Productivity and redox influences on the late Ordovician ‘Katian Extinction’ and ‘early Silurian Recovery’

Abstract

The development of large paleobiological databases has provided an opportunity to reassess the pattern and timing of marine biodiversity changes during the Late Ordovician, which has newly revealed a major decline in biodiversity during the middle to late Katian Stage (448.7–446.7 Ma) and a rapid faunal recovery during the latest Hirnantian to early Telychian stages (444.7437.1 Ma), herein termed the “Katian Extinction” (KE) and the “Early Silurian Recovery” (ESR), respectively. Being only recently recognized, the kill mechanism of the KE and the environmental conditions associated with the ESR have received little attention to date. Here, we report whole-rock iron isotopic compositions (δ56FeIRMM-14) for the YD-1 drillcore in South China and, combined with other geochemical proxies, use them to investigate environmental influences on the KE and ESR. Iron isotopes are more sensitive to fluctuations in the oxic-to-suboxic part of the redox spectrum than commonly used proxies (e.g., Fe-speciation and trace-metal data) because sediment δ56Fe increases in the oxic zone as a result of 56Fe uptake by Fe(III), whereas it decreases in the suboxic zone due to fractionation in favor of 55Fe during dissimilatory iron reduction. The Katian Extinction coincided with decreasing δ56FeIRMM-14 (from +0.35‰ to 0.00‰) as well as increasing Corg/P and MoEF, supporting an anoxia-driven biocrisis. Enhanced rates of organic carbon accumulation (OCAR) and biogenic silica accumulation (SiAR) from the Rhuddanian to mid-Aeronian were potentially induced by widespread upwelling in the Yangtze Sea linked to the post-Hirnantian eustatic transgression. Our study provides insight into the causes of the Katian Extinction, which was an integral part of the multi-phased Late Ordovician Mass Extinction, and documents the environmental conditions in the Yangtze Sea in which the Early Silurian Recovery occurred.