Distribution, abundance and carbon isotopic composition of gaseous hydrocarbons in Big Soda Lake, Nevada: An alkaline, meromictic lake

0016.7037/X3/122107-08$03.00/O

Geochmrcrr PI Cumochimm AC/U Vol. 41, pp. 2107-21 14 0 Pergamon Press Ltd.1983. Printed inU.S.A.

Distribution, abundance and carbon isotopic composition of gaseous hydrocarbons in Big Soda Lake, Nevada: An alkaline, meromictic lake RONALD S. OREMLAND U.S. Geological Survey, Menlo Park, CA 94025 and DAVID J. DES MARAIS NASA Ames Research Center, Moffett Field, CA 94035 (Received February

15, 1983; accepted in revised form August 26, 1983)

Abstract-Distribution and isotopic composition (6°C) of low molecular weight hydrocarbon gases were studied in Big Soda Lake (depth = 64 m), an alkaline, meromictic lake with permanently anoxic bottom waters. Methane increased with depth in the anoxic mixolimnion (depth = 20-35 m), reached uniform concentrations (55 PM/I) in the monimolimnion (35-64 m) and again increased with depth in monimolimnion bottom sediments (>400 PM/kg below I m sub-bottom depth). The 8’3C[CH4] values in bottom sediment below 1 m sub-bottom depth (c-70 per mil) increased with vertical distance up the core (6”C[CH4] = -55 per mil at sediment surface). Monimolimnion b”C[CH.,] values (-55 to -61 per mil) were greater than most 6”C[CH.J values found in the anoxic mixolimnion (92% of samples had G”C[CH.,] values between -20 and -48 per mil). No significant concentrations of ethylene or propylene were found in the lake. However ethane, propane, isobutane and n-butane concentrations all increased with water column depth, with respective maximum concentrations of 260, 80, 23 and 22 nM/l encountered between 50-60 m depth. Concentrations of ethane, propane and butanes decreased with depth in the bottom sediments. Ratios of CH4/[C2H6 + CJHs] were high (250-620) in the anoxic mixolimnion, decreased to -161 in the monimolimnion and increased with depth in the sediment to values as high as 1736. We concluded that methane has a biogenic origin in both the sediments and the anoxic water column and that C,-C, alkanes have biogenic origins in the monimolimnion water and shallow sediments. The changes observed in Gi3C[CH4] and CH4/(C2H6 + CrHs) with depth in the water column and sediments are probably caused by bacterial processes. These might include anaerobic methane oxidation and different rates of methanogenesis and C, to Cq alkane production by microorganisms.

INTRODUCTION

HABITATSFAVORABLE for the early, depositional stages of oil and gas formation probably include aquatic environments with permanently anoxic bottom waters (e.g., meromictic lakes) whose sediments receive large inputs of organic matter from their productive surface waters (DIDYK et al., 1978; DEMAISONand MOORE, 1980). In addition, hydrocarbon production may be enhanced by hypersaline conditions (DEGENS and PALUSKA, 1979). Geological evidence indicates that certain lacustrine-derived oil shales, such as the Tertiary Green River Formation in Colorado and Utah, evolved from sediments of meromictic lakes with bottom waters (monimolimnions) that were anoxic, hypersaline and in addition, were highly alkaline (BRADLEY, 193 1; SMITH and ROBB, 1973; ROEHLER, 1974; TISSOT el al.. 1978; DEMAISONand MOORE, 1980). However, only a few studies have characterized the low molecular weight gaseous hydrocarbons found within possible present-day analogues of such ancient anoxic, hypersaline environments. SACKETT et al. (1979) reported that methane, ethane and propane, found in the Orca Basin (Gulf of Mexico) brinewaters and sediments appeared to have a biogenic origin. POTTS (1979) observed production of ethylene by microorganisms present in the water column of Solar Lake (Sinai). Thus, aside from the studies just mentioned, there is

a gap in our knowledge with respect to the gaseous hydrocarbons present in hypersaline and anoxic waters, especially highly alkaline ones. In this paper, we report on the presence of low molecular weight hydrocarbons and their carbon isotopic compositions (613C) in Big Soda Lake, an alkaline, moderately hypersaline, meromictic lake. STUDY

SITE

DESCRIPTION

Big Soda Lake is located in western Nevada near Fallon (about 560 km east of San Francisco). Meromixis was caused by irrigation practices which have induced an 18 m rise in the lake level since 1907, leaving a chemocline present at 35 m depth (HUTCHINSON, 1937; KOENIGet a/., 1971; Rust& 1972; KIMMEL et al., 1978).The lake is about 64 m deep at its center and has a surface area of I .6 km-‘. The stratification is caused by dense, saline bottom waters of the monimolimnion (total dissolved solids = 87 g/liter below 35 m) underlying less dense surface waters of the mixolimnion (total dissolved solids = 26 g/liter above 20 m) RUSH, 1972; KHARAKA el al.,1984).Details on the chemistry ofthe lake will be published elsewhere (KHARAKA et al., 1984). The waters are alkaline (pH = 9.7), and the bottom waters are anoxic. Monimolimnion waters contain abundant sulfate (6.5 g/liter), free sulfide plus reduced sulfur compounds (420 mg/liter), ammonia (50 mg/liter), dissolved organic carbon (60 mg/liter) and are well buffered (alkalinity = 410 meq/liter) (KHARAKA et ul., 1984). During periods of thermal stratification (spring through fall). dissolved oxygen disappears at about 19-20 m (oxycline). During winter, the upper 29 m are nearly isothermal and

2107

R. S. Oremland and D. J. Des Marais

2108

oxygen is mixed down to 28 m (CLOERNet al., 1983a). Sea- erson grab was opened and the sediments were poured into sonal changes in the lake’s zonation along with the terms a plastic pan. One hundred fifty ml of the sediment were used to describe the various zones have been summarized in then placed in a wide mouth Edenmeyer flask (250 ml) that Fig. 1. already held 50 ml of distilled water. The 5asks were sealed A bloom of pennate diatoms occurs in the mixolimnion with black rubber stoppers and were vigorously shaken for during winter which is followed (during summer and fall) by 15 min. About 5 min elapsed from the time the sediment the growth of a dense plate of photosynthetic bacteria (Ecgrab was tripped to the time the sediments were sealed in tothiorhodospiru vaculata; H. TRUPER,pers. commun.) at a the 5asks. After shaking, the vapor phase of the extraction depth ofabout 21 m (CLOERNet al.. 1983a). Integrated annual flasks was transferred to serum vials. Thirty ml of the gas water column productivity has been estimated to be 500 gC/ ph.ase were removed by syringe while simultaneously injecting 30 ml of distilled water into the flasks to avoid presm* (CLOERNet al., 1983b) and is subject to nitrogen or trace metal limitation during spring through fall (AXLER etal., sure differentials. Vials were injected as described above. Sediments captured in ‘the piston core were extruded into 1978;PRLSCUet al., 1982). Sediments taken from the monimolimnion are soft, fluid, and generally green in color (due subcores and sediment sub-samples (- 18 ml or -20 g) to abundant chlorophylls; G. RAU, pets. commun.). Cores were quickly dispensed into pre-weighed vials (vol = 67 ml) taken from the monimolimnion exhibit numerous colored which contained 10 ml of 10 N NaOH (to kill microorganisms). Vials were next capped with black rubber stoppers bands (green, red, brown, black), possibly caused by the sinking of bacteria and algae from the water column. Numerous pen(no. 5) and stored in this fashion until analysis for C,-Ch nate diatoms and actively motile bacteria can be observed hydrocarbons (within 2 weeks). upon examination of these sediments with a microscope. For later determination of d’%Z,methane and other hyMonimolimnion sediments have methanogenic bacterial acdrocarbons (ethane and propane) were extracted in the field tivity (OREMLAND et al., 1982a). The organic carbon content and stored. For extraction of CH4, graduated prescription of these sediments is about 3% by wt. (S. ROBINSON and G. bottles (500 ml) were tilled with 440 ml of lake water and RAU, pets. commun.). The 6’% ofdissolved inorganic carbon capped with a recessed, black rubber stoppers (no. 3). The is -0.7 to -1.2 per mil above the chemocline and -2.8 per gas phases (approx. 65 ml) were next displaced by flushing to the atmosphere with ultra high purity He for 1.5 min mil below (KHARAKA et al., 1984). The 6°C of organic carbon (50~ = 150 ml/min). Next, methane was extracted from in monimolimnion sediments (upper 2 cm) ranges from -20.5 the lake water by vigorously shaking the bottles (15 mm). to -22 per mil (G. RAU and S. ROBINSON, pers. commun.). The helium gas phase which now contained the extracted methane was subsampled by transfer into test tubes (18 METHODS AND MATERIALS X 150 mm; vol = 25 ml; sealed with no. 1 recessed butyl Dissolved oxygen was determined initially (Oct., 1980 rubber stoppers). This was accomplished by injecting 50 ml and Apr., 1981) by use of a “Hach Kit” (ALSTERBERG, of distilled water into the bottles while simultaneously al1925). Subsequent determinations were performed with sul- lowing a 50 ml glass syringe inserted into the gas phase to fide resistant electrodes and by modifications of the WinkIer fill with the disulaced asses. The captured gas uhase was technique (CARPENTER, 1965) designed to eliminate sulfide next injected into the-inverted, distilled-w%er%lled test interference (results have been presented by CLOERN et al.. tubes. Displaced water exited the tube via a venting needle 1983a). Gaseous hydrocarbons were extracted from lake (20g) inserted through the rubber stopper. Tubes were water by a modification of the procedure of RUDD et al. stored in the inverted position with about 1 ml of distilled (1974). Thirty ml of lake water followed by either 30 ml of water left lying over the stopper (to insure a seal). Just prior air or high purity Nz were drawn up into a 50 ml plastic to analysis, 5 ml of 2 N NaOH were injected into the test syringe (total syringe volume = 70 ml). The syringe was tubes to create a positive pressure in the tubes and to sorb fitted with a 22 gauge needle, the needle was inserted into carbon dioxide. The extraction procedure was tested for any a rubber bung and then the syringe was vigorously shaken possible procedural isotope fractionation by using degassed for 3 min. The entire 30 ml of vapor phase was next purged lake water allowed to equilibrate overnight with a gas phase through a serum vial (vol = 7 ml) that was sealed with a of methane of known isotopic composition. No differences recessed, butyl rubber stopper (no. 1) and vented with anwere observed between the composition of this methane and other 22 gauge needle. Hydrocarbon gases were stored for its parent source. Blanks were also run using degassed lake at least 3 weeks in these vials without significant loss (
SPRING-FALL

WINTER

Mixolimnion (Aerobic) 20m -

35m-

-291n -35m

64m-

-64m

FIG.1.Seasonal variations in the limnologicaf properties of Big Soda Lake (from CLOERN etal., 1983a). Surface water temperature varies from a maximum of 24°C (summer) to a minimum of 4°C during winter. Temperature of the monimolimnion is always 13°C. Oxycline refers to the depth at which dissolved oxygen is no longer detectable.

Light hydrocarbons stopper, the gas phase (vol = 250 ml) was flushed for -4 min with ultra high purity helium (flow = 200 ml/min), and then placed on a reciprocal shaker at high speed for 15 min. After shaking, the gas phase was transferred to a 1 liter bottle by water displacement, Thus, four flasks (total liquid vol. = 1.5liters) were extracted from depth to yield a 1 liter gas sample of extracted hydrocarbons in helium. The method was checked for procedural isotopic fractionation and hydrocarbon blanks as indicated for methane. No differences in 6’jC were detected against hydrocarbon standards and no background ethane or propane was found in the blanks. For determination of d’3C[CH.+] extracted from the piston core, the sample vial headspace gas was transferred into 18 X I50 mm test tubes by displacement with distilled water as outlined above.

Quantijcaiion ofdissolvedhydrocarbons and derermination of 6°C Hydrocarbons (methane through butane) in the storage vials were analysed by flame ionization gas chromatography (KVENVOLDEN and REDDEN, 1980; OREMLAND, 1981). The methods used to determine the 613C of methane, ethane and propane have been published elsewhere (HAYES et al., 1977; MATTHEWS and HAYES, 1978; DES MARAIS, 1978; OREMLAND et al, 1982a). For analysis of G’3C[CZH,] and 6”C[C3Hs], the 1 liter bottle was immersed in a dry ice/ acetone bath to IN)wer the vapor pressure of water and the rubber stopper was penetrated with a 22 gauge needle attached to a piece of flexible stainless steel tubing. The tubing was connected to a 5 cc loop which was immersed halfway in liquid nitroge:r. This loop was part of a Valco 8-port valve injection system of the gas chromatograph. The contents of the bottles were drawn through the chilled loop, trapping the ethane and larger hydrocarbons in the loop with 100% efficiency and pumping away the noncondensible gases into a vacuum system fitted with a liquid nitrogen trap. The hydrocarbons in the loop were then warmed and injected into the gas chromatograph-combustion system using the Valco &port valve. Carbon isotope ratios (613C) of samples are expressed as: fS’?C ““Lnown= where R is “C/“C bonate standard.

R““how”

(

------1 RPDB

1

x1000

and PDB is the Peedee

Recovery ofmethanogenic

Belemnite

car-

bacteria from the water column

To test whether methanogenic bacteria were present in the water column, water samples were recovered from 33 m depth during Feb. 1982. Water was gently drained from the 7 liter Niskin sampler into 4 liter glass bottles. The bottles were filled to overflowing and sealed with black rub-

in Big Soda Lake

2109

ber stoppers. Care was taken to avoid air bubbles. Upon return to shore, the bottles were briefly opened and 100 ml of water were removed in order to generate a gas phase in the bottles. All manipulations took place under a flow of nitrogen gas. After resealing, the bottle stoppers were punctured with a nitrogen gassing syringe (and venting needle) and purged for 20 min (flow = 150 ml/min) in order to remove traces of oxygen. Substrates were added to two bottles in order to promote methanogenesis at concentrations of 2 mM (methanol) and I mM (trimethylamine). Twobromoethanesulfonic acid (BES). a specific inhibitor of methanogens (GUNSALUS ef al., 1978; BALCH and WOLFE. 1979; OREMLAND, 1981) was added to one bottle (5 mM) to block methane production. One bottle did not receive any additions. The bottles were incubated statically at 12°C for 3 months during which time methane in the headspaces was periodically analysed by gas chromatography (see above).

RESULTS The

concentrations

of dissolved

oxygen

and

C, to

in Big Soda Lake during October, 1980 are listed in Table I. Dissolved oxygen decreased precipitously with depth below 15 m (the thermocline) and was not detected below 20 m. Conversely, methane increased with depth below I5 m and achieved uniform concentrations (about 55 PM/ 1) in the monimolimnion. No significant concentrations of ethylene or propylene (e.g. >3 nM/I) were detected in either the water column or in sediments recovered with a Pederson grab. Concentration profiles of C, to C4 alkanes appear consistent with a source in the sediments. Ratios of CH4/(C2Hs + C3Hs) were highest (333) at the chemocline (35 m) and then decreased to 16 1 at a depth of 60 m. Similar results were obtained when C,-C4 hydrocarbon abundances in the water column were measured in Oct. 1982. Profiles of dissolved methane and ethane concentrations observed during July 198 1, are shown in Fig. 2A. At this time of the year, the concentration of dissolved oxygen was uniformly 250 PM/L above a depth of 12 m but decreased linearly to 30 PM/I at 20 m and was not detected beneath 21 m (CLOERN et al., 1983a). Both methane and ethane had constant concentrations in the monimolimnion, but their concentrations decreased drastically above 35 m. Ethane, however, reached uniform background concentra-

C4 alkanes

at various

depths

2110

R. S. Oremland and D. J. Des Marais CH, 0.1 0fi-m) 0 -

Ei n

(gmoles/L) 10

1.0 ,,,qn, A,

I

I ,,I

111,

100 I

/ “‘“1

I

I

i

!

I

..,

50

o=CH4

t

A=C2Hg

A b4

60 c

I

701”“1

1

10

100

1000

0

C2Hg (omoles/L)

I

/I

I

1

/

400

200

J

-70

600

-50

-30

-10

h13C (CH,I

CH41C2Hs

FIG. 2. A) Depth distribution of dissolved methane and ethane, B) Ratios of CH4/C2H6 with depth and C) G’3C[CHJ with depth in Big Soda Lake during July, 1981. Error bars associated with 6’3C[CH,,] values indicate 95% confidence intervals. Samples above 24 m could not be analysed for 6”C[CH,,] because they were below detection limits (1 gg C/5 ml extracted gas). the oxycline was present at 20 m.

C3 to C4 hydrocarbons were not determined.

tions in the waters above 32 m. Concentrations

of methane decreased only slightly between 32 to 24 m, but decreased drastically and reached uniformly low values in waters near the surface (above 8 m). The difference between the methane and ethane concentration profiles caused the ratio of CH4/C$&, to vary with depth. Values of CH.&H6 were about 25 in the waters above 8 m, increased rapidly between the depth of 12 m and 34 m (values ranged from 185 at 16 m to 638 at 24 m) and decreased to constant values ( 174- 184) below the chemocline at 35 m (Fig. 2B). We have consistently observed high CH4/C2H6 or CH4/(C2H6 + CsH8) values at these intermediate depths of the lake (usually within the anoxic portion of the mixolimnion; see Fig. 1), regardless of season. Methane concentrations within the anoxic portion of the mixolimnion vary as a consequence of seasonal

mixing, depth of the oxyciine and water column biological activity (CLOERN et al., 1983a; Fig. 3A). Carbon isotopic compositions of methane in the monimolimnion were relatively constant with depth and ranged between -55 and -61 per mil (Fig. 2C). However, methane in the anoxic portion of the mixolimnion (20-35 m) was isotopically much heavier than the methane in the monimohmnion. The greatest 13C enrichment was encountered just above the chemocline at 32 m (-20.3 per mil) and represents a shift of as much as 39 per mil relative to the dt3C[CH4] values measured in the monimolimnion. Methane at depths between 24 and 35 m was always isotopically heavier than that in monimolimnion, although the magnitude of the difference was seasonally variable (Fig. 3B). Isotopic compositions of ethane (-27.0 f 0.3) and propane (-24 + 0.5) in the

-

ow 1.1 Apn

A

A July

1981 1981

0 Feb. 1982

cgau

11 May 1982 + July 7982 0 Oct.

A / -60

/ -50 d3C

/ -40

/ 30

1982 I -20.-

fCH4)

FIG. 3. Seasonal variations in A) the concentration of methane and B) the values of 6’3C(CH4] with depth in the anoxic water column of Big Soda Lake.

2111

Light hydrocarbons in Big Soda Lake TABLE 3: Formation water samples taken

monimolimnion (at 50 M) were determined during May 1982. The distribution of Cl-C4 hydrocarbons and of 613C[CH4] with depth in sediments taken from beneath the monimolimnion (62 m) are shown in Table 2. Methane abundance was constant in the upper 50 cm (113-120 PM/kg), but then increased steadily with depth, reaching a maximum value of 4 18 FM/ kg at 160 cm. By contrast, ethane, propane, isobutane and n-butane exhibited highest concentrations at the top of the core (10 cm) and decreased steadily with depth. Therefore, as a consequence of these distributions, CH4/(C2H6 + C3H8) values were lowest at the sediment surface (e.g.. 190 at 10 cm) but increased steadily with depth, reaching a maximum value of 1736 at 160 cm (Table 2). In addition, 6’3C[CH4] values became progressively lighter with increasing depth down the core. Thus, 613C[CH4] values encountered at the surface (e.g. -55 per mil at IO cm) were about 19 per mil heavier than G’3C[CH4] values at the bottom of the core (e.g. -74 per mil at 175 cm). Concentrations of ethylene and propylene remained uniformly low with core depth and were about 7-20 fold less abundant than their respective alkanes. Incubation of samples of lake water collected at a depth of 33 m in February 1982 demonstrated the presence of methanogenic bacteria in the anoxic waters of the lake just above the chemocline. Bottles containing additions of methanol or trimethylamine produced methane during the incubation (Table 3). No significant increases or decreases in the concentrations of methane were observed in either the unamended or BES-inhibited bottles. The absence of methanogenic activity in the flask lacking substrates additions, therefore, may have been caused by a seasonal dearth of methanogenic substrates in the water column.

of

oistr‘bur~on

C,

to C4 hydrocarbons

and

1" 25 5" 65 85 105 125 135 160 175

WI

CLH4

114 12" 113 18" 251 325 39" 287 418 345

0.022 0.022 0.017 O.OL8 0.020 0.036 0.029 0.015 0.023 u.039

bND = not determinedi CVal”es given for

6

C,CH4,

CA

C3"8

0.432 0.400 0.334 0.3,4 cl.292 0.220 0.242 0.180 0.185 0.184

0.019 0.015 0.013 0.015 0.016 0.013 0.029 0.009 0.019 0.038

0.169 0.145 0.119 0.106 0.099 0.062 0.015 il.059 0.051 0.066

indicate

95x

C&J

confidence

-~NONE

Gas

Phase

BESb

w

(umles)

l?iAC

2

32

30

21

19

16

32

30

24

22

35 39

30 30

34 34

123 368

26 47

47

32

34

I.068

III

57 69

31 28

35 33

1,246 1,307

385 75iJ

6'3C[CH41

"Moles/Kg Sub-bottom Depth (cm)

in

incubated 1982.

in marine sediments (e.g. MARTENS and BERNER, 1974), and, because bubbles were not visibly present in the upper 1 m of sediment, transport of methane out of the sediments is probably diffusion-controlled (KIPPHUT and MARTENS, 1982). Also, the presence of fine laminae in the shallow sediments argues for a lack of disturbance by bubbles. Methane encountered within these deeper sediments is of bacterial origin. Sediments taken from the monimolimnion have been previously shown to have methanogenic activity (OREMLAND et al., 1982a). In addition, methane encountered below 1 m sub-bottom depth in monimolimnion sediments (where bubbles were observed) had isotopically “light” values of 6’3C[CH.J (< -70 per mil) that are characteristic of most accumulations of microbially produced methane (FUEX, 1977; CLAYPOOLet al., 1980; SCHOELL, 1980; RICE and CLAYPOOL, 198 1). Finally, the high (> I 100) values of CH4/(C2H6 + C,H,) encountered below 1 m sub-bottom depth usually indicate gas of biogenic origin in environments of this type (BERNARDet ul., 1976). Thus, methane in the lake has a biogenic rather than thermogenic origin even though the geochemical “signals” usually considered (e.g. G’3C[CH4] and CH4/[C2H6 + C3H8]) vary widely within the vertical column studied (Table 1, 2; Fig. 2. 3). By contrast, concentrations of C2-C4 alkanes decrease with depth in the sediment (Table 2), implying

The deeper sediments (below 1 m sub-bottom depth) of the monimolimnion of Big Soda Lake appear to be the major source of methane. The large concentration gradient for methane near the surface of the sediments (Table 2) is similar to those observed

2:

netilane

ja

Additions:

DISCUSSION

TABLE

(days

Time

of methane by anaerobically from 33m during February,

in the sed,ments

of Big

Soda

Lake.a

sediment

L-Q"10

u.043 0.031 u.020 0.014 0.010 0.005 0.007 ND 0.006 0.010

limits.

!rC4HlO

0.018 "."I" 0.005 0.004 O.O"4 ND6 ND ND 0.004 O.008

($k&) 19" 22" 751 429 642 ,152 1232 ,201 1736 ,390

6'3c w41~ -55 + -55 T -6,iZ -64 i -69 + -7" T -72 + -73 T -71 T -74 T _

2 2 2 i 0.5 2 0.5 0.5 2

2112

R. S. Oremland and D. J. Des Marais

an origin either near the top of the core or within the monimolimnion waters. Therefore, these C,-C, alkanes are probably formed by low-temperature chemical and/or microbiological reactions (DAVIS and SQUIRES, 1954; HUNT et al., 1980; OREMLAND,198 1; VOGELet al., 1982; WHELAN et al., 1980) rather than by thermogenic reactions. The 6’% of ethane and propane cannot yet be used to infer their origin. The 6°C values of these compounds in the monimolimnion (6’3C[CzH6] = -27; 6”C[C,Hs] = - 24) indeed are similar to values observed in environments where thermogenic hydrocarbon production occurs (STAHL and CAREY, 1975; DES MARAISet al., I98 1). However, isotopic measurements of ethane and propane produced by microorganisms have not been reported. Relatively high values of CH4/(CzH6 + C,Hs) or CH4/C2H6 are consistently observed within the anoxic mixolimnion (Tables 1 and 2; Fig 2B). These high values reflect a decrease above the monimolimnion in the abundance of the Cz to C, alkanes which exceeds the decrease observed for methane. Although this might be interpreted to indicate preferential bacterial consumption of higher hydrocarbons, it more likely indicates that biogenic methane is produced in the anoxic mixolimnion. The successful recovery of methanogenic bacteria from 33 m (Table 3) and the fact that methanogenesis occurs in the anoxic water column of other meromictic lakes (WINFREY and ZEIKUS, 1979) support this interpretation. In addition, “C-methylamine is converted to 14C-methane during the course of in situ incubation of Big Soda Lake anoxic waters (OREMLAND, unpub. data). A most interesting feature of the lake is that the 613Cabundances in methane increase upward through the sediment column (Table 2) and further increase upward through the water column (Fig. 2C; Fig. 3B). The maximum enrichment in 13Cis observed between the “lightest” sediment 613C[CH4] value (-74 per mil at 174 cm) and the heaviest 6’3C[CH4] value (-20.3 per mil at 32 m) encountered in the anoxic mixolimnion. Similar observations were reported in anoxic sediment cores taken from the Santa Barbara Basin (DOSE and KAPLAN, 1981). Because there are no apparent sources of thermogenic gas in Big Soda Lake that would contribute isotopically “heavy” methane (e.g. 613C[CH4] > -50 per mil) to the system, the progressive vertical increase of G’3C[CH4] appears to be due to microbial processes. Assuming that no methane is formed vice reduction of carbon dioxide (OREMLANDet al., 1982a), and that the 6°C of organic carbon in the sediments (about -20 per mil) is typical of the lake in general, the apparent differences in isotopic composition between the methane and its organic precursors are about: 0 to -20 per mil (in the anoxic mixolimnion), -35 to -40 per mil (in monimohmnion surface sediment and waters) and -54 per mil (in monimolimnion sediment below 1 m sub-bottom depth). Carbon isotope fractionation associated with methanogenesis yields methane that is depleted in 13Crelative to the substrate. Methanol-grown enrich-

ment cultures produce methane with a fractionation factor of about 1.070 (ROSENFELDand SILVERMAN, 1959; OREMLAND et al., 1982a), while methane is formed by Hz plus COz grown cells with fractionation factors ranging from about 1.023 for Methanohacterium thermoautotrophicum to about 1.055 for Methanobacterium bryantii (GAMES et cd.,1978). However, due to closed system effects, the observed 613Cdifference between product and reactant can be much smaller than the maximum differences predicted by fractionation factors (ROSENFELDand SILVERMAN, 1959; GAMES et al., 1978). This can occur in nature if the pool sizes of methane precursors are small relative to rates of bacterial consumption of substrates and formation of methane. In high-sulfate environments, such as the upper sediments and water column of Big Soda Lake, methanogenesis appears to proceed primarily from compounds like methanol or methylamines (OREMLANDE~al., 1982a). Presumably this is due to removal of most of the acetate and/ or H2 by competitive sulfate-reducing bacteria (OREMLAND et al., 1982b; OREMLAND and POLC‘IN. 1982). Therefore, a closed-system effect may occur in Big Soda Lake if the turnover time of methane precursors (methanol, methylamine, etc.) is rapid. In addition, competition between different groups of microorganisms (e.g. sulfate-reducers vs. methanogens) for the same substrate (e.g. acetate) may result in only isotopically “heavy” substrate being available to the disadvantaged microbes (i.e. methanogens). Initial consumption of acetate, for example, by the thermodynamically more efficient sulfatereducers may result in residual acetate that is enriched in 13C.Thus, the slower-growing methanogens could subsequently metabolise relatively heavy substrate and yield methane of relatively heavy isotopic composition. This mechanism was postulated by SitVERMAN and OYAMA (1968) for producing heavy 613C[CH4]values observed during the course of methanogenesis in incubated soils. Thus, “C-depleted methane, such as that generated in the laboratory by pure bacterial cultures may not always be achieved by the mixed microbial communities encountered in nature. Another microbial mechanism which may contribute to the heavy 6’%[CH4] values encountered in the lake is methane oxidation. Aerobic methaneoxidizing bacteria are active in the water columns of stratified lakes (RUDD et al., 1974) and activity has been detected in the aerobic mixolimnion of Big Soda Lake (N. IVERSEN,pers. commun.). It has been well established that these organisms favor oxidation of the light isotope, thereby enriching residual unoxidized methane in 13Cby fractionation factors as large as those in methanogenesis (SILVERMANand OYAMA. 1968; BARKER and FRITZ, 1981; COLEMAN r’t d.. 1981). Aerobic oxidation of methane, however, can occur only at or above the depth of the oxycline, which is usually present at 20 m during spring to fail but descends to 29 m during winter (Fig. I : CLOERN et al., 1983a). Thus, downward diffusion from the

2113

Light hydrocarbons in Big Soda Lake oxycline of residual, unoxidized methane may have contributed somewhat to the large enrichments of 13C in methane encountered in the anoxic mixolimnion. However, very heavy I~‘~C[CH~]values (e.g. -40 to -20 per mil; Fig. 2C, 3B) were consistently observed in this region at times of the year when the oxycline was located at 20 m. Therefore, it is unlikely that downward diffusion of heavy unoxidized methane significantly influences the 6’3C[CH4] values in the anoxic mixolimnion, and certainly not in the sediments and waters of the monimolimnion. Consumption of methane by anaerobic bacteria (e.g. sulfate-reducers) has been invoked to explain concentration profiles observed for methane in anoxic marine environments (REEBURGH, 1976; BARNES and GOLDBERG, 1976; REEBURGH and HEGGIE, 1977; SACKETTet al., 1979). It has also been offered as an explanation for 13C-enriched methane found in anoxic Santa Barbara Basin sediments ( DOOSEand KAPLAN, 198 1). Biological experiments with anaerobic waters and sediments have demonstrated conversion of 14CH4 to 14C02 (PANGANIBAN et ul., 1979; REEBURGH, 1980; ZEHNDERand BROCK, 1980; IVERSON and BLACKBURN, 198 1; DEVOL, 1983), however it is not as yet clear if this causes a net consumption of methane. Whether anaerobic methane oxidation is mediated by sulfate-reducers (DAVIS and YARBROUGH, 1966), is a “back-reaction” carried out by methanogens (ZEHNDER and BROCK, 1979), or is the responsibility of other microbes is as yet an open question. To date, no cultures of these organisms have been reported. However, anaerobic methane oxidation has been detected in both the water column and sediments of Big Soda Lake (N. IVERSEN, pcrs. commun.). Therefore, the anoxic mixolimnion and the monimolimnion sediments of the lake appear to be zones of both methane production and anaerobic oxidation. If, due to high rates of substrate utilization, methanogenesis in these zones yields relatively heavy methane, (enclosed system effect) and if a portion of it is oxidized anaerobically, the residual methane may be isotopically very heavy. We find this explanation the most plausible mechanism by which isotopically heavy methane can be generated in the anoxic regions of Big Soda Lake. The results obtained thus far have important implications for interpretation of geochemical data. Uncritical use of parameters such as G13C[CH4] and CH4/(C2H6 + C3H,) to categorize gases with regard to their mode of origin (e.g. thermogenic vs. biogenic) can be clouded by numerous microbiological effects. Intimate knowledge of the chemical, physical, hydrological, ecological, and microbial processes occurring in environments where low molecular weight hydrocarbons occur is necessary to help identify the origin of the gases. Acknowledgment.~-We are grateful for the excellent technical assistance provided by C. Culbertson, J. Duff, T. Vogel, M. Golan-Boc and M. Stallard. We wish to thank N. Iversen, M. Klug, G. Rau, Y. Kharaka and S. Robinson for

helpful discussions and insights. We are particularly thankful to R. Smith, G. Demaison, G. Claypool, J. Cloem, D. Rice and J. Hayes for their constructive comments and manuscript review.

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