The 5th International Conference of Geobiology

         The Cryogenian Period comprises two global glaciations–Sturtian and Marinoan–separated by an interglacial interval. Molecular clock estimates and biomarker records suggest major bio-innovation milestones during Cryogenian, characterized by the ecological dominance of green algae and the first appearance of Demospongiae. However, previous studies indicate a temporal decoupling between the termination of the Sturtian glaciation and the proliferation of green algae, and the environmental drivers of these events are debated. The Datangpo Formation, deposited in the rift Nanhua Basin during the Cryogenian interglacial interval, serves as a critical stratigraphic archive for investigating palaeoceanographic changes. Particularly, we focus on shallow-water cap carbonate depositions in horst settings and deep-water black shale depositions in graben settings of the basal Datangpo Formation to reconstruct the spatiotemporal evolution of marine redox dynamics in Nanhua Basin immediately following the termination of the Sturtian glaciation. 'Stable' uranium isotope compositions (δ²³⁸U expressed as the deviation of sample ²³⁸U/²³⁵U ratio from the same ratio of the standard CRM145) integrating with multiple geochemical proxies were used to evaluate marine redox conditions during this critical time interval.

                The cap carbonate deposited at shallow-water horst setting yielded a large negative excursion of δ²³⁸U (as low as -1‰), whereas the black shales deposited at deep-water graben setting lacked authigenic U enrichment and δ²³⁸U variation beyond the range of the upper continental crust. These observations suggest a dramatic expansion of marine anoxia during the initial stage of the Cryogenian interglacial. A temporally correlative carbonate succession in Mongolia yielded a similar negative δ²³⁸U excursion, suggesting that the expansion of anoxia likely has broad spatial significance. Such pervasive anoxia may have impeded for the diversification of complex ecosystems in the wake of the termination of the Sturtian glaciation.