The carbon isotope excursion on GSSP candidate section of Lopingian–Guadalupian boundary

An isotopic stratigraphically well-documented outcrop in Penglaitan, Guangxi Province, South China, has been proposed as a candidate GSSP for the Lopingian–Guadalupian boundary. Correlatable outcrops from the west wing (Tieqiao section) and east wing (Penglaitan section) of the Laibin Syncline exhibit synchronous excursions in carbon isotopes. The isotopic excursions (δ13C) show the best placement of the boundary may lie at the base of Bed 6k which coincides with the Clarkina postbitteri conodont zone and with eustatic change. δ13C increases during the uppermost Guadalupian (Jinogondolella granti conodont zone from the top of 3c to the base of 6i). A δ13C peak value of 5‰ is located at the transition between these two conodont zones and is suggested as a proxy for the transition from transgression to regression. A gradual depletion of carbon isotopes occurs in the C. postbitteri zone from Bed 6e to 7b, and this gradual δ13C excursion also suggests the sequences around the Guadalupian–Lopingian boundary at the candidate section are conformable. During the middle-later C. postbitteri zone a 3.5‰ dramatic δ13C depletion is recorded at the Tieqiao section, but only a 2‰ depletion at the deeper facies Penglaitan section, synchronous with conodont zones that mark eustatic changes.