Constraining Shallow Seawater Oxygenation for the Yangtze Platform During the Early Cambrian

Abstract

AbstractWhile atmospheric oxygenation occurred largely in two major steps, the oxygenation of Earth's oceans is assumed to be more complicated, shifting repeatedly between oxic and anoxic conditions. The exact timeline and causal drivers of these changes, which seemingly culminated in the well‐ventilated oceans we see today are not well understood. This study presents Cr isotope and rare earth element + yttrium (REE + Y) data combined with V/Cr ratios for the calcareous Yanjiahe and shaley Shuijingtuo Formations from the shelf of the Yangtze Block, South China, to improve our understanding of marine redox conditions during the early Cambrian. We measured authigenic δ53Cr values of up to 1.38‰ for the carbonates of the Yanjiahe Formation as well as Ce anomalies as low as 0.57 suggesting deposition under oxic conditions. The Shuijingtuo Formation is characterized by authigenic δ53Cr values as high as 1.27‰ at its base, but shows a strong increase in detrital content upwards, masking authigenic Cr isotope values and dissolved marine REE + Y profiles. The upper Shuijingtuo Formation is characterized by V/Cr ratios as high as 27, which indicate anoxic conditions. Hence, our data support a prevalence of oxic seawater conditions in the shallow seawater of the Yangtze Block during the Ediacaran‐Cambrian transition, which shifted repeatedly to anoxic during the early Cambrian likely as a result of sea level rise and a redox‐stratified ocean.