LIP volcanism (not anoxia) tracked by Cr isotopes during Ocean Anoxic Event 2 in the proto-North Atlantic region
LIP volcanism (not anoxia) tracked by Cr isotopes during Ocean Anoxic Event 2 in the proto-North Atlantic region
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DOI：
10.1016/j.gca.2022.06.016
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发表时间：
2022-06
影响因子：
5
通讯作者：
L. Nana Yobo;C. Holmden;A. Brandon;K. Lau;J. Eldrett;S. Bergman
中科院分区：
文献类型：
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作者：
L. Nana Yobo;C. Holmden;A. Brandon;K. Lau;J. Eldrett;S. Bergman
铬是一种氧化还原敏感元素，由于Cr（VI）-Cr（III）转化，其在地球表面环境中表现出大范围的同位素组成。铬的这种性质已被利用作为海洋沉积物地球氧化历史的示踪剂。然而，使用Cr的古氧化还原应用由于其复杂的循环而难以实施，这造成海水δ 53 Cr值的空间变异性。铬的主要输入，如海底火山活动，以掩盖氧化还原过程的潜在变化，进一步阻碍了应用。在中白垩纪海洋缺氧事件2（OAE 2）期间，沉积物δ 53 Cr值的负偏移的两个先前的报告证明了这些并发症。在海洋沉积物中观察到的负位移与预期的正位移相冲突，这是由于缺氧沉积汇的扩张引起的同位素轻铬（III）的下降增加。在这项研究中，一个海洋碳酸盐岩序列的芯从鹰福特组在美国得克萨斯州，在西部内陆海道的南部，描绘了负1.5‰ δ 53 Cr偏移发生在两个步骤，与第二个步骤达到峰值最低值与同位素未分馏的火成岩源无法区分。与已发表的δ 53 Cr记录相反，每个步骤在地层学上都与火山活动增加的喷发频率和/或强度的代用证据相匹配，这些证据使用了先前从同一岩心测量的187 Os/188 Os、87 Sr/86 Sr和Os浓度代用指标，支持了火山来源的Cr向海洋的更高输入作为负Cr同位素偏移的驱动因素。
Chromium is a redox sensitive element that exhibits a large range of isotopic compositions in Earth’s surface environments because of Cr(VI)-Cr(III) transformations. This property of Cr has been exploited as a tracer of Earth’s oxygenation history using marine sediments. However, paleoredox applications using Cr are difficult to implement due to its complicated cycling, which creates spatial variability in seawater δ53Cr values. Applications are further hindered by the potential for variability in the major inputs of Cr, such as submarine volcanism, to mask redox processes. Two previous reports of negative excursions in sedimentary δ53Cr values during the middle Cretaceous Ocean Anoxic Event 2 (OAE 2) demonstrate these complications. Observed negative shifts in marine sediments conflict with the positive shifts expected in response to the increased drawdown of isotopically light Cr(III) prompted by the expansion of anoxic depositional sinks. In this study, a marine carbonate succession cored from the Eagle Ford Formation in Texas, USA, in the southern part of the Western Interior Seaway, depicts the negative 1.5‰ δ53Cr excursion occurring in two steps, with the second step reaching peak minimum values indistinguishable from isotopically unfractionated igneous sources. In contrast to published δ53Cr records, each step stratigraphically matches proxy evidence for increased eruption frequency and/or intensity of volcanic activity using combined187Os/188Os,87Sr/86Sr and Os concentration proxies previously measured from the same core, supporting higher inputs of volcanically sourced Cr to the oceans as the driver for the negative Cr isotope excursion.