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 (<−70 per mil) increased with vertical distance up the core (δ¹³C[CH₄] = −55 per mil at sediment surface). Monimolimnion δ¹³C[CH₄] values (−55 to −61 per mil) were greater than most δ¹³C[CH₄] values found in the anoxic mixolimnion (92% of samples had δ¹³C[CH₄] 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+ C3H8]">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)">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.