Redox history of the Three Gorges region during the Ediacaran and Early Cambrian as indicated by the Fe isotope
Redox history of the Three Gorges region during the Ediacaran and Early Cambrian as indicated by the Fe isotope
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DOI：
10.1016/j.gsf.2017.02.005
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发表时间：
2018
影响因子：
8.9
通讯作者：
Y. Sawaki;Miyuki Tahata;T. Komiya;T. Hirata;Jian Han;D. Shu
中科院分区：
文献类型：
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作者：
Y. Sawaki;Miyuki Tahata;T. Komiya;T. Hirata;Jian Han;D. Shu
埃迪卡拉纪-寒武纪的过渡以许多事件为特征，如大型多细胞后生动物的出现和地表环境的扰动。根据地质证据，有人提出，这种转变与大气氧水平的增加相吻合，而大气氧水平是生命进化的关键。尽管可以从沉积铁矿物的组成中推断出古代的氧化还原条件，但这种方法并不一定适用于所有岩石。在地球系统中，由于铁对氧化还原条件的敏感性，铁的循环引起了相当大的兴趣。含铁矿物的铁同位素组成揭示了古大洋铁旋回的信息。遗憾的是，现有的埃迪卡拉纪-寒武纪海洋的铁同位素数据有限，为了克服这一不足，我们在三峡地区中国南部的埃迪卡拉纪-早寒武世沉积序列中钻探了一个化石。用激光烧蚀多收集器电感耦合等离子体质谱分析了钻芯中黄铁矿颗粒的铁同位素比值(δ56/54Fe)。结果表明，δ56/54Fe的变化很大，从−1.6到1.6‰，并在许多序列中观察到正的铁同位素比值。黄铁矿中δ56/54Fe呈阳性，表明海水中的亚铁部分被氧化，表明三峡海水在埃迪卡拉纪和早寒武世是含铁的。然而，在陡山沱组4段和水井坨组底部的有机富碳黑色页岩中，聚集的黄铁矿颗粒的δ56/54Fe值接近于零，这表明在这些时期，海洋曾短暂地受到硫化物条件的支配。值得注意的是，δ56/54Fe值为负值，低于−1‰，可被解释为DIR的特征。在埃迪卡拉纪最大的碳同位素负异常期间，DIR也可能在一定程度上促进了有机质的再矿化。
The Ediacaran–Cambrian transition is characterized by numerous events such as the emergence of large multi-cellular metazoans and surface environmental disturbances. Based on geological evidence, it has been proposed that this transition coincided with the increase in the atmospheric oxygen level that was key to the evolution of life. Even though ancient redox conditions can be inferred from the composition of sedimentary iron mineral species, this method is not necessarily applicable to all rocks. In the Earth system, the cycling of iron is of considerable interest owing to its sensitivity to redox conditions. Information regarding the paleo-oceanic iron cycle is revealed in the iron isotopic composition of iron-bearing minerals. Unfortunately, only limited iron isotopic data exists for Ediacaran- to Cambrian-period oceans.To circumvent this deficiency, we drilled a fossiliferous Ediacaran to Early Cambrian sedimentary succession in the Three Gorges region, South China. We analyzed the iron isotope ratios (δ56/54Fe) of pyrite grains in the drill cores using laser ablation multi collector inductively coupled plasma mass spectrometry. The results demonstrate large variations inδ56/54Fe, from −1.6 to 1.6‰, and positive iron isotope ratios are observed in many successions. The presence of positiveδ56/54Fe in pyrite indicates that the ferrous iron in the seawater was partially oxidized, suggesting that seawater at Three Gorges was ferruginous during the Ediacaran and Early Cambrian periods. However, aggregated pyrite grains in organic carbon-rich black shales at Member 4 of the Doushantuo Formation and the base of the Shuijingtuo Formation yield near-zeroδ56/54Fe values; this suggests that the ocean was transiently dominated by sulfidic conditions during these periods. Notably negativeδ56/54Fe values, lower than −1‰, can be interpreted as a signature of DIR. The DIR also might contribute in part to the re-mineralization of organic matter during the largest negative carbon isotope anomaly in the Ediacaran.