Testing the Precambrian reverse weathering hypothesis using a 1-billion-year record of shales

With solar luminosity 30% lower than the present day, the Precambrian Earth should have been vulnerable to glaciation, yet the Proterozoic sedimentary record indicates largely ice-free conditions. It has been proposed that elevated rates of reverse weathering, i.e the precipitation of authigenic clay minerals from seawater with simultaneous release of CO2, maintained atmospheric CO2 at sufficient high to compensate for the reduced solar luminosity (Isson and Planavsky, 2018). This hypothesis has remained untested because of the difficulty in differentiating authigenic clays from detrital or burial diagenetic clays. This mode of origin distinction is not possible using bulk techniques. Here we employ a novel, scanning electron microscope based mineral mapping approach (Rafiei et al., 2020) to identify and quantify the proportion of authigenic clays in twelve Proterozoic marine shale formations from Vindhyan and Chhattisgarh basins of India, representing a 1-billion-year record of marine shales. We find that eight out of twelve formations, contain definite authigenic clays, with up to 45 wt% of the sample being comprised of authigenic illite. We conclude that the widespread occurrence and high abundance of authigenic illite provides strong empirical support for an enhanced reverse weathering throughout much of the Proterozoic.