Abstract:

After nearly a billion years with no evidence for glaciation, ice
advanced to equatorial latitudes at least twice between 717 and
635 Mya. Although the initiation mechanism of these Neoproterozoic
Snowball Earth events has remained a mystery, the broad
synchronicity of rifting of the supercontinent Rodinia, the emplacement
of large igneous provinces at low latitude, and the onset of
the Sturtian glaciation has suggested a tectonic forcing. We present
unique Re-Os geochronology and high-resolution Os and Sr isotope
pro!les bracketing Sturtian-age glacial deposits of the Rapitan
Group in northwest Canada. Coupled with existing U-Pb dates, the
postglacial Re-Os date of 662.4 ± 3.9 Mya represents direct geochronological
constraints for both the onset and demise of a Cryogenian
glaciation from the same continental margin and suggests a 55-My
duration of the Sturtian glacial epoch. The Os and Sr isotope data
allow us to assess the relative weathering input of old radiogenic
crust and more juvenile, mantle-derived substrate. The preglacial
isotopic signals are consistent with an enhanced contribution of
juvenile material to the oceans and glacial initiation through
enhanced global weatherability. In contrast, postglacial strata
feature radiogenic Os and Sr isotope compositions indicative of
extensive glacial scouring of the continents and intense silicate
weathering in a post–Snowball Earth hothouse.