Mo–Cr isotope evidence for a reducing Archean atmosphere in 3.46–2.76 Ga black shales from the Pilbara, Western Australia
Mo–Cr isotope evidence for a reducing Archean atmosphere in 3.46–2.76 Ga black shales from the Pilbara, Western Australia
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DOI：
10.1016/j.chemgeo.2012.12.018
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发表时间：
2013-02
期刊：
影响因子：
3.9
通讯作者：
M. Wille;O. Nebel;M. V. Kranendonk;R. Schoenberg;I. Kleinhanns;M. Ellwood
中科院分区：
文献类型：
--
作者：
M. Wille;O. Nebel;M. V. Kranendonk;R. Schoenberg;I. Kleinhanns;M. Ellwood
利用海洋沉积物中氧化还原敏感元素的分布及其稳定同位素变化来追踪大氧化事件（GOE; 2.4-2.2Ga）前后大气可能的氧化作用。基于各种地球化学示踪剂的对比数据集被用来支持和反对2.4Ga之前大气中游离氧的上升，并且部分氧化环境的证据仍然难以捉摸。为了确定太古宙的大气氧含量，我们收集了四个黑色页岩层位3.46 ~ 2.76Ga的Mo和Cr的稳定同位素数据，并结合了主要元素和微量元素。各沉积单元内Mo、Cr、U元素特征主要受大陆输入的影响。Mo同位素值和Mo和U元素特征都表明缺氧条件，没有这些元素氧化还原循环的迹象。这与先前基于已发表的硫同位素数据（Ohmoto et al., 2006）和~3.45Ga氧化赤铁矿产状的解释相矛盾（Hoashi et al., 2009）。样品组内的铬浓度模式受大陆物源组成控制，而不是受自生铬海水贡献控制。这一解释得到了所有样品组中相对均匀的Cr同位素组成的支持，并且与高温控制的Cr同位素比率一致。由于Cr的自生/地壳富集因子要小得多，因此与Mo同位素相比，用Cr同位素识别碎屑占主导地位的沉积物中的氧化还原循环过程可能不那么敏感。
The distribution of redox-sensitive elements and their stable isotope variations in marine sediments has been employed to track the possible oxygenation of the atmosphere at, and before, the Great Oxidation Event (GOE; 2.4–2.2Ga). Contrasting datasets have been used to advocate for and against the rise of free oxygen in the atmosphere prior to 2.4Ga based on various geochemical tracers, and evidence for a partially oxidized environment remains elusive. Herein, we present stable isotope datasets for Mo and Cr, in conjunction with major and trace elements from four black shale horizons spanning a time interval from 3.46 to 2.76Ga in order to ascertain atmospheric oxygen levels through the Archean. The Mo, Cr and U elemental signatures within all sedimentary units are dominated by continental input. Both Mo isotopic values and Mo and U elemental signatures suggest anoxic conditions with no indication of redox cycling of these elements. This contradicts previous interpretations of an oxidized atmosphere based on published sulfur isotopic data (Ohmoto et al., 2006) and oxidized hematite occurrences at ~3.45Ga (Hoashi et al., 2009). Cr concentration patterns within the sample suites are controlled by continental provenance composition rather than by an authigenic Cr seawater contribution. This interpretation is supported by the relatively homogeneous Cr isotopic compositions across all sample suites and is consistent with high temperature controlled Cr isotopic ratios. Identification of redox cycling processes within detrital dominated sediments with Cr isotopes is likely to be less sensitive compared to Mo isotopes, due to the much smaller authigenic/crustal enrichment factors for Cr.