Progressive expansion of seafloor anoxia in the Middle to Late Ordovician Yangtze Sea: Implications for concurrent decline of invertebrate diversity

The Great Ordovician Biodiversification Event (GOBE) achieved its peak during the Middle Ordovician, likely in association with climatic cooling and a rise of atmospheric O2. However, unstable redox states developed widely in contemporaneous epeiric seas, challenging previous assumptions about sustained oceanic oxygenation driven by deep-ocean ventilation in the aftermath of Ordovician cooling. Here, we investigate two Middle-Upper Ordovician shale-dominated successions from intra-shelf basin and slope settings of the Yangtze Sea, South China. Negative shifts in bulk delta 15N and productivity proxies reveal temporal changes in the pattern of nutrient cycling under moderate-to-high productivity conditions, which were likely sustained by increased upwelling of cold and nutrient-rich deep waters in sync with contemporaneous climatic cooling. As a result, marine anoxia likely expanded in deep waters of the Yangtze Sea, as reflected in enrichments of highly reactive iron and redox-sensitive metals (Mo, U). Although moderate Mo abundances (25-50 ppm) and delta 98/95Mo values (+0.4 to +0.8%o) within the euxinic interval suggest that bottom waters had low sulfide concentrations, the delta 98/95Mo values are comparable with those of coeval euxinic facies (- +0.6%o). Furthermore, our modeling results show that low delta 98/95Mo can be produced when sulfidic sinks dominate Mo removal, suggesting that Middle-Late Ordovician oceans may have been less oxygenated than previously assumed, at least for the continental seas of South China, Baltica and Laurentia. These environmental changes may provide a new explanation for the -50% decline of invertebrate species following the peak of GOBE. Our findings thus link the progressive expansion of marine anoxia and climatic cooling to a profound biotic change during the Mid -Late Ordovician Period.(c) 2022 Elsevier B.V. All rights reserved.