Salinity variations of the inner Yangtze Sea during the Ordovician-Silurian transition and its influences on marginal marine euxinia

The Ordovician-Silurian transition (OST) is a critical geological interval, during which dramatic climatic, environmental and biological changes occurred. Although expanded euxinic conditions have been regarded as one of the main causes of Late Ordovician mass extinction (LOME), the controls of euxinia remain the topic of debate. In this study, we evaluate the paleosalinity conditions of the inner Yangtze Sea (IYS) during the OST using a combination of paleosalinity proxies (B/Ga, Sr/Ba, and S/TOC). Our results reveal the climate-driven salinity variations during the OST, with lower salinity conditions in glacial interval derived from increasing fresh-water contribution in the IYS. In combination with available redox data, these salinity perturbations can be further related to the development of euxinia in the OST IYS. The existence of the pre-Hirnantian density stratified water columns limited diffusion of weathering-produced sulfate into deep water thereby favoring the formation of a mid-depth "euxinic wedge" redox structure in the IYS. In contrast, the desalinization of the IYS in glacial interval weakened the density stratification and facilitated the transportation of weathering-produced sulfate into deeper water. The associated strengthening and expansion of euxinic conditions during the Katian/Hirnantian transition may have triggered the first phase of the LOME. However, the relatively oxic conditions and vigorous hydrological circulations in glacial maximum may promoted the sulfate accumulation and distribution in the IYS. In post-glaciation, the invasion of open ocean seawater into the low-salinity inner Yangtze Sea not only rebuilt the halocline but enlarged the sulfate reservoir in the IYS, triggering the expansion of euxinic conditions in the anoxic basin. Our findings suggest that establishment of euxinic conditions in the IYS during OST was influenced by variations of seawater salinity and that these events may have exerted strong control on evolutionary patterns during this critical period in Earth history.