Molecular and isotopic compositions of void gases from Sites 618 and 619 were determined. The gases were predominantly methane with the C1/C2 ratios averaging 4,000 and 26,000 at Sites 618 and 619, respectively. The delta13C-C1 values at Site 618 were nearly constant, ranging from - 70.1 to - 73.7 per mil. delta13C-CH1 values for Site 619 between 76 and 178 meters sub-bottom ranged from - 94.8 to - 70.8 per mil, becoming progressively heavier with depth. The molecular and isotopic compositions at both sites are characteristic of biogenic methane. Small spheres (~ 1-2 mm diameter) of gas hydrates were observed in the Orca Basin between 20 and 40 m sub-bottom and were often associated with sandy sections. The hydrate was predominantly methane with a delta13-C1 of - 7 per mil and trace amounts of ethane, propane, and carbon dioxide. The isotopic value for the methane was similar to that of the nearest gas pocket, indicating that hydrate was formed from predominantly biogenic methane without isotopic fractionation.

Supplement to: Pflaum, Ronald C; Brooks, James M; Cox, H Benjamin; Kennicutt, Mahlon C; Sheu, Der-Duen (1986): Molecular and isotopic analysis of core gases and gas hydrates, Deep Sea Drilling Project Leg 96. In: Bouma, AH; Coleman, JM; Meyer, AW; et al. (eds.), Initial Reports of the Deep Sea Drilling Project, Washington (U.S. Govt. Printing Office), 96, 781-784