Effect of massive gas hydrate formation on the water isotopic fractionation of the gas hydrate system at Hydrate Ridge, Cascadia margin, offshore Oregon
Effect of massive gas hydrate formation on the water isotopic fractionation of the gas hydrate system at Hydrate Ridge, Cascadia margin, offshore Oregon
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俄勒冈州近海卡斯卡迪亚边缘水合物岭大量天然气水合物形成对天然气水合物系统水同位素分馏的影响
DOI：
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发表时间：
2006
期刊：
影响因子：
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通讯作者：
W. Borowski
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作者：
H. Tomaru;M. Torres;R. Matsumoto;W. Borowski
由于天然气水合物优先富集重水同位素，因此，从含天然气水合物沉积物中采集的孔隙沃茨的δ 18 O和δD值可以提供有关天然气水合物丰度和形成机制的信息。在ODP航次204期间，从海波海脊深层（40至125 mbsf）天然气水合物矿床中取样的孔隙水沃茨显示，由于岩心回收期间天然气水合物的不稳定性，溶解Cl−减少，18 O和D富集。从大量数据集（n = 30个样品）估算的氧和氢同位素分馏因子（αO = 1.0025和αH = 1.022）与实验测定值一致。相比之下，来自海脊顶部（n = 32）浅层样品（<25 mbsf）的孔隙沃茨高度富集溶解Cl−，并贫化18 O和D，这与块状天然气水合物矿床的形成速度相一致，其速度快于通过平流或扩散去除这些异常的速度。卤水中的水同位素分馏因子明显低于实验测定值，αO为1.0010（平均值为1.0012），αH为1.008（平均值为1.008）。我们讨论了几个因素，可能会导致这种异常分馏，并建议水合物晶格中的低气体占有率（高水合数）可能是负责所观察到的小分馏。如果是这样的话，氧和氢分馏可以作为天然气水合物形成过程中的水化数的指标。
Because gas hydrate is preferentially enriched in the heavy water isotopes, the δ18O and δD values of pore waters collected from gas hydrate–bearing sediment can provide information on the abundance and mechanisms of gas hydrate formation. Pore waters sampled from deep‐seated (40 to 125 mbsf) gas hydrate deposits in Hydrate Ridge during ODP Leg 204 show depletion in dissolved Cl− and enrichments in 18O and D due to gas hydrate destabilization during core recovery. The oxygen and hydrogen isotopic fractionation factors (αO = 1.0025 and αH = 1.022) estimated from an extensive data set (n = 30 samples) correspond to experimentally determined values. In contrast, pore waters from shallow samples (<25 mbsf) at the ridge summit (n = 32) are highly enriched in dissolved Cl− and depleted in 18O and D, consistent with formation of massive gas hydrate deposits at rates faster than those at which these anomalies would be removed by advection or diffusion. The water isotopic fractionation factors in the brine are significantly lower than those experimentally determined, with αO of 1.0010 (average value of 1.0012) and αH of 1.008 (average value of 1.008). We discuss several factors that may be causing this anomalous fractionation and suggest that low gas occupancy in hydrate lattice (high hydration number) may be responsible for the observed small fractionation. If this were the case, the oxygen and hydrogen fractionation may serve as an indicator of hydration number during formation of gas hydrate in natural systems.