Paired U And Mo Isotope Evidence For Pervasive Anoxia In The Cryogenian Early Interglacial Ocean

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