Manganese enrichment in the Gowganda Formation of the Huronian Supergroup: A highly oxidizing shallow-marine environment after the last Huronian glaciation

摘要

Oxidative precipitation and authigenic enrichment of the redox sensitive element Mn in sedimentary rocks can serve as a proxy for the release of high levels of O_2 during the Great Oxidization Event (GOE). Here we investigate Mn abundance in sedimentary rocks of the 2.45-2.22Ga Huronian Supergroup, Canada. We found authigenic Mn enrichments with high Mn/Fe ratios following the appearance of Fe oxides in the Firstbrook Member of the Gowganda Formation of the Huronian Supergroup, which was deposited immediately after the last Huronian glaciation. The Mn-bearing minerals in the Firstbrook Member are spessartine-rich almandine and Mn-bearing chlorite, which are likely to have been formed through post-depositional diagenesis and/or metamorphism using Mn oxides precipitated in the ocean at the time of deposition. When assuming the solution equilibrium between the atmosphere and shallow oceans, oxidative Mn precipitation requires that atmospheric O2 be higher than ~10-2 times the present atmospheric level (PAL). The cumulative Mn amount per unit area in the Firstbrook Member is comparable in magnitude to that in the Mn deposits in the Hotazel Formation of the Transvaal Supergroup, South Africa. Our results suggest an appearance of highly active aerobic biosphere immediately after the last Huronian glaciation, supporting the hypothesis that climatic recovery from the Huronian glaciation accelerated the GOE.