Distinguishing the Isotopic Signals of Nitrate Assimilation and Denitrification Along the GEOTRACES GP15 Pacific Meridional Transect

The biogeochemical processes shaping the fluxes of nitrogen (N) in the tropical to subarctic Pacific are illuminated by nitrate isotope ratios (δ¹⁵N and δ¹⁸O) along the GEOTRACES GP15 section. In the equatorial and tropical Pacific, nitrate δ¹⁵N and δ¹⁸O are 2–4‰ higher in the thermocline than in deeper waters. This widespread elevation is driven by both nitrate assimilation in Southern Ocean surface waters and denitrification in the eastern tropical Pacific. In addition to this, a poleward increase in the δ¹⁵N of surface nitrate from the Equator to 10°S is generated by the progressive consumption of nitrate upwelled at the Equator and transported southward. This process leads to increases in the δ¹⁵N of phytoplankton biomass and a strong meridional nitrate isotopic gradient in the thermocline due to regeneration of sinking N. North of the Equator, an analogous gradient is barely detectable due to weaker northward flow and a compressed spatial scale for nitrate drawdown. By contrast, between 5°N and 40°N, the nitrate isotope gradients are dominated by the processes driving the oceanic fixed N budget: denitrification and N₂ fixation. High-δ¹⁵N and -δ¹⁸O nitrate in the tropics is advected from the oxygen deficient zone where denitrification occurs, whereas in the adjacent subtropics, low-δ¹⁵N thermocline nitrate suggests a response by N₂ fixation that is not observed in the south. The asymmetry of the north and south tropical Pacific gradients in thermocline nitrate isotopes has implications for efforts to reconstruct the N cycle in the past.