Organic Geochemistry of Sediments in Nearshore 

Areas of the Mississippi and Atchafalaya Rivers: 

I. General Organic Characterization 

By William H. Orem, Robert J. Rosenbauer, Peter W. Swarzenski, Harry E. Lerch, 

Margo D. Corum, and Anne L. Bates 

Open-File Report 2007-1180 

U.S. Department of the Interior 

U.S. Geological Survey 

1
 U.S. Department of the Interior 

 DIRK KEMPTHORNE, Secretary 

 U.S. Geological Survey 

 Mark D. Myers, Director 

 U.S. Geological Survey, Reston, Virginia 2007 

 Revised and reprinted: 2007 

 For product and ordering information: 

 World Wide Web: http://www.usgs.gov/pubprod

 Telephone: 1-888-ASK-USGS 

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 its natural and living resources, natural hazards, and the environment: 

 World Wide Web: http://www.usgs.gov

 Telephone: 1-888-ASK-USGS 

 

 Suggested citation: 

 Orem, W, and others, 2007, Organic geochemistry of sediments in nearshore areas of the 

 Mississippi and Atchafalaya Rivers: I. general organic characterization: U.S. Geological 

 Survey Open-File Report 2007-1180. 

 Any use of trade, product, or firm names is for descriptive purposes only and does 

 not imply endorsement by the U.S. Government. 

 Although this report is in the public domain, permission must be secured from the 

 individual copyright owners to reproduce any copyrighted material contained within 

 this report. 

 

2
 Table of Contents 

 Page 

List of Figures..................................................................................................................................... 3 

List of Tables...................................................................................................................................... 4 

Conversion Factors............................................................................................................................. 5 

Abstract............................................................................................................................................... 6 

I. Introduction..................................................................................................................................... 7 

II. Study Area and Sampling............................................................................................................... 9 

III. Analytical Methods....................................................................................................................... 14 

 III.A. Elemental Analysis of Sediments...................................................................................... 14 

 III.B. Sediment C1-C6 Hydrocarbon (Gas) Analysis................................................................... 16 

IV. Results........................................................................................................................................... 18 

 IV.A. Elemental Composition of Sediments............................................................................... 18 

 IV.B. Hydrocarbon Gases C1-C6................................................................................................. 48 

V. Discussion...................................................................................................................................... 49 

VI. Conclusions................................................................................................................................... 63 

Acknowledgments............................................................................................................................... 64 

References Cited................................................................................................................................. 65 

List of Figures

Fig. 1. Lower Mississippi and Atchafalaya River coring sites, March 2001, 

 oxic and hypoxic coring sites June and September 2002, and June 2003 coring 

 sites. Numbers next to designated sites refer to numbers listed in Table 1......................... 10 

Fig. 2. Multicorer for collection of near surface sediment cores.................................................... 15 

Fig. 3. Total carbon (% dry wt.) and inorganic carbon (% dry wt.) contents of 

 sediments from cores collected in the lower Mississippi and Atchafalaya 

 River estuaries in 2001........................................................................................................ 24 

Fig. 4. Organic carbon content (% dry wt.) of sediments from cores collected in 

 the lower Mississippi and Atchafalaya River estuaries in 2001.......................................... 25 

Fig. 5. Total nitrogen content (% dry wt.) of sediments from cores collected in 

 the lower Mississippi and Atchafalaya River estuaries in 2001.......................................... 26 

Fig. 6. Total phosphorus contents (% dry wt.) of sediments from cores collected in 

 the lower Mississippi anf Atchafalaya River estuaries in 2001.......................................... 27 

Fig. 7. Total sulfur contents (% dry wt.) of sediments from cores collected in the 

 lower Mississippi and Atchafalaya River estuaries in 2001............................................... 28 

Fig. 8. Atomic C/N (organic carbon/total nitrogen) of sediments from cores collected 

 in the lower Mississippi and Atchafalaya River estuaries in 2001................................... 29 

Fig. 9. Atomic C/P (organic carbon/total phosphorus) of sediments from cores collected 

 in the lower Mississippi and Atchafalaya River estuaries in 2001..................................... 30 

3
Fig. 10. Atomic N/P (total nitrogen/total phosphorus) of sediments from cores collected 

 in the lower Mississippi and Atchafalaya River estuaries in 2001......................................31 

Fig. 11. Atomic C/S (organic carbon/total sulfur) of sediments from cores collected 

 in the lower Mississippi and Atchafalaya River estuaries in 2001..................................... 32 

Fig. 12. Organic elemental data from June 2002 hypoxic-1(●) and oxic-1 (○) sites in the 

 lower Mississippi River system; all results are reported as % dry wt. ............................... 44 

Fig. 13. Organic elemental atomic ratios from June 2002 hypoxic-1(●) and oxic-1 (○) 

 sites in the lower Mississippi River system........................................................................ 45 

Fig. 14. Organic elemental data from the lower Mississippi River system for cores collected 

 in September 2002 from the hypoxic-2 site by multicore (●) and vibracore (■), and 

 the oxic-2 site by multicore (○) and vibracore (□);all results are reported as % dry wt..... 46 

Fig. 15. Elemental atomic ratios from the lower Mississippi River system for cores collected 

 in September 2002 from the hypoxic-2 site by multicore (●) and vibracore (■), and 

 the oxic-2 site by multicore (○) and vibracore (□).............................................................. 47 

Fig. 16. Organic elemental data from cores MR-J03-3 (○) and MR-J03-5 (●) collected off 

 the lower Mississippi River System; all results are reported as % dry wt.......................... 50 

Fig. 17. Elemental atomic ratios of sediments from cores MR-J03-3 and MR-J03-5 collected 

 off the lower Mississippi River System.............................................................................. 51 

Fig. 18. Free (red) and bound (blue) methane concentrations (ppm/g dry wt.) in sediments 

 from cores collected in the lower Mississippi and Atchafalaya River estuaries in 2001.....54 

Fig. 19. Free (red) and bound (blue) ethane concentrations (ppm/g dry wt.) in sediments from 

 cores collected in the lower Mississippi and Atchafalaya River estuaries in 2001............. 55 

Fig. 20. Free (red) and bound (blue) propane concentrations (ppm/g dry wt.) in sediments from 

 cores collected in the lower Mississippi and Atchafalaya River estuaries in 2001............. 56

Fig. 21. Free (red) and bound (blue) butane concentrations (ppm/g dry wt.) in sediments from 

 cores collected in the lower Mississippi and Atchafalaya River estuaries in 2001............. 57

Fig. 22. Free (red) and bound (blue) pentane concentrations (ppm/g dry wt.) in sediments from 

 cores collected in the lower Mississippi and Atchafalaya River estuaries in 2001............. 58

Fig. 23. Free (red) and bound (blue) hexane concentrations (ppm/g dry wt.) in sediments from 

 cores collected in the lower Mississippi and Atchafalaya River estuaries in 2001..............59 

List of Tables

Table 1. Location and description of coring sites in the lower Mississippi 

 and Achafalaya River estuaries........................................................................................... 11 

Table 2. Elemental C, N, P, and S contents of sediments from 2001 USGS 

 cores in the Mississippi River and Atchafalaya River estuaries..........................................20 

Table 3. Elemental C, N, P, and S contents of sediments from 2002-2003 USGS 

 cores in the lower Mississippi River System .......................................................................35 

Table 4. Concentrations of C1 to C6 hydrocarbon gases in sediments from cores 

 collected in the lower Mississippi and Achafalaya River estuaries.....................................52 

Table 5. Accumulation rates (AR) for total carbon (TC), total nitrogen (TN), and 

 total sulfur (TS) at sites MR-J03-3 and MR-J03-5..............................................................62 

4
5

Conversion Factors 

Inch/Pound to SI 

Multiply By To obtain 

Length 

foot (ft) 0.3048 meter (m) 

Volume 

gallon (gal) 3.785 liter (L) 

cubic foot (ft3) 28.32 liter (L) 

Flow rate 

cubic foot per second (ft3/s) 28.32 liter per second (L/s) 

gallon per minute (gal/min) 0.06309 liter per second (L/s) 

Mass 

ounce avoirdupois (oz) 28.35 gram (g) 

pound avoirdupois (lb) 0.4536 kilogram (kg) 

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows: 

°F = (1.8×°C) +32 

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: 

°C = (°F-32)/1.8 

Altitude, as used in this report, refers to distance above the vertical datum. 

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25°C). 

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L). 

SI to Inch/Pound 

Multiply By To obtain 

Length 

meter (m) 3.281 foot (ft) 

Volume 

liter (L) 0.2642 gallon (gal) 

liter (L) 0.03531 cubic foot (ft3

) 

Flow rate 

liter per second (L/s) 0.03531 cubic foot per second (ft3

/s) 

liter per second (L/s) 15.85 gallon per minute (gal/min) 

Mass 

gram (g) 0.03527 ounce avoirdupois (oz) 

kilogram (kg) 2.205 pound avoirdupois (lb) 
Organic Geochemistry of Sediments in Nearshore Areas of the 

Mississippi and Atchafalaya Rivers: I. General Organic 

Characterization 

By William H. Orem, Robert J. Rosenbauer, Peter W. Swarzenski, Harry E. Lerch, 

Margo D. Corum, and Anne L. Bates

Abstract 

 This report presents results on the general organic characteristics of sediment cores collected 

from the coastal zone of the Mississippi River system, including distributions of the important 

nutrient elements (C, N, P, and S). This was part of a larger study conducted from 2001-2005 to 

examine the delivery of sediment-associated contaminants to the Gulf of Mexico by the 

Mississippi River system, funded by the USGS Coastal and Marine Geology Program. 

Companion reports emphasize organic contaminants (Rosenbauer and others, 2006), and metals 

(Swarzenski and others, 2006). The level of contamination within the deltaic system of the 

Mississippi River system was determined through the collection of sediment cores from 

interdistributary bays, and offshore in the Gulf of Mexico, including the zone of hypoxia. Results 

provide the basis for reconstructing contaminant inventories from which to develop historic 

perspectives on nutrient loading and hypoxia, and to better understand how sediment-hosted 

contaminants either directly or indirectly move through biota and ultimately affect ecosystem 

health. 

 Concentrations of C, N, P, and S in sediments varied by a factor of 10 between sites, and in 

down core profiles. Nearshore cores collected in 2001 proved to have erratic downcore C, N, P, 

and S profiles and sediment deposition rates, suggesting a high energy regime controlled more 

by variability in river flow rather than by geochemical processes and reactions within the system. 

These results focused further coring activities further offshore. Atomic C/N ratios suggest that 

organic matter deposited at all sites is a mix of microbial (algal) and terrestrial (vascular plant) 

remains, but with algal material dominant. Concentrations of total sulfur in sediments from cores 

in the zone of hypoxia were often higher than those in nearby zones with oxic water columns. 

Corresponding atomic C/S ratios were typically lower in sediments from sites in the zone of 

hypoxia compared to nearby sites with oxic water columns, and thus atomic C/S values may be 

useful as a proxy for identifying sites impacted by hypoxic conditions in the water column and 

for examining historical trends in hypoxia. At one site examined in this study, maximum hypoxic 

conditions were observed in the mid 1960’s. The organic elemental composition (C, N, P, and S) 

of sediments was also used to guide sample selection for contaminant analysis, and to normalize 

the contaminant data to organic C content of the sediments. 

 Dissolved hydrocarbon gases in sediments showed a dominance of methane, but identifiable 

concentrations of ethane and hexane, and trace concentrations of propane, butane, and pentane 

were also detected. All dissolved gases except hexane were dominated by “bound” gas, gas 

released only after agitation of the sediment in a blender. Hexane, in contrast was observed 

mostly as free gas, determined by headspace analysis.

6
I. Introduction 

 The Mississippi is one of the Earth’s great river systems, ranking seventh worldwide for water 

discharge (580 km3 yr-1), and sixth for suspended sediment discharge (200 million metric tons yr1

), (Meade, 1995). The Mississippi River (MR) and its tributaries drain nearly half of the 

conterminous United States (Spink and others, 1998). Discharge from the Mississippi and 

Atchafalaya Rivers (the two principal mouths of the Mississippi River system) accounts for 

about 90% of the freshwater input to the Gulf of Mexico, and is also the major source of 

sediments, nutrients, and pollutants to the Gulf (Walker, 1996). It is also worth noting that 

approximately 90% of commercial fisheries in the Gulf of Mexico come from the so-called 

fertile crescent, the area affected directly by freshwater runoff (including nutrients) from the 

Mississippi River (Lohrenz and others, 1997). Unlike most of the Earth’s other large river 

systems, however, the Mississippi is highly regulated by manmade structures. About a third of 

the total flow to the Mississippi River is diverted at the Old River Control Structure into the 

Atchafalaya River. The construction of dams and reservoirs along the Mississippi/Missouri River 

system has resulted in substantial decreases in total suspended matter concentrations 

(Vorosmarty and others, 1997). Levees along the lower Mississippi play a major role in 

channeling the river and preventing the river mouth from movement within its natural floodplain 

(Delaney and Craig, 1997). Partly as a result of these various control structures along the MR 

system, large flood events (e.g. the Missouri River flood of 1993) effectively decrease total 

suspended matter concentrations in the lower river in subsequent years by scouring and flushing 

out recently deposited riverbed sediments. 

 Human activities on land account for 80% or more of marine pollution, and rivers are the 

major conduits through which contaminants of terrestrial origin are transported to the sea 

(Meade, 1995; Howarth and others, 2000). Thus, research efforts have focused on the fluvial 

discharge of global rivers to estuaries and the coastal ocean. Surprisingly, while the Amazon and 

Yellow Rivers have been systematically studied, the MR system has only recently been 

comprehensively investigated (Meade, 1995). As a result, research is needed on many aspects of 

the MR system, such as the role of seasonal storage and resuspension in the off-continent 

material flux budget (especially nutrients and other pollutants), the interplay between sediment 

supply, subsidence and wetland loss, and the geographic extent of historic/modern hypoxia (e.g. 

low levels of dissolved oxygen in surface water, from 1-30% saturation). 

 Previous work (Meade, 1995) has shown that the Mississippi River system, which drains one 

of the most industrialized corridors of the world, exhibits surprisingly low concentrations of 

many dissolved elements. This observation has been found to be generally true for other turbid 

rivers of the world. In turbid rivers, many contaminants are likely to be associated with 

suspended sediments, and an accurate assessment of the overall health of such a river is thus 

incomplete if only dissolved constituents are examined. River-borne contaminants (dissolved 

and particulate) discharged into the Gulf of Mexico originate from a wide range of natural, 

industrial, municipal and agricultural inputs. Meade (1995) reports that the combined discharge 

from industrial and municipal point sources accounts for up to 2 % of the total discharge of the 

Mississippi River. This delivery of riverine contaminants impacts coastal ecosystem health and 

productivity, and plays an important role in the formation and intensity of seasonal hypoxic (e.g. 

low oxygen level) events that occur in the Mississippi Bight. 

7
 To address the topic of the delivery of sediment-associated contaminants to the Gulf of 

Mexico by the Mississippi River system, a study was initiated in 2001 by the USGS Coastal and 

Marine Geology Program. This study examined organic contaminant deposition and 

biogeochemical transformations across river-ocean salinity gradients and in recent deltaic 

sediments. The level of contamination within the deltaic systems was determined through the 

collection of sediment cores from interdistributary bays, and offshore in the Gulf. Results 

provide the basis for reconstructing contaminant inventories from which to develop historic 

perspectives on nutrient loading and hypoxia, and to better understand how sediment-hosted 

contaminants directly or indirectly move through biota and ultimately affect ecosystem health. 

Our overall hypothesis is that differential sedimentation of terrigenous, sediment-hosted 

contaminants in the high energy environment of the lower Atchafalaya and Mississippi Rivers 

and their respective receiving basins and flood plains can significantly alter the environmental 

fate and ultimate availability of these constituents. This differential sedimentation is primarily 

due to the discharge of Atchafalaya River waters into a broad, shallow inner shelf environment, 

while Mississippi River waters discharge into the Gulf of Mexico, beyond the shelf break (Fig. 

1.). 

 To address the issue of present and historic hypoxia, we are comparing the sediments from 

sites where the water column is chronically hypoxic to sites that are chronically oxic. Shallow 

sediment cores provide a window into past hypoxic/anoxic events off the Mississippi River. 

Specific biomarkers and other redox proxies when combined with radiometric sediment dating 

(

210Pb and 137Cs) will provide historical information of hypoxic events and associated ecosystem 

shifts. Such a record is invaluable in order to place current hypoxic events into an historical 

perspective (i.e. the anthropogenic imprint on the extent/duration of hypoxia). 

 Few datasets of recent sedimentary processes and organic contaminant distributions and 

loading of the active Mississippi River Delta currently exist. In this report, we address the 

general organic geochemistry of sediments from inshore and offshore sites. Results include 

distributions of the important nutrient elements (C,N,P, and S) and other constituents in sediment 

cores from the inshore and offshore MR (Fig. 1). Other reports emphasize organic contaminants 

(Rosenbauer and others,, 2007), and metals (Swarzenski and others, 2006). The delta is in a 

destructive phase of evolution as evidenced by the rapid conversion of wetlands to open marine 

environs. Sediments deposited in the natural levees and interfluves are now being eroded and 

contaminants stored in these sediments are potentially being released back into the open 

coastal/marine environment. Results from this study attempt to fill this information gap. 

8
9

II. Study Area and Sampling 

 The main study area includes the lower Mississippi and Atchafalaya River Deltas (Fig. 1). 

Sediment coring was conducted in 2001, 2002, and 2003 to examine the organic matter 

composition of sediments. Initial priority was given to sampling and understanding organic 

matter deposited in fine grained sediments in the study area in the context of sediment-hosted 

contaminant transport down the Mississippi and Atchafalaya Rivers. A list of all sites sampled 

for this study is presented in Table 1. Sampling in 2001 focused on five nearshore sites: 2 off the 

lower Mississippi River and 3 near the mouth of the Atchafalaya River (Fig. 1). From the initial 

coring effort in 2001, we observed highly irregular and erratic deposition rates in the region 

closest to the Mississippi and Atchafalaya Rivers (Swarzenski and others, 2006). This indicates a 

highly energetic and geochemically unpredictable depositional system that is controlled more by 

the sheer physical energy of flow variability rather than by geochemical processes and reactions 

within the system. Based on the initial results, our strategy evolved to concentrate on sampling 

sediments more distant from the MR delta. 

 During 2002 and 2003, we collected a suite of undisturbed sediment cores that span a wide 

range of sedimentation rates, degree of hypoxia, total organic carbon concentrations, and 

variable influences of the MR system (Fig. 1). These cores provide the opportunity to reconstruct 

organic matter deposition and organic contaminant inventories and time series across transects 

that should better constrain off-continent material flux and examine the role of water column 

hypoxia in relation to contaminant bioavailablility and biologic uptake. A mix of short and long 

cores were collected for organic matter analysis in this 
June 2003 Sites 

(10) MR JO3-3 

(11) MR JO3-5 

sediment 

deposition 

zone 

Shelf Break 

March 2001 Sites September 2002 Sites 

(3,4) Atchafalaya 

North 

(5) Atchafalaya 

South 

(2) South Pass 

(1) Pass Raphael 

(7, 9) Hypoxic-1 

Hypoxic-2 

(6, 8) Oxic-1 

Oxic-2 

sediment 

movement 

off shelf 

Fig. 1. Map of lower Mississippi and Atchafalaya River showing coring sites, March 2001 , oxic 

and hypoxic coring sites June and September 2002, and June 2003 coring sites. Numbers next to 

designated sites refer to numbers listed in Table 1. Note the location of the shelf break near the 

Mississippi River mouth, facilitating movement of sediment into deep water, compared to large 

zone of sediment deposition near discharge of Atchafalaya River. 

10
Table 1. Location and description of coring sites in the lower Mississippi and Achafalaya River estuaries. 

Site Date Lat. Long. Figure Description 

March 2001 Cores

(1) Mississippi River 3/14/2001 29 13.014 N 89 07.222 W 2 Brackish water site on east side of 

Pass Raphael Site 

Core 010314-5 

(2) Mississippi River 3/14/2001 29 04.383 N 89 06.933 W 2 Marine site at southern tip of delta 

South Pass Site 

Core 010314-6 

(3) Atchafalaya River 3/19/2001 29 30.468 N 91 21.630 W 2 Mouth of Atchafalaya River as it 

North Site 1 Achafalaya Bay; western side of a 

Core 010319-1 group of barrier islands 

(4) Atchafalaya River 3/20/2001 29 27.402 N 91 20.298 W 2 Close to Atchafalaya River North 

North Site 2 but somewhat further southeast 

Core 010320-1 

(5) Atchafalaya River 3/18/2001 29 23.100 N 91 25.098 W 2 Leeward side of barrier islands se

South Site Atchafalaya Bay from the Gulf of 

Core 010318-1 

June 2002 Cores

(6) Mississippi River 6/11/2002 29 20.387 N 89 54.088 W 2 Multicore collected at site in 

Oxic - 1 

continuously oxic surface water zone. 

(7) Mississippi River 6/12/2002 28 56.107 N 90 18.139 W 2 

Multicore collected at site in hypoxic 

Hypoxic - 1 surface water zone. 

11
12

Table 1. Continued. 

Site Date Lat. Long. Figure Description 

September 2002 Cores

(8) Mississippi River 9/17/2002 29 21.984 N 89 55.052 W 2 Multicore collected at site in 

Oxic-2 

continuously oxic surface water zone. 

Core RCE 100_03A 

(8) MR Oxic - 2 9/17/2002 29 21.984 N 89 55.052 W 2 Vibracore collected at site in 

Vibracore 

continuously oxic surface water zone. 

Core RCE 101B Orca-2 

(9) MR Hypoxic - 2 9/18/2002 28 56.059 N 90 18.257 W 2 

Multicore collected at site in hypoxic 

Multicore surface water zone. 

Core RCE 102_02A 

(9) MR Hypoxic - 2 9/18/2002 28 56.059 N 90 18.257 W 2 

Vibracore collected at site in hypoxic 

Vibracore surface water zone. 

Core RCE_103B - Hypo2 

June 2003 Cores

(10) Mississippi River 6/19/2003 28 37.762 N 90 00.620 W 3 Multicore collected from area wit

MR-J-03-3 water column; water depth 226 m 

(11) Mississippi River 6/20/2003 28 55.501 N 90 22.501 W 3 Multicore collected from area wit

MR-J-03-5 hypoxic water column; water dept
study. Short multicores (up to 50 cm depth) were collected using an Ocean Instruments, Inc. multicorer (Fig. 2), to examine processes at the sediment/water interface and changes in deposition during 

the last century. Longer vibracores, up to 300 cm depth were collected to examine geochemistry over 

longer time scales. Sediments were extruded in a vertical orientation on shipboard or shortly after 

return to land. Clean techniques were used during sediment extrusion and sectioning to avoid 

contamination. Short multicores were sectioned in 1 cm intervals to the bottom of the core, while 

longer vibracores were sectioned every 2 cm down to 50 cm, and in 5 cm increments below this. 

Sediment sections were immediately transferred to clean glass jars (I-Chem jars), frozen on dry ice, 

and shipped frozen back to laboratory facilities (Reston, VA and Menlo Park, CA) for organic 

geochemical studies. 

 Any use of trade, product, or firm names is for descriptive purposes only and does not imply 

endorsement by the U.S. Geological Survey. 

13
III. Analytical Methods 

A. Elemental Analysis of Sediments 

 Total and organic carbon (TC and OC), total nitrogen (TN), and total sulfur (TS) contents of 

sediments were determined using a Leco 932 CNS Analyzer (Leco Corporation, St. Joseph, MI, USA). 

TC, TN, and TS were measured directly, after drying the sediment overnight at 60° C. OC was 

determined after removal of inorganic carbon (IC), mostly carbonates, which constitutes a variable 

fraction (0-50%) of the TC content of sediments from these cores. We used an acid vapor method to 

remove the IC, slightly modified from that of Hedges and Stern (1984) and Yamamuro and Kayanne 

(1995). Sediment samples (5 to 6 mg) were weighed into prebaked (450° C) silver cups, placed in an 

acid vapor chamber (dessicator with beakers of concentrated HCl in the bottom), allowed to react for a 

minimum of 48 hrs, dried (60° C), and analyzed. In addition to samples, blanks and calcium carbonate 

standards were also placed in the acid vapor chamber. Blanks consisted of a prebaked (450° C) empty 

silver cup, and were used to monitor organic C contamination during the procedure. Calcium carbonate 

standards were used to visually determine when the acid vapor treatment was 

14
Fig. 2. Multicorer for collection of near surface sediment cores. 

15
complete, and as a check on the efficiency of the acid vapor treatment. All samples were analyzed at 

least induplicate. We have used this approach previously to examine TC, OC, IC, TN, and TS 

concentrations in marine sediments (Orem and others, 1999). Analytical precision (percentage relative 

standard deviation) was about 2% for TC, 4% for OC and TS, and 3% for TN. IC is reported as the 

calculated difference between TC and OC (e.g. %TC – %OC = %IC ). 

 Total phosphorus (TP) concentrations in sediments were determined by the method of Aspila and 

others (1976), slightly modified for work in clastic sediments from the lower MR System. Samples 

were dried overnight (60° C), cooled to room temperature in a dessicator, weighed, and placed in 

precleaned (soaked in 10% HCl overnight, rinsed with deionized/distilled water, and baked at 450° C) 

ceramic crucibles. Generally, 0.4-0.6 g of the sediment was used for TP analysis. Weighed sediment 

samples were baked at 550° C for 2 hrs., cooled, then transferred into clean plastic centrifuge cones 

containing 45 ml of 1 M HCl. All plastic and glassware used for TP analysis was cleaned by soaking in 

10% HCl overnight, followed by rinsing with deionized/distilled water. The empty crucibles were 

rinsed with 5 ml of 1 M HCl and the rinse was added to the centrifuge cones for a final volume of 50 

ml of 1 M HCl. The samples were extracted in the 1 M HCl for 16 hrs. on a shaker to dissolve the 

phosphate. An aliquot of each extract was centrifuge-filtered using Millipore ultrafree-CL HVPP lowbinding Durapore centrifuge filters (0.45 μm pore size), then neutralized with a 0.1 M NaOH solution, 

and transferred to plastic test tubes. The filtered aliquots were analyzed for phosphate using the 

standard phospho-molybdate method (Strickland and Parsons, 1972), and a Brinkman PC900 

fiberoptic colorimeter. Analytical precision (percentage relative standard deviation) for the TP analysis 

is ± 3%. 

 Elemental ratios (C/N, C/P, N/P, C/S) reported are atomic ratios, calculated after conversion of 

weights of each parameter to molar units. 

 

B. Sediment C1-C6 Hydrocarbon (Gas) Analysis 

 The concentrations of C1

-C6 hydrocarbons (methane to hexane) were determined by headspace 

analysis. Sediments from cores collected at various sites off the Mississippi and Atchafalaya Rivers 

were sectioned into 4 or 5 cm intervals, and each section transferred to metal cans. Each metal can had 

a top containing a silicone rubber septum that was secured in a hole in the top with epoxy cement. A 

small amount of gluteraldehyde was added to the sediment in each can (to retard microbial activity), 

and the top was secured on the paint can. 

 Cans containing the sediment were returned to the laboratory in Reston, VA for gas analysis. A 

known volume of gas (generally 1 ml) in the headspace of the can was extracted using a gas tight 

syringe and analyzed by gas chromatography with flame ionization detection (GC-FID), using an 

Aglient 6890N GC equipped with a 30 meter x .32 mm ID Carbonplot column, and a constant 

hydrogen carrier gas flow of 4ml/min. Analytical conditions for the GC-FID analysis were as follows: 

(1) initial column temperature of 40° C at injection; (2) ramping to 350°C at 15° C/min.; (3) a final 

hold at 350 ° C for 5 min. Concentrations of C1-C6 hydrocarbon gases were determined using an 

external standard method, generated using C1

-C6 gas standards of 100 ppm and 1000 ppm (Supelco). 

Gas blanks were determined from analysis of degassed water head space. The headspace in each can 

was analyzed for gaseous hydrocarbons (C1-C6) at least in duplicate. The average gas content from 

these analyses represents the “free” gas present. 

16
 After completion of the “free” gas analysis, the volume of headspace in each can was determined by 

filling with degassed water, and the contents of each can were transferred to a specially modified 

stainless steel blender for “bound” gas determination using degassed water. The blender was a 

commercial product, modified to include a septum that was attached with epoxy cement to a hole in the 

top of the blender. Degassed water was added to the sample in the blender to adjust the blender 

headspace to approximately 45 mL. The sediment sample in the blender was homogenized for 5 min. 

at high speed, and a 1 ml sample of gas in the headspace was removed from the septum in the top of 

the blender in a gas tight syringe for analysis. Concentrations of “bound” hydrocarbons were 

determined by GC-FID using the method described for the “free” hydrocarbon analysis. The volume of 

headspace in the blender was determined for each sample by adding degassed water. The terms “free” 

and ‘bound” hydrocarbon gas are used operationally here, and imply only ease of extraction, not 

necessarily the molecular state of the gaseous hydrocarbons in the sediments. After subsampling for 

“bound” hydrocarbons, the sample was transferred from the blender to a preweighted plastic 500 mL 

beaker, and dried in oven at 110 C for 48-72 hours. The beaker was then weighed and dry weight of 

sample determined. Final quantification of the gas content of each sediment sample was calculated 

using the GC-FID data, the headspace volumes, and the dry weight of each sediment sample 

(determined following the “bound” hydrocarbon analysis). Concentrations are reported as ppm gas per 

g sediment (dry weight basis). 

17
IV. Results 

 

A. Elemental Composition of Sediments 

 2001 coring sites - The elemental composition (TC, OC, IC, TN, TP, TS) of the sediments from 

cores collected in 2001 from various locations in the lower Mississippi and Atchafalaya River Deltas is 

presented in Table 2. All TC, OC, IC, TN, TP, and TS concentrations are reported as percent of total 

sediment on a dry weight basis. Atomic C/N, C/P, C/S, and N/P ratios are also presented in Table 2. 

Results are plotted in Figs. 3-11. Total C contents in these cores ranged from 0.5% to 5.5%, OC 

contents from 0.2% to 3.9%, and IC from below detectable levels to 4.5%. The highest total C contents 

were observed at the surface of the Atchafalaya South site core, but this was primarily inorganic C in 

composition. Highest organic C contents were observed at the Pass Raphael site in the lower 

Mississippi River. Downcore variations in total, organic and inorganic C contents were as great or 

greater than the site to site variations, and downcore profiles were irregular with subsurface maxima 

common in most of these cores. 

 TN values range from 0.03% to 0.32% in the 2001 cores from the lower Mississippi and 

Atchafalaya Rivers (Table 2). The TN data tended to follow the OC data, and the highest total N values 

were observed at the Pass Raphael site. Downcore profiles for TN generally followed those for OC, 

and were irregular with depth with numerous subsurface maxima in the cores. TS values ranged from 

<0.001% to 1.25% in these cores. The highest TS values were observed at the Pass Raphael site (as 

were TN and OC), and TS values tended to follow those for OC at each core site. TP values ranged 

from 0.032% to 0.073% in these cores. Highest TP values were observed at the South Pass and Pass 

Raphael sites. Downcore TP profiles did not resemble those for OC, TN, and TS, suggesting that 

factors influencing deposition and biogeochemical cycling of P are somewhat different than those for 

C, N, and S. 

 Atomic C/N ratios (Table 2 and Fig. 8) range from 7 to 18, but with most values between 8 and 

15. Atomic C/N values in the range of 8-15 are usually indicative of a mix of algal and vascular plant 

material contributing to the organic matter in the sediments (Tissot and Welte, 1984). The Atchafalaya 

north and south sites had the overall lowest atomic C/N values. The Atchafalaya north 2 site had some 

of the highest atomic C/N values. This site is near the mouth of the Atchafalaya River and should 

receive significant riverine discharge of terrestrially derived (vascular plant) organic matter from 

wetlands along the river and from further upstream. In several of the cores, subsurface maxima in OC 

content are correlated with maxima in atomic C/N, indicating that these maxima are largely driven by 

input of terrestrially derived (vascular plant) organic matter.C/N ratios (Table 2 and Fig. 8) range from 

7 to 18, but with most values between 8 and 15. 

 Atomic C/P (Table 2 and Fig. 9) values range from 27 to 167 in these cores, but with most values 

between 30 and 75. This range of values for atomic C/P is somewhat low for algal- or vascular plantderived organic matter (e.g. pure algae should have a C/P ratio of 106), suggesting a source of excess P 

entering the lower Mississippi River Delta area (Tissot and Welte, 1984). The highest atomic C/P 

values were observed at the Pass Raphael site. This is largely driven by the high OC values, especially 

over the 10-60 cm depth interval. Atomic N/P ratios ranged from 2.1 to 11.5 (Table 2 and Fig. 10), 

which are also relatively low relative to atomic N/P values expected for algal- and vascular plantderived organic matter (Tissot and Welte, 1984). The atomic N/P values also suggest a source of 

18
19

excess P to these sites. Atomic N/P values were highest at the Pass Raphael site, similar to atomic C/P 

values. Downcore C/P and N/P values generally follow the OC profiles. 

 Atomic C/S values (Table 2 and Fig. 11) have a large range of 5.5 to 2480, but with most values 

between 25 and 100. The lowest atomic C/S values were observed at the South Pass site, and at Pass 

Raphael above 60 cm. The low atomic C/S values here are largely driven by high TS concentrations. 

The Atchafalaya sites have generally lower TS concentrations, and higher atomic C/S values. The Pass 

Raphael site has an unusual atomic C/S vertical profile, with a sharp change at about 60 cm depth. 

 Oxic/Hypoxic sites – The elemental composition (TC, OC, IC, TN, TP, and TS) and elemental 

ratios (atomic C/N, C/P, C/S, and N/P) of organic matter in sediments from the hypoxic and oxic sites 

sampled in June and September 2002 (hypoxic-1 and 2, and oxic-1 and 2 on the map in Fig. 1) are 

presented in Table 3 and in Figs. 12-15. At the oxic sites, TC concentrations in sediments range from 

about 1% to 10% in all cores, compared to a range of only 1.6% to 0.9% at the hypoxic sites. Organic 

C contents ranged from 0.6% to 10% at the oxic sites, and from 0.4% to 1.4% at the hypoxic site. 

Inorganic C varied from undetectable (<0.01%), up to 0.5% at the hypoxic sites and 0.9% at the oxic 

sites. Average values for TC, OC, and IC, respectively, for all samples were: 2.76%, 2.56%, and 0.24% 

at the oxic sites; and 1.08%, 0.721%, and 0.37% at the hypoxic sites. The higher TC and OC content of 

sediments at the oxic site was driven largely (but not exclusively) by a large maximum in total and 

organic C (up to 10% TC and OC) in the 25-100 cm depth interval at the oxic site 

(Fig. 14). The hypoxic site also exhibited an increase in TC and OC over the 25-100 cm depth interval, 

but the increase was 6-7x smaller than that observed at the oxic site. 

 TN concentrations in sediments ranged from 0.07% to 0.7% at the oxic sites, and 0.05% to 0.1% 

at the hypoxic sites (Table 3, Figs. 12 and 14). Average values for TN for all oxic- and hypoxic-1 and 

2 samples were: 0.2% at the oxic sites and 0.09% at the hypoxic sites. Downcore profiles for TN 

paralleled those for OC at both the oxic and hypoxic sites. As with TC and OC, the higher TN content 

of sediments at the oxic sites was largely, but not exclusively, due to the large increase in organic 

matter deposited in the 25-100 cm depth interval at the oxic sites. 

 In contrast to carbon and nitrogen, TP values were comparable at the oxic and hypoxic sites: range 

of 0.03% to 0.07% at the oxic sites (average for all samples, 0.05%); and a range of 0.05% to 0.07% at 

the hypoxic sites (average for all samples, 0.06%), (Table 3). The downcore profile for TP did not 

exhibit the large maximum in the 25-100 cm interval that was observed for TC, OC and TN (Figs. 12 

and 14). 

 TS concentrations in sediments ranged from 0.1% to 3% at the oxic sites, and from 0.03% to 1.5% 

at the hypoxic sites (Table 3). Average TS concentrations for all samples were about 0.7% at the oxic 

site and about 0.4% at the hypoxic site. Sediments from hypoxic sites would be expected to have 

higher TS concentrations relative to oxic sites, assuming comparable organic matter contents. This was 

observed at the hypoxic 1 and oxic 
20

Table 2. Elemental composition (% dry wt.) of sediments from 2001 USGS cores in the Mississippi and Atchafalaya Rivetotal C (TC), organic C (OC), inorganic C (IC), total N (TN), total P (TP), total S (TS) and atomic C/N, C/P, C/S, and N/P 

Sample Depth TC OC IC TN TP TS C/N C/P C/S 

(cm) (%) (%) (%) (%) (%) (%) (Atomic) (Atomic) (Atomic

Mississippi River 0-2 2.11 1.88 0.23 0.184 0.0733 0.191 11.9 66.1 26.3 Pass Raphael Site 2-4 1.38 0.951 0.43 0.131 0.0609 0.178 8.46 40.3 14.3 

Core 010314-5 4-6 0.955 0.586 0.37 0.080 0.0443 0.286 8.54 34.1 5.47 

6-8 1.64 1.28 0.36 0.140 0.0543 0.444 10.7 60.8 7.70 

8-10 2.81 2.46 0.36 0.249 0.0576 0.932 11.5 110 7.05 

10-12 2.18 1.98 0.20 0.194 0.0581 0.765 11.9 87.9 6.91 

12-14 1.80 1.68 0.12 0.178 0.0642 0.424 11.0 67.5 10.6 

14-16 1.93 1.80 0.13 0.186 0.0716 0.470 11.3 64.8 10.2 

16-18 1.91 1.77 0.14 0.183 0.0719 0.434 11.3 63.5 10.9 

18-20 1.88 1.72 0.16 0.186 0.0678 0.532 10.8 65.4 8.63 

20-25 2.22 2.08 0.14 0.193 0.0784 0.526 12.6 68.4 10.6 

25-30 2.05 1.99 0.06 0.197 0.0701 0.654 11.8 73.2 8.12 

30-35 2.32 2.19 0.13 0.211 0.0599 0.834 12.1 94.3 7.01 

35-40 3.18 3.03 0.15 0.265 0.0628 1.15 13.3 124 7.03 

40-45 3.61 3.48 0.13 0.288 0.0621 1.06 14.1 144 8.76 

45-50 3.76 3.94 0.00 0.316 0.0607 1.25 14.5 167 8.41 

50-60 1.31 1.17 0.14 0.117 0.0537 0.447 11.7 56.2 6.99 

60-70 0.774 0.638 0.14 0.064 0.0502 0.021 11.6 32.8 81.1 

70-80 0.809 0.588 0.22 0.061 0.0523 0.015 11.2 29.0 105 

80-90 1.03 0.838 0.20 0.090 0.0549 0.054 10.8 39.4 41.4 

90-100 0.925 0.773 0.15 0.064 0.0536 0.026 14.1 37.2 79.4 

100-106 0.870 0.644 0.23 0.055 0.0543 0.062 13.6 30.6 27.7 

Mississippi River 0-2 1.90 1.52 0.38 0.158 0.0644 0.199 11.2 60.7 20.4 

South Pass Site 2-4 2.00 1.58 0.42 0.167 0.0630 0.196 11.0 64.7 21.5 

Core 010314-6 4-6 2.32 1.74 0.58 0.208 0.0769 0.226 9.76 58.3 20.6 

6-8 2.09 1.56 0.53 0.180 0.0666 0.236 10.1 60.4 17.6 
21

Table 2. Continued 

Sample Depth TC OC IC TN TP TS C/N C/P C/S 

(cm) (%) (%) (%) (%) (%) (%) (Atomic) (Atomic) (Atomic

Mississippi River 8-10 2.65 2.00 0.65 0.187 0.0659 0.279 12.5 78.3 19.1 South Pass Site 10-12 1.37 1.03 0.34 0.118 0.0616 0.170 10.2 43.1 16.2 

Core 010314-6 12-14 2.60 2.11 0.49 0.234 0.0688 0.305 10.5 79.1 18.5 

14-16 2.03 1.59 0.44 0.164 0.0690 0.208 11.3 59.4 20.4 

16-18 2.07 1.61 0.46 0.179 0.0672 0.194 10.5 61.8 22.2 

18-20 2.23 1.69 0.54 0.151 0.0654 0.222 13.0 66.6 20.3 

20-25 1.86 1.52 0.35 0.158 0.0659 0.201 11.2 59.5 20.2 

25-30 2.25 1.78 0.47 0.217 0.0686 0.296 9.56 66.9 16.0 

30-35 2.39 2.10 0.29 0.240 0.0692 0.303 10.2 78.2 18.5 

35-40 2.19 1.68 0.51 0.171 0.0589 0.291 11.4 73.6 15.4 

40-45 1.26 0.873 0.39 0.106 0.0635 0.183 9.60 35.4 12.7 

45-50 2.06 1.47 0.59 0.166 0.0646 0.252 10.3 58.6 15.6 

50-60 1.56 1.04 0.52 0.143 0.0640 0.218 8.48 41.9 12.7 

60-70 1.62 1.19 0.43 0.175 0.0660 0.227 7.93 46.5 14.0 

70-74 2.43 2.06 0.37 0.227 0.0663 0.267 10.6 80.1 20.6 

Atchafalaya River 0-2 1.57 1.26 0.31 0.138 0.0634 0.080 10.6 51.2 42.0 

North Site 2-4 1.06 0.721 0.34 0.090 0.0528 0.034 9.34 35.2 56.6 

Core 010319-1 4-6 1.01 0.727 0.28 0.093 0.0529 0.028 9.12 35.4 69.3 

6-8 1.45 1.10 0.35 0.135 0.0526 0.080 9.50 53.9 36.7 

8-10 1.78 1.45 0.33 0.173 0.0597 0.126 9.77 62.6 30.7 

10-12 1.34 1.09 0.24 0.127 0.0538 0.068 10.0 52.2 42.8 

12-14 1.07 0.806 0.26 0.098 0.0521 0.042 9.59 39.9 51.2 

14-16 0.85 0.594 0.26 0.078 0.0511 0.046 8.88 30.0 34.5 

16-18 1.21 0.986 0.23 0.114 0.0626 0.071 10.1 40.6 37.1 

18-20 0.806 0.619 0.19 0.072 0.0508 0.020 10.0 31.4 82.6 

20-25 0.800 0.511 0.29 0.060 0.0457 0.056 9.93 28.8 24.4 
22

Table 2. Continued 

Sample Depth 

(cm) 

TC 

(%) 

OC 

(%) 

IC 

(%) 

TN 

(%) 

TP 

(%) 

TS 

(%) 

C/N 

(Atomic) 

C/P 

(Atomic) 

C/S 

(Atomic

Atchafalaya River North Site 

Core 010319-1 

Archafalaya River 

North Site 2 Core 010320-1 

25-30 30-35 

35-40 

40-45 45-50 

50-60 

60-70 

0-2 

2-4 4-6 6-8 

8-10 

10-12 

12-14 

14-16 

16-18 

18-20 

20-25 

25-30 

30-35 

35-40 

40-45 

45-50 

50-60 

60-70 

0.855 0.753 

0.502 

0.808 0.855 

1.07 

1.28 

1.74 

0.998 0.785 0.675 0.754 

1.11 

1.13 

0.508 

0.734 

0.934 

1.09 

1.08 

1.10 

1.29 

1.38 

1.19 

1.10 

0.991 

0.625 0.536 

0.325 

0.596 0.613 

0.807 

1.03 

0.719 

0.719 0.369 0.441 0.401 

0.891 

0.723 

0.206 

0.471 

0.804 

0.874 

1.05 

1.09 

1.33 

1.48 

1.17 

0.940 

0.579 

0.230.22

0.18

0.210.24

0.260.25

1.03

0.280.420.230.35

0.22

0.41

0.30

0.26

0.13

0.22

0.03

0.01

0 

0 

0.02

0.16

0.41

0.079 0.064 

0.046 

0.083 0.060 

0.100 0.120 

0.088 

0.046 0.045 0.041 0.041 

0.081 

0.082 

0.032 

0.045 

0.062 

0.070 

0.096 

0.100 

0.127 

0.140 

0.093 

0.094 

0.096 

0.0494 0.0426 

0.0362 

0.0537 0.0465 

0.0532 0.0548 

0.0597 

0.0480 0.0322 0.0389 0.0392 

0.0499 

0.0464 

0.0322 

0.0382 

0.0458 

0.0501 

0.0540 

0.0557 

0.0571 

0.0641 

0.0519 

0.0521 

0.0548 

0.026 0.071 

<0.001 

<0.001 <0.001 

<0.001 

0.024 

0.062 

0.041 0.024 0.026 0.031 

0.036 

0.030 

0.014 

0.024 

0.042 

0.034 

0.044 

0.046 0.084 

0.074 

0.082 

0.047 

0.046 

9.23 9.77 

8.24 

8.37 11.9 

9.41 

10.0 

9.53 

18.2 9.56 12.5 11.4 

12.8 

10.3 

7.51 

12.2 

15.1 

14.6 

12.8 

12.7 12.2 

12.3 

14.7 

11.7 

7.03 

32.6 32.4 

23.2 

28.6 34.0 

39.1 

48.5 

31.0 

38.6 29.6 29.2 26.4 

46.0 

40.2 

16.5 

31.8 

45.3 

45.0 

50.1 

50.5 60.1 

59.5 

58.1 

46.5 

27.2 

64.2 20.2 

>868 

>1590 >1640 

>2150

114 

31.0 

46.8 41.0 45.3 34.5 

66.1 

64.3 

39.3 

52.4 

51.1 

68.6 

63.7 

63.3 42.3 

53.4 

38.1 

53.4 

33.6 
23

Table 2. Continued 

Sample Depth TC OC IC TN TP TS C/N C/P C/S 

(cm) (%) (%) (%) (%) (%) (%) (Atomic) (Atomic) (Atomic

Archafalaya River 70-80 0.978 0.619 0.36 0.078 0.0480 0.044 9.25 33.2 37.6 North Site 2 80-90 1.17 0.891 0.28 0.099 0.0533 0.037 13.8 43.1 64.3 

Core 010320-1 

Atchafalaya River 0-2 3.41 1.08 2.33 0.100 0.0418 0.086 12.6 46.6 33.5 South Site 2-4 4.63 0.918 3.71 0.092 0.0375 0.032 11.6 49.3 76.6 

Core 010318-1 4-6 5.51 1.01 4.50 0.087 0.0362 0.016 13.5 80.9 168 

6-8 5.02 0.856 4.16 0.072 0.0383 0.009 13.9 56.7 254 

8-10 4.62 0.931 3.69 0.085 0.0431 0.001 12.8 61.2 2480 

10-12 2.39 0.840 1.55 0.111 0.0568 0.146 8.82 43.4 15.4 

12-14 1.73 1.02 0.71 0.122 0.0581 0.143 9.75 56.7 19.0 

14-16 1.79 1.06 0.73 0.129 0.0580 0.210 9.58 84.9 13.5 

16-18 2.14 0.940 1.20 0.120 0.0544 0.144 9.14 63.4 17.4 

18-20 2.75 1.03 1.70 0.124 0.0560 0.097 9.69 58.0 28.3 

20-25 1.61 1.27 0.34 0.152 0.0618 0.134 9.74 65.4 25.3 

25-30 1.45 1.13 0.31 0.140 0.0616 0.124 9.41 54.0 24.3 

30-35 1.45 1.15 0.30 0.140 0.0614 0.135 9.58 53.2 22.7 

35-40 1.39 1.06 0.33 0.139 0.0633 0.131 8.89 47.9 21.6 

40-45 1.45 1.18 0.28 0.159 0.0613 0.144 8.65 47.5 21.9 

45-50 1.54 1.21 0.33 0.155 0.0621 0.167 9.10 60.1 19.3 

50-60 1.59 1.33 0.26 0.178 0.0648 0.167 8.71 65.8 21.3 

60-70 nd nd nd nd nd nd nd nd nd 

70-80 1.55 1.21 0.33 0.163 0.0644 0.130 8.66 65.0 24.8 

80-85 1.49 1.18 0.32 0.159 0.0641 0.129 8.65 57.1 24.4 

85-bt 1.56 1.25 0.30 0.175 nd 0.156 8.33 nd 21.4 
Fig. 3. Total carbon (% dry wt.) and inorganic carbon (% dry wt.) contents of sediments from cores 

collected in the lower Mississippi and Atchafalaya River estuaries in 2001. 

 

24
Fig. 4. Organic carbon content (% dry wt.) of sediments from cores collected in the lower 

Mississippi and Atchafalaya River estuaries in 2001. 

25
Fig. 5. Total nitrogen content (% dry wt.) of sediments from cores collected in the lower 

Mississippi and Atchafalaya River estuaries in 2001. 

26
Fig. 6. Total phosphorus contents (% dry wt.) of sediments from cores collected in the lower 

Mississippi and Atchafalaya River estuaries in 2001. 

27
Fig. 7. Total sulfur contents (% dry wt.) of sediments from cores collected in the lower Mississippi 

and Atchafalaya River estuaries in 2001 (note different scale for Pass Raphael site). 

 

 

 

28
Fig. 8. Atomic C/N (organic carbon/total nitrogen) of sediments from cores collected in the lower 

Mississippi and Atchafalaya River estuaries in 2001. 

29
Fig. 9. Atomic C/P (organic carbon/total phosphorus) of sediments from cores collected in the 

lower Mississippi and Atchafalaya River estuaries in 2001. 

30
Fig. 10. Atomic N/P (total nitrogen/total phosphorus) of sediments from cores collected in the 

lower Mississippi and Atchafalaya River estuaries in 2001. 

31
Fig. 11. Atomic C/S (organic carbon/total sulfur) of sediments from cores collected in the lower 

Mississippi and Atchafalaya River estuaries in 2001. 

32
1 sites (Fig. 12). Sediments at the oxic 2 site, however, had much higher TS concentrations 

compared to hypoxic 2, reflecting the generally higher organic C contents in the oxic 2 sediments 

(Fig. 14). Downcore profiles of TS generally paralleled those for C and N at all oxic and 

hypoxic sites. 

 Overall atomic C/N and C/P values were relatively low at both the oxic and hypoxic sites, 

reflecting greater deposition of microbially-derived (algal) organic matter in these sediments. 

Atomic C/N, C/P, and N/P values were generally higher at the oxic coring sites (Oxic-1 and -2), 

compared to the hypoxic coring sites (Hypoxic-1 and -2), (Figs. 13 and 15). This is consistent 

with a greater proportion of terrestrially-derived organic matter at the oxic sites, though 

deposition of algal material still dominated at the oxic sites. The range of atomic C/N, C/P, and 

N/P values were: 2-23, 30-829, and 3-49, respectively in sediments from the oxic sites, and 6-14, 

21-61, and 2-5, respectively in sediments from the hypoxic sites. Average atomic C/N, C/P, and 

N/P values were: 12.5, 168, and 11.8, respectively in sediments from the oxic sites, and 9.7, 32, 

and 3.4 in sediments from the hypoxic sites. Significant changes in downcore atomic C/N, C/P, 

and N/P values (maxima and minima in the vertical profiles) at all sites, largely reflect changes 

in organic carbon deposition over time. 

 Atomic C/S values ranged from 3.5 to 23 in sediments from oxic site cores, and from 1.3 to 

75 in sediments from the hypoxic cores. Average atomic C/S values were 9.9 for all oxic site 

samples, and 12.8 for all hypoxic site samples. The shallow multicore samples (@ 0-30 cm) had 

average atomic C/S values of 10.5 at the oxic sites and 7.0 at the hypoxic sites. Lower atomic 

C/S values were anticipated in sediments from the hypoxic sites, reflecting possible microbial 

sulfate reduction in the water column and additional deposition of metal sulfides in sediments. 

This was certainly the case at the hypoxic 1 versus oxic 1 sites (Fig. 13). Lower atomic C/S 

values were also observed at oxic 2 compared to hypoxic 2 (Fig. 15). However, comparison of 

atomic C/S values at the oxic 2 and hypoxic 2 sites was complicated by a large difference in 

organic carbon contents between the two locations. The downcore atomic C/S profiles were very 

irregular at the oxic 2 and hypoxic 2 sites. Atomic C/S values showed an overall decrease with 

depth at the oxic 2 and hypoxic 2 sites, probably reflecting both biodegradation of OC and 

buildup of metal sulfides in the sediments from microbial sulfate reduction. 

June 2003 core sites – Elemental C,N,S data and atomic C/N and C/S ratios for sediments 

from cores MR-J03-3 and MR-J03-5 (location map in Fig. 1) are presented in Table 3 and 

plotted in Figs. 16 and 17. Core MR-J03-5 is located near the 2002 hypoxic-1 and 2 core sites 

(Table 1, Fig. 1), and is considered to be in the zone of hypoxia in the Gulf of Mexico near the 

Mississippi River delta (Rabalais and others, 2001 and 2002). Core MR-J03-3 is located further 

offshore, and is considered to have an oxic water column. Only TC was determined in these 

cores. Sediment concentrations of TC have a tight range of 1.4% to 1.7% (average for all 

samples of 1.44%) in core MR-J03-3, and a somewhat larger range of 0.6% to 1.7% (average for 

all samples of 1.13%) in core MR-J03-5. TC concentrations in the MR-J03-3 core were similar 

to those in the oxic-1 core (Fig 12), but different from those in the oxic-2 core, which had a 

pronounced maximum in TC beginning at about 25 cm depth (Fig. 14). Core MR-J03-5 had TC 

concentrations similar to those in the the hypoxic-1 and 2 cores. The downcore profile of TC in 

core MR-J03-3 is relatively constant, with a slight peak at about 3 cm, and several other minor 

peaks downcore. Core MR-J03-5 has a much more variable downcore profile, with a maximum 

TC value at about 3 cm, and a minimum at about 7 cm. Below 3 cm, core MR-J03-3 had higher 

TC concentrations compared to core MR-J03-5. 

33
34

 TN concentrations ranged from 0.13% to 0.22% (average for all samples of 0.17%) in core 

MR-J03-3, and from 0.05% to 0.19% (average for all samples of 0.09%) in core MR-J03-5. As 

with TC, concentrations of TN in sediments from core MR-J03-3 were similar to those in the 

oxic-1 core, but lower than those in oxic-2. This was primarily due to the large maximum in TC 

and TN in the 25-100 cm increment in the oxic-2 core. The MR-J03-5 core has sedimentary TN 

concentrations similar to those at hypoxic-1 and 2. Downcore profiles of TN parallel those for 

TC in both the MR-J03-3 and -5 cores. 

In contrast to TC and TN, which are higher in the MR-J03-03 core, TS values were 

distinctly higher in the MR-J03-5 core compared to the MR-J03-3 core. TS concentrations 

ranged from 0.16% to 0.58% in the MR-J03-5 core (average for all samples of 0.31%), compared 

to 0.16% to 0.24% in the MR-J03-3 core (average for all samples of 0.20%). The downcore 

profile of TS in the MR-J03-3 core resembled that for TC and TN, whereas the downcore profile 

for TS in the MR-J03-5 core was distinctly different from 
Table 3. Elemental composition (% dry wt.) of sediments from 2002-2003 USGS cores in the lower Mississippi River estu

total C (TC), organic C (OC), inorganic C (IC), total N (TN), total P (TP), total S (TS), and atomic C/N, C/P, C/S, and N/P 

Sample Depth TC OC IC TN TP TS C/N C/P C/S 

(cm) (%) (%) (%) (%) (%) (%) (Atomic) (Atomic) (Atomic) (

Oxic 1 0-1 1.77 1.18 0.59 0.157 0.0630 0.349 8.76 48.3 9.02 

Multicore 1-2 1.22 0.816 0.40 0.118 0.0490 0.196 8.06 42.9 11.1 

2-3 1.59 0.974 0.62 0.145 0.0476 0.262 7.83 52.8 9.92 

3-4 2.12 1.67 0.45 0.177 0.0497 0.409 11.0 86.6 10.9 

4-5 2.11 1.61 0.50 0.185 0.0501 0.507 10.1 82.9 8.48 

5-6 1.99 1.68 0.31 0.181 0.0546 0.448 10.8 79.3 10.0 

6-7 1.80 1.53 0.27 0.179 0.0475 0.531 9.97 83.1 7.69 

7-8 1.81 1.48 0.33 0.168 0.0497 0.505 10.3 76.8 7.82 

8-9 1.61 1.41 0.20 0.165 0.0484 0.476 9.96 75.1 7.91 

9-10 1.86 1.69 0.17 0.180 0.0480 0.498 10.9 90.8 9.06 

10-11 1.74 1.55 0.19 0.177 0.0463 0.557 10.2 86.3 7.43 

11-12 2.00 1.80 0.20 0.198 0.0471 0.643 10.6 98.6 7.47 

12-13 1.69 1.45 0.24 0.169 0.0489 0.535 10.0 76.5 7.23 

13-14 1.74 1.53 0.21 0.166 0.0505 0.801 10.7 78.1 5.10 

14-15 1.73 1.46 0.27 0.173 0.0484 0.788 9.84 77.8 4.94 

15-16 1.80 1.61 0.19 0.187 0.0484 0.867 10.0 85.8 4.96 

16-17 1.78 1.47 0.31 0.183 0.0468 0.825 9.37 81.0 4.76 

17-18 1.70 1.50 0.20 0.198 0.0478 0.759 8.83 80.9 5.28 

18-19 1.82 1.55 0.27 0.209 0.0501 0.718 8.65 79.8 5.76 

19-20 2.11 1.73 0.38 0.262 0.0524 0.778 7.70 85.1 5.94 

20-21 3.15 3.23 0 0.391 0.0568 0.979 9.63 147 8.81 

21-22 2.06 1.72 0.34 0.360 0.0499 0.768 5.57 88.9 5.98 

22-23 1.26 0.991 0.27 0.316 0.0480 0.647 3.66 53.2 4.09 

23-24 1.02 0.717 0.30 0.376 0.0473 0.499 2.22 39.1 3.84 

24-25 1.05 0.793 0.26 0.457 0.0499 0.503 2.02 41.0 4.21 

35
Table 3. Continued 

Sample Depth TC OC IC TN TP TS C/N C/P C/S 

(cm) (%) (%) (%) (%) (%) (%) (Atomic) (Atomic) (Atomic) 

Oxic-2 0-1 1.95 1.67 0.28 0.195 0.0660 0.268 9.99 65.2 16.6 

Multicore 1-2 1.44 1.08 0.36 0.135 0.0547 0.233 9.33 50.9 12.4 

2-3 1.94 1.39 0.55 0.158 0.0550 0.312 10.2 65.2 11.9 3-4 2.00 1.54 0.46 0.174 0.0522 0.364 10.3 76.1 11.3 4-5 2.47 1.81 0.66 0.181 0.0566 0.301 11.7 82.5 16.0 

5-6 2.20 1.96 0.24 0.186 0.0650 0.392 12.3 77.8 13.3 

6-7 2.26 1.86 0.40 0.191 0.0564 0.383 11.4 85.0 13.0 

7-8 2.53 1.92 0.61 0.195 0.0525 0.356 11.5 94.3 14.4 

8-9 2.19 1.83 0.36 0.179 0.0517 0.360 11.9 91.3 13.6 

9-10 2.24 1.76 0.48 0.185 0.0545 0.433 11.1 83.3 10.8 

10-11 2.50 2.02 0.48 0.171 0.0565 0.363 13.8 92.2 14.8 

11-12 1.58 0.920 0.66 0.102 0.0529 0.134 10.5 44.8 18.3 

12-13 1.90 0.960 0.94 0.102 0.0487 0.110 11.0 50.8 23.3 

13-14 2.35 2.34 0.01 0.175 0.0438 0.480 15.6 138 13.0 

14-15 2.54 2.42 0.12 0.181 0.0444 0.630 15.6 140 10.2 

15-16 2.17 2.18 0 0.172 0.0449 0.505 14.8 125 11.5 

16-17 2.45 2.26 0.19 0.208 0.0483 0.660 12.7 121 9.14 

17-18 3.00 2.83 0.17 0.217 0.0455 0.948 15.2 160 7.97 

18-19 3.50 3.38 0.12 0.241 0.0422 1.06 16.4 206 8.51 19-20 3.72 3.74 0 0.257 0.0389 0.924 17.0 248 10.8 

20-21 3.01 3.01 0 0.206 0.0393 0.821 17.0 198 9.79 

21-22 2.97 3.36 0 0.206 0.0401 0.902 19.0 216 9.94 

22-23 2.44 2.44 0 0.180 0.0419 0.923 15.8 150 7.06 

23-24 3.81 4.08 0 0.247 0.0399 1.20 19.3 264 9.08 

24-25 3.52 3.56 0 0.248 0.0393 1.17 16.7 234 8.12 

25-26 3.96 4.13 0 0.267 0.0400 1.19 18.0 266 9.26 

26-27 3.42 3.52 0 0.236 0.0404 1.05 17.4 225 8.95 

36
Table 3. Continued 

Sample Depth 

(cm) 

TC 

(%) 

OC 

(%) 

IC 

(%) 

TN 

(%) 

TP 

(%) 

TS 

(%) 

C/N 

(Atomic) 

C/P 

(Atomic) 

C/S 

(Atomic) 

Oxic-2 

Multicore 

Oxic-2 

Vibracore 

Hypoxic-1 

Multicore 

27-28 28-29 29-30 

30-31 

31-32 32-33 

5-10 

10-15 15-20 25-30 50-55 75-80 

100-105 

125-130 

150-155 

175-180 

200-205 

225-230 

250-255 

0-1 

1-2 

2-3 

3-4 

4-5 

5-6 

4.00 5.43 6.61 

6.02 

8.61 9.63 

2.27 

3.21 3.67 3.53 9.84 

10.25 

1.01 

1.27 

1.06 

1.15 

1.36 

1.24 

1.08 1.09 

0.981 

1.11 

1.08 

1.01 

1.09 

3.99 5.99 6.71 

6.67 

8.53 10.1 

2.04 

3.02 3.94 3.79 10.2 10.3 

0.852 

0.658 

0.807 

0.822 

0.833 

0.860 

0.983 0.619 

0.622 

0.708 

0.700 

0.581 

0.630 

0.01 

0 0 

0 

0.08 

0 

0.23 0.19 

0 0 0 0 

0.16 

0.61 

0.25 

0.33 

0.53 

0.38 

0.10 0.47 

0.36 

0.40 

0.38 

0.43 

0.46 

0.268 

0.323 0.366 

0.343 

0.461 

0.516 

0.201 

0.236 

0.268 0.265 0.718 0.743 

0.070 

0.074 

0.104 

0.085 

0.106 

0.105 

0.112 0.112 

0.111 

0.135 

0.106 

0.100 

0.110 

0.0409 

0.0411 0.0378 

0.0346 

0.0345 

0.0314 

0.0493 

0.0380 

0.0286 0.0425 0.0330 0.0333 

0.0463 

0.0560 

0.0552 

0.0565 

0.0630 

0.0618 

0.0581 0.0548 

0.0550 

0.0560 

0.0503 

0.0571 

0.0551 

1.00 

0.9230.898

0.803

1.20 

1.46 

0.638

1.89 

2.96 2.28 2.08 2.48 

0.339

0.491

0.474

0.417

0.583

0.173

0.3390.332

0.398

0.402

0.360

0.288

0.505

17.4 

21.6 21.4 

22.7 21.6 

22.8 

11.8 

14.9 

17.1 16.7 16.6 16.2 

14.2 

10.4 

9.05 

11.3 

9.16 

9.55 

10.2 6.44 

6.53 

6.12 

7.70 

6.78 

6.68 

252 

376 458 

497 638 

829 

107 

205 

355 230 797 798 

47.4 

30.3 

37.7 

37.5 

34.1 

35.9 

43.6 29.1 

29.2 

32.6 

35.9 

26.2 

29.5 

10.6 

17.3 19.9 

22.2 19.0 

18.5 

8.53 

4.26 

3.55 4.44 13.1 11.1 

6.71 

3.58 

4.54 

5.26 

3.81 

13.3 

7.74 4.98 

4.17 

4.70 

5.19 

5.38 

3.33 

37
Table 3. Continued 

Sample Depth TC OC IC TN TP TS C/N C/P C/S 

(cm) (%) (%) (%) (%) (%) (%) (Atomic) (Atomic) (Atomic) 

Hypoxic-1 6-7 1.21 0.737 0.47 0.131 0.0544 0.646 6.56 34.9 3.04 

Multicore 7-8 1.11 0.650 0.46 0.106 0.0548 0.837 7.15 30.6 2.07 

8-9 1.16 0.738 0.42 0.117 0.0545 0.981 7.36 34.9 2.01 

9-10 1.19 0.741 0.45 0.108 0.0531 0.995 8.00 36.0 1.99 

10-11 1.16 0.719 0.44 0.113 0.0558 0.986 7.42 33.2 1.95 

11-12 1.18 0.787 0.39 0.113 0.0546 1.46 8.12 37.2 1.44 

12-13 1.22 0.801 0.42 0.119 0.0562 1.04 7.85 36.8 2.06 

13-14 1.62 1.13 0.49 0.128 0.0545 1.09 10.3 53.5 2.77 

14-15 1.45 0.943 0.51 0.103 0.0538 0.888 10.7 45.2 2.83 

15-16 0.931 0.496 0.44 0.074 0.0537 0.639 7.82 23.8 2.07 

16-17 0.917 0.454 0.46 0.084 0.0546 0.943 6.30 21.4 1.28 17-18 0.979 0.546 0.43 0.088 0.0546 1.10 7.24 25.8 1.32 18-19 1.05 0.633 0.42 0.098 0.0522 1.34 7.53 31.3 1.26 

Hypoxic-2 0-1 1.09 0.748 0.34 0.091 0.0653 0.184 9.58 29.5 10.8 

Multicore 1-2 1.04 0.730 0.31 0.094 0.0630 0.157 9.06 29.9 12.4 

2-3 1.06 0.629 0.43 0.082 0.0612 0.262 8.94 26.5 6.41 

3-4 1.03 0.635 0.40 0.073 0.0590 0.267 10.1 27.8 6.35 

4-5 1.08 0.622 0.46 0.070 0.0590 0.235 10.4 27.2 7.06 

5-6 1.04 0.614 0.43 0.068 0.0593 0.231 10.5 26.7 7.09 

6-7 1.01 0.551 0.46 0.057 0.0586 0.178 11.3 24.2 8.26 

7-8 1.00 0.556 0.44 0.068 0.0587 0.163 9.54 24.4 9.10 

8-9 0.967 0.576 0.39 0.059 0.0588 0.175 11.4 25.3 8.78 

9-10 0.992 0.551 0.44 0.060 0.0588 0.151 10.7 24.2 9.74 

10-11 0.997 0.541 0.46 0.065 0.0593 0.174 9.71 23.5 8.30 

11-12 1.02 0.550 0.47 0.065 0.0589 0.170 9.87 24.1 8.64 

12-13 0.985 0.553 0.43 0.060 0.0611 0.155 10.7 23.3 9.52 

13-14 0.993 0.544 0.45 0.059 0.0601 0.145 10.8 23.3 10.0 

38
Table 3. Continued 

Sample Depth TC OC IC TN TP TS C/N C/P C/S 

(cm) (%) (%) (%) (%) (%) (%) (Atomic) (Atomic) (Atomic) 

Hypoxic-2 14-15 0.986 0.547 0.44 0.060 0.0576 0.125 10.6 24.5 11.7 

Multicore 15-16 1.01 0.545 0.46 0.062 0.0598 0.126 10.2 23.5 11.5 

16-17 0.992 0.548 0.44 0.062 0.0598 0.134 10.3 23.6 10.9 

17-18 1.00 0.537 0.46 0.063 0.0608 0.138 9.94 22.8 10.4 

18-19 1.00 0.555 0.44 0.063 0.0611 0.104 10.3 23.4 14.2 

19-20 1.01 0.564 0.45 0.065 0.0610 0.124 10.1 23.8 12.1 

20-21 0.997 0.543 0.45 0.061 0.0592 0.129 10.4 23.6 11.2 

21-22 0.990 0.560 0.43 0.059 0.0602 0.127 11.1 24.0 11.8 

22-23 0.969 0.537 0.43 0.063 0.0589 0.093 9.94 23.5 15.4 

23-24 0.969 0.541 0.43 0.063 0.0555 0.129 10.0 25.1 11.2 

24-25 1.04 0.562 0.48 0.066 0.0642 0.145 9.93 22.6 10.3 

25-26 1.26 0.774 0.49 0.090 0.0662 0.220 10.0 30.2 9.39 

Hypoxic-2 5-10 1.00 0.799 0.20 0.085 0.0555 0.108 11.0 37.1 19.7 

Vibracore 10-15 1.03 0.842 0.19 0.086 0.0587 0.108 11.4 37.0 20.8 

15-20 0.965 0.798 0.17 0.066 0.0580 0.091 14.1 35.5 23.4 

20-25 1.04 0.866 0.17 0.082 0.0625 0.121 12.3 35.7 19.1 

25-30 1.18 1.06 0.12 0.103 0.0577 0.165 12.0 47.4 17.1 

50-55 1.40 1.15 0.25 0.118 0.0624 0.238 11.4 47.5 12.9 

75-80 1.23 1.08 0.15 0.120 0.0616 0.284 10.5 45.2 10.2 

100-105 1.16 1.36 0 0.117 0.0589 0.066 13.6 59.5 55.0 

125-130 0.878 0.924 0 0.084 0.0549 0.033 12.8 43.4 74.7 

150-155 0.930 0.994 0 0.106 0.0596 0.069 10.9 43.0 38.4 

175-180 1.28 1.12 0.16 0.107 0.0598 0.099 12.2 48.3 30.2 

200-205 1.28 1.09 0.19 0.120 0.0593 0.095 10.6 47.4 30.6 

225-230 1.18 1.32 0.14 0.126 0.0556 0.154 12.2 61.2 22.9 

39
Table 3. Continued 

Sample Depth TC OC IC TN TP TS C/N C/P C/S 

(cm) (%) (%) (%) (%) (%) (%) (Atomic) (Atomic) (Atomic) 

MRJ03-3A 0-1 1.54 nd nd 0.216 nd 0.162 8.31* nd 25.4* 

1-2 1.47 nd nd 0.170 nd 0.200 10.1* nd 19.6* 2-3 1.68 nd nd 0.133 nd 0.196 14.7* nd 22.9* 3-4 1.54 nd nd 0.169 nd 0.174 10.6* nd 23.6* 4-5 1.48 nd nd 0.173 nd 0.169 9.98* nd 23.4* 5-6 1.46 nd nd 0.175 nd 0.164 9.73* nd 23.8* 6-7 1.46 nd nd 0.173 nd 0.173 9.84* nd 22.5* 7-8 1.44 nd nd 0.171 nd 0.168 9.82* nd 22.9* 8-9 1.48 nd nd 0.177 nd 0.176 9.75* nd 22.4* 

9-10 1.52 nd nd 0.166 nd 0.171 10.7* nd 23.7* 

10-11 1.62 nd nd 0.158 nd 0.174 12.0* nd 24.8* 11-12 1.51 nd nd 0.160 nd 0.184 11.0* nd 21.9* 12-13 1.44 nd nd 0.159 nd 0.179 10.6* nd 21.5* 13-14 1.56 nd nd 0.159 nd 0.177 11.4* nd 23.5* 14-15 1.45 nd nd 0.158 nd 0.171 10.7* nd 22.6* 15-16 1.49 nd nd 0.159 nd 0.169 10.9* nd 23.5* 16-17 1.48 nd nd 0.156 nd 0.178 11.1* nd 22.2* 17-18 1.47 nd nd 0.158 nd 0.188 10.8* nd 20.9* 18-19 1.47 nd nd 0.157 nd 0.189 10.9* nd 20.8* 19-20 1.48 nd nd 0.192 nd 0.212 8.99* nd 18.6* 20-21 1.50 nd nd 0.173 nd 0.210 10.1* nd 19.1* 21-22 1.38 nd nd 0.164 nd 0.198 9.81* nd 18.6* 22-23 1.37 nd nd 0.163 nd 0.211 9.80* nd 17.3* 23-24 1.40 nd nd 0.164 nd 0.193 9.95* nd 19.4* 24-25 1.40 nd nd 0.170 nd 0.198 9.60* nd 18,9* 25-26 1.39 nd nd 0.168 nd 0.215 9.65* nd 17.2* 26-27 1.40 nd nd 0.170 nd 0.228 9.60* nd 16.4* 

40
Table 3. Continued 

Sample Depth TC OC IC TN TP TS C/N C/P C/S 

(cm) (%) (%) (%) (%) (%) (%) (Atomic) (Atomic) (Atomic) 

MRJ03-3A 27-28 1.39 nd nd 0.166 nd 0.215 9.76* nd !7.2* 

28-29 1.39 nd nd 0.172 nd 0.233 9.42* nd 15.9* 29-30 1.38 nd nd 0.173 nd 0.229 9.30* nd 16.1* 30-31 1.39 nd nd 0.171 nd 0.217 9.48* nd 17.1* 31-32 1.40 nd nd 0.179 nd 0.210 9.12* nd 17.8* 32-33 1.39 nd nd 0.178 nd 0.220 9.11* nd 16.9* 33-34 1.38 nd nd 0.178 nd 0.220 9.04* nd 16.7* 34-35 1.41 nd nd 0.179 nd 0.213 9.18* nd 17.7* 35-36 1.36 nd nd 0.177 nd 0.213 8.96* nd 17.0* 36-37 1.40 nd nd 0.183 nd 0.210 8.92* nd 17.8* 37-38 1.48 nd nd 0.175 nd 0.206 9.86* nd 19.2* 38-39 1.37 nd nd 0.178 nd 0.205 8.98* nd 17.8* 39-40 1.44 nd nd 0.190 nd 0.242 8.84* nd 15.9* 40-41 1.40 nd nd 0.175 nd 0.225 9.33* nd 16.6* 41-42 1.36 nd nd 0.176 nd 0.218 9.01* nd 16.6* 42-43 1.37 nd nd 0.178 nd 0.216 8.98* nd 16.9* 43-44 1.36 nd nd 0.176 nd 0.195 9.01* nd 18.6* 44-45 1.39 nd nd 0.180 nd 0.213 9.00* nd 17.4* 45-46 1.40 nd nd 0.177 nd 0.199 9.22* nd 18.8* 

46-47 1.37 nd nd 0.178 nd 0.203 8.98* nd 18.0* 

MRJ03-5A 0-1 1.47 nd nd 0.164 nd 0.346 10.4* nd 11.3* 

1-2 1.63 nd nd 0.188 nd 0.378 10.1* nd 11.5* 2-3 1.71 nd nd 0.162 nd 0.320 12.3* nd 14.3* 

3-4 1.62 nd nd 0.151 nd 0.343 12.5* nd 12.6* 

4-5 0.895 nd nd 0.070 nd 0.202 14.9* nd 11.8* 

41
Table 3. Continued 

Sample Depth TC OC IC TN TP TS C/N C/P C/S 

(cm) (%) (%) (%) (%) (%) (%) (Atomic) (Atomic) (Atomic) 

MRJ03-5A 5-6 0.841 nd nd 0.077 nd 0.204 12.7* nd 11.0* 

6-7 0.641 nd nd 0.051 nd 0.164 14.6* nd 10.4* 7-8 0.919 nd nd 0.064 nd 0.286 16.7* nd 8.58* 8-9 1.08 nd nd 0.063 nd 0.535 20.0* nd 5.39* 

9-10 1.22 nd nd 0.089 nd 0.544 16.0* nd 5.99* 

10-11 1.18 nd nd 0.110 nd 0.448 12.5* nd 7.03* 11-12 1.28 nd nd 0.084 nd 0.579 17.8* nd 5.90* 12-13 0.969 nd nd 0.081 nd 0.470 14.0* nd 5.50* 13-14 1.06 nd nd 0.151 nd 0.356 8.19* nd 7.95* 14-15 1.10 nd nd 0.057 nd 0.331 22.5* nd 8.87* 15-16 0.867 nd nd 0.065 nd 0.300 15.6* nd 7.71* 16-17 1.54 nd nd 0.050 nd 0.307 35.9* nd 13.4* 

17-18 0.972 nd nd 0.085 nd 0.289 13.3* nd 8.98* 

18-19 1.06 nd nd 0.098 nd 0.266 12.6* nd 10.6* 

19-20 1.07 nd nd 0.070 nd 0.218 17.8* nd 13.1* 

20-21 0.936 nd nd 0.040 nd 0.194 27.3* nd 12.9* 

21-22 1.04 nd nd 0.087 nd 0.220 13.9* nd 12.6* 

22-23 1.25 nd nd 0.049 nd 0.163 29.7* nd 20.5* 

23-24 1.05 nd nd 0.059 nd 0.236 20.8* nd 11.9* 

24-25 0.939 nd nd 0.089 nd 0.291 12.3* nd 8.61* 

25-26 nd nd nd nd nd nd nd nd nd 

26-27 1.08 nd nd 0.072 nd 0.202 17.5* nd 14.3* 

27-28 1.54 nd nd 0.125 nd 0.378 14.4* nd 10.9* 

28-29 0.924 nd nd 0.081 nd 0.144 13.3* nd 17.1* 

29-30 1.02 nd nd 0.180 nd 0.267 6.61* nd 10.2* 30-31 1.16 nd nd 0.064 nd 0.323 21.1* nd 9.59* 

42
43

Table 3. Continued 

Sample Depth TC OC IC TN TP TS C/N C/P C/S 

(cm) (%) (%) (%) (%) (%) (%) (Atomic) (Atomic) (Atomic) 

MRJ03-5A 31-32 0.986 nd nd 0.032 nd 0.216 35.9* nd 12.2* 

32-33 1.10 nd nd 0.126 nd 0.251 10.2* nd 11.7* 

* Ratio calculated using TC rather than OC 
Fig. 12. Organic elemental data from June 2002 hypoxic-1(●) and oxic-1 (○) sites in the lower 

Mississippi River system; all results are reported on a dry weight basis. 

44
Fig. 13. Organic elemental atomic ratios from June 2002 hypoxic-1(●) and oxic-1 (○) sites in the lower 

Mississippi River system. 

45
Fig. 14. Organic elemental data from the lower Mississippi River system for cores collected in 

September 2002 from the hypoxic-2 site by multicore (●) and vibracore (■), and the oxic-2 site by 

multicore (○) and vibracore (□). All results are on a dry weight basis. 

46
Fig. 15. Elemental atomic ratios from the lower Mississippi River system for cores collected in 

September 2002 from the hypoxic-2 site by multicore (●) and vibracore (■), and the oxic-2 site by 

multicore (○) and vibracore (□). All results are on a dry weight basis. 

47
that for TC and TN. In particular, the MR-J03-5 core exhibited pronounced maxima in TS from 0-4 

cm, and especially in the 7-20 cm interval. 

 The high TS values and the downcore peaks in TS in the MR-J03-05 core are also evident in the 

elemental C/S plot (Fig. 17). Atomic C/S values average 10.8 for all samples in the MR-J03-5 core, 

compared to an average atomic C/S of 19.7 for the MR-J03-3 core. The lower atomic C/S values in 

the MR-J03-5 core compared to MR-J03-3 core, reflect the position of the former in the general 

zone of hypoxia. Atomic C/S values ranged from 16-25 for all sediment samples from the MR-J03-

3 core, and 5-20 in sediments from the MR-J03-5 core. The downcore atomic C/S profile in the 

MR-J03-5 core shows a minimum in atomic C/S centered about 10 cm depth, reflecting elevated TS 

values. In contrast, the MR-J03-3 core shows a gradual decline in atomic C/S with depth, but with 

no major excursions. At depths below about 25 cm, atomic C/S values in cores MR-J03-3 and -5 

approach each other, but with the values in the MR-J03-5 core still generally lower. 

 Atomic C/N values were higher overall at the MR-J03-5 site than the MR-J03-3 site. Average 

atomic C/N values for all samples were 9.87 and 16.7 for the MR-J03-3 and -5 cores, respectively. 

The MR-J03-3 core had a relatively smooth downcore atomic C/N profile, with perhaps a slight 

decline in atomic C/N with depth. The downcore atomic C/N profile in the MR-J03-5 core, in 

contrast, had an irregular downcore profile with a great degree of variability in atomic C/N. The 

higher atomic C/N values at the MR-J03-5 site probably reflect greater input of terrestrial organic 

matter to the sediments at this more inshore site (Fig. 1), and the irregularity in the profile reflects 

changing discharge patterns in the lower Mississippi River system. 

C. Hydrocarbon Gases (C1

-C6)

 Concentrations of hydrocarbon gases (C1-C6) were determined in the 2001 cores, and results are 

presented in Table 4 and in Figs. 18-23. Methane (C1) dominates the hydrocarbon gas distributions 

in all cores, generally accounting for >90% of the total hydrocarbon gases present. Ethane, propane, 

and hexane have significant concentrations in most cores, while butane and pentane are 

undetectable in many core intervals. Concentration ranges (dry wt. basis) for total hydrocarbons 

(“bound” plus “free”) are as follows: methane (C1) 8 to 11,689 ppm/g sediment , ethane (C2) 0.05 to 

1.39 ppm/g sediment, propane (C3

) <0.01 to 0.52 ppm/g sediment, butane (C4) <0.01 to 0.24 ppm/g 

sediment, pentane (C5) <0.01 to 1.06 ppm/g sediment, and hexane (C6) <0.01 to 5.29 ppm/g 

sediment. The highest hydrocarbon gas concentrations (mostly methane) were observed at the 

Atchafalaya South site, and the lowest at Pass Raphael. 

 “Bound” hydrocarbons constitute a significant fraction of the total hydrocarbon gases in most 

sediments. Butane and pentane detected in these samples was all present as “bound” gas. The 

percentage of “bound” methane was variable, and ranged from <3% to >99% of the total, but was 

more often the dominant form of methane in these samples, except at the Atchafalaya River South 

site. Ethane and propane were also present primarily as “bound” gas. In contrast, hexane in most 

samples is present as ‘free” gas. 

 The vertical profiles of methane, the overwhelmingly dominant hydrocarbon gas, generally 

show increasing concentration with depth. This is most apparent at the Atchafalaya South site, 

where concentrations of methane sharply increase below 20 cm. In contrast, ethane, propane, and 

hexane concentrations generally increase with depth. Too little data is available for butane and 

propane to delineate vertical concentration trends. 

48
V. Discussion 

 Results from this study provide information on the general organic characteristics of sediments 

from coastal Louisiana off the Mississippi and Atchafalaya River systems, including spatial and 

downcore distributions of the important organic elements: C, N, P, and S. These data are useful in 

interpreting the relative proportions of autochthonous and allochthonous organic matter being 

deposited in this region, nutrient loading stimulating algal production in the coastal zone, and the 

presence of hypoxic conditions. In addition to this direct utility, results are used in interpreting and 

normalizing organic contaminant data obtained as part of this study, and reported elsewhere 

(Rosenbauer and others, 2007). 

 Our initial work in 2001 focused on sites directly off the mouths of the Mississippi and 

Atchafalaya Rivers. However, these nearshore sites proved to have erratic deposition rates 

suggesting a high energy regime controlled more by variability in river flow rather than by 

geochemical processes and reactions within 

49
Fig. 16. Organic elemental data from cores MR-J03-3 (○) and MR-J03-5 (●) collected off the lower 

Mississippi River System. Results are reported as % on a dry weight basis. 

50
Fig. 17. Elemental atomic ratios of sediments from cores MR-J03-3 and MR-J03-5 collected off the 

lower Mississippi River System. 

51
Table 4. Concentrations of C1 to C6 hydrocarbons in sediments from cores collected in the lower Mississippi and Achafal

ppm gas/g sediment (dry wt.) as free (FR), bound (BD), and total (TOT) gas. 

Sample Depth Methane (C1) Ethane (C2) Propane (C3) Butane (C4) Pentane (

(cm) FR BD TOT FR BD TOT FR BD TOT FR BD TOT FR BD 

Mississippi River 0-4 37.5 47.1 84.6 0.22 0.86 1.08 0.14 0.38 0.52 0.00 0.14 0.14 0.00 0.00 Pass Raphael Site 4-8 115 63.6 179 0.30 1.09 1.39 0.18 0.00 0.18 0.00 0.00 0.00 0.00 0.00 Core 010314-5 8-12 11.3 38.3 49.6 0.10 0.53 0.63 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 

12-16 4.37 12.1 16.5 0.00 0.52 0.52 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 16-20 16.2 15.3 31.5 0.15 0.46 0.61 0.00 0.36 0.36 0.00 0.00 0.00 0.00 0.00 20-25 11.2 8.50 19.7 0.62 0.54 1.16 0.00 0.37 0.37 0.00 0.00 0.00 0.00 0.00 25-30 5.42 65.3 70.7 0.16 0.35 0.51 0.00 0.40 0.40 0.00 0.11 0.11 0.00 0.00 30-35 6.99 64.2 71.2 0.14 0.24 0.38 0.00 0.18 0.18 0.00 0.00 0.00 0.00 0.00 35-40 10.5 68.3 78.8 0.00 0.41 0.41 0.00 0.13 0.13 0.00 0.00 0.00 0.00 0.00 40-45 1.92 60.0 61.9 0.00 0.18 0.18 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 45-50 2.34 55.9 58.2 0.00 0.19 0.19 0.00 0.09 0.09 0.00 0.00 0.00 0.00 0.00 

Mississippi River 0-4 13.0 57.2 70.2 0.00 0.25 0.25 0.00 0.19 0.19 0.00 0.00 0.00 0.00 0.00 South Pass Site 4-8 27.3 63.7 91.0 0.05 0.00 0.05 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Core 010314-6 8-12 34.8 95.4 130 0.13 0.20 0.33 0.06 0.23 0.29 0.00 0.00 0.00 0.00 0.00 

12-16 51.0 66.2 117 0.05 0.14 0.19 0.05 0.27 0.32 0.00 0.00 0.00 0.00 0.00 16-20 116 70.8 187 0.24 0.15 0.39 0.09 0.21 0.30 0.00 0.00 0.00 0.00 0.00 20-25 51.3 63.1 114 0.00 0.08 0.08 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 25-30 2.91 56.3 59.2 0.00 0.17 0.17 0.00 0.08 0.08 0.00 0.00 0.00 0.00 0.00 30-35 31.2 118 149 0.00 0.10 0.10 0.00 0.03 0.03 0.00 0.00 0.00 0.00 0.00 35-40 25.2 156 181 0.00 0.12 0.12 0.00 0.05 0.05 0.00 0.00 0.00 0.00 0.00 40-45 173 308 481 0.05 0.08 0.13 0.00 0.04 0.04 0.00 0.02 0.02 0.00 0.00 45-50 119 1890 2010 0.02 0.11 0.13 0.00 0.11 0.11 0.00 0.05 0.05 0.00 0.00 

Atchafalaya River 0-5 11.3 93.5 105 0.00 0.79 0.79 0.00 0.33 0.33 0.00 0.14 0.14 0.00 1.06 North Site 1 5-10 3.20 211 214 0.12 0.7 0.82 0.00 0.34 0.34 0.00 0.13 0.13 0.00 0.07 Core 010319-1 10-15 8.43 499 507 0.06 0.52 0.58 0.00 0.25 0.25 0.00 0.11 0.11 0.00 0.05 

15-20 16.6 818 835 0.00 0.33 0.33 0.00 0.21 0.21 0.00 0.11 0.11 0.00 0.00 20-25 8.34 1090 1100 0.07 0.91 0.98 0.00 0.44 0.44 0.00 0.18 0.18 0.00 0.08 

52
53

Table 4 Continued 

Sample Depth Methane (C1) Ethane (C2) Propane (C3) Butane (C4) Pentane (

(cm) FR BD TOT FR BD TOT FR BD TOT FR BD TOT FR BD 

Atchafalaya River 25-30 68.5 2070 2140 0.08 0.84 0.92 0.00 0.39 0.39 0.00 0.17 0.17 0.00 0.06 North Site 1 30-35 22.9 1600 1620 0.00 0.42 0.42 0.00 0.22 0.22 0.00 0.08 0.08 0.00 0.00 Core 010319-1 35-38 24.1 1580 1607 0.19 0.56 0.75 0.00 0.15 0.15 0.00 0.07 0.07 0.00 0.00 Atchafalaya River 0-5 8.14 26.9 35.0 0.00 1.30 1.30 0.00 0.46 0.46 0.00 0.24 0.24 0.00 0.00 South Site 5-10 8.42 20.0 28.4 0.00 0.88 0.88 0.10 0.34 0.44 0.00 0.00 0.00 0.00 0.00 Core 010318-1 10-15 1.58 6.67 8.23 0.00 0.48 0.48 0.00 0.19 0.19 0.00 0.07 0.07 0.00 0.00 

15-20 11.4 5.08 16.5 0.00 0.20 0.20 0.05 0.12 0.17 0.00 0.00 0.00 0.00 0.00 20-25 207 21.8 229 0.00 0.37 0.37 0.00 0.18 0.18 0.00 0.00 0.00 0.00 0.00 25-30 4150 121 4270 0.16 0.40 0.56 0.12 0.14 0.26 0.00 0.00 0.00 0.00 0.00 30-40 5780 3580 9360 0.05 0.81 0.86 0.00 0.25 0.25 0.00 0.10 0.10 0.00 0.00 40-50 5000 6680 11700 0.08 0.29 0.37 0.00 0.13 0.13 0.00 0.00 0.00 0.00 0.00 50-60 4350 7200 11500 0.00 0.33 0.33 0.00 0.15 0.15 0.00 0.00 0.00 0.00 0.00 60-70 6160 2760 8920 0.07 0.13 0.20 0.00 0.09 0.09 0.00 0.00 0.00 0.00 0.00 70-80 4280 4140 8420 0.00 0.46 0.46 0.00 0.16 0.16 0.00 0.08 0.08 0.00 0.00 
Fig. 18. Free (red) and bound (blue) methane concentrations (ppm/g dry wt.) in sediments from 

cores collected in the lower Mississippi and Atchafalaya River estuaries in 2001. 

54
Fig. 19. Free (red) and bound (blue) ethane concentrations (ppm/g dry wt.) in sediments from cores 

collected in the lower Mississippi and Atchafalaya River estuaries in 2001. 

55
Fig. 20. Free (red) and bound (blue) propane concentrations (ppm/g dry wt.) in sediments from cores 

collected in the lower Mississippi and Atchafalaya river estuaries in 2001. 

56
Fig. 21. Free (red) and bound (blue) butane concentrations (ppm/g dry wt.) in sediments from cores 

collected in the lower Mississippi and Atchafalaya River estuaries in 2001. 

57
Fig. 22. Free (red) and bound (blue) pentane concentrations (ppm/g dry wt.) in sediments from cores 

collected in the lower Mississippi and Atchafalyaa River estuaries in 2001. 

58
Fig. 23. Free (red) and bound (blue) hexane concentrations (ppm/g dry wt.) in sediments from cores 

collected in the lower Mississippi and Atchafalaya River estuaries in 2001. 

59
the system. Elemental organic analyses, and hydrocarbon gas studies also indicated erratic 

conditions, with possible seasonal removal and redeposition of sediment in the upper 10-20 cm at 

the 2001 sites. Vertical profiles of C, N, P, and S are often very erratic in the upper 20 cm of 

sediment (upper 50 cm at Pass Raphael site), (Figs. 3-7). Methane concentrations are near zero in 

the upper sections of the 2001 cores (except at the Atchafalaya North site), before increasing 

suddenly at depths between 20 and 50 cm (Fig. 16). These results support the idea of rapid sediment 

deposition and removal by strong currents in the upper portions of these sediments. 

 Ratios of the elemental composition of organic matter in sediments (atomic C/N, C/P, and N/P) 

are frequently useful in distinguishing the source of organic matter to sediments (Macko and others, 

1993; Tissot and Welte, 1984). At most sites from this study, atomic C/N values did not usually 

exceed 15, and in many instances were <10. Atomic C/N values in this range suggest a mix of 

autochthonous (algal) and allochthonous (terrestrially-derived) sources of organic matter being 

deposited in the sediments, but with autochthonous sources dominating. Atomic C/P and N/P ratios 

are also relatively low overall (generally <100 and 10, respectively), again suggestive of the 

dominance of autochthonous organic matter. This conclusion is consistent with previous work on 

organic matter in sediments from the offshore Mississippi River system (Goni and others, 1998). 

This result is somewhat surprising considering the proximity of the Mississippi and Atchafalaya 

River deltas to the offshore sampling sites, and the expected transport of terrestrially derived 

organic matter down the river. However, much of the authochthonous organic matter derived from 

the river discharge may be carried further offshore. In addition, the river system discharges 

dissolved nutrient elements into the nearshore area, which may stimulate algal productivity here. 

 Delivery of excess nutrients (especially N) from the Mississippi River system, and stratification 

of the water column related to climatic conditions and seasonal changes in freshwater discharge 

from the Mississippi River system are thought to be the main factors responsible for the 

development of hypoxic conditions in the coastal Gulf of Mexico near the Mississippi and 

Atchafalaya River deltas (Justic and others, 1995; Wiseman and others, 1997). A large focus of the 

sampling effort in this study was coring in this hypoxic zone (sites Hypoxic-1 and 2, and MR-J03-

05), and nearby areas that were considered to have had oxic conditions in the water column (sites 

Oxic-1 and 2, and MR-J03-3), (see Table 1, and Fig. 1). Atomic C/S values of sediments may be 

useful as an indicator of anoxic and hypoxic conditions in aquatic systems (Cornwell and Sampou, 

1995). Hypoxic or anoxic conditions in the water column in marine environments may allow 

anaerobic microbial processes such as sulfate reduction to occur. The reactive sulfide produced as a 

byproduct of microbial sulfate reduction may then react with iron and precipitate iron sulfide 

minerals to the underlying sediments. This “extra” metal sulfide delivered to sediments in zones 

with anoxic/hypoxic water columns increases sedimentary sulfur concentrations, and results in 

lower atomic C/S values in the sediments. We observed elevated TS concentrations and low atomic 

C/S values at Hypoxic-1 compared to Oxic-1, and at MR-J03-5 compared to MR-J03-3, confirming 

that our Hypoxic-1 and MR-J03-5 sites were indeed located in the zone of hypoxia. We did not 

observe higher sedimentary TS or lower atomic C/S at the Hypoxic-2 compared to Oxic-2, 

suggesting that the Hypoxic-2 site, while located in the general zone of hypoxia, was not hypoxic. 

This may reflect some degree of patchiness in the overall zone of hypoxia in this region. Results do 

suggest that TS concentrations and atomic C/S values may be useful indicators of hypoxia in the 

overlying waters, and downcore variations in TS and atomic C/S may provide historical context to 

hypoxia in a given region. 

60
61

 In regard to historical context, the TS and atomic C/S data from the MR-J03-5 core (Figs. 16 and 

17) indicate a peak in hypoxia, indicated by a maximum in TS and a minimum in atomic C/S, 

centered at about 10 cm depth. Sedimentation rates and sediment mass accumulation rates for this 

and other sites in this study were determined using vertical distributions of 239, 240Pu, 137Cs, and 210Pbex

(= 210Pbtot – 226Ra), (Swarzenski and others, 2006). The linear sedimentation rate for hypoxic site MRJ03-5 was determined as: 0.26 cm yr-1 (210Pb-based) to 0.36 cm yr-1 (239, .240Pu peak-based); much 

slower than that at oxic site MR-J03-3: 1.24 cm yr-1 (210Pb-based) to 0.99 cm yr-1 (239, 240Pu peakbased). The estimated sedimentation rates at MR-J03-5 place the peak in hypoxia (based on TS and 

atomic C/S ratios) at this site in the mid 1960’s. Atomic C/S values actually increased (increasing 

oxic conditions) between the mid 1960’s and about 1990, before decreasing again in years since 

about 1990. Above about 22 cm (e.g. since about 1920 in the MR-JO3-5 core), atomic C/S values at 

the hypoxic MR-J03-5 site have been significantly lower than those at the oxic MR-J03-3 site. 

Sedimentation rates were not available for the Hypoxic-1 core, although this site also had a 

minimum in atomic C/S values at a depth of 10-20 cm, with increasing values toward the surface 

(Fig. 13). 

 Mass accumulation rates of sediments from sites MR-J03-3 and MR-J03-5 (Fig. 1) were also 

determined by Swarzenski and others (2006): 0.87 g cm-2

 yr-1 (210Pb-based) to 0.69 (239,240Pu peakbased) at MR-J03-3, and 0.30 g cm-2

 yr-1 (210Pb-based) to 0.38 g cm-2 yr-1 (239, 240Pu peak-based) at MRJ03-5. Average values for TC, TN, and TS in these cores were: 1.44%, 0.171%, and 0.198%, 

respectively at MR-J03-3, and 1.13%, 0.092%, and 0.305%, respectively at MR-J03-5. These values 

allow average accumulation rates for TC, TN, and TS to be calculated at these sites; results of these 

calculations are shown in Table 5. Accumulation of TC, TN, and TS were all higher at MR-J03-3, 

reflecting the much higher mass accumulation rates at this site, which is located along the 

Mississippi River Canyon and may receive direct sediment discharge funneled down the canyon 

(Coleman and others, 1981; Penland and Boyd, 1981). Despite higher TS concentrations at MRJ03-5, the MR-J03-3 site has slightly higher overall TS accumulation due to the much higher mass 

of sediment accumulated here. 

 Results of hydrocarbon gas analysis of sediments from the 2001 cores show that methane is the 

dominant hydrocarbon gas. Total concentrations of the different hydrocarbon gases in sediments 

from this study were comparable to those determined in other nearshore coastal sediments (Martens 

and Berner, 1977; Claypool and Kvenvolden, 1983; Judd, 2004). Ratios of C1/C2 + C3 have been 

used to distinguish biogenic from thermogenic gas, with biogenic gas having values >1,000 and 

thermogenic gas having values <100 (Bernard and others, 1977 and 1978; Fleischer and others 

2001). The C1

/C2 + C3 ratio in these sediments ranges from about 10 to >20,000, suggesting both 

biogenic and thermogenic gas is present (Anderson and Bryant, 1990; Bouma and Roberts, 1990). 

However, microbial methane utilization may be skewing the C1

/C2 + C3 values in some samples 

giving the gas composition the appearance of being thermogenic (Oremland, 1981; Orphan and 

others, 2001) 
Table 5. Accumulation rates (AR) for total carbon (TC), total nitrogen (TN), and total sulfur (TS) at sites MR-J03-3 and M

TC TC TN TN TS T

AR (210Pb) AR (239,240Pu) AR (210Pb) AR (239,240Pu) AR (210Pb) AR (2

Site (mgC cm-2yr-1) (mgC cm-2yr-1) (mgN cm-2yr-1) (mgN cm-2yr-1) (mgS cm-2yr-1) (mgS c

MR-J03-3 12.5 9.9 1.5 1.2 1.7 

MR-J03-5 3.4 4.3 0.3 0.3 0.9 

62
VI. Conclusions 

 This report presents results of general organic analysis of sediments from a series of cores 

collected in the offshore Mississippi River system, including: organic elemental analysis (C, N, P, 

and S) and hydrocarbon gas analysis. Results of organic contaminants, trace element distributions, 

and radiochemical dating of the same sediment cores are presented elsewhere (Rosenbauer and 

others, 2007; Swarzenski and others, 2006). 

 Concentrations of C, N, P, and S in sediments varied by a factor of 10 between sites, and in 

down core profiles. Nearshore cores collected in 2001 proved to have erratic downcore C, N, P, and 

S profiles and sediment deposition rates, suggesting a high energy regime controlled more by 

variability in river flow rather than by geochemical processes and reactions within the system. 

These results focused further coring activities further offshore. Atomic C/N results suggest that 

organic matter deposited at all sites is a mix of microbial (algal) and terrestrial (vascular plant) 

remains, but with algal material dominant. Concentrations of total sulfur in sediments from cores in 

the zone of hypoxia were often higher than those in nearby zones with oxic water columns. 

Corresponding atomic C/S ratios were typically lower in sediments from sites in the zone of 

hypoxia compared to nearby sites with oxic water columns, and atomic C/S values may be useful as 

a proxy for identifying sites impacted by hypoxic conditions in the water column and for examining 

historical trends in hypoxia. At one site examined in this study, maximum hypoxic conditions were 

determined to have occurred in the mid 1960’s, based on analysis of atomic C/S values in a dated 

core. The organic elemental composition (C, N, P, and S) of sediments was also used to guide 

sample selection for contaminant analysis, and to normalize the contaminant data to organic C 

content of the sediments. 

 Dissolved hydrocarbon gases in sediments showed a dominance of methane, but identifiable 

concentrations of ethane and hexane, and trace concentrations of propane, butane,.and pentane were 

also detected. All dissolved gases except hexane were dominated by “bound” gas, gas released only 

after agitation of the sediment in a blender. Hexane, in contrast was observed mostly as free gas, 

determined by headspace analysis.

63
Acknowledgments 

 Funding for this project was provided by the U.S. Geological Survey, Coastal and Marine 

Geology Program (John Haines, Program Coordinator). Several colleagues participated in the 

collection and sectioning of cores during various cruises for this study, we especially acknowledge 

Marci Marot, Chris Reich, Brian Kindinger, Nancy Dewitt, and Marissa Beck for their assistance. 

The captain and crew of the R/V Longhorn and Steve Rabalais (Louisiana Universities Marine 

Consortium) provided essential logistical support. Curtis Palmer and Sandra Neuzil (USGS, Reston, 

VA) provided helpful reviews that greatly improved the quality of this report. 

64
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