NEOPROTEROZOIC PALEOGEOGRAPHY AND THE ONSET OF CRYOGENIAN SNOWBALL EARTH

Through most of Earth history the balance between sources and sinks of carbon dioxide have been maintained such that climate is not in a Snowball state. This balance was disrupted at the Tonian-Cryogenian boundary leading to the ca. 717 Ma onset of the Sturtian Snowball Earth glaciation. Understanding the forcings that led to ice-albedo runaway are central to understanding Earth's long term climate and habitability. As chronostratigraphy of Tonian successions have improved, there is no longer a close temporal association between a deeply negative carbon isotope excursion and the onset of Sturtian glaciation—challenging hypotheses that have invoked that record as giving insight to biogeochemical forcing of cooling. High-precision geochronology has now revealed that the rapid main phase of emplacement of the Franklin large igneous province was not synchronous with the onset of Sturtian glaciation, but preceded it by ~2 Myr. These data challenge the hypothesis that radiative forcing from erupted aerosols was a tipping point into Snowball Earth. Additionally, the Sturtian was not a one-off event, but was followed by the Marinoan Snowball Earth glaciation in less than 20 Myrs. These advances lend support to hypotheses that connect cooling to increases in global weatherability through changes in Tonian paleogeographic boundary conditions. The position and break-up of the supercontinent Rodinia at low-latitudes, the formation and amalgamation of volcanic arc terranes in the tropics, and the emplacement of low-latitude large igneous provinces have all been invoked as contributing to more efficient silicate weathering globally and therefore decreasing carbon dioxide levels. Reconstructions of Neoproterozoic paleogeography are key for evaluating such hypotheses. In this contribution, we present updated reconstructions of Neoproterozoic global paleogeography that integrate advances in the database of paleomagnetic poles and of the evolution of continental margins including rifting and orogenesis. These reconstructions provide a framework to gain insight into paleogeographic change that set the stage for the onset of Cryogenian low-latitude glaciation.