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

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

doi:10.1016/0016-7037(83)90035-2

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

R.S. Oremland·D.J. Des Marais

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Abstract

Distribution and isotopic composition (δ 13 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 μM/l) in the monimolimnion (35–64 m) and again increased with depth in monimolimnion bottom sediments (>400 μM/kg below 1 m sub-bottom depth). The μ 13 C [ CH 4 ] values in bottom sediment below 1 m sub-bottom depth (<−70 per mil) increased with vertical distance up the core ( δ 13 C [ CH 4 ] = −55 per mil at sediment surface). Monimolimnion δ 13 C [ CH 4 ] values (−55 to −61 per mil) were greater than most δ 13 C [ CH 4 ] values found in the anoxic mixolimnion (92% of samples had δ 13 C [ CH 4 ] 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 CH 4 [C 2 H 6 + C 3 H 8 ] "> CH 4 [C 2 H 6 + C 3 H 8 ] 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 2 -C 4 alkanes have biogenic origins in the monimolimnion water and shallow sediments. The changes observed in δ 13 C [ CH 4 ] and CH 4 (C 2 H 6 + C 3 H 8 ) "> CH 4 (C 2 H 6 + C 3 H 8 ) with depth in the water column and sediments are probably caused by bacteria] processes. These might include anaerobic methane oxidation and different rates of methanogenesis and C 2 to C 4 alkane production by microorganisms.

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R.S. Oremland, D.J. Des Marais. 1983. Distribution, abundance and carbon isotopic composition of gaseous hydrocarbons in Big Soda Lake, Nevada: An alkaline, meromictic lake. https://doi.org/10.1016/0016-7037(83)90035-2

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