Unusually high stable carbon isotopic values of methane from low organic carbon Mars analog hypersaline environments

Details Unusually high stable carbon isotopic values of methane from low organic carbon Mars analog hypersaline environments Unusually high stable carbon isotopic values of methane from low organic carbon Mars analog hypersaline environments

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p13b1389k&link_type=abstract

American Geophysical Union, Fall Meeting 2010, abstract #P13B-1389

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[0424] Biogeosciences / Biosignatures And Proxies, [0456] Biogeosciences / Life In Extreme Environments, [4840] Oceanography: Biological And Chemical / Microbiology And Microbial Ecology, [6225] Planetary Sciences: Solar System Objects / Mars

Scientific paper

Motivated by the Mars rovers’ findings of past hypersaline environments and the discovery of methane in the atmosphere of Mars, we examined methanogenesis in hypersaline ponds in Baja California Sur and in the Don Edwards National Wildlife Refuge in northern California. Methane-rich bubbles were observed to be released from below gypsum/halite crusts in these environments. The stable carbon isotopic composition of these bubbles ranged from about -30 to -40 ‰. Methane with these relatively high isotopic values would typically be considered non-biogenic, however incubations of crust and sediments samples over time resulted in the production of methane. We therefore undertook a series of measurements aimed at understanding the isotopic composition of methane in these environments. The concentrations and isotopic composition of the particulate organic carbon (POC) in these environments were measured. POC content was low (relative to most methane-producing sedimentary environments), generally less than 1%, and always less than 2% of the total mass. The isotopic composition of the POC ranged from -13 to -22 ‰. To determine the substrates used by the methanogens, 13C-labeled trimethylamine (TMA), monomethylamine, methanol, acetate and bicarbonate were added to incubation vials and the methane produced was monitored for 13C content. The main substrates used by the methanogens in these hypersaline environments were the non-competitive substrates, the methylamines and methanol. When unlabeled, but isotopically known, TMA was added to incubation vials in varying concentrations, the isotopic composition of the methane produced also varied. Little, if any, difference in the isotopic composition between the TMA and methane occurred at the lowest TMA concentration (10 µM final concentration). The lowest methane δ13C values (and so greatest fractionation between methane and TMA) occurred when the most TMA was added (1000 µM final concentration). This change in the isotopic composition of the methane produced with varying TMA concentrations, along with the high in situ methane δ13C values, suggest that the methanogens in these environments are operating at low substrate concentrations. Apparently, substrate limitation decreases isotopic fractionation during methanogenesis, which results in the abnormally high biogenic methane δ13C values.

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