THE TECTONIC CONTEXT OF TONIAN TO CRYOGENIAN ENVIRONMENTAL AND EVOLUTIONARY CHANGE: INSIGHTS FROM RE-OS GEOCHRONOLOGY AND WEATHERING PROXIES

The Tonian and Cryogenian periods (1000 – 635 Ma) were intervals of great environmental and evolutionary change. Strata of this age record the diversification of eukaryotes, large perturbation to nearly every geochemical cycle, and multiple and long-lasting global glaciations, all set against a backdrop of large-scale tectonic reorganization resulting in the low-latitude distribution of ~25 paleocontinents. Fundamental to our understanding of these evolutionary and geological phenomena is an accurate and precise geochronological framework. Recent studies have indicated that enhanced basaltic weathering at low-latitudes may control large-scale and long-term climate change as well as playing a dominant role in the C, P, and Fe cycles. By coupling initial Os (Os_i) and Sr isotope stratigraphy with new geochronological constraints through this interval in Earth history, we can better understand the role of tectonics in climate forcing and biological change.

Here, we present new Re-Os ages as well as Sr and Os isotope proxy records for mid-Cryogenian sedimentary strata from Laurentia, Congo and Mongolia. These data constrain the onset of deposition of pre- and post-Snowball Earth glacial horizons as well as reveal the dominant local weathering regimes in these basins and aid in correlating horizons with well-preserved and rich microfossil assemblages. New Re-Os geochronology data from the Mwashya subgroup, Zambia provides a maximum depositional age for the overlying glaciogenic deposits of the Grand Conglomerate that are inconsistent with previous suggestions of a pre-Sturtian age. Additionally, new Re-Os ages from the Callison Lake Formation of the Coal Creek inlier, Yukon, Canada and the Chuar Group of the Grand Canyon, Arizona confirm correlations between the two and provide depositional age constraints for the onset of sedimentation in these Laurentian basins. New Os_i and Sr isotope values from the post-Sturtian Tayshir Member, Tsagaan Oloom Formation of Mongolia can be compared with equivalent datasets from NW Canada to evaluate global weathering regimes in the aftermath of a Snowball Earth epoch. By combing Os_i and Sr isotope records with new geochronological constraints we can begin to construct a record of changes in the silicate weathering regime through 300 Myrs of Neoproterozoic Earth history.