Coupled Mo and U isotopes constrain oceanic ventilation during the early Ediacaran

The early Ediacaran witnessed the termination of the Marinoan Ice Age, morphological diversification of macroscopic eukaryotes, and possible global-ocean oxygenation, reflecting dynamic relationships among changes in climate, oxygenation state, and biological evolution during this period, although debate continues regarding the nature of these relationships. To better constrain global-ocean redox conditions following the Marinoan Snowball Earth event and their potential controls, we investigated coupled molybdenum (Mo) and uranium (U) isotopic compositions from euxinic black shales of Member II of the Ediacaran Doushantuo Formation at the basinal Yuanjia section, South China. Upsection, the δ 98 Mo values increase from −1.5‰ to +0.6‰, while δ 238 U values decrease from +0.8‰ to −0.1‰, showing a strong negative correlation (R 2 = 0.60, p < 0.05). Iron speciation data indicate persistently euxinic watermass conditions, while Mo and U enrichment factors and Cd/Mo versus Co × Mn data suggest a restricted-basin setting. This isotopic pattern suggests that basinal restriction and global redox states exerted a combined control on trace-metal behavior. Enhanced basinal restriction likely suppressed water-column ventilation and intensified sulfate reduction, leading to increased H 2 S concentrations, reduced Mo-isotopic fractionation, and a shift in U reduction from the sediment–water interface to sediment porewaters. The high δ 238 U in the lower study interval indicates a transient oxygenation event followed by partial deoxygenation of the early Ediacaran ocean. These results highlight dynamic redox fluctuations driven by multiple factors during postglacial recovery. This study provides an example of the differential sensitivity of Mo and U isotopes under a range of redox conditions and demonstrates that the combined application of their isotopic compositions offers a more nuanced view of watermass dynamics than the use of a single isotopic system in reconstructing paleo–ocean redox conditions.