Spatiotemporal variations of nitrogen isotopic records in the Arabian Sea

Abstract. Available reports of dissolved oxygen, δ¹⁵N of nitrate (δ ¹⁵N_NO3) and δ¹⁵N of total nitrogen (δ¹⁵N_bulk) for trap material and surface/downcore sediments from the Arabian Sea (AS) were synthesized to explore the AS' past nitrogen dynamics. Based on 25 μmol kg⁻¹ dissolved oxygen isopleth at a depth of 150 m, we classified all reported data into northern and southern groups. By using δ¹⁵N_bulk of the sediments, we obtained geographically distinctive bottom-depth effects for the northern and southern AS at different climate stages. After eliminating the bias caused by bottom depth, the modern-day sedimentary δ¹⁵N_bulk values largely reflect the δ¹⁵N_NO3 supply from the bottom of the euphotic zone. Additionally to the data set, nitrogen and carbon contents vs. their isotopic compositions of a sediment core (SK177/11) collected from the most southeastern part of the AS were measured for comparison. We found a one-step increase in δ¹⁵N_bulk starting at the deglaciation with a corresponding decrease in δ¹³C_TOC similar to reports elsewhere revealing a global coherence. By synthesizing and reanalyzing all reported down core δ¹⁵N_bulk, we derived bottom-depth correction factors at different climate stages, respectively, for the northern and southern AS. The diffusive sedimentary δ¹⁵N_bulk values in compiled cores became confined after bias correction revealing a more consistent pattern except recent 6 ka. Such high similarity to the global temporal pattern indicates that the nitrogen cycle in the entire AS had responded to open-ocean changes until 6 ka BP. Since 6 ka BP, further enhanced denitrification (i.e., increase in δ¹⁵N_bulk) in the northern AS had occurred and was likely driven by monsoon, while, in the southern AS, we observed a synchronous reduction in δ¹⁵N_bulk, implying that nitrogen fixation was promoted correspondingly as the intensification of local denitrification at the northern AS basin.