Abstract The methane (CH4) emission from the Arctic Ocean is crucial to understand global carbon cycle. Here, we investigated sulfate (SO42−) in pore water and compositional and isotopic gas signatures at ARAON Mounds (hydrate/nonhydrate-bearing sites) and background site in the Chukchi Sea. Sulfate-methane transition (SMT) did not reach at the background site but occurred at shallow depths (≤3.3 m below the seafloor) at all ARAON Mounds sites. The SO42− profiles at ARAON Mounds also clearly indicate the unsteady state due to upward gas migration by high flux at the hydrate-bearing sites compared to the nonhydrate-bearing sites. The isotopic signatures of gas samples at the hydrate-bearing sites and below the SMT at the nonhydrate-bearing sites reflect thermogenic source transported across at least 1 km through faults/fractures in the Chukchi Sea. The headspace (HS) gas samples above/near the SMT at the nonhydrate-bearing sites are affected by the biogenic CH4 with enriched 12C; they indicate biogenic or thermogenic/biogenic mixed sources. The thermogenic gases below the SMT at ARAON Mounds have high C1/C2+ ratios (>300), much higher than those of normal thermogenic gases in offshore shallow sediments ( The carbon isotopic fractionation (ec = δ13CCO2 - δ13CCH4) in HS samples of the background site and ARAON Mounds above the SMT are consistent with the biogenic gas range generated via microbial CO2 reduction. However, ec below the SMT is anomalously low (13–42‰) and is higher at the hydrate-bearing sites than at the nonhydrate-bearing sites. We postulate that this low ec is explained by the two-phase fluid transport model of Kim et al. (2012) and that gas hydrates highly influence this value. We suggest that ec can be used as a powerful geochemical proxy for the upward gas migration and gas hydrate occurrence in shallow marine sediment systems.

中文翻译：

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楚科奇海 ARAON Mound 浅层沉积物的上升气源和后生过程

摘要 北冰洋的甲烷 (CH4) 排放对于了解全球碳循环至关重要。在这里，我们研究了 ARAON Mounds（含水合物/非水合物地点）和楚科奇海背景地点的孔隙水中的硫酸盐 (SO42−) 以及成分和同位素气体特征。硫酸盐-甲烷转变 (SMT) 未到达背景站点，但发生在所有 ARAON Mounds 站点的浅层深度（海底以下 ≤ 3.3 m）。ARAON Mounds 的 SO42− 剖面也清楚地表明，与非水合物场地相比，水合物场地的高通量气体向上迁移导致的不稳定状态。含水合物地点和不含水合物地点的 SMT 下方的气体样品的同位素特征反映了通过楚科奇海的断层/裂缝输送至少 1 公里的热源。不含水合物地点的 SMT 上方/附近的顶空 (HS) 气体样品受到富含 12C 的生物 CH4 的影响；它们表示生物源或热源/生物源混合源。ARAON Mounds SMT下方的热成因气体具有较高的C1/C2+比（>300），远高于近海浅层沉积物中的正常热成因气体（背景HS样本中的碳同位素分馏（ec = δ13CCO2 - δ13CCH4）） SMT 上方的站点和 ARAON Mounds 与通过微生物 CO2 还原产生的生物气范围一致。然而，SMT 下方的 ec 异常低 (13–42‰) 并且在含水合物站点处高于非水合物站点我们假设这种低 ec 是由 Kim 等人的两相流体传输模型解释的。(2012) 并且天然气水合物对这个值有很大影响。我们建议 ec 可用作浅海沉积系统中向上运移和天然气水合物存在的强大地球化学代理。