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The low molecular weight hydrocarbon gases detected in buried sediments may be useful indicators of the thermal alteration of organic matter in sediments. Hydrocarbon gases can be produced either by bacterial fermentation of, or thermochemical degradation of organic matter. Claypool and Kaplan (1974) have suggested that gases derived from these two sources can be distinguished by chemical composition of the gas and by the δC value of the methane. In general, biologically produced gas in marine sediments has a δC value for methane between -9O°/°o and -60 °/oo and has a very low concentration of higher hydrocarbons. Thermal degradation of organic matter will add isotopically heavy methane and higher hydrocarbons to the gas in the sediment. Higher stages of thermal alteration (incipient metamorphism) may degrade even the low molecular weight hydrocarbon gases, resulting in methane with a <5C value between -5O°/oo and -30°/oo (Stahl, 1974). The DSDP gases previously analyzed by Claypool et al. (1973) were primarily of biogenic origin, but increasing amounts of thermochemically produced gas were observed with increase in depth of burial. In this paper, we report similar mixtures of biogenically produced and thermochemically produced gases in sediments from Site 368 and Hole 369A.

Interstitial Gas Analysis of Sediment Samples from Site 368 and Hole 369A