Terminal Mesoproterozoic (1.1–1.0 Ga) shallow ocean oxygenation and the rise of crown-group eukaryotes

Abstract At the end of the Mesoproterozoic Era (1.1–1.0 Ga), crown-group eukaryotes including rhodophytes and chlorophytes diversified and began to dominate the marine ecosystem. It is commonly thought that the oxygenation of Earth's surface environment was the driver behind this eukaryotic evolution and ecosystem change, but there is currently little evidence for an increase in biospheric oxygenation across the Meso-Neoproterozoic transition. Here, we report mineralogical and geochemical data from the ca. 1.1 Ga Nanfen Formation, North China, to explore possible causal relationships between marine redox conditions and terminal Mesoproterozoic biotic innovation. Elevated Ba concentrations and the occurrence of authigenic barite in the Nanfen Formation indicate an increase in seawater sulfate concentrations, likely caused by enhanced oxidative weathering of the continents. The increase of carbonate I/(Ca + Mg) ratios from ~0 μmol/mol to ~15 μmol/mol, coupled with a negative shift in carbonate δ13C, indicates oxidation of iodide and dissolved organic carbon as a result of enhanced water column oxygenation on the North China Platform. These geochemical trends occur coincident with increased P/Al ratios, suggesting that enhanced P bioavailability ultimately drove more extensive oxygenation. These results, in combination with highly fractionated carbonate Cr isotope data from likely time-equivalent strata in West Africa and extensive Mn deposits in Western Australia, suggest widespread oxic shallow ocean conditions during the terminal Mesoproterozoic. All together, existing evidence suggests that shallow ocean oxygenation likely created favorable conditions for the diversification of crown-group eukaryotes at ca. 1.1 Ga.