Sources and flux of natural gases from Mono Lake, California

Abstract

The ability to identify a formation mechanism for natural gas in a particular environment requires consideration of several geochemical factors when there are multiple sources present. Four primary sources of methane have been identified in Mono Lake. Two of these sources were associated with numerous natural gas seeps which occur at various locations in the lake and extend beyond its present boundary; the two other gas sources result from current microbiological processes. In the natural gas seeps, we observed flow rates as high as 160 moles CH4 day−1, and estimate total lakewide annual seep flux to be 2.1 × 106 moles CH4. Geochemical parameters (δ13CH4,δDCH4,CH4/[C2H6+ C3H8]) andδ14CH4measurements revealed that most of the seeps originate from a paleo-biogenic (δ13CH4 = about −70%.). natural gas deposit of Pleistocene age which underlies the current and former lakebed. Gas seeps in the vicinity of hot springs had, in combination with the biogenic gas, a prominent thermogenic gas component resulting from hydrothermal alteration of buried organic matter.Current microbiological processes responsible for sources of natural gas in the lake included pelagic meth- anogenesis and decomposition of terrestrial grasses in the littoral zone. Methanogenesis in the pelagic sediments resulted in methane saturation (2–3 mM at 50 cm; δ13CH4 = about −85%.). Interstitial sulfate decreased from 133 mM at the surface to 35 mM by 110 cm depth, indicating that sulfate-reduction and methanogenesis operated concurrently. Methane diffused out of the sediments resulting in concentrations of about 50 μM in the anoxic bottom waters. Methane oxidation in the oxic/anoxic boundry lowered the concentration by >98%, but values in surface waters (0.1–1.3μM) were supersaturated with respect to the atmosphere. The δ13CH4 (range = −21.8 to −71.8%.) of this unoxidized residual methane was enriched in 13C relative to methane in the bottom water and sediments. Average outward flux of this methane was 2.77 × 107 moles yr−1. A fourth, but minor source of methane (δ13CH4 = −55.2%.) was associated with the decomposition of terrestrial grasses taking place in the lake's recently expanded littoral zone.