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首页> 外文期刊>Journal of Palaeogeography >Trace and rare earth elemental geochemistry of carbonate succession in the Middle Gaoyuzhuang Formation, Pingquan Section: Implications for Early Mesoproterozoic ocean redox conditions
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Trace and rare earth elemental geochemistry of carbonate succession in the Middle Gaoyuzhuang Formation, Pingquan Section: Implications for Early Mesoproterozoic ocean redox conditions

机译：平泉段高玉庄组中段碳酸盐岩演替的痕迹和稀土元素地球化学：对早中元古代海洋氧化还原条件的影响

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Abstract The concentrations of redox-sensitive trace elements, such as uranium (U), vanadium (V), molybdenum (Mo), cobalt (Co), chromium (Cr) and rare earth elements (REE + Y) were determined in a given carbonate succession in the Gaoyuzhuang Formation (~1.56 Ga), which spans depths from outer shelf to intertidal, to explore the Early Mesoproterozoic ocean redox conditions. The values of the Zr-normalized redox-sensitive trace element concentrations and some relevant ratios show obvious changes from bottom to top in the succession. Samples from the outer shelf setting (M1 interval) demonstrate significantly enhanced values in Zr-normalized redox-sensitive trace element concentrations and relevant ratios (the peaks of Mo/U, V/Cr and Ni/Co ratios larger than 8, 4.25, and 7, respectively). Authigenic framboidal pyrites were also found within oncolite-like carbonate concretions and surrounding host rocks in this interval. These all indicate a euxinic state in the outer shelf environment. Less enrichment of Zr-normalized redox-sensitive elemental abundances and a mild decrease in the values of geochemical ratios were present in the inner shelf environment (M2 interval) (the V/Cr and Ni/Co ratios fall into a range of 2.5-4.25 and 4-5, respectively), suggesting dysoxic conditions dominant in the inner shelf setting. Samples from the shallower subtidal and intertidal settings (M3 and {M4} intervals) are mostly invariable with much lower values of Zr-normalized redox-sensitive elements and relevant ratios, with the V/Cr and Ni/Co ratios typically near or less than 2 and 5 respectively, indicative of oxic conditions in the high-energy subtidal/intertidal zones. A remarkable negative Ce anomaly exhibited in the shale-normalized {REE} + Y diagram in the {M4} interval may provide evidence in support of the hypothesis. Taken together, our results suggest a relatively shallow chemocline in the Early Mesoproterozoic ocean: the transitions between euxinic, dysoxic and oxic may occur in quiet-water outer shelf and highenergy subtidal zone, respectively. The presence of euxinic ocean bottom waters is compatible with low concentrations of seawater sulfate and reduced levels of atmospheric oxygen during this period. The extreme environmental conditions induced by these anoxic oceans could have been responsible for the delayed oxygenation of the biosphere and hindered the evolution of multicellular life.
机译：摘要在给定的条件下测定了对铀，钒，钼，钴，铬和铬等氧化还原敏感的痕量元素的浓度。高油庄组（〜1.56 Ga）的碳酸盐岩演替活动范围从外陆架到潮间带，跨越中元古代生代的海洋氧化还原条件。 Zr归一化氧化还原敏感的痕量元素浓度的值和一些相关的比值显示从下到上依次变化。外部架子设置（M1间隔）的样品显示Zr归一化氧化还原敏感的痕量元素浓度和相关比率（Mo / U，V / Cr和Ni / Co比率的峰值大于8、4.25和7）。在此区间内，在类似陨石的碳酸盐凝结物中和周围的宿主岩石中也发现了自生的黄铁矿黄铁矿。这些都表明在外部架子环境中处于一种富余状态。在内部层架环境（M2区间）中，Zr归一化的氧化还原敏感元素丰度富集程度降低，地球化学比值出现轻微下降（V / Cr和Ni / Co比值落在2.5-4.25之间）和分别为4-5），这表明内架环境中的低氧环境占主导地位。潮间带和潮间带较浅的区域（M3和{M4}间隔）的样品大部分不变，而Zr归一化氧化还原敏感元素的值和相关比率要低得多，V / Cr和Ni / Co比率通常接近或小于图2和图5分别表示高能潮下/潮间带的有氧条件。在{M4}区间的页岩标准化{REE} + Y图中表现出的显着负Ce异常可能为支持该假设提供了证据。两者合计，我们的研究结果表明，中元古生代海洋中的趋化层较浅：在安宁，低氧和有氧之间的过渡可能分别发生在静水外层架和高能潮下带。在此期间，存在富余的海洋底水与低浓度的海水硫酸盐和减少的大气氧含量相兼容。这些缺氧海洋引起的极端环境条件可能是造成生物圈氧化延迟的原因，并阻碍了多细胞生命的发展。

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