Volcanism-driven marine eutrophication in the end-Ordovician: Evidence from radiolarians and trace elements of black shale in South China

A variety of recent work has addressed the key factors in the establishment of widespread seafloor anoxia at the time of the Late Ordovician Mass Extinction (LOME), but the trigger for marine eutrophication remains under debate. Herein we investigated two shale-dominated successions across the Ordovician-Silurian boundary in South China and identified three intervals characterized by abundant radiolarians and frequent volcanic ash beds, as well as elevated values of P/Al and Baexcess, through the late Katian to late Rhuddanian. These results suggest a strong positive correlation between intensive volcanism and increased primary productivity in the surface ocean. Moreover, high enrichment of redox-sensitive trace elements like Mo and U, together with smaller and more consistent size of pyrite framboids, suggest dominantly anoxic conditions in bottom waters of the end-Ordovician Yangtze Sea. Thus, these lines of evidence point towards a hypothesis of volcanism-driven marine eutrophication, in which increased nutrient levels sourced from volcanism were transported into the oceans and thereby stimulated oceanic primary productivity. This in turn caused expansion of seafloor anoxia, and extinction of marine animals that survived the global temperature fall and marine regression resulting from the end-Ordovician glaciation.