The Global Stratotype Section and Point (GSSP) for the base- Sakmarian Stage (Cisuralian, Lower Permian)

The Sakmarian Stage represents a critical time interval because the Late Paleozoic Ice Age reached its acme during this time. After extensive studies, we herein use the First Appearance Datum (FAD) of the conodont Mesogondolella monstra within the lineage from M. uralensis→M. monstra→ M. manifesta at Bed 26/3 of the Usolka section in the southern Urals, Russia, as the primary signal for the base-Sakmarian GSSP. The first occurrence of the conodont Sweetognathus binodosus within the lineage from Sw. aff. merrilli→Sw. binodosus is used as an auxiliary marker for the base of the Sakmarian Stage. An extrapolated age for this GSSP is 293.52 ± 0.17 Ma produced by a Monte Carlo simulation based on two ash beds dated by chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS). The 87Sr/86Sr isotopic value based on conodont apatite for the base of the Sakmarian Stage is approximately 0.70787. Two minor negative excursions in δ13Ccarb are present at the boundary levels and the upper one is consistent with the GSSP level. The documentation of fusulinaceans, ammonoids, brachiopods and palynology also aid intercontinental and marine-terrestrial correlation. The Usolka section meets the requirements for the GSSP and a monument has been established; correlation with sections in North America and South China is also well documented.