Methanogenic calcite, 13C-depleted bivalve shells, and gas hydrate from a mud volcano offshore southern California
Methanogenic calcite, 13C-depleted bivalve shells, and gas hydrate from a mud volcano offshore southern California
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DOI：
10.1130/g22098.1
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发表时间：
2006-02-01
期刊：
影响因子：
5.8
通讯作者：
Powell, CL
中科院分区：
文献类型：
--
作者：
Hein, JR;Normark, WR;Powell, CL
位于南加州近海24公里处800米深的一座泥火山中，甲烷和硫化氢从浅埋甲烷水合物上方的冷渗漏中喷发出来。在2.1米的活塞岩心中发现了双壳类、自生方解石和甲烷水合物。两种双壳贝类的文石贝壳异常地缺乏C-13(至-千分之19的三角洲(13)C)，这是迄今发现的最缺乏C-13的海洋大型动物贝壳。活的和死的标本的碳同位素都表明，它们部分使用了从厌氧氧化甲烷中提取的碳来建造贝壳。增量(13)C值变化很大，但大多数都在-12‰到-19‰的范围内。这种变异性可能对识别地质记录中的冷渗漏水合物系统具有诊断意义。自生方解石在岩心中含量丰富，向下至甲烷水合物顶部的次底部约为1.5。方解石亏损C-13(Delta(13)C=-46‰~-58‰)，表明厌氧氧化甲烷产生的碳是方解石的主要来源。甲烷的来源包括来自中新统源岩的地质油气藏，以及盆地沉积物中有机质的生物和热源降解。氧同位素表明，在天然气水合物解离和强甲烷通量的影响下，方解石大部分是与周围底水的同位素平衡以外形成的。泥火山沉积物中的高金属含量表明，沿着下伏断层循环的流体对基岩进行了淋溶，这也允许大量的化石甲烷。
Methane and hydrogen sulfide vent from a cold seep above a shallowly buried methane hydrate in a mud volcano located 24 km offshore southern California in 800 m of water. Bivalves, authigenic calcite, and methane hydrate were recovered in a 2.1 m piston core. Aragonite shells of two bivalve species are unusually depleted in C-13 (to -19 parts per thousand delta(13)C), the Most C-13-depleted shells of marine macrofauna yet discovered. Carbon isotopes for both living and dead specimens indicate that they used, in part, carbon derived from anaerobically oxidized methane to construct their shells. The delta(13)C values are highly variable, but most are within the range -12 parts per thousand to -19 parts per thousand. This variability may be diagnostic for identifying cold-seep-hydrate systems in the geologic record. Authigenic calcite is abundant in the cores down to similar to 1.5 in subbottom, the top of the methane hydrate. The calcite is depleted in C-13 (delta(13)C = -46 parts per thousand to -58 parts per thousand), indicating that carbon produced by anaerobically oxidized methane is the main source of the calcite. Methane sources include a geologic hydrocarbon reservoir from Miocene source rocks, and biogenic and thermogenic degradation of organic matter in basin sediments. Oxygen isotopes indicate that most calcite formed out of isotopic equilibrium with ambient bottom water, under the influence of gas hydrate dissociation and strong methane flux. High metal content in the mud volcano sediment indicates leaching of basement rocks by fluid circulating along an underlying fault, which also allows for a high flux of fossil methane.