1. Highly fractionated Hg isotope evidence for dynamic euxinia in shallow waters of the Mesoproterozoic ocean.
- Author
-
Wu, Yaowen, Tian, Hui, Yin, Runsheng, Chen, Di, Grasby, Stephen E., Shen, Jun, Li, Tengfei, Ji, Sui, and Peng, Ping'an
- Subjects
- *
WATER depth , *EUPHOTIC zone , *SEAWATER , *ANOXIC waters , *ISOTOPES , *ATMOSPHERIC nitrogen , *MERCURY - Abstract
Studies of Mesoproterozoic ocean chemistry have converged on a widely accepted scenario where a stable shallow layer of oxic waters occurred over deeper anoxic ferruginous waters, with mid-depth euxinic waters restricted to continental margins. However, several lines of evidence suggest shallow waters may not always have remained oxic, but fluctuated between oxic, anoxic, and euxinic conditions. Here, we present new Hg-C-S geochemistry data of ca. 1.5–1.4 Ga sediments in the Yanliao Basin to better understand the redox structure of shallow water environments, especially for the photic zone. In this study, positive Δ 199 Hg values suggest atmospheric Hg(II) deposition into oxic surface waters, whereas abnormally negative Δ 199 Hg values indicate enhanced sequestration of gaseous Hg(0) and photo-reduction of reduced sulfur-bound Hg(II) in response to photic zone euxinia. Our highly fractionated Δ 199 Hg values reveal a new scenario, in which the extent of euxinia is more dynamic than generally thought, and may expand from the mid-depth to the photic zone, likely linked to the superimposed operation of enhanced oxidative weathering input of SO 4 2 − and high OM flux in the water column. Given that euxinic waters may accelerate the sequestration of bio-essential elements, it is suggested that the dynamic euxinia of shallow water played a significant role in delaying the diversification of eukaryotes in the Mesoproterozoic oceans. Our study also highlights that Hg isotopes may act as a promising new proxy to trace redox changes in the surface ocean over geological time. • Negative Δ 199 Hg suggest photo-reduction of sulfur-bound Hg(II) in the photic zone. • The extent of euxinia was dynamic from the mid-depth to the photic zone. • Dynamic euxinia of shallow waters may delay the diversification of eukaryotes. • Hg isotopes may act as a new proxy to record redox changes in the surface ocean. [ABSTRACT FROM AUTHOR]
- Published
- 2023
- Full Text
- View/download PDF