Methane is the major hydrocarbon gas measured in Vacutainer samples from Holes 603D and 613 ( C1/sumCn > 0.999). In Hole 613 the concentration of this dry hydrocarbon gas is highest (7.4 x 10 **5 ppm max.) in the upper 60 to 120 m, then decreases erratically to low trace levels by 261 m sub-bottom (lower Pliocene). No gas accumulations were observed in older sediments. Methane from both holes is strongly depleted in both 13C (d13C, - 75 to -85 per mil) and deuterium (D/H, - 175 to -262 per mil), indicating the biogenic origin of the methane. The C and H isotopic compositions support methanogenesis via the CO2-reduction pathway; this is also corroborated by the dissolved-sulfate and alkalinity minima at these depths. The relationship between D/H of the methane and coexisting interstitial water from Site 613 further show the methanogenesis to be primarily by CO2 reduction.

Supplement to: Whiticar, Michael J; Faber, Eckhard (1987): Carbon and hydrogen isotopes in gas samples from Leg 95, Holes 603D and 613. In: Poag, CW; Watts, AB; et al. (eds.), Initial Reports of the Deep Sea Drilling Project, Washington (U.S. Govt. Printing Office), 95, 647-650