Whole rock and pyrite geochemistry of the low-grade metamorphic sediments trace the redox conditions in the ocean during the Tonian

The Tonian Period captures critical co-evolutionary transitions between marine redox conditions and early life, but the sediments that have undergone low-grade metamorphism complicate investigations into contemporaneous marine redox conditions. In this study, comprehensive analysis of whole rock and pyrite chemistry in slates of the Tongtawan Formation (∼800 Ma) is conducted to reconstruct oceanic redox conditions and evaluate diagenetic/metamorphic impacts on these sediments. The size of framboidal pyrites mostly ranges from 3.33 to 7.67 μm, indicating a predominantly ferruginous deep ocean, with minor occurrences of euxinic or dysoxic conditions. However, depressed ratios of highly reactive iron to total iron (i.e., Fe_HR/Fe_T mostly <0.38) in whole rock seem to indicate oxic conditions. This is attributed to the consumption of highly reactive iron due to chloritization during diagenesis/metamorphism. Pyrite morphology and diagnostic trace element ratios confirm its syngenetic to early-diagenetic origins, validating pyrite chemistry as a robust proxy for contemporaneous seawater composition. In situ LA-ICP-MS analyses document pronounced enrichments of elements (e.g., Co, Ni, Mo, and Mn) in pyrites, exceeding typical Tonian and Mesoproterozoic records. These enrichments reflect enhanced inventory in the ocean due to atmospheric and oceanic oxygenation—with Mo exhibiting the most diagnostic response—and additional fluxes from the erosion of mafic/ultramafic rocks and from hydrothermal inputs linked to the mantle plume and Rodinia breakup (e.g., Co, Ni, and Mn). This indicates Tonian oceans possessed a significantly expanded element reservoir during the Tonian, displaying enhanced oxidation levels despite largely ferruginous deep waters against a backdrop of increasing atmospheric oxygen.