Isotopic composition of interstitial fluids and origin of methane at DSDP Leg 84 Holes@en

CH4 and CO2 species in pore fluids from slope sediments off Guatemala show extreme 13C-enrichment (d13C of -41 and +38 per mil, respectively) compared with the typical degree of 13C-enrichment in pore fluids of DSDP sediments (d13C of - 60 and + 10 per mil). 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 d18O with decrease in chlorinity. As against seawater d18O values of 0 and chlorinity of 19 per mil, the water collected from decomposed gas hydrate from Hole 570 had a d18O of + 3.0 per mil and chlorinity of 9.5 per mil. The isotopic compositions of pore-fluid constituents change gradually with depth in Hole 568 and discontinuously with depth in Hole 570.危地马拉外陆坡沉积物孔隙流体中的甲烷(CH₄)与二氧化碳(CO₂)组分，相较于深海钻探计划(Deep Sea Drilling Project, DSDP)沉积物孔隙流体典型的¹³C富集程度（二者的δ¹³C分别为-60‰与+10‰），呈现出极端的¹³C富集特征：甲烷的δ¹³C为-41‰，二氧化碳的δ¹³C为+38‰。这种异常的同位素组成被认为源自有机质的微生物降解作用，同时可能伴随与天然气水合物(gas hydrates)形成相关的额外同位素分馏过程。除甲烷与二氧化碳展现出的同位素分馏效应外，孔隙水的δ¹⁸O值随氯度降低呈现系统性升高趋势。对比海水的δ¹⁸O值（0‰）与氯度（19‰），从570号钻孔分解的天然气水合物中采集的水样，其δ¹⁸O值为+3.0‰，氯度为9.5‰。568号钻孔内孔隙流体组分的同位素组成随深度呈渐变特征，而570号钻孔内该组成则随深度呈现不连续变化。