Nitrogen isotopic constraints on marine primary productivity across the Ordovician–Silurian transition

The Ordovician-Silurian transition (OST) witnessed the second-largest mass extinction event during the Phanerozoic. However, while the decline of animal species is well documented, research on primary productivity during the OST remains qualitative, despite its importance in regulating ecosystem growth. In this study, we explored the potential of nitrogen isotope as a potential proxy for primary productivity in the oligotrophic oceans during the OST. We present nitrogen isotope (δ¹⁵N) data from a drill core (L203 well) in South China, combined with literature data from 17 sections from different latitudes around the world. Primary productivity is calculated independently based on sedimentary carbon contents and deposition rates, using the Knies equation. The data show that on a global scale, primary productivity is strongly negatively correlated with nitrogen isotopes. Low levels of primary productivity concur with increased δ¹⁵N, which suggests a weakening of nitrogen fixation, and vice versa. Moreover, using the δ¹⁵N data constraints primary productivity, this study reveals the differential mechanisms of organic matter accumulation in the Wufeng and Lungmachi formations, with the former primarily driven by high productivity, and the latter by enhanced preservation under anoxic conditions. This study provides new theoretical support and practical examples for using nitrogen isotope constraints paleoceanographic primary productivity.