(Table 1) Hydrocarbon (C1-C5) and carbon dioxide content of interstitial gas samples, with the carbon isotope compositions of methane and carbon dioxide, from the holes drilled in the Gulf of California (DSDP Leg 64)

Interstitial gases from the Deep-Sea Drilling Project (DSDP-IPOD) Leg-64 Sites 474, 477-479 and 481 in the Gulf of California have been analyzed by gas chromatography and stable-isotope mass spectrometry in order to evaluate their compositions in terms of biogenic and thermogenic sources.The hydrocarbon gas (C1-C5) concentrations were comparable to the shipboard data and no olefins could be detected. The d13C data for the CH4 confirmed the effects of the thermal stress on the sedimentary organic matter, in that the values were typically biogenic near the surface and become more depleted in 12C vs. depth at Sites 474, 478 and 481. The CH4 at Site 477 was the heaviest in 13C and at Site 479 it did not show a predominant high-termperature component. The CO2 at depth from most sites was of a primarily thermogenic origin from carbonate decomposition.