Redox of Neoproterozoic Snowball Earth Carbonate Depositional Environments

Abstract:

Neoproterozoic Snowball Earth episodes (~717–635 Ma) are temporally associated with the expansion of complex life. However the relationship between these climate perturbations and ecosystem changes is poorly constrained, including the impact of glaciations on oxygenation. In this study, we characterize changes in carbonate redox through and surrounding the Cryogenian Snowball Earth episodes as preserved in NE Svalbard. We characterize the distribution and redox state of Fe and Mn through X-ray absorption near edge structure (XANES) spectroscopy and mapping carried out at the Stanford Synchrotron Radiation Lightsource. Carbonate geochemistry is susceptible to alteration through diagenesis and burial, so we compare Fe and Mn redox across petrographic textures and assess the extent of water-rock alteration through carbonate clumped isotope (∆₄₇) analysis. Carbonates precipitated across glacial and non-glacial climate states in facies ranging from deep basins below wave base to peritidal and terrestrial settings. Changes in mineral δ¹⁸O and ∆₄₇ values of most facies are more consistent with precipitation from diverse fluid sources than pervasive fluid-buffered alteration. The earlier, Sturtian-correlated glaciation of the Petrovbreen Formation contains syn-glacial carbonates with reduced Fe across petrographic textures of all facies examined, despite the cooccurrence of an exposure surface hosting sulfate pseudomorphs. Terrestrial and lacustrine facies of the Wilsonbreen Formation that make up the later, Marinoan-correlated glacial deposits contain minerals with oxidized Fe. The mineralizing environment of Marinoan deposits was likely suboxic, given the abundance of reduced Mn incorporated into carbonates. In non-glacial facies, associations between inferred water depth and Fe redox are largely consistent, with Fe-oxides present in shallow peritidal facies and reduced Fe-carbonates and sulfides at greater depths. These results do not show a clear transition in redox among non-glacial facies surrounding these Snowball Earth episodes, and highlight the potential heterogeneity of oxygenation within Cryogenian glacial facies.