by Francis Macdonald, Tony Prave, Catherine Rose

The Cryogenian Period (720-635 Ma) is arguably the most pivotal time in Earth history marked by the extraordinary fluctuations in planetary climate regulation of Snowball Earth, changes in surface redox conditions unmatched since the Great Oxidation Event, and the dual biospheric milestones of algae becoming the dominant marine producers and, ultimately, the advent and initial diversification of metazoa. Many conceptual models have been advanced to explain the genesis of these events and gain deeper knowledge of the processes that drove, and the conditions that characterized, such global transformations. Understanding how the Cryogenian Earth system functioned, and identifying the mechanisms that enabled our planet to exit that state, is one of science’s greatest challenges. GRIND-CRY is designed to address that challenge via creation of a worldwide core archive of Cryogenian strata. This archive will deliver the necessary and sufficient temporal resolution and the biological, geological, paleontological, and geochemical data to answer the outstanding questions of: what caused Snowball Earth and why are the durations of the two global glaciations (Sturtian, Marinoan) so different?; what drove oxygenation-deoxygenation of the oceans?; did extraordinary climate and redox changes generate mass extinction and form ecological bottlenecks?; when and where is the first appearance of animal biomarkers?; why do large amplitude C-isotope excursions bracket Snowball Earth glaciations?; how were oceanic nutrient, sulfate and oxygen levels impacted?; what were the conditions that ended the Cryogenian and transformed the Earth system into the Ediacaran-Cambrian world of oxygenated oceans and ecosystems inhabited by diverse and widespread animals? A September 2023 ICDP-approved Virtual Workshop attended by 107 researchers worldwide identified the Cryogenian successions of northern Namibia, western Brazil and southern Oman as the key primary GRIND-CRY targets. A total of 12 drill sites (5 in Namibia, 4 in Brazil, 3 in Oman) will recover a total of c. 6 km of core, inclusive of banded iron formation in the Urucum and Chuos formations of Brazil and Namibia, respectively, the Marinoan and Sturtian glaciations and intervening non-glacial strata of the iconic Otavi Group Namibia, and the Mirbat Group strata in Oman that yielded the earliest known animal biomarkers. GRIND-CRY utilizes the successful GRIND-ECT drilling strategies, protocols and educational outreach programs to contribute directly to ICDP’s Prime Science Themes of Geodynamic Processes, Georesources, Environmental Change and Geohazards (extreme climate change).

• Determine what biogeochemical conditions generated Snowball Earth glaciations and enabled their attendant non-glacial interlude Establish the first-appearance-datum, extent and diversification of animal biomarkers (e.g. demosponge sterols) Refine knowledge about the timing and tempo of Cryogenic biospheric evolution, the magnitude of biotic turnover, and the nature/causes of extinctions Construct continuous, high-resolution records of C-O-S-Ca-Mg isotopes to test and create models that explain some of the highest amplitude fluctuations of the global C and S cycles in Earth history and why these bracket Snowball Earth glaciations Establish what were the key drivers and restrictions (nutrient fluxes, oscillating oxygen levels, etc.) of marine primary productivity Obtain the geochemical data (stable and non-traditional isotopes, redox sensitive element ratios, trace elements) to test and refine models of ocean oxygenation Construct a finely resolved age model utilizing U-Pb and Re-Os techniques to establish timings, rates and durations of the major biogeochemical (C isotope excursions, first occurrence of biomarkers) and climatic events of Cryogenian time

Biosphere, Cryogenian, Earth History, NEOPROTEROZOIC

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