Paired U and Mo isotope evidence for pervasive anoxia in the Cryogenian early interglacial ocean

Abstract

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 O2 in global seawater. To provide more robust constraints on the extent of global-ocean oxygenation during the mid-Cryogenian Period, we analyzed paired U (δ238U) and Mo (δ98Mo) isotope compositions of organic-rich shale in the basal Datangpo Formation from two separate drillcores in South China. This unit yielded an average δ238U of −0.39 ± 0.23‰ (2SD; relative to standard CRM145), which is significantly lower than the average δ238U (+0.02 ± 0.12‰) of euxinic sediments of the modern restricted Black Sea and well below a modeled δ238U of +0.2‰ for modern unrestricted euxinic facies (e.g., Saanich Inlet). The U/Al ratios of our samples indicate that ~ 40% of U was derived from detrital materials (δ238Udetrital = ~−0.3‰), suggesting that the δ238U of authigenic U is even lower than the measured bulk-sample values. Taking into consideration the 0.4–0.8‰ enrichment of 238U in euxinic sediments relative to seawater, the δ238U values of the Datangpo shales correspond to contemporaneous seawater δ238U of −1.20‰ to −0.73‰. Owing to mass balance considerations, these low seawater δ238U values are indicative of widespread development of anoxia in the Cryogenian interglacial ocean (~658–654 Ma). These shales yield mostly low δ98Mo values (+0.56 ± 0.46‰), although the youngest samples have values as high as +1.21 ± 0.23‰ (relative to NIST SRM 3134 = +0.25‰), all of which are significantly lower than modern seawater δ98Mo (+2.34 ± 0.10‰). The Datangpo shales either directly record the global seawater δ98Mo signature or provide a minimum estimate thereof (in the event of local influences). The consistently low δ98Mo values of the Datangpo shales thus confirm widespread anoxia in the Cryogenian interglacial ocean. These findings based on paired δ238U-δ98Mo analyses thus provide key information regarding global-ocean redox conditions during the interval immediately preceding the rise of macroscopic and morphologically complex animals