Nitrogen isotope evidence for a redox-stratified ocean and eustasy-driven environmental evolution during the Ordovician–Silurian transition

Abstract

The Ordovician-Silurian (O/S) transition was a critical geological interval, during which significant biological events and dramatic environmental changes occurred. Extensive research has proven that changes in the ocean environment are one of the main causes of events that affect the biosphere. However, the redox structure and related geochemical evolution of the ocean during the O/S transition remain under debate. N isotopes are thought to have significant potential for further elucidating the oceanic redox structure and related geochemical evolution during the O/S transition, and are an effective supplement to the former environmental proxies of bottom sediments. This study presents high-resolution N isotope data from a well-sequenced drill core across the O/S transition from the Lower Yangtze Platform, South China, showing an interesting isotopic variation with high δ 15 N values (+1‰ to +4‰) during the Late Katian to Hirnantian ages of the Ordovician and much lower values (−1‰ to +1‰) during the Rhuddanian Age of the earliest Silurian. This δ 15 N variation is mostly controlled by ocean redox conditions and the corresponding changes in the nitrate reservoir. Combined with previously reported δ 15 N data, a redox-stratified ocean with a relatively deep redox transition zone was reconstructed according to the distribution of δ 15 N profiles across different palaeogeographic locations. The lower δ 15 N values and more reducing conditions in deep water sections demonstrate that glacio–eustatic fall, rather than increased upwelling, was likely the main driver of the global redox change during the Late Ordovician. Furthermore, a global δ 15 N decrease during the Late Katian may be associated with the debated Boda warm event, and the up to six positive δ 15 N excursions observed during the Hirnantian might suggest the cyclicity of the glaciation process. • δ 15 N can be correlated globally during the Ordovician-Silurian (O/S) transition. • Oceanic δ 15 N was controlled by water redox conditions during the O/S transition. • Positive δ 15 N excursions with a low δ 15 N baseline support a redox-stratified ocean. • δ 15 N records indicate Hirnantian redox evolution driven by eustatic sea-level fall. • Global δ 15 N variations correspond with a Boda warm event and multistage Hirnantian glaciation.