A high-resolution record of the late Hirnantian to Aeronian marine redox change in South China and its relationship with the record of graptolite biodiversity

Abstract

Originating in the late Hirnantian (latest Ordovician) and extending into the Rhuddanian (early Silurian), the global Rhuddanian Oceanic Anoxic Event (Rhuddanian-OAE), characterized by habitat-specific anoxic-euxinic seafloor conditions led to the deposition of widespread graptolitic black shales. However, the intricate relationship between habitat-specific seafloor redox conditions and graptolite diversity record of the Rhuddanian-OAE remains enigmatic. Here, we present a continuous sequence of seafloor redox records obtained from a core spanning from the upper Katian (Ordovician) to the middle Aeronian (Silurian) in the Changning region of SE Sichuan Province. This region represents the Inner Yangtze Sea within the upper Yangtze Platform of South China. Following the glaciation peak of the Hirnantian, our study unveils the evolution of bottom waters, transitioning from oxic to permanently euxinic, intermittently euxinic, anoxic, and suboxic–oxic transitional states. This evolution is supported by independent evidence, including trace metals (Mo, U, V) and Corg/TP ratios, lithology, and TOC content. Local factors and global seawater drove redox changes of bottom water in Changning. In contrast to the increasing oxygenation in Changning and the northern hemisphere's suboxic bottom waters in Yukon, Canada, the bottom water of the southern hemisphere in the early Aeronian maintained a euxinic environment, potentially influencing the graptolite paleogeography of the early Aeronian. The graptolite diversity record was likely influenced by both paleoecological changes in the preferred habitat of deeper-water species combined with taphonomic processes that influenced their preservation potential.