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REDOX ENVIRONMENT RESTORATION OF OCEAN WITH THE Fe/Mn RATIO OF CARBONATE ROCKS |
CAI Qing-qing1, YAN Bao-qiang2, WANG Yong-zhi3 |
1. Liaoning Experimental High School, Shenyang 110031, China; 2. Liaoning Institute of Geological Survey, Shenyang 110080, China; 3. Jilin University, Changchun 130000, China |
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Abstract 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.
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Received: 16 January 2019
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