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Evidence for methane isotopic bond re-ordering in gas reservoirs sourcing cold seeps from the Sea of Marmara
Earth and Planetary Science Letters ( IF 5.3 ) Pub Date : 2021-01-01 , DOI: 10.1016/j.epsl.2020.116619
T. Giunta , J. Labidi , I.E. Kohl , L. Ruffine , J.P. Donval , L. Géli , M.N. Çağatay , H. Lu , E.D. Young

Abstract The measurement of methane clumped isotopologues ( Δ 13 CH 3 D and Δ 12 CH 2 D 2 ) allows exploring isotope bond ordering within methane molecules, and may reveal equilibrium temperatures. Whether such temperature reflects the formation or re-equilibration temperature of the methane is not well understood, but would have critical implications for the use of methane clumped isotopologues as geo-thermometers. Here we investigate gas bubbles from vigorous emissions at cold seeps (n = 14) in the Sea of Marmara, Turkey. These cold seeps are sourced from deeper sedimentary reservoirs. Conventional geochemical tracers such as carbon and hydrogen bulk isotopic ratios (13C/12C and D/H) or n-alkane molecular ratios, suggest these gases reflect various degrees of mixing between thermogenic and microbial sources. Some samples would generally be considered purely microbial in origin ( C 1 / C 2 + > 1500 ; δ 13 C − 60 ‰ ). We report measurements of Δ 13 CH 3 D and Δ 12 CH 2 D 2 showing that a fraction of those gases are in internal thermodynamic equilibrium, with the abundances of the two mass-18 isotopologues indicating concordant temperatures of ∼90 °C and ∼130 °C. These concordant temperatures are recorded by gases of putative microbial and thermogenic origin; the temperatures of equilibration are irrespective of the formation mechanism of the gases. We conclude that the two high-temperatures recorded by Δ 13 CH 3 D and Δ 12 CH 2 D 2 are best explained by non-enzymatic re-equilibration at two local subsurface temperatures. First principles suggest that unequal rates of exchange are possible. Disequilibrium signatures where the two isotopologues yield discordant apparent temperatures are exhibited by other samples. In those cases the data define a trend of variable Δ 13 CH 3 D at nearly constant Δ 12 CH 2 D 2 . These signatures are enigmatic, and we investigate and reject multiple possible explanations including mixing, diffusion or Anaerobic Oxidation of Methane. Different rates of re-equilibration between the two rare isotopologues are implied, although lacks experimental foundation at present. In general, all of these data point towards re-equilibration of the mass-18 methane isotopologues as an important process.

中文翻译:

来自马尔马拉海冷泉的气藏中甲烷同位素键重新排序的证据

摘要 甲烷聚集同位素体(Δ 13 CH 3 D 和Δ 12 CH 2 D 2 )的测量允许探索甲烷分子内的同位素键排序,并可能揭示平衡温度。这种温度是否反映了甲烷的形成或重新平衡温度尚不清楚,但对使用甲烷聚集同位素体作为地球温度计具有重要意义。在这里,我们调查了土耳其马尔马拉海冷泉 (n = 14) 处剧烈排放产生的气泡。这些冷渗流来自更深的沉积储层。传统的地球化学示踪剂,如碳和氢本体同位素比(13C/12C 和 D/H)或正烷烃分子比,表明这些气体反映了热源和微生物源之间不同程度的混合。一些样品通常被认为是纯微生物来源的(C 1 / C 2 + > 1500 ; δ 13 C − 60 ‰)。我们报告了 Δ 13 CH 3 D 和 Δ 12 CH 2 D 2 的测量结果,表明这些气体中的一小部分处于内部热力学平衡状态,两种质量为 18 的同位素体的丰度表明温度为 ~90 °C 和 ~130 ℃。这些一致的温度由假定的微生物和产热来源的气体记录;平衡温度与气体的形成机制无关。我们得出结论,Δ 13 CH 3 D 和Δ 12 CH 2 D 2 记录的两个高温最好通过在两个局部地下温度下的非酶促再平衡来解释。首要原则表明,不平等的汇率是可能的。其他样品表现出两种同位素体产生不一致的表观温度的不平衡特征。在这些情况下,数据定义了Δ 13 CH 3 D 在几乎恒定的Δ 12 CH 2 D 2 处的变化趋势。这些特征是神秘的,我们调查并拒绝了多种可能的解释,包括甲烷的混合、扩散或厌氧氧化。尽管目前缺乏实验基础,但暗示了两种稀有同位素之间不同的再平衡率。一般而言,所有这些数据都表明质量为 18 的甲烷同位素体的重新平衡是一个重要过程。我们调查并拒绝了多种可能的解释,包括甲烷的混合、扩散或厌氧氧化。尽管目前缺乏实验基础,但暗示了两种稀有同位素之间不同的再平衡率。一般而言,所有这些数据都表明质量为 18 的甲烷同位素体的重新平衡是一个重要过程。我们调查并拒绝了多种可能的解释,包括甲烷的混合、扩散或厌氧氧化。尽管目前缺乏实验基础,但暗示了两种稀有同位素之间不同的再平衡率。一般而言,所有这些数据都表明质量为 18 的甲烷同位素体的重新平衡是一个重要过程。
更新日期:2021-01-01
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