USGS Professional Paper 1686-B

Organic-Carbon Sequestration in Soil/Sediment of the Mississippi River Deltaic Plain—Data; Landscape Distribution, Storage, and Inventory; Accumulation Rates; and Recent Loss, Including a Post-Katrina Preliminary Analysis

Chapter B of Soil-Carbon Storage and Inventory for the Continental United States

This report is available online in pdf format (16.5 MB): USGS Professional Paper 1686-B (Opens the PDF file in a new window. )

Data for appendixes 1-5 are available online in Excel and ASCII format.

By Helaine W. Markewich, Gary R. Buell, Louis D. Britsch, John P. McGeehin, John A. Robbins, John H. Wrenn, Douglas L. Dillon, Terry L. Fries, and Nancy R. Morehead

U.S. Geological Survey Professional Paper 1686-B, 253 pages (Published April 2007)


Mississippi River deltaic plain soil/sediment organic carbon (SOC) storage in the surface meter of soil/sediment.

Soil/sediment of the Mississippi River deltaic plain (MRDP) in southeastern Louisiana is rich in organic carbon (OC). The MRDP contains about 2 percent of all OC in the surface meter of soil/sediment in the Mississippi River Basin (MRB). Environments within the MRDP differ in soil/sediment organic carbon (SOC) accumulation rate, storage, and inventory. The focus of this study was twofold: (1) develop a database for OC and bulk density for MRDP soil/sediment; and (2) estimate SOC storage, inventory, and accumulation rates for the dominant environments (brackish, intermediate, and fresh marsh; natural levee; distributary; backswamp; and swamp) in the MRDP.

Comparative studies were conducted to determine which field and laboratory methods result in the most accurate and reproducible bulk-density values for each marsh environment. Sampling methods included push-core, vibracore, peat borer, and Hargis1 sampler. Bulk-density data for cores taken by the "short push-core method" proved to be more internally consistent than data for samples collected by other methods. Laboratory methods to estimate OC concentration and inorganic-constituent concentration included mass spectrometry, coulometry, and loss-on-ignition. For the sampled MRDP environments, these methods were comparable. SOC storage was calculated for each core with adequate OC and bulk-density data. SOC inventory was calculated using core-specific data from this study and available published and unpublished pedon data linked to SSURGO2 map units. Sample age was estimated using isotopic cesium (137Cs), lead (210Pb), and carbon (14C), elemental Pb, palynomorphs, other stratigraphic markers, and written history. SOC accumulation rates were estimated for each core with adequate age data.

Cesium-137 profiles for marsh soil/sediment are the least ambiguous. Levee and distributary 137Cs profiles show the effects of intermittent allochthonous input and/or sediment resuspension. Cesium-137 and 210Pb data gave the most consistent and interpretable information for age estimations of soil/sediment deposited during the 1900s. For several cores, isotopic 14C and 137Cs data allowed the 1963–64 nuclear weapons testing (NWT) peak-activity datum to be placed within a few-centimeter depth interval. In some cores, a too old 14C age (when compared to 137Cs and microstratigraphic-marker data) is the probable result of old carbon bound to clay minerals incorporated into the organic soil/sediment. Elemental Pb coupled with Pb source-function data allowed age estimation for soil/sediment that accumulated during the late 1920s through the 1980s. Exotic pollen (for example, Vigna unguiculata and Alternanthera philoxeroides) and other microstratigraphic indicators (for example, carbon spherules) allowed age estimations for marsh soil/sediment deposited during the settlement of New Orleans (1717–20) through the early 1900s.

For this study, MRDP distributary and swamp environments were each represented by only one core, backswamp environment by two cores, all other environments by three or more cores. MRDP core data for the surface meter soil/sediment indicate that (1) coastal marshes, abandoned distributaries, and swamps have regional SOC-storage values >16 kg m–2; (2) swamps and abandoned distributaries have the highest SOC storage values (swamp, 44.8 kg m–2; abandoned distributary, 50.9 kg m–2); (3) fresh-to-brackish marsh environments have the second highest site-specific SOC-storage values; and (4) site-specific marsh SOC storage values decrease as the salinity of the environment increases (fresh-marsh, 36.2 kg m–2; intermediate marsh, 26.2 kg m–2; brackish marsh, 21.5 kg m–2). This inverse relation between salinity and SOC storage is opposite the regional systematic increase in SOC storage with increasing salinity that is evident when SOC storage is mapped by linking pedon data to SSURGO map units (fresh marsh, 47 kg m–2; intermediate marsh, 67 kg m–2; brackish marsh, 75 kg m–2; and salt marsh, 80 kg m–2).

MRDP core data for this study also indicate that levees and backswamp have regional SOC-storage values <16 kg m–2. Group-mean SOC storage for cores from these environments are natural levee (17.0 kg m–2) and backswamp (14.1 kg m–2).

An estimate for the SOC inventory in the surface meter of soil/sediment in the MRDP can be made using the SSURGO mapped portion of the coastal-marsh vegetative-type map (13,236 km2, land-only area) published by the Louisiana Department of Wildlife and Fisheries and U.S. Geological Survey (1997). This area has a SOC inventory (surface meter) of 677 Tg (slightly more than 2 percent of the 30,289 Tg SOC inventory for the MRB). The MRDP (6,180 km2, land-only area) has an estimated SOC inventory of 397 Tg. Most of the MRDP is located within the SSURGO mapped coastal marshlands. The entire MRDP, including water, has an area of about 10,800 km2. Using the ratio of total MRDP area to SSURGO mapped MRDP area as an adjustment, the MRDP SOC inventory is estimated at 694 Tg. This larger estimate of 694 Tg for the SOC inventory is probably more realistic, because it is reasonable to assume that the marsh sediments overlain by shallow water have comparable SOC storage to that of the adjacent land areas.

MRDP core data for this study indicate that there is some variability in long-term SOC mass-accumulation rates for centuries and millennia and that this variability may indicate important geologic changes or changes in land use. However, the consistency of the range in rates of SOC accumulation through time suggests a remarkable degree of marsh sustainability throughout the Holocene, including the recent period of significant marsh modification/channelization for human use. One example of marsh sustainability is its present ability to function as a SOC sink even with Louisiana’s large-scale coastal land loss during the last several decades. With coastal-marsh restoration efforts, this sink potential will increase.

Looking to the future, a total of 1,101 g m–2 yr–1 SOC is projected to be lost from all of coastal Louisiana (U.S. Army Corps of Engineers, Louisiana Coastal Area (LCA) subprovinces 1–4; not just the MRDP) through coastal erosion from year 2000 to 2050. This translates to a projected SOC-loss rate of about 0.20 percent per year.

The recent Hurricanes Katrina and Rita, which devastated the Louisiana coast during late August and late September 2005, transformed about 259 km2 (100 mi2) of marsh to open water (U.S. Geological Survey, 2005). To the extent that some or all of this land loss is permanent, this result equates to a SOC loss of about 15 Tg. This estimate is based on the year-2000 15,153-km2 land area for the LCA study area that includes LCA subprovince 4. Using the year-2000 land area, the LCA study area had an estimated SOC inventory of 858 Tg. The estimated 15 Tg SOC loss attributable to Hurricanes Katrina and Rita is 1.7 percent of the year-2000 LCA inventory and 2.3 percent of the year-2000 MRDP inventory. If this SOC loss is included in the projection for the year 2050, then the MRDP would either remain a source with a net SOC loss of 3 Tg or become a weak sink with a net SOC gain of 4 Tg. These estimates are lower bounds for potential SOC flux because they are only for the surface meter of landmass.

1Hargis, T.G., and Twilley, R.R., 1994, Improved coring device for measuring soil bulk density in a Louisiana deltaic marsh: Journal of Sedimentary Research, v. 64A, no. 3, p. 681–683
2SSURGO is the acronym for the Soil Survey Geographic Database developed and distributed by U.S. Department of Agriculture, Natural Resources Conservation Service. A SSURGO dataset includes digital map data, attribute data, and metadata





Purpose and Scope

Climate, Physiography, Geology, and Vegetation




Vegetation and Land-Use Change

Summary of Methods

Field Methods

Sampling Devices

Sampling for Comparison of Methods

Laboratory Methods

Approach to Developing Chronostratigraphy

Data Reduction Methods

Data and Results

Bulk-Density Measurements

Marsh and Swamp

Bulk-Density Comparisons—Bayou Perot Intermediate-Marsh and Lake Salvador Fresh-Marsh Cores

Comments on Marsh-Core Bulk-Density Data

Tangipahoa Swamp Cores TN1b and TN1c

Natural Levee, Distributary, and Backswamp

St. Landry Backswamp Core SL1b

Organic-Carbon Measurements and Soil/Sediment Carbon Trends

Marsh and Swamp

Natural Levee, Distributary, and Backswamp

Cesium-137 and Potassium-40 Measurements

Differences in Cesium-137s and Potassium-40 by Environment


Natural Levee and Distributary



Post-1963–64 Trends in Soil/Sediment Deposition in Marsh, Levee, and Backswamp

Carbon-14 Measurements

St. Mary Fresh-Marsh Cores SM1a, SM1b, and SM1c

Terrebonne Brackish-Marsh Cores TB2a and TB2c

Bayou Sauvage Distributary

Natural Levee, Backswamp, and Swamp

Measurements for Inorganic Elemental Constituents

Differences in Inorganic Elemental Constituents by Environment

Marsh—Major and Minor Elements

Backswamp—Major and Minor Elements

Swamp—Major and Minor Elements

Palynomorph and Other Microstratigraphic Marker Data

Palynomorph Age Indicators

Vegetation Classification

Common Cover Types


St. Mary Fresh-Marsh Vibracore SM1c

SM1c Zone A: Sample Depth 268.0 Centimeters

SM1c Zone B: Samples Depths 266.7, 246.1, 227.3, 217.2, 201.9, 189.2, and 179.1 Centimeters

SM1c Zone C: Sample Depths 168.3, 161.3, 148.0, 133.4, 122.6, and 111.8 Centimeters

SM1c Zone D : Sample Depths 96.5 and 6.4 Centimeters

St. Bernard Brackish-Marsh Vibracore SB1c

SB1c Zone A: Sample Depth 396.0 Centimeters

SB1c Zone B: Sample Depths 305.0, 316.0, 365.0, and 384.0 Centimeters; and SB1c Zone C: Sample Depths 214.0, 235.0, 265.0, and 285.0 Centimeters

SB1c Zone D: Sample Depths 127.0, 166.0, 195.0, and 205.0 Centimeters

SB1c Zone E: Sample Depths 85.0 and 106.0 Centimeters

SB1c Zone F: Sample Depths 44.0 and 69.0 Centimeters; and SB1c Zone G: Sample Depths 4.0 and 11.0 Centimeters

Terrebonne Brackish-Marsh Vibracore TB2c

TB2c Zone A: Sample Depths 532.9, 433.4, and 357.9 Centimeters

TB2c Zone B: Sample Depths 283.9, 243.4, and 199.9 Centimeters

TB2c Zone C: Sample Depths 146.5, 129.8, 93.6, 67.2, and 66.1 Centimeters


St. Martin Backswamp Push-Core MR1d

MR1d Paleoenvironment and Age

St. Landry Backswamp Push-Core SL1c

SL1c Paleoenvironment and Age

Swamp—Tangipahoa Swamp Push-Core TN1d

TN1d Paleoenvironment and Age

Soil/Sediment Organic Carbon Landscape Distribution

Estimation of Soil/Sediment Organic Carbon Inventory

Geographic Trends in Soil/Sediment Organic Carbon Inventory

Mississippi River Deltaic Plain Soil/Sediment Organic Carbon Storage by Environment

Data Comparisons

Depth Trends in Organic-Carbon Concentration

Accumulation Rates—Temporal Trends in Mississippi River Deltaic Plain Soil/Sediment Organic Carbon Sequestration

Bomb-Spike Cesium-137

Bomb-Spike Delta Carbon-14

Cosmogenic Carbon-14

Palynomorphs and Other Stratigraphic Markers

Uncertainties in Age Assignments

Recent Trends in Soil/Sediment Organic Carbon Loss—Effects of Land Loss

Land-Loss Estimates

Estimates of Soil/Sediment Organic Carbon Loss and Potential Gain

Soil/Sediment Organic Carbon Source or Sink

Using Cesium-137 Data

Using Carbon-14 Data

Hurricanes Katrina and Rita— Effects on Soil/Sediment Organic Carbon Projections

Summary and Conclusions





Appendix 1

Appendix 2

Appendix 3

Appendix 4

Appendix 5

Appendix 6




This report is available online in pdf format (16.5 MB): USGS Professional Paper 1686-B (Opens the PDF file in a new window. )
To view the PDF document, you need the Adobe Acrobat® Reader installed on your computer. (A free copy of the Acrobat® Reader may be downloaded from Adobe Systems Incorporated.)

For more information, please contact Helaine W. Markewich.

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