Carbon isotope compositions of the Late Permian carbonate rocks in southern China: their variations between the Wujiaping and Changxing formations
Abstract The carbon isotope compositions of the Upper Permian marine carbonate rocks in southern and eastern Guizhou and central Guangxi of southern China have been investigated. The results reveal a δ13C anomaly at the boundary between the Wujiaping and Changxing Formations of the Upper Permian. The δ13C values in limestones of the Wujiaping formation are mostly within the range of 3.5 and 5.0‰, whereas the δ13C values in limestones of the Changxing Formation are mostly less than 3.0‰, ranging from 0.2 to 3.1‰. The carbon isotope negative excursion at the boundary between these two formations is helpful to differentiate the two stages (Wuchiapian and Changxingian) of the Late Permian in southern China if substantiated by sufficient fossil materials. This also means that the δ13C values of the carbonates were decreasing throughout the Late Permian, and this change was accompanied by some rapid falls, such as those in the Permian–Triassic boundary and possibly in the Wujiapingian and Changxingian boundary. During the Late Permian, the consumption of organic carbon by the oxidation of the previously formed coals, as a result of dry climates and tectonic uplifting, must have exceeded the burial of organic carbon (as in coal). In addition, lack of coal deposits during the late Tatarian on the Gondwana fragments suggests a global reduced coal accumulation. Consequently, more 12C-rich CO2 was released into the atmosphere, which might contribute to the δ13C depletion seen in Upper Permian marine carbonates.