Isotopic composition of nitrate in the Western Tropical Pacific Ocean: Biogeochemical signals and their transport

The Western Tropical Pacific Ocean has a complicated circulation system and plays a significant role in regulating global marine nitrogen cycles and budgets. Powerful insights can be gained by using nitrate isotope (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) techniques. In this study, nitrate concentrations and δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- values were obtained along meridional (130°E) and zonal (20°N) transects in the Western Tropical Pacific Ocean. The δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- values ranged from 2.9 to 10.9 ‰ and from 2.0 to 5.7 ‰, respectively. For the water mass in the thermocline originating in the North Pacific, the simultaneously elevated δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- values indicated nitrate assimilation. The relatively low δ¹⁵N-NO₃^- values in the intermediate water indicated the remineralization of sinking organic matter produced by N₂ fixation at shallow depths, but these signals exhibited a delay in downward propagation in the North Pacific subtropical gyre. For water masses originating in the Southern Hemisphere, the variations in nitrate isotopes might be attributed mainly to physical mixing, including diapycnal mixing between the thermocline and intermediate water and lateral advection from the Equatorial Undercurrent. Furthermore, according to end-member mixing calculations, at least 62 % of the nitrate in the water of the North Subsurface Countercurrent was derived from the Southern Hemisphere, whereas North Pacific Tropical Water, South Pacific Tropical Water and nutrient-rich upwelling accounted for 50 %, 12 % and 38 %, respectively, of the nitrate in the water of the North Equatorial Countercurrent.