Sedimentation rate calculations and isotopic analysis on sediment cores along the IODP Expedition 311 transect@en

The oceanographic and tectonic conditions of accretionary margins are well-suited for several potential processes governing methane generation, storage and release. To identify the relevant methane evolution pathways in the northern Cascadia accretionary margin, a four-site transect was drilled during Integrated Ocean Drilling Program Expedition 311. The d13C values of methane range from a minimum value of -82.2 per mil on an uplifted ridge of accreted sediment near the deformation front (Site U1326, 1829 mbsl, meters below sea level) to a maximum value of -39.5 per mil at the most landward location within an area of steep canyons near the shelf edge (Site U1329, 946 mbsl). An interpretation based solely on methane isotope values might conclude the 13C-enrichment of methane indicates a transition from microbially- to thermogenically-sourced methane. However, the co-existing CO2 exhibits a similar trend of 13C-enrichment along the transect with values ranging from -22.5 per mil to +25.7 per mil. The magnitude of the carbon isotope separation between methane and CO2 (Ec = 63.8 ± 5.8) is consistent with isotope fractionation during microbially mediated carbonate reduction. These results, in conjunction with a transect-wide gaseous hydrocarbon content composed of > 99.8% (by volume) methane and uniform dDCH4 values (-172 per mil ± 8) that are distinct from thermogenic methane at a seep located 60 km from the Expedition 311 transect, suggest microbial CO2 reduction is the predominant methane source at all investigated sites. […]

增生型大陆边缘（accretionary margin）的海洋学与构造条件，十分适配多种调控甲烷生成、赋存与释放的潜在过程。为明确卡斯卡迪亚北部增生型大陆边缘的甲烷演化途径，综合大洋钻探计划（Integrated Ocean Drilling Program, IODP）第311次航次期间布设了一条包含4个站位的断面开展钻探。甲烷的δ¹³C值（原文标注为d13C）变化范围为：在变形前锋附近的增生沉积物抬升脊处（U1326站位，水深1829 mbsl，即海平面以下1829米），其最小值为-82.2‰；而在陆架边缘附近陡坡峡谷区域内最靠陆的位置（U1329站位，水深946 mbsl），其最大值为-39.5‰。若仅基于甲烷同位素数值开展解释，可能会得出"甲烷的¹³C富集指示微生物成因甲烷向热成因甲烷转化"的结论。然而，共生的CO₂沿该断面同样呈现¹³C富集的趋势，其数值范围为-22.5‰至+25.7‰。甲烷与CO₂之间的碳同位素分馏幅度（Ec = 63.8 ± 5.8）与微生物介导的碳酸盐还原过程中的同位素分馏特征相符。上述结果结合全断面气态烃类组分中甲烷体积占比＞99.8%，以及均一的δD-CH₄值（原文标注为dDCH4，为-172‰±8）——该数值与距离第311航次断面60 km处一处海底冷泉的热成因甲烷特征显著不同——表明在所调查的全部站位中，微生物介导的CO₂还原作用是甲烷生成的主要来源。[…]