Dynamic evolution of the Ediacaran ocean across the Doushantuo Formation, South China

Abstract

Oceanic redox conditions and atmospheric oxygenation likely played a crucial role in the evolution of macroscopic multicellular eukaryotes during the early Ediacaran period. However, the oxidation mechanism and magnitude of the Ediacaran ocean–atmosphere system remain controversial. To constrain the oceanic redox conditions and the contemporaneous oxygenation of the atmosphere, we present a comprehensive investigation of redox-sensitive elements (e.g., Mo and U), Fe speciation and S isotopes of pyrite from the platform, slope and basin sections across the Doushantuo Formation in South China. Our results suggest a redox-stratified Ediacaran ocean with a fluctuating chemocline from the slope to platform location across the Doushantuo Formation. In particular, euxinic/intermittently euxinic conditions developed not only at the platform and slope but also in the deep basin. Furthermore, these euxinic conditions indicate that high sulfate concentrations may have accumulated not only at the ending of the Ediacaran Doushantuo Formation but also at the middle and beginning. Thus, these results suggest that the extensive euxinic conditions associated with the continuous oceanic sulfate input were in response to progressive oxygenation of the atmosphere during the early Ediacaran period. Integrated with previously published results, if the dissolved organic carbon (DOC) reservoir existed, different mechanisms may be responsible for the oxidation of deep ocean in each part of the studied sections across the Doushantuo Formation. One mechanism is oxidation by sulfate through a bacterial sulfate reduction (BSR) process under anoxic conditions. Another mechanism is oxidation by dissolved free oxygen under oxic/suboxic conditions. Finally, a dynamic evolution model of the Ediacaran ocean–atmosphere system across the Doushantuo Formation, South China, was suggested.