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首页> 外文期刊>Geochemistry, geophysics, geosystems >Rock magnetic and geochemical evidence for authigenic magnetite formation via iron reduction in coal-bearing sediments offshore Shimokita Peninsula, Japan (IODP Site C0020)
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Rock magnetic and geochemical evidence for authigenic magnetite formation via iron reduction in coal-bearing sediments offshore Shimokita Peninsula, Japan (IODP Site C0020)

机译：岩石磁性和地球化学证据通过铁减少煤沉积物脱岸沉积物沉积物，日本（IODP网站C0020）

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摘要

Sediments recovered at Integrated Ocean Drilling Program (IODP) Site C0020, in a fore-arc basin offshore Shimokita Peninsula, Japan, include numerous coal beds (0.3-7 m thick) that are associated with a transition from a terrestrial to marine depositional environment. Within the primary coal-bearing unit (approximate to 2 km depth below seafloor) there are sharp increases in magnetic susceptibility in close proximity to the coal beds, superimposed on a background of consistently low magnetic susceptibility throughout the remainder of the recovered stratigraphic sequence. We investigate the source of the magnetic susceptibility variability and characterize the dominant magnetic assemblage throughout the entire cored record, using isothermal remanent magnetization (IRM), thermal demagnetization, anhysteretic remanent magnetization (ARM), iron speciation, and iron isotopes. Magnetic mineral assemblages in all samples are dominated by very low-coercivity minerals with unblocking temperatures between 350 and 580 degrees C that are interpreted to be magnetite. Samples with lower unblocking temperatures (300-400 degrees C), higher ARM, higher-frequency dependence, and isotopically heavy Fe-56 across a range of lithologies in the coal-bearing unit (between 1925 and 1995 mbsf) indicate the presence of fine-grained authigenic magnetite. We suggest that iron-reducing bacteria facilitated the production of fine-grained magnetite within the coal-bearing unit during burial and interaction with pore waters. The coal/peat acted as a source of electron donors during burial, mediated by humic acids, to supply iron-reducing bacteria in the surrounding siliciclastic sediments. These results indicate that coal-bearing sediments may play an important role in iron cycling in subsiding peat environments and if buried deeply through time, within the subsequent deep biosphere.
机译：在综合海洋钻井计划（IODP）网站C0020中恢复的沉积物，在日本陆舰夏洛塔半岛（Mapshiin Shimokita半岛）中包括许多煤层（0.3-7米厚），与从陆地到海洋沉积环境的过渡相关。在初级煤轴承单元（大致2公里深度下方）内，煤床紧密邻近磁化率急剧增加，在恢复的地层序列的其余部分始终如一的低磁化率的背景下叠加。我们研究了磁化率变异性的来源，并在整个芯记录中表征了主要磁体组合，使用等温剩余磁化（IRM），热退磁，中间空气倒置磁化（ARM），铁形状化和铁同位素。所有样品中的磁性矿物组合由非常低矫顽力的矿物质在350和580摄氏度之间的解释为磁铁矿之间的脱胀温度。在煤轴承单元（1925和1995 MBSF之间）的一系列岩性上，具有较低的解锁温度（300-400摄氏度），更高的臂，更高频率依赖性和同位素重的FE-56的样品表示良好的存在-GreateAnigenic磁铁矿。我们认为铁还原细菌在埋葬和孔隙水中的埋葬和相互作用期间促进了煤轴承单元内的细粒磁铁矿的生产。煤/泥炭在由腐殖酸介导的埋葬期间作为电子供体的来源，以供应围绕硅质沉积物的冰冷菌。这些结果表明，在随后的深层生物圈内，煤轴承沉积物可能在铁循环中发挥重要作用。

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