Overwriting of sedimentary magnetism by bacterially mediated mineral alteration
Overwriting of sedimentary magnetism by bacterially mediated mineral alteration
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DOI：
10.1130/g39706.1
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发表时间：
2018-04
期刊：
影响因子：
5.8
通讯作者：
Y. Ebert;R. Shaar;S. Emmanuel;N. Nowaczyk;M. Stein
中科院分区：
文献类型：
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作者：
Y. Ebert;R. Shaar;S. Emmanuel;N. Nowaczyk;M. Stein
海洋和湖泊沉积物是全球古地磁资料的重要来源。虽然通常认为碎屑氧化铁记录了沉积物中大部分的磁性信号，但细菌硫酸盐还原过程中形成的硫化铁也可以代表沉积物磁性的重要来源。了解硫酸盐还原如何影响沉积磁性对于解释古地磁记录至关重要。在这里，我们展示了三种不同类型的磁性颗粒可以通过细菌硫酸盐还原产生，每一种都以不同的方式影响大块沉积物的磁性。我们将磁力显微镜和电子探针显微分析相结合，对冰期和间冰期死海沉积物中的磁性矿物提取物进行了成像。在抑制细菌硫酸盐还原作用的干冰间冰期沉积物中，我们发现了具有强粒间磁相互作用的灰长岩型树状体（fe3s4）。相比之下，湿冰期沉积物经历了广泛的硫酸盐还原，黄铁矿（FeS 2）是主要的硫化物相。冰川黄铁矿的高分辨率磁成像显示，在黄铁矿中，灰长岩以单畴颗粒的形式存在。我们还发现，当钛磁铁矿颗粒经过细菌介导的蚀变形成黄铁矿时，原始磁性颗粒被分成更小的区域，这可能有助于通过这些磁畴的重组获得二次磁化。我们的结果提供了一个以前未记载的机制，细菌介导的蚀变可以覆盖原始碎屑磁记录。
Marine and lacustrine sediments represent an important source of global paleomagnetic data. Although it is usually assumed that detrital iron oxides record most of the magnetic signal in sediments, iron sulfides—which form during bacterial sulfate reduction—can also represent a significant source of sedimentary magnetism. Knowing how sulfate reduction impacts sedimentary magnetism is critical to the interpretation of paleomagnetic records. Here, we show that three distinct types of magnetic particles can be produced by bacterial sulfate reduction, each of which impacts the bulk sediment magnetism in a distinct way. We combined magnetic force microscopy and electron probe microanalysis to image magnetic mineral extracts from Dead Sea sediments from a glacial period and an interglacial period. In sediments from the dry interglacial period, during which bacterial sulfate reduction was suppressed, we found greigite framboids (Fe 3 S 4) with strong intergrain magnetic interactions. Contrastingly, in sediments from the wet glacial period, which experienced extensive sulfate reduction, pyrite (FeS 2) is the dominant sulfide phase. High-resolution magnetic imaging of glacial pyrite reveals that greigite is present as single-domain particles within the pyrite. We also found that as titanomagnetite grains undergo bacterially mediated alteration to form pyrite, the original magnetic grains become divided into smaller regions, which potentially facilitates acquisition of secondary magnetization by the reorganization of these magnetic domains. Our results provide a previously undocumented mechanism by which bacterially mediated alteration can overwrite primary detrital magnetic records.