Diagenetic Alteration of Magnetic Signals by Anaerobic Oxidation of Methane Related to a Change in Sedimentation Rate
Diagenetic Alteration of Magnetic Signals by Anaerobic Oxidation of Methane Related to a Change in Sedimentation Rate
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DOI：
10.1016/j.gca.2005.02.004
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发表时间：
2005-08
影响因子：
5
通讯作者：
N. Riedinger;Kerstin Pfeifer;S. Kasten;J. Garming;C. Vogt;C. Hensen
中科院分区：
文献类型：
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作者：
N. Riedinger;Kerstin Pfeifer;S. Kasten;J. Garming;C. Vogt;C. Hensen
对阿根廷和乌拉圭沿海大陆边缘三个地点的沉积物进行了地球化学和岩石磁学调查，以研究甲烷厌氧氧化（AOM）驱动的铁矿物成岩蚀变。阿根廷西部盆地代表了研究非稳态过程的合适沉积环境，因为它具有高度动态的沉积条件。矿物学和散装固相数据表明，沉积物主要由铁矿物含量高的陆源物质组成。作为这些沉积物的典型特征，可以观察到磁化率 (κ) 的明显最小值。孔隙水数据表明，这些磁化率最小值与硫酸盐/甲烷转变 (SMT) 的当前深度一致，其中 HS− 是由 AOM 过程生成的。释放的 HS− 与丰富的铁（羟基）氧化物发生反应，导致硫化铁沉淀，同时磁化率几乎完全丧失。地球化学数据模拟表明，该地区的磁记录很大程度上受到更新世/全新世过渡期间发生的平均沉降速率（SR）急剧变化的影响。我们假设在冰期/间冰期过渡期间平均 SR 的急剧下降导致 SMT 固定在特定深度。停滞明显增强了不同沉积区间内铁（羟基）氧化物的成岩溶解作用。这一假设得到了数值模型的进一步证实，其中平均 SR 从冰期期间的 100 cm kyr−1 下降到全新世的 5 cm kyr−1 ，并且来自下方的甲烷通量固定为恒定值。为了获得观测到的地球化学和磁性模式，SMT 必须保持在固定位置约 9000 年。该计算值与更新世/全新世过渡的时间密切相关。模型结果还表明，恒定的高平均 SR 会导致稳态条件下孔隙水硫酸盐的上凹轮廓。
Geochemical and rock magnetic investigations of sediments from three sites on the continental margin off Argentina and Uruguay were carried out to study diagenetic alteration of iron minerals driven by anaerobic oxidation of methane (AOM). The western Argentine Basin represents a suitable sedimentary environment to study nonsteady-state processes because it is characterized by highly dynamic depositional conditions. Mineralogic and bulk solid phase data document that the sediment mainly consists of terrigenous material with high contents of iron minerals. As a typical feature of these deposits, distinct minima in magnetic susceptibility (κ) are observed. Pore water data reveal that these minima in susceptibility coincide with the current depth of the sulfate/methane transition (SMT) where HS− is generated by the process of AOM. The released HS− reacts with the abundant iron (oxyhydr)oxides resulting in the precipitation of iron sulfides accompanied by a nearly complete loss of magnetic susceptibility. Modeling of geochemical data suggest that the magnetic record in this area is highly influenced by a drastic change in mean sedimentation rate (SR) which occurred during the Pleistocene/Holocene transition. We assume that the strong decrease in mean SR encountered during this glacial/interglacial transition induced a fixation of the SMT at a specific depth. The stagnation has obviously enhanced diagenetic dissolution of iron (oxyhydr)oxides within a distinct sediment interval. This assumption was further substantiated by numerical modeling in which the mean SR was decreased from 100 cm kyr−1 during glacial times to 5 cm kyr−1 in the Holocene and the methane flux from below was fixed to a constant value. To obtain the observed geochemical and magnetic patterns, the SMT must remain at a fixed position for ∼9000 yrs. This calculated value closely correlates to the timing of the Pleistocene/Holocene transition. The results of the model show additionally that a constant high mean SR would cause a concave-up profile of pore water sulfate under steady state conditions.