Pre-NOE seafloor oxidation archived in authigenic mineral assemblage in late Tonian marine sediments, South China

Neoproterozoic Oxygenation Event (NOE) is significant oxidation of surface Earth environment on the eve of the origin of metazoan. Marine oxygenation of NOE, supported by multiple redox-sensitive proxies, is suggested to start in the interglacial period of Cryogenian snowball Earth ice ages. Paleoenvironmental conditions before Neoproterozoic Oxygenation Event were recorded in marine deposits in late Tonian ocean. We investigated marine authigenic mineral assemblages in fine-grained siliciclastic successions (<758 Ma), below Sturtian-age Chang’an diamictite (i.e., >720 Ma), deposited in deep-water basin, in South China. The authigenic mineral assemblages, occur as lenticular concretion, consist of sparry calcite, equant Fe-Mn-dolomite, and radial barite fans. There is sharp contact between Fe-rich zone and Mg-rich zone in the equant dolomites. The carbonate isotopes of authigenic carbonate minerals yield a highly ¹³C-depleted variation range from -15‰ to -20‰ (relative to V-PDB). In addition, there is scarce pyrite in concretion and host rock of siltstone whereas radial barite fans exist closely with dolomite. The barites yield consistent δ³⁴S values of ~+27.5‰ (relative to V-CDT). The results suggest that there was possibly significant Fe-Mn reduction-driven organic oxidation in early-diagenetic sediment under a bottom-water condition beneficial to the formation of manganese and iron oxidant/hydroxide. Moreover, the occurrence of authigenic sulfate with modern seawater-like δ³⁴S is interpreted as the consequence of widespread sulfide re-oxidation at late-Tonian seafloor. We link authigenic mineral assemblage with sporadic seafloor oxidation in deep-water basin before Neoproterozoic Oxygenation Event.