Elevated marine productivity triggered nitrogen limitation on the Yangtze Platform (South China) during the Ordovician-Silurian transition

The Ordovician-Silurian transition (OST) coincided with global environmental and biological changes, including perturbations of the carbon and sulfur cycles, oceanic redox changes, and a mass extinction event, but their effects on the contemporaneous marine nitrogen cycle remain poorly known. In this study, we measured bulk nitrogen isotopes in the Datianba and Shuanghe OST sections of South China, which accumulated in the semi-restricted inner Yangtze Sea. The uniformly low delta N-15(bulk) (mostly -1 to +1 parts per thousand) of Wufeng and Longmaxi Formation black shales (representing the pre- and post-Hirnantian intervals) imply large losses of bioavailable nitrogen (i.e., nitrate and ammonium) via denitrification/anammox as well as strong nitrogen fixation by diazotrophs. Persistence of low delta N-15(bulk) values within the Guanyinqiao Bed (representing the Hirnantian glacial interval) indicates that denitrification/anammox remained strong even during this major glacio-eustatic fall and deep-ocean ventilation event. We hypothesize that elevated primary productivity caused by upwelling was a key factor producing a shallow redoxcline in the inner Yangtze Sea. By comparing our nitrogen data with records from other cratons, we suggest that oceanic anoxia was prevalent globally during the Late Katian to early Rhuddanian, and that ocean-redox conditions were spatially heterogeneous during the Hirnantian glaciation. These considerations imply that bioavailable nitrogen was the limiting factor on marine primary productivity during the OST, and that oceanic anoxia may have contributed to the Late Ordovician mass extinction, especially on the Yangtze Platform.