Global Hg cycle over Ediacaran–Cambrian transition and its implications for environmental and biological evolution

Abstract

The Ediacaran–Cambrian (E–C) transition witnessed remarkable environmental changes, the extinction of the Ediacaran biota, and subsequent rapid diversification of Cambrian animals. However, the linkages between environmental and biological evolution are still under debate at this critical time. Here, we present new Hg abundance and Hg isotopes in sediments from South China, which are then combined with those published from the Indian craton to explore the co-evolution of environment and complex life during the E–C transition. In both areas, high Hg/TOC ratios and near-zero Δ199Hg of the Late Ediacaran sediments suggest enhanced volcanism, whereas relatively high Hg/TOC ratios and positive Δ199Hg shift upsection indicate volcanic-sourced atmospheric Hg(II) deposition in the earliest Cambrian. The dramatically decreasing Hg/TOC ratios and positive Δ199Hg of early Cambrian Age 2 to 3 sediments indicate scavenging of dissolved seawater Hg(II) by organic matter particulates. Our Hg results suggest volcanism may have likely played a significant role in the extinction of Ediacaran biota and global negative carbon excursions near the E–C boundary. Furthermore, our Hg data provides new evidence of extensive OM burial in the early Cambrian Age 2 to 3 oceans, leading to a rapid increase of Earth-surface O2 levels that coincided with appearance of more complex large-body animals. Our study provides new insights of the global Hg cycle into the co-evolution of the environment and complex life at this critical time.