Chromium isotopes track redox fluctuations in Proterozoic successions of the Chapada Diamantina, São Francisco craton, Brazil

Abstract

The Chapada Diamantina region in the São Francisco craton of eastern Brazil is composed of sedimentary successions containing both Mesoproterozoic and Neoproterozoic carbonate levels, making it a key natural laboratory for understanding the fluctuations of Earth’s biogeochemical cycles during its middle age. The ca. 1.4–1.2 Ga Caboclo Formation stromatolites yielded unfractionated δ53Crauth (authigenic) (~–0.54‰ to +0.08‰). Ediacaran cap carbonates and phosphatic stromatolites of the Salitre Formation, on the other hand, yielded fractionated δ53Crauth reaching as high as +0.51‰, suggesting the input of 53Cr-rich Cr(VI), first delivered through meltwater-induced post–snowball Earth fluctuating redox conditions and then through weathering and mobilization under a fully oxygenated environment. The acquired data set highlights the very distinct redox conditions throughout the Proterozoic and reinforces the suggestion that after the Cryogenian global glaciations, Earth’s atmosphere and hydrosphere became progressively oxygenated during the Ediacaran-Cambrian transition.