The arc of the Snowball: U-Pb dates constrain the Islay anomaly and the initiation of the Sturtian glaciation
In order to understand the onset of Snowball Earth events, precise geochronology and chemostratigraphy are needed on complete sections leading into the glaciations. While deposits associated with the Neoproterozoic Sturtian glaciation have been found on nearly every continent, time-calibrated stratigraphic sections that record paleoenvironmental conditions leading into the glaciation are exceedingly rare. Instead, the transition to glaciation is normally expressed as erosive contacts with overlying diamictites, and the best existing geochronological constraints come from volcanic successions with little paleoenvironmental information. We report new stratigraphic and geochronological data from the upper Tambien Group in northern Ethiopia, which indicates that the glacigenic diamictite at the top of the succession is Sturtian in age. U-Pb zircon dates obtained from two tuffaceous siltstones that are 74 and 84 m below the diamictite are 719.68 ± 0.46 Ma and 719.68 ± 0.56 Ma (2σ), respectively. We also report a U-Pb date of 735.25 ± 0.25 Ma from a crystal-rich tuff located 2 m above the nadir of a high-amplitude, basin-wide, negative δ13C excursion previously correlated with the Islay anomaly. This age for the anomaly agrees with Re-Os age constraints from Laurentia, suggesting that the δ13C signal is globally synchronous and preceded the Sturtian glaciation by ∼18 m.y. The interval between the Islay anomaly and Sturtian glaciation is recorded in the Tambien Group as an ∼600 m succession of predominantly shallow-water carbonates and siliciclastics with δ13C values recording a prolonged period at +5‰, followed by an interval of lower, but still positive, values leading up to the glaciation. Our data are consistent with synchronous global onset of the Sturtian glaciation at ca. 717 Ma. Shallow-water carbonates in strata directly below the first diamictite suggest that glacial onset was rapid in terranes of the Arabian-Nubian Shield.