| Abstract: | Chemical and isotopic compositions have been measured on 62 microbial gases from Tertiary hemipelagic sediments in the Middle America Trench off Guatemala and from decaying kelp and surf grass currently accumulating in Scripps Submarine Canyon off southern California. Gases from the Middle America Trench have been generated primarily by the reduction of carbon dioxide; methane δ13C varies from −84‰ to −39‰, methane δD varies from −208‰ to −145‰, and carbon dioxide δ13 C varies from −27‰ to +28‰. Gases from Scripps Submarine Canyon have been generated primarily by acetate dissimilation; methane δ13 C varies from −63‰ to −43‰, methane δD varies from −331‰ to −280‰, and carbon dioxide δ13C varies from −17‰ to +3‰.Methane δ13C values as heavy as −40‰ appear to be uncommon for gases produced by carbon dioxide reduction and, in the Middle America Trench, are associated with unusually positive carbon dioxide δ13C values. However, based on the 25‰ intramolecular fractionation between acetate car☐yl carbon and methyl carbon estimated from the Scripps Submarine Canyon data, methane produced by acetate dissimilation may commonly have heavy δ13C values. The δD of methane derived from acetate is more negative than natural methanes from other origins. Microbial methane δD values appear to be controlled primarily by interstitial water δD and by the relative proportions of methane derived from carbon dioxide and acetate.The chemical and isotopic compositions of microbial gas and thermogenic gas overlap, making it difficult to determine the origins of many commercial natural gases from methane δ13C and C2+ hydrocarbon concentrations alone. Measurements of methane δD and carbon dioxide δ13C can provide useful additional information, and together with ethane δ13C data, help identify gases with mixed microbial and thermogenic origins. |
