Combined Effects of Pacific Water Intrusion and Beaufort Gyre in Shaping Dual Nitrate Isotope Patterns in the Western Arctic Ocean

Abstract Nitrate is a primary limiting nutrient for biological production in the Arctic Ocean. In the Arctic Ocean, nitrogen and oxygen isotope ratios of nitrate (δ 15 N–NO 3 − and δ 18 O–NO 3 − ) serve as powerful tracers for distinguishing nitrate sources and constraining key biogeochemical processes. Hydrological parameters, nutrients, and δ 15 N–NO 3 − and δ 18 O–NO 3 − were collected during the summer cruise of 2024 in the western Arctic Ocean to examine the vertical patterns of dual nitrate isotopes and their driving mechanisms. An anomalous vertical decoupling of δ 15 N‒NO 3 − and δ 18 O–NO 3 − in the western Arctic basin was identified, which is distinct from patterns observed across most global oceans. This arises from the intrusion of Pacific Water (PW), which is modified by benthic denitrification and remineralization processes on the Bering‐Chukchi shelf, and carries a high δ 15 N signal (∼7–8‰) and a low δ 18 O signal (∼0‰) into the basin area. The average δ 15 N–NO 3 − and δ 18 O–NO 3 − values in PW are 7.7 ± 0.5‰ and −0.4 ± 0.6‰, respectively, compared to the mean values of 5.2 ± 0.4‰ and 1.5 ± 0.3‰ in Atlantic Water. Furthermore, PW becomes trapped by the anticyclonic circulation of the Beaufort Gyre (BG), resulting in prolonged residence time that amplifies regional nitrogen loss and remineralization signals. Under climate warming, the enhanced intrusion of PW and the evolution of BG are expected to substantially reshape regional nitrogen cycling, with broad implications for Arctic marine ecosystems.