(Table 1) Concentrations and carbon isotopic compositions of CH4 and CO2 at DSDP Site 76-533

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, sediment cores commonly contained pockets caused by the expansion of gas upon core recovery. The carbon isotopic composition (d13C per mil relative to PDB standard) of CH4 and CO2 in these gas pockets has been measured, resulting in the following observations: (1) d13C-CH4 values increase with depth from approximately -94 per mil in the uppermost sediment to about -66 per mil in the deepest sediment, reflecting a systematic but nonlinear depletion of 12C with depth. (2) d13C-CO2 values also increase with depth of sediment from about -25 per mil to about -4 per mil, snowing a depletion of 12C that closely parallels the trend of the isotopic composition of CH4. The magnitude and parallel distribution of d13C 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.

在深海钻探计划（Deep Sea Drilling Project）第76航次533站位前400米的沉积物中，已鉴定出的主要气态含碳组分为甲烷（CH₄）与二氧化碳（CO₂）。在海底以下约25米深度以下，沉积物岩芯中普遍存在因岩芯回收过程中气体膨胀形成的气袋。已对这些气袋内甲烷与二氧化碳的碳同位素组成（相对于PDB标准（Pee Dee Belemnite Standard）的δ¹³C，单位为千分）进行了测定，得到如下观测结果：(1) δ¹³C-CH₄值随沉积物深度增加，从表层沉积物的约-94‰增至最深层沉积物的约-66‰，反映出¹²C随深度发生系统性但非线性的贫化。(2) δ¹³C-CO₂值同样随沉积物深度增加，从约-25‰升至约-4‰，呈现出与甲烷同位素组成趋势高度一致的¹²C贫化现象。甲烷与二氧化碳的δ¹³C值的变化幅度与平行分布特征，与"甲烷由有机质来源的二氧化碳经微生物还原作用生成"这一假说相符。上述结果表明，该站位天然气水合物中赋存的甲烷与二氧化碳均为生物成因。