Sedimentary manganese carbonate deposits as faithful proxies of ancient ocean redox fluctuations: Insights from the Permian Zunyi Mn deposits, South China

Abstract Sedimentary manganese (Mn) carbonate deposits, i.e., the land-based major Mn mineral resources on Earth, are of great scientific value for palaeoceanographic research because Mn is a redox-sensitive metal whose deposition is restricted by oceanic redox conditions. The genesis of Mn carbonates remains controversial and the palaeoceanographic implication behind sedimentary Mn carbonate deposits is unclear. Here, we investigate the precipitation mechanisms, spatiotemporal redox, and hydrographic dynamics of the Middle Permian Zunyi Mn carbonate deposits in South China. The preserved original Fe-Mn oxide residues within Mn carbonate nodules, coupled negative δ98MoNIST+0.25 values (mean: −1.59‰), negative δ13Ccarbonate values (mean: −7.68‰), and variable Ce anomalies (0.21–1.71), collectively support that Mn(II) carbonates were formed via the diagenetic reduction of original Mn(III/IV) oxides. Multiple lines of evidence (e.g., lithofacies, authigenic Mo and U enrichments, Ce anomalies, δ34Ssulfide, and δ98MoNIST+0.25) indicate that the deep-waters in the Qianzhong rift basin rapidly shifted from euxinic-anoxic to suboxic-oxic conditions during Mn deposition. This shift was driven by the rapid shallowing and exposure of the basin induced by continuous crustal uplift and global regression. These insights support an “oxidation expansion model” for the genesis of the Zunyi Mn deposits, which involves the downward expansion of the redoxcline triggering swift oxidation of anoxic deep waters and in situ precipitation of Mn(III/IV) oxides, and subsequently transforming to Mn(II) carbonates via diagenetic reduction. This study concludes that ancient Mn carbonate deposits are reliable proxies for paleoceanic redox fluctuations, and an overview of Mn carbonate deposits in the broad context of geological history supports their close genetic connection with oceanic redox perturbations.