Making up for Lost Time: Stratigraphic and Geochemical Implications of a Long-Lived Sturtian Glaciation

Abstract:

Geochronology provides a direct test of many models pertaining to the nature of Cryogenian climate change. Here we present new U-Pb and Re-Os geochronological constraints on Sturtian glacial deposits from Zambia, Arctic Alaska, Mongolia, South China, and throughout Laurentia. We present these new ages in stratigraphic, sedimentological and geochemical context to integrate data from other key localities, such as Namibia and Australia, and to construct a new age model for the Cryogenian Period. We find that there is no evidence for a global, pre-717 Ma glaciation. Instead, Sturtian glacial deposits appeared on all of these cratons by 717 Ma following widespread basaltic volcanism. We then review the nature and duration of the >50 Myr long Sturtian glacial epoch. In many localities, strata deposited during this interval are characterized by two massive diamictite units separated by diamictite-free, siliciclastic strata. These intervening clast-poor deposits are commonly interpreted to represent an interglacial period, however, alternatively they formed during a long-lived glaciation as newly rifted margins subsided and detached from the ice-grounding line. In the aftermath of the Sturtian glaciation, high sedimentation rates made up for lost time, and filled pent-up accommodation space, creating thick Cryogenian non-glacial interlude deposits on many margins. We discuss implications for geochemical cycles and the interpretation of carbon isotope anomalies.