Petrography and geochemistry of the Permian-Triassic boundary interval, Yangou section, South China: Implications for early Griesbachian seawater δ13CDIC gradient with depth

The carbon isotopic composition (delta C-13(carb)) recorded in shelf carbonates has been widely used as a proxy for the isotopic composition (delta C-13(DIC)) of surface ocean water to establish paleocean chemistry and circulation patterns. However, delta C-13(carb) values do not necessarily preserve the delta C-13(DIC), due to post-depositional diagenetic alteration. In order to examine the early Griesbachian surface-to-deep delta C-13(DIC) gradient with depth, the diagenetic features of the Permian-Triassic boundary interval (beds 18 to 35) from Yangou section, located in the Yangtze carbonate platform interior, South China, are delineated to compare with those of the slope GSSP Meishan section. The petrographic and geochemical observations show that the early Griesbachian carbonates in the Yangou section underwent pervasive dolomitization in its early diagenetic history. Three types of early replacement dolomites and one type of dolomite cement are present. The dolomite crystals display internal zonation, with high-Ca calcian dolomite (HCD) core being encased successively by calcite and an outermost Fe-rich HCD cortex. The initial dolomitization took place in anoxic seawater, and underwent subsequent diagenetic system involved with meteoric water. The two most negative delta C-13 values in claystones of Beds 21-3 and 35 are probably related to meteoric diagenesis. Above and/or below the meteorically influenced beds, the dolomite and calcite have uniformly positive delta C-13 values. The primary carbon isotopic compositions are probably preserved in the early Griesbachian carbonate from the platform Yangou section, which could probably be related to the poor formation of the outermost Fe-rich HCD cortex. Compared to the slope carbonate from the Meishan section, the platform carbonate from the Yangou section has lower primary delta C-13(carb) values. It is estimated that the delta C-13(DIC) gradient with depth between Yangou and Meishan is less than the previously suggested. The results highlight the need for evaluation of local delta C-13(carb) as record of delta C-13(DIC) in paleoseawater, and carry important implications for understanding the Permian-Triassic carbonate successions throughout the world. (C) 2017 Elsevier B.V. All rights reserved.