The Onset of the Sturtian Snowball Earth: New Geochronology and Chemostratigraphy from the Tambien Group of Ethiopia

The Snowball Earth hypothesis proposes that the onset of global glaciation was a rapid event, particularly at low-latitudes, due to a runaway ice-albedo instability in the climate system. Currently, the Neoproterozoic Sturtian Snowball Earth Glaciation is constrained to have been ongoing at low-latitudes in present day NW Canada at 716.47 ± 0.24 Ma based on a U-Pb date from a tuff within glacial diamictite (Macdonald et al., 2010). A date from a rhyolite underlying this diamicitite of 717.43 ± 0.14 Ma has been inferred to precede the glaciation, although this age is not a strict maximum for the initiation of the snowball Earth since evidence of glaciation may have been obscured in volcanic units. The Tonian-Cryogenian Tambien Group of northern Ethiopia is a mixed carbonate-siliciclastic sequence deposited in an arc proximal basin that conformably culminates in diamictite associated with the Sturtian Glaciation. This study exploits the presence of tuffs suitable for high precision U-Pb zircon geochronology interbedded with recently discovered exposures of the transition into the snowball Earth. Dates from tuffs stratigraphically <100 m below massive diamictite enable a test of synchronicity for the initiation of the Sturtian Glaciation when compared with data from NW Canada and S China. Furthermore, tuffs in the proximity of the chemostratigraphically defined Islay negative δ13C anomaly give the opportunity to develop new U-Pb dates to test the prediction from Re-Os dating of black shales in NW Canada that the Islay significantly predates the Sturtian ice age, decoupling the sharpest negative downturns in inorganic δ13C values from glacial initiation. These data also augment and temporally constrain the pre-glacial 87Sr/86Sr curve, informing ongoing modeling efforts of global weathering and seawater chemistry as they relate to hypotheses of the role of large igneous province emplacement and paleogeography in the initiation of Snowball Earth glaciation.