Coupling of N2 Fixation and Denitrification and Their Spatial Extent in the Arabian Sea Over the Last Glacial‐Interglacial Period

Abstract

The ocean nitrogen cycle is influenced by climatic conditions, which regulate atmospheric greenhouse gases and generate feedback effects. These feedbacks occur through the release of nitrous oxide and alterations in the fixed‐nitrogen inventory, which in turn regulates carbon sequestration. The Arabian Sea hosts an intense oxygen minimum zone, making it a critical region for understanding marine biogeochemical cycling. Understanding past variability in nitrogen cycling offers crucial insights into the relationship between ocean circulation and productivity, which is essential for predicting future biogeochemical changes. Here, we present reconstructions of spatiotemporal variation in water column denitrification and N2 ${\mathrm{N}}_{2}$ fixation, the influence of oceanic ventilation, and the role of monsoonal variability on nitrogen cycling in the Arabian Sea. Based on our sedimentary nitrogen isotope ( δ $\delta $15N) records, we found N2 fixation and denitrification to be concomitantly high during the Last Glacial Maximum (LGM; ∼23–19 ka) in the northern and southeastern Arabian Sea. In contrast, both processes weakened during the deglaciation. The dominance of denitrification, with the decrease in dissolved oxygen as a result of the reduced influx of Antarctic Intermediate Water, appears to have begun in the early Holocene. On millennial timescales, denitrification intensity is mainly controlled by subsurface ventilation by water masses from the Southern Ocean. Despite temporal variations, both processes remained more intense in the northern Arabian Sea. Our findings underscore the sensitivity of the nitrogen cycle in the Arabian Sea to climate change, with continued warming causing denitrification to intensify, providing positive feedback to the climate.