Dynamic redox conditions in Mesoproterozoic shallow seawater: Constraints from carbonate fabrics and geochemistry

The delayed eukaryote evolution during the mid-Proterozoic may have been linked to prolonged low oxygen levels during this period. Two short oxygenation events have recently been identified at ca. 1.57 Ga and ca. 1.4 Ga, but the redox conditions during the intervening interval remain poorly constrained. In this study, we conducted mineralogical and geochemical investigations on three sections of the Wumishan Formation (ca. 1.52−1.47 Ga) that were deposited between the two oxygenation events in the Yanliao basin. The results revealed two distinct changes in carbonate mineralogy and precipitation style associated with concurrent changes in carbonate I/(Ca + Mg): a transition from intervals with positive I/(Ca + Mg) anomalies and abundant water-column carbonate mud to intervals with no I/(Ca + Mg) anomalies and abundant seafloor precipitates, suggesting a shift from suboxic to anoxic conditions in shallow seawater. In both of the suboxic intervals, I/(Ca + Mg) values reached 0.5−2.6 μmol/mol, with negative Ce anomalies (Ce/Ce*(SN) = 0.69; data from literature) occurring in the later interval (ca. 1.48 Ga). The results suggest significant redox fluctuations in the shallow seawater during the Mesoproterozoic. However, these pulsed oxygenation episodes may not have been sufficient to sustain the continuous evolution of early eukaryotes.