Coupling of the recovery of earliest Silurian sponges and ocean redox conditions: Evidence from South China

Many aspects of the Late Ordovician Mass Extinction (LOME) aftermath and recovery have been puzzling due to heterogeneities in tempo and triggering mechanisms. Benthic fossil groups, which are the most severely affected by oxidative stress, offer the best opportunities for understanding both biological and ecological recovery after the LOME. In recent studies, deep-water sponge assemblages (which may have had high physiological tolerance to oxygen deviations) have been reported widely across South China in the immediate aftermath of the extinction interval. In order to further explore the lateral and temporal distributions of sponges, and ecological effects of benthic recovery during this critical interval, this study presents new Llandovery sponge assemblages recovered from two sections in Hunan Province, South China, accompanied by geochemical analyses. The sponge communities are preserved by pyritic spicule replacements and shows a relatively deep-water affinity (estimated around 60–150 m in depth) comparable to several previously reported assemblages in South China, and consistent with the observed graptolite ecology. Geochemical analysis of total organic carbon (TOC) and major and trace element composition in both sections show similar trends, indicating a shift in redox state of the bottom-water from persistent anoxia or intermittent euxinia in the earliest Rhuddanian, becoming oxygenated in the early Aeronian. Based on the present study and previous biological and geochemical data from South China, the distribution of early Silurian sponge assemblages in South China demonstrates a gradual expansion towards deeper regions when ocean redox conditions ameliorated. This study confirms the continuity of the end-Ordovician sponge faunas and taxa, both laterally across South China, and temporally through the early Silurian. The pioneering colonization of sponges in low-oxygen environments after the LOME may have set the stage for the subsequent recovery of other benthic organisms.