A new paleoenvironmental index for anoxic events——Mo isotopes in black shales from Upper Yangtze marine sediments

• Published in: Science China. Earth sciences Vol. 54; no. 7; pp. 1024 - 1033
• Main Authors: Zhou, Lian, Su, Jie, Huang, JunHua, Yan, JiaXing, Xie, XiNong, Gao, Shan, Dai, MengNing, Tonger
• Format: Journal Article
• Language: English
• Published: Heidelberg SP Science China Press 01.07.2011; Springer Nature B.V
• Subjects: Bioturbation; Diagenesis; Earth and Environmental Science; Earth Sciences; Environment; Islands; Isotopes; Marine biology; Marine sediments; Ordovician; Oxygenation; Paleoecology; Permian; Research Paper; Sediments; Shales; Silurian; Trace elements; Water circulation; 同位素; 地球化学参数; 氧化还原条件; 海洋沉积物; 环境指数; 缺氧事件; 长江上游; 黑色页岩; Yangtze River; black shale; proxy for paleo-redox conditions; molybdenum isotopes; Upper Yangtze
• ISSN: 1674-7313; 1869-1897
• DOI: 10.1007/s11430-011-4188-z

This paper investigates the high-solution of Mo isotopes and uses trace-element analyses for fresh and representative black shales and siliceous shales collected from the transition between the Late Ordovician and the Early Silurian at the Wangjiawan section in Yichang and the Late Permian Dalong Formation in the Shangsi Section of Sichuan. The applicability of different geochemical parameters used as paleo-oxygenation indices are also compared. The preliminary results show that V/（V＋Ni）, Uauth （auth U）, V/Cr, Ceanom and U/Th have a scattered variation range, but most samples plot within the suboxic-anoxic fields. The suboxic-anoxic environment was dominant during the deposition and formation of the two anoxic facies. These redox indicators show little correspondence to the δ98M0 values. The U/Mo ratio can be used as a potential proxy for the paleo-redox conditions due to the possibility that Mo is enriched relative to U at different redox gradients during early diagenesis. This evidence is more significant for the euxinicity condition and corresponds to positive δ98M0 （〉1.5%o） values with low U/Mo ratios. This evidence is likely related to the depositional conditions near the boundary between anoxic and euxinic environ- ments, which are characterised by low bioturbation or water circulation. Other samples reveal a wide scatter of U/Mo ratios and δ98M0 〈1.5%0. These results are likely due to punctuated improvements in oxygenation with intense bioturbation or water circulation, which led to the redistribution of trace element.