Boron isotope evidence for Ordovician-Silurian Transition postglacial marginal marine salinity with implications for redox conditions and biotic recovery

Anoxic conditions in marginal marine settings play a critical role in paleoceanographic and paleoecological studies. Understanding the mechanisms underpinning anoxia requires insights into watermass conditions in the study area. While salinity is a fundamental property of water, proxies for reconstructing paleosalinity and its influence on redox conditions in ancient systems are often overlooked. To illustrate this, we focus on the Yangtze Marginal Sea during the Ordovician-Silurian Transition (OST), specifically the Early Silurian postglacial anoxia. We utilized proxy records for salinity and redox conditions archived in organic-rich cherts and black shales deposited in the Yangtze Block, South China, between ca. 447.62–438.49 million years ago. Our study is the first to report the variation in bulk boron (B) isotopic (δ¹¹B) values during the OST. Analyzing paleosalinity data, such as δ¹¹B and B/gallium (Ga) along with redox indicators, such as molybdenum (Mo), uranium (U), and vanadium (V), provides constraints on the continuous vertical hydrological variability across the central Yangtze Marginal Sea. Our analysis suggests that the watermass in the study area was close in salinity to seawater during the Late Ordovician due to a well-established connection with the ocean. This was followed by a decline in salinity in the Early Silurian resulting from an influx of glacial melt water. Postglacial warming-induced freshwater input and sea-level change subsequently contributed to strong seawater stratification and elevated primary productivity. This disruption in the exchange between shallow and deep waters led to the development of anoxia in the Early Silurian Yangtze Sea, a phenomenon that has significantly impacted Earth's evolutionary history. Our study provides fresh insights into the Late Ordovician Mass Extinction event and the subsequent biotic recovery. In addition, we underscore the importance of integrating redox analyses with salinity reconstructions in explaining the mechanisms of anoxic environments.