NEW ZIRCON U-PB GEOCHRONOLOGY ON THE STURTIAN DEGLACIATION IN SOUTH CHINA

The Cryogenian Snowball Earth hypothesis was conceived as the sedimentary and paleomagnetic evidences suggested an equatorial ice line during the widespread Sturtian and Marinoan glaciations. This hypothesis predicts that the Snowball Earth should be terminated synchronously. While the Marinoan deglaciation has been well constrained at ca. 635 Ma worldwide, the timing for the Sturtian deglaciation is yet to be determined. Here we present a new zircon U-Pb date from an ash bed within the basal part of the interglacial Datangpo Formation in South China. The ash bed is dated at ca. 663.8 Ma by SIMS and is further refined to ca. 661.0 Ma by CA-ID-TIMS. This new CA-ID-TIMS zircon U-Pb date is regarded as the minimum timing for the Sturtian deglaciation in South China.

Radiometric dates from the post-Sturtian strata include ca. 661.0 Ma from South China, 662.4 ± 3.9 Ma from NW Canada, 659.0 ± 4.5 Ma from Mongolia, and 657.2 ± 5.4 Ma from Australia. Although these dates are usually regarded as identical within uncertainties and consistent with the Snowball Earth hypothesis, they are not sufficient to prove a synchronous Sturtian deglaciation. If the onset, as well as the termination, of the Sturtian glaciation was synchronous worldwide as the hypothesis predicted, the timing for the Sturtian deglaciation is constrained between 663.03 ± 0.11 Ma from South Australia and ca. 661.0 Ma from South China, and the duration of the Sturtian glaciation is 54~56 myr, given the onset timing of the Sturtian glaciation is ca. 717 Ma. However, as mentioned above, more high precision geochronological dates and geochemical proxies are needed to prove the globally synchronous Sturtian deglaciation.