Assessing the utility of Fe/Al and Fe-speciation to record water column redox conditions in carbonate-rich sediments
Assessing the utility of Fe/Al and Fe-speciation to record water column redox conditions in carbonate-rich sediments
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DOI：
10.1016/j.chemgeo.2014.05.031
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发表时间：
2014-08-29
期刊：
影响因子：
3.9
通讯作者：
Wood, R. A.
中科院分区：
文献类型：
--
作者：
Clarkson, M. O.;Poulton, S. W.;Wood, R. A.
基于铁丰度（Fe/Al）和铁形态的地球化学指标已被广泛应用于海洋沉积物中，以便通过地质年代揭示古沉积氧化还原条件。然而，到目前为止，这些铁的代用物只与现代和古代的细粒硅屑海洋沉积物进行了校准。这显然限制了它们的使用，特别是与富含碳酸盐的沉积物和岩石有关。为了解决这个问题，我们在这里通过三种方法探讨了铁基氧化还原代理在碳酸盐岩中的适用性。首先，我们收集了现代海洋沉积物的铁/铝数据，以研究碳酸盐含量和沉积环境对铁富集程度的影响。其次，我们扩展了这一方法，对现代和古代海洋沉积物在含氧和含氧（缺氧硫化物）水柱条件下沉积的新的和现有的铁形态数据进行了汇编。最后，我们展示了来自石灰岩和白云岩配对样品集的新数据，以证明深埋白云化对铁物种形成氧化还原代理的潜在意义。在氧化条件下沉积的现代海洋沉积物中，铁/铝与碳酸盐含量没有关系。这些沉积物的平均Fe/Al比值为0.55 +/- 0.11，较高的比值可能归因于早期成岩作用下铁向沉积物-水界面的稳态再活化。相反，显著的铁/铝富集是水柱铁矿物形成和沉积的结果，要么是在缺氧条件下，要么是由于缺氧热液流体输入到缺氧海水中。铁形态数据也显示与碳酸盐含量没有直接相关性，而是根据总铁（Fe- t）和有机碳含量区分出三种类型。无论碳酸盐含量如何，在含氧水柱条件下沉积的沉积物，铁- t >0.5 wt.%，通常低于Fe-HR/Fe-T硅塑性参考阈值（0.38），以区分缺氧环境。同样与硅屑校准一致的是，缺氧水柱条件下沉积的富含碳酸盐的沉积物往往具有Fe-HR/Fe-T比值>0.38。相比之下，富氧碳酸盐沉积物具有低铁- t (
Geochemical proxies based on Fe abundance (Fe/Al) and Fe-speciation have been widely applied to marine sediments in order to unravel paleo-depositional redox conditions though geological time. To date, however, these Fe proxies have only been calibrated in relation to modern and ancient fine-grained siliciclastic marine sediments. This clearly limits their use, particularly in relation to carbonate-rich sediments and rocks. To address this, we here explore the applicability of Fe-based redox proxies in carbonates through three approaches. First, we have compiled Fe/Al data for modern marine sediments to investigate variability in Fe-enrichments as a function of carbonate content and depositional setting. Second, we have expanded this approach with a compilation of new and existing Fe-speciation data for modern and ancient marine sediments deposited under oxic and euxinic (anoxic sulfidic) water column conditions. Finally, we show new data from paired limestone and dolomite sample sets to demonstrate the potential significance of deep burial dolomitization on the Fe-speciation redox proxy.Modern marine sediments deposited under oxic conditions show no relationship between Fe/Al and carbonate content. These sediments have an average Fe/Al ratio of 0.55 +/- 0.11, with some higher values potentially being attributable to steady-state early diagenetic remobilization of Fe towards the sediment-water interface. In contrast, significant Fe/Al enrichments occur as a consequence of water column Fe mineral formation and deposition, either under anoxic conditions, or due to input of anoxic hydrothermal fluids into oxic seawater. Iron speciation data also show no direct correlation with carbonate content, and instead three groups can be distinguished based on total Fe (Fe-T) and organic C contents. Sediments deposited under oxic water column conditions, with Fe-T >0.5 wt.%, generally plot below the lower Fe-HR/Fe-T siliciclastic reference threshold (0.38) for distinguishing anoxic environments, regardless of carbonate content. Also consistent with siliciclastic calibrations, carbonate-rich sediments deposited under anoxic water column conditions tend to have Fe-HR/Fe-T ratios >0.38. In contrast, oxic carbonate-rich sediments with low Fe-T (