Dynamic redox conditions in Mesoproterozoic shallow seawater: Constraints from carbonate fabrics and geochemistry

Longfei Sun, Xiaoying Shi, Xiqiang Zhou, Limin Zhou, Kangjun Huang, Lei Xu, Baozeng Xie, Xinqiang Wang, Dongjie Tang
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Abstract

The delayed eukaryote evolution during the mid-Proterozoic may have been linked to prolonged low oxygen levels during this period. Two short oxygenation events have recently been identified at ca. 1.57 Ga and ca. 1.4 Ga, but the redox conditions during the intervening interval remain poorly constrained. In this study, we conducted mineralogical and geochemical investigations on three sections of the Wumishan Formation (ca. 1.52−1.47 Ga) that were deposited between the two oxygenation events in the Yanliao basin. The results revealed two distinct changes in carbonate mineralogy and precipitation style associated with concurrent changes in carbonate I/(Ca + Mg): a transition from intervals with positive I/(Ca + Mg) anomalies and abundant water-column carbonate mud to intervals with no I/(Ca + Mg) anomalies and abundant seafloor precipitates, suggesting a shift from suboxic to anoxic conditions in shallow seawater. In both of the suboxic intervals, I/(Ca + Mg) values reached 0.5−2.6 μmol/mol, with negative Ce anomalies (Ce/Ce*(SN) = 0.69; data from literature) occurring in the later interval (ca. 1.48 Ga). The results suggest significant redox fluctuations in the shallow seawater during the Mesoproterozoic. However, these pulsed oxygenation episodes may not have been sufficient to sustain the continuous evolution of early eukaryotes.
中新生代浅层海水的动态氧化还原条件:碳酸盐结构和地球化学的制约因素
原生代中期真核生物进化的延迟可能与这一时期长期的低氧水平有关。最近在约 1.57 Ga 和约 1.4 Ga 发现了两次短暂的含氧事件。1.57Ga和约1.4Ga,但其间的氧化还原条件仍未得到很好的解释。在本研究中,我们对燕辽盆地两次含氧事件之间沉积的乌蒙山地层(约 1.52-1.47 Ga)的三个剖面进行了矿物学和地球化学研究。结果表明,碳酸盐矿物学和沉淀方式的两种明显变化与碳酸盐I/(Ca + Mg)的同时变化有关:从I/(Ca + Mg)异常为正值、水柱碳酸盐泥丰富的区段过渡到I/(Ca + Mg)异常为负值、海底沉淀物丰富的区段,这表明浅海海水从亚缺氧条件转向缺氧条件。在两个亚缺氧区间,I/(Ca + Mg)值都达到了 0.5-2.6 μmol/mol,在较晚的区间(约 1.48 Ga)出现了负的 Ce 异常(Ce/Ce*(SN) = 0.69;数据来自文献)。这些结果表明,在中新生代,浅海海水发生了明显的氧化还原波动。然而,这些脉冲氧合事件可能不足以维持早期真核生物的持续演化。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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