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

Distribution and isotopic composition (δ¹³C) 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 μ¹³C[CH₄] values in bottom sediment below 1 m sub-bottom depth (CH₄[C₂H₆+ C₃H₈] 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 C₂-C₄ alkanes have biogenic origins in the monimolimnion water and shallow sediments. The changes observed in δ¹³C[CH₄] and CH₄(C₂H₆+ C₃H₈) 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 C₂ to C₄ alkane production by microorganisms.