Mercury anomalies across the Ediacaran-Cambrian boundary: Evidence for a causal link between continental erosion and biological evolution
Mercury anomalies across the Ediacaran-Cambrian boundary: Evidence for a causal link between continental erosion and biological evolution
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埃迪卡拉纪-寒武纪边界的水星异常：大陆侵蚀与生物进化之间因果关系的证据
DOI：
10.1016/j.gca.2021.04.011
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发表时间：
2021
影响因子：
5
通讯作者：
Wang Wei
中科院分区：
文献类型：
--
作者：
Liu Ze-Rui Ray;Zhou Mei-Fu;Wang Wei
海洋氧化还原状态的显著扰动被认为是埃迪卡拉动物群向寒武纪动物群进化的主要原因之一。然而，这些跨越埃迪卡拉纪-寒武纪（E-C）边界的生态系统变化的驱动力仍然是一个有争议的问题。本文报道了印度小喜马拉雅地区Mussoorie剖面E-C沉积物中汞含量和汞同位素的地层变化。晚埃迪卡拉纪Krol D和E段白云岩的Hg/Al比值从基底向上逐渐增大，Δ199Hg值逐渐减小，这很可能是由于陆源汞长期大量输入到E - c海洋所致。下Tal群上覆的硅质磷质岩相对于Mo具有较高的Re富集水平，表明早寒武纪陆架的亚缺氧沉积环境与埃迪卡拉生物群的消失是一致的。沉积有机质氧化风化过程中大气对o2的消耗和浅水富营养化可能是造成这种o2缺乏的原因，其证据包括晚ediacaran白云岩的δ202Hg和δ13C耦合值以及早寒武世磷质岩的Cd/TOC和Zn/TOC比值较高。上覆的黑色页岩Hg和微量元素（Cd和Zn）含量降低，Δ199Hg值升高，表明在寒武纪多样化主要阶段之前的寒武纪第二阶段，陆地通量和海水养分显著下降。因此，我们认为大陆侵蚀的增强和普遍的上升流导致了大陆架浅海氧化还原状态和营养物质含量的重大变化，从而促进了E-C边界的生物进化。
Significant perturbations in ocean redox states have been thought to be one of the major causes for the biological evolution from the Ediacara biota to the Cambrian fauna. However, the driving force of these changes in the ecosystem across the Ediacaran–Cambrian (E–C) boundary is still a matter of debate. In this study, we reported stratigraphic variations of Hg content and Hg isotopes in E–C sediments from the Mussoorie section, Lesser Himalaya, India. Dolomites from the late Ediacaran Krol D and E members have increasing Hg/Al ratios and decreasing Δ199Hg values from the base upwards, which were most likely originated from a prolonged and enormous terrestrial-Hg input into the E–C ocean. The overlying cherty phosphorites in the lowest Tal Group are highly rich in Re relative to Mo, indicative of a suboxic-anoxic depositional environment on the earliest Cambrian shelf that was coincident with the disappearance of the Ediacara biota. Such an O2-deficient condition was possibly led by a consumption of atmospheric O2during the oxidative weathering of sedimentary organic matter and shallow-water eutrophication, with evidence from the coupled δ202Hg and δ13C values in late-Ediacaran dolomites and high Cd/TOC and Zn/TOC ratios in early Cambrian phosphorites. The overlying black shales have decreasing contents of Hg and micronutrients (Cd and Zn) and increasing values of Δ199Hg, representing notable declines of terrestrial flux and seawater nutrient at the Cambrian Stage 2 before the major phase of Cambrian diversification. Therefore, we conclude that enhanced continental erosion and pervasive upwelling had led to major changes in redox state and nutrient content in shallow seawaters on shelves, which in turn had promoted the biological evolution across the E–C boundary.