Geochemistry of hydrate gas and water at DSDP Hole 84-570

Molecular and isotopic measurements of gas and water obtained from a gas hydrate at Site 570, DSDP Leg 84, are reported. The hydrate appeared to be Structure I and was composed of a solid framework of water molecules enclosing methane and small amounts of ethane and carbon dioxide. Carbon isotopic values for the hydrate-bound methane, ethane, and carbon dioxide were -41 to about -44, -27, and -2.9 per mil, respectively. The d13C-C1 values are consistent with void gas values that were determined to have a biogenic source. A significant thermogenic source was discounted because of high C1/C2 ratios and because the d13C-CO2 values in these sections were also anomalously heavy (or more positive) isotopically, suggesting that the methane was formed biogenically by reduction of heavy CO2 . The isotopically heavy hydrate d13C-C2 is also similar to void gas isotopic compositions and is either a result of low-temperature diagenesis producing heavy C2 in these immature sediment sections or upward migration of deeper thermogenic gas. The salinity of the hydrate water was 2.6 per mil with dDH2O and d18OH2O values of +1 and +2.2 per mil, respectively.本数据集报道了深海钻探计划（Deep Sea Drilling Project, DSDP）第84航次570站位一处天然气水合物的气体与水体分子及同位素测定结果。该水合物属结构I型（Structure I），由包裹甲烷、少量乙烷与二氧化碳的水分子固体骨架构成。赋存于该水合物中的甲烷、乙烷及二氧化碳的碳同位素值分别为-41‰至约-44‰、-27‰及-2.9‰。其δ¹³C-C₁值与经测定判定为生物成因的孔隙气（void gas）的同位素结果相符。鉴于较高的C₁/C₂比值，且该层位的δ¹³C-CO₂同位素值异常偏高（即同位素组成更偏正），故排除了显著热成因来源的可能性，表明甲烷是通过还原重质CO₂的生物途径生成的。同位素组成偏重的水合物δ¹³C-C₂值同样与孔隙气体的同位素组成相近，其成因可能为未成熟沉积层段的低温成岩作用生成了重质C₂，或是深部热成因气体向上运移所致。该水合物晶格水的盐度为2.6‰，其δD-H₂O与δ¹⁸O-H₂O值分别为+1‰及+2.2‰。