Estuarine and Coastal Marine Science (1974) 2, 407-415

Methane, Carbon Dioxide and Dissolved Sulfate from Interstitial Water of Coastal Marsh Sediments

Thomas Whelan Coastal Studies Institute, Louisiana State University, Baton Rouge, Louisiana 70803, U.S.A.

Received 5 April 1974

The depth distributions of dissolved CH,, total CO*, Cl- and SO,*- in interstitial waters are reported from three subenvironments in the coastal marsh sediments of south Louisiana. Dissolved CH, ranged from 8.0 x 10-3

to 7.2 ml/l, with each core demonstrating a slight increase in concentration with depth. The highest concentrations of methane were observed in the northern subenvironment of the marsh, which was low in salinity. Extraction under vacuum of methane from whole sediment samples indicated that a significant amount of gas is present as bubbles. Total CO* ranges from I IO to 410 ml/l. Carbon dioxide, extracted under vacuum, yielded Si3C PD s values of - 18.9 and -22*5%,, for the southern and northern areas, respectively.

The X0, concentration in two areas was greater than or equal to that predicted by the depletion of dissolved S0,2- from microbial reduction processes. In one core, however, the X02 concentration was less than could be accounted for by the S0,2- deficit. In general, where S0,2- reduction was active or where the dissolved S0,2- concentration was low, the concen- tration of CH, increased. Relatively small vertical gradients of dissolved CH, and ZC02, the low hydrostatic head above the sediment column and the high degree of unconsolidation suggest that gas is migrating upward through the sediment column. Coastal marshes may be an important source of methane to the atmosphere and waters of coastal environments.

Introduction

Investigations concerning the origin and distribution of sedimentary gas have become impor- tant recently for several reasons. In situ production of CH, and CO,, initiated by early transformations of sedimentary organic matter, is known to be an important link in certain diagenetic processes such as carbonate cementation (Garrison et al., 1969; Allen et al., 1969; Presley & Kaplan, 1968; Whelan & Roberts, 1973) and metal sulfide precipitation (Nissen- baum et al., 1972). In addition, anomalous acoustical behavior has been correlated with the existence of gas bubbles in marine sediments (Jones et al., 1964; Levin, 1962). It has been suggested that the concentration of CH, in estuarine interstitial waters is regulated by bubble formation which is followed by removal from the sediment column (Reeburgh, 1969). The presence of shallow sedimentary gas may have a pronounced effect on sediment stability, directly by entrapment of gas bubbles in unconsolidated sediment or indirectly by initiation of certain diagenetic features such as carbonate cements and nodules.

408 T. Whelan

Several mechanisms have been proposed for the production of CH, in marine sediments. Koyama (1964) demonstrated that in paddy soils CH, was generated primarily by bacterial fermentation of low-molecular-weight organic acids. It was postulated that in the reducing sediments of Saanich Inlet CH, was produced from biochemical reduction of preformed CO, subsequent to the termination of SOa2- reduction; this process yielded total CO, (X0,) in the interstitial waters enriched in 13C with respect to sea water (Nissenbaum et al., 1972).

Coastal marsh environments are typified by organic-rich reducing sediments with a high gas content. The purpose of this paper is to investigate the depth distribution and variability of CH,, total CO, and dissolved SOd2- in three subenvironments of the marsh sediments of south Louisiana.

Study site and methods

Location

Core samples were taken from three subenvironments associated with the Barataria Bay complex of southern Louisiana (Figure I) during November 1972 and September 1973.

The general environmental setting and descriptions of stations AL (Airplane Lake) and JTF (John the Fool) have been described by Ho & Lane (1973). Airplane Lake, in the south- ern area, is a semi-enclosed interdistributary water body surrounded by marsh in which tidal flushing is restricted. Salinities average from zz to ~5%~. John the Fool Lake, in the northern area, is also a semi-enclosed lake surrounded by marsh. Salinities, however, are much lower, averaging from 5 to IO%,,. In both areas, Spartina alterniflora or S. putens and montmorillonite clay are the most abundant plant and sediment types, respectively. The

km

Figure I. Location map showing the study areas.

CH4, COa and SOde- in coastal marsh water 409

sedimentary sequence in both areas indicate that marsh sediments extend about I m below the sediment-water interface. Below this depth sediments become coarser with sand and silt interbeded, a feature which is representative of levee and bay environments.

Station MB (Macoin Bay) was chosen in order to compare methane concentrations in a well-flushed marsh sediment with those in the two more restricted marsh types. Macoin Bay is a relatively large body of water which is directly influenced by daily movement of sea water through Caminada Pass from the Gulf of Mexico. The sediments of Macoin Bay are more typical of the bay-bottom type, interbedding of silt and sand occurring within a clay sequence.

All core samples were taken in '-1.5 m of water. These sediments remain under water throughout the year, even though water levels may fluctuate seasonally 30-40 cm.

Coring technique

Two piston cores of approximately 6o-70 cm and three of IOO-IIS cm were taken in 7*6-cm PVC core barrels. Each core was quickly sealed with a rubber stopper before it was removed from the water in order to minimize gas loss and air contamination. After pistons and rubber stoppers for each core were secured in place, the cores were stored on ice in the field and kept at 4 “C until laboratory analysis, about one week later.

Methods of analysis

Intervals of approximately IO cm were extruded directly from the core barrel into N,- operated sediment squeezers (Reeburgh, 1967). Twenty milliliters of interstitial water from each section was squeezed into two ro-ml plastic syringes. Ten milliliters was used for dissolved methane analysis and IO ml was saved for Cl-, SOd2- and X0, measurements. Dissolved methane was stripped from the water by purging with He for 5 min (Reeburgh, 1968). The gases were routed through an activated charcoal trap cooled with dry ice acetone, at -70 “C, to quantitatively adsorb methane. The trap was heated to 90 “C with a hot- water bath to release methane and a I-CC sample was injected into a gas chromatograph operated with a flame ionization detector. The precision and accuracy of methane analysis by this method were comparable to that obtained with the multi-phase equilibrium tech- nique described by McAuliffe (1971) The stripping technique was found to be linear for dissolved methane concentrations higher than observed in the northern marsh areas.

Methane was extracted from two 6o-cm cores collected in November 1972 by extruding the entire sample into a vessel evacuated to 5 mmHg and sweeping the expanded gas through the -70 “C activated charcoal trap to adsorb CH,. The CO, released under vacuum from the same sample was condensed with liquid nitrogen and collected in glass break seals for carbon isotope ratio determination. The WC values are given relative to the PDB standard.

Total CO, (X0,) was determined on the second ro-ml sample of interstitial water by infrared adsorption of CO, released by acidification using a total carbon analyzer.

Dissolved sulfate was determined on separate s-ml aliquots by the EDTA titration methods described by Vogel (1961).

Results

The results presented in Table I show that chlorinity in the interstitial water was about IO%,, less at station JTF than in the two southern areas. The northern marshlands of Louisiana are typified by fresher waters owing to limited sea water iniIux. In core MB,

410 T. Whelan

chlorinity increased to 15.1%~ at 50 cm from the surface value of 11.8%~ and dropped to 14.6x,, at 90 cm. The Cl- ion concentration remained fairly constant with depth in the other two cores.

TABLE I. Depth distribution of several dissolved components in the interstitial waters

Station Soda- Cl- Sod*- deficit” x0, depth (cm) (%o) (%o) (md 6-f)

O-5 0.83 12.5 5-11 1’57 12.6

22-27 0.67 12.8

42-47 0.53 12.6

52-57 0.42 12.6 62-67 0.48 12.7

7277 0.43 12.8 83-88 0’22 12.7

113-118 0.3 I 12.6

AL-I I 9’50 16.25 I .80 16.33

11.62 17.67

O-5 1’59 11.8 16-22 I .88 12.6

27-32 I ‘67 13.6

37-42 I.74 14.6

47-52 I .66 15.1

57-62 I’59 14’9 67-72 1’73 14.8 77-82 1.64 14.7 88-93 1.62 14.6

12.65 17’92 13’92 17’50 13’35 18.08

14.02 17’50 16.10 17’50 14’98 18.33

MB-18 0.64 7.08

-1.32 5’00 2.31 5’42 3.00 6.42 4.60 7’33 5’07 8.92 3.36 8.33 4.20 9.00 4’30 9.08

JTF-4 2-8 0’04 3’4 4’52 13’92 8-13 0’02 3’3 4.62 14’50

13-18 0.04 3’3 4’32 14’50 23-28 0.08 3’4 4.06 13’92 33-38 0’0 3’3 - 13’33 43-48 0.13 3’3 3.38 13’33 53-58 0.04 3’7 4’92 14’17 64-69 0’0 3’3 - x4.17 74-79 0’0 3’3 - 11.67

84-89 0’0 3’3 - 10’00

2’0 x 10-l 1’5 x 10-l 2.3 x 10-l 2.3 X 10-l 2.8 x 10-l 5’3 x 10-l 4.2 x 10-l 8.2 x 10-l

14.0X 10-l

8.8 x IO+ 8.2 x IO-~ 8.0 x IO-~ 8.0 x 10-S 8.0 x IO-~

12.8x 10-3 9’4X 10-S

12.7x IO-~ 12’1 x 10-3

3’2 2.8 5’2 5’2

2:;

7’2 7’3 1’2 1.6

‘SO,e- deficit mM = Cl- x0 sample X 0.14--S0,~- x0 sample

96.06

x Ios.

Dissolved SOa2- ranged from 0.22 to 1.57%~ in core AL, from 1.88 to 1’59%,, in core MB and 0.13 to ox0 in core JTF. There was at least an order of magnitude difference between the SOd2- concentration in cores JTF and MB.

The SOd2- deficit was calculated in order to compare total CO, concentrations that could be produced entirely by bacterial SOd2- reduction. The millimolar (mM) deficit in SO, was derived from the difference between the observed SOe2- concentration and that calculated from the sea water ratio of S0,2-/C1-, as shown in Table I. The greatest increase in SOa2- deficit was observed in core AL. The interstitial water at 8 cm was depleted by 1.8 mM in SOd2-, while at 85 cm the SO, 2- deficit was 16.1 IIIM. The SO,s- deficit ranged from - 1.32 to 5.07 mM in core MB-18 and from co to 4.92 mM in core JTF.

Figures 2, 3 and 4 and Table I illustrate the depth distributions of methane and total CO, in cores MB, AL and JTF, respectively. Methane ranged from 8 x IO-~ to 12.8 x IO-~ ml/l

CH,, CO2 and S0,2- in coastal marsh water 411

in core MB, from 1.5 x 10-l to 14.0 x 10-l ml/l in core AL, and from 1.2 to 7.3 ml/l in core JTF. There were approximately three orders of magnitude difference between the concentration of methane in cores JTF and MB. Total CO, ranged from 5.0 to 9.08 mM in core MB, from 1625 to 18.33 mM in core AL and from 10.0 to 14’5 mM in core JTF.

CH,(ml/l IWx10-8)

Figure 2. Depth profile of methane and carbon dioxide concentrations from core MB-I%

Table z demonstrates the variability of dissolved CH,, total CH, and a comparison of 613C values of CO, extracted by the vacuum degas technique. Core AL-IO contained 3-4 times less dissolved CH, than AL-r I at the corresponding depth intervals. The sediment in

ZCO,(mL/l IW) 300 320 340 360 L 380 400

/ 1 I

7 - t I

25- hi, I

\ .

Z 0

-1 ‘x0,"

g 50- l \ \

t 91 ./’ .s -‘\ \ f f i 0 75- i’

0 \. i .

100 - I

,

0 05

,y4,

IO

CH,(ml/t IW)

Figure 3. Depth profile of methane and carbon dioxide concentrations from core AL-II.

412 T. Whelan

core AL-IO was highly porous and contained numerous root fibers. It is possible that this sediment was more closely associated with the water column and hence more oxidizing and not representative of the sediments under investigation. Similar concentrations of CH, were noted at both depths in JTF-4 and JTF-6. Total CH,, as determined by the vacuum

ZCO,(ml/L IWI

I I 2 4 6 ;

CH,(mL/l IW)

Figure 4. Depth profile of methane and carbon dioxide concentrations from core JTF-4.

degas technique, is higher in both cases than the corresponding value for dissolved CH,, indicating that a significant portion of CH, is present in the gas or bubble phase. The P3C values were determined on CO, released under vacuum (both dissolved and bubble phases). Cores JTF-I and JTF-z were about 3x,, more negative than cores AL-3 and AL-4. The

TABLE 2. Comparison of total CHI (vacuum degas method), dissolved CHI and PC of CO, in marsh sediments from sites JTF and AL

Dissolved CHI W/g sed.)”

Total CHd P3C values W/g se0 of coa

by vacuum degas method

AL-IO 0.03 0.05 - AL-I r 0.13 0’22 -

AL-3 and AL-4b - - 0.29 -18.9

JTF-4 3’1 5.2 - JTF-6 3’7 3’4 -

JTF-I and JTF-2b - - 8.3 -22.5

’ Assuming that these sediments contain 50 o/0 water by weight, the values reported for methane in milliliters per liter of interstitial water (Table I) can be expressed as mg/g of dry sediment. b Average of CH, concentration from cores taken in November 1972; all other data reported here were taken in September 1973.

CH4, CO2 and SOd2- in coastal marsh water 413

overall enrichment of 12C in these samples with respect to typical marine- and fresh-water bicarbonate indicates that the main source of sedimentary CO, was from decomposition of organic carbon.

Discussion

The production of sedimentary CH, is highly variable in nearshore and coastal environ- ments. In this study, three orders of magnitude difference in the concentration of dissolved methane were observed between the sediments in the northern, low-salinity marsh and those in the open marsh of the southern area. Emery & Hoggan (1958) reported CH, concentra- tions of 0.24 ml/l and 0.047 ml/l in interstitial waters from two separate marsh sediments. Concentrations of dissolved CH, in Chesapeake Bay sediments ranged from o near the sediment-water interface to ISO ml/l at IOO cm (Reeburgh, 1969). It was proposed that the concentration of CH, was regulated by ebullition from the sediment as bubbles. Comparison of the total and dissolved CH, concentrations reported in Table 2 indicates that a significant fraction of sedimentary methane exists as gas bubbles trapped within the sediment column. The difference in water temperature between November 1972 and September 1973 was too small to account for the difference in methane concentration.

Total CO, concentration in core AL (the highest X0, of the three areas) was about 2.5 times lower at IOO cm than in Chesapeake Bay sediments (Reeburgh, 1969) and 2 to 5 times higher than reported for nearshore marine sediments (Presley & Kaplan, 1968) at IOO cm. The excess total CO, in the interstitial waters of non-carbonate deposits presumably originates from microbial degradation of organic matter rather than from solution of CaC03.

Assuming that microbial sulfate reduction can be represented by the equation

each molecule of SOh2- reduced should yield two molecules of bicarbonate. If all the inter- stitial X0, was produced from this reaction, the millimolar deficiency in interstitial S0,2-, as calculated from the difference between the interstitial ratio of S0,2-/C1- and the sea water ratio of S0,2-/C1-, should be one-half the concentration of X0,. The data in Table 2

demonstrate that in core JTF there was about 1.5 times as much X0, as could be accounted for by sulfate reduction processes alone. In core MB, approximately all the X0, could be accounted for by SO, reduction, and in core AL there was less X0, than could be accounted for by this process.

Methane may be produced by several mechanisms, including anaerobic fermentation of organic matter (Rosenfeld & Silverman, 1959; Emery & Hoggan, 1958) and chemical reduction of preformed CO, (Nissenbaum et al., 1972). It was postulated that the kinetic isotope effect associated with this reaction

C02+4H2+CH,+2H20

yielded X0, enriched in WC up to +17.3%~ WYSUS PDB. The sediments which had the most positive 613C values of dissolved X0, were devoid of measurable SO,2- and S2-, contained the highest concentration of X0, and had positive E, values. The 61X! values reported in Table 2 were -22.5x,, for JTF and -18.9%~ for AL; they indicate that this mechanism of CO, reduction was not operating under the same conditions as for Saanich Inlet sediments discussed by Nissenbaum et al. (1972). However, the data in this study indicate that a relationship exists between the amount of SOa2-and the amount of CH, present. The concentration of CH, was highest in core JTF, where SOa2- (total) was lowest,

414 T. Whelan

and CH, was lowest in core MB, where interstitial SO, was highest. In AL, SOd2- demon- strated the greatest change with depth, as did the CH, concentration. These trends are consistent with a set of competitive reactions for molecular or organically available hydrogen between !Y- and CO,, yielding H,S, or metal sulfides, until S2- becomes low or absent; then CO, reduction may occur.

The gradients of dissolved CH,, X0, and SO,2- in the interstitial water of all three cores taken from the marsh were considerably smaller than reported for the other marine environments previously mentioned. The vertical gradient in SOd2- concentration was significant only in core AL, which was taken in a closed interdistributary bay. It appears likely that marsh sediments, which are geologically Recent deposits and extend to only about I m below the sediment-water interface, are in a steady state of gas production and that a significant quantity of gas exists as bubbles migrating toward the sediment-water interface. The high water content of these sediments, their uniform degree of unconsolida- tion, and the relatively shallow water column (small hydrostatic head) above the sediment lend support to this proposition. More information is needed concerning the concentration and distribution of CH, in the water and in the atmosphere above the marsh in order to estimate rates of gas production associated with coastal marshlands.

Acknowledgements

I wish to thank Dr Brian Eadie of Texas A & M University for isotope ratio determination. This study was supported by the Geography Programs, Office of Naval Research, through

the Coastal Studies Institute, Louisiana State University, under Contract Nooorq-69-A- OZII-0003, Project NR 388 002.

References

Allen, R. C., Garvish, E., Friedman, G. M. & Sanders, J. E. 1969 Aragonite cemented sandstone from the outer continental shelf off Delaware Bay: submarine lithification mechanism yields product resembling beachrock.gournal of Sedimentary Petrology 39, 136-149.

Emery, K. 0. & Hoggan, D. 1958 Gases in marine sediments. Bulletin of the American Association of Petroleum Geologists 4, 2174-2188.

Garrison, R. E., Luternaur, L. J., Grill, E. U., McDonald, R. D. & Murray, J. W. 1969 Early diagenetic cementation of Recent sands, Fraser River delta, British Columbia. Sedimentology 12, 27-46.

Ho, C. L. & Lane, J. 1973 Interstitial water composition in Barataria Bay (Louisiana) sediment. Estuarine and Coastal Marine Science I, 125-135.

Jones, J. L., Leslie, C. B. & Barton, L. E. 1964 Acoustic characteristics of underwater bottoms.Journal of the Acoustical Society of America 36, 154-157.

Koyama, T. 1964 Gaseous metabolism in lake sediments and paddy soils. In Advances in Organic Geochemistry pp. 363-375. (Colombo, U. & Hobson, G. D., eds.) MacMillan, New York.

Levin, F. K. 1962 The seismic properties of Lake Maracaibo. Geophysics 27, 35-47. McAuliffe, C. 1971 Gas chromatographic determination of solutes by multiple phase equilibrium.

Chemical Technology I, 46. Nissenbaum, A., Presley, B. J. & Kaplan, I. R. 1972 Early diagenesis in a reducing fjord, Saanich Inlet,

British Columbia. I. Chemical and isotopic changes in major components of interstitial water. Geochimica and Cosmochimica Acta 36, 1007-1027.

Presley, B. J. & Kaplan, I. R. 1968 Changes in sulfate, calcium and carbonate from interstitial water of nearshore sediments. Geochimica and Cosmochimica Acta 32, 1037-1048.

Reeburgh, W. S. 1967 An improved interstitial water sampler. Limnology and Oceanography IZ, 163- 165.

Reeburgh, W. S. 1968 Determination of gases in sediments. Environmental Science and Technology 2, 140-141.

Reeburgh, W. S. 1969 Observations of gases in Chesapeake Bay sediments, Limnology and Oceanogruphy x4,368-375.

CH4, CO2 and SOda- in coastal marsh water 415

Rosenfeld, W. D. & Silverman, S. R. 1959 Carbon isotope fractionation in bacterial production of methane. Science 130, 1658-1659.

Vogel, A. T. 1961 Quantitative Inorganic Analysis 3rd edition, 446-447. London, Longmans. Whelan, T. & Roberts, H. H. 1973 Carbon isotope composition of carbonate nodules from a fresh

water swamp. Journal of Sedimentary Petrology 43, 54-58.