Abstract Cold seep environments are characterized by methane-rich fluid migration and discharge at the seafloor. These environments are also intimately linked to microbial communities, which oxidize methane anaerobically, increase alkalinity and promote authigenic carbonate precipitation. We have analyzed a suite of methane-derived authigenic carbonate (MDAC) crusts from the North and Barents Sea using stable and clumped isotopes (δ13C, δ18O, δ44Ca, and Δ47) to characterize the sources of fluids as well as the environment of carbonate authigenesis. We additionally assess the potential of MDACs as a Δ47-based paleotemperature archive. The MDACs occur as three main textural-mineralogic types: micritic Mg-calcite cements, micritic aragonite cements and cavity filling aragonite cements. We find that micritic Mg-calcite cements have low δ13CVPDB values (−30 to −47‰), high δ44CaSW values (−0.4 to −0.8‰), and Δ47-temperatures (0–6 °C) consistent with shallow sub-seafloor precipitation in isotopic equilibrium. Micritic aragonite cements and cavity filling aragonite cements both have a wider range in δ13CVPDB values (−18 to −58‰), lower δ44CaSW values (−0.8 to −1.6‰) and a larger range in Δ47-based apparent temperatures (–2 – 25 °C) with samples displaying equilibrium and disequilibrium clumped isotope values. The range in apparent temperatures as well as δ44CaSW values seen in the aragonite MDACs suggest two kinetic processes: a kinetic isotope effect (KIE) due to the incomplete equilibration of carbon and oxygen isotopes among DIC species from the different sources of DIC (i.e., seawater, methane-sourced DIC and DIC residual to CO2 degassing or diffusion) and a KIE due to a fast, irreversible precipitation affecting the cations, particularly Ca, bound to carbonate mineral. Our results improve the understanding of kinetic effects on clumped isotope temperatures in MDACs and demonstrate how the multi-isotopic approach combined with textural-mineralogic criteria can be used to identify MDACs for accurate paleotemperature reconstructions.

中文翻译：

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甲烷冷渗环境中自生碳酸盐的稳定和团块同位素表征

摘要 冷渗环境的特点是富含甲烷的流体在海底运移和排放。这些环境也与微生物群落密切相关，微生物群落厌氧氧化甲烷，增加碱度并促进自生碳酸盐沉淀。我们使用稳定的团块同位素（δ13C、δ18O、δ44Ca 和 Δ47）分析了一套来自北海和巴伦支海的甲烷衍生自生碳酸盐 (MDAC) 地壳，以表征流体的来源以及碳酸盐自生环境. 我们还评估了 MDAC 作为基于 Δ47 的古温度档案的潜力。MDACs 以三种主要的结构矿物类型出现：泥晶镁方解石胶结物、泥晶文石胶结物和空腔填充文石胶结物。我们发现泥晶镁方解石水泥具有低 δ13CVPDB 值（-30 至 -47‰）、高 δ44CaSW 值（-0.4 至 -0.8‰）和 Δ47 温度（0-6°C）与浅层海底一致同位素平衡中的沉淀。泥晶文石胶结物和空腔填充文石胶结物的 δ13CVPDB 值范围更广（-18 至 -58‰），δ44CaSW 值更低（-0.8 至 -1.6‰），而基于 Δ47 的表观温度范围更大（–2 – 25 °C)，样品显示平衡和不平衡聚集同位素值。在文石 MDAC 中观察到的表观温度范围以及 δ44CaSW 值表明存在两个动力学过程：由于来自不同 DIC 来源（即海水）的 DIC 物种之间碳和氧同位素的不完全平衡导致的动力学同位素效应 (KIE) , 甲烷来源的 DIC 和 DIC 残留到 CO2 脱气或扩散）和 KIE，这是由于快速、不可逆的沉淀影响与碳酸盐矿物结合的阳离子，特别是 Ca。我们的结果提高了对 MDAC 中聚集同位素温度的动力学影响的理解，并展示了如何将多同位素方法与结构矿物学标准相结合来识别 MDAC，以进行准确的古温度重建。