Redox changes in the outer Yangtze Sea (South China) through the Hirnantian Glaciation and their implications for the end-Ordovician biocrisis

Abstract

Two pulses of faunal mortality occurred during the Late Ordovician mass extinction (ca. 445 Ma). This biocrisis is recorded in Hirnantian strata of South China as a stepwise extinction of graptolites in both the hydrologically semi-restricted inner and open outer Yangtze Sea. Although expanded marine euxinia is widely regarded as the main cause of the biocrisis, the spatial-temporal pattern and driving mechanisms behind redox changes, as well as the extent to which they influenced marine faunas, remain unclear. Here, we present a study of mid-shelf and outer-shelf sections of the less-studied outer Yangtze Sea, based on an integrated suite of geochemical proxies [i.e., iron speciation, pyrite δ34S (δ34Spy), and major- and trace-element data] to provide insight into changes in marine redox conditions, chemical weathering rates, and primary productivity across the Hirnantian Glaciation. Iron speciation ratios and trace-element enrichment factors show that euxinia appeared in mid-shelf settings during the late Katian, subsequently expanded into deeper waters, and then diminished during the Hirnantian Glaciation and expanded again thereafter. Expansions of euxinia across the outer Yangtze Sea prior to and after the Hirnantian Glaciation may have been the result of both elevated primary productivity, as indicated by elevated organic carbon accumulation rates (OCAR), and increased sulfate weathering inputs due to preglacial and postglacial enhanced fresh-rock exposure on land. However, we propose that the local development of euxinia was controlled mainly by sulfate availability, which depended on continental weathering intensity—a hypothesis supported by strong covariant relationships between the chemical index of alteration (CIA, a weathering proxy), redox proxies, and δ34Spy values. The contraction of oceanic euxinia during the main glacial interval was caused by a reduction in continental weathering intensity; contemporaneously weaker euxinia, as well as higher δ34Spy values, in outer-shelf relative to mid-shelf areas may have been due to limited terrestrial sulfate supply in deeper sites and small sulfate reservoir in the open ocean during the glaciation. Persistence of euxinia in the outer Yangtze Sea after the termination of the main glacial interval is evidence of a growing sulfate reservoir in early Silurian oceans, probably due to increased continental weathering. Furthermore, the comparision of the redox and fossil records indicates that elevated extinctions among mesopelagic graptolites coincided with enhanced euxinia in the outer Yangtze Sea, supporting the hypothesis that redox changes were the main stressor of both benthic fauna and some zooplankton. Our results highlight the interrelated influences of climate, continental weathering, and riverine sulfate fluxes on marine redox variations and the biotic crisis of the Late Ordovician.