Perspectives on Proterozoic surface ocean redox from iodine contents in ancient and recent carbonate
Perspectives on Proterozoic surface ocean redox from iodine contents in ancient and recent carbonate
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DOI：
10.1016/j.epsl.2017.01.032
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发表时间：
2017-04-01
影响因子：
5.3
通讯作者：
Lyons, Timothy W.
中科院分区：
文献类型：
--
作者：
Hardisty, Dalton S.;Lu, Zunli;Lyons, Timothy W.
元古界是真核生物出现和最初记录到的多样化的时代。对这些事件至关重要的浅海环境中的氧气水平低于今天的水平，尽管低了多少还存在争议。在这里，我们使用浅海相灰岩和白云岩中的碘酸盐(氧化的碘物种)的浓度来生成元古界近地表海洋氧化还原条件的第一个全面记录。碘代用品对局部氧气的可获得性和与缺氧水域的相对接近程度都很敏感。为了评估我们方法的有效性，使用了新近系-第四系碳酸盐岩来证明成岩作用通常会减少，而且不太可能增加碳酸盐碘含量。尽管有可能失去成岩作用，但元古界碳酸盐碘的最大水平相对于太古宙的碳酸盐碘水平是高的，特别是在古元古代和新元古代的洛马芒迪和舒拉姆碳同位素漂移期间。对于舒拉姆异常，与新近系-第四纪碳酸盐岩的比较表明，成岩作用不是所观察到的碘趋势的原因。元古代碳酸盐岩相对于显生界的基线低碘水平与浅层氧-缺氧界面有关。地表水中的氧浓度至少会间歇性地高于支持真核生物所需的阈值。然而，来自中元古代浅水碳酸盐岩的低碘诊断数据，相对于围栏时间段的数据，与动态化学跃层和缺氧水域的数据是一致的，这些水域将断续向上和横向混合进入浅海。这种氧化还原的不稳定性可能对早期真核生物的多样化和扩张构成了挑战，创造了一种不利于动物出现的进化格局。(C)2017爱思唯尔B.V.保留所有权利。
The Proterozoic Eon hosted the emergence and initial recorded diversification of eukaryotes. Oxygen levels in the shallow marine settings critical to these events were lower than today's, although how much lower is debated. Here, we use concentrations of iodate (the oxidized iodine species) in shallowmarine limestones and dolostones to generate the first comprehensive record of Proterozoic near-surface marine redox conditions. The iodine proxy is sensitive to both local oxygen availability and the relative proximity to anoxic waters. To assess the validity of our approach, Neogene-Quaternary carbonates are used to demonstrate that diagenesis most often decreases and is unlikely to increase carbonate-iodine contents. Despite the potential for diagenetic loss, maximum Proterozoic carbonate iodine levels are elevated relative to those of the Archean, particularly during the Lomagundi and Shuram carbon isotope excursions of the Paleo-and Neoproterozoic, respectively. For the Shuram anomaly, comparisons to Neogene-Quaternary carbonates suggest that diagenesis is not responsible for the observed iodine trends. The baseline low iodine levels in Proterozoic carbonates, relative to the Phanerozoic, are linked to a shallow oxic-anoxic interface. Oxygen concentrations in surface waters would have at least intermittently been above the threshold required to support eukaryotes. However, the diagnostically low iodine data from mid-Proterozoic shallow-water carbonates, relative to those of the bracketing time intervals, are consistent with a dynamic chemocline and anoxic waters that would have episodically mixed upward and laterally into the shallow oceans. This redox instability may have challenged early eukaryotic diversification and expansion, creating an evolutionary landscape unfavorable for the emergence of animals. (C) 2017 Elsevier B. V. All rights reserved.