Above-ambient levels of methane and higher hydrocarbons were detected in the atmosphere of the Mono Basin. These gases emanated from several different sources, including natural gas seeps (thermogenic and biogenic), and methanogenic activity in sediments. Seeps were distributed over nearly 33% of the lake bottom and were also present in the exposed former lakebed. They originated from one or more natural gas deposits that underlie the basin. Seeps associated with hot springs had a thermogenic character, whereas the others had the characteristics of bacterially formed gases. The radiocarbon content of methane in all seep gases was low (5-11% modern carbon), indicating an age of greater than about 20,000 years.

Dissolved methane increased with depth in the sediments, ultimately reaching saturation levels (1-3 mM). The outward diffusive flux of methane into the anoxic bottom waters elevated concentrations to 60 µM during the 1984-1988 meromictic interval. The radiocarbon content of hypolimnion methane was equivalent to that of the dissolved inorganic carbon (~80% modern carbon), indicating derivation from current methanogenic activity. Oxidation by anaerobic bacteria was the major sink for this methane. Methane that escaped oxidation transited to the epilimnion where it attained supersaturation (0.1-3.0 µM) relative to the atmosphere. Measured outward fluxes over a 2-year period were highly variable, ranging from 0.01 to 4.82 mmoles CH₄ m^-2 d^-1.

Bacterial activity was detected in the sediments of the hypolimnion High rates of sulfate reduction (~200 µmoles L^-1 d^-1) were measured in the upper 5-cm interval but decreased > 10-fold with depth. The highest rate of methanogenic activity was ~1 µmole L^-1 d^-1, as measured by reduction of ¹⁴CO₂. Bacteria metabolized dimethylsulfide, trimethylamine, acetate, and glucose to primarily CO₂ and some CH₄. Turnovers of glucose (k = 0.16 d^-1) and trimethylamine (k = 0.09 d^-1) were highest in the upper 2.5-cm interval and decreased as much as 10-fold with depth. These results confirm that a low rate of methanogenic activity occurs in this extreme environment and is the source of methane to the hypolimnion.