Cerium stable isotope ratios in ferromanganese deposits and their potential as a paleo-redox proxy
Cerium stable isotope ratios in ferromanganese deposits and their potential as a paleo-redox proxy
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DOI：
10.1016/j.gca.2016.02.025
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发表时间：
2016-05
影响因子：
5
通讯作者：
R. Nakada;Y. Takahashi;M. Tanimizu
中科院分区：
文献类型：
--
作者：
R. Nakada;Y. Takahashi;M. Tanimizu
稀土元素（REE）模式中的铈（Ce）异常已被用来估计古海洋环境的氧化还原状态。铈是独特的，因为它在氧化条件下形成四价阳离子，与其他以三价状态存在的稀土元素相反。这种特性导致相对于邻近的稀土元素，铈的浓度过高或过低。然而，使用Ce异常作为古氧化还原代理没有很好地校准。研究表明，Ce异常与Ce稳定同位素比值（δ142Ce）的耦合是区分氧化还原条件和亚氧氧化还原条件的更定量指标。我们的研究结果表明，在连续形成的铁（Fe）和锰（Mn）沉淀从温泉水的稀土元素模式没有任何相关的变化，重铈同位素的逐步富集。Mn析出相的δ 142 Ce值比Fe析出相的δ 142 Ce值重约0.35‰，这与实验预测相一致。3种不同形成过程的海相铁锰矿床δ 142 Ce值为：水成（+0.25‰）>成岩（+0.10‰）>热液（+0.05‰），也反映了其形成环境的氧化还原条件。这些观察结果表明，铈稳定同位素比值产生更多的定量信息氧化还原状态比稀土模式单独。因此，我们建议，这种新的代理可以成功地利用重建海洋氧化还原状态，特别是从轻微的氧化高氧条件下，如大氧化事件（GOE）。
The cerium (Ce) anomaly observed in rare earth element (REE) patterns has been used to estimate the redox state of paleo-marine environments. Cerium is unique because it forms tetravalent cations under oxic conditions, in contrast to the other REEs that occur in a trivalent state. This characteristic leads to anomalously high or low Ce concentrations relative to neighboring REEs. However, the use of Ce anomaly as a paleo-redox proxy is not well calibrated. This study shows that coupling of the Ce anomaly and Ce stable isotope ratio (δ142Ce) is more quantitative redox proxy to distinguish suboxic and oxic redox conditions. Our results revealed a progressive enrichment in heavy Ce isotopes in consecutive formations of iron (Fe) and manganese (Mn) precipitate from hot spring water without any associated change in REE patterns. The δ142Ce values of Mn precipitates were approximately 0.35‰ heavier than those of the Fe precipitates, which was consistent with experiment-based predictions. The δ142Ce values of marine ferromanganese deposits with three different formation processes were hydrogenetic (+0.25‰) > diagenetic (+0.10‰) ⩾ hydrothermal (+0.05‰), which also reflects redox conditions of their formation environment. These observations suggest that the Ce stable isotope ratios yield more quantitative information regarding redox state than REE patterns alone. We thus suggest that this novel proxy can be successfully utilized to reconstruct marine redox states, particularly from slightly oxic to highly oxic conditions such as the Great Oxidation Event (GOE).