Carbon isotopic composition of Early Triassic marine carbonates, Eastern Sichuan Basin, China

Abstract

As the interval following the biggest Phanerozoic mass extinction at the end of Permian, the Early Triassic is characterized by an immature, poorly functioned ecosystem. The effects of these extreme environmental conditions can be mirrored by the changes in the δ13C record of marine carbonates. However, the details about the carbon isotopic composition and evolution of the Early Triassic seawater remain poorly understood. A dataset of new δ13C and δ18O values as well as selected major and trace element data (including concentrations of CaO, MgO, Mn, and Sr) was obtained from 113 marine carbonate samples collected in Eastern Sichuan Basin. The isotopic and elemental data are used to evaluate effects of thermochemical sulfate reduction on δ13C. The δ13C values of a few samples affected by thermochemical sulfate reduction were corrected. By combining carbonate δ13C results in our investigated sections, we constructed a composite curve of the Lower Triassic. The results reveal a δ13C anomaly of carbonate rocks throughout the Early Triassic, accompanied by some rapid jumps and falls, such as those from approximately −2‰ to the extremely high value of 8‰ within a period of about 5 Ma. The Early Triassic δ13C profile derived from Eastern Sichuan Basin shows a close correspondence with Guandao section in Guizhou Province, whereas it yields an excursion pattern differing from Chaohu section in Anhui Province of the Lower Yangtze region (with the δ13C value from the minimum around −6‰ to the maximum near 4‰). The higher δ13C values and the positive carbon isotope excursions in the Lower Triassic from Eastern Sichuan Basin were most likely a consequence of the principal environmental change that may include: (1) the barren land surface due to the absence of vegetation, (2) the loss of diverse marine invertebrate groups and marine carbonates, (3) the thriving of bacteria, algae and methanogenus in ocean ecosystems, and (4) the local effect of the repositories of isotopically light carbon occurred in the form of methane hydrates. This process ended at the Early-Middle Triassic boundary, which indicates that the biotic recovery started at the end of the Lower Triassic. The terrestrial vegetation and marine invertebrates, as the regulators for carbon cycle, are irreplaceable and demand strong protections.