Anomalous weathering trends during the Early Silurian warming: Implications for the biotic crisis and recovery

Abstract

The Late Ordovician mass extinctions were divided into two distinct stages, with the second stage occurring during the Late Hirnantian. However, biotic recovery was delayed until the Aeronian and the triggering factors are still unknown. To comprehend the connections between paleoclimate, marine redox states, and biotic recovery in the Early Silurian, we analyzed temporal variations in chemical weathering intensity using multi-geochemical proxies for the shales of the Longmaxi Formation in South China. The results reveal a significant increase in chemical weathering intensity from the Rhuddanian warming phase to the Aeronian cooling phase. This increase is anomalous, considering it coincides with the Early Silurian's climate cooling. This study posits that Rhuddanian sediments exhibited low chemical weathering intensity and more congruent weathering, while Aeronian sediments were characterized by intermediate chemical weathering intensity and incongruent weathering. Given that the gradual enrichment of oxygen within bottom waters can be supported by Corg/P and Corg-S-Fe, this study suggests that congruent weathering-induced high paleoproductivity is the primary factor accelerating anoxia in the Rhuddanian. In summary, the persistent marine anoxia of seawater thus triggered the deposition of organic-rich shales and delayed biotic recovery in the Rhuddanian.