The Cryogenian Period included at least two long-lived global Snowball glaciations, the Sturtian and Marinoan, which were separated by a non-Snowball interval that lacks evidence for persistent ice cover. Non-Snowball Cryogenian sediments record the earliest possible rise of chlorophyte algae, high magnitude perturbations to the carbon cycle, and possible evidence for a substantial increase in atmospheric oxygen. The shallow marine Datangpo Formation and the correlative, deep-marine Xiangmeng Formation document continuous, shale-dominated sequences that were deposited in the Nanhua Basin on the Yangtze Craton (South China) during the non-Snowball interval. Previous studies have extrapolated estimates of seawater molybdenum isotope (δ98Mo) compositions for the non-Snowball using data from the lower Datangpo Fomation and the Arena Formation of east Greenland, which either constitute only a small portion of the total thickness of non-Snowball deposits, or are difficult to assess based on the likelihood of isotopic fractionation during non-quantitative Mo drawdown. To address these limitations, we report bulk rock Fe speciation data in combination with published redox sensitive trace element (Mo and U) concentration data from euxinic marine sedimentary rocks that were deposited throughout the non-Snowball interval. Local syn-depositional and diagenetic effects during Mn‑carbonate precipitation are considered during interpretations of regional redox datasets, and these insights provide a base framework for the robust interpretation of new δ98Mo data from euxinic sediments. The result is a better constrained and continuous record of global ocean palaeoredox change throughout the non-Snowball interval. The global Mo dataset shows an increase in δ98Mo from +0.67 ± 0.06‰ to +1.12 ± 0.09‰, and a contemporaneous decrease in maximum Mo concentrations in euxinic sediments. Using these data, we employ a single reservoir model to evaluate the evolving spatial extent of different oceanic redox states. When assuming a post-Sturtian decrease in weathering input, the model results suggest that substantial anoxic seafloor was maintained throughout the non-Snowball, while euxinic seafloor area decreased, mirroring trends in regional redox proxy data. Our geochemical data and modelling further support the proposal that both temperature and redox conditions in marine environments may have been more hospitable in the latter part of the non-Snowball interlude, broadly contemporaneous with the earliest possible evidence for the emergence of animal life.
CORE (COnnecting REpositories)
CORE (COnnecting REpositories)