Department of Environmental Science, Stockholm University, 106 91 Stockholm, Sweden;
Department of Geological Sciences, Stockholm University, 106 91 Stockholm, Sweden.
Proc Natl Acad Sci U S A. 2021 Mar 9;118(10). doi: 10.1073/pnas.2019672118.
The East Siberian Arctic Shelf holds large amounts of inundated carbon and methane (CH). Holocene warming by overlying seawater, recently fortified by anthropogenic warming, has caused thawing of the underlying subsea permafrost. Despite extensive observations of elevated seawater CH in the past decades, relative contributions from different subsea compartments such as early diagenesis, subsea permafrost, methane hydrates, and underlying thermogenic/ free gas to these methane releases remain elusive. Dissolved methane concentrations observed in the Laptev Sea ranged from 3 to 1,500 nM (median 151 nM; oversaturation by ∼3,800%). Methane stable isotopic composition showed strong vertical and horizontal gradients with source signatures for two seepage areas of δC-CH = (-42.6 ± 0.5)/(-55.0 ± 0.5) ‰ and δD-CH = (-136.8 ± 8.0)/(-158.1 ± 5.5) ‰, suggesting a thermogenic/natural gas source. Increasingly enriched δC-CH and δD-CH at distance from the seeps indicated methane oxidation. The ΔC-CH signal was strongly depleted (i.e., old) near the seeps (-993 ± 19/-1050 ± 89‰). Hence, all three isotope systems are consistent with methane release from an old, deep, and likely thermogenic pool to the outer Laptev Sea. This knowledge of what subsea sources are contributing to the observed methane release is a prerequisite to predictions on how these emissions will increase over coming decades and centuries.
东西伯利亚北极大陆架蕴藏着大量的淹没碳和甲烷 (CH)。近年来，由于覆盖其上的海水升温以及人为增温的影响，全新世变暖导致海底永冻层解冻。尽管过去几十年已经广泛观察到海水 CH 的升高，但不同海底隔室（如早期成岩作用、海底永冻土、甲烷水合物和底层热成因/自由气）对这些甲烷释放的相对贡献仍不清楚。拉普捷夫海观测到的溶解甲烷浓度范围为 3 至 1500 nM（中位数为 151 nM；过饱和约 3800%）。甲烷稳定同位素组成表现出强烈的垂直和水平梯度，两个渗漏区的源特征为 δC-CH = (-42.6 ± 0.5)/(-55.0 ± 0.5) ‰和 δD-CH = (-136.8 ± 8.0)/(-158.1 ± 5.5) ‰，表明其为热成因/天然气源。随着远离渗漏区距离的增加，δC-CH 和 δD-CH 的丰度逐渐增加，表明甲烷的氧化作用。在渗漏区附近，ΔC-CH 信号明显贫化（即，古老）（-993 ± 19/-1050 ± 89‰）。因此，所有三个同位素系统都表明甲烷是从一个古老、深层且可能是热成因的储层释放到拉普捷夫海外部的。了解海底源对观测到的甲烷释放的贡献是预测未来几十年和几个世纪这些排放将如何增加的前提。
