Quantifying N₂ fixation and its contribution to export production near the Tonga-Kermadec Arc using nitrogen isotope budgets

The spatial distribution of marine di-nitrogen (N-2) fixation informs our understanding of the sensitivities of this process as well as the potential for this new nitrogen (N) source to drive export production, influencing the global carbon (C) cycle and climate. Using geochemically-derived delta N-15 budgets, we quantified rates of N-2 fixation and its importance for supporting export production at stations sampled near the southwest Pacific Tonga-Kermadec Arc. Recent observations indicate that shallow (<300 m) hydrothermal vents located along the arc provide significant dissolved iron to the euphotic zone, stimulating N-2 fixation. Here we compare measurements of water column delta N-15(NO3+NO2) with sinking particulate delta N-15 collected by short-term sediment traps deployed at 170 m and 270 m at stations in close proximity to subsurface hydrothermal activity, and the delta N-15 of N-2 fixation. Results from the delta N-15 budgets yield high geochemically-based N-2 fixation rates (282 to 638 mol N m(-2) d(-1)) at stations impacted by hydrothermal activity, supporting 64 to 92% of export production in late spring. These results are consistent with contemporaneous N-15(2) uptake rate estimates and molecular work describing high Trichodesmium spp. and other diazotroph abundances associated with elevated N-2 fixation rates. Further, the delta N-15 of sinking particulate N collected at 1000 m over an annual cycle revealed sinking fluxes peaked in the summer and coincided with the lowest delta N-15, while lower winter sinking fluxes had the highest delta N-15, indicating isotopically distinct N sources supporting export seasonally, and aligning with observations from most other delta N-15 budgets in oligotrophic regions. Consequently, the significant regional N-2 fixation input to the late spring/summer Western Tropical South Pacific results in the accumulation of low-delta N-15(NO3+NO2) in the upper thermocline that works to lower the elevated delta N-15(NO3+NO2) generated in the oxygen deficient zones in the Eastern Tropical South Pacific.