Mercury anomalies across the Ediacaran–Cambrian boundary: Evidence for a causal link between continental erosion and biological evolution

Abstract

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 O2 during 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.