Reconstruction of oceanic paleosalinity during the Late Ordovician in the inner Yangtze Sea of South China

Abstract

The Ordovician-Silurian Transition (OST) was a key geological period, which recorded the Late Ordovician Mass Extinction (LOME), volcanic eruption, oceanic anoxia, rapid sealevel fluctuations, glacial event and carbon isotope excursion. Recent studies also identify a late Katian extinction pulse as part of the LOME. During the OST, the inner Yangtze Sea (IYS) was tectonically restricted, but the controlling factors of salinity heterogeneity in the basin and their impact on LOME remain debated. Carbonate sedimentation dominated the IYS in the Late Ordovician, thus complicating continuous salinity reconstructions because most paleosalinity proxies are calibrated for shales and are less readily applied to carbonates. Based on carbon-isotope stratigraphy, we used geochemical proxies (e.g., elemental and the new proxy “excess B") to assess the relative salinity variations in the IYS during the OST. The carbonate salinity proxies revealed a paleosalinity trend like that observed in shale (Decreases and then increases near the Hirnantian). Combining multiple paleoenvironmental proxies and previously published data, we evaluated the contribution of paleosalinity changes to LOME. Our δ¹³C_org and δ¹³C_carb records the Guttenberg positive isotope carbon excursion (GICE) and the Hirnantian Isotopic Carbon Excursion (HICE). The paleosalinity proxies, along with others, allowed the division of the OST into four Units where salinity was controlled by sealevel changes, the Kwangsian Orogeny, and climate-induced changes in terrestrial runoff. The reduction in species and abundance supports the occurrence of the Katian Extinction in the IYS. The expansion of anoxic seawater was not the only factor controlling the LOME, but salinity likely has played a crucial role.