Geochmrcrr PI Cumochimm AC/U Vol. 41, pp. 2107-21 14 0 Pergamon Press Ltd. 1983. Printed in U.S.A. 0016.7037/X3/122107-08$03.00/O 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 zyxwvutsrqponmlkjihgfedcbaZ (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 moni- molimnion 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 HABITATS FAVORABLE for the early, depositional stages of oil and gas formation probably include aquatic en- vironments 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; DEMAISON and 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, hyper- saline environments. SACKETT et al. (1979) reported that methane, ethane and propane, found in the Orca Basin (Gulf of Mexico) brinewaters and sediments ap- peared to have a biogenic origin. POTTS (1979) ob- served 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, mer- omictic 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; KOENIG et 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 monimolim- nion (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. Monimolim- nion 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