利用碳酸盐岩的Fe/Mn比值恢复海洋的氧化还原状态

碳酸盐岩的Fe/Mn元素比值，作为一项新的地球化学指标，可以用于恢复海洋的氧化还原状态.在氧化条件下，Fe³⁺和Mn⁴⁺均不可溶，因此氧化海水中的溶解Fe和Mn的含量均很低.Fe³⁺和Mn⁴⁺在还原条件下可以被细菌还原为可溶的Fe²⁺和Mn²⁺，而氧化还原电位的计算表明，Mn⁴⁺的还原要早于Fe³⁺的还原，因此细菌的Mn还原过程发生在沉积物的更浅层.可溶的Fe²⁺和Mn²⁺向上扩散到海水中，替代碳酸盐岩晶格里的Ca²⁺，因此碳酸盐岩晶格中的Fe²⁺和Mn²⁺的含量受控于来自沉积物孔隙水的扩散，而后者又与水岩界面的氧化还原状态相关.因此可以预测，随着海水变得逐渐缺氧，碳酸盐岩中的Fe/Mn比值会逐渐增高.为了验证这一假说，我们分析了中元古代高于庄组白云岩的Fe/Mn比值.研究发现，几乎所有的样品的Fe/Mn比值介于20～30之间，显著高于泥盆纪末期深水碳酸盐岩和浅水台地碳酸盐岩的Fe/Mn比值.高于庄组碳酸盐岩高的Fe/Mn比值一方面可能指示了中元古代低的大气氧气浓度和海洋的广泛缺氧，也可能反映了白云岩形成于缺氧的沉积物空隙水里.

REDOX ENVIRONMENT RESTORATION OF OCEAN WITH THE Fe/Mn RATIO OF CARBONATE ROCKS

As a new geochemical index, the Fe/Mn ratio of carbonate rocks can be used to restore the redox state of ocean. Both Fe³⁺ and Mn⁴⁺ are insoluble under the oxidation condition, so the dissolved Fe and Mn contents in oxidized seawater are very low. Fe³⁺ and Mn⁴⁺ can be reduced to soluble Fe²⁺ and Mn²⁺ by bacteria under the reduction condition, and the calculation of redox potential shows that the reduction of Mn⁴⁺ is earlier than that of Fe³⁺, thus the bacteria-induced Mn reduction process occurs in the shallower layer of sediments. The soluble Fe²⁺ and Mn²⁺ diffuse upward into seawater to replace the Ca²⁺ in lattice of carbonate rocks. The contents of Fe²⁺ and Mn²⁺ are controlled by the diffusion of porewater in sediments, while the latter is related to the redox state of water-rock interface. Therefore, it can be predicted that the Fe/Mn ratio in carbonate rocks would gradually increase as seawater becomes less oxygenated. The Fe/Mn ratio of dolomites in the Mesoproterozoic Gaoyuzhuang Formation is analyzed to test the hypothesis. It is found that the Fe/Mn ratio of almost all samples is 20-30, significantly higher than that of carbonate rocks in Late Devonian deep water and shallow water platform, which may indicate the low atmospheric oxygen concentration and wide oxygen depletion in ocean in Mesoproterozoic, and the dolomites were formed in the anoxic sediment porewater.