

A. A. Jakkula 55-1 01/12/1955-02/27/1955 53 TAMU Drummond 1956

A. A. Jakkula 55-3 03/29/1955-04/07/1955 13 TAMU Drummond 1956

A. A. Jakkula 55-8 07/04/1955-07/06/1955 6 TAMU Drummond 1957

Caryn* 96 05/17/1956-05/18/1956 7 WHOI Wennekens 1959

Unknown (U of M.) G5705 04/01/1957-04/07/1957 0 Wennekens 1959; 7 stations, no

data found

Gerda* 10/20/1957 5 NODC

Gerda* 02/22/1958-02/24/1958 6 NODC

Hidalgo* 58-H-1 03/27/1958-04/01/1958 10 TAMU McLellan 1959; McLellan 1960;

Nowlin et al. 1969; Nowlin 1972

Hidalgo 58-H-4 05/14/1958-06/28/1958 20 TAMU McLellan 1959; McLellan 1960;

Nowlin et al. 1969; Nowlin 1972

Gerda* 04/11/1958-04/25/1958 10 NODC

Gerda* 09/30/1958-10-01/1958 11 NODC

Hidalgo* 59-H-2 02/18/1959-02/26/1959 10 TAMU McLellan 1960; Nowlin et al.

1969; Nowlin 1972

Hidalgo* 60-H-6 05/19/1960-05/28/1960 15 TAMU

Gerda* 6110 03/01/1961-03/04/1961 10 MIAMI

Gerda 6110 04/30/1961 1 NODC

Gerda* 6110 11/18/1961-11/20/1961 4 NODC

Hidalgo* 61-H-6 03/23/1961-04/07/1961 6 TAMU

Hidalgo* 61-H-9 05/23/1961-06/06/1961 16 TAMU

Hidalgo* 61-H-16 10/15/1961-10/17/1961 5 TAMU

Hidalgo* 61-H-19 11/11/1961-11/13/1961 5 TAMU

Hidalgo* 62-H-1 01/17/1962-01/19/1962 5 TAMU

Hidalgo* 62-H-3 02/14/1962-03/31/1962 90 TAMU McLellan & Nowlin 1962;

McLellan & Nowlin 1963;

Nowlin & McLellan 1967;

Nowlin et al. 1969; Nowlin

1972; Ichiye et al. 1973

Gerda* 04/24/1962-04/25/1962 3 NODC

Hidalgo* 62-H-4 05/12/1962-05/25/1962 10 TAMU

Gerda* 06/07/1962-06/14/1962 14 NODC

Alaminos* 12/07/1963-12/08/1963 4 NODC

Alaminos* 64-A-2 01/17/1964-01/25/1964 17 TAMU Nowlin et al. 1969; Nowlin 1972

Olenty* 02/24/1964-03/17/1964 9 NODC

Alaminos* 64-A-3 02/27/1964-03/02/1964 7 TAMU Nowlin et al. 1969; Nowlin 1972

Obdorsk* SRT-R9029 05/23/1964-06/14/1964 12 ARI

Alaminos* 65-A-1 02/07/1965 4 TAMU Leipper 1968b

Akademik A. Kovalevsky* 03/28/1965-05/31/1965 45 ARI

Alaminos* 65-A-11 08/11/1965-08/21/1965 23 TAMU Leipper 1968a

Alaminos* 65-A-13 09/12/1965-09/23/1965 59 TAMU Leipper 1968a; Ichiye et al. 1973

Alaminos* 10/02/1965-10/07/1965 6 NODC

Geronimo* 6 11/01/1965-11/02/1965 6 NMFS-M

Alaminos* 01/09/1966-01/27/1966 9 NODC

Pillsbury* 01/31/1966 1 MIAMI

Alaminos* 66-A-3 02/11/1966-02/20/1966 42 TAMU Leipper 1968b

Undaunted* 06/02/1966-06/06/1966 12 NODC
12

Table 3.1

Cruises with Bottle Oxygen Data (continued)

Vessel Cruise ID Dates

(mm/dd/yyyy)

No.

Stns

Data

Source

References

Alaminos* 66-A-8 06/06/1966-06/22/1966 14 TAMU Nowlin et al. 1969; Nowlin

1972; Leipper 1970

J. E. Pillsbury* 6609 06/12/1966-06/22/1966 2 MIAMI

SRT-R9029* CUBA I 08/12/1966 3 FRC

Alaminos 66-A-11 08/05/1966-08/07/1966 10 TAMU Leipper 1968a; Ichiye et al.

1973

Alaminos* 66-A-15 11/06/1966-11/12/1966 9 TAMU Leipper 1968a; Ichiye et al.

1973

Alaminos 66-A-15 10/28/1966-11/04/1966 29 TAMU Leipper 1968a; Ichiye et al.

1973

SRT-R9029* CUBA III 01/22/1967-01/23/1967 3 FRC

Peto* CUBA III 02/18/1967 4 FRC

SRT-R9029* CUBA III 05/06/1967 3 FRC

Mikhail Lomonosov* 20 02/21/1967 1 ARI

Mikhail Lomonosov* 20 03/02/1967 1 ARI

Alaminos* 67-A-4 06/11/1967-07/01/1967 24 TAMU Nowlin et al. 1969; Nowlin

1972; Ichiye et al. 1973

Alaminos* 67-A-6 08/05/1967-08/20/1967 28 TAMU Leipper 1968a

SRT-R9074* CUBA II 08/26/1967 4 FRC

SRT-R9074* CUBA II 09/23/1967-09/24/1967 3 FRC

SRT-R9074* CUBA II 10/23/1967 3 FRC

Alaminos* 67-A-8 09/09/1967-09/12/1967 8 TAMU Nowlin et al. 1969; Nowlin

1972

SRT-R9074* CUBA IV 01/15/1968-01/16/1968 3 FRC

SRT-R9074* CUBA IV 02/02/1968-02/03/1968 3 FRC

SRT-R9074* CUBA IV 04/11/1968-04/12/1968 3 FRC

Alaminos* 68-A-2 02/16/1968-03/05/1968 26 TAMU Leipper 1968b

J. E. Pillsbury* P-6803 04/11/1968-04/27/1968 13 MIAMI

Alaminos* 68-A-5 04/22/1968-05/13/1968 16 SUSIO

Sardina* CUBA IV 06/29/1968 4 FRC

Alaminos* 68-A-8 08/18/1968-09/03/1968 21 TAMU Leipper 1968c

Alaminos* 68-A-9 09/11/1968-09/15/1968 8 TAMU

SRT-R9029* CUBA IV 09/12/1968 3 FRC

SRT-R9029* CUBA IV 10/15/1968-10/16/1968 4 FRC

Atlantis II* 11/16/1968-11/25/1968 3 BLOS/

MIAMI

SRT-R9112* CUBA V 02/13/1969 3 FRC

SRT-R9112* CUBA V 04/06/1969-04/07/1969 3 FRC

SRT-R9112* CUBA V 05/25/1969-05/26/1969 2 FRC

Trident* 68 04/19/1969-04/24/1969 8 URI

Alaminos* 69-A-7 05/01/1969-05/18/1969 14 SUSIO

Alaminos* 69-A-8 06/08/1969-06/15/1969 10 TAMU

Kane* 939014 06/10/1969-09/09/1969 148 NOO

Alaminos* 69-A-10 07/12/1969-07/29/1969 21 TAMU

Akademik N. Knipovich* CUBA V 07/13/1969-07/14/1969 4 FRC

SRT-R9075* CUBA V 08/02/1969-08/03/1969 2 FRC

SRT-R9075* CUBA V 09/06/1969-09/07/1969 3 FRC

SRT-R9075* CUBA V 10/14/1969 3 FRC

Alaminos* 69-A-12 09/09/1969-09/18/1969 6 SUSIO

Alaminos* 70-A-3 02/05/1970-03/03/1970 4 SUSIO

Alaminos* 70-A-6 04/04/1970-04/21/1970 61 TAMU

Alaska* 05/01/1970-05/11/1970 8 TAMU

Tursiops* T7015 05/02/1970-05/11/1970 8 FSU

Alaminos* 70-A-7 05/05/1970-05/11/1970 12 SUSIO

Alaminos* 70-A-9 06/17/1970-06/21/1970 9 SUSIO

Alaminos* 70-A-14 10/22/1970-11/01/1970 20 SUSIO

Virgilio Uribe* C12 (7012) 10/31/1970-11/13/1970 39 NODC

Virgilio Uribe VU7012 10/31/1970-11/13/1970 13 NODC

Virgilio Uribe* VU7102 01/18/1971-01/21/1971 6 NIO

Aliot (SRT-M8005)* 6189 03/15/1971 1 ARI

Aliot (SRT-M8005)* 3234 03/20/1971-03/21/1971 7 CITI

Aliot (SRT-M8005)* 3237 07/18/1971 5 CITI

Virgilio Uribe* VU7108 04/28/1971-05/09/1971 19 NIO

Virgilio Uribe VU7108 04/28/1971-05/10/1971 31 NIO

Virgilio Uribe* 711 05/24/1971-06/09/1971 39 UN

Virgilio Uribe* VU7114 07/21/1971-07/27/1971 19 NIO

Virgilio Uribe* 712 08/04/1971-09/03/1971 42 UN

Foton(SRT-M8024)* 3314 10/05/1971-10/07/1971 16 CITI

Foton* 11/04/1971-11/07/1971 10 CITI
13

Table 3.1

Cruises with Bottle Oxygen Data (continued)

Vessel Cruise ID Dates

(mm/dd/yyyy)

No.

Stns

Data

Source

References

Foton(SRT-M8024)* 3315 11/04/1971-11/05/1971 6 CITI

Foton(SRT-M8024)* 3238 10/06/1971-10/07/1971 3 CITI

Foton(SRT-M8024)* 3239 11/06/1971 3 CITI

Virgilio Uribe* VU7120 10/10/1971-10/17/1971 20 NIO

Virgilio Uribe* 713 10/27/1971-11/10/1971 38 UN

Virgilio Uribe* VU7202 01/11/1972-01/19/1972 21 NIO

Virgilio Uribe VU7202 01/10/1972-01/19/1972 17 NIO

Antares* 02/18/1972-02/24/1972 9 NODC

Alaminos* 72-A-7 03/26/1972-03/28/1972 2 TAMU Morrison & Nowlin 1977

Virgilio Uribe* 721 04/25/1972-05/19/1972 38 UN

Alaminos* 72-A-9 05/02/1972-05/21/1972 57 TAMU Morrison et al. 1973;

Morrison & Nowlin 1977

Tursiops* 11 05/16/1972-05/18/1972 7 FSU

Andrey Vilkitsky* 03/17/1973 1

Antares* 03/23/1973-03/24/1973 3

Akademik Kurchatov* 14 04/03/1973-04/07/1973 4 IO

Alaminos* 73-A-8 05/18/1973-06/03/1973 14 SUSIO

Virgilio Uribe* VU7310 05/23/1973-06/05/1973 27 UN

Researcher* 10/06/1976-11/14/1976 89 AOML Berberian and Cantillo 1976

Gyre* 76-G-11 11/11/1976-11/16/1976 2 TAMU Key 1981

Gyre 77-G-14 12/05/1977-12/06/1977 2 TAMU

Gyre 78-G-03 04/04/1978-04/12/1978 42 TAMU Morrison et al. 1983

Gyre* 04/06/1982 1

Suncoaster* 09/18/1982 1

Suncoaster* GM01 03/09/1983-03/20/1983 35 SAIC SAIC 1986

Suncoaster* SC8310 11/12/1983-11/18/1983 41 SAIC SAIC 1986

Cape Florida* CF8405 05/06/1984-05/17/1984 39 SAIC SAIC 1986; SAIC 1987

Pelican* PN-8502 10/22/1985-10/25/1985 16 SAIC SAIC 1988

Justo Sierra* JS085 03/19/1985 1

Altair* AL8601 01/24/1986-02/04/1986 45 SAIC SAIC 1988

Pelican* PN8713 04/08/1987 2 SAIC SAIC 1989

Pelican PN8713 04/07/1987-04/10/1987 22 SAIC SAIC 1989

Pelican PN8715 04/25/1987-04/26/1987 15 SAIC SAIC 1989

Pelican PN8803 07/17/1987 11 SAIC SAIC 1989

Pelican PN8814 11/15/1987-11/16/1987 9 SAIC SAIC 1989

Gyre* 87G04 04/12/1987-04/16/1987 11 TAMU

Gyre* 87G10 10/27/1987-10/28/1987 3 TAMU

Gyre* 87G11 11/18/1987-11/23/1987 12 TAMU

Gyre* 87G12 11/29/1987- 12/05/1987 11 TAMU

Gyre* 88G05 10/171988-10/23/1988 19 TAMU

Pelican PN8820 02/09/1988-02/10/1988 15 SAIC SAIC 1989

Pelican PN8903 07/21/1988 16 SAIC SAIC 1989

Gyre* 89G06 05/18/1989-05/201989 4 TAMU

Gyre* 89G15 11/13/1989-1117/1989 10 TAMU Biggs 1989

Gyre* 90G04 02/21/1990 1 TAMU

Gyre* 90G05 02/28/1990 1 TAMU

Gyre* 90G10 07/14/1990-07/23/1990 12 TAMU Biggs 1990

Gyre* 90G15 07/13/1990-07/15/1990 6 TAMU Biggs 1991a

Gyre* 10/13/1990-10/15/1990 7 TAMU

Gyre* 91G02 03/05/1991-03/09/1991 8 TAMU Biggs 1991b

Gyre* 91G04 06/15/1991-06/25/1991 11 TAMU Biggs 1991c

Gyre* 92G04 04/08/1992 1 TAMU

Gyre* 92G05 05/01/1992-05/08/1992 7 TAMU LATEX: Jochens et al. 1998;

Nowlin et al. 1998a, 1998b

Gyre* 92G07 06/22/1992-06/25/1992 3 TAMU

Gyre* 92G08 08/01/1992-/0807/1992 12 TAMU LATEX: Jochens et al. 1998;

Nowlin et al. 1998a, 1998b

Gyre* 92G10 10/05/1992 2 TAMU

J. W. Powell* 92P10 11/05/1992-11/12/1992 8 TAMU LATEX: Jochens et al. 1998;

Nowlin et al. 1998a, 1998b

Gyre* 93G01 01/08/1993-01/14/1993 10 TAMU

Gyre* 93G02 02/06/1993-02/12/1993 12 TAMU LATEX: Jochens et al. 1998;

Nowlin et al. 1998a, 1998b

J. W. Powell* 93P06 04/28/1993-05/10/1993 21 TAMU LATEX: Jochens et al. 1998;

Nowlin et al. 1998a, 1998b

J. W. Powell* 93P11 07/28/1993-08/06/1993 21 TAMU LATEX: Jochens et al. 1998;

Nowlin et al. 1998a, 1998b

J. W. Powell* 93P14 11/10/1993-11/21/1993 23 TAMU LATEX: Jochens et al. 1998;

Nowlin et al. 1998a, 1998b
14

Table 3.1

Cruises with Bottle Oxygen Data (continued)

Vessel Cruise ID Dates

(mm/dd/yyyy)

No.

Stns

Data

Source

References

Gyre* 94G01 04/25/1994-05/07/1994 40 TAMU LATEX: Jochens et al. 1998;

Nowlin et al. 1998a, 1998b

Gyre* 94G03 05/17/1994 1 TAMU Biggs 1994a

Gyre* 94G05 07/19/1994-07/20/1994 8 TAMU Biggs 1994b

Gyre* 94G07 08/16/1994-08/19/1994 4 TAMU Biggs 1994c

Gyre* 94G08 10/20/1994-10/24/1994 33 TAMU Biggs 1995a

J. W. Powell* 94P10 07/28/1994-08/07/1994 41 TAMU LATEX: Jochens et al. 1998;

Nowlin et al. 1998a, 1998b

Gyre* 94G09 11/03/1994-11/13/1994 49 TAMU LATEX: Jochens et al. 1998;

Nowlin et al. 1998a, 1998b

Gyre* 95G03 06/13/1995-06/16/1995 7 TAMU Biggs 1995b

Gyre* 97G14 11/17/1997-11/26/1997 47 TAMU NEGOM: Jochens et al. 2002

Gyre* 98G05 05/06/1998-05/15/1998 47 TAMU NEGOM: Jochens et al. 2002

Gyre* 98G10 07/27/1998-08/06/1998 42 TAMU NEGOM: Jochens et al. 2002

Gyre* 98G15 11/14/1998-11/24/1998 45 TAMU NEGOM: Jochens et al. 2002

Gyre* 99G07 05/17/1999-05/27/1999 43 TAMU NEGOM: Jochens et al. 2002

Gyre* 99G08 08/18/1999-08/27/1999 43 TAMU NEGOM: Jochens et al. 2002

Gyre* 99G12 11/13/1999-11/21/1999 44 TAMU NEGOM: Jochens et al. 2002

Gyre* 00G04 04/16/2000-04/26/2000 48 TAMU NEGOM: Jochens et al. 2002

Gyre* 00G05 05/04/2000-06/17/2000 44 TAMU DGoMB: Rowe 2005

Gyre* 00G08 07/29/2000-08/07/2000 46 TAMU NEGOM: Jochens et al. 2002

Gyre* 01G05 06/03/2001-06/18/2001 12 TAMU DGoMB: Rowe 2005

Various SEAMAP 06/02/1982-09/23/2003 319 SEAMAP http://www.gsmfc.org/seamap.

html

* denotes cruise data were used in statistical analyses; for cruises listed twice, one listing gives the

number of stations used in statistical analyses and the other is the number not used

Key to Data Sources:

AOML: NOAA, Atlantic Oceanographic and Meteorological Laboratories

ARI: Atlantic Research Institute of Fishing Econ. and Ocean. (ATLATNIRO), USSR

BLOS: Bigelow Laboratory for Ocean Science, ME

CCF: Collection, Carlsberg Foundation, Charlottenlund, Denmark

CITI: Cuban Institute of Technological Investigations, Havana, Cuba

DUKE: Duke University

FRC: Fisheries Research Center, Havana, Cuba

FSU: Florida State University

IO: Institute of Oceanology, AS USSR, Moscow

MHI: Marine Hydrophysical Institution, USSR

MIAMI: Rosenstiel School of Marine and Atmospheric Science, University of Miami

NIO: National Institute of Fisheries, Mexico

NMFS-M: NOAA, National Marine Fisheries Service, Miami, FL

NMFS-G: NOAA, National Marine Fisheries Service, Galveston, TX

NODC: NOAA, National Oceanographic Data Center

NOO: U.S. Naval Oceanographic Office, Bay St. Louis, MS

NOS: NOAA, National Ocean Service, Norfolk, VA

POI: Pacific Oceanological Institute (VLADIVOSTOK)

SAIC: Science Applications International Corporation

SEAMAP: Southeast Area Monitoring and Assessment Program

SIO: Scripps Institute of Oceanography

SUSIO: State University System of Florida, Institute of Oceanography, St. Petersburg, FL

TAMU: Texas A&M University

UN: University National of Mexico, Institute of Geophysics, Mexico City

URI: University of Rhode Island

USCG: U.S. Coast Guard

WCC: Woodward-Clyde Consultants

WDCB: World Data Center B

WHOI: Woods Hole Oceanographic Institution
15

Table 3.2

Cruises with Oxygen Sensor Data

(The data sources are the data originator, if known, or the data repository, usually NODC, from

which the data were obtained.)

Vessel Cruise ID Dates

(mm/dd/yyyy)

No.

Stns

Data

Source*

Justo Sierra JS085 03/05/1985-03/23/1985 27 MMS_DEEPW

OREGON II CRU_199 04/22/1992-05/23/1992 88 MMS_DEEPW

R/V Gyre 92G05 05/01/1992-05/08/1992 13 MMS_LATEXA

R/V Gyre 92G08 08/01/1992-08/07/1992 19 MMS_LATEXA

Pelican CRUISE2 08/23/1992-08/12/1992 31 MMS_DEEPW

R/V Gyre 92G10 11/05/1992-11/12/1992 15 MMS_LATEXA

Pelican CRUISE3 11/10/1992-11/21/1992 28 MMS_DEEPW

OREGON_II CRU_203 01/06/1993-02/11/1993 63 MMS_DEEPW

R/V Gyre 93G02 02/06/1993-02/12/1993 19 MMS_LATEXA

Pelican CRUISE4 02/13/1993-02/24/1993 30 MMS_DEEPW

J.W. Powell 93P06 04/28/1993-05/11/1993 39 MMS_LATEXA

OREGON_II CRU_204 05/06/1993-06/15/1993 105 MMS_DEEPW

Pelican CRUISE5 05/25/1993-06/03/1993 30 MMS_DEEPW

J.W. Powell 93P11 06/28/1993-08/06/1993 37 MMS_LATEXA

Pelican CRUISE6 08/28/1993-09/05/1993 27 MMS_DEEPW

J.W. Powell 93P14 11/10/1993-11/21/1993 37 MMS_LATEXA

Pelican CRUISE7 12/05/1993-12/12/1993 23 MMS_DEEPW

J.W. Powell 94P10 07/27/1994-08/07/1994 52 MMS_LATEXA

R/V Gyre 94G09 11/02/1994-11/13/1994 51 MMS_LATEXA

SHOYOMARU 1991-01 05/08/1994-05/13/1994 10 MMS_DEEPW

OREGON_II CRU_209 04/16/1994-06/09/1994 78 MMS_DEEPW

R/V Gyre 96G06 10/12/1996-10/27/1996 12 MMS_DEEPW

Justo Sierra CANEK_0 12/11/1996-12/18/1996 35 DEEPSTAR

Justo Sierra CANEK_1 05/25/1997-06/04/1997 52 DEEPSTAR

R/V Gyre 97G08 08/07/1997-08/21/1997 11 MMS_DEEPW

R/V Gyre 97G14 11/16/1997-11/26/1997 48 MMS_NEGOM

R/V Gyre 98G05 11/16/1997-11/26/1998 48 MMS_NEGOM

Justo Sierra CANEK_2 03/30/1998-04/06/1998 61 DEEPSTAR

R/V Gyre 98G10 07/26/1998-08/06/1998 43 MMS_NEGOM

R/V Gyre 98G15 11/13/1998-11/24/1998 46 MMS_NEGOM

Justo Sierra CANEK_3 01/31/1999-02/03/1999 28 DEEPSTAR

R/V Gyre 94G01 04/24/1999-05/07/1994 50 MMS_LATEXA

R/V Gyre 99G07 05/20/1999-05/27/1999 46 MMS_NEGOM

R/V Gyre 99G08 08/18/1999-08/27/1999 44 MMS_NEGOM

Justo Sierra CANEK_4 09/04/1999-09/11/1999 89 DEEPSTAR

R/V Gyre 99G12 11/13/1999-11/21/1999 45 MMS_NEGOM

R/V Gyre 00G04 04/16/2000-04/26/2000 49 MMS_NEGOM

R/V Gyre 00G05 05/04/2000-06/17/2000 45 MMS_DGOMB

R/V Gyre 00G08 07/29/2000-08/07/2000 47 MMS_NEGOM

Justo Sierra CANEK_5 06/25/2000-07/03/2000 117 DEEPSTAR

R/V Gyre 01G05 06/02/2001-06/18/2001 13 MMS_DGOMB

Justo Sierra CANEK_6 05/03/2001-06/14/2001 72 DEEPSTAR

R/V Gyre 02G03 04/02/2002-04/02/2002 1 MMS_DGOMB

R/V Gyre 02G07 06/04/2002-06/13/2002 6 MMS_DGOMB

* MMS_DEEPW: Nowlin et al. (2001); DiMarco et al. (2001)

 MMS_LATEXA: Nowlin et al. (1998a, 1998b); Jochens et al. (1998)

 MMS_DGOMB: Rowe (2005)

 MMS_NEGOM: Jochens and Nowlin (1998, 1999, 2000); Jochens et al. (2002)

 DEEPSTAR: CICESE and Mitchell (2002)

Figure 3.1 shows the locations of the stations at which bottle oxygen data have been assembled.

These are presented in six parts covering time periods of the order of a decade. Figure 3.2 shows

the locations of the stations at which oxygen sensor data have been assembled.
16

Figure 3.1. Locations of bottle oxygen stations for the deepwater Gulf of Mexico. Shown are

stations made (a) before 1950, (b) 1950 to 1960, (c) 1960 to 1970, (d) 1970 to 1980,

(e) 1980 to 1990, and (f) 1990 through 2001. Bathymetry contours are 200 m and

3000 m. Not all stations in final data base are shown.
17

Figure 3.1. Locations of bottle oxygen stations for the deepwater Gulf of Mexico. (continued)
18

Figure 3.1. Locations of bottle oxygen stations for the deepwater Gulf of Mexico. (continued)
19

98N 96N 94N 92N 90N 88N 86N 84W 82N 80N

20N

22N

24N

26N

28N

30N

18N

Figure 3.2. Locations of stations with dissolved oxygen sensor data in the deepwater Gulf of

Mexico. Bathymetry contours are 200 and 3000 m.

3.2 Data Quality Assurance and Quality Control

Dissolved oxygen data basically fall into two types: 1) discrete values from iodometric analysis

(classical Winkler or variants) of water samples collected from Niskin or Nansen bottles, or 2)

continuous values from Beckman polarographic oxygen sensors or other sensors mounted on

CTD/STD packages. Both data types were assembled. The quality of dissolved oxygen

measurements made using the classical Winkler (1888) technique and its variants (e.g.,

Carpenter 1965a, 1965b, 1966; see also Strickland and Parsons 1972) depends to a large degree

on the skill of the analyst and application of analysis techniques. For example, variations in

water sampling techniques (including those caused by day/night lighting on the deck and weather

conditions under which sampling is done), reagent preparation and use, and visual starch

endpoint determinations by different eyes can result in large or small variations in results (see,

e.g., Culberson 1991; Culberson et al. 1991) The quality of the polarographic oxygen sensor

data, however, depends on the quality of a careful calibration using bottle data. Without such

calibration, they are of unknown quality. Most polarographic oxygen sensor data did not show

evidence of having been calibrated post-cruise with the appropriate bottle data. Such calibration

is a very time consuming process that is beyond the scope of the contract. Therefore, such

calibrations were not done on the continuous sensor data. These data were included in the

database with available metadata. However, they were not used in the synthesis.
20

Each data file was formatted by station in accordance with a set format, including standard

headers containing the metadata. The metadata includes the date/time, latitude, longitude, station

number, total water depth, and other information as available. Figure 3.3 shows an example of a

header. The columnar data consists of depth, dissolved oxygen, and associated data quality flag.

The files include, if known, temperature and salinity data and their quality flags. Some nutrient

data also are included, but their quality is unknown and was not checked.

For each data set, the metadata were reviewed for accuracy and completeness. Corrections were

made as available information from publications and/or reports allowed. Seafloor depths, either

those provided by the data originator or from the U.S. Navy’s Digital Bathymetric Data Base 2-

min (DBDB2), were compared to the deepest data point in each cast for the given location.

Discrepancies were resolved by accepting the bottom depth that was at or exceeded the deepest

data point. Where available and not in conflict with the deepest datum, the originator's water

depth is reported in the metadata. The longitude and latitude were plotted and checked to see if

the locations were reasonable. Dates were examined for reasonableness as compared to other

station dates on a given cruise.

Most, if not all, determinations of dissolved oxygen concentrations derived from water-bottle

samples were made using the Winkler (1888) method or a variant due to Carpenter (1965a,

1965b, 1966). Winkler-based methods are the usual method used for the time period covered in

the database. Moreover, there generally was insufficient information available from the data

sources to determine the particular variant of the method applied, so none are reported in the

metadata.

Although continuous dissolved oxygen sensors have been developed and are in common use

today, data from polarographic oxygen sensors are often questionable due to problems associated

with the probe response. The response is nonlinear and sensitive to (a) strong oxygen gradients,

(b) the decent rate of the instrument package, and (c) oxygen depletion around the probe tip in

non-pumped systems. The sensor also suffers from significant pressure hysteresis (large

differences between the upcast and downcast dissolved oxygen profiles). Relatively complex

correction algorithms have been developed to correct for these effects (Owens and Millard

1985), but often with unsatisfactory results. The instrument performs best in the deeper portions

of casts made in deep water. Any polarographic oxygen sensor data that have not been calibrated

using dissolved oxygen bottle data from Winkler-type analytical techniques are unreliable for use

in the study analysis. Information was generally lacking on whether such calibrations had been

applied. So, the polarographic oxygen sensor data were retained in the Oxygen Archive, but no

QA/QC was applied. The data were not used in the reanalysis or synthesis interpretations.

Dissolved oxygen sensors that have become available in the last several years may make more

reliable measurements (see e.g., the DeepStar data set across the Yucatan Channel); however, the

few data sets from these sensors were retained as is in the database with no further evaluation.

All data are retained in the database, including suspect data. The suspect data are handled

through the use of flags. A value of zero denotes the datum passes the QA/QC tests. A value of

one denotes the datum is suspect, and a two denotes no QA/QC was applied.

Temperature, salinity, and dissolved oxygen data from the Deepwater Archive had undergone

QA/QC processing as part of the Deepwater Study. These data had been screened for range and

historical envelopes (see Nowlin et al. 2001 for discussion of QA/QC tests applied to

temperature, salinity, and dissolved oxygen). The quality flags assigned to these data were

retained in the Oxygen Archive, unless further QA/QC applied under this study classified a data

point as suspect.
21

Last Modification Date is 04-AUGUST-2003.

The flag, -99.0000, in a data cell means no data are available.

The flag, 0, in a quality cell means data are of good quality.

The flag, 1, in a quality cell means data are suspect.

The flag, 2, in a quality cell means data have not been quality

controlled.

COUNTRY : DENMARK

OCL INTERNAL CRUISE CODE : 8

LATITUDE : 21.5700

LONGITUDE : -86.3200

DATE (YYYY/MM/DD) : 1922/02/01

TIME (GMT) : 04:00:00

OCL INTERNAL PROFILE CODE : 20124

NUMBER OF DEPTH LEVELS : 14

NODC ACCESSION NUMBER : 7101220

NODC PROJECT CODE : UNKNOWN

NODC INSTITUTION : COLLECTION CARLSBERG FOUND. DANA

EXPEDIT.(CHARLOTTENLUND

WOD98 UNIQUE STATION NUMBER : UNKNOWN

SHIP NAME : DANA II

ORIGINATORS CRUISE CODE : UNKNOWN

ORIGINATORS STATION CODE : UNKNOWN

ORIGINATOR STATION NUMBER : 122

SUBMITTING INSTITUTION : COLLECTION CARLSBERG FOUND. DANA

EXPEDIT.(CHARLOTTENLUND

SUBMITTING PI : UNKNOWN

DEPTH PRECISION : UNKNOWN

BOTTOM DEPTH (METERS) : 1035 as_given

T-S PROBE : UNKNOWN

PRIMARY INVESTIGATOR : UNKNOWN

COLUMN 1 DEPTH : METERS

COLUMN 2 TEMPERATURE : DEGREES CELSIUS

COLUMN 3 SALINITY : PSS or PPT

COLUMN 4 OXYGEN : MILLILITERS PER LITER

COLUMN 5 NITRITE : NOT-MEASURED

COLUMN 6 NITRATE : NOT-MEASURED

COLUMN 7 PHOSPHATE : NOT-MEASURED

COLUMN 8 SILICATE : NOT-MEASURED

COLUMN 9 SALINITY II : NOT-MEASURED

COLUMN 10 T-S QUALITY FLAG : SUSPECT = 1 WHEN SIGMA-THETA > 2.3 std.dev.

FROM HISTORICAL MEAN, 0 = PASSED, 2 = UNTESTED

COLUMN 11 OXY QUALITY FLAG : SUSPECT = 1 WHEN OXYGEN > 2.3 std.dev.

FROM HISTORICAL MEAN, 0 = PASSED, 2 = UNTESTED

COLUMN 12 NO2 QUALITY FLAG : SUSPECT = 1 WHEN NITRITE > 2.3 std.dev.

FROM HISTORICAL MEAN, 0 = PASSED, 2 = UNTESTED

COLUMN 13 NO3 QUALITY FLAG : SUSPECT = 1 WHEN NITRATE > 2.3 std.dev.

FROM HISTORICAL MEAN, 0 = PASSED, 2 = UNTESTED

COLUMN 14 PO4 QUALITY FLAG : SUSPECT = 1 WHEN PHOSPHATE > 2.3 std.dev.

FROM HISTORICAL MEAN, 0 = PASSED, 2 = UNTESTED

COLUMN 15 SIO3 QUALITY FLAG : SUSPECT = 1 WHEN SILICATE > 2.3 std.dev.

FROM HISTORICAL MEAN, 0 = PASSED, 2 = UNTESTED

*END*

Figure 3.3. Example of a data file header for the dissolved oxygen bottle data.
22

The QA/QC of the bottle data consisted of three tiers of checks. The primary QA/QC focused on

removal of duplicate data sets and the flagging of unrealistic wild point outliers. The secondary

QA/QC focused on examination of the bottle data for and flagging of unreasonable data points as

defined by statistical limits based on standard deviations. The tertiary QA/QC consisted of

identification of problem points during the analysis and synthesis phase of the study. Each tier is

discussed briefly below.

Primary QA/QC: Checks were made for duplicate data sets, with duplicates being removed.

Data were plotted by depth, sigma-theta level, or potential temperature and examined for overall

reasonableness. Physically unrealistic data points (spikes or wild points) or profiles were flagged

as suspect.

Secondary QA/QC: This tier of QA/QC used the database to develop a mean and standard

deviation for binned data. Data that were outside 2.3 standard deviations (2.3σ) from the mean

based on depth were flagged as suspect. A histogram of data by depth was constructed (Figure

3.4) to identify depth bins for determination of means and standard deviations. The depths bins

selected were: 50-m increments from 0 to 1000 m depths; 250-m increments from 1000 to 2000

m; 500-m increments from 2000 to 3000 m; and a bottom bin including all data below 3000 m.

Table 3.3 gives the statistics that were used. These were determined for the whole Gulf based on

the uncorrected data available in January 2003 from over 2200 stations. Figure 3.5 shows the

mean and 2.3σ used for this step.

Tertiary QA/QC: The tertiary QA/QC step was a product of the analysis and synthesis phase

of the study. The few data points that were determined to be unreliable during this phase of the

study were flagged as suspect.

Discussion of QA/QC results: Presented here are several examples of data sets and how they fit

within the wild point, 2.3σ, and suspect profile criteria. These examples are intended to alert the

users of the database to beware of and use the quality flags for the data points and to examine the

data themselves to determine whether additional QA/QC should be applied for the intended use.

First is shown one of the data sets identified by Nowlin et al. (1969) as being of good quality.

These data are from a summer 1967 Alaminos cruise in the eastern and central Gulf. Station

locations are shown in the top panel of Figure 3.6. The bottom panels of Figure 3.6 show the data

by depth and grouped by east and west locations, divided at –90°W longitude. Note the

characteristic low level of scatter in the data below 1500 m that was noted by Nowlin et al.

(1969). Compare the larger scatter of the winter 1962 Hidalgo data located below 1500 m, that

were noted by Nowlin et al. (1969) to be of questionable quality (Figure 3.7). The data that lie

outside the 2.3σ contours are readily apparent in these plots. However, Nowlin et al. (1969) state

that the large scatter seen at the various depths, which includes that scatter within 2.3σ contours

shown, makes all dissolved oxygen data from this cruise suspect. The Oxygen Archive, however,

does not have these data flagged as suspect unless they are outside the 2.3σ limits.

A more recent data set was collected over the northern Gulf aboard the R/V Gyre in May/June

2000. Station locations are given in the top panel of Figure 3.8. Note, in the bottom panels of the

figure that the scatter below 1500 m is similar to that of the 1967 Alaminos cruise. The profiles

also suggest that the profile, indicated by the arrow, from a station in the western Gulf may be

suspect. After further inspection, the cast was retained in the data base but was excluded from the

set of profiles used to form the canonical set (see Section 3.3 below).

Many of the data sets do not sample below 1500 m, even when the water depth is much greater.

An example is shown in Figure 3.9. The stations are located in the southwestern Gulf, and the
23

Figure 3.4. Histogram of bottle oxygen data count in 50-m bin increments. Inset shows a

close-up of the counts for bins 32-76.
24

Table 3.3

Bin Statistics for Bottle Oxygen Data in the Gulf of Mexico Used in QA/QC

Bin interval

(m)

No.

Points

Mean

(mL·L-1)

Standard

Deviation

(mL·L-1)

Maximum

(mL·L-1)

Minimum

(mL·L-1)

Mean

Depth of

Data (m)

 0 to 50 7263 4.69 0.37 7.47 2.08 18.9

 50 to 100 5533 4.37 0.63 6.79 1.63 73.2

 100 to 150 3795 3.61 0.65 6.16 1.77 122.4

 150 to 200 2750 3.28 0.48 5.49 2.05 173.5

 200 to 250 2009 3.09 0.42 4.99 1.89 218.5

 250 to 300 1683 3.03 0.47 5.58 1.52 275.9

 300 to 350 1198 2.94 0.44 4.86 1.51 314.9

 350 to 400 1102 2.88 0.40 4.69 1.48 380.9

 400 to 450 1028 2.83 0.33 4.09 1.26 415.5

 450 to 500 1033 2.85 0.31 5.81 1.73 482.0

 500 to 550 674 2.86 0.30 6.18 2.18 513.5

 550 to 600 574 2.92 0.25 4.31 1.96 581.3

 600 to 650 530 2.96 0.28 4.46 1.53 614.0

 650 to 700 449 3.13 0.39 6.82 1.78 678.4

 700 to 750 549 3.21 0.28 4.36 2.13 722.0

 750 to 800 609 3.34 0.37 4.65 1.49 775.8

 800 to 850 400 3.47 0.38 4.43 1.18 812.0

 850 to 900 185 3.64 0.45 6.75 2.10 879.2

 900 to 950 263 3.77 0.35 4.81 1.94 921.7

 950 to 1000 536 3.99 0.35 5.53 1.89 982.2

1000 to 1250 1134 4.33 0.42 7.16 2.56 1109.3

1250 to 1500 581 4.73 0.33 5.56 2.66 1398.6

1500 to 1750 299 4.86 0.31 5.63 3.15 1562.8

1750 to 2000 230 4.94 0.42 5.95 1.88 1893.3

2000 to 2500 231 5.04 0.37 6.02 3.19 2218.0

2500 to 3000 128 5.00 0.30 5.66 3.37 2761.5

3000 to 4000 76 4.94 0.31 5.54 3.84 3274.3

data were collected in fall 1970 from the Mexican ship Virgilio Uribe. This lack of sampling

below 1500 m was a common occurrence, particularly in data sets from 1970 to the present. Only

10% of stations in water depths greater than 1500 m sampled 95% of the available water column.

This number decreases rapidly with increasing ocean depth. For example, only 5% of stations

deeper than 2500 m sampled 95% of the total water depth.

There also are a number of data sets that are unusual, but which were not flagged as suspect. One

"discrete" data set that was examined was from the 1970 Tursiops cruise near the region of the

Loop Current inflow (Figure 3.10). Each profile consists of far more data than is generally

collected by discrete samplers, and we know of no probe-derived oxygens from as early as 1970.

It is likely this is a case of data that were interpolated to standard depths, rather than actual data.

But there is insufficient information available to determine this for certain.
25

Figure 3.5. Oxygen statistics by depth bin. Shown are the mean and 2.3σ statistics for the

whole Gulf (black; circles), west of 90°W (red; squares), and east of and including

90°W (green; triangles). Symbols show the middle of each depth bin. Numbers on

the right give the number of data points for each bin. The mean depths of the

oxygen minimum layer are approximately 415 m for the whole Gulf, 525 m for the

eastern Gulf, and 400 m for the western Gulf.
26

Figure 3.6. Locations of stations (top) and oxygen data (bottom) for R/V Alaminos cruise, 67-A04, on 11 June - 1 July 1967. Historical mean and 2.3σ are shown.
27

Figure 3.7. Locations of stations (top) and oxygen data (bottom) for R/V Hidalgo cruise, 62-H03, on 14 February - 31 March 1962. Historical mean and 2.3σ are shown.
28

Figure 3.8. Locations of stations (top) and oxygen data (bottom) for R/V Gyre cruise, 00-G-05,

on 4 May - 17 June 2000. Historical mean and 2.3σ are shown.
29

Figure 3.9. Locations of stations (top) and oxygen data (bottom) for the Virgilio Uribe cruise,

C12, on 31 October - 13 November 1970. Historical mean and 2.3σ are shown.
30

Figure 3.10. Locations of stations (top) and oxygen data (bottom) for the Tursiops cruise, T7015,

on 2 - 11 May 1970. Historical mean and 2.3σ are shown.
31

The temporal nature of the sampling was examined to determine if there was bias in the seasons

in which samples were taken. Table 3.4 shows the number of stations with dissolved oxygen

samples from bottles by decade and month. Shown are both the numbers for all the stations in the

whole Gulf and for stations that had at least one sample taken at or below 1500 m depth.

Sampling in the whole Gulf and at or deeper than 1500 m was conducted, respectively, 34 and

40% of the time in spring (March-May), 25 and 20% in summer (June-Aug), 26 and 21% in fall

(Sept-Nov), and 15 and 16% in winter (Dec-Feb). Although summer is most frequently sampled

and winter least, the distribution in time is reasonable for analysis purposes.

Table 3.4

Number of Stations With Bottle Dissolved Oxygen by Month and Decade

Decade Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total

All stations

<1950 0 29 33 24 5 0 0 0 0 0 0 0 91

1950s 28 42 11 23 22 8 41 28 18 10 0 27 258

1960s 38 153 92 70 72 146 56 155 176 21 27 4 1010

1970s 65 11 16 130 262 49 24 30 13 126 172 0 898

1980s 66 64 44 74 64 12 40 18 11 55 80 13 541

1990s 14 15 8 58 189 29 62 132 8 43 219 0 777

2000+ 0 0 0 48 26 31 14 53 9 1 0 11 193

Total 211 314 204 427 640 275 237 416 235 256 498 55 3768

Stations with samples collected at or below 1500 m

<1950 0 8 6 3 3 0 0 0 0 0 0 0 20

1950s 16 23 6 8 5 3 2 14 9 0 0 2 88

1960s 14 19 41 28 15 52 18 13 22 3 1 0 226

1970s 0 0 4 64 58 3 0 1 1 40 37 0 208

1980s 29 3 4 0 1 0 0 0 0 1 16 2 56

1990s 2 0 0 0 0 1 1 1 0 1 0 0 6

2000+ 0 0 0 0 11 16 0 0 0 0 0 0 27

Total 61 53 61 103 93 75 21 29 32 45 54 4 631

3.3 Box Model Initialization Data Sets

Two sets of canonical profiles of dissolved oxygen, temperature, and salinity were developed for

use in initialization of the box model discussed in Section 5. One was for the Yucatan Channel;

the other was for the deep Gulf basin. All oxygen values were based on Winkler titrations, i.e.,

no continuous probe values were used. The Yucatan Channel profile was constructed from

profiles collected within the box with corners at 21°N, 87°W and 22°N, 84.3°W. Table 3.5

shows values for the Yucatan Channel; Table 3.6 shows values for the deep Gulf basin.
32

Table 3.5

Yucatan Channel Canonical Profiles for Temperature, Salinity, and Dissolved Oxygen

Depth

(m)

Potential

Temperature

(°C)

Salinity

Dissolved

Oxygen

(mL·L-1)

0. 27.47 35.95 4.56

10. 27.39 36.06 4.56

100. 24.64 36.32 4.15

200. 19.44 36.53 3.57

500. 11.31 35.31 3.00

800. 6.41 34.41 3.33

1150. 4.49 34.49 4.50

1500. 4.15 34.97 4.87

1750. 4.06 34.97 5.09

2000. 3.98 34.98 5.43

Table 3.6

Deep Gulf Basin Canonical Profiles for Temperature, Salinity, and Dissolved Oxygen

Depth

(m)

Potential

Temperature

(°C)

Salinity

Dissolved

Oxygen

(mL·L-1)

0. 26.83 35.71 4.65

10. 26.38 35.72 4.74

100. 20.29 36.40 3.68

200. 15.41 36.00 3.06

500. 8.67 35.06 2.84

800. 5.68 34.90 3.61

1150. 4.45 34.95 4.48

1500. 4.12 34.97 4.91

1750. 4.07 34.97 4.95

2000. 4.04 34.97 4.99

3000. 3.99 34.97 5.08

3500. 3.97 34.97 4.90
33

The canonical dissolved oxygen profiles were constructed through a series of steps designed to

eliminate bad cruises, bad profiles, and bad points from the data base leaving only the best-ofthe-best data. These data then were averaged by depth, smoothed, spline-fit to 1-m intervals, and

smoothed again to produce the final representative profiles. Each profile was considered three

times. Only profiles that were accepted all three times were used to compute the canonical

profiles.

In each pass, the ensembles of dissolved oxygen versus depth and dissolved oxygen versus

potential temperature were plotted in adjacent panels. Individual profiles were over-plotted in a

different color one at a time and were accepted or rejected by eye. After the initial screening, the

results were examined by cruise. Cruises with large numbers of rejected profiles were identified

and all profiles from that cruise were removed from further consideration. In the second and third

screening, individual profiles were eliminated on a profile-by-profile basis by comparing them to

the ensemble plot of profiles that had passed the preceding step. Only profiles that were accepted

three times were used in the averaging and smoothing steps. The number of profiles that passed

the three screens was 161 for the Yucatan Channel and 906 for the deep Gulf basin.

The QA/QC processing of the temperature, salinity, and dissolved oxygen data resulted in the

assignment of a quality control flag to each data point. If any of the temperature, salinity or

dissolved oxygen values at a given depth within an accepted profile were flagged bad, then all

three of the values were rejected for use in the canonical data set. The remaining data then were

averaged by depth and smoothed with sequential applications of a moving box car of 3, 5, 7, 11,

and 33 points. The resulting profile was spline-fit to 1-m bins and smoothed one final time with a

33-point moving average.
35

4 DISSOLVED OXYGEN IN THE DEEPWATER GULF OF MEXICO

Dissolved oxygen is one of the most intensely studied components of seawater. It is essential for

life to many marine organisms. Broecker (e.g., 1974) has categorized it as being intermediate in

limiting oceanic biologic processes. The dissolved oxygen concentration can vary, even on local

scales, from undetectable (anoxic) to supersaturated with respect to atmospheric concentration.

Biological processes both produce oxygen via photosynthesis and consume it via respiration.

Abiotic redox reactions also can lead to oxygen consumption. Physical processes such as air-sea

exchange and benthic oxygen demand may cause further changes in oxygen concentration.

Different water masses have differing oxygen concentrations depending on their source and

history. Movement and mixing of different water masses therefore give further texture to the

distribution of oxygen in the ocean. For most major ocean basins this leads to the "classic"

dissolved O2 depth profile shown in Figure 4.1. In this section, these biological, chemical and

physical processes, which control the concentration and distribution of dissolved oxygen, are

examined in the Gulf of Mexico based on our general understanding of these oceanic processes

and the data we have gathered as part of this project.

O2 (μM)

0 50 100 150 200 250 300 0

1000

2000

3000

4000

5000

Figure 4.1. Dissolved O2 in the North Atlantic Ocean (based on Millero 1996, Fig. 6.9).

Our discussion starts with Section 4.1, which presents background information on the physical

oceanography, circulation, and water masses of the Gulf of Mexico and a discussion of flushing

time. These processes set the major features of dissolved oxygen distribution in the Gulf of

Mexico. The brief review presented here is intended to highlight the physical oceanographic setting;

it is not intended to be a comprehensive review (see Nowlin et al. 2000). The sources and sinks of

dissolved oxygen in the deepwater of the Gulf of Mexico are identified in Section 4.2, and the

operative physical and biogeochemical processes are discussed. Section 4.3 presents the results of a

reanalysis of the historical oxygen database in the deepwater Gulf of Mexico. The potential effects

of oil and gas discharges are considered in Section 4.4. Depth (m)
36

4.1 Physical Oceanographic Background

The distribution of dissolved oxygen in the water column of the deepwater Gulf of Mexico

(hereinafter "Gulf") is influenced by circulation, which transports waters and the oxygen they

contain throughout the Gulf. The mean circulation in the upper 1000 m has been observed to be

generally anticyclonic, except perhaps in the Bay of Campeche, with many cyclonic and anticyclonic

eddies present at any given time (Figure 4.2), while sparse data sets and model results (Figure 4.3)

suggest the mean circulation in the deeper waters is cyclonic, but also with many cyclonic and

anticyclonic eddies present (e.g., data: Molinari et al. 1978; Hofman and Worley 1986; Hamilton

1990; Hamilton and Lugo-Fernandez 2001; Nowlin et al. 2001; models: Welsh and Inoue 2000;

Sturges et al. 2004). Water masses are not formed in the Gulf of Mexico, but water masses from

the Atlantic move into the Gulf through the Yucatan Channel. With a sill depth of approximately

2000 m, this passage is deep enough to allow deep water masses, which have relatively high

dissolved oxygen concentrations, to enter the Gulf. These are the source of replenishment of

dissolved oxygen to the deep Gulf waters. This section explores these factors in more detail.

4.1.1 Brief Review of Physical Oceanography of the Deepwater Gulf of Mexico

Reviewing the historical current database, Nowlin et al. (2001) found that the highest maximum and

mean current speeds were near the sea surface with maxima reaching up to 200 cm·s-1 in the eastern

Gulf and 100 cm·s-1 in the western Gulf. They also found that speeds decreased with depth, tending

toward a minimum of approximately 5-10 cm·s-1 mean speed at depths of 800-1000 m, and that

speeds increased somewhat with depth below that level, likely due to bottom intensification of

currents (see also Molinari and Morrison 1988; Cooper et al. 1990; and Hamilton et al. 1997).

Maximum currents below 1500 m have been observed to reach 20-50 cm·s-1 and for a few

measurement locations to reach nearly 100 cm·s-1 (Nowlin et al. 2001). Hamilton and LugoFernandez (2001) found recurring episodes of energetic currents of 40-50 cm·s-1, with maximum

velocities sometimes exceeding 85 cm·s-1, about 10 m above bottom in ~2000 m depth near the

Sigsbee Escarpment. Thus, deep waters of the Gulf are not quiescent, and the oxygen-rich deep

waters from the Yucatan Channel and the southeastern Gulf can be carried into the northern and

southwestern Gulf to renew those waters.

There are two principal forcing functions of the circulation in the upper 800 to 1000 m: the Loop

Current system, including the Loop Current itself, Loop Current eddies (LCEs) and other

circulation phenomena derived from them (Figure 4.4), and wind stress (overviews are given in

Nowlin et al. 2001 and Continental Shelf Associates, Inc. 2000). The Loop Current system begins

with the Yucatan Current that enters the Gulf from the Caribbean Sea through the Yucatan Channel.

It then separates from the Campeche Bank, becoming the Loop Current. The Yucatan Current is

westward intensified (e.g., Cochrane 1963). Estimates of the Loop Current transport vary from 22-

33 Sv (e.g., Ochoa et al. 2001 and Sheinbaum et al. 2002 and references cited therein). There can be

flows southward out of the Gulf at the western and eastern edges of Yucatan Channel (e.g.,

Cochrane 1963; Hansen and Molinari 1979; Ochoa et al. 2001; Sheinbaum et al. 2002). Ochoa et

al. (2001) estimated southward transports of 8 Sv.

As it enters the Gulf, the Yucatan Current extends to the sill depth of approximately 2000 m. Its

continuation in the Gulf, which is called the Loop Current, carries within it water masses from the

global ocean. The Loop Current extends into the eastern Gulf and then exits through the Florida

Straits. The net transport out of the Florida Straits has been estimated at approximately 32 Sv

(Hofmann and Worley 1986) upstream of the sill and 32 ± 3 Sv downstream of the sill (Schmitz

and Richardson 1968; Baringer and Larsen 2001).
37

98W 96W 94W 92W 90W 88W 86W 84W 82W 80W

18N

20N

22N

24N

26N

28N

30N

50 cm s-1

Figure 4.2. Near-surface drifter velocity estimates. Estimates were made for each 0.5° x 0.5° bin

based on averaging all drifter velocity estimates in that bin for the period 1989-1999.

Shown are the 200-, 1000-, and 3000-m isobaths. (from Nowlin et al. 2001)

As it extends into the Gulf, the Loop Current may be confined to the southeastern Gulf (Figure 4.4,

lower panel) or may extend well into the northeastern or north central Gulf (Figure 4.4, upper

panel). It can interact with the northern slope (see e.g., Huh et al. 1981; Paluszkiewicz et al. 1983).

Bunge et al. 2002 showed that transports below ~1000 m in the Yucatan Channel were well

correlated to the surface extension of the Loop Current and suggested this relationship results in

deep exchange of Caribbean and Gulf waters. Fratantoni et al. (1998) concluded that Loop Current

perturbations affect the formation and evolution of Tortugas eddies in the southern Straits of

Florida. Extensions of the Loop Current into the northern Gulf can result in the separation of an

anticyclonically circulating Loop Current Eddy (LCE) that then may migrate into the western Gulf,

if it does not reattach to the Loop current (e.g., Maul 1976; Vukovich 1995; Sturges and Leben

2000).

Currents associated with the Loop Current and its eddies extend down to at least the sill depth of

the Florida Straits (somewhat less than 800 m) and decrease with depth. Weatherly (2004) found

that currents in the Loop Current and its LCEs extended down to a depth of at least 900 m. Using

data from 1972 and 1973 to determine mass and salt balances, Maul (1976) estimated that

approximately 10% of the water above 700 m that entered through the Yucatan Channel was

exchanged with the waters of the Gulf before flowing out the Florida Straits. His results likely were

influenced by an LCE separation event that occurred during the measurements.
38

98W 96W 94W 92W 90W 88W 86W 84W 82W 80W

20N

22N

24N

26N

28N

30N

50 cm s-1

Figure 4.3. Record-length mean current vectors at the first sigma level above the bottom.

Constructed from 1993-1999 University of Colorado Princeton Ocean Model output.

Shown are the 200, 1000, 2000, and 3000-m isobaths. (from Nowlin et al. 2001)

The anticyclonic LCEs separate from the Loop Current at intervals ranging from 3 to 17 months,

with a peak in frequency of separation near 12 months and secondary peaks at 9 and 6 months

(e.g., see Sturges 1994; Sturges and Leben 2000). The LCEs can remain in the eastern Gulf for

some time after their formation and can reattach to the Loop Current (e.g., Vukovich 1995). Most

eventually move westward, with typical translation speeds of approximately 5 km·d-1 (Elliot 1982,

see also Hamilton et al. 1999), and reach the western edge of the Gulf of Mexico basin. The LCEs

have current cores and water masses similar in properties to waters of the Caribbean type that

comprise most of the Loop Current waters. Thus, their migration westward carries water with these

properties into the western Gulf. These anticyclonic, mesoscale current rings have average lifetimes

longer than one year (Elliot 1982) and may spawn cyclonic rings during interaction with one

another or with the continental slope (e.g., Vidal et al. 1994). Hamilton et al. (2002) reported

cyclones moved on and off the slope in association with large anticyclones and that cycloneanticyclone eddy pairs were a mechanism for exchange of material between the shelf and deep

waters (see also Hamilton 1992 on cyclonic eddies). The migration of the LCEs and other eddies

represent one mechanism for transport of Loop Current waters throughout the Gulf of Mexico.

The second forcing function is wind stress, which primarily affects the upper waters of the Gulf of

Mexico. This forcing generates two main classes of phenomena. First, low frequency regional wind

patterns can generate low frequency regional circulation patterns. Sturges (1993) suggests a wind-
39

Figure 4.4. Sea surface height fields in the Gulf of Mexico. Fields are for (upper) 20 September

1999 showing extension of Loop Current into the northeastern Gulf and formation of

LCE Juggernaut and (lower) 14 November 1999 showing westward migration of

Juggernaut, with the Loop Current in the southeastern Gulf, and an older anticyclone

in the western Gulf.
40

driven anticyclonic circulation occurs over the west and central Gulf with a westward intensified

boundary current adjacent to the western slope of the Gulf. Vázquez de la Cerda (1993) has shown

that seasonal variations in the wind fields over the Gulf generate a cyclonic circulation over the

Campeche Bay in the southwestern Gulf (see also Vázquez de la Cerda et al. 2005).

The second class of wind-forced currents consists of episodic current events that are forced by

highly energetic atmospheric events such as tropical cyclones, extratropical cyclones, cold air

outbreaks, and other frontal passages. The strongest of these includes hurricanes which can

produce currents in the mixed layer that exceed 150 cm·s-1, and when combined with wave currents,

may exceed 300 cm·s-1 (Nowlin et al. 2000). Hurricane induced currents at 200 m have been

observed to be approximately 100 cm·s-1 in several instances, and even at 700 m may reach speeds

of 15-20 cm·s-1 (e.g., Brooks 1983). These storms can inject air bubbles deep into the upper water

column, enhancing the dissolved oxygen content of the water.

Below about 800 m, the direction and speeds of deep currents are not well understood. Model

output and geostrophic calculations based on reasonable choices of reference levels suggest the

long-term mean circulation of the deep Gulf is cyclonic (e.g., Nowlin et al. 2001; Welsh and Inoue

2002; Sturges et al. 2004). This cyclonic circulation also seems to be intensified offshore of the

steep topography of the Sigsbee Escarpment in the north central Gulf, the Campeche shelf, and the

west Florida shelf (e.g., see Weatherly 2004). Hamilton (1990) and Sturges et al. (1993) suggested

that currents below 1000 m may be strongly influenced by the Loop Current through excitation of

energetic currents associated with topographic Rossby waves. These currents were barotropic, being

highly coherent in the vertical, and exhibited bottom intensification. Evidence suggests these deep

currents may be driven by the Loop Current or separation of an LCE from the Loop Current.

Additional information on the currents beneath 800-1000 m comes from numerical models. Model

studies suggest that both cyclonic and anticyclonic eddies are present, perhaps as pairs, in the deep

basin waters (Sturges et al. 1993; Welsh and Inoue 2000; Nowlin et al. 2001; see Figure 4.3). The

eddies form in the eastern Gulf under the Loop Current or newly formed LCEs. Guided by

topography, they then propagate into the western Gulf. Some model results suggest deep layer

eddies form, often as a pair consisting of an anticyclone and a stronger cyclone, in a response to a

westward moving newly formed anticyclonic LCE in the upper layer (e.g., Welsh and Inoue 2000).

4.1.2 Description of Water Masses in the Gulf of Mexico

Surface waters in the Gulf are subject to heat and freshwater exchanges and winds. These processes

may affect the oxygen concentrations in the surface waters by altering the rate at which oxygen is

exchanged with the atmosphere, which is the main source of oxygen in the surface waters. Although

the buoyancy forcing by river discharge that affects the nearshore coastal currents is a form of

thermohaline forcing that is known to be important over the shelves of the Gulf, no thermohaline

forcing of consequence or water mass formation are known to occur in the deepwater region of the

Gulf (Nowlin et al. 2000). However, Nowlin and Parker (1974) suggested that, during winter time

cold air outbreaks, there may be formation over the shelves of a water type with characteristics

similar to the waters just below the core of the Subtropical Underwater. They further suggested this

water was dense enough to flow over the Gulf slopes at the intermediate depths just below the

Subtropical Underwater core. This water type would not reach the deep Gulf, and hence would not

be a source of oxygen to the deep waters.

In general then, water masses in the Gulf are the global ocean water masses that enter and exit the

Gulf through the Yucatan Channel and the Florida Straits. The sill depths of these bathymetric

features are approximately 2000 and 800 m, respectively. The sills essentially control which water

masses enter and leave the Gulf. Source waters for oxygen in the deep part of the water column

mainly originate in the Atlantic Ocean (Morrison and Nowlin 1982). They are modified as they

spread from the Caribbean Sea into the Gulf.
41

There are five water masses carried into the Gulf by the Loop Current (Table 4.1). These are

described briefly from top to bottom. The Subtropical Underwater is characterized by a salinity

maximum at its core. It occurs at typical depths of 150-250 m in the eastern Gulf (Morrison and

Nowlin 1977) and, if present, between 0-250 m in the western Gulf (Morrison et al. 1983). Below

the Subtropical Underwater is 18°C Sargasso Sea water, which is characterized by an oxygen

maximum between 3.6-3.8 mL·L-1 typically at depths of 200-400 m in the Loop Current waters in

the eastern Gulf (Morrison and Nowlin 1977). Within the general decrease of oxygen with depth

that occurs in the upper 1000 m, the presence of the oxygen maximum of the 18°C Sargasso Sea

water together with any shallow sub-surface local maximum creates a double oxygen minimum

structure in the eastern Gulf (Shiller 1999). This oxygen maximum appears to erode away as the

waters mix into the western Gulf (Morrison et al. 1983).

The oxygen minimum layer in the Gulf of Mexico develops outside the Gulf and enters through the

Yucatan Channel as an established water mass feature (Wüst 1964). This water mass is the Tropical

Atlantic Central Water, which is characterized by an oxygen minimum of below 3 mL·L-1. It occurs

at depths of 450-700 m in the eastern Gulf (Morrison and Nowlin 1977) and 250-500 m in the

western Gulf (Morrison et al. 1983). The oxygen minimum layer spreads across the Gulf of

Mexico and can intersect the seabed on the continental slope (Kennicutt 2000). Figure 4.5 shows

data collected in May/June 2000 during a cruise of the MMS-sponsored Northern Gulf of Mexico

Continental Slope Habitats and Benthic Ecology (DGOMB) Study. The Tropical Atlantic Central

Water is seen as the oxygen minimum zone centered about the 27.15 sigma-theta density surface.

These recent data are consistent with findings of the earlier studies.

Other water masses in the deep water Gulf include the Antarctic Intermediate Water, which occurs

between 700-1000 m depth in the eastern Gulf and 500-1100 m in the western Gulf, and a modified

upper North Atlantic Deep Water, which is between 900-1200 m in the eastern Gulf and 1000-1100

m in the western Gulf (Nowlin and McLellan 1967; Morrison and Nowlin 1977; Morrison et al.

1983). These deeper water masses do not have oxygen as their chief characteristic water property.

Nowlin et al. (1969) found that, from approximately 1500 m to the bottom, dissolved oxygen

distributions suggest no clearly discernible horizontal variation and exhibit only slight vertical

gradients. They found mean concentrations of approximately 5 mL·L-1. Temperature and salinity

below the sill depth exhibit similar distributions (McLellan and Nowlin 1963; Nowlin and

McLellan 1967). Data from the early 1960s and the 1970s, as well as data collected during

DGOMB, indicate that the deep water of the Gulf of Mexico is homogeneous with relatively

constant dissolved oxygen content (see Figure 4.5); this observation is examined further in Section

4.3.

The deep water masses of the Gulf mainly enter with the Loop Current through the Yucatan

Channel and then flow into the Gulf interior to fill the deep basins. Using silica as a tracer, Carder

et al. (1977) found a bolus of renewal waters, 200-300 m thick, moved from the sill region into the

Gulf, sank rapidly to the bottom at 3500 m with some mixing, and penetrated at least 350 km into

the Gulf. To explain observed temperature-salinity distributions, Maul et al. (1985) suggested upper

North Atlantic Deep Water flows into the Gulf on the east side of the Yucatan Channel, cyclonically

flows around the Gulf basin, and returns to the channel to flow southward on the west side. Sturges

et al. (2004) used temperature to infer cyclonic circulation at the 2000-m depth. Model results of

Welsh and Inoue (2002) seem to support this idea. They propose a conceptual model for ventilation

of the deep waters. The relatively cold, salty, oxygen-rich waters that enter the Gulf at depth through

the Yucatan Channel are downwelled in the eastern basin and then circulated cyclonically into the

rest of the Gulf. Older, warmer water is upwelled onto the Campeche Bank beneath the western

limb of the Loop Current.
42

Table 4.1

Water Masses of the Caribbean Sea and Gulf of Mexico

Water Mass Depth

(m)

σθ

(kg·m-3)

Characteristic

Feature

Characteristic

Range

Eastern Caribbean1

SUW-LC 150-250 25.40 Smax

SUW

18°C W 200-400 26.50 O2 max

TACW 400-700 27.15 O2 min

AAIW 600-800 27.30 NO3 max

AAIW 700-900 27.40 PO4 max

AAIW 600-900 27.40 Smin

AAIW 800-1000 27.50 SiO3 max

UNADW

1100-1600

at sill

at sill

at sill

27.70

27.75

27.75

27.75

Smax

SiO3 min

PO4 min

NO3 min

Eastern Gulf of Mexico2

SUW-LC 150-250 25.40 Smax 36.7-36.8

SUW 150-250 25.40 Smax 36.4-36.5

18°C W 200-400 26.50 O2 max 3.6-3.8 ml·l-1

TACW 400-700 27.15 O2 min 2.85-3.25 ml·l-1

AAIW na na NO3 max na

AAIW 700-900 27.40 PO4 max 1.8-2.5 mg-at·l-1

AAIW 800-1000 27.50 Smin 34.86-34.89

AAIW 27.50 SiO3 max *

UNADW

MIX** 900-1200 27.70 SiO3 max 23-25 mg-at·l-1

Western Gulf of Mexico3

SUW-LC na na na na

SUW 0-250 25.40 Smax 36.4-36.5

18°C W na na na na

TACW 250-400 27.15 O2 min 2.5-2.9 ml·l-1

AAIW 500-700 27.30 NO3 max 29-35 mg-at·l-1

AAIW 600-800 27.40 PO4 max 1.7-2.5 mg-at·l-1

AAIW 700-800 27.50 Smin 34.88-34.89

AAIW

UNADW

MIX** na na na na

1 - Morrison and Nowlin 1982 2 - Morrison and Nowlin 1977; Nowlin and McLellan 1967

3 - Morrison et al. 1983; Nowlin and McLellan 1967

SUW-LC = Subtropical Underwater within the Loop Current and Caribbean Sea

SUW = Subtropical Underwater in the Gulf but outside the Loop Current

18°C W = 18°C Sargasso Sea Water; TACW = Tropical Atlantic Central Water

AAIW = Antarctic Intermediate Water; UNADW = Upper North Atlantic Deep Water

**MIX = Mixture of low silicate UNADW and very high silicate Caribbean Mid-Water

*high SiO3 in AAIW and MIX waters results in broad SiO3 maximum approximately from 27.50 to 27.70
43

0 1 2 3 4 5 6

Dissolved Oxygen (mL L-1)

28

27

26

25

24

23

22

21

20

Oxygen

Minimum

Zone

0 1 2 3 4 5 6

3200

3000

2800

2600

2400

2200

2000

1800

1600

1400

1200

1000

800

600

400

200

0

Dissolved Oxygen (mL L-1)

Oxygen

Minimum

Zone

Figure 4.5. Dissolved oxygen profiles for DGOMB cruise 1 in May/June 2000. Shown are

profiles in density (left) and depth (right). Lines in the left panel are the mean and ±2.3

standard deviations based on a combination of data from several recent cruises

(DGOMB cruise 1, ten LATEX A cruises, and nine NEGOM-COH cruises).

4.1.3 Flushing Time

The flushing time, also called residence time, is the time required to replace an amount of a

substance (or volume of water) equal to the amount stored in the reservoir (e.g., Broecker and Peng

1982). It can be expressed as

τ = M/R

where τ is the flushing time, M is the mass or the amount of a substance in the reservoir, and R is

the flux or the rate of supply or removal of the substance to or from the reservoir (e.g., Libes 1992;

Pilson 1998). Two major assumptions in this calculation are that the reservoir is well mixed and the

concentration of the substance in the reservoir does not change.

The rate of supply of oxygen to the deep Gulf depends on the flux of oxygen both into and out of

the various depth zones of the Gulf. This flux depends on the concentration of dissolved oxygen in

the source waters, the transport of the source waters into the Gulf interior, and the rate of removal

from respiration and detrital decay/remineralization. The concentrations in the source waters are

relatively well known for the various depths, but the transports and consumptive rates within depth

layers are not.

Depth (m)

Sigma-theta (kg m-3)
44

Nowlin (1972) suggested the relatively uniform dissolved oxygen, temperature, and salinity of the

waters below 1500 m indicate that either the deep waters have common sources or the residence

time is great enough for exchange processes to erode gradients. There appear to be no

computations in the literature of the residence time of oxygen or of deep waters based on oxygen

concentrations in the deepwater Gulf of Mexico. However, El-Sayed et al. (1972) made a very

rough calculation of the minimum residence time for waters between 100 and 750 m based on

oxidation of carbon produced in the euphotic zone. They found the minimum residence time was 3

years, but that the actual residence time must be much longer because of the simplicity of the

computation made.

There have been several papers that considered residence times for water in the deep layers of the

Gulf based on other properties. Shiller (1999) provided a summary of the literature on residence

times of deep Gulf waters. Using silica data from Carder et al. (1977) for the eastern Gulf and from

Tréguer et al. (1995) for the world ocean, he estimated a residence time of 50 years for deep Gulf

waters. Shiller (1999) reported that the carbon-14 data of Mathews et al. (1973) indicated a

residence time of 270 years in the western Gulf. Schink (private communication) used

radiochemistry to estimate the turnover to be on the order of 70 yr. A modeling study by Welsh and

Inoue (2000) suggested relatively short residence times of approximately 10 years for the waters of

the lower layer. However, in their more recent modeling study, Welsh and Inoue (2002) suggest the

residence time of the deep water is about 100 years. They base this on flux calculations and the

analysis of Buerkert (1997) of barium concentrations in the deep Gulf.

4.2 Sources and Sinks of Dissolved Oxygen

As described in the introduction to this chapter, the distribution of dissolved oxygen in the ocean is

dependent on the complex interplay between biological, chemical, and physical processes. Because

it is a biologically active gas, dissolved oxygen is a non-conservative property of sea water. In order

to develop a model that describes the vertical and horizontal patterns of oxygen in the deepwater

Gulf of Mexico, the major sources and sinks of oxygen were identified by reviewing available

literature (e.g., Riley 1951, Shiller 1999, and Kennicutt 2000 and references therein). Dissolved

oxygen in the deepwater Gulf of Mexico can be traced to global ocean water masses, to physical

processes such as advection and mixing, as described in Section 4.1, and to consumption through

biogeochemical processes in the water column and in surficial sediments. The sources and sinks of

oxygen are summarized in Table 4.2, together with their vertical spheres of influence.

For purposes of this study, the shelf is defined to be that part of the Gulf in water depths of 200 m

or less. The wide continental shelves of the Gulf tend to isolate the deepwater Gulf of Mexico from

many coastal processes (Kennicutt 2000) that might otherwise be sources or sinks for oxygen. This

includes the direct effect of river discharges on air-sea interactions and river-oceanic water mixing.

No water masses of consequence are formed over the shelf. Discharges from the Mississippi River

have significant impact on the oxygen concentrations in the water over the Louisiana shelf, both

from the increased water stratification caused by the buoyancy effects of the fresh water and from

the increased nutrients carried by riverine water into the ocean. Such nutrients then become available

to fuel an increase in organic matter production that contributes to oxygen consumption, and even

hypoxia, over the shelves. However, these effects are limited mainly to the inner shelf inshore of

water depths less than about 60 m (e.g., Rabalais and Turner 2001).

Under some circulation regimes, sediments discharged from the Mississippi River can increase the

quantities of particulate matter in the open ocean over the slope in the central Gulf. As the organic

matter in these particles sinks and decays, the oxygen consumption in this region is expected to

increase. However, little of the Mississippi River plume is transported into the deep Gulf. Shiller

(1999) examined dissolved oxygen concentrations, averaged over 2° squares, at the depth of the

oxygen minimum. He suggested that proximity to continental margins may affect oxygen

concentrations in intermediate-depth waters. This would result from an increase in oxygen
45

consumption processes associated with the more productive margins as compared to the less

productive open ocean. The effects of shelf processes on oxygen concentrations are examined in

Section 4.3.

Table 4.2

 Sources and Sinks of Dissolved Oxygen in the Deepwater Gulf of Mexico

Vertical Zone Process Description

Euphotic Zone Transport of water masses

Horizontal advection

Vertical mixing

Diffusion

Exchange with the atmosphere (air-sea interface)

Oxygen production by primary productivity (photosynthesis)

Aerobic microbial respiration

Macrofauna, meiofauna respiration

Anaerobic bacterial respiration in microenvironments (particles)

Organic matter oxidation

POC, DOC, CO2, reduced species (H2S, Fe2+, Mn2+, NH4+) rain rate

Below the Euphotic Zone Transport of water masses

Horizontal advection

Vertical mixing

Diffusion

Aerobic microbial respiration

Macrofauna, meiofauna respiration

Fishes, zooplankton respiration

Anaerobic bacterial respiration in microenvironments (particles)

Organic matter oxidation

POC, DOC, CO2, Reduced species (H2S, Fe2+, Mn2+, NH4+) rain rate

Water/Sediment Interface Sediment-water diffusive flux

Water column macrofauna, meiofauna respiration

Water column aerobic microbial respiration

Water column anaerobic bacterial respiration in microenvironments

(particles)

Benthic oxygen demand

Epifauna respiration

Sediment aerobic microbial respiration

Sediment macrofauna, meiofauna respiration

Sediment anaerobic microbial respiration (in the oxygen free zone)

POC burial

Other Oil seeps

Oil spills

Petroleum extraction (produced waters, drilling fluids, drill cuttings)

Other anthropogenic processes
46

There are three basic regimes with differing physical-biogeochemical processes that affect dissolved

oxygen concentrations in the water column (Figure 4.6). First is the surface layer, which

encompasses the euphotic zone, the mixed layer, and the interface with the atmosphere. In the deep

ocean, it typically extends from the sea surface down to ~100 to 200-m depth. In this zone,

exchanges with the atmosphere add or remove oxygen from the surface water and wind events mix

well-oxygenated surface waters to the bottom of the mixed layer. Photosynthesis adds oxygen

locally in this zone while respiration from both animals and particulate decay processes removes it.

The second regime extends from the bottom of the surface layer to just above the water-sediment

interface. It consists of three components defined by the sill depths of the Yucatan Channel (~2000

m) and the Florida Straits (~800 m). This part of the water column includes the main thermocline

and the deep waters. Here oxygen generally is removed through respiration and particulate decay

processes and the oxygen minimum zone is found. The third regime is the water-sediment interface

at the sea floor. It too is dominated by respiration and particulate decay processes that remove

oxygen. Physical processes associated with advection and vertical mixing of water masses, and, to a

comparatively negligible extent, with diffusion import or export oxygen in all three regimes. In

some cases, water movement can offset the results of local processes of photosynthesis and

respiration. Below are summarized the essentials of the processes that affect dissolved oxygen

concentrations and distributions in the oceans.

4.2.1 Physical Processes in the Gulf of Mexico

The basic control on dissolved oxygen concentrations in near-surface waters is exchange with the

atmosphere. The flux of oxygen between the atmosphere and the ocean depends on wind speed, air

and sea surface temperature, salinity, surface films, wave action, and bubble injection rates (e.g.,

Richards 1965; Riley and Chester 1971; Broecker and Peng 1982; Pilson 1998). In some cases,

waters within the photic zone can become supersaturated with respect to oxygen. This is caused by

air injection or photosynthesis (e.g., Broecker and Peng 1982). The exchange occurs across a thin

layer of the atmosphere-surface water interface. Near surface waters usually are at or near

equilibrium concentrations.

Oxygen saturation levels depend on the relative partial pressures of the air and water media and,

hence, on factors that control gaseous solubility. Solubility is a function of temperature, salinity,

barometric pressure, and atmospheric humidity. Of these, the temperature and salinity of the water

are the principal factors. Humidity may be negligible in controlling solubility. Solubility decreases

with increases in temperature or salinity and increases with increases in pressure. Other factors that

influence oxygen concentrations include surface conditions (turbulence), presence and extent of

surface organic or inorganic films, kinetics of dissolution, and surface heating (Riley and Chester

1971). Oil spills would change the structure of the surface films, and would generally decrease the

solubility of the water and thereby decrease air-sea exchanges of oxygen (NRC 2003).

At the air-sea interface, by definition equilibrium implies that the exchange of oxygen molecules in

both directions is equal; it does not imply the process is stagnant. The rate of exchange between the

gaseous and liquid phases is controlled by the rate of molecular diffusion through two boundary

layers, one in the gaseous phase and one in the liquid phase. Molecular diffusion in the gaseous

phase is several thousand times greater than in the liquid phase. Thus, when the boundary layer

thickness in the water decreases with increasing turbulence, the exchange rates between the two

media are greatly increased. Note also that wave action can result in the injection of bubbles deep

into the upper water column, increasing oxygen concentrations in the water column due to increased

surface area and oxygen undersaturation at depth. When these waters return to the surface they can

be supersaturated.
47

Figure 4.6. Schematic of vertical zones in the Gulf of Mexico. Inflow and outflow from the

Yucatan Channel (YC) and the Florida Straits (FS) are shown on the right with their

sill depths. Advection into and within each box and the exchanges at the air-sea can

be either direction depending on conditions. Exchanges at the water-sediment

interfaces generally are consumptive. Decay of detritus and remineralization (D/R)

and respiration (R) are consumptive processes with rates that decay exponentially

with depth. Photosynthesis (Ph) is a source.
48

Due to the larger vertical gradients in concentration, vertical fluxes generally greatly exceed

horizontal fluxes. However, under conditions that usually prevail at sea, turbulence produced by

wind ensures that the layer above the seasonal thermocline is in thermodynamic equilibrium with

the sea surface. Oxygen can be transported from the upper layer to the deeper parts of the ocean by

vertical advective processes; transport by molecular diffusion is negligible.

Below the surface layer, the major source of dissolved oxygen in the deepwater Gulf of Mexico is

the horizontal advection of well-oxygenated water masses from the Loop Current (e.g., Morrison

and Nowlin 1977; Shiller 1999) and vertical and horizontal advection below the Yucatan Channel

sill depth (e.g., Welsh and Inoue 2002). The distribution and characteristics of these water masses

were discussed in Section 4.1 above. In Section 4.3 below, typical values of oxygen concentrations

seen at the various depths are discussed. Because no water masses of consequence are formed in

the Gulf, ventilation of the deep waters from thermohaline circulation does not occur.

4.2.2 Biogeochemical Processes in the Gulf of Mexico

Dissolved oxygen is a non-conservative constituent of water that is affected by biological and

chemical processes that occur throughout the water column. In surface waters, the major source of

dissolved oxygen is from the atmosphere, with net local enhancements from photosynthetic

biological productivity. However, in subsurface waters, which are the main focus of this study,

dissolved oxygen is consumed through biogeochemical processes, with replenishment due to

physical processes of horizontal and vertical advection and mixing of the water masses. Rates of

consumption of dissolved oxygen in the water column can be estimated from changes in apparent

oxygen utilization (AOU) with changing water mass structure in deep waters.

Biologically induced changes predominantly occur after the water mass is no longer in direct

exchange with the atmosphere. The main causes of in situ changes in the oxygen concentrations of

the sea water are the competing processes of photosynthesis and respiration. In the upper waters,

where light is sufficient, photosynthesis by phytoplankton usually predominates. It leads to the

removal of CO2 and the liberation of oxygen, according to the formula

6CO2 + 6H2O = (C6H2O6) + 6O2,

where the left side represents uptake and the right side release (Chester 2003). Surface values of

dissolved oxygen in the Gulf of Mexico are presented in Section 4.3.

Photosynthetic production of oxygen can be a significant source of oxygen (e.g., see Craig and

Hayward 1987 in a study in the North Pacific Ocean). Indeed, in regions of intense biological

activity, this can result in the development of a water layer supersaturated with dissolved oxygen.

Productivity seaward of the shelf edge is generally low in the Gulf (see Biggs and Ressler 2001,

and references therein), so it is not expected that there would be wide-spread supersaturation

associated with biological activity in the Gulf. However, Biggs and Ressler (2001) identified "hot

spots" of primary production along the continental margin. These were localized, sometimes

temporally persistent, areas associated with mesoscale features such as cyclonic/anticyclonic eddies.

Below the point where the net productivity is zero (called the compensation point or compensation

depth), the dissolved oxygen is only consumed by the respiration of decaying plant materials,

animals, and bacteria. Respiration is the reverse of the above chemical equation. Biological

respiration can be aerobic or anaerobic in microenvironments. Anaerobic respiration occurs in areas

that are oxygen deficient, leading to denitrification or sulfate reduction processes that utilize nitrate

and sulfate instead of dissolved oxygen as the electron acceptors. Iron and manganese oxides can

also serve as electron acceptors. The present circulation system in the deep Gulf results in welloxygenated deep waters, so anaerobic respiration is limited to microenvironments (i.e., inside

particles) or small stagnant water pockets near the sea floor. Oxygen is consumed by the
49

heterotrophic activity of organisms (primarily bacteria) utilizing organic matter such as particulate

organic carbon (POC) and dissolved organic carbon (DOC) in the water column, as well as

hydrocarbons and associated gases that enter the water from seeps along the northern continental

slope (e.g., MacDonald 2002; De Beukelaer et al. 2003). The ultimate limiting factor in oxygen

consumption is the amount of metabolizable organic carbon available.

Oxygen is involved in the remineralization of organic matter to its constitutive elements. The

standard representation of remineralization is represented by the Redfield equations. The Redfield

ratio was formulated as a P:N:C:O2 of 1:16:106:138; the P:O2 ratio of 1:138 was estimated using

assumed ratios of C:O2 of 1:1 and N:O2 of 1:2 (Redfield et al. 1963). (But see Anderson et al.

1986, Anderson and Sarmiento 1994, Anderson 1995, and Hedges et al. 2002, which suggest the

Redfield ratios for O2 may be in error.) Takahashi et al. (1985) revised these estimates for

thermocline waters of the Atlantic and Indian oceans and determined them to be

1:16:122(±18):172. As the organic material sinks down in the water column, it is oxidized and

consumes the oxygen approximately in proportion to the ratios expressed by the Redfield ratios.

Thus, the oxidation of carbon and nitrogen by oxygen can be represented, for example, by

CH2O + O2 = CO2 + H2O

NH3 + 2O2 = HNO3 + H2O

The rate of oxygen consumption generally decreases with water depth because this rate depends

primarily on the concentration of metabolizable organic matter, which in turn decreases with

increasing depth below the thermocline. Thus, consumption in deep waters is slow because little

organic matter suitable for consumption reaches these depths. The organic matter that reaches deep

waters has been substantially reworked as it sinks through the long water column, leaving a

relatively oxidation-resistant residue (e.g., Hedges et al. 2002).

The Yucatan channel sill depth of approximately 2000 m acts as a barrier to the very cold waters of

2-3 °C that exist in the Atlantic at depths of about 2-3.8 km. Therefore, water temperatures in the

deep Gulf below the sill depth, which are 4-5°C, are warmer than water temperatures at similar

depths in the Atlantic. This likely increases respiration rates in the deep Gulf as compared to similar

depths in the Atlantic (Rowe et al. 2003).

Oxygen consumption rates for the Gulf apparently have not been calculated. However, Riley (1951)

estimated a rate of oxygen consumption in the water column from an evaluation of oxygen

distributions for the Atlantic ocean between 54°N and 45°N (Table 4.3). Jenkins (1982) used

helium tracers to determine oxygen utilization rates in a region in the eastern subtropical North

Atlantic in the upper 1000 m, but below the euphotic zone. He compared his results to those of

Riley (1951) and concluded Riley's estimates for the upper waters were accurate to within 10-20%,

but that Riley's deeper water values were suspect. Jenkins (1998) used new data from the eastern

subtropical North Atlantic and new analysis techniques to determine oxygen utilization rates in the

upper 600 m. His results showed that oxygen consumption decreased exponentially with depth.

Pilson (1998) estimated the vertical distribution of oxygen consumption for the world ocean based

on extrapolations from sediment trap data and concluded these estimates were in good agreement

with those of Riley (1951).

Although these rates of consumption are rough estimates and may be underestimates, they point to

two facts of importance for the Gulf of Mexico. First, the rate of consumption decreases almost

exponentially with depth. Second, even the oxygen in the deep sea will be used up given enough

time, unless it is replaced. Due to the rapid resupply of oxygen by advection of the oxygen-rich

water masses entering the Gulf, consumptive processes do not exhaust deep-water dissolved

oxygen. But without such ventilation the deep waters would soon become stripped of oxygen, as

occurs in places such as the Black Sea.
50

Table 4.3

Estimated Rate of Dissolved Oxygen Consumption in the Atlantic Ocean Between 45°N and 54°N

(from Riley 1951)

Mean depth

(m)

O2 consumption

(μmol kg-1 yr-1)

200 9.1

280 3.5

370 2.2

510 2.4

700 1.5

1000 0.6

1250 0.2

1500 0.07

2000 0.06

2500-4000 0.006

Processes in the water column where dissolved oxygen reacts with dissolved inorganic matter,

including sulfides, Fe2+, and Mn2+, also consume oxygen. Oxygen consumption from these

processes are not well understood and likely are negligible in the water column (e.g., Rowe 2005).

The oxygen minimum typically observed in vertical profiles is created when the rate of oxygen

production is exceeded by the rate of oxygen consumption (e.g., Riley 1951). This minimum

develops in regions of high primary productivity. The biological production in the deepwater Gulf,

however, is not high enough that the input of sinking and decaying organic detritus reduces the

oxygen to low levels in that part of the basin. Instead, the oxygen minimum layer is transported

horizontally into the Gulf as part of the water masses entering with the Loop Current (Metcalf

1976; Morrison and Nowlin 1977; Wüst 1964). The dynamics of biological production and

oxygen consumption within the Gulf then contribute to maintenance of the oxygen minimum.

Where low oxygen waters intersect sediments on the slope, there is potential for localized reduction

of oxygen to suboxic levels, depending on the amount of metabolizable organic material available.

An oil spill would contribute to localized decreases in oxygen concentrations (see Section 4.2.3

below; e.g., Carney 2001). The oxygen minimum in the Gulf exhibits concentrations of 2.5-2.9

mL·L-1 in the western Gulf (Morrison et al. 1983). These concentrations are quite high compared to

oxygen minimum zones in other parts of the world, where concentrations drop below 0.5 mL·L-1

(e.g., Rogers 2000; Gage et al. 2000).

Near the seafloor, the same respiration reactions occur as in the water column, but at significantly

accelerated rates due to the general presence of a benthic nepheloid layer rich in organic matter (e.g.,

Karl et al. 1976). Below this layer, oxygen is transported into the sediments by exchange across the

interface that is driven by the physical forces of advection and diffusion and the biologic processes

of bioturbation and bioirrigation. Oxygen can be consumed in both abiotic and biotically driven

redox reactions with reduced components within sediments that may diffuse into the overlying

water. These include H2S, Fe2+, and Mn2+. The net apparent consumption of dissolved oxygen by

sediments is commonly referred to as "benthic oxygen demand" (BOD) and biologists commonly

equate it to net sediment heterotrophic activity. Results of geochemical studies from DGOMB data
51

found that O2 concentrations in sediments decreased in a highly variable manner in different parts

of the Gulf, reaching undetectable values at depths from <1 mm to ~13 cm below the sedimentwater interface (Rowe 2005). This consumption of oxygen by sediments is typical of oceanic

sediments. Jahnke and Jackson (1992) estimated the benthic oxygen flux from organic carbon

burial rates for the Atlantic and Pacific oceans. Their results estimated the flux in the Gulf and

adjacent Atlantic to be 0.06 – 0.24 mol O2·m-2·yr-1. The main source of the oxygen to the sediments

is from the overlying water. If the transport of dissolved oxygen to these waters is slow or absent,

as may occur in sea floor depressions or brine pools, localized pockets of oxygen-depleted waters

can be created and anoxic processes can take place in the sediments (see e.g., Pilson 1998).

4.3 Data Reanalysis

For the data analyses below, only bottle data that passed the primary and secondary QA/QC

processing were used. For these analyses, 35792 data points from 2790 separate stations were used

(see Table 3.1 for a list of the cruises). Figure 4.7 shows the locations of these stations relative to

different sub-regions of the Gulf of Mexico.

98W 96W 94W 92W 90W 88W 86W 84W 82W 80W

20N

22N

24N

26N

28N

30N

NW

SW

NE

SE

YC

FS

Figure 4.7. Locations of 2790 stations with at least one good oxygen datum in the Gulf of

Mexico. These data were used in the computations of the basic statistics. Geographical

regions are shown, including the Yucatan Channel (YC) and the Florida Straits (FS).

Bathymetry contours are 200, 1000, 2000 and 3000 m.
52

4.3.1 Basic Statistics

Regional Statistics: The mean and standard deviation were computed by depth bins for the whole

Gulf of Mexico as well as for sub-regions within the Gulf. Statistics were calculated for the data

that were in the Oxygen Archive as of 22 June 2004 (see Table 3.1).

Figure 4.8 shows the vertical profiles of the mean for the whole Gulf, as well as the Yucatan

Channel and Bay of Campeche sub-regions. Table 4.4 provides the statistics. The mean dissolved

oxygen concentration profile for the whole Gulf of Mexico follows the classical pattern with high

values near surface, decreasing values below this to an oxygen minimum, and generally increasing

values with depth below. The profiles for the Yucatan Channel and the Bay of Campeche follow a

similar pattern.

Figure 4.8 also shows the standard error of the mean for each bin. The standard error is defined as

€

σ x = σ

n

where σ is the standard deviation and n is the number of data points or independent samples. The

standard deviations for all the profiles are highest (0.3-0.6 mL·L-1) in the upper few hundred meters,

where atmospheric exchanges, inputs from photosynthesis, and oxygen consumption of organic

matter are important factors (Table 4.4). The standard deviations are lower (0.1-0.3 mL·L-1) below

this part of the water column, where such processes are greatly diminished.

Note that application of more stringent exclusion criteria to the data set than were applied in this

study would result in rejection of more data points and consequently less differences in means

between sub-regions and smaller standard deviations. For example, exclusion of data from the 1962

Hidalgo cruise, 62-H-3, results in mean concentrations that generally are elevated on the order of

0.01 mL·L-1 for many bins than the mean shown in Table 4.4. The low mean in the bottom bin of

the Bay of Campeche increases to nearly that for the whole Gulf, but the sample size is halved. This

cruise had significant scatter in the data (see Figure 3.7), although most of the points were within

2.3 standard deviations of the mean and so were not rejected in the QA/QC. Although exclusion of

these data would slightly change the magnitudes of the means and standard deviations, the general

patterns discussed here remain unchanged.

The Yucatan Channel mean profile, which reflects the profile of the source water masses entering

the Gulf, has several differences of note from the profile for the whole Gulf. First, the dissolved

oxygen concentrations in the Yucatan Channel tend to be higher throughout most of the water

column than those of the whole Gulf. Because there are no deep water masses formed in the Gulf

and oxygen is consumed with time below the upper layer that interacts with the atmosphere, this

difference suggests that the waters in the Gulf interior are not immediately replenished by the

inflowing water masses. The mean profile for the upper 750 m in the Bay of Campeche also

suggests this replenishment may be relatively slow, as these waters are depleted in oxygen relative

to the mean.

Note, however, that between approximately 750-1000 m the oxygen values in the Yucatan Channel

are less than those for the whole Gulf. This is evidence of a general vertical rise of isopycnals,

carrying their water masses, that occurs in the Gulf because of "mass adjustments associated with

the current regime" (Morrison et al. 1983) as the water masses are circulated westward by the

currents (see, e.g., Table 4.1). Thus, the more oxygenated waters below 1000 m in the Yucatan

Channel rise in depth as they spread into the Gulf giving the observed higher oxygen

concentrations just below the oxygen minimum in the waters within the Gulf.
53

Figure 4.8. Mean and standard error of bottle oxygen data by depth bin for the whole Gulf (WG;

green), Yucatan Channel (YC; red), and Bay of Campeche (BC; blue). The standard

error of the mean is the black horizontal line at the mid-point of each bin. The number

of data points, by bin, are given. The Bay of Campeche is south of and including 22°N

latitude.
54

Table 4.4

Dissolved Oxygen Mean and Standard Deviation in mL·L-1 by Bin for the Whole Gulf of Mexico

and the Yucatan Channel, Bay of Campeche, and Florida Straits Sub-regions

Region: Whole Gulf Yucatan Channel Bay of Campeche Florida Straits

Bin Range

(m)

NPTS Mean Std.

Dev.

NPTS Mean Std.

Dev.

NPTS Mean St.

Dev.

NPTS Mean St.

Dev.

0 – 50 7311 4.70 0.31 664 4.56 0.22 955 4.53 0.32 432 4.57 0.25

50 – 100 5670 4.40 0.59 479 4.42 0.33 602 4.31 0.60 249 4.29 0.46

100 - 150 3971 3.60 0.63 353 4.06 0.44 325 3.21 0.59 176 3.70 0.60

150 – 200 2797 3.26 0.43 322 3.66 0.26 257 2.80 0.24 152 3.36 0.43

200 – 250 2066 3.09 0.39 189 3.56 0.24 264 2.68 0.20 64 3.22 0.36

250 – 300 1732 3.03 0.44 204 3.60 0.24 196 2.54 0.16 70 3.21 0.30

300 – 350 1248 2.94 0.43 123 3.51 0.34 116 2.49 0.16 48 3.17 0.37

350 - 400 1151 2.88 0.38 119 3.31 0.28 147 2.50 0.14 50 2.98 0.25

400 – 450 1058 2.82 0.29 109 3.15 0.26 79 2.53 0.16 44 2.93 0.18

450 – 500 1050 2.85 0.26 125 3.08 0.23 128 2.59 0.14 52 2.82 0.24

500 – 550 727 2.85 0.24 114 3.00 0.21 78 2.67 0.17 37 2.91 0.20

550 – 600 597 2.92 0.20 80 2.95 0.17 92 2.81 0.18 25 2.95 0.21

600 – 650 544 2.95 0.24 67 2.94 0.20 35 2.89 0.21 16 2.94 0.33

650 – 700 470 3.12 0.28 72 2.96 0.24 48 3.05 0.24 17 3.07 0.35

700 – 750 515 3.20 0.27 62 3.04 0.20 70 3.20 0.26 15 3.23 0.27

750 – 800 620 3.35 0.31 74 3.17 0.29 88 3.39 0.21 24 3.21 0.36

800 – 850 429 3.49 0.35 53 3.33 0.29 20 3.58 0.28 15 3.23 0.27

850 – 900 199 3.64 0.36 51 3.48 0.33 27 3.76 0.27 8 3.55 0.48

900 – 950 273 3.78 0.31 48 3.67 0.33 43 3.81 0.27 9 3.84 0.32

950 – 1000 543 4.01 0.30 45 3.99 0.29 77 3.98 0.25 12 3.82 0.39

1000 – 1250 1159 4.35 0.36 134 4.36 0.37 124 4.34 0.32 22 4.43 0.30

1250 – 1500 608 4.77 0.25 83 4.82 0.29 105 4.69 0.26 13 4.73 0.30

1500 – 1750 345 4.89 0.24 66 5.03 0.26 18 4.87 0.22 9 4.91 0.29

1750 – 2000 233 4.96 0.31 49 5.16 0.32 24 4.75 0.32

2000 - 2500 236 5.06 0.31 24 5.57 0.17 29 4.95 0.28

2500 – 3000 152 5.03 0.20 14 5.04 0.24

3000 - bottom 88 5.00 0.25 4 4.71 0.39

Whole Gulf (n=35792) NPTS = number of points

Yucatan Channel (n=3709): NW corner 22°N 87°W; SE corner 21°N 84°W (see Figure 4.7)

Bay of Campeche (n=3965): West of 90°W and south of 22.5°N exclusive

Florida Straits (n=1559): NW corner 25°N 83°W; SE corner 23°N 80°W (see Figure 4.7)

The Yucatan Channel profile also has oxygen concentrations at depths greater than about 1500 m

that are high compared to the mean profiles within the Gulf. This is due to stations in the database

that are located south of the sill. Data below about 2000 m in the Yucatan Channel are from stations

located south of the sill. Waters at these depths are higher in oxygen than those at comparable

depths within the Gulf because their source is the North Atlantic Deep Water with its high oxygen

maximum core in the Caribbean of 5.5-6 mL·L-1 (Wüst 1964).

Second, there is a double oxygen minimum seen in the Yucatan Channel profile that indicates the

presence of the oxygen maximum in the 18°C Sargasso Sea water near 300 m depth. Although this

water mass may be present in individual profiles within the southeastern and, rarely, northeastern

Gulf, it is not present in the mean in other profiles and even is weak in the mean profile for the
55

Florida Straits data within the Gulf (see also Morrison et al. 1983; Morrison and Nowlin 1977).

This suggests 18°C Sargasso Sea water is eroded quickly as it mixes into the Gulf.

Third, the oxygen minimum of the Yucatan Channel is at approximately 600 m, which is deeper in

than the whole Gulf at approximately 400 m or the Bay of Campeche at about 300 m. This is

evidence of the general rise of isopycnals into the western Gulf. The depletion of oxygen in the

whole Gulf and Bay of Campeche minimums relative to that of the Yucatan Channel is evidence of

the age of the water. As the water mass circulates into the Gulf interior, the oxygen in this upper

part of the water column is being consumed as organic matter decays. So, these profiles together

suggest that the replenishment of the oxygen from the source waters into the Gulf interior is slow.

The rates of replenishment, however, cannot be determined from the data available.

Examination of profiles from sub-regions of the Gulf of Mexico provides additional insights into

the vertical structure of dissolved oxygen. For this purpose, the Gulf was divided into four

quadrants, going counterclockwise: southeast, northeast, northwest, and southwest. Figure 4.9

shows the vertical profiles of the means for these quadrants and gives the standard error of the

mean for each bin. Table 4.5 gives the mean, standard deviation, and number of data points.

The upper 1000 m exhibits the greatest differences between the four sub-regions. The southeast

profile is similar to that of the Yucatan Channel, with the oxygen maximum associated with the

18°C Sargasso Sea water weakly evident at about 300 m and the deeper oxygen minimum at about

600 m. The mean profiles for the three other regions do not have the 18°C water maximum.

Moving counterclockwise through the quadrants from the southeast to the southwest, the oxygen

minimum becomes shallower and the concentration at the minimum becomes less. This is evidence

for the differences between the sub-regions in the replenishment rate of oxygen from the Loop

Current system relative to the consumption rate in the Gulf interior. The mean profile for the

southeast sub-region, much of which is influenced directly by the Loop Current approximately 70%

or more of the time (SAIC 1989), is very similar to that for the Yucatan Channel (compare the

Yucatan Channel column of Table 4.4 with the Southeast column of Table 4.5). For example, the

oxygen minimum is 2.94 mL·L-1 at 600-650 in the Yucatan Channel and 2.96 mL·L-1 at 550-650 in

the southeast Gulf. This strong match in the mean profiles indicates the southeast sub-region is

replenished at a rate consistent with the rate of flows of the source waters into the Gulf.

The northeast sub-region, some of which also is influenced directly by the Loop Current but at less

than 70% of the time (SAIC 1989), has less oxygen in the oxygen minimum (2.77 mL·L-1), which

itself occurs higher in the water column at 400-450 m. The mean profile in the northwest subregion is similar in structure to that in the northeast sub-region, but with lower oxygen

concentrations for given depths. For example, the oxygen minimum is at 400-450 m, as in the

northeast, but has a lower mean of 2.65 mL·L-1. This similarity likely reflects the movement of Loop

Current eddies, which carry source waters, into the western Gulf. One analysis shows the presence

of these eddies in the northwestern Gulf 5-15% of the time (SAIC 1989). The southwest quadrant

has the lowest oxygen concentrations of all with the structure also higher in the water column (2.52

mL·L-1 at 350-400 m for the oxygen minimum). This decrease in oxygen concentration in the

oxygen minimum is evidence that the oxygen concentrations from the source waters of the upper

water column are being reduced by the local processes of oxygen consumption during organic

decay as they move from the source region at the Yucatan Channel into the western Gulf.

To explore this, the western Gulf was divided into the Bay of Campeche (south of and including

22°N) and north-central western Gulf. Figure 4.10 shows profiles of means for the upper 1000 m

of the two sub-regions. The standard error also is shown. The dissolved oxygen distribution in the

Bay has an average minimum value significantly less than that in the north-central western Gulf

(2.49 mL·L-1 at 300-350 m in Campeche Bay versus 2.63 mL·L-1 at 400-450 m in the north-central
56

Figure 4.9. Mean and standard error of bottle oxygen data by depth bin for the southeast (SE:

red), northeast (NE; gray), northwest (NW; blue), and southwest (SW; green)

quadrants of the Gulf of Mexico. The standard error of the mean is the black

horizontal line at the mid-point of each bin. The number of data points, by bin, are

given. The boundaries of the quadrants are illustrated in Figure 4.7.
57

Table 4.5

Dissolved Oxygen Mean and Standard Deviation in mL·L-1 by Bin for the Southeast, Northeast,

Northwest, and Southwest Sub-regions of the Gulf of Mexico

Region: Southeast Northeast Northwest Southwest

Bin Range

(m)

NPTS Mean Std.

Dev.

NPTS Mean Std.

Dev.

NPTS Mean St.

Dev.

NPTS Mean St.

Dev.

0 – 50 2093 4.62 0.26 2021 4.78 0.31 1812 4.80 0.27 1385 4.57 0.32

50 – 100 1483 4.46 0.43 1823 4.32 0.62 1450 4.39 0.69 914 4.46 0.58

100 - 150 1136 3.99 0.51 1377 3.46 0.51 931 3.40 0.65 527 3.50 0.75

150 – 200 969 3.61 0.37 877 3.19 0.32 540 2.99 0.28 411 2.91 0.31

200 – 250 623 3.47 0.33 616 3.06 0.28 429 2.88 0.22 398 2.77 0.26

250 – 300 598 3.46 0.35 533 2.95 0.29 311 2.75 0.18 290 2.59 0.18

300 – 350 392 3.37 0.40 469 2.84 0.28 209 2.69 0.16 178 2.54 0.18

350 - 400 387 3.24 0.34 374 2.82 0.24 175 2.67 0.15 215 2.52 0.14

400 – 450 332 3.10 0.29 363 2.77 0.18 192 2.65 0.15 171 2.57 0.16

450 – 500 415 3.04 0.24 313 2.82 0.17 124 2.73 0.17 198 2.59 0.15

500 – 550 324 2.98 0.22 160 2.83 0.20 107 2.73 0.17 136 2.67 0.18

550 – 600 265 2.96 0.18 119 2.98 0.19 73 2.95 0.22 140 2.79 0.18

600 – 650 213 2.96 0.24 129 3.05 0.24 104 2.90 0.24 98 2.85 0.20

650 – 700 192 3.03 0.29 137 3.25 0.25 63 3.24 0.22 78 3.03 0.26

700 – 750 178 3.12 0.28 158 3.30 0.22 72 3.22 0.26 107 3.17 0.27

750 – 800 240 3.23 0.34 150 3.44 0.30 80 3.49 0.27 150 3.37 0.22

800 – 850 176 3.35 0.33 95 3.65 0.35 87 3.59 0.33 71 3.48 0.29

850 – 900 113 3.57 0.39 24 3.74 0.33 29 3.71 0.25 33 3.76 0.27

900 – 950 115 3.73 0.34 51 3.88 0.30 45 3.81 0.26 62 3.80 0.25

950 – 1000 162 3.93 0.34 177 4.07 0.26 73 4.09 0.31 131 3.98 0.26

1000 – 1250 448 4.34 0.38 280 4.40 0.32 196 4.39 0.37 235 4.30 0.33

1250 – 1500 238 4.80 0.25 106 4.78 0.26 86 4.73 0.23 178 4.73 0.25

1500 – 1750 161 4.94 0.25 59 4.82 0.24 64 4.86 0.19 61 4.83 0.21

1750 – 2000 105 5.09 0.29 42 4.93 0.17 35 4.92 0.31 51 4.74 0.30

2000 - 2500 87 5.24 0.34 48 4.99 0.16 37 5.02 0.14 64 4.89 0.28

2500 – 3000 28 5.10 0.18 39 5.08 0.11 30 5.03 0.17 55 4.96 0.25

3000 - bottom 9 5.03 0.25 18 5.13 0.12 12 5.08 0.20 49 4.93 0.27

Southeast Gulf of Mexico (n=11482): East of 90°W inclusive and south of 25°N inclusive NPTS=number of points

Northeast Gulf of Mexico (n=10558): East of 90°W inclusive and north of 25°N exclusive

Northwest Gulf of Mexico (n=7366): West of 90°W exclusive and north of 25°N exclusive

Southwest Gulf of Mexico (n=6386): West of 90°W exclusive and south of 25°N inclusive

western Gulf). Standard deviations are ~0.15 mL·L-1 and the standard errors are ~0.01 mL·L-1

suggesting that the means are statistically different. This is evidence of a minimal or slow exchange

of waters from the northwestern Gulf with those of Campeche Bay, such as might occur in the

presence of a persistent cyclonic feature in the Bay (Vázquez de la Cerda et al. 2005).

Mean profiles below 1000 m suggest oxygen levels are less at a specified depth the farther from the

inflowing source waters a station is located. At the deepest bin (≥3000 m), however, there is a hint

that the waters in the southeast sub-region may be lower in oxygen than those in the northeast or

northwest. Unfortunately, this may be an artifact of the very small number of samples. If real, it

likely would be related to the poorly understood deep circulation underlying the Loop Current

(Nowlin et al. 2001).
58

Figure 4.10. Mean oxygen concentrations by bins for the Bay of Campeche (BC; blue) and the

northwest Gulf of Mexico (NW; red). All data used were west of 90°W, with the Bay

of Campeche using those data south of and including 22.5°N and the northwest Gulf

using all other data. The standard error of the mean is the horizontal line at the midpoint of each bin depth. The number of data points, by bin, are given.
59

Uniformity of Deep Concentrations: Nowlin et al. (1969) undertook a careful and detailed

examination of dissolved oxygen data sets from the deep waters of the Gulf of Mexico. They were

motivated by findings from three cruises in 1966 and 1967 that seemingly contradicted the study of

McLellan and Nowlin (1963). That earlier study, based on data from the 1962 cruise 62-H-3 of the

Hidalgo, had suggested there were large gradients in dissolved oxygen below approximately 1500-

m depth. Nowlin et al. (1969) conducted a careful comparison of dissolved oxygen data from 62-

H-3 and the 1964 cruises, 64-A-2 and 64-A-3, of the Alaminos with those from the 1935 cruise of

the Atlantis, the 1958 and 1959 cruises of Hidalgo (58-H-4, 58-H-1, 59-H-2), and the 1966 and

1967 cruises of the Alaminos (66-A-8, 67-A-4, 67-A-8). They concluded that the data from the

1962 and 1964 cruises were based on faulty sampling or poor analysis techniques. They

determined the data sets from the 1935, 1958, 1959, 1966, and 1967 cruises were good. Results of

their analysis of these good data showed there was no clearly discernable horizontal variation in

dissolved oxygen in the deep waters of the Gulf. Nowlin et al. (1969) argued this was consistent

with the horizontal uniformity of salinity and potential temperature in the data from the 1966 and

1967 cruises and that McLellan and Nowlin (1963) found in the 62-H-3 data. They also found that

there were only slight vertical gradients below the depth of the Yucatan sill, and that the mean

dissolved oxygen concentrations of the deep waters at or below 1500 m was 4.99 mL·L-1 (excluding

the 1962 data). Table 4.6 lists the mean, standard deviation, and standard error of the mean for each

of the cruises examined by Nowlin et al. (1969). They found no evidence that oxygen

concentrations had changed during the time period covered.

Table 4.6

Historical Statistics for Dissolved Oxygen At or Below 1500 m Depth in the Gulf of Mexico

(after Nowlin et al. 1969; Wüst data are in the core of the NADW, based on data from 1932-1937)

Data set identifier Mean

(mL·L-1)

Standard

deviation

(mL·L-1)

Standard

error

(mL·L-1)

Number of

data points

Atlantis 1935 4.96 0.134 0.028 23

58-H-1 5.08 0.124 0.028 19

58-H-4 4.87 0.054 0.020 7

59-H-2 5.01 0.055 0.013 17

66-A-8 4.99 0.048 0.011 19

67-A-4 Gulf basin 5.03 0.077 0.010 55

67-A-8 5.01 0.111 0.024 21

67-A-4 Yucatan basin 5.61 0.099 0.030 11

Wüst (1964) Yucatan basin 5.64 0.33 0.076 19

62-H-3 4.62 0.280 0.026 114

64-A-2 and 3 4.58 0.482 0.059 67

Statistics for data at or below 1500 m depth were computed using 1054 data points from 461

different stations that passed QA/QC processing. Figure 4.11 shows the locations of the stations.

The statistics for selected sub-regions are presented in Table 4.7.
60

98W 96W 94W 92W 90W 88W 86W 84W 82W 80W

20N

22N

24N

26N

28N

30N

NW

SW

NE

SE

YC

FS

Figure 4.11. Locations of 461 stations with at least one good oxygen datum in depths ≥1500 m in

the Gulf of Mexico. Geographical regions are shown, including the Yucatan Channel

(YC) and the Florida Straits (FS). Bathymetry contours are 200, 1000, 2000 and

3000 m.

The mean dissolved oxygen concentration for the whole Gulf is 4.97 mL·L-1, which is similar to the

4.99 mL·L-1 found by Nowlin et al. (1969) for "good" data. The standard deviation, however, is

approximately twice that of the Nowlin et al. (1969) study. This is due to the more rigorous data

processing used in that earlier study as compared to this study, which rejected only the most clearly

suspect data points. However, a rigorous processing was applied to the data base to provide a good

data set for use in the box model (see Section 3.3). The mean of this data set is 4.98 mL·L-1 with a

standard deviation comparable to that of Nowlin et al. (1969). This and the standard errors of the

means suggest that, although the standard deviations for the selected sub-regions are approximately

0.2-0.3 mL·L-1, inferences based on the means can be reasonably drawn.

To examine the issue of horizontal uniformity of oxygen concentration below 1500 m, a first

division compared the eastern with the western Gulf. The mean concentration in the eastern Gulf is

greater than that in the western Gulf by 0.12 mL·L-1. A Student-t test was applied to examine

whether there was a statistically meaningful difference between the means in these two regions. The

null hypothesis was

H0: μ1 – μ2 = 0
61

where μ1 and μ2 represent the two population means. At the 95% confidence level, the null

hypothesis is rejected, suggesting that the two regions have different mean dissolved oxygen

concentrations at depth.

Table 4.7

Statistics for Dissolved Oxygen At or Below 1500 m Depth in the Gulf of Mexico from Data in the

Oxygen Archive

(Regions of the Gulf are shown in Figures 4.7 and 4.11)

Region Mean

(mL·L-1)

Standard

deviation

(mL·L-1)

Standard

error

(mL·L-1)

Number of

data points

Number of

unique

stations

Whole Gulf 4.97 0.27 0.008 1054 461

Whole Gulf – Box Model 4.98 0.09 0.009 97 43

East 5.03 0.28 0.011 596 268

West 4.90 0.26 0.012 458 193

Yucatan Channel above sill 5.09 0.30 0.028 116 61

Southeast outside YC 5.00 0.26 0.016 251 117

Northeast 4.96 0.21 0.015 206 90

Northwest 4.95 0.22 0.016 178 75

Southwest 4.87 0.27 0.016 280 118

The means of the deep water dissolved oxygen concentrations for the five sub-regions (Table 4.7)

suggest the oxygen concentrations may decrease slightly going from the source waters in the

Yucatan Channel into the southeast, northeast/northwest, and southwest Gulf. The greatest

difference, of 0.22 mL·L-1, is between the inflowing source waters at the Yucatan Channel and the

waters of the southwest Gulf. Although this difference is comparable to the standard deviation, the

standard error of the mean suggests these means are different. A Student-t test indicates that, at the

95% confidence level, the means are not from the same population. The means for waters of the

northeast and northwest are within 0.01 mL·L-1 of each other, and the Student-t test supports that

they are not different. Application of the Student-t test to the other pairs of sub-regions results in

rejection of the null hypothesis of the same mean. The standard errors also support this, with the

northeast and northwest values evidencing overlap in the means plus/minus the standard error and

the southeast and southwest values evidencing no overlap. These statistics suggest that there may be

a horizontal variation in the deep Gulf waters and that these waters may essentially be divided into

three basic parts: southeastern Gulf, northern Gulf, and southwestern Gulf.

The relatively high mean concentration in the southeast Gulf is an indication that oxygen may be

rapidly replenished from the source waters. The relatively low mean concentration in the southwest

Gulf suggests these waters are not flushed as rapidly as the rest of the Gulf. However, from the

standard deviations of 0.2-0.3 mL·L-1, these statistics may also simply be the result of differences in

oxygen titration methods, variations by analyst in the visual determination of the end point of the

titrations, poor data collection, or poor sample analysis.
62

4.3.2 Patterns of Dissolved Oxygen Distribution

As seen in the statistical profiles, the fundamental distributions of dissolved oxygen are caused by

the distribution of the inflowing source waters into the Gulf interior by the circulation with relatively

small modifications caused by local inputs of oxygen in the photic zone and mixed layer and by

oxygen consumption throughout the water column. These source waters consist of a number of

water masses, including the oxygen minimum waters of the Tropical Atlantic Central Water (mean

of ~2.9 mL·L-1 at 300-700 m depths) and the well-oxygenated waters of the Antarctic Intermediate

Water (at 500-1000 m depths). Vertically, the structure everywhere within the Gulf consists of welloxygenated waters (~4.5 mL·L-1 or higher) in the mixed layer that occurs approximately in the

upper 100 m, declining concentrations to an oxygen minimum (generally < 3 mL·L-1) ranging from

~300 m to 700 m, and increasing concentrations to well-oxygenated deep waters that are fairly

uniform (~5 mL·L-1) below about 1500 m. This pattern was seen by Morrison and Nowlin (1977)

in data from the eastern Gulf on cruise 72-A-9 in May 1972 and by Morrison et al. (1983) in data

from the western Gulf on cruise 78-G-3 in April 1978. Those papers provide detailed discussion of

the vertical distributions of oxygen in the Gulf. Horizontally, dissolved oxygen concentrations in

waters in the mixed layer are variable, depending as they do on the variable air-sea exchanges,

photosynthetic activity, and proximity to regions with relatively heavy sedimentation (i.e., river

outflows). Concentrations in waters below the mixed layer tend to decrease slightly the farther away

the water is from the source waters inflowing at the Yucatan Channel, confirming that there is no

water mass formation in the Gulf to replenish the dissolved oxygen consumed by the decay of

organic matter. The source and sink system appears to have been in equilibrium over the 80 years

of observations.

Structure of Dissolved Oxygen Distributions: Examination of vertical sections of dissolved oxygen

concentrations with depth from selected cruises provides a means for visualization of the patterns of

distributions seen in the mean profiles. To provide examples of the patterns of dissolved oxygen

distributions, here are examined vertical sections from cruises in 1958 and 2000-2001.

In 1958, the research vessel Hidalgo conducted a number of cruises to examine the circulation and

properties of the waters in the Gulf of Mexico. Results from two of these cruises, 58-H-1 and 58-

H-4, are considered here. These cruises represent snapshots of the oxygen distribution associated

with the then-current stage of the Loop Current intrusion into the eastern Gulf and Loop Current

rings into the western Gulf. Figure 4.12 shows the locations of three transects from these cruises.

Figure 4.13 is a vertical section of dissolved oxygen across the Yucatan Channel from cruise 53-H4; all samples were collected on 18 May 1958. Figure 4.14 shows the distribution of dissolved

oxygen concentrations in the eastern Gulf from the Mississippi River Delta to Havana, Cuba,

collected 25-28 June 1958 on cruise 58-H-4. Figure 4.15 presents the vertical distribution in the

western Gulf along –94.5°W longitude from data collected on 58-H-1 on 23-30 March 1958.

These three transects are from McLellan (1960).

These three distributions have the same basic vertical distribution of dissolved oxygen throughout

the water column. The dissolved oxygen values are relatively high near the surface, decrease to a

minimum, and increase with depth below the minimum. Although the depth of the minimum is

variable, being higher in the water column in the Yucatan Channel, the minimum occurs near the

sigma-t of 27.1 on all three transects (see McLellan 1960). This exemplifies the tendency of water

masses to flow along isopycnal surfaces, as well as the general uplift of those isopycnals from east

to west in the Gulf.

The vertical section of dissolved oxygen for the Yucatan Channel (Figure 4.13) shows marked

tilting of the isolines of dissolved oxygen in the upper 500 m. This tilting upward to the west of

isolines also is present in the vertical sections (not shown here) of temperature, salinity, and sigma-t.

It is indicative of the strong northward flow of the Yucatan Current in the upper waters that is
63

98N 96N 94N 92N 90N 88N 86N 84W 82N 80N

20N

22N

24N

26N

28N

30N

8 9 10 1112

18N

Yucatan Channel

Cruise 58-H-4

18 May 1958

Western Gulf (-94.5W)

Cruise 58-H-1

23-30 March 1958

Eastern Gulf

Cruise 58-H-4

25-28 June 1958

Figure 4.12. Station locations for Hidalgo cruises 58-H-1 and 58-H-4. Bathymetry contours are

200, 1000, 2000 and 3000 m.

intensified to the west. Note also that the sill depth of about 2000 m allows the well-oxygenated

waters of > 4 mL·L-1 that are below about 1000 m depth to flow into the Gulf. These results are

seen in other transects across the Yucatan Channel, including most recently that shown in Ochoa et

al. (2001) from a data set collected in 1997. That recent transect also exhibits a pattern in the vertical

distribution of dissolved oxygen that is similar to that in Figure 4.13, including the tilted isolines of

oxygen and the presence of an oxygen minimum of < 3 mL·L-1 at depths of 300 m to 800 m.

The vertical distribution of dissolved oxygen in the eastern Gulf reflects the basic structure of the

oxygen content of the source waters inflowing at the Yucatan Channel. Figure 4.14 also reflects

patterns in distribution caused by a different circulation regime than that of the Yucatan Channel.

There appears to be a clockwise circulation around station 90, as indicated by the depressed isolines

at that station. There also appears to be an eastward flow south of station 88, again indicated by the

depression in isolines. This structure suggests that either the Loop Current "made a wide sweep and

entered the Florida Strait from the northwest" (McLellan 1960) or that a Loop Current Eddy had

detached with its centerline located near station 90 and the Loop Current had turned eastward south

of station 88. The result of this circulation is that, in the upper 800 m to 1000 m, the isolines of

dissolved oxygen are deeper in the water column near the center of the feature at station 90 and

deepen south of station 88. The oxygen minimum had values generally greater than 2.7 mL·L-1 at
64

Figure 4.13. Vertical distribution of dissolved oxygen across the Yucatan Channel, Cruise 58-H-4,

18 May 1958. (after McLellan 1960)
65

Figure 4.14. Vertical distribution of dissolved oxygen in the Eastern Gulf, from the Mississippi

River Delta to Havana, Cruise 58-H-4, 25-28 June 1958. (after McLellan 1960)
66

Figure 4.15. Vertical distribution of dissolved oxygen in the Western Gulf, along -94.5°W

longitude, Cruise 58-H-1, 23-30 March 1958. (after McLellan 1960)

depths ranging from ~350 m to 500 m at different parts of the section due to the circulation. Below

about 1000 m the waters are well-oxygenated, with the highest concentration being 4.9 mL·L-1 near

bottom.

In contrast to the eastern Gulf in June 1958 (Figure 4.14), the western Gulf in 1958 had no

evidence of strong circulation features such as Loop Current Eddies, that depress (anticyclones) or

raise (cyclones) isolines. The temperature, salinity, and sigma-t sections (not shown) suggest a

general anticyclonic circulation with an eastward counterflow near the Mexican coast and a

westward flow on the Texas shelf (McLellan 1960). The isopleths of dissolved oxygen exhibit only

small vertical uplifts and depressions, rather than the strong depressions or uplifts that were seen in

association with the strong circulation features of the eastern Gulf. The oxygen minimum in the

western Gulf had values generally from 2.5 to 2.8 mL·L-1 at sigma-t level of about 27.1 and depth

of ~400 m. This pattern differs from that of the eastern Gulf; this reflects the differences in

circulation, as well as the decrease in oxygen concentrations due to the greater net consumption of
67

oxygen that occurs in the western Gulf because those waters are farther from the source waters than

those of in the eastern Gulf. Maximum oxygen concentrations below 2500 m were > 5.0 mL·L-1.

The near surface values of the three transects exhibit a number of differences, which likely reflect

different processes at work. Seasonal differences in the air-sea exchanges are suggested by

comparison of the relatively lower, more stratified oxygen values (~4.5 mL·L-1) of the upper waters

in the summertime transect in the eastern Gulf (Figure 4.14) with the relatively higher, well-mixed

concentrations (~5 mL·L-1) of the early spring transect in the western Gulf (Figure 4.15). Of the

three transects, the maximum oxygen values near surface were > 5.9 mL·L-1 and occurred over the

Texas shelf in the western Gulf transect (Figure 4.15). These waters had temperatures that were

relatively cold (~16°C) and salinities that were relatively low (< 35 to as low as 32.7), exemplifying

the typical wintertime conditions in this region that are favorable for increased oxygen uptake by the

ocean in air-sea exchanges. Variability in photosynthetic activity is indicated by the local subsurface oxygen maximum at about 50-100 m depth at station 91 offshore of the Mississippi River

Delta (Figure 4.15). This would be an area that could have higher summertime primary productivity,

with increased photosynthesis, associated with nutrient inputs from the Mississippi River (see e.g.,

Jochens et al. 2002).

The MMS-sponsored project, DGOMB, provided an opportunity to sample much of the northern

Gulf to near bottom. Data from two cruises were examined: Cruise 1 in 4 May – 17 June 2000 and

Cruise 2 in 2-18 June 2001. Figure 4.16 shows the sea surface height during each cruise with the

stations from a transect selected from each cruise. The June 2000 cruise crossed the northern half

of a Loop Current Eddy that was in the western Gulf (Figure 4.16 upper panel). The dissolved

oxygen section for this transect (Figure 4.17) shows the characteristic deepening isolines from the

outer edge (station RW1) to the center (station RW6). The basic vertical structure remains the same,

as seen in the Hidalgo cruises and as found by Morrison and Nowlin (1977) for the eastern Gulf in

1972 and Morrison et al. (1983) for the western Gulf in 1978. The near-surface waters are welloxygenated, as is typical for waters that have contact with the atmosphere and photosynthetic

activity. The core of the oxygen minimum occurs on the sigma-theta surface of 27.15. The welloxygenated deep waters occur below about 1000 m. Oxygen concentrations below 1500 m were ≥

4.9 mL·L-1 for this transect. The June 2001 cruise sampled the eastern Gulf. Although there were a

number of eddy features (anticyclonic and cyclonic) in the area sampled, no transect crossed such

features (Figure 4.16 lower panel). A transect consisting of the selected stations from the cruise was

constructed and is presented in Figure 4.18. The station spacing is too wide to allow conclusions

about the effects of circulation on the dissolved oxygen distributions. However, this figure shows,

once again, the consistent pattern of the vertical structure of dissolved oxygen in the Gulf. The

oxygen minimum again is on the sigma-theta surface of 27.15. Note the high oxygen

concentrations for the deepest waters, below the sill depth of the Yucatan Channel; these exceed 5

mL·L-1 at the samples below 1500 m. This is indicative of the continued replenishment of dissolved

oxygen to the deep waters from the only source available–overflowing waters from the Yucatan

Channel.

Patterns from oxygen/sigma-theta plots: The water masses tend to have their characteristic property

cores along specific isopycnals (see, e.g., Wüst 1964; Wennekens 1959; Morrison and Nowlin

1977; and Morrison et al. 1983). The data sets from the May/June 2000 and June 2001 DGOMB

cruises on the R/V Gyre were compared to those from the 1972 Alaminos cruise 72-A-9 examined

by Morrison and Nowlin (1977) and the 1978 Gyre cruise 78-G-3 examined by Morrison et al.

(1983). DGOMB data were split into east and west to match, respectively, the 72-A-9 and 78-G-3

data sets from those two regions. Figure 4.19 shows the dissolved oxygen concentrations versus

sigma-theta, with the black symbols being from the older cruises and the red data being from the

DGOMB cruises.
68

Figure 4.16. Sea surface height for TOPEX/ERS2 analysis for 13 May 2000 and 10 June 2001.

Station locations for transects shown in Figures 4.17 and 4.18 are shown.
69

Figure 4.17. Dissolved oxygen on the western-most transect of DGOMB cruise DG1, 4 May - 17

June 2000. Dashed line shows the sigma-theta density surface associated with

selected water masses (Table 4.1). Dots show sampling locations.
70

Figure 4.18. Dissolved oxygen on DGOMB cruise DG2, 2-18 June 2001. Dashed lines show the

sigma-theta density surfaces associated with selected water masses in the Gulf (Table

4.1). Station locations are shown in Figure 4.16.
71

Figure 4.19. Dissolved oxygen on density surfaces for the eastern and western Gulf of Mexico.

Data from the east Gulf are located at or east of -90°W; data from the west are west

of -90°W. Specific cruises shown are the May 1972 Alaminos cruise (72-A-9), April

1978 Gyre cruise (78-G-3), and May/June 2000 and June 2001 Gyre cruises (00-G05 and 01-G-05) of the DGOMB project. Shown are sigma-theta density surfaces

for the 18°C Sargasso Sea Water (26.5 kg·m-3), Tropical Atlantic Central Water

(27.15 kg·m-3), and Antarctic Intermediate Water (27.4 kg·m-3).
72

Comparing the upper and lower panels of Figure 4.19, it is seen that the basic patterns between the

east and west Gulf are the same. The horizontal variability in the upper waters is large, reflecting

differences in the air-sea exchanges, photosynthetic inputs, and oxygen consumption rates of the

near-surface waters. In contrast, the horizontal variability of the deep waters, below the Antarctic

Intermediate Water with a salinity minimum core on the 27.4 kg·m-3 sigma-theta surface, is small.

The DGOMB cruises in 2000 and 2001 did not sample south of approximately 27°N in the eastern

Gulf. This likely resulted in the lack of samples in the 18°C Sargasso Sea Water that occurs along

sigma-theta surface of 26.5 kg·m-3. This water mass, however, is evident in some of the stations

from the 1972 Alaminos cruise (Figure 4.19, upper panel). Morrison et al. (1983) found that this

water mass does not occur in the western Gulf; this is confirmed by the DGOMB data (Figure

4.19, lower panel). The core of the oxygen minimum has been found by Wüst (1964), Morrison

and Nowlin (1977), and Morrison et al. (1983) to be on the 27.15 kg·m-3 sigma-theta surface

throughout the Gulf. The DGOMB data set confirms that this is still the case.

Temporal Variations: In their detailed comparison of oxygen data sets from 1935, 1958, 1959,

1962, 1964, 1966, and 1967 (see 4.3.1 above), Nowlin et al. (1969) found no evidence that

dissolved oxygen concentrations from waters at or below 1500 m had changed during the time

period covered. The comparison of the data sets from 1972 and 1978 with those from 2000-2001,

as well as the results described above in the statistical section, provide additional evidence that no

changes have occurred, even higher in the water column. To consider this issue further, the data

were divided into six periods: pre-1950, 1950i to 1960, 1960i to 1970, 1970i to 1980, 1980i to

1990, and 1990i to 2001, where the "i" indicates that data from this year were included for the

period. Data then were divided into east and west and binned as before, and then statistics were

computed. Figure 4.20 shows the mean dissolved oxygen values by decade. No clearly discernible

temporal trends are present. For example, considering the deepest values, in both the east and west,

the decade of 1960-1970 is lowest in mean concentration, 1990-2001 is in the middle, and 1970-

1980 is highest. For another example, considering the oxygen minimum bin of 450-500 m in the

eastern Gulf, the lowest mean concentration is in 1950-1960 (with the smallest sample size), the

next lowest is 1980-1990, and the highest is 1970-1980. The oxygen minimum bin of 300-350 m

in the western Gulf has a different pattern, with the lowest concentration being in 1970-1980 and

the highest being 1950-1960. These results together with results from other analyses in this study

and from the early investigators indicate that the processes that maintain the oxygen distributions

have not changed over the 80 years encompassed by the available data sets.

Influence from shelves on deep ocean oxygen distributions: To examine effects of shelf processes

on the deepwater oxygen distributions, the northern Gulf was divided into the 2° longitude swaths

of 86-88°W, 88-90°W, 90-92°W, 92-94°W, and 94-96°W. The data within these swaths north of

25°N then were separated by total water depth into regions bounded by the 200-1000 m, 1000-2000

m, and 2000-3000 m isobaths. Data in the upper 1000 m then were binned in 50-m deep bins and

the mean and standard deviation for each bin were computed. The pattern of oxygen distribution in

the 86-88°W swath (Figure 4.21), which includes DeSoto Canyon, suggests that lower oxygen

concentrations might occur in the shallower total water depths at and above the oxygen minimum

zone. The swath for 88-90°W shows a similar order, but with less difference between the relatively

high and low oxygen regions. However, because the deep water regions in these swaths encompass

territory that is directly influenced by the Loop Current, this result more likely is due to the

occurrence of higher oxygen concentrations from those source waters in these deepwater regions

(2000-3000 m and 3000 m-bottom) than from influence of the shelves. Additionally, this pattern is

not clear when the standard deviations, which are large, are considered. Below the oxygen minimum

in this swath and for all swaths west of 90°W, there is no discernible pattern (Figure 4.21). Thus,

little if any influence from the shelves are seen in the oxygen distributions in the northern Gulf.
73

Figure 4.20. Mean dissolved oxygen in the Gulf of Mexico by decade. Data from the eastern Gulf

(red) are located east of and including -90°W. Data from the western Gulf (black) are

located west of -90°W.
74

Figure 4.21. Mean dissolved oxygen in 50-m depth bins for selected total water depths in the

northern Gulf of Mexico. Only data north of 25°N were used.

The LATEX A (Nowlin et al. 1998a, 1998b) and NEGOM (Jochens et al. 2002) programs provide

information on the dissolved oxygen distributions over the shelves and insights into possible

exchanges with the deepwater. As with Nowlin and Parker (1974), the LATEX A and NEGOM

programs showed no water mass formation over the shelves. This is for the simple reasons that the

shelf waters never become cold or saline enough to create the dense waters necessary to flow off the

shelf and ventilate the deep waters. Profiles along the shelf edge (LATEX A and NEGOM) and

1000-m isobath (NEGOM) indicate that the upper water masses, such as the Subtropical

Underwater and the Tropical Atlantic Central Water, may be present at the shelf edge and that

circulation features such as cyclonic and anticyclonic eddies, squirts, and jets may result in

exchange of waters between the shelf and the open ocean (see Sahl et al. 1997; Nowlin et al. 1998a,

1998b; Jochens et al. 2002). However, these exchanges are limited to the upper few hundred meters

and do not extend into the deep waters.

The results of the LATEX A and NEGOM programs show that the processes over the shelves have

little influence on the oxygen distributions in the upper waters of the deepwater Gulf. Any influence

on the oxygen content of the lower waters would have to be from transport of organic material off
75

the shelf into the deep basins, but the oxygen data do not suggest that this contributes in any

unusual way to the normal oxygen consumption patterns.

These results suggest there is no strong influence of shelf processes on dissolved oxygen

concentrations in the deepwater Gulf. Rather, the effect of the processes at the continental margin

on the oxygen distributions in the open ocean waters seems to be related to processes of off shelf

circulation interactions with the topography, such as uplift of isopycnals from east to west in the

upper 1000 m in response to the general circulation pattern of the Gulf.

4.4 Effects of Oil and Gas Inputs on Dissolved Oxygen Concentrations

A natural hydrocarbon spill experiment in the Gulf of Mexico has been ongoing for millions of

years in the form of naturally occurring seeps that introduce oil at many locations throughout the

basin. The mean dissolved oxygen concentrations in the deep Gulf have not noticeably changed

over the last 80 years, and the stability of the oxygen distribution in the deep Gulf suggests the

natural experiment has not resulted in large depletions in the oxygen reservoir of the deep Gulf (see

Section 4.3). For this reason, the Gulf of Mexico system must be considered in equilibrium with

regard to dissolved oxygen concentrations, with ventilation processes balancing local consumption

processes. For example, on a basin scale, the Gulf dissolved oxygen system has not shown any

effect from the ~150,000 tonnes of hydrocarbons typically input to the Gulf each year from natural

and anthropogenic sources. However, on local spatial and short-term scales, the effects of such

inputs could be profound.

In this section, we discuss how oil and gas inputs can affect the concentration of dissolved oxygen

in the deepwater region of the Gulf of Mexico. Rather than an exhaustive listing of every process

that can potentially affect dissolved oxygen concentration, we limit our discussion to those

processes considered most significant in deep water. We begin the section with a discussion of the

dissolved oxygen content of the Gulf of Mexico as a function of water depth. Next, is a general

discussion of the sources of petroleum in the ocean followed by a discussion of the general

processes that can affect dissolved oxygen concentration in the water column, including the effects

of drilling fluids, drill cuttings, and discharge of produced water. A brief summary of the IXTOC I

and other case study spills is next, followed by a set of simple calculations of the impact on

dissolved oxygen concentrations by extreme oil input into the ocean. Last, a simple model is

presented to quantify the amounts of oil needed to produce specified changes in the oxygen content

of the Gulf of Mexico.

Background: A series of three reports by the National Research Council details our progressive

understanding of the sources, fates, and effects of petroleum in the ocean (NRC 1975, 1985, 2003).

Information from the 1985 and 2003 reports was used in considering the effects on dissolved

oxygen concentration of oil and gas inputs, although this issue was not directly considered in these

reports. These reports detail the many sources of petroleum in the ocean: anthropogenic inputs

from extraction, transportation, and consumption of petroleum and natural hydrocarbon seeps.

From available data for 1990-1999, the average annual inputs of petroleum hydrocarbons to the

Gulf from anthropogenic sources are approximately 2000 tonnes from extraction with 1700 tonnes

being from produced waters, 1600 tonnes from transportation, and 6800 tonnes from consumption

(NRC 2003). These anthropogenic discharges can occur either at specific sites (e.g., extractive

inputs), or over wide areas (e.g., consumption inputs). Most of these inputs are near surface and so

have little likelihood of directly impacting the dissolved oxygen in the deep waters.

Natural oil seeps in the offshore Gulf of Mexico are estimated to contribute approximately 95% of

the total oil inputs (NRC 2003). The seepage rates are estimated to be 140,000 ± 60,000 tonnes per

year (NRC 2003), based on an estimate of Mitchell et al. (1999) of 70,000 ± 30,000 tonnes per

year for the northern Gulf. However, these natural discharges occur in specific regions and usually

have relatively low and chronic release rates.
76

The geologic record indicates that natural hydrocarbon seepage has been active in the Gulf of

Mexico for tens of millions of years (MacDonald 2002). At the sites of this natural leakage of

hydrocarbons, localized low oxygen conditions can occur in the upper meter or so of the sediments

and, hence, likely would influence oxygen concentrations of the waters at the sediment-water

interface. This effect is caused by oxygen consumption by the chemosynthetic communities that

live off the hydrocarbon seeps (see MacDonald et al. 1995; MacDonald 2002). However, dissolved

oxygen concentrations measured in the water column within a few cm above seeps are at levels

typical of the bottom waters (MacDonald and Schroeder 1993). Additionally, concentrated brine

pools are associated with the seeps, and these pools are anoxic (see MacDonald 2002). However,

these are localized features that do not appear to have impacted the general oxygen concentrations in

the deepest waters of the Gulf. Chemosynthetic communities are active at the seep sites. Such

communities require access to oxygenated waters because chemosynthesis, which is a pathway for

carbon fixation, consumes oxygen (MacDonald 1992; MacDonald et al. 1995; MacDonald 2002).

In the Gulf, this access is provided by the well-oxygenated bottom waters (MacDonald et al. 1995).

Thus, the chemosynthetic communities in the Gulf are additional evidence of ventilation of the

deepest Gulf waters.

The relative inputs from the various sources, shown in Figure 4.22, indicate that typical, annual

hydrocarbon inputs to the Gulf are dominated by the natural sources. As seen in Section 4.3, the

dissolved oxygen concentration in the deep waters of the Gulf are maintained by a balance of

oxygen inputs from circulation of well-oxygenated deep waters inflowing from the Caribbean Sea

at the Yucatan sill and oxygen consumption. This balance is maintained even in the presence of a

substantial natural input of hydrocarbons to the system. Although these inputs do not appear to

have perturbed the dissolved oxygen system of the basin, they can have extreme (positive and

negative) effects locally (see, e.g., discussions of chemosynthetic communities that live off the oil

seeps in the Gulf; MacDonald 1992, 2002).

93.09%

1.13%

0.20%

1.06%

4.52%

Natural Oil Seeps

Extraction: produced waters

Extraction: spills and other

Transportation

Consumption

Figure 4.22. Percent of annual average hydrocarbon discharge by natural and anthropogenic

sources. Approximate discharges are: 140,000 tonnes from natural oil seeps, 1,700

tonnes from extraction-produced waters, 300 tonnes from extraction-other, 1,600

tonnes from transportation, and 6,800 tonnes from consumption (NRC 2003).
77

The input of hydrocarbons from anthropogenic sources generally are small compared to the natural

input and so also are not expected to perturb the dissolved oxygen system of the deep Gulf.

Nevertheless, they can be expected to have possibly extreme local effects. Furthermore, catastrophic

oil spills, such as IXTOC I (discussed below), can introduce quantities of hydrocarbons that are

several times greater than the inputs from natural sources (Figure 4.23). As discussed below, the

effects of such a massive spill on the total dissolved oxygen system of the Gulf does not cause a

major perturbation to that system. However, local effects on dissolved oxygen concentrations can be

extreme as the hydrocarbons released and the organic matter from the resulting mortality decay.

The factors involved with these local processes, both from typical anthropogenic discharges and

from catastrophic spills are many and complex.

22.35%

0.27%

0.05%

0.26%

1.09%

75.99%

Natural Oil Seeps

Extraction: produced waters

Extraction: spills and other

Transportation

Consumption

IXTOC I

Figure 4.23. Percent of annual average hydrocarbon discharge by natural and anthropogenic

sources compared to the discharge from the IXTOC I oil spill. Approximate

discharges are: 476,000 tonnes from the IXTOC I spill, 140,000 tonnes from natural

oil seeps, 1,700 tonnes from extraction-produced waters, 300 tonnes from extractionother, 1,600 tonnes from transportation, and 6,800 tonnes from consumption (NRC

1985; NRC 2003).

Processes that affect the fate of hydrocarbon discharges into the oceanic environment include

weathering (evaporation, emulsification, and dissolution), oxidation (chemical and biological), and

transport (advection and spreading, dispersion and entrainment, sinking and sedimentation,

partitioning and bioavailability, and shoreline stranding with possible tarball formation). Each of

these processes is examined in detail by the NRC (2003).

The NRC (1975, 1985, 2003) reports also provide detailed information on the effects of

hydrocarbons on the oceanic environment, primarily to biological marine communities or individual

organisms. NRC (2003) notes that the level of impact depends on a complex set of factors, such as

rate of release, type of petroleum, physical properties at the site of discharge and/or contact with

land, and local biological characteristics of the ecosystems that are exposed. Of particular interest to

our study is the effect on dissolved oxygen concentrations in the water column of hydrocarbons
78

discharged into the deepwater oceanic environment of the Gulf of Mexico. It is expected that the

rates of oxygen consumption due to hydrocarbon oxidation would vary for each of the factors

described above and that the environment would vary for different parts of the water column in a

deepwater release. To assess the effects on dissolved oxygen would require a complex oil spill

model beyond the scope of this study.

Another important factor in assessing effects is whether the oil spill is at or near the sea surface or

is a subsurface release. The vertical structure of subsurface discharges extends from the point of

discharge to the sea surface. It is thought to consist of a jet phase near the discharge point at the sea

floor, then a plume phase with its own complex dynamics in the next several hundred meters,

followed by a post-terminal phase where the rise is driven by the buoyancy of individual drops or

particles and associated hydrodynamic drag, and eventually a surface slick phase where the

hydrocarbon reaches the surface (see NRC 2003 for details). NRC (2003) reports that most of the

oil from a deep discharge would rise to the surface over several hours although some oil would be

dissolved and/or emulsified; most if not all of the natural gas, if present, likely would dissolve into

the water. This would impact the longevity of the residence of the hydrocarbon in the water column,

and so its availability for oxidation. Also hydrate formation is possible for deep spills with natural

gas present; formation of hydrates would lower the buoyancy of the hydrocarbon and increase the

amount of time the hydrocarbon takes to reach the surface. However, Johansen et al. (2002) report

that hydrates did not form as predicted in a controlled spill in 844 m of water 125 km off the coast

of Norway. Clearly, local conditions again will determine the nature and extent of the impacts.

It is expected that the rates of oxygen consumption would vary for each of the different phases of

flow in the water column, but no data are available on what these rates would be in each phase.

However, decay takes time to occur, so the impact in the water column on the oxygen concentration

will depend highly on how long labile hydrocarbons are present in the water column. It is likely that

most of the hydrocarbon will rise to the surface within hours of release, so that direct oxygen

consumption most likely will occur at the surface. Emulsification may retain a portion of the

hydrocarbon in the water column for extended periods, but, coupled with the transports by

circulation, the chemistry of such mixtures will control and likely slow the rate of oxygen

consumption within a particular region. The distance from the point of discharge to the surface

determines the extent of the region in which mixing of oil with water can occur and how much time

is available for the soluble oil fractions to dissolve into the water column (NRC 2003). Once at the

surface, different processes, including clean-up, act on the hydrocarbon. At the surface, the air-sea

exchange of oxygen, which is impacted by the hydrocarbon slick, can offset part or all of the

oxidation processes that remove oxygen from the water.

Effect on dissolved oxygen concentrations in the water column: The input of oil and gas can affect

dissolved oxygen concentration in the water through photo-oxidation as well as microbial oxidation

(NRC 2003). The photo-oxidation process is limited by the presence of sunlight and the optical

qualities of the incident light, water, and hydrocarbons. It is necessarily a near-surface process. The

importance of this process for consumption of oxygen at the surface is unclear. However, NRC

(2003) reports it is unimportant in a mass balance sense. It will have no effect in the deep waters.

A more important process impacting oxygen concentrations is microbial oxidation. It acts

throughout the water column, and so directly affects oxygen concentrations in the deep waters. The

extent to which microbial oxidation occurs is dependent on many environmental factors including

water temperature, chemical composition of the oil, abundance and species of microorganisms,

available dispersal mechanisms for hydrocarbons that might remove them prior to their oxidation,

available nutrients (nitrogen and phosphorus), salinity, and pH (NRC 1985; Von Oudenhoven et al.

1983). Oxidation rates of hydrocarbons are mostly limited by their solubility and dispersion.
79

Oil can be dispersed in the ocean by several mechanisms. As a result, for oil spills in the open

ocean, most of the oil does not come ashore, but rather dissipates in situ. In the open ocean, 10-75%

of spilled oil can be evaporated from the surface within a few days (NRC 2003). This again,

however, is dependent on many parameters such as chemical composition of the petroleum,

meteorological and oceanographic conditions, and the surface thickness and water depth of the spill.

Higher wind speeds and air and water temperatures greatly increase the rate at which oil evaporates

(Chapman 1985). Temperature can affect both the rate of evaporation at the surface and the rate of

microbial growth and activity and therefore, the biodegradation rate which in turn affects the

oxidation rate at all levels within the water column. Differences in biodegradation rates exist even

among different types of hydrocarbons. For example, tars and resins are typically resistant to

biological attack, while paraffins, alkanes, alkynes, and monoaromatics are readily biodegradable

(e.g., Von Oudenhoven et al. 1983; NRC 1985).

At the surface, the interaction of waves, winds, and currents can lead to the formation of patches of a

thick foamy substance called mousse. This consists of an emulsion of oil in water, with the water

component typically being about 80-90%. The formation of the mousse is believed to slow the

biodegradation processes of the oil. Because of this, the formation of mousse is considered to be a

mechanism for transport of hydrocarbons over long distances. It is thought that the formation of

mousse may also slow the degradation process by limiting the amount of light that can penetrate the

surface layer and by slowing evaporation rates. Gibbs (1976) notes that mousse also limits

biodegradation by reducing the rate of diffusion of oxygen and nutrients into the interior of the

mousse volume. In addition, mousse produces much thicker surface layers. Oxygen limitation of

biodegradation of oil slicks at the surface is thought unlikely because there is direct contact with the

atmosphere. However, oxidation limitation may occur when the oil is dispersed in the water column,

but nutrient limitation of biodegradation is probably more likely (NRC 1985).

The oxygen concentration can be further lowered as biological organisms associated with the

oxidation of the hydrocarbons die and decompose in the water column and in the benthos. Again,

this is unlikely to be important in well-oxygenated water, but could be important if the oil sinks,

particularly in stratified water close to shore (i.e., shallow waters) where organic sediments exist and

the oxygen concentrations are low.

Because of the wide range of hydrocarbon types and differing environmental conditions, the rate of

oxygen consumption by oxidation is not clear. However, a rate of oxidation of oil in seawater was

found by ZoBell (1969) to be 0.2 to 2 mg oil per liter per day at 20°C and at concentrations of 0.5

to 50 ml oil per liter of seawater. Adding oil above 50 ml per liter did not change the rate of

oxidation. This compares to oxidation rates reported by Gibbs (1976) of 0.082 mg oil per liter per

day at 14°C (0.03 mg oil per liter per day at 4°C). The latter values assume a theoretical oxygen

demand of 3.5 g O2 per g of oil. It is also well known that the rate of biodegradation increases as

the size of dispersed oil droplets gets smaller, presumably because of the increased surface area for

bacterial settlement and activity. The microbes must access the dissolved oxygen and nitrogen from

the water as well as the carbon in the oil (P. Chapman, personnel communication).

Effect of drilling fluids on dissolved oxygen concentrations: Drilling fluids can affect local

concentrations of dissolved oxygen through organic enrichment near the seabed. There are three

major categories of drilling fluids: water-based muds, oil-based muds, and synthetic-based muds.

Locally, the impact of oil-based drilling fluids on benthic communities can be severe and toxic (see

e.g., Avanti Corporation 1997a); discharge of oil-based muds and associated cuttings has been

prohibited in the U.S. waters. Discharge of water-based muds and associated cuttings is allowed.

Although synthetic-based drilling muds themselves are not allowed to be discharged, the drill

cuttings may be rinsed and then discharged into the ocean with a residue of the mud remaining on

them. Impacts on dissolved oxygen concentrations from drilling fluids have two sources: impacts

from the biodegradation of any organic material in muds or associated cuttings that may be
80

discharged and physical accumulation of cuttings (or muds if discharged) at the seafloor that

smothers benthic communities which then degrade. The organic material content of the muds and

cuttings, and their solubility and bioavailability, and hence the possible effects on dissolved oxygen

concentrations, varies with the characteristics of the mud used and the formations drilled.

The impact of drilling muds and cuttings on dissolved oxygen concentrations in the water column

and at the seafloor also will be highly dependent on the local environmental conditions, including

the water depth, energetics of local currents and waves, substrate type, and location of the discharge

point (e.g., near-surface or near sea floor), as well as the nature and volume of the discharges

(Avanti Corporation 1997b). Neff et al. (2000) reviewed drilling fluids, with emphasis on syntheticbased muds, and their environmental risks. They found that cuttings piles rarely occurred in

deepwater or high-energy marine environments, but that where muds and cuttings accumulated

(field studies examined were in water depths <600 m), adverse effects occurred to the local benthic

communities. The drilling muds and cuttings smother the benthic communities that are downcurrent of the discharge point. The resulting organic material undergoes aerobic degradation

processes that lead to oxygen depletion in the water column immediately above the sediments and

depletion to hypoxic or anoxic conditions in the surficial sediment layers (Avanti Corporation

1997a; Neff et al. 2000). Finally, there can be an emergence of sediment anaerobic microbial

communities (Avanti Corporation 1997a; Neff et al. 2000). Synthetic-based drilling-muds are

believed to have high oxygen demands during biodegradation, but there is as yet no consensus on

their biodegradation (Avanti Corporation 1997a). Neff et al. (2000) report that, relative to syntheticbased fluids, biological effects are likely to be limited to sediments near the discharging platform,

with the organic enrichment and resulting oxygen depletion as the main mechanisms of adverse

impact, and that within 3-5 years of cessation of discharge the oxygen concentrations will have

increased to levels that would allow recovery of the ecosystem affected. Rates of oxygen

consumption are unclear and highly dependent on localized factors.

Summary of the IXTOC I case: When considering the effects of an oil spill on the environment, it is

instructive to consider one of the most famous and most studied of all oil spills in an ocean

environment. The IXTOC I production platform of PEMEX was on the continental shelf of

Campeche Bay in the southern Gulf of Mexico. From 3 June 1979 to 23 March 1980,

approximately 476,000 tonnes of crude oil were spilled from this platform into the open ocean over

the continental shelf, making the IXTOC I spill the second largest marine spill event (NRC 2003),

second only to the deliberate release of oil from multiple sources during the 1991 Gulf War. The

water depth at this location was 52 m; the spill occurred 14 m above bottom. The oil rose to the

surface forming a plume that emanated from the spill site. At the spill site a fire cone 25-30 m in

diameter consumed some of the released oil. The slick was estimated at 1 mm thick and 300 m wide

and stretched several hundred miles northwest to the Texas coast, following wind driven currents

(NRC 1985). Oil concentrations in the water column during September 1979 ranged from 10.6

mg·L-1 a few hundred meters from the blowout to less than 0.005 mg·L-1 at a distance of 80 km

from the blowout (Boehm and Fiest 1982). Comparatively, maximum oil concentrations from the

Ekofisk blowout in the North Sea and Amoco Cadiz and Argo Merchant tanker spills were 0.30,

0.35, and 0.45 mg·L-1, respectively. These values were comparable to concentrations found at the

edge of the oil plume of IXTOC. The higher concentrations at IXTOC are attributed to the

subsurface release (Boehm and Fiest,1982).

Approximately 10% of the spilled oil from IXTOC I was recovered. Microorganisms consumed 30

to 50% of the spill, and the balance of the oil (40-60%) was removed through weathering processes

such as dispersal in the water column or evaporation to the atmosphere. Between 0.5 to 3% of the

spilled hydrocarbons remained in the bottom sediments near the spill. About 3000 to 4000 metric

tons of oil washed onto the beaches of southern Texas. At the time of the spill there was no

information on how the local biota were affected by the spill because there were no pre-spill data

with which to compare post-spill data. It was observed that within 25 km of the spill, local microbe
81

communities were replaced with hydrocarbon utilizing populations. However, nutrients were

limiting, so little degradation was noted (NRC 1985).

Worst case computations of impact: ZoBell (1969) found that the aqueous dissolved oxygen in

320,000-400,000 L of seawater would be consumed by just one liter of oil, i.e., in English units one

barrel of oil would oxidize 40 acre-feet of seawater. For perspective, the IXTOC I spill was

3,332,000 barrels, while the Exxon Valdez oil spill was 262,000 barrels. ZoBell's value, based purely

on molecular balancing of chemical equations, makes many assumptions that are practically

unrealistic. The assumptions include complete mixing of the oil with the entire volume of the

available seawater, unlimited time for the oxidation to take place, absolutely no contact with the

atmosphere for oxygen replenishment, and no processes such as burning, clean-up, or stranding to

remove the oil from the water. However, it does provide a good starting point from which to gain

some insights concerning the amount of oil necessary to consume all of the oxygen in the waters of

the Gulf of Mexico deeper than 200 m.

Since ZoBell’s ratio of seawater to oil is approximately 1:106 and if the volume of the Gulf of

Mexico is assumed at approximately 2.352 M km3 with an average oxygen concentration of 5

mL·L-1, it would require a spill of 6 trillion liters (31 billion barrels) to consume every atom of

dissolved oxygen in that volume. This is equivalent to approximately the cargo of 63 thousand oil

tankers. For perspective, the world's estimated crude oil reserves are ~1.2 trillion barrels (Energy

Information Administration 2004), the U.S. reserves are ~23 billion barrels (Energy Information

Administration 2002), and Gulf of Mexico U.S. reserves are ~5.2 billion barrels (Crawford et al.

2003). Thus, it is not possible that any spill in the Gulf of Mexico will completely oxidize the

resident waters. However, locally and short term, the effect of an oil spill on oxygen concentration

has the potential to be significant.

Two weeks after the Amoco Cadiz tanker spill (March 1978; South-Western Channel on the

Portsall Rocks, three miles off the coast of Brittany, France; 223,000 metric tons) biodegradation

rates were estimated to be between 0.01 and 0.027 mg oil per liter per year. At that time after the

spill, the reduction of dissolved oxygen saturation in surface waters downstream of the spill ranged

from 1 to 4.6%. The only possible explanation of the observed deficits is biodegradation (Aminot

1979).

Effects of discharge of produced waters: Produced waters are waters that are produced during

normal oil and gas drilling and production operations. Currently, produced waters can be

discharged with an average concentration 29 mg oil and grease L-1 per month. Using ZoBell’s

oxidation rate and assuming no replenishment or ventilation of oxygenated waters, the amount of

produced water that would be needed to draw down the dissolved oxygen content in the water

column below 800 m by 10 percent is estimated. If the total volume of the Gulf of Mexico below

800 m is 16x106 km3 then it would take approximately 16x101 1L of oil to reduce the dissolved

oxygen by 10 percent. Given the allowable discharge concentration, to discharge this quantity of oil

would require discharge of approximately 8x101 3L of produced water. If it is assumed that the

average Gulf of Mexico produced waters discharge rate is 250 ML of fluid per day (NRC 2003), it

would take roughly 320 thousand days or 876 years to discharge the required amount of produced

water to reduce the dissolved oxygen content in the water column below 800 m by 10 percent.

Content of Oxygen Reservoirs: The dissolved oxygen content of the Gulf of Mexico at specified

depth layers is presented in Table 4.8. The layers are based on hydrographic (water mass)

properties found in the Gulf of Mexico basin and the depth layers that define the sills of the two

ports into the Gulf. A clear oxygen minimum is seen in the 200 to 800-m box. This minimum is

associated with the Tropical Atlantic Central Water entering the Gulf through the Yucatan Channel

(see Section 4.1.2 for details). The rationale for the delineation of the Gulf into these layers is more

fully discussed in Chapter 5.1 when the Box Model is presented.
82

Table 4.8

Reservoirs and Their Dissolved Oxygen Content per km2

Reservoir Depth Range

(m)

Thickness

(m)

Volume

per km2

(TL*·km-2)

Average O2

(mL·L-1)

O2 Content

per km2

(Mmol-O2

·km-2)

1 0-200 200 0.2 4.18 37.4

2 200-800 600 0.6 2.99 80.2

3 800-1500 700 0.7 4.15 130

4 1500-2000 500 0.5 4.89 109

5 2000-4000 2000 2.0 5.01 448

*1 km3= 109 m3, 1 m3= 103 L, so 1 km3= 101 2 L = 1 TL

At NTP (0°C) 1 mg O2 = 0.7 mL, 1 mg O2 = 62.5 μmol O2 so 1 mL O2 = 44.7 μmol O2

Simple model of impact: To further investigate the impact of an oil spill on water column oxygen

concentration, a first-order estimate of oxidation in an idealized system is developed. The approach

presented utilizes a simple box model. This model is designed to focus on the potential effect on

dissolved oxygen concentrations of major discharges of oil to the marine environment.

The objective of the following calculation is to present a simple system that can be used to make

estimates of the quantities of petroleum that would have to be oxidized in order to produce specified

changes in the oxygen content in closed reservoirs (boxes) of specified size and initial oxygen

concentration.

The dissolved oxygen distributions in the Gulf of Mexico for which the average oxygen content can

be calculated define the reservoir depth ranges (Table 4.8). The reservoir volumes are determined by

thickness (depth range) and planar size (km2). From their average oxygen content, a concentration

of dissolved O2 in megamoles km-2 can be calculated. From this and the planar size of the reservoir,

the total amount of oxygen can then be calculated. This approach allows one to scale calculations

from an immediate area of influence caused by the injection of petroleum to a basin-wide estimate.

The assumptions are that the reservoir behaves as a box that is closed (no fluxes in or out) and no

processes other than the oxidation of petroleum (carbon to carbon dioxide) occur within the box.

Obviously both of these assumptions are false, so this calculation should be regarded as an upper

limit.

A central consideration is how to connect a volume of added petroleum (and/or hydrocarbon gases)

to oxygen consumption. This involves two factors that are the number of moles (λ) of oxygen

needed to oxidize one mole of petroleum carbon (pet-C) and the number of moles of pet-C per unit

volume (V) of petroleum (Cpet-C).

pet-C + λO2  CO2 + H2O

The number of moles of oxygen consumed is then:

ΔO2 = λ Cpet-CV.
83

For simple sugar (CH2O) λ is 1 and for methane (CH4) λ is 2. Here the preliminary estimate of λ

= 1.6 is used.

In sugar C is 40 wt.% and in methane C is 75 wt. %. Kinghorn (1983) gives typical ranges of wt.%

C in petroleum of 84 to 87% and specific gravity of from 0.81 to 0.985 g cm-3. Mid-range values of

85 wt. % C and 900 g L-1 yield 765 g pet-C L-1 = 64 mol pet-C L-1. Combining this with λ gives a

value of about 102 mol-O2 consumed per liter of petroleum. Petroleum volume is often quoted in

barrels. 1 "US petroleum barrel" is 42 gallons or (x 4.55 L gallon-1) 191 L. Therefore, about 2 x104

mol-O2 (0.02 Mmol) are consumed per barrel of petroleum. Based on the O2 reservoirs given in

Table 4.8, the number of barrels of petroleum per km2 area necessary to produce 1) a 10% change

in O2, 2) hypoxia (2 mL O2 L-1), and 3) anoxia (complete O2 consumption) have been calculated

(Table 4.9). [Note that 100 barrels of oil is equivalent to a layer of oil 0.5 mm thick over 1 km2.]

Table 4.9

Number of Barrels of Petroleum per km2 to Produce a Given Change in Different Reservoirs

Reservoir 10% Loss Hypoxia Anoxia

1 187 975 1,870

2 401 1,320 4,010

3 650 3,360 6,500

4 505 3,220 5,050

5 2,240 13,450 22,400

The following points should be considered when investigating this model. Our approach is very

simplistic and has no real world analogy. Essentially all the model does is titrate a volume of

oxygenated water with oil derived – carbon producing an upper limit for O2 consumption in a

closed system. In fact, any closed basin with no exchange will go anoxic if there is enough labile

carbon and time. The mitigating circumstances in the offshore Gulf of Mexico are many and

include:

• First, the oil will be found as a surface slick with only the oil at the oil/water interface

subject to degradation. Even for a spill at depth, it is unlikely that significant mixing of oil-inwater occurs unless there are very turbulent conditions. So the titration would be limited by

the rate of oxidation across the liquid/liquid interface.

• Second, the losses to evaporation would be significant in the first hours of a spill

reducing surface area and lability (see fourth below).

• Third, dissolution of components of the oil is very sparing, limiting actual physical

transport into the water volume – you have the titrant and the receptor but they have to be

brought physically together.

• Fourth, the microbial community needs to be "turned on". While hydrocarbon-oxidizing

ability is present, the microbial community is dormant. Some field experiments suggest it

takes days to weeks to induce exponential growth in the indigenous microbial community, by
84

that time physical mixing and other processes have completely altered the hydrocarbons and

moved them around significantly.

• Fifth, if the rate of bio-oxidative removal is similar to the rate of oxygen production or

transport (advection/diffusion) – one could titrate forever with the greater Gulf essentially

supplying an unlimited amount of oxygen.

• Sixth, nutrients may well be limiting, N and P in particular. Therefore, complete

oxidation is unlikely unless there is a source of nutrients. Carbon far exceeds the other

essential elements (including micronutrients such as iron).

• Seventh, the thicker the oil slick, the more it will damp wave activity and retard oxygen

diffusion across the sea-air interface, but the slick itself will not be uniformly thick so this will

vary considerably. Also, because it is the oil at the interface that affects the oxygen

concentration (see 1 above), a thick slick of a given volume will cover less area than a thinner

one of the same volume, thus bacterial activity, and hence oxygen reduction, will take longer.

As a final computation, Table 4.10 estimates the upper limit of dissolved oxygen consumption if all

the hydrocarbon discharged from the various natural and anthropogenic sources was oxidized. The

estimated maximum oxygen consumed then is compared to the total oxygen in selected volumes

within the Gulf at an assumed average concentration as given in Table 4.11. To determine possible

basin scale catastrophic effects from total O2 consumption by the various sources (Table 4.10) on

the total O2 available in the selected volumes requires several major assumptions. First, the volume

must be assumed to be closed, with no replenishment from the atmosphere, photosynthesis, or

advection, and the hydrocarbon must be well mixed within the volume. Second, all the hydrocarbon

from the selected source must be confined within the selected volume. Third, all the hydrocarbon

must be labile, and it must be fully oxidized within the selected water volume. These assumptions

are not realistic, but do allow a zeroth order comparison to determine if the effects from the

anthropogenic sources could be substantial on a basin scale.

The comparison itself shows that basin-scale effects from any of the annual discharges are

negligible in all of the specified volumes, including the oxygen minimum zone and the deep Gulf

waters below 1500 m. Even IXTOC-like spills would have minimal effects, as such a spill would

consume at most only 1/10,000th of the available O2 in the selected volumes. For example compare

the total O2 of 1.27E7 Mmol-O2 in the oxygen minimum zone in the southeast Gulf with the total

O2 of 1.29E3 Mmol-O2 consumed for an IXTOC-sized spill fully oxidized within and totally

confined to that volume.

These computations make the major point that anthropogenic activities, even those of catastrophic

spills, will not have major basin scale effects on the levels of dissolved oxygen in the Gulf of

Mexico. The reader is reminded that the focus here has been on oxygen consumption, and so this

conclusion may not apply to other environmental effects that may result from anthropogenic

activities. Any major effects on dissolved oxygen from these activities will be localized, where they

could be substantial within a few kilometers of the source. Neither the simple models in this

subsection nor the simple box model of Section 5 were designed to address such localized effects.

Summary: The "worst" case computations and the simple model show that the catastrophic draw

down of dissolved oxygen in the deep region of the Gulf of Mexico would not occur without a

breakdown of the earth's thermohaline circulation, which provides the ventilation of dissolved

oxygen to the deep waters in the Gulf. The ventilation and replenishment processes in the Gulf of

Mexico prevent basin wide effects. However, the impact on dissolved oxygen concentrations of

hydrocarbon inputs can be local. Determination of the nature and extent of such local effects

depends on a complex system of interactions between the chemical nature and fate of the

discharged material, the rates of dissolved oxygen replenishment at the site, and the environmental
85

conditions existing at the discharge point and within the region of impact of the discharge plume.

To estimate these local effects requires specific information on the discharge and its environmental

setting and/or a complex modeling effort to tie together the many physical, chemical, biological, and

geological factors that control the processes of degradation of hydrocarbons in the ocean.

Table 4.10

Dissolved Oxygen Consumed by Total Oxidation of Selected Hydrocarbon Sources

Hydrocarbon

Source

Hydrocarbon

Quantity

(106

barrelsa)

Oxygen

Consumed

(Mmol-O2) b

Natural Seeps1 0.98 378

Extraction-Produced Waters1 0.0119 4.59

Extraction-Other1 0.0021 0.81

Transportation1 0.0112 4.32

Consumption1 0.0476 18.38

IXTOC I spill2 3.332 1,286

U.S. Gulf of Mexico Proved

& Unproved Reserves3 5,225 2.02E+06

U.S. Proved Reserves4 22,677 8.75E+06

1 NRC 2003

2

 NRC 1985, 2003

3

 Crawford et al. 2003

4

 Energy Information Administration 2002

a

 assumes 1 tonne = 294 gallons and 1 barrel = 42 gallons

b

 assumes 1 liter of petroleum oxidizes 102 mol-O2; 1 Mmol = 106 mol
86

Table 4.11

Dissolved Oxygen Content by Reservoir

(Whole Gulf is only that part of the Gulf of Mexico in water depths of 200 m or greater, i.e., the

shelf is excluded.)

Reservoir

Description

Depth

Range

(m)

Volume

(106 km3)

Oxygen

Concentration

(mL·L-1)

Oxygen

Content

(106 MmolO2) a

Whole Gulf

Upper Layer 0-200 0.239 4.18 44.7

Oxygen Minimum 200-800 0.557 2.99 74.4

Between Sills 800-1500 0.549 4.15 102

Deep Layer ≥ 1500 1.007 5.00 225

Surface to Bottom 0-bottom 2.352 4.97 522

Oxygen Minimum

Southeast Gulf 200-800 0.095 2.99 12.7

Northeast Gulf 200-800 0.129 2.99 17.2

Northwest Gulf 200-800 0.113 2.99 15.1

Southwest Gulf 200-800 0.214 2.99 28.6

Deep Layer

Southeast Gulf ≥ 1500 0.136 5.00 30.4

Northeast Gulf ≥ 1500 0.243 5.00 54.4

Northwest Gulf ≥ 1500 0.142 5.00 31.8

Southwest Gulf ≥ 1500 0.482 5.00 107.8

a

 assumes 1 mL O2 = 44.7 μmol-O2
87

5 BOX MODEL

A box model was constructed that modeled the complexity of the distribution of oxygen in the

deep water region of the Gulf of Mexico. The model, despite taking a simple approach to a

complex problem, provides useful insights into how the distribution of dissolved oxygen in the

deepwater Gulf might be established and maintained. Among those insights are the following

key findings on the importance of transport, reactive loss, and primary productivity:

1. It is highly probable that the oxygen profiles in the Gulf are determined by a unique

combination of the transport process associated with circulation and mixing and the

reactive processes associated with biogeochemical cycles. It is unlikely that a multitude

of oceanographic and biogeochemical states could describe the observed oxygen profiles.

2. The transport through the Yucatan Channel plays the most significant role in controlling

the oxygen profiles in the deep waters of the Gulf. The model suggests that

approximately 30% of the Yucatan flow is diverted into the Gulf, although this may be a

model artifact. Unfortunately the model is unable to provide an independent estimate of

the flux necessary to ventilate the deep Gulf waters below the 2000-m sill depth of the

Yucatan Channel. This vertical mass flux is critical to maintaining the deep oxygen

profiles. But, because the actual mechanisms for transport of deep waters into the Gulf

interior and for vertical mixing and ventilation of waters below the Yucatan sill depth are

not well known, the necessary vertical flux of mass cannot be resolved by the box model.

3. The oxidation of carbon in the water column plays a secondary, but important, role to that

of transport in maintaining oxygen profiles in the upper 800 m of the Gulf. The model is

incapable of ascertaining its importance below 800 m.

4. The oxidation at the sediment interface plays a minor, but necessary, role in maintaining

the oxygen below 1500 m. The model is incapable of ascertaining its importance above

1500 m. By design, it has no effect in the top 200 m.

5. The net production of oxygen in the surface layer is consistent with the rates reported for

air-sea exchanges and photosynthetic inputs at the latitude associated with the Gulf.

6. The oxygen content in the Gulf would decrease to half of its present value in 10 years

were the Gulf to be isolated from any source of oxygen, be it transport, primary

productivity, or atmospheric exchange.

These insights should provide the basis for specifying the additional observations and modeling

needed to more accurately understand the processes that control the distribution of dissolved

oxygen in the deepwater Gulf of Mexico.

5.1 Introduction

The goal of the "Simple Box Model" Task is to construct a box model that adequately represents

the inputs to and outputs from the deepwater Gulf of Mexico in a manner that reflects the

complexity of the distribution of oxygen. In essence, the focus of the model is to gain insights

into how the distribution of dissolved oxygen in the deepwater Gulf is established and apparently

maintained over time and space, thus providing an improved basis for specification of

observations needed to more accurately determine/predict changes in the oxygen content and

structure of the basin.

Our hypothesis is that the distribution of oxygen within the water column is maintained by a

combination of two major processes: the transport processes associated with circulation and

mixing, and the reactive processes associated with biogeochemical cycles. This simple box

model consists of five elements that will allow us to address how the distribution might be

maintained:
88

1. Vertical and horizontal spatial definition of the boxes.

2. Estimates of the mass transport between and through boxes, in particular how much of

the Loop Current flow must be diverted into the interior of the Gulf.

3. The sources and sinks of oxygen consisting of

a). Physical – atmospheric and water masses

b). Biological – photosynthesis and respiration

c). Chemical – abiotic oxygen consumption

d). Geological – hydrocarbon seeps and vents

e). Anthropogenic – hydrocarbon releases

4. The governing equations describing the accumulation, transport, and reaction of oxygen.

5. The inclusion of conservative tracers, i.e., temperature and salinity, to close the mass

transport balance.

The level of detail chosen to describe each of the five elements defines the complexity of the

model. It is highly dependent on the availability of data and understanding of the physical and

biogeochemical processes active within the Gulf. As stated in Section 4, the data available to

determine flux rates are sparse and knowledge of the circulation in the deep Gulf below about

1000 m is poor. Thus, this box model is limited to essentially zero-order approximations.

Nonetheless, we believe that it provides insights into the gaps in knowledge and how they might

be resolved (see Section 6).

The model setup, as well as the underlying assumptions of a box model, are discussed in Section

5.2. Model equations are defined in Section 5.3. The results of the baseline model run are given

in Section 5.4. Several scenarios are presented in Section 5.5. The conclusions are in Section 5.6.

5.2 The Model Setup

The model setup section addresses the first three elements outlined in the introduction. These are

discussed in the following three subsections. Section 5.2.1 addresses various aspects of the

vertical and horizontal spatial definition of the boxes, Section 5.2.2 estimates the mass transport

between and through the boxes, and Section 5.2.3 discusses the sources and sinks of oxygen

relative to the box model.

5.2.1 Definition of the Boxes

A box model requires that the spatial extent of the individual boxes be defined. These are defined

in terms of horizontal and vertical sizes, which in this box model are defined as

a. Horizontal Box Size: There are insufficient high quality oxygen data, extending below

2000 m, to allow realistic division of the Gulf into east and west boxes. Thus, a single

basin model is posed that has no western or eastern basin, but encompasses the entire

Gulf. In essence, the mechanisms that maintain a standard dissolved oxygen profile will

be examined. Hence, only the vertical division of boxes must be defined.

b. Vertical Box Size: The available bottle dissolved oxygen data taken in the Yucatan

channel are shown versus depth in Figure 5.1. The general vertical structure in the

channel, as well as numerous profiles taken across the Gulf, show similar regions of

recognizable trends in the oxygen distribution. Based on these trends, the single basin

model is subdivided into five vertical layers (see Figure 5.2) that are intended to mirror

the expected biological (reactive processes) and physical processes (transport processes).

The box model was based on these five vertical layers. The layers are shown

schematically in Figure 5.3 along with presumed transport pathways. Details of each box,

including volume and surface area, are presented in Table 5.1.
89

Figure 5.1. Dissolved oxygen versus depth for the available data in the Yucatan Channel.
90

Figure 5.2. Cumulative volume by depth of the deepwater Gulf of Mexico and the vertical

delineation between layers. Details about the volume of each layer and the

associated area of the sediment interface are provided in Table 5.1.
91

V5 C5

C4(in)

u4(in)

V4 C4

w45

C3(in)

u3(in)

V3 C3 u3(out)

d34

C2(in)

u2(in)

V2 C2 u2(out)

d23

C1(in)

u1(in)

V1 C1 u1(out)

d12

u4(out)

Yucatan 

Sill Depth

Florida Straits 

Sill Depth

d45

Figure 5.3. Box model configuration and notation used for the dissolved oxygen mass balance.

The arrows represent fluxes. An option is included to allow for a deep return flow

through the Yucatan Channel, i.e., u4(out). Below the 2000-m sill depth of the

Yucatan Channel, there is no horizontal flux into or out of the box.
92

Table 5.1

Volume, Thickness, and Sediment Surface Areas for Each Layer in the Basin-wide Box Model

of the Gulf of Mexico

Box Number &

Description

Volume

(105 km3)

Layer

Thickness

(m)

Sediment

Surface Area

(103 km2)

1. Surface 2.040 200 - 2. Oxygen Minimum 5.653 600 3.085

3. Oxygen Increase 5.559 700 4.423

4. Oxygen Constant 3.300 500 2.242

5. Bottom 6.922 1800 5.874

The five vertical layers of the box model are

i. The surface layer encompasses that portion of the water column influenced by

air-sea exchange of oxygen and the photic zone. It ranges from the surface to 200

m. This represents the layer most impacted by surface processes and features such

as winds and the thermocline. It includes the photic zone that extends to depths of

approximately 60-100 m. It is dominated by the flux of oxygen between the

atmosphere and the ocean, as well as the photosynthetic production of oxygen and

the rapid biological consumption of oxygen by organic carbon metabolism.

ii. The oxygen-minimum layer ranges from 200 to 800 m. This layer extends from

the bottom of the photic zone to the accepted sill depth of the Florida Straits. This

is the region that contains the oxygen minimum zone. The lower depth also

corresponds to the upper depths of the Antarctic Intermediate Water. It is also

approximately the maximum depth to which the isopycnals within a ring are

depressed. The oxygen minimum at 400-600 m is in part a relic of processes

occurring upstream; this layer also is primarily a region of oxygen consumption.

iii. The oxygen-increase layer ranges from 800 to 1500 m, from the sill depth of

the Florida Straits to the approximate depth at which dissolved oxygen no longer

increases significantly in concentration with vertical depth.

iv. The constant oxygen layer ranges from 1500 to 2000 m, the approximate sill

depth of the Yucatan Channel. This is a region of maximum oxygen concentration

below the surface layer.

v. The bottom layer ranges from 2000 m to 3800 m, i.e., from the Yucatan

Channel sill depth to the deepest depth of the Gulf of Mexico. This is a region

where the dissolved oxygen concentration changes very little with increasing

depth.

c. Well-Mixed Volumes for Each Box: The implied assumption in any box model is that

the volume in each box is so well mixed that any changes are instantly communicated to
93

the entire box. Thus, the boxes should be made small enough relative to the processes

being studied so that this assumption is not grossly violated.

d. Rigid-Lid Approximation: Another implied assumption in this box model is that the

volume in each box must remain constant over time. This constraint implies that either

the time scale is long enough so that temporal changes in the surface elevation average to

zero or the volume changes associated with the surface elevation variations are so small

compared to the bathymetric volume of the Gulf that they can be neglected. Both

conditions hold. The bathymetric volume of the Gulf, estimated from a standard

bathymetry data set, is 2.352 x 106 km3. The corresponding surface area is 1.631 x 106

km2. A 1% change in the volume of the Gulf would correspond to a 15-m change in the

surface elevation. Changes in the surface elevation are the result of changes in the Loop

Current transport, tides, the river input, precipitation, and evaporation. Altimetry from the

TOPEX/Poseidon and ERS-2 satellites show the deviation in surface elevation across the

Gulf rarely exceeds ±1 m, or no more than a 0.07% change in total volume.

Consequently, the rigid lid approximation holds.

e. Exclusion of the Shelf: In this box model, the shelf is not included and the deep ocean

exchange with and processes associated with the shelf are neglected. This assumption is

based primarily on the small volume of the shelf relative to the remainder of the Gulf.

Only 8.5% of the Gulf’s volume occurs in depths from the surface to 200 m and, of that

volume, 15.8% occurs on the shelf. It is assumed that the contribution of processes over

the shelf and any exchanges from land to the dissolved oxygen concentrations in the deep

Gulf interior can be reasonably neglected. Furthermore, the contribution from the shelf

will be implicitly included in the oxygen concentration initial conditions, to be discussed

later. Excluding the shelf is a reasonable assumption given the crude resolution of this

box model and the knowledge that the continental slope acts as an effective barrier to

transport across the shelf.

5.2.2 Mass Transport

A box model makes no specific distinction as to the source of the inflow, the destination of the

outflow, or how much vertical transport there has to be. It essentially integrates the individual

details into a net transport. To make the model physically realistic, the major inflow, outflow,

and vertical fluxes must be judiciously specified.

a. Inflow Estimates: The major inflow into the box model, designated u1(in), u2(in), u3(in),

and u4(in), (see Figure 5.3) is assumed to be diverted from the Loop Current. Thus, the

question is: exactly how much is diverted. Altimeter fields from the TOPEX/Poseidon and

ERS-2 satellites show clear indications that the Loop Current exchanges fluid with the

interior of the Gulf of Mexico. In this model, that exchange is parameterized as the unknown

β, a number that defines the fraction of the total Loop Current flow diverted into the interior.

It will be shown that the parameter β can be reasonably constrained by the physical

restrictions inherent in this model.

The total flux through the Yucatan Channel is fairly well known, but the vertical distribution

of that flux, averaged across the channel, is not. To avoid the need for a reliable and

extensive current meter data set in the Yucatan to accurately define the vertical distribution,

we take two approaches. In the first, the horizontal flux is apportioned in a systematic and

reproducible way using a polynomial velocity profile of the simple form

n u(z) = mz
94

The velocity is a maximum at the surface and is zero at the bottom, This simplified profile is

assumed to be adequate for the purposes of this zero order box model. The coefficient m is

determined by the total flux

1

1

+

+ = n

total H

n

m

q

where H is the total depth and the total flux is specified.

In the second approach the work of Sheinbaum et al. (2002) is used to make the flux

estimates. They made direct current observations across the Yucatan Channel for a period of

ten months and presented the structure of the mean along-channel velocity field as their

Figure 2a. In the absence of the actual data, a graphical representation of the flux profile was

extracted from their figure. For the case of a Yucatan Channel inflow of 30 Sv and a sill

depth of 2000 m, the profiles for various choices of n as well as the extracted Sheinbaum

profile, are shown in Figure 5.4. Table 5.2 lists the actual flux values pertinent to each box.

Increasing the order of the polynomial n effectively forces more of the flux to occur closer to

the surface.

b. Outflow Estimates: The major outflow from the box model rejoins the Loop Current before

exiting the Gulf through the Florida Straits. The manner in which this box model is designed,

coupled with the continuity constraint, ensures that the outflow is a dependent variable.

Consequently, if 30 Sv enters the Gulf through the Yucatan Channel, then 30 Sv must exit.

Part of that 30 Sv is diverted into the Gulf interior, the realm of this box model, and if the

water level in the Gulf is to remain constant over a long period of time, it leaves the interior

of the Gulf and reenters the Loop Current. We should also note that the 800-m sill depth of

the Florida Straits effectively guarantees that only boxes 1 and 2 can have this outflow, i.e.,

u1(out) and u2(out). But because there is evidence (Bunge et al. 2002) of a deep return flow

to the Caribbean through the Yucatan Channel, an outflow is included for boxes 3 and 4,

u3(out) and u4(out), to allow for a return flow that is a box model unknown.

c. Vertical Transports: Because the vertical velocities are much smaller than the horizontal

velocities, the vertical flux of mass in the ocean is extremely difficult to accurately measure.

For the purposes of this model, the vertical flux could be roughly estimated from the

hydrostatic continuity balance. Originally we had anticipated doing just this, but quickly

discovered this approach can be quite misleading if the scaled parameters are incorrectly

estimated. To avoid this problem, the transport between the upper three boxes was

determined as an output from the box model and are graphically shown in Figure 5.3 as d12, d23, and d34. Because of the difference in sill depth between the Yucatan and the Florida

Straits, and the small Yucatan return flow, continuity requires that the majority of the inflow

to boxes 3 and 4 must be diverted vertically. The vertical transport, w45, is the only means by

which box 5 can be ventilated.

5.2.3 Sources and Sinks of Dissolved Oxygen

a. Physical: Each of the boxes will have, as a source and a sink, the physical transport of

oxygen into and out of the box. Transport alone, however, is insufficient to maintain the

distribution of oxygen within the water column.

b. Photosynthesis and Atmospheric Transfer: The rate of photosynthesis within the surface

layer and the transfer of atmospheric oxygen into the surface layer are difficult to

independently quantify from the literature. Consequently, the combined photosynthetic

production and net atmospheric input of oxygen into the surface layer are determined as an

output from the box model.
95

Figure 5.4. Velocity profiles of the Yucatan Channel inflow. Shown are the four cases of an

assumed polynomial form, u(z) ~ zn, and an estimate based on the direct current

measurements across the Yucatan Channel made by Sheinbaum et al. (2002).
96

Table 5.2

Fluxes of the Yucatan Channel Inflow

(Fluxes in Sv are through boxes 1-4 of the model from the Yucatan Channel (YC) under selected

choices of polynomial order.)

Flux

(Sv)

Sheinbaum et al.

(2002)

n=2 n=3 n=4

-

n=5

YC1 12.01 8.13 10.32 12.28 14.06

YC2 13.72 15.39 15.80 15.38 14.54

YC3 3.03 6.01 3.77 2.30 1.39

YC4 1.15 0.469 0.117 0.029 0.007

YC total 30 30 30 30 30

c. Biogeochemical Processes in the Water Column: The oxidation of particulate and

dissolved organic carbon within the water column results in the consumption of oxygen. It is

assumed that this is the primary pathway for the reactive loss of dissolved oxygen. It is

modeled, not as a second order reaction in oxygen and carbon, i.e., r = kV[O2][C], but as a

simple first order reaction in oxygen, i.e., r = kwV[O2]. Thus, all possible biogeochemical

reactions are parameterized into a single reaction with one rate constant. The alternative of

including all possible reactions is untenable in a box model, particularly in this case where

many of the rates are unknown. Furthermore, a second order reaction would require a

separate box model to track carbon, a task that is beyond the scope of this simple box model.

To account for the fact that the availability of labile carbon does decrease rapidly with

increasing depth and is probably quite refractive in the deep Gulf, the first order reaction is

only included in the upper two boxes and selectively turned off in the remaining three deeper

boxes. This selection was not entirely arbitrary as experimentation with the model quickly

showed that if the reaction was included below 800 m, the demarcation between the second

and third box, then reasonable results were not possible.

d. Biogeochemical Processes at the Sediment-Water Interface: The consumption of oxygen

at the sediment surface is the combination of the transfer of oxygen from the bulk to the

surface followed by a sediment-mediated reaction. The rate at which this occurs can be

modeled as

€

V

d [O2 ] surface

dt = Akm [O2 ]bulk − [O2] ( surface ) −r [O2 ] surface

where V is the volume, A is the surface area, km is the mass transfer coefficient from bulk to

surface, r is the rate of disappearance by the sediment-mediated reaction, [O2]bulk is the

known oxygen concentration in solution, and [O2]surface is the unknown concentration at the

surface. The consumption of oxygen can be either mass transport limited or reaction limited.

If the sediment is porous and the oxygen has to diffuse into the pores before it can react, then

the rate is probably mass transport limited. In the interests of simplicity, a mass transport

limited rate is assumed for this model, so that
97

€

V

d [O2] surface

dt = Akm [O2 ] bulk − [O2 ] ( surface )

If it further is assumed that the surface concentration is a linear fraction of the bulk, then

€

V

d [O2 ] bulk

dt = Akm

(1− x)

x [O2 ] bulk = Akb [O2 ] bulk

This has the same mathematical form as a first order reaction. Consequently, there is no

mathematical difference between the first order water column reaction discussed above and

this mass transport limited process. In fact, it is no different than a first order reaction

occurring at the sediment-water interface. The rate constant and the mass transfer coefficient

are mathematically identical and no independent information can be extracted from this box

model by including both water column and sediment processes together in a single box.

Because each box is ideally well mixed, all of the fluid by definition must contact the

sediment before it leaves the box. Unless the fluid spends a lot of time in the box, i.e., the

overturning age is quite large, then complete contact with the sediment is unlikely. An

option has been included to scale down the contribution expected from the sediment. To

account for the fact that a water column reaction is imposed in the upper two boxes, it is only

for the deepest two boxes that a sediment contribution can be included. Experimentation with

the model resulted in determining the values that minimized the error of the model.

e. Geological and Anthropogenic: The presence of natural hydrocarbon seeps and vents and

the inadvertent introduction of hydrocarbon from human activities into the Gulf are a

potential sink for oxygen. Because the model only allows for a first order reactive loss of

oxygen and does not independently follow the fate of carbon, this sink cannot be modeled

directly. However, it can be estimated by an assumed increase in the rate constant of kw, an

approach attempted in the scenarios section.

5.3 The Model Equations

Before establishing the equations that will be used to mathematically model the distribution of

oxygen in the Gulf of Mexico, key points from Section 5.2 are summarized. The distribution of

oxygen within the water column is controlled by a combination of transport processes associated

with circulation and mixing and by reactive processes associated with major biogeochemical

cycles occurring in the water column and across the water-sediment interface. The box model

has five boxes in the vertical, as shown in Figure 5.3. The difference in the sill depths between

the Yucatan and the Florida Straits will have a significant effect on the results. The governing

equations can now be defined, the unknowns specified, and a solution attempted.

5.3.1 Governing Equations

The basic equation governing tracer properties in each layer of this box model is posed in terms

of a simple balance between accumulation, transport, and reaction, represented here as:

( ) ( ) dt

dC V = uinCin − uoutC − d C − r C (5.1)

Here C is the tracer concentration, denoted somewhat generically because it is meant to represent

any tracer, not necessarily just dissolved oxygen. The variable uin is the horizontal flow into the
98

box, i.e., the inflow that brings with it material of concentration Cin. The variable uout is the

horizontal flow out of the box that carries with it material of concentration C that is within the

box. There is also a vertical flux either into or out of (but not both) the top of the box, as well as

a vertical flux into or out of the bottom of the box. The variable d(C) is meant to represent this

vertical flux of material between overlying boxes. Details of how this is mathematically

expressed will be presented below. The variable r(C) represents the first order process, either a

water column reaction, photosynthesis in the photic zone, or a sediment mediated reaction, that

either produces or consumes oxygen. Equation (5.1) simply states that the accumulation of a

tracer, VdC/dt, i.e., the time rate of change of the tracer in the box of volume V, is the result of

two major processes: the transport processes associated with circulation and mixing, uin·Cin –

uout·C - d·C, and the reactive processes associated with biogeochemical cycles, r(C). As a

consequence of the well-mixed assumption, C is the concentration of the tracer everywhere

within the box; consequently it is also the concentration of the tracer leaving the box. If the

transport of incoming tracer Cin exceeds the loss by outflow and reaction, then the concentration

in the box will grow in time, that is, until something changes. But, however interesting that

evolution might be, time dependence is well beyond the scope of this simple box model. Steady

state is assumed so that VdC/dt = 0.

The fate of oxygen is of primary interest to this study, and its fate is largely, but not completely,

controlled by transport. Getting those transports correct, even within the context of this zeroorder box model, is pivotal. A box model based on tracking oxygen as the sole tracer is entirely

feasible, and was an approach we initially investigated. But this approach can be improved

upon. Oxygen is not a conservative tracer; the reactive processes play an important role that

must be accounted for by the box model. This complicates the box model in that both transport

and reaction must be determined simultaneously if oxygen is used as the only tracer. If

additional tracers can be utilized, tracers that are conservative and are simply transported with

the fluid, then it is advantageous to make use of them in order to obtain a more reliable estimate

of the physical transport. We decided to make use of the extensive set of temperature and

salinity profiles in the Gulf as those conservative tracers.

The expansion of equation (5.1) for oxygen, temperature, and salinity gives the following set of

equations for each of the five boxes:

Box 1

( ) 1 1 1 1 12 1 2 1 1 1 1

1 1 1 1 12 1 2

1 1 1 1 12 1 2

1 1 12

: 0 { , }

: 0 { , }

: 0 { , }

: 0

Oxygen balance u O u O d O O k V O V r O

Heat balance u T u T d T T

Salt balance u S u S d S S

Continuity u u d

w P

in in out

in in out

in in out

in out

⋅ = − + − +

⋅ = − +

⋅ = − +

= − +

(5.2)
99

Box 2

2 2 2 2 23 2 3 12 1 2 2 2

2 2 2 2 23 2 3 12 1 2

2 2 2 2 23 2 3 12 1 2

2 2 23 12

: 0 { , } { , }

: 0 { , } { , }

: 0 { , } { , }

: 0

Oxygen balance u O u O d O O d O O V k O

Heat balance u T u T d T T d T T

Salt balance u S u S d S S d S S

Continuity u u d d

w

in in out

in in out

in in out

in out

⋅ = − + − −

⋅ = − + −

⋅ = − + −

= − + −

(5.3)

Box 3

3 3 3 3 34 3 4 23 2 3 3 3 3

3 3 3 3 34 3 4 23 2 3

3 3 3 3 34 3 4 23 2 3

3 3 34 23

: 0 { , } { , }

: 0 { , } { , }

: 0 { , } { , }

: 0

Oxygen balance u O u O d O O d O O A k O

Heat balance u T u T d T T d T T

Salt balance u S u S d S S d S S

Continuity u u d d

b

in in out

in in out

in in out

in out

⋅ = − + − −α

⋅ = − + −

⋅ = − + −

= − + −

(5.4)

Box 4

4 4 4 4 34 3 4 45 5 4 4 4

4 4 4 4 34 3 4 45 5

4 4 4 4 34 3 4 45 5

4 4 34 45

: 0 { , }

: 0 { , }

: 0 { , }

: 0

Oxygen balance u O u O d O O d O A k O

Heat balance u T u T d T T d T

Salt balance u S u S d S S d S

Continuity u u d d

b

in in out

in in out

in in out

in out

⋅ = − − + −α

⋅ = − − +

⋅ = − − +

= − − +

(5.5)

Box 5

45 4 45 5 5 5 5

45 4 45 5

45 4 45 5

45 45

: 0

: 0

: 0

: 0

Oxygen balance w O d O A k O

Heat balance w T d T

Salt balance w S d S

Continuity w d

b

in

in

in

⋅ = − −α

⋅ = −

⋅ = −

= −

(5.6)

Here On is the dissolved oxygen concentration, Sn is the salinity, and Tn is the temperature within

each box; Vn is the volume; An is the sediment surface area; un

in is the horizontal flux into each

box and un

out is the horizontal flux out of each box; d12, d23, d34 and d45 are the vertical fluxes

between boxes, kw is the first order rate constant in the bulk, kb is the mass transfer coefficient at

the water-sediment interface; αn is the scaling factor controlling the contribution of the sediment

process, and rP(O2) is the combined photosynthetic production of and net atmospheric input of

oxygen into the surface layer.

For each box, there is a balance of mass transported into and out of the box, known as continuity.

For the sake of consistent notation, both horizontal and vertical fluxes are designated positive if

they flow into a box and negative if they flow out. All vertical fluxes are given as dij, which
100

designates the flux between two adjacent boxes i and j. In addition to continuity, there is a tracer

balance. A conservative tracer is swept along by the flow and is neither created nor destroyed.

For the purposes of this model, we assume that salt and heat are ideal. Removed from the

influences of the surface where freshwater can be added or the heat content increased by solar

radiation, this assumption holds quite well. At the surface, we accept the small errors introduced

by this assumption because on the overall scale of this box model those errors are negligible.

The notation dij{Ci,Cj} is our shorthand for the vertical transport of tracer C. The value of the

transport depends on whether the unknown vertical flux dij flows up toward the surface, a

positive quantity, or down toward the bottom, a negative quantity. For example, if d23 is positive,

then it will carry tracer C3 up to box 2, but if d23 is negative, then it will carry tracer C2 down to

box 3. The notation dij{Ci,Cj} is meant to mathematically represent these possibilities, which the

box model solves in an iterative manner.

As we have already noted, it is extremely difficult to determine a physical value for the vertical

flux into or out of box 5. An examination of the equation set for box 5 shows that including

salinity and temperature does not solve the problem. If the two salinities are in fact different,

then the salt balance equation cannot be satisfied unless w45 is different from d45. If the two

temperatures are in fact different, then the heat balance equation cannot be satisfied unless w45 is

different from d45. But continuity clearly requires that w45 is equal to d45. The oxygen balance

can be satisfied only because it is nonconservative, i.e., the sediment mediated reaction

consumes enough oxygen so that the two concentrations can be different. The box model solves

for w45 as an unknown. This difficulty in constraining the vertical flux highlights a significant

knowledge gap in determining how the distribution of oxygen within the water column is

maintained, that of the vertical fluxes below the 2000-m depth.

5.3.2 Specification of Knowns and Unknowns

Considering the equation set (5.2 to 5.6), we could estimate values for the vertical fluxes and

determine the resulting oxygen concentrations, to then be compared against the known values, or

the known oxygen concentrations could be used to place a bound on the vertical fluxes. The

latter approach is chosen and the unknowns are as follows:

1) The horizontal outflows from each box, u1(out), u2(out), u3(out), and u4(out);

2) The inflow into box 4, u4(in), and into box 5, w45. 2) The vertical fluxes, d12, d23, d34, and d45;

3) The production of oxygen in the photic zone by some combination of photosynthesis and

atmospheric transfer, and;

4) The two rate constants controlling the reactive loss of oxygen in the water column, kw, and at the sediment-water interface, kb.

The knowns are the concentration of salinity, temperature, and oxygen and the horizontal inflows

to the upper four boxes and are specified a priori. Table 5.3 shows the values of temperature,

salinity, and oxygen used for the inflow through the Yucatan Channel and for the Gulf interior.

Section 3.3 discusses how these values were obtained. The value used for each box was the mean

value of the 1-m resolved data averaged over the depth of the box.

An examination of the oxygen data explains the rational for the design of flow into the box. Note

specifically that the oxygen concentration in box 4, 4.95 mL L-1, is actually less than that of box

5, 5.04 mL L-1. The only reasonable means by which the concentration in box 5 (which receives

no horizontal transport from the Yucatan Channel inflow because the box lies below the sill

depth) could be higher than the overlying box is if there is an alternative, higher concentration

source. The likely source is the 5.12 mL L-1 oxygen in the lowest box of the Yucatan. Perhaps the

contention of Welsh and Inoue (2000) that Loop Current rings are the mechanism for driving the

deep circulation is correct and eddies are in fact a major mechanism for transport of water to the
101

deep Gulf. In order to formulate a reasonable box model we had to include a mechanism for

injecting high oxygen concentration fluid below 2000 m.

Table 5.3

Values of Temperature, Salinity, and Dissolved Oxygen Used for the Yucatan Channel Inflow

and Gulf Interior

Box

Number

Temperature

(°C)

Salinity Dissolved

Oxygen

(mL·L-1)

Yucatan Channel

1 24.38 36.32 4.11

2 11.72 35.50 3.17

3 4.76 34.94 4.31

4 4.06 34.97 5.12

Gulf Interior

1 20.74 36.23 3.87

2 9.27 35.20 3.02

3 4.60 34.94 4.42

4 4.07 34.97 4.95

5 4.00 34.97 5.04

Florida Straits

1 22.68 36.25 3.97

2 10.88 35.38 3.08

5.3.3 Additional Details

To maintain a constant volume, the following set of continuity constraints must hold, beginning

with the overall continuity balance:

in in in in out out out out u1 + u2 + u3 + u4 = u1 + u2 + u3 + u4 (5.7)

The horizontal inflow to each box is determined by the fraction of the inflow from the Yucatan

Channel that leaves the Loop Current and enters the Gulf interior. This fraction is the parameter,

β, which can range from 0 to 1 and is determined by minimizing the error in the box model,

where the error is yet to be defined. The equation set is:
102

4 45 4

3 3

2 2

1 1

u w YC

u YC

u YC

u YC

in

in

in

in

+ =

=

= ⋅

= ⋅

β

β

 (5.8)

It is recognized that the difference in sill depths between the Yucatan Channel and the Florida

Straits forces all of the flow below 800 m to enter the Gulf interior. The implied assumption is

that the Yucatan Channel return flux, YCR, originates in the Gulf and is not a countercurrent to

the Loop Current. In this model YCR is determined as part of the box model.

The portion of the Yucatan Channel flow that does not enter the Gulf becomes, according to this

model, the Loop Current. It is determined by continuity, so that

( )

( )

0

0

1

1

4

3

2 2

1 1

=

=

= −

= −

LC

LC

LC YC

LC YC

β

β

 (5.9)

Because all of the Loop Current flow below the sill depth of the Florida Straits is assumed to be

diverted into the Gulf interior, LC3 and LC4 are zero. The Loop Current consisting of LC1 and

LC2 does not enter the box model domain, but serves as a transport path, or conduit, to bring

fluid to the box model. Fluid leaves the box model and ‘rejoins’ the Loop Current as it exits

through the Florida Straits. In this manner continuity is maintained. The final concentration of

oxygen, temperature, and salinity in the Florida Straits is not an independent variable, but simply

the result of mixing so that

R

R

out out

out in out

out in out

FS FS FS YC YC

u u YC

C FS LC C u C

C FS LC C u C

+ = = −

+ =

⋅ = ⋅ + ⋅

⋅ = ⋅ + ⋅

1 2

3 4

2 2 2 2 2 2

1 1 1 1 1 1

(5.10)

Here FS1 and FS2 are the Florida Straits fluxes, which are determined by the box model, and

C1(out) and C2(out) are the concentrations. The Yucatan return flow is the output of the box

model between the two sill depths, 800 m at the Florida Straits, and 2000 m at the Yucatan

Channel. In order to derive equation (5.10) we assumed that during the Loop Current’s transit

from the Yucatan Channel to the Florida Straits, the tracer concentrations did not change by

reaction or through vertical mixing. This is a robust assumption given the fairly rapid transit of

the Loop Current compared to the time required to flush the Gulf interior. Therefore any

concentration differences actually measured at the Florida Straits are solely due to mixing of the

Loop Current with water leaving the Gulf interior. The fact that the oxygen concentrations,

temperature, and salinity at the Florida Straits are between that of the Yucatan Channel and the

Gulf interior (see Table 5.3) is confirmation that this assumption is reasonable.

The turnover times for each box, i.e., the time for the incoming flow to completely exchange the

water volume, is of interest. The turnover times are defined as follows:
103

45

5

5

4 45

4

4

3 34

3

3

2 23

2

2

1 12

1

1

d

V

u d

V

u d

V

u d

V

u d

V

in

in

in

in

=

+ =

+ =

+ =

+ =

τ

τ

τ

τ

τ

5.3.4 Solution Strategy

To summarize, two independent parameters, consisting of the shape of the velocity profile and

the parameter β, must be specified. Values for known quantities, such as the concentrations of

oxygen, salinity and temperature and the volume and sediment surface area of each box, also

must be specified a priori. Equations 5.2 through 5.10 represent a system of linear equations that

can be combined into the compact form

A∗ x = b (5.11)

where x is the vector containing the 15 unknowns, arranged so that the order of magnitude of

each term is similar





































































































⋅

⋅

⋅

=

2 1

2

1

2

1

54

45

34

23

12

4

4

3

2

1

r (O ) V

k A

k V

FS

FS

w

d

d

d

d

u

u

u

u

u

x

p

b

w

in

out

out

out

out

With the exception of d12, d23, and d34, each of the unknowns must be positive. This provides an

additional constraint on the system of equations. The vector b contains the known part of each
104

equation and A is the matrix of coefficients derived from rearranging (5.2) through (5.10). This

particular system of equations is classified as overdetermined, i.e., there are ten more equations

than unknowns. This means there is no value for x that exactly satisfies equation (5.11), but there

is a value that minimizes the error of the model. One could reduce the number of equations to

exactly match the number of unknowns, but that would require a decision as to which equations

to eliminate and would significantly reduce the confidence in the answer. Consequently, all the

equations are retained and a least squares solution method is used to find the values for x that

minimizes the error in reproducing the vector b, i.e.,

ε = A∗ x − b

where the error, ε, is minimized with respect to the parameter β, the fraction of Yucatan Channel

inflow diverted into the Gulf interior. Therefore, varying the parameter β until the error becomes

as small as possible further reduces the error to a global minimum. This is the procedure used in

this study to establish a set of values for such parameters as transport and rate constants.

5.4 Baseline Model Run

To determine the values for the transports, vertical fluxes, reaction rates, and net surface

productivity that best reproduces the known oxygen concentrations, a series of model runs were

completed. These runs were made to establish the baseline, i.e., the values for the transports,

vertical fluxes, reaction rates, and photosynthetic productions that best minimizes the error. For

each velocity profile, a value for the parameter β was found over the range from zero to one that

resulted in a global minimum for the error. Using this "minimum β", the resulting values for the

unknowns are shown in Table 5.4 for each of the five velocity profiles.

The most striking result from the baseline model run is that many of the box model unknowns

are independent of the velocity profile. Consider that:

1. Approximately 26 – 30% of the assumed 30 Sv Yucatan Channel flow is diverted into the

interior; the remainder becoming the Loop Current. Ochoa et al. (2001) found that the

net average transport through the Yucatan Channel was close to 25 Sv, consisting of an

inflow of 33 Sv and a return flow of 8 Sv. Whether by coincidence or not, the 8 Sv return

flow is 25% of the 33 Sv inflow and surprisingly close to the 26 – 30% value.

2. The flux out of the Gulf interior, between the surface and 200 m, is in the range from 4 to

5 Sv. This flux combines with the Loop Current to become the surface part of the Florida

Straits flux.

3. The flux out of the Gulf interior, between 200 and 800 m, is 5 Sv. This flux combines

with the Loop Current to become the deep part of the Florida Straits flux.

4. The vertical flux crossing the 200-m depth is towards the surface and is ~1 Sv.

5. The vertical flux crossing the 800-m depth is towards the surface and at 2.1 to 2.3 Sv.

6. The residence time, or the overturning age, of the volume between the surface and 200 m

is 15 to 30 yrs.

7. The residence time, or the overturning age, of the volume between the 200 and 800 m is

25 to 30 yrs.

8. The first order rate constant parameterizing the oxidation of particulate and dissolved

organic carbon is 0.065 yr-1, determined from the kwV1 value after dividing by the volume

of box 1. The rate constant simply means that if the Gulf were cutoff from all sources of

oxygen, then it would take ~10 years for the oxygen concentration to decrease by onehalf as the result of reactive loss. Given the surface oxygen concentration of 3.87 mL L-1,

the estimated oxygen consumption rate in the surface layer is therefore 5.3 μmol kg-1 yr-1.

This value is an order of magnitude smaller than the estimated rate of consumption

estimated by Riley (1951) as presented in Table 4.3. However it is quite consistent with
105

the results of Jenkins (1982), who used tritium-helium3 dating to get a rate of 5

μmole/kg/yr. Subsequent studies all come up with the same number usually in the range

of 1 to 5 μmole/kg/yr at 900 – 1000 m depth.

9. Based on the first order rate constant parameterizing the loss of oxygen at the sediment

interface, an equivalent rate constant for reactive loss in the waters of the deep Gulf is

~0.001 yr-1. Given the oxygen concentration of 5.04 mL L-1 for the deep Gulf, the

estimated oxygen consumption rate in the deep Gulf is 0.1 μmol kg-1 yr-1. This

corresponds to a residence time on the order of 50 years. Fiadiero and Craig (1978)

calculated an oxygen utilization rate below 1000 m of 2 μL/L/yr, or 0.09 μmole/kg/yr,

which is consistent with the value determined here.

10. The net primary productivity and atmospheric exchange for the upper 200 m of the entire

Gulf is between 1.6 to 1.9 x 1010 mL s

-1. This corresponds to 2.2 to 2.6 mol m-2 yr-1. These values are consistent with the fluxes reported by Bopp et al. (2002) for various

regions of the world’s oceans and in particular the latitude associated with the Gulf of

Mexico.

Again we reiterate, these results hold regardless of the assumed velocity profile. This is evidence

of the self-consistency of the model.

Table 5.4

Box Model Output from Baseline Runs

Description Units n=1 n=2 n=3 n=4 Sheinbaum

et al. (2002)

βmin 0.29 0.29 0.29 0.26 0.30

u1

out Sv 2.22 3.16 4.01 4.29 4.82

u2

out Sv 5.36 5.95 5.92 5.05 5.01

u3

out Sv 5.29 2.70 1.05 0.18 1.05

u4

out Sv 3.51 1.49 048 0 1.00

u4

in Sv 1.26 0.32 0.08 0.02 0.76

d12 Sv 0.58 0.83 1.04 1.12 1.25

d23 Sv 2.10 2.29 2.36 2.15 2.12

d34 Sv -1.62 -1.02 -0.36 0.03 0.14

d45, w54 Sv 0.63 0.16 0.04 0.01 0.38

FS1 Sv 6.36 9.06 11.49 13.59 13.40

FS2 Sv 15.10 17.06 17.33 16.65 14.78

YCR Sv 8.80 4.19 1.53 0.18 2.05

kwV1 Sv 0.402 0.457 0.470 0.427 0.423

kbA2 Sv 0.036 0.009 0.002 0.0006 0.022

rp(02)V1 mL·L-1 Sv 1.585 1.808 1.865 1.707 1.684

τ1 years 29 20 16 15 13

τ2 years 30 26 26 29 29

τ3 years 20 30 47 75 56

τ4 years 33 74 228 4300 109

τ5 years 350 1390 5,535 22,560 570

ε, error 4.916 3.495 2.250 1.950 1.578
106

There are several other items to note from Table 5.4. First, the smallest total error is obtained for

the most realistic velocity profile, the Sheinbaum profile. Since this profile was determined

graphically from a figure, actual flux values would lead to better estimates of the unknowns.

Secondly, the box model determined a Yucatan return flux of 2.05 Sv for the Sheinbaum velocity

profile, which is surprisingly close to the 2 Sv found by Ochoa et al. 2001. Third, the flux into

the bottom most box, w45, while less than 0.6 Sv, still varies are over a wide range depending on

the assumed velocity profile. Recent model studies by Welsh and Inoue (2000) support an

extremely rapid turnover (~10 yr) of the bottommost layer, with rings as the mechanism for

driving the deep circulation. On the other hand, Schink (private communication) used

radiochemistry to estimate the turnover to be on the order of 70 yr. None of the box model

turnover times (or residence times) for box 5 shown in Table 5.4 are close to these independent

values. This implies that the vertical flux, w45, into box 5 must be much larger than even the n=1

velocity profile allows. In order to obtain a turnover time of 100 years, the flux through box 5

would have to be 2.2 Sv. Assuming that 1.75 Sv came from the bottom 500 m of a Loop Current

eddy over a two-year spin down period and at any one time 3 warm-core eddies were in the Gulf,

then the initial size of each eddy would have to be ~300 km in diameter. This is a probable eddy

diameter, but a turnover time of ten years calls for a significantly greater flux and either many

more warm-core eddies or larger diameter eddies. An improvement to this box model might

incorporate a generic eddy that injected fluid below the sill depth, contained Loop Current water

properties, and would allow for significant vertical fluxes.

5.5 Scenarios

Next we examine the dependence of dissolved oxygen concentrations on the Loop Current

transport, the rate of primary productivity, the rate of remineralization, and the rate of sediment

reaction. The values in Table 5.4 serve as the baseline for testing scenarios, such as how the

oxygen concentrations might change given a change in photosynthesis. The complete set of

equations, described by (5.2) through (5.6), can be greatly simplified to only track the transport,

production, and reactive loss of oxygen since the other values are now known. This simplified

equation set is:

( ) 1 1 1 1 12 1 2 1 1 1 1 0 { , }

1

u O u O d O O k V O V r O

Box

w P

in in out = − + − +

⋅

2 2 2 2 23 2 3 12 1 2 2 2 2 2 2 0 { , } { , }

2

u O u O d O O d O O V k O A k O

Box

w b

in in out = − + − − −α

⋅

3 3 3 3 34 3 4 23 2 3 3 3 3 3 3 0 { , } { , }

3

u O u O d O O d O O V k O A k O

Box

w b

in in out = − + − − −α

⋅ (5.12)

4 4 4 4 34 3 4 45 5 4 4 4 0 { , }

4

u O u O d O O d O A k O

Box

b

in in out = − − + −α

⋅

0 45 4 45 5 5 5 5

5

w O d O A k O

Box

b

in = − −α

⋅

The transports, rate constants, and oxygen production in the surface layer are found in Table 5.4.

Both the water column and sediment-water interface reactions are given with the understanding

that only one of the two can be included.
107

Discussion of Model Runs

First examined was how the errors in the model affect the estimated oxygen concentrations in

each of the boxes. As noted above, the unknowns will not completely satisfy the governing

equations. If the transports, rate constants, and oxygen production in the surface layer in Table

5.4 are input to equation (5.12), then the oxygen concentrations in each box can be determined.

The results are shown in Table 5.5 for each of the five velocity profiles.

Table 5.5

Model Estimated Dissolved Oxygen Concentrations by Box

Dissolved

Oxygen

(mL·L-1)

Known n=1 n=2 n=3 n=4 Sheinbaum

et al. (2002)

Box 1 3.87 3.86 3.86 3.86 3.86 3.86

Box 2 3.02 2.99 2.99 2.99 2.99 3.00

Box 3 4.42 4.31 4.31 4.31 4.32 4.34

Box 4 4.95 4.66 4.52 4.48 4.95 4.95

Box 5 5.04 5.03 5.03 5.02 5.04 5.03

If there were no errors in the box model, then the predicted oxygen concentrations for each of the

velocity profiles would exactly match the expected values. As it is, the match is quite good,

except for box 3. The model has difficulty in getting the oxygen concentration in box 3 high

enough. In fact, the concentration of 4.31 mL·L-1 is that of the incoming Yucatan oxygen

concentration. The only way to raise the value is to bring fluid up from the lower box, box 4

where the oxygen concentration is 4.95. But the temperature distribution is in the opposite

direction and forces fluid down from box 3 to 4. This is the case for the n = 1, 2, and 3 velocity

profiles, where the d34 vertical flux is negative. This mismatch might be corrected with a

different distribution of boxes in the vertical. It also points out how averaging the vertical

distribution of a tracer concentration over a large depth can lead to problems.

To examine a series of scenarios and still recognize the inherent errors in this box model, the

concentrations were corrected by the error implied in Table 5.5.

For the first scenario, the role transport alone plays was examined. There is no production of

oxygen in the surface layer, there are no biogeochemical processes consuming oxygen, and the

transports are given by the values in Table 5.4. The results of solving equation (5.12) given these

constraints are shown in Table 5.6 for each of the velocity profiles. They indicate that transport

alone is sufficient to maintain the oxygen in box 3 (800 to 1500 m). This was a design constraint

found by running the model a sufficient number of times to determine reasonable results that

could not be obtained if any reaction was allowed in this box (see Section 5.2.3c). Above 800 m

significant reactive losses must act to reduce the oxygen concentrations and below 1500 minor

reactive losses must exist.
108

Table 5.6

Role of Transport in Establishing Dissolved Oxygen Concentrations

Dissolved

Oxygen

(mL·L-1)

Known n=1 n=2 n=3 n=4 Sheinbaum

et al. (2002)

Box 1 3.87 4.00 4.00 4.00 4.00 4.00

Box 2 3.02 3.58 3.58 3.58 3.59 3.58

Box 3 4.42 4.42 4.43 4.43 4.43 4.43

Box 4 4.95 5.04 5.00 4.99 5.12 5.12

Box 5 5.04 5.13 5.13 5.14 5.13 5.13

The role that photosynthesis plays was examined next. Here, there is no photosynthetic

production or atmospheric exchange of oxygen in the surface layer, but there is reactive loss. The

rate constants and transports are given in Table 5.4. The results of solving equation (5.12) given

these constraints are shown in Table 5.7. The oxygen concentration in the surface layer is

reduced by reactive loss from the 4.00 mL·L-1 that transport mixes into the surface layer to a

value that is lower than the measured concentration of 3.87 mL·L-1. The remaining boxes are

unaffected because the vertical flux into the surface layer is positive upwards for every velocity

profile (see d12 in Table 5.4). Therefore, a change in the surface layer cannot be communicated

downward and the surface layer is in essence isolated from the underlying boxes as long as the

net vertical flux is upwelling. It may well be that there are local regions where the flux is

downwelling, but the nature of the box model essentially assumes a global net flux.

The contribution of the sediment interface reaction was next examined. For this scenario, there is

no sediment reaction. The rate constant for the water column reaction, the net production of

oxygen in the surface layer, and the transports are given in Table 5.4. The results of solving

equation (5.12) given these constraints are shown in Table 5.8. The only significant difference is

seen in the bottom two boxes where the sediment reaction occurs. A slight increase in the

concentrations is seen.

A crude estimation of what would happen if there was a doubling of carbon in the water column

can be made by doubling the rate constant for the water column reaction in the top two boxes and

including it in the bottom three boxes (Table 5.9). This assumes that there would be significant

labile particulate carbon left by the time it settled below the second box. As we have noted

previously, this model cannot directly track the fate of hydrocarbon in the water column unless

carbon is tracked simultaneously with oxygen. Nevertheless, this scenario shows a dramatic

decrease in the oxygen concentrations, especially at depth. This reinforces our findings that the

water column reaction actually contributes very little below 800 m in depth. It also infers that an

introduction of carbon at depth has the potential to significantly decrease the oxygen content if

the system is closed.
109

Table 5.7

Scenario of No Oxygen Production in the Surface Layer

Dissolved

Oxygen

(mL·L-1)

Known n=1 n=2 n=3 n=4 Sheinbaum

et al. (2002)

Box 1 3.87 3.26 3.37 3.45 3.50 3.54

Box 2 3.02 3.02 3.02 3.02 3.02 3.02

Box 3 4.42 4.42 4.42 4.42 4.42 4.42

Box 4 4.95 4.95 4.95 4.95 4.95 4.95

Box 5 5.04 5.05 5.05 5.06 5.04 5.04

Table 5.8

Role of Sediment-water Reactions in Maintaining Dissolved Oxygen Concentrations

Dissolved

Oxygen

(mL·L-1)

Known n=1 n=2 n=3 n=4 Sheinbaum

et al. (2002)

Box 1 3.87 3.87 3.87 3.87 3.87 3.87

Box 2 3.02 3.02 3.02 3.02 3.02 3.02

Box 3 4.42 4.42 4.42 4.43 4.43 4.43

Box 4 4.95 5.04 5.00 4.99 5.12 5.12

Box 5 5.04 5.13 5.13 5.14 5.13 5.13

Table 5.9

Role of Carbon Doubling in Maintaining Dissolved Oxygen Concentrations

Dissolved

Oxygen

(mL·L-1)

Known n=1 n=2 n=3 n=4 Sheinbaum

et al. (2002)

Box 1 3.87 3.25 3.31 3.36 3.37 3.42

Box 2 3.02 2.50 2.43 2.34 2.23 2.30

Box 3 4.42 3.95 3.68 3.33 2.95 3.18

Box 4 4.95 3.58 2.82 1.74 0.14 2.37

Box 5 5.04 1.62 0.49 0.14 0.04 1.09
110

Finally, an estimate was made of the decay half-life of the oxygen. Here the questions asked are:

"What would happen if there was no more transport of oxygen from the Yucatan into the Gulf

and no more photosynthesis?" and "How long would it take for the oxygen to decay?" For the

water column reaction the decay of oxygen is given by

k VC

dt

dC V = − w

which has the solution

k t w C t C t e− ( ) = ( ) 0

The concentration drops to half of its initial value at the half-life, given as

wk

t

ln 2

1 2 =

Similarly, the half-life for the sediment contribution is

V

A k

t

b

ln 2

1 2 =

The results for each of the velocity profiles are shown in the Table 5.10. The large variation in

the sediment half-life is not surprising given the range of mass transfer coefficients shown in

Table 5.4 and the large uncertainty in the vertical flux. The time necessary for the oxygen

concentration in the water column to decrease to half of its initial value ranges from 9.5 to 11.2

years, depending on the velocity profile. This range is entirely due to the range in rate constants

reported in Table 5.4.

Table 5.10

Time Necessary for Dissolved Oxygen Concentration to Decrease by 50% If There Is No

Transport

Half-life

(years)

n=1 n=2 n=3 n=4 Sheinbaum

et al. (2002)

Water column 11.2 9.8 9.5 10.4 10.6

Sediment 123 500 2,025 7,836 200

5.6 Conclusions

Despite its apparent complexity, this is a very simple box model. However, while it is a zeroorder approach, the results should not be summarily dismissed as zero-order. By reason of the

model's design philosophy, the results provide a reasonable bound to the problem of how oxygen
111

is maintained in the deep Gulf. The model was deliberately developed with the least amount of

external information possible. In this way we were assured that the principles of physics and

biology would constrain and guide the box model development, rather than some assumed

knowledge about the system. In every case, as the box model evolved to what is reported here,

the model had to be adjusted in ways that were then found to be realistic. For example, the water

column reaction had to be severely attenuated below 800 m in order to obtain reasonable results.

This is representative of the rapid consumption of labile carbon in the upper water column.

The results from the box model can be roughly separated into two parts, findings that are most

likely independent of the specific details of the box model, and findings that may not be

independent. Regarding independent findings, the box model indicates to us that the oxygen

profiles in the Gulf are most likely determined by a unique combination of a) the transport

process associated with circulation and mixing and b) the reactive processes associated with

biogeochemical cycles. It is unlikely that there are a multitude of oceanographic and

biogeochemical states that could lead to the observed oxygen profiles. From a mathematical

perspective this means this is a boundary value problem. From a physical perspective it means

that extra-basin effects, in particular the transport through the Yucatan Channel, have a

significant role in controlling the oxygen profiles in the Gulf. This is seen in how the shape of

the assumed velocity profile is critical to determining how well the box model reproduces the

observed oxygen concentrations. Obtaining accurate flux information in the Yucatan, and for that

matter, through the Florida Straits, should be a priority if the box model is to be improved.

The results of the box model suggest that ventilation of the deep Gulf waters below the 2000-m

sill depth of the Yucatan Channel is key to maintaining the dissolved oxygen distributions. First,

there is the matter of the lower oxygen concentration in the Gulf above 2000 m. The source of

the higher concentration oxygen in the deep Gulf waters must come directly from a water mass

that contains properties similar to the lowest level of the Yucatan Channel. Independent

estimates of the residence time in the deep Gulf that range from 10 to 70 years indicate that the

flux below the 2000 m depth has to be much larger than the box model estimates of 0.01 to 0.6

Sv. In its present design, the vertical flux across the 2000-m depth is constrained by the modeled

flow at depth through the Yucatan Channel. This constraint could be eased by allowing for an

'autonomous' source of high oxygen concentration water. The vertical flux could be determined

by the box model, but, as the model is presently formulated, to do so would presume that the

sediment-mediated rate constant controls the flux. Considering equation (5.12) for box 5, the

vertical flux would be given by the following:

4 5

5 5 5

45 O O

A k O

w in

b

− = α

Here we see that the flux is entirely dependent on the value for the sediment rate constant. Since

this is the only equation defining the flux, it is unlikely to provide a statistically reliable answer.

Because the mechanisms for transport of deep water into the Gulf interior and for vertical mixing

below the sill depth are unknown, the necessary flux cannot be resolved by the box model.

Determining the flux will require a much more sophisticated numerical model than a box model.

There are three results from the box model that are quite enlightening, though they may not be

independent of the box model design. First, approximately 30% of the Yucatan Channel flow is

diverted into the interior; the remainder outflowing as the Loop Current. Secondly, the first order

rate constant parameterizing the oxidation of particulate and dissolved organic carbon is 0.065

yr-1. Given the surface oxygen concentration of 3.87 mL·L-1, the estimated oxygen consumption

rate is therefore 5.3 μmol kg-1 yr-1. This value is comparable to the estimated rate of consumption

found by Riley (1951); seen Table 4.3. Third, the net primary productivity and atmospheric

exchange for the upper 200 m of the entire Gulf is between 2.2 to 2.6 mol m-2 yr-1.
113

6 DISCUSSION, CONCLUSIONS, AND RECOMMENDATION

This section provides a brief discussion of issues associated with dissolved oxygen in the Gulf of

Mexico (Section 6.1), conclusions from the study (Section 6.2), identification of significant gaps in

data and knowledge (Section 6.3), and a recommendation for further study (Section 6.4).

6.1 Discussion

Seven questions were developed to evaluate what this study has revealed about the types and rates

of processes occurring in the deep Gulf of Mexico that affect the levels of dissolved oxygen in the

deep water and about the balance that maintains this level. The knowledge that was obtained in this

study for each question also was examined.

What major processes affect the levels of oxygen in the deepwater Gulf of Mexico?

The major processes that affect the dissolved oxygen concentrations in the Gulf of Mexico have

been known for over 50 years. This study's findings are consistent with earlier studies. The major

sources are water mass transport into the basin from the Caribbean Sea (source waters for the deep

Gulf), the atmospheric-oceanic exchange of oxygen at the air-sea interface (ocean generally in

equilibrium with the atmosphere for the environmental temperature and salinity conditions), and

primary productivity and its consequent photosynthesis input of oxygen (surface phenomenon with

downward advection only within the waters mixed by atmospheric processes). Aside from transport

of water masses out of the basin, the major sinks are removal by respiration of organic matter in

both the water column and the sediments. Other processes, such as chemical removal or organic

enrichments from oil and gas seeps, are minor or only potentially of local importance.

The distribution of the data sets, particularly within the deep waters of the Gulf, is relatively sparse

in time and space. However, the relative uniformity of the temperature and salinity in the deep

waters below about 1500 m and the slow rates of oxygen consumption that occur at those depths

suggest that the distribution is adequate to resolve the horizontal and vertical oxygen distributions in

those deep waters.

What is the relative importance of these processes? What are the timeframes for these processes –

hours, days, years, decades, centuries?

The relative importance of the processes that maintain the dissolved oxygen distributions through

the water column are dependent on the depth. In essence, the Gulf can be viewed as a two layer

system when it comes to considering major processes of dissolved oxygen replenishment. These

are the upper layer, which is approximately the upper 200 m, and the lower layer, which extends

from approximately 200 m to the bottom. The major process of oxygen replenishment in the upper

layer is through air-sea exchanges of oxygen and mixing by wind and wave action. This process is

continually present and maintains the upper waters at or near equilibrium concentrations. Input of

oxygen by photosynthesis can be important in this layer, but it is local in nature. It can result in

supersaturation of waters, particularly below the actual air-sea interface.

The only process for renewal of dissolved oxygen to the deep water is advection into the Gulf

interior of inflowing source waters. There is no deep water ventilation by water mass formation and

thermohaline circulation. The processes that advect the inflowing waters into the interior are not well

understood. The transport into the basin at the Yucatan Channel is balanced by the outflow at the

Florida Straits and recirculation out the Yucatan Channel, and although the transport out the Florida

Straits is reasonably understood, the recirculation is not. Further, the transport mechanisms that

move the well-oxygenated deep waters from the Yucatan Channel into the Gulf interior, particularly

into the western Gulf, are not understood.
114

The major sink of dissolved oxygen throughout the water column is the oxidation of organic

material. This process is not unique to the Gulf, nor does the Gulf have unique aspects, except for

the localized hydrocarbon seeps and anoxic brine pools. These seeps and pools do not affect the

dissolved oxygen levels in the water column except within a few centimeters above them. This

oxidation of organic material also operates, in essence, in two different layers. The first layer again

is generally in the upper 200 m, which is where, in the world's ocean, approximately 90% of the

organic matter from photosynthesis is oxidized (Millero 1996; Riley 1951). Below this layer and in

particular below the oxygen minimum zone, which can occur as deep as 700 m in the Gulf, the

oxidation rate is assumed to decrease exponentially. The water column respiration, however, is not

well measured; no direct measurements in the Gulf were found. At the sediment-water interface, the

oxygen consumption processes of the sediments can be of local importance in drawing down the

oxygen concentrations within a few cm of the sediment surface. In a totally closed ocean basin, this

consumption over time could result in anoxic conditions in substantial portions of the deep layer,

such as seen in the Black Sea. However, the Gulf of Mexico basin is not such a closed system, and

oxygen removed by these processes is replenished by the advection into the interior of the oxygenrich water masses.

The temporal rates of the processes involved in maintaining the oxygen distributions are not well

known. However, given the continual input of natural hydrocarbons and the maintenance of the

well-oxygenated deep waters, it is likely that the intra-basin effects of consumption are in balance

with the extra-basin processes that transport into the deep Gulf the oxygen-rich water masses that

are formed at high latitudes. Flushing times of the deep Gulf are uncertain, in part because the

processes of advection into this region are not well understood.

What are the relevant spatial and temporal scales of these processes?

The station location maps show that most areas within the deepwater Gulf have been sampled at

least once. Although more stations were taken in summer (~30-40%), sufficient numbers were

taken in the other seasons (15-26% each) to reduce the seasonal bias in sampling. However, the

existing data base remains lacking in both the spatial and temporal coverage necessary to address

questions of scales of the processes. A crucial factor that is missing is a good understanding of the

processes of circulation in the deep waters of the Gulf. The circulation in the upper 1000 m is

reasonably well-defined, but the circulation and its variability in lower waters are poorly measured

and, so, are not well understood. Most of the sampling for dissolved oxygen has been on cruises

with fairly wide spacing between stations, and very few cruises sampled significant portions of the

Gulf quasi-synoptically. Vertically, the sampling is sparse below 1500 m, but the relative uniformity

of the concentrations suggests that the sampling done has been adequate to reveal the major

distributions.

Because the dissolved oxygen system in the Gulf of Mexico operates very much as it does in the

world's ocean, it is unlikely that major processes or potentially catastrophic events have been missed

by the sampling. It is likely that events or distributions that occur over very short timeframes or

small spatial scales may have not been sampled; but their importance also is likely to be small to

negligible for the broad oxygen distributions. However, they may be important for local

considerations, such as timeframes of several years or less over a region of several kilometers or

less. For example, hurricanes can have effects on the distribution of oxygen, both through the

injection of bubbles into the water column and from storm-induced upwelling (e.g., see Ichiye

1972). As another example, the brine pools in the Gulf are anoxic, however, their major effects

appear to occur on small scales (cm) in their immediate vicinity, and the replenishment of dissolved

oxygen through the general circulation that transports oxygen rich waters into these areas appears

to be sufficient to prevent the anoxic conditions from growing in extent. However, the available data

base does not allow a clear determination of the rates of replenishment or assessment of the

possibility that one time events set the distribution and then the profiles begin to degrade.
115

Do we expect oil and gas related discharges to significantly alter oxygen levels in the deep Gulf?

The oil and gas remaining proven and unproven reserves for the U.S. Gulf of Mexico have been

estimated by MMS at 5,225 million barrels of oil and 28,699 billion cubic feet of gas (Crawford et

al. 2003). Using the barrels of oil equivalent (BOE) conversion factor of 5,620 cubic feet gas/BOE,

yields a BOE of 5,107 million barrels from the gas. Thus, the total quantity of hydrocarbons in

these reserves is approximately 10 billion barrels. Complete oxidation of these oil and gas reserves

would consume 0.76% of the standing oxygen supply. By comparison the annual discharges from

extraction (drill fluids including synthetic-based mud, drill cuttings, produced waters, accidental

spills from platforms, etc.) would consume 0.000001% of the standing oxygen supply. These

effects are not significant alterations of the oxygen supply, unless the Gulf becomes a closed

system that is not ventilated by the inflowing oxygen-rich water masses through the Yucatan

Channel.

There can be local effects, such as near platforms where cuttings might be discharged, smothering

the benthic communities, and resulting in draw down of oxygen just above the sediments (NRC

2003; Neff et al. 2000). However, the database is insufficient to evaluate local events and their effect

on oxygen distributions in the immediate vicinity. The database does not include the sampling to

give the details of dissolved oxygen distributions in deep water. The nature of the hydrocarbon

material, the lability of the material, its chemistry and properties within the temperature and salinity

structure of the deep water, the location of its discharge in the water column, its likely movement

(rising to the surface, staying near the sea floor or at a density level in the water column if formed

into dense enough emulsions with water or sediments, advecting to distant locations through

circulation processes)--all these factors and more influence the extent of the local impact of such

material on dissolved oxygen concentrations. The environmental conditions at such locations also

would play a substantial role in controlling the magnitude of the local effects. For example, the

likely impact of currents at the deep water location may be to disperse cuttings in deep water before

they accumulate, or lack of light or nutrients may hinder growth of the organisms that oxidize the

organic material.

Does the box model indicate any unrecognized sources or sinks for oxygen in the Gulf of Mexico? 

Are there regional differences?

The limitations of the data and gaps in knowledge, including a paucity of specific information on

the deep circulation and transport rates, lack of availability of specific rates of consumption within

the water column, and insufficient high quality oxygen data sets for all water depths, regions, and

sediment-water interface, resulted in development of a first order simple box model for the whole

Gulf. The model therefore was not capable of testing to determine whether there were unrecognized

sources or sinks or regional differences. It also was not designed to test local conditions. Two

issues that might be interesting to address with a model are (1) what the intra-basin variations might

be and what causes them and (2) determination of whether micro-scale environments, such as the

brine pools, might drive mesoscale features. However, limitations of the data and gaps in

knowledge and, consequently, this simple box model are not adequate to address these issues in

detail.

Do oxygen distributions indicate any areas unusually sensitive or resilient to oxygen depletion and

what are the characteristics that make these areas unique?

Evaluation of the database suggests that the deep waters in the southwest Gulf may contain older

water that is slightly less well-oxygenated than the southeast or northern Gulf. However, the data

are not of sufficiently high quality or quantity to determine this definitively. If real, this distribution

makes sense because the southwest Gulf likely is ventilated by transport more slowly than are the

other areas. The deep circulation is not well known and there are no definitive studies on the age of

the deep waters and their residence time. The available oxygen database cannot address these

issues. The data base also does not reveal localized unusual areas, although other studies have

found such features as anoxic brine pools and natural hydrocarbon seeps with chemosynthetic (and

oxygen using) communities in local areas.
116

What information is missing to develop a quantitative inventory of oxygen in the basin?

The major information missing is a thorough understanding of the deep water circulation, which

controls how and at what rates the well-oxygenated (~5 mL·L-1) deep waters inflowing at the

Yucatan Channel are transported into the Gulf interior to ventilate the waters below ~1500 m.

Additionally, definition of the rates of consumption with depth are specifically needed for the Gulf,

with its large hydrocarbon input through the seeps. To address local effects, detailed studies of the

fates and effects of anthropogenic inputs near discharge points are needed.

6.2 Conclusions

The deep waters of the Gulf of Mexico Basin are ventilated by the circulation and mixing into the

Gulf interior of the well-oxygenated water masses inflowing at the Yucatan Channel. Consequently,

the Gulf of Mexico Basin is in no danger of becoming anoxic due either to natural processes or oil

and gas production. Nature has been providing more oil to the basin in the form of seeps than is

being discharged by humans, and yet the values of dissolved oxygen throughout the water column

are stable. This is a natural experiment of geological age. Decreased oxygen levels, however, could

occur on localized bases.

Sources of Dissolved Oxygen in the Deepwater Gulf of Mexico

• The Gulf of Mexico is a semi-enclosed sea with two ports. The major inflow port is at the

Yucatan Channel, which has a sill depth that is deep enough (~2000 m) to allow the transport of

oxygen rich waters into the Gulf. The Loop Current brings these waters in. The major outflow port

at the Florida Straits is shallow enough (~800 m) that the deeper, oxygen rich waters can mix into

the interior of the Gulf, rather than flowing directly out with the Loop Current.

• The source of dissolved oxygen to the upper waters (~100-200 m) in the Gulf of Mexico is

mainly from exchanges with the atmosphere with local contributions from photosynthesis.

• The source of dissolved oxygen to the deep waters is the transport of oxygen-rich water

masses into the Gulf of Mexico from the Caribbean Sea through the Yucatan Channel. There is no

water mass formation in the Gulf of Mexico to replenish the deep oxygen concentrations. So, the

deep circulation of the Gulf and its associated mixing are the mechanisms that replenish the deep

oxygen.

Sinks of Dissolved Oxygen

• The major sink of oxygen in the Gulf, as elsewhere in the world's oceans, is oxidation of

organic matter.

• The oxygen minimum zone, which generally is found in the Gulf between 300-700 m, is

derived from two mechanisms. First, the Tropical Atlantic Central Water with the oxygen minimum

at its core brings in waters that are depleted from processes occurring outside the Gulf. The decay

of organic matter that occurs in the Gulf itself augments this minimum by the same processes.

However, the productivity of the Gulf is not high enough to create extreme oxygen minimum zones

that are seen in other parts of the world's ocean, such as in the Arabian Sea.

Data Sets

• Vertical sets of dissolved oxygen data, from bottle samples, were collected at approximately

4000 stations in the Gulf of Mexico.

• These data were collected from 1922 through 2001, when this study ceased data assembly.

The full water column was most likely to have been sampled on cruises during the decades of the

1950s, 1960s, and 1970s. Only the decades of the 1960s and 1970s had station coverage

throughout the Gulf basin.
117

• The southwest Gulf is less sampled than other areas.

• Waters at or below 1500 m are not well sampled, with less than 500 stations sampling at

those depths.

• Data quality is variable, even within cruises. This likely is due to different analysis

techniques used in the older data sets and differences in the analysts' visual identification of the end

points of titration, as well as poor sampling and/or laboratory analysis.

• Profiles of dissolved oxygen from sensors were obtained for approximately 1800 stations.

It is not clear whether these have been properly calibrated with bottle data.

Spatial and Temporal Changes in Oxygen Levels

• Early studies, using known high quality data sets, found that below approximately 1500 m

there was "no clearly discernable horizontal variation in dissolved oxygen in these waters" and

"only slight vertical oxygen gradients" (Nowlin et al. 1969).

• Analysis of the additional data sets used in this study suggest the deep Gulf waters may

have slightly different oxygen values in three different regions. However, the quality of the data sets

is variable enough that this finding is not definitive. The three regions are:

- The southeast Gulf has deep oxygen concentrations most similar to those of the inflowing

source waters. The close contact of the southeastern Gulf waters with the inflowing Loop Current

waters likely results in more rapid replenishment of oxygen than elsewhere in the Gulf. This

replenishment would be through mechanisms of direct water mass transport and the effects of

mixing induced as water flows over the sill or otherwise interacts with the topography in the region.

- The deep waters of the northern Gulf have slightly less oxygen than those in the southeast,

suggesting the flushing time there is longer. However, both the northeast and northwest Gulf waters

have similar concentrations, and so the transport and mixing mechanisms, which carry oxygen rich

waters into these regions, are the same for these two regions.

- The southwestern Gulf has deep oxygen concentrations that are less than the northern

Gulf waters, suggesting a longer flushing time there.

• Comparison of data sets from the 1970s with those from 2000/2001 indicates there has not

been any discernible change in the oxygen structure in the Gulf of Mexico. This suggests that the

transport mechanisms that replenish the oxygen are adequate to balance the oxygen consumption

from decay of organic matter, including that from oil seeps and anthropogenic sources.

Influences of Shelf Processes on Deepwater Dissolved Oxygen Concentrations

• Data sets available are not adequate to study the details of the effects of shelf-deep ocean

exchanges on the oxygen levels.

• No water mass is formed on the shelves. Thus, there are no shelf waters dense enough to

sink down into the deep ocean of the Gulf. Any shelf-deep ocean exchanges will impact only the

upper waters.

• Transport of organic matter off the shelf, particularly in the region of the Mississippi River

Delta, could have local effects on the oxygen concentrations in the deep Gulf if the material sinks to

depth before decaying.
118

Effects of Natural Hydrocarbons on Dissolved Oxygen Concentrations

• The NRC (2003) reports an estimated 140,000 ± 60,000 tonnes of hydrocarbons are

introduced into the Gulf of Mexico from natural seeps each year. This seepage begins at the sea

floor and rises through the water column to the surface, so it potentially can affect the total water

column. Although this natural leakage of hydrocarbons into the Gulf apparently has been occurring

for millions of years, no significant large-scale perturbations in the dissolved oxygen content of the

deep Gulf waters were observed by this study to result from this input. However, localized low

oxygen conditions are reported to occur in the upper meter or so of the sediments and, hence, it is

likely that oxygen concentrations of the waters at the sediment-water interface may be depleted.

• Chemosynthetic communities are active at the seep sites. Because they require access to

oxygenated waters, their presence is additional evidence of ventilation of the deepest Gulf waters.

Effects of Oil and Gas Activities on Dissolved Oxygen Concentrations

• Knowledge of many factors is necessary to make a thorough assessment of the effects of

various oil and gas activities on dissolved oxygen concentrations in the Gulf of Mexico. These

factors include (1) the environmental conditions at the discharge location such as currents at all

affected depths, surface and internal wave action, water temperatures, and local biological

characteristics of the ecosystems that are exposed, (2) the type and rate of the discharge, and (3) the

chemical nature of the material discharged. This is a highly complex problem that requires a more

sophisticated approach that can be accomplished in this study.

• Effects on dissolved oxygen concentrations in the deepwater Gulf of anthropogenic

discharges most likely would be most substantial at the sediment-water interface at a discharge site

or the sea surface reached by the plume, not within the water column itself. Effects are expected to

be local, but potentially could be severe for short periods. Further study is needed to examine such

effects because they are localized and complex.

• The NRC (2003) reports anthropogenic sources of hydrocarbons are estimated at

approximately 2000 tonnes from extraction (1700 from produced waters), 1600 tonnes from

transportation, and 6800 tonnes from consumption. These inputs are widespread throughout the

Gulf. Thus, their impact on dissolved oxygen concentrations is negligible over the large scale in the

deepwater Gulf of Mexico. The localized effects of discharges, however, might be measurable and,

in some cases, substantial. Most of these discharges occur at or near the sea surface, so there would

be essentially no effect on the dissolved oxygen concentrations of the deep waters. An exception

might be the secondary effects from any resulting increase in the quantity of decaying organic

material, such as from growth of organisms that utilize the hydrocarbons, that sinks through the

water column.

• Catastrophic oil spills can introduce hydrocarbons at 2-3 times the rate of the natural seeps.

The effect of such a spill on the dissolved oxygen concentrations would depend on many factors,

including chemical characteristics of the spilled material, location in the water column and nature of

the spill, residence time in the water column of labile components of the hydrocarbon as the spilled

material rises toward the sea surface, and environmental conditions at the spill site, including sites to

which the material may be transported. In the large scale, the impact is expected to be minimal,

although, again, local effects on oxygen levels could be substantial.

Modeling Results

The results of the simple box model developed in this study provided insights into the importance

of transports, reactive losses, and primary productivity. Transport is the primary process controlling

the oxygen profiles, followed in importance by respiration of organic material in the water column.

The oxidation of organic matter in the sediment plays a minor role. Specifically:
119

• The oxygen profiles in the Gulf likely are determined by a unique combination of the

transport process associated with circulation and mixing and the reactive processes associated with

biogeochemical cycles. It is unlikely that a multitude of oceanographic and biogeochemical states

could describe the observed oxygen profiles. This implies that a model, if properly designed and

constrained, is mathematically capable of determining a unique answer.

• The transport through the Yucatan Channel plays the most significant role in controlling the

oxygen profiles in the deep Gulf. The model suggests that 30% of the inflow is diverted into the

Gulf. However, the modeled Yucatan Channel flow at depth is not sufficient itself to maintain the

oxygen profiles in the deepwater Gulf. Because the mechanisms for transport of deep waters into

the Gulf interior and for vertical mixing in waters below the sill depth are unknown, the necessary

flux cannot be resolved by the box model.

• The oxidation of carbon in the water column plays a secondary, but important, role to that of

transport in maintaining oxygen profiles in the upper 800 m of the Gulf. It was not found to be

necessary below 800 m.

• The model is incapable of simultaneously determining, within a box, oxidation rates in the

water column and oxidation rates at the sediment interface. Some judgment is necessary to choose

which process is to be modeled within each box.

• The oxidation at the sediment-water interface plays a minor, but necessary, role in

maintaining the oxygen below 1500 m. It was not found to be necessary above 1500 m. By design,

it has no effect above 200 m.

• The net production of oxygen in the surface layer is consistent with the rates reported for

the latitude associated with the Gulf of Mexico.

• The oxygen content in the Gulf would decrease to half of its present value in 10 years if the

Gulf was isolated from all sources of dissolved oxygen, be it transport, primary productivity, or

atmospheric exchange.

6.3 Knowledge and Data Gaps

This study has revealed a number of gaps in data and knowledge regarding dissolved oxygen in the

deepwater Gulf of Mexico that are of scientific interest. These include

1. What is the rate of consumption of dissolved oxygen throughout the water column,

particularly below the mixed layer and also in regions with natural hydrocarbon seeps?

2. What is the age of the water masses, below the mixed layer, in the various regions of the

Gulf of Mexico? In particular, what are the ages of the inflowing water masses at the Yucatan

Channel as compared to those in the southeast, northeast, northwest, and southwest quadrants of the

Gulf interior? What is the age of the outflowing Florida Strait waters?

3. What is the circulation below 1500 m that results in the replenishment of the deep dissolved

oxygen throughout the Gulf? What mechanisms are there for horizontal advection in the deep

waters of the Gulf? What are the vertical fluxes involved and what mechanisms cause them? How

do deep waters from the Yucatan Channel mix into the interior of the Gulf? What mechanisms

operate in the four quadrants of the Gulf and how do they compare?

4. What is the Gulf-wide horizontal structure of dissolved oxygen distributions below 1500 m

and in the oxygen minimum zone?
120

5. How will dissolved oxygen concentrations in the Gulf of Mexico change through time with

global warming?

6. What is the carbon cycle in the deepwater Gulf of Mexico and how is it maintained?

Each of these can be addressed by a combination of (1) high quality field measurements of

dissolved oxygen, currents, and other parameters specific to the particular question and (2) detailed

modeling studies using coupled physical-biogeochemical, primitive equation models. Some of these

issues would require repeated measurements every 5 years or so to monitor whether the system is

changing.

The gap of greatest application to the goals and responsibilities of the Minerals Management

Service, however, concerns understanding the details of the effects of hydrocarbons on dissolved

oxygen concentrations in specific local situations. This study has shown that, on large scales, the

dissolved oxygen concentrations have not changed over the 80 years during which data have been

collected. This is despite increasing anthropogenic inputs of hydrocarbons to the environment.

However, the data sets available are not adequate to determine whether local changes may have

occurred or to determine the local effects of specific anthropogenic inputs. Furthermore, the

complex interactions between the physical, chemical, biological, and geological factors, associated

with both the anthropogenic input itself and the environmental conditions, that determine whether

specific anthropogenic inputs will have substantial local effects on the dissolved oxygen content of

the local oceanic ecosystem, are not well understood or modeled.

6.4 Recommendation

To determine whether anthropogenic activities are locally affecting dissolved oxygen levels will

require local monitoring of dissolved oxygen. A study designed to assess the nature and extent of

the effects on dissolved oxygen concentrations of discharges at selected sites of oil and gas

exploration and production operations in the deepwater Gulf, or perhaps at natural seeps to provide

a natural experiment to determine important parameters for later modeling studies, should be

undertaken. Monitoring of discharges should include measurements to determine the fate of drill

cuttings, drilling fluids, produced waters, and similar discharges. Local environmental conditions,

such as currents, waves, temperature, salinity, nutrients, carbon, particulate matter, and other

chemical properties should be monitored at the selected locations to allow determination of the

importance of the various complex of factors involved and to provide information that would allow

development of an oil spill model that could assess possible effects on dissolved oxygen of a

hypothetical subsurface blowout. Only if substantive effects are shown to occur should detailed

modeling, process, or other studies be undertaken.
121

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The Department of the Interior Mission

As the Nation's principal conservation agency, the Department of the Interior has responsibility 

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