Large carbon isotope variability during methanogenesis under alkaline conditions
Large carbon isotope variability during methanogenesis under alkaline conditions
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DOI：
10.1016/j.gca.2018.06.007
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发表时间：
2018-09
影响因子：
5
通讯作者：
H. M. Miller;Nabil Chaudhry;M. Conrad;M. Bill;S. Kopf;A. Templeton
中科院分区：
文献类型：
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作者：
H. M. Miller;Nabil Chaudhry;M. Conrad;M. Bill;S. Kopf;A. Templeton
高碳同位素值（δ 13 CCH 4> −40‰）已被广泛用作碱性岩石流体中甲烷非生物形成的证据，特别是在蛇纹岩系统中。然而，同位素分馏微生物产甲烷过程中是相对欠研究在高pH值。从Samail蛇绿岩中的超碱性地下流体中，以评估CH 4的碳和氢同位素值如何根据pH值和碳酸盐矿物来源（NaHCO 3或CaCO 3）而变化。氢同位素分馏αH20/CH 4（1.46-1.66）在不同pH值下没有变化。相反，碳同位素分馏αCO2/CH 4的范围为1.028 - 1.089。碳同位素分馏随着pH值的增加而增加，13 C的最大亏损为-85 ‰。然而，在pH ≥ 9时，添加CaCO 3的实验中，13 C的消耗显著减少，产生的δ 13 CCH 4高达−28‰。为了评估δ 13 CCH 4的巨大变异性，我们开发了一个稳态模型来评估碳酸盐溶解速率、细胞吸收CO2和不可逆CH 4产生如何影响甲烷生成过程中的净同位素分馏。在无机碳含量高、产甲烷速率慢的模拟碱性蛇纹岩成岩液中，δ 13 CCH 4的含量高，δ 13 CCH 4的含量低。然而，当碳酸盐溶解的速率比细胞吸收的速率慢时，会发生小的碳同位素分馏，导致相对较高的δ 13 CCH 4值（> 100 - 35‰），传统上被解释为纯粹的“非生物”。因此，微生物甲烷可以产生在地球上的碳有限的镁铁质和超镁铁质岩石的环境和潜在的其他行星机构，但它可能是难以确定同位素生物甲烷时，矿物碳酸盐是主要的碳源。
High carbon isotope values (δ13CCH4> −40‰) have widely been used as evidence that methane in alkaline rock-hosted fluids was formed abiotically, particularly in serpentinizing systems. However, isotope fractionation during microbial methanogenesis is relatively understudied at high pH. We isolated a hydrogenotrophicMethanobacteriumsp. from hyperalkaline subsurface fluids in the Samail ophiolite to assess how carbon and hydrogen isotope values of CH4varied depending upon pH and carbonate mineral source (NaHCO3or CaCO3). The hydrogen isotope fractionation αH20/CH4(1.46–1.66) did not vary across pH. In contrast, the expressed carbon isotope fractionation, αCO2/CH4, ranged from 1.028 to 1.089. Carbon isotope fractionation increased with pH, reaching a maximum13C depletion of −85‰. However, the13C depletion significantly diminished at pH ≥ 9 for CaCO3-amended experiments, generating δ13CCH4as high as −28‰. To evaluate the large variability in δ13CCH4, we developed a steady-state model to assess how the rates of carbonate dissolution, cellular uptake of CO2and irreversible CH4production can affect the net isotope fractionation during methanogenesis.Methanobacteriumsp. can produce highly depleted δ13CCH4in simulated alkaline serpentinizing fluids when dissolved inorganic carbon levels are high and methanogenesis rates are slow. However, small carbon isotope fractionation occurs when rates of carbonate dissolution are slower than cellular uptake, leading to relatively high δ13CCH4values (>∼−35‰) that are traditionally interpreted to be purely “abiotic”. Thus, microbial CH4can be produced in carbon-limited mafic and ultramafic rock-hosted environments on Earth and potentially other planetary bodies, but it may be difficult to isotopically identify biogenic methane when mineral carbonates are the dominant carbon source.