Anoxic Diagenesis and Methane Generation in Sediments of the Blake Outer Ridge, Deep Sea Drilling Project Site 533, Leg 76
Anoxic Diagenesis and Methane Generation in Sediments of the Blake Outer Ridge, Deep Sea Drilling Project Site 533, Leg 76
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DOI：
10.2973/dsdp.proc.76.109.1983
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发表时间：
1983-11
期刊：
影响因子：
--
通讯作者：
G. Claypool;C. Threlkeld
中科院分区：
文献类型：
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作者：
G. Claypool;C. Threlkeld
测定了布莱克外脊最上面400 m沉积物的间隙水和气体样本中SC^"、HCO 3”、CO2和CH 4的浓度以及δS和δC值。这些测量提供了与有机质分解产生的电子的厌氧呼吸成岩过程的详细解释的基础。在海底以下很浅的深度（< 1米），沉积物是厌氧的。硫酸盐还原仅限于沉积物的最高15米，并导致氧化碳从沉积物中的显着外流。在硫酸盐还原区的底部，向上扩散的CH 4被氧化，显然与SO 2+还原有关。通过CO2还原产生CH 4是15 m深度以下最重要的代谢过程。在15 ~ 100 m深度范围内，CO_2的去除比CO_2的输入快，HCO_3 ~-浓度略有下降，δ C值正移40‰。共存的CH_4、CO_2和HCO_3 ~+之间的差异与CH_4和溶解CO_2之间的动力学分馏以及CO_2和HCO_3 ~+之间的平衡分馏是一致的。在深度大于100 m时，CO2的输入速率（δ C = -25‰）超过CO2转化为CH 4的去除速率（δ C为60 - 65‰）的2倍。这导致溶解的HCO 3-浓度增加，同时保持HCO 3-的δ C相对恒定在-110 ‰。非稳态沉积导致有机碳含量明显较高，150 m以下孔隙水的碱度异常高（70 meq I”）。这些高碱性被认为与自发脱羧反应有关，而不是与生物过程有关。在200 ~ 400 m深度范围内，HCOf浓度普遍降低，而δC不变，这可能反映了自生碳酸盐沉淀的增加。输入-输出碳同位素质量平衡计算，碳酸盐体系平衡结合观察到的CO2-CH 4的比例在气相中，独立地表明，CH 4浓度的顺序为100 mmol kg-1的存在于孔隙沃茨的布莱克外脊沉积物。据信该量的CH 4不足以使孔隙沃茨饱和并使CH 4·6 H2O气体水合物稳定。这些计算的结果与238米天然气水合物的物理回收相冲突，并与这些沉积物中天然气水合物存在的间接证据（地震反射体，沉积物起泡，盐度和氯度随深度略有下降，以及压力岩心筒观察）相冲突。如果CH 4的产生被限制在相对薄的（1-10米）深度间隔内，并且在整个沉积柱的所有深度处不均匀地发生，或者如果另一个产甲烷过程（例如，乙酸发酵）是气体的主要贡献者。
Concentrations and δS and δC values were determined on SC^", HCO3", CO2, and CH4 in interstitial water and gas samples from the uppermost 400 m of sediment on the Blake Outer Ridge. These measurements provide the basis for detailed interpretation of diagenetic processes associated with anaerobic respiration of electrons generated by organic-matter decomposition. The sediments are anaerobic at very shallow depths (< 1 m) below the seafloor. Sulfate reduction is confined to the uppermost 15 m of sediment and results in a significant outflux of oxidized carbon from the sediments. At the base of the sulfate reduction zone, upward-diffusing CH4 is being oxidized, apparently in conjunction with SC^~ reduction. CH4 generation by CO2 reduction is the most important metabolic process below the 15-m depth. CO2 removal is more rapid than CO2 input over the depth interval from 15 to 100 m, and results in a slight decrease in HCO3~ concentration accompanied by a 40‰ positive shift in δ C. The differences among coexisting CH4, CO2, and HCOf are consistent with kinetic fractionation between CH4 and dissolved CO2, and equilibrium fractionation between CO2 and HCO3". At depths greater than 100 m, the rate of input of CO2 (δ C = -25‰) exceeds by 2 times the rate of removal of CO2 by conversion to CH4 (δ C of 60 to 65‰). This results in an increase of dissolved HCO3" concentration while maintaining δ C of HCO3" relatively constant at -IlO‰. Non-steady-state deposition has resulted in significantly higher organic carbon contents and unusually high (70 meq I") pore-water alkalinities below 150 m. These high alkalinities are believed to be related more to spontaneous decarboxylation reactions than to biological processes. The general decrease in HCOf concentration with constant δC over the depth interval of 200 to 400 m probably reflects increased precipitation of authigenic carbonate. Input-output carbon isotope-mass balance calculations, and carbonate system equilibria in conjunction with observed CO2-CH4 ratios in the gas phase, independently suggest that CH4 concentrations on the order of 100 mmol kg" are present in the pore waters of Blake Outer Ridge sediments. This quantity of CH4 is believed to be insufficient to saturate pore waters and stabilize the CH4 6H2O gas hydrate. Results of these calculations are in conflict with the physical recovery of gas hydrate from 238 m, and with the indirect evidence (seismic reflectors, sediment frothing, slightly decreasing salinity and chlorinity with depth, and pressure core barrel observations) of gas-hydrate occurrence in these sediments. Resolution of this apparent conflict would be possible if CH4 generation were restricted to relatively thin (1-10 m) depth intervals, and did not occur uniformly at all depths throughout the sediment column, or if another methanogenic process (e.g., acetate fermentation) were a major contributor of gas.