The Tonian and Cryogenian Periods

The Tonian and Cryogenian periods together span from 1000 to c. 635.5 Ma and are currently chronometrically divided at 720 Ma. The early Tonian followed the amalgamation of the Rodinia supercontinent and is a time for which the stratigraphic, chemostratigraphic, and fossil record is relatively sparse and poorly dated. The initiation of intracratonic basins on many cratons c. 850 Ma, while Rodinia was still intact, is responsible for a much richer late Tonian record. This record preserves evidence for eukaryotic diversification and the first documented pronounced negative carbon isotope anomaly in the Neoproterozoic—the Bitter Springs Anomaly. Much of the second half of the Tonian Period is characterized by high carbon isotope values (δ13C of carbonate >5‰), but recent studies indicate that at least one and probably two deep negative δ13C excursions occurred after c. 740 Ma, the latter immediately preceding the onset of Cryogenian glaciation. This glaciation appears to have initiated globally at c. 717 Ma, based on consistent, high-precision U–Pb zircon ages from multiple sedimentary successions. These ages will support formal definition of the Global Stratotype Section and Point for the base of the Cryogenian System. This first Cryogenian glaciation, commonly referred to as the Sturtian glaciation, was long-lived, ending c. 660 Ma. Because the second and briefer late Cryogenian (i.e., Marinoan) glaciation is known to have initiated prior to 639 Ma and ended c. 635.5 Ma, the Cryogenian nonglacial interval must have been relatively short-lived (c. 20 Myr). Nevertheless, this interval is well represented on many cratons, due in part to the formation of widespread rift basins and passive margins as Rodinia began to break up. Although molecular clock and biomarker data suggest that the earliest animals had appeared by this time, no unambiguous metazoan fossils have been recovered from Cryogenian strata, which show low overall fossil diversity.