Doubly substituted isotopologues of methane hydrate (13CH3D and 12CH2D2): implications for methane clumped isotope effects, source apportionments and global hydrate reservoirs
Doubly substituted isotopologues of methane hydrate (13CH3D and 12CH2D2): implications for methane clumped isotope effects, source apportionments and global hydrate reservoirs
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DOI：
10.1016/j.gca.2021.08.027
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发表时间：
2021-08
影响因子：
5
通讯作者：
Naizhong Zhang;G. Snyder;Mang Lin;Mayuko Nakagawa;A. Gilbert;N. Yoshida;R. Matsumoto;Y. Sekine-Y.-Seki
中科院分区：
文献类型：
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作者：
Naizhong Zhang;G. Snyder;Mang Lin;Mayuko Nakagawa;A. Gilbert;N. Yoshida;R. Matsumoto;Y. Sekine-Y.-Seki
最近开发的甲烷团块同位素技术为甲烷生物地球化学提供了新鲜和新颖的见解，这是通过其他技术（例如常规稳定同位素测定和碳氢化合物分子组成测量）无法获得的。尽管如此，控制天然甲烷样品的聚集同位素特征（Δ13CH3D 和 Δ12CH2D2）的控制过程和机制仍然是一个活跃的研究领域。在这里，我们提出了甲烷水合物中的成对簇同位素测量，甲烷水合物是沿着大陆边缘广泛分布的主要甲烷储层，在全球碳循环和气候系统中发挥着重要作用。我们的研究旨在为甲烷团块同位素效应的基本过程及其回答有关天然天然气水合物堆积中发现的甲烷来源和迁移的基本问题的潜力提供新的线索。天然气水合物样品是从日本海东缘的五个浅海沉积物地点回收的，其中大多数样品的Δ13CH3D和Δ12CH2D2温度范围为~15至~170°C，显然与预期的甲烷形成温度相匹配。这些聚集的同位素特征沿平衡线的分布最好是通过在 165 ± 15 °C 温度下形成的平衡热成因甲烷和在 1–2 °C 下平衡的生物甲烷的混合效应来解释，这可能是由缓慢的产甲烷作用、甲烷厌氧氧化 (AOM) 或它们的组合引起的。气体迁移/扩散、水合物形成和解离对Δ13CH3D和Δ12CH2D2值的影响微不足道。通过将聚集同位素结果与其他传统方法相结合，鉴定了产热和两个微生物末端成员及其同位素组成，并且还量化了每个末端成员的相对贡献。结果不仅证明了甲烷团块同位素数据可用于识别天然甲烷样品中潜在的端元，而且还揭示了更传统的碳同位素方法可能大大低估了全球天然气水合物储层中存在的热成因甲烷的比例。来源解析精度的提高使我们能够更好地了解天然气水合物的形成历史和聚集机制，以及天然气水合物解离在过去地质事件中所起的作用。估计的热成因端元地层温度可进一步应用于重建海洋沉积环境热成因甲烷形成时的古地温梯度。
Recently developed methane clumped isotope techniques provides fresh and novel insights into methane biogeochemistry, which have been unobtainable through other techniques such as conventional stable isotope determinations and molecular composition measurements of hydrocarbons. Nonetheless, the governing processes and mechanisms which control the clumped isotope signatures (Δ13CH3D and Δ12CH2D2) of natural methane samples remain an active area of investigation. Here, we present paired clumped isotope measurements in methane hydrate, which is a major methane reservoir widely distributed along the continental margins and which plays an important role in the global carbon cycle and climate system. Our study aims to shed new light into the fundamental processes of methane clumped isotope effects and their potential to answer fundamental questions regarding the source and migration of methane found in naturally occurring gas hydrate accumulations.Gas hydrate samples were recovered from five shallow marine sediment sites on the eastern margin of the Japan Sea and most of them present Δ13CH3D and Δ12CH2D2temperatures ranging from ∼15 to ∼170 °C that apparently match expected methane formation temperatures. The distribution of these clumped isotope signatures along the equilibrium line is best explained by the mixing effect of equilibrated thermogenic methane formed at temperatures of 165 ± 15 °C and biogenic methane equilibrated at 1–2 °C, which may result from slow methanogenesis, anaerobic oxidation of methane (AOM), or their combination. The influences of gas migration/diffusion, hydrate formation and dissociation on Δ13CH3D and Δ12CH2D2values are insignificant. By combining clumped isotope results with other traditional approaches, a thermogenic and two microbial end-members as well as their isotopic compositions were identified and the relative contribution of each end-member was also quantified. The results not only demonstrate the applicability of methane clumped isotope data to identify potential end-members in natural methane samples, but also reveal that more conventional carbon isotope approaches may significantly underestimate the fraction of thermogenic methane present in global gas hydrate reservoirs. Improvements in the accuracy of source apportionment enable us to better understand the formation history and mechanisms of gas hydrate accumulation, as well as the role played by gas hydrate dissociation in past geological events. The estimated formation temperatures of thermogenic end-member can be further applied in reconstruction of the paleo geothermal gradient at the time when the thermogenic methane was formed at marine sedimentary environment.