Controls on calcium isotope fractionation in sedimentary porewaters
Controls on calcium isotope fractionation in sedimentary porewaters
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DOI：
10.1016/j.epsl.2009.01.011
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发表时间：
2009-03
影响因子：
5.3
通讯作者：
B. Teichert;N. Gussone;M. Torres
中科院分区：
文献类型：
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作者：
B. Teichert;N. Gussone;M. Torres
大洋钻探计划（ODP）期间钻探的会聚大陆边缘缺氧沉积物中孔隙水和自生碳酸盐的钙同位素组成首次揭示了控制碎屑海洋、富含有机物的沉积环境中钙地球化学的不同过程。在海厄里脊（卡斯卡迪亚边缘，近海俄勒冈州/美国）第204航次期间钻探的4个地点，在甲烷厌氧氧化过程中，硫酸盐被消耗，在沉积剖面的上部米内，有机物通过硫酸盐还原被消耗。这些反应通过生成碳酸氢盐促进自生碳酸盐的沉淀，这反映在钙浓度的显著降低中。虽然在碳酸盐沉淀过程中观察到Ca同位素分馏，但ODP Leg 204孔隙流体中的Ca浓度与Ca同位素分离，这似乎主要受完全覆盖碳酸盐形成效应的轻Ca同位素释放控制。不同的过程，如有机结合钙的释放，离子交换和离子对的形成可能是负责释放轻钙。更深的沉积部分，其他过程，如灰蚀变影响钙同位素组成的孔隙水。两个地点，钻入更深的核心的增生棱镜，揭示了性质的流体与海洋基底的反应。这些深部流体具有较高的Ca浓度和较低的δ44/40 Ca比值。
The calcium isotopic composition of porewaters and authigenic carbonates in the anoxic sediments of a convergent continental margin drilled during Ocean Drilling Program (ODP) provides first insight into the different processes that control Ca geochemistry in clastic marine, organic-rich sedimentary environments. In 4 sites drilled during Leg 204 at Hydrate Ridge (Cascadia Margin, offshore Oregon/USA), sulfate is consumed during anaerobic oxidation of methane and of organic matter via sulfate reduction within the upper meters of the sedimentary section. These reactions promote the precipitation of authigenic carbonates through the generation of bicarbonate, which is reflected in a pronounced decrease in calcium concentration. Although Ca isotope fractionation is observed during carbonate precipitation, Ca concentration in the pore fluids from ODP Leg 204 is decoupled from Ca isotopy, which seems to be mainly controlled by the release of light Ca isotopes that completely overprint the carbonate formation effect. Different processes, such as the release of organically bound Ca, ion exchange and ion pair formation may be responsible for the released light Ca. Deeper within the sedimentary section, additional processes such as ash alteration influence the Ca isotopic composition of the porewater. Two sites, drilled into the deeper core of the accretionary prism, reveal the nature of fluids which have reacted with the oceanic basement. These deep fluids are characterized by relatively high Ca concentrations and low δ44/40Ca ratios.