Concentrations and carbon isotopic compositions of CH4 and CO2 in gas from sediments of the Blake Outer Ridge, Deep Sea Drilling Project Leg 76

The principal gaseous carbon-containing components identified in the first 400 m of sediment at Deep Sea Drilling Project Site 533, Leg 76, are methane (CH4 ) and carbon dioxide (CO2 ). Below a sub-bottom depth of about 25 m, sedi ment cores commonly contained pockets caused by the expansion of gas upon core recovery. The carbon isotopic com position (δ13C %0 relative to PDB standard) of CH4 and CO2 in these gas pockets has been measured, resulting in the following observations: (1) δ 1 3CC H 4 values increase with depth from approximately -94% in the uppermost sediment to about -66% in the deepest sediment, reflecting a systematic but nonlinear depletion of 12C with depth. (2) δ 1 3Cc θ 2 values also increase with depth of sediment from about - 25% to about - 4%, snowing a depletion of 12C that closely parallels the trend of the isotopic composition of CH4 . The magnitude and parallel distribution of δ 13C values for both CH4 and CO2 are consistent with the concept that the formation of the CH4 resulted from the microbiological reduction of CO2 from organic substances. These results imply that CH4 and CO2 incorporated in gas hydrates at this site are biogenic.