Isotopic composition of interstitial fluids and origin of methane in slope sediment of the Middle America Trench, Deep Sea Drilling Project Leg 84.

CH₄ and CO₂ species in pore fluids from slope sediments off Guatemala show extreme ¹³C-enrichment (delta ¹³C of -41 and +38per mille, respectively) compared with the typical degree of ¹³C-enrichment in pore fluids of DSDP sediments (delta ¹³C of -60 and +10per mille). 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 CH₄ and CO₂, the pore water exhibits a systematic increase in delta ¹⁸O with decrease in chlorinity. As against seawater -¹⁸O values of O and chlorinity of 19per mille, the water collected from decomposed gas hydrate from Hole 570 had a -¹⁸O of +3.0per mille and chlorinity of 9.5per mille. The isotopic compositions of pore-fluid constituents change gradually with depth in Hole 568 and discontinuously with depth in Hole 570. -Authors