Marine Redox Conditions in the Early Cambrian Ocean: Insights from the Lower Cambrian Phosphorite De

来源 :Journal of Earth Science | 被引量 : 0次 | 上传用户:ccwawa
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It is generally considered that a significant change in oceanic redox conditions occurred during the Ediacaran–Cambrian transition. However, there are currently two major conflicting views on the degree of oxygenation of deep water(oxic vs. ferruginous) during this interval. To date, the oxygenation conditions of the Early Cambrian ocean have not been well constrained. The oxygenation magnitude and mechanism of the Early Cambrian ocean could be critical to the significant biological evolution of the “Cambrian Explosion”. To constrain the Early Cambrian oceanic redox environment, we conducted an integrated study on iron and sulfur isotopes and redox-sensitive elements(Mo, U, and V) of Lower Cambrian phosphorite deposits from two shallow sections(Meishucun and Gezhongwu) and a deeper water section(Zunyi) from the Yangtze Platform, South China. The near zero δ~(56)Fe values from the two shallow sections studied here reflect oxic conditions in the lower phosphorite deposition. An obvious positive shift in δ~(56)Fe and redox-sensitive element content was observed in the middle parts of the two shallow water sections, which might reflect loss of light iron by dissimilatory iron reduction during early diagenesis under suboxic shallow water in the platform. However, the highly positive δ~(56)Fe values in the deep section could reflect a lower oxidation degree of dissolved Fe(II) under anoxic deep water. The data suggest redox-stratified oceanic conditions during the Early Cambrian, in which completely oxygenated shallow water(platform) coexisted with anoxic deep water(slope). We propose that prolonged upwelling of dissolved organic carbon(DOC)-, Fe(II)- and phosphorus-rich anoxic deep water in a redox-stratified ocean could have increased exchange with the open ocean, resulting in major phosphorite deposition in oxic-suboxic conditions. The progressive oxygenation of the ocean may have facilitated the Early Cambrian biotic diversification. It is generally considered that a significant change in oceanic redox conditions occurred during the Ediacaran-Cambrian transition. However, there are currently two major conflicting views on the degree of oxygenation of deep water (oxic vs. ferruginous) during this interval. To date, the oxygenation conditions of the Early Cambrian ocean have not been well constrained. The oxygenation magnitude and mechanism of the Early Cambrian ocean could be critical to the significant biological evolution of the “Cambrian Explosion.” To constrain the Early Cambrian oceanic redox environment, we conducted an integrated study on iron and sulfur isotopes and redox-sensitive elements (Mo, U, and V) of Lower Cambrian phosphorite deposits from two shallow sections (Meishucun and Gezhongwu) and a deeper water section (Zunyi) from the Yangtze Platform, South China. The near zero δ ~ (56) Fe values ​​from the two shallow sections called here reflect oxic conditions in the lower phosphorite deposition. An obvi ous positive shift in δ ~ (56) Fe and redox-sensitive element content was observed in the middle parts of the two shallow water sections, which might reflect loss of light iron by dissimilatory iron reduction during early diagenesis under suboxic shallow water in the platform However, the highly positive δ ~ (56) Fe values ​​in the deep section could reflect a lower oxidation degree of dissolved Fe (II) under anoxic deep water. The data suggest redox-stratified oceanic conditions during the Early Cambrian, in which completely Oxyated shallow water (platform) coexisted with anoxic deep water (slope). We propose that prolonged upwelling of dissolved organic carbon (DOC) -, Fe (II) - and phosphorus-rich anoxic deep water in a redox-stratified ocean could have increased exchange with the open ocean, resulting in major phosphorite deposition in oxic-suboxic conditions. The progressive oxygenation of the ocean may have facilitated the Early Cambrian biotic diversification.
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