Local and global controls on carbon and sulfur isotope records in the Middle Ordovician Epeiric sea successions of the Taebaeksan Basin, Korea

Abstract

Although the sedimentary carbonate carbon isotope record is generally considered a reliable archive of long-term changes in the global carbon cycle, the influences of local variability in carbon cycling and diagenesis frequently overwhelm its global controls, particularly in shallow platform environments. During the Middle Ordovician period, the middle Darriwilian isotopic carbon excursion (MDICE) has been identified worldwide, including the Taebaeksan Basin in the Sino-Korean Block. However, in the eastern part of the Taebaeksan Basin, the positive δ13Ccarb trend associated with the MDICE was disrupted by negative δ13Ccarb anomalies, indicating the alternating prevalence between local and global controls. By comparing paired carbon and sulfur isotope signatures, we demonstrate the involvement of two distinct mechanisms in this local event: a gradual decrease in δ13Ccarb closely parallels that of organic carbon, and a negative δ13Ccarb peak, superimposed on the long-term δ13Ccarb decrease, coincides with the positive sulfur isotope shift in pyrite, without a corresponding change in δ13Corg. The latter indicates the formation of 13C-depleted authigenic carbonate during early diagenesis, associated with microbial sulfate consumption, whereas post-depositional processes cannot fully account for the coupled negative shifts in δ13Ccarb and δ13Corg values. Instead, this simultaneous divergence from the MDICE event may be due to limited exchange of dissolved inorganic carbon with open ocean water, coupled to an increase in terrestrial runoff, as shown by changes in the clay mineral assemblage. While generally aligning with sedimentological and paleontological investigations regarding the formation of restricted environments in the eastern part of the Taebaeksan basin, this interpretation challenges the previously presumed timing and duration of platform restriction. Our findings highlight that the spatial variability of isotopic records may compromise their fidelity as global proxies but offer valuable insights into the evolution of depositional environments and platform geometry, often obscured by sporadic outcrop exposure.