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STUDY TITLE: Deepwater Program: Understanding the Processes that Maintain the 

Oxygen Levels in the Deep Gulf of Mexico 

REPORT TITLE: Understanding the Processes that Maintain the Oxygen Levels in the 

Deep Gulf of Mexico, Synthesis Report 

CONTRACT NUMBER: 1435-01-02-CT-85080 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREAS: Western, Central, and Eastern Gulf of Mexico 

FISCAL YEARS OF PROJECT FUNDING: 2002, 2003, 2004, and 2005 

COMPLETION DATE OF REPORT: May 2005 

COSTS: FY 2003 $116,576; FY 2004 $37,754; CUMULATIVE PROJECT COST: 

$154,330 

PROJECT MANAGER: Ann E. Jochens 

AFFILIATION: Texas A&M University 

ADDRESS: Department of Oceanography, 3146 TAMU, College Station, TX 77843-

3146 

PRINCIPAL INVESTIGATORS: Leslie C. Bender, Steven F. DiMarco, Matthew K. 

Howard, Ann E. Jochens, Mahlon C. Kennicutt II, John W. Morse, and Worth D. Nowlin, 

Jr. 

KEY WORDS: Dissolved oxygen, Gulf of Mexico 

BACKGROUND: With the increase of oil and gas activities in the deepwater Gulf of 

Mexico, the Minerals Management Service (MMS), Gulf of Mexico Outer Continental 

Shelf Region, seeks a greater understanding of the environment in Gulf waters deeper 

than 400 m to aid in making decisions about the protection of the marine environment. 

Because discharges of oil or gas can consume oxygen during their 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. This 

summary presents the main results of the Dissolved Oxygen Study. 

OBJECTIVES: 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. 

DESCRIPTION: The study area is that part of the Gulf of Mexico in water depths ≥ 400 

m. Vertically, it extends from the sea surface to sea floor. The four project components 

were: (1) conduct a data search, synthesis, and reanalysis of available historical oxygen 

data collected from water depths greater than 400 m throughout the entire Gulf of 

Mexico basin (note 200 m was used to encompass the entire area offshore of the shelf 
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edge); (2) develop 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) identify data gaps including 

an evaluation of the quality of the data and how representative they are of the Gulf’s 

basin environment; and (4) propose methods for filling the data gaps which could 

include field work. 

An inventory of dissolved oxygen data sets was compiled, and as many of these data 

sets as possible were obtained from MMS, the National Oceanographic Data Center, 

other Federal and State agencies, national laboratories, universities, Mexican 

institutions, and the private sector. When available, the temperature and salinity data 

associated with the oxygen data also were obtained and assembled into the database. 

Data were processed for quality assurance/quality control (QA/QC). Data were 

compiled into a digital database on CD-ROM. Dissolved oxygen data basically fall into 

two types: 1) discrete values from iodometric analysis of water samples collected from 

Niskin or Nansen bottles, or 2) continuous values from polarographic oxygen sensors or 

other sensors mounted on CTD/STD packages. Both data types were assembled. The 

number of stations with discrete oxygen data is approximately 3,600 from over 200 

cruises. There are about 1,400 CTD casts with oxygen sensor data, but the data quality 

is unknown. Results of the data reanalysis, box model study, gaps identification, and 

recommendations are presented in the Study Results section below. 

SIGNIFICANT CONCLUSIONS: The dissolved oxygen database, covering the period 

1922-2001, is adequate to study basin-scale distributions in the Gulf of Mexico, but not 

local distributions or anthropogenic effects thereon. Sources of dissolved oxygen are 

air-sea exchanges at the sea surface and local contributions from photosynthesis in the 

photic zone (the upper 100-200 m). For deeper waters, the major source is transport of 

relatively dense, oxygen-rich water masses through the Yucatan Channel into the Gulf 

interior. The major sink of oxygen in the Gulf at all depths is oxidation of organic matter. 

Because the deep waters are well-ventilated by transport processes, the Gulf of Mexico 

Basin is in no danger of becoming anoxic due either to natural processes, oil and gas 

production, or other anthropogenic effects. Decreased oxygen levels, however, could 

occur in localized areas. Comparison of data sets by decade indicates there has not 

been any discernible change in the vertical or horizontal distribution of dissolved 

oxygen, suggesting 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. A study of the effects of specific 

anthropogenic activities on local dissolved oxygen concentrations is recommended to 

determine the extent, nature, and significance of any such localized effects. The 

mechanisms that transport oxygen-rich water masses from the Yucatan Channel into 

the Gulf interior at depths greater than approximately 1,000 m are not well-understood, 

and a study to determine these Gulf-wide processes would be useful for studies of 

pollutant transports at depth. 

STUDY RESULTS: 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 

(~2,000 m) to allow the transport of relatively dense, oxygen-rich deep source waters 
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from the Atlantic Ocean into the Gulf from the Caribbean Sea. 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 sources of dissolved oxygen in the upper waters (~100-200 m) in the Gulf of 

Mexico are from exchanges with the atmosphere everywhere at the sea surface, mixing 

mainly by wind and wave action, and local contributions from photosynthesis in the 

photic zone. The source of dissolved oxygen to the deep waters is the transport and 

mixing of relatively dense, 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. 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 detritus from living organisms, natural hydrocarbon seeps 

that are prevalent in the Gulf, continental detritus washed into the ocean through river 

runoff, and anthropogenic inputs. The rate of oxygen consumption by organic material 

has not been measured in the Gulf of Mexico. 

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. The productivity of the Gulf is not high enough to 

create extreme oxygen minimum zones such as seen elsewhere. 

Data analysis suggests Gulf waters ≥ 1500 m may have slightly different oxygen values 

in three different regions: southeastern Gulf with mean O2 concentrations of 4.99 mL·L1

, northern Gulf with a mean of 4.95 mL·L-1, and southwestern Gulf with mean of 4.87 

mL·L-1. However, the quality of the data is variable enough that this finding is not 

definitive. Comparison of data sets from the 1970s with those from 2000/2001 and 

comparison of decadal mean oxygen profiles indicate there has not been any 

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

Gulf. This suggests that transport mechanisms that replenish oxygen are adequate to 

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

and anthropogenic sources. 

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 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 direct 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 only if the 

material sinks to depth before decaying. But, the database is not adequate to assess 

this likely local effect. 
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Although the natural leakage of hydrocarbons, estimated at 140,000±60,000 tonnes per 

year, has been occurring for millions of years, dissolved oxygen concentrations 

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

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

content of 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, oxygen concentrations at the sediment-water interface may be 

depleted at the seeps or associated brine pools. The oxygen database is not adequate 

to assess associated local effects. Because they require access to oxygenated waters, 

the presence of chemosynthetic communities at seep sites is additional evidence of 

ventilation of deepest Gulf waters. 

Anthropogenic sources of hydrocarbons to the Gulf are reported at annual estimates of 

2000 tonnes from extraction, 1,600 tonnes from transportation, and 6,800 tonnes from 

consumption. Complete oxidation of the U.S. Gulf oil and gas reserves, which are on 

the order of 10 billion barrels, would consume <1% of the standing oxygen supply. By 

comparison annual discharges from extraction would consume 0.000001% of the 

standing oxygen supply. These effects are not significant alterations of the oxygen 

supply. Catastrophic oil spills can introduce hydrocarbons at 2-3 times the rate of 

natural seeps, but such an input will not significantly alter the oxygen supply. There 

may be local effects from anthropogenic discharges, but the oxygen database is not 

adequate to assess them. 

To determine whether anthropogenic activities are locally affecting dissolved oxygen 

levels 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 is recommended. 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. 

STUDY PRODUCTS: 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. 

Jochens, A. E., M. K. Howard, and S. F. DiMarco. 2004. Recent observations of water 

properties in the Gulf of Mexico. Contributed presentation, 27 January 2004. AGU 

Ocean Ocean Sciences 2004, Portland, OR, 25-30 January 2004. Eos, Trans. AGU, 

84(52), Ocean Sciences Meeting Supplement, Abstract OS31F-01, OS75-76. 
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CD-ROM, Dissolved Oxygen Data Base for the Gulf of Mexico. 2005. Deepwater 

Program: Understanding the Processes that Maintain the Oxygen Levels in the Deep 

Gulf of Mexico. 
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