Redox conditions and nitrogen cycling in the Late Ordovician Yangtze Sea (South China)

Abstract

Abstract The late Ordovician was a critical interval in Earth s history, during which changes in the biosphere, climate and environment occurred. In this study, we analyse the nitrogen isotope composition and multiple geochemical indices of Ordovician-Silurian (O S) shales in the Yangtze Sea (South China) to elucidate the evolution of nitrogen cycling and sedimentary environments. The nitrogen isotope values of Wufeng shale samples (Upper Ordovician) range from −0.4‰ to +1.6‰, and those of Longmaxi shale samples (Lower Silurian) range from +0.6‰ to +3.1‰. According to the longitudinal distribution of nitrogen isotope, there was a positive shift from the Wufeng Formation to Longmaxi Formation. Moreover, the redox-sensitive element ratios and nitrogen isotopes were strongly related. These relationships imply that the pre- and post-Hirnantian intervals of nitrogen cycle perturbation were directly related to changes in the oceanic redox conditions in the Yangtze Sea region. During the depositional stage of the Wufeng Formation (pre-Hirnantian) the water column was deep, causing anaerobic reduction in the sedimentary environment. The nitrogen cycle was dominated by anaerobic nitrogen fixation with little isotopic fractionation; thus, the δ15N values are light. In contrast, during the depositional stage of the Longmaxi Formation (post-Hirnantian), global cooling and tectonic uplift led to sea-level change, and sedimentation was increasingly affected by the supply of terrigenous clastic materials. The oxygen content of the water column increased, and redox stratification was maintained dynamically. Denitrification and anaerobic ammonium oxidation (anammox) played important roles in the Longmaxi Formation nitrogen cycle, leading to strong isotopic fractionation. Nitrogen isotopes are a reliable indicator of palaeo-environmental changes and have great value for the restoration of palaeo-depositional conditions and the comparative analysis of global Ordovician-Silurian geological events.