The delayed diversification of early animals until the middle Ediacaran Period, despite their likely origin in the Cryogenian Period or earlier, has been attributed to low levels of dissolved O-2 in global seawater. To provide more robust constraints on the extent of global-ocean oxygenation during the mid-Cryogenian Period, we analyzed paired U (delta U-238) and Mo (delta Mo-98) isotope compositions of organic-rich shale in the basal Datangpo Formation from two separate drillcores in South China. This unit yielded an average delta U-238 of -0.39 +/- 0.23 parts per thousand. (25D; relative to standard CRM145), which is significantly lower than the average delta U-238 (+0.02 +/- 0.12 parts per thousand.) of euxinic sediments of the modern restricted Black Sea and well below a modeled delta U-238 of +0.2 parts per thousand for modern unrestricted euxinic facies (e.g., Saanich Inlet). The U/Al ratios of our samples indicate that similar to 40% of U was derived from detrital materials (delta U-238(detrital) = similar to -0.3 parts per thousand), suggesting that the delta U-238 of authigenic U is even lower than the measured bulk-sample values. Taking into consideration the 0.4-0.8 parts per thousand enrichment of delta U-38 in euxinic sediments relative to seawater, the delta U-238 values of the Datangpo shales correspond to contemporaneous seawater delta U-238 of -1.20 parts per thousand. to -0.73 parts per thousand. Owing to mass balance considerations, these low seawater delta U-238 values are indicative of widespread development of anoxia in the Cryogenian interglacial ocean (similar to 658-654 Ma). These shales yield mostly low delta Mo-98 values (+0.56 +/- 0.46 parts per thousand.), although the youngest samples have values as high as +1.21 +/- 0.23 parts per thousand. (relative to NIST SRM 3134 = +0.25 parts per thousand), all of which are significantly lower than modern seawater delta Mo-98 (+2.34 +/- 0.10 parts per thousand). The Datangpo shales either directly record the global seawater delta Mo-98 signature or provide a minimum estimate thereof (in the event of local influences). The consistently low delta Mo-98 values of the Datangpo shales thus confirm widespread anoxia in the Cryogenian interglacial ocean. These findings based on paired delta U-238-delta Mo-98 analyses thus provide key information regarding global-ocean redox conditions during the interval immediately preceding the rise of macroscopic and morphologically complex animals
