24. ISOTOPIC COMPOSITION OF INTERSTITIAL FLUIDS AND ORIGIN OF METHANE IN SLOPE SEDIMENT OF THE MIDDLE AMERICA TRENCH, DEEP SEA DRILLING PROJECT LEG 841

C H 4 and CO 2 species in pore fluids from slope sediments off Guatemala show extreme 13C-enrichment (δ13C of- 4 1 and +38‰, respectively) compared with the typical degree of 13C-enrichment in pore fluids of DSDP sediments (δ1 3C of- 60 and + lO‰). These unusual isotopic compositions are believed to result from microbial decomposition of organic matter, and possibly from additional isotopic fractionation associated with the formation of gas hydrates. In addition to the isotopic fractionation displayed by CH 4 and CO2, the pore water exhibits a systematic increase in δ 1 8 θ with decrease in chlorinity. As against seawater δ 1 8 θ values of 0 and chlorinity of 19‰, the water collected from decom-posed gas hydrate from Hole 570 had a δ 1 8 θ of + 3.0 ‰ and chlorinity of 9.5‰. The isotopic compositions of pore-flu-id constituents change gradually with depth in Hole 568 and discontinuously with depth in Hole 570