Diagenetic Dissolution of Biogenic Magnetite in Surface Sediments of the Benguela Upwelling System
Diagenetic Dissolution of Biogenic Magnetite in Surface Sediments of the Benguela Upwelling System
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DOI：
10.1007/s005310050293
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发表时间：
2000-03
影响因子：
2.3
通讯作者：
K. Hilgenfeldt
中科院分区：
文献类型：
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作者：
K. Hilgenfeldt
本格拉上升流系统表层水中强烈的初级生物生产力为纳米比亚附近大陆坡的海底提供了大量的有机物，并在顶部沉积层中维持了极端浓度的产生磁铁矿的细菌。因此，生物磁铁矿迄今为止是这些矿床中磁信号的主要载体，这也是因为陆源亚铁磁性矿物的输入非常少。沉积物柱中的还原条件导致细菌磁铁矿部分选择性溶解在主要矿化层下方几厘米处。通过高分辨率岩石磁性分析和透射电子显微镜详细记录了这种成岩过程。浓度依赖性和粒度敏感的磁性参数，例如磁化率、实验室赋予的剩磁和磁滞数据，揭示了沉积物上部 10 厘米内亚铁磁性矿物含量显着下降，同时亚铁磁性矿物组合逐渐向下粗化，从顶部厘米处主要磁性单畴颗粒到更深地层中的多畴颗粒。电子显微镜观察既可以根据形状和粒度明确识别细菌磁铁矿，又可以追踪深度对生物磁性矿物成分的溶解影响。
Intense primary biologic productivity in the surface waters of the Benguela upwelling system provides a high supply of organic matter to the sea floor at the continental slope off Namibia and sustains extreme concentrations of magnetite producing bacteria in the top sediment layers. Biogenic magnetite is thus by far the dominant carrier of the magnetic signal in these deposits also because of a very minor input of terrigenous ferrimagnetic minerals. Reducing conditions in the sediment column cause a selective dissolution of the bacterial magnetite fraction just a few centimeters below the main mineralization horizon. This diagenetic process is documented in detail by high-resolution rock magnetic analyses and transmission electron microscopy. Concentration dependent and grain-size sensitive magnetic parameters, such as susceptibility, laboratory imparted remanences, and hysteresis data, reveal a significant drop in ferrimagnetic mineral content within the upper 10 cm of the sediments accompanied by a gradual downward coarsening of the ferrimagnetic mineral assemblage from primarily magnetic single-domain particles in the top centimeters to multi-domain grains in deeper strata. Electron microscope observations enable both an unequivocal identification of bacterial magnetite on the basis of shape and grain-size and to trace dissolution effects on the biogenic magnetic mineral component to depth.