The dual isotopes of deep nitrate as a constraint on the cycle and budget of oceanic fixed nitrogen

Abstract

We compare the output of an 18-box geochemical model of the ocean with measurements to investigate the controls on both the mean values and variation of nitrate δ 15N and δ 18O in the ocean interior. The δ 18O of nitrate is our focus because it has been explored less in previous work. Denitrification raises the δ 15N and δ 18O of mean ocean nitrate by equal amounts above their input values for N 2 fixation (for δ 15N) and nitrification (for δ 18O), generating parallel gradients in the δ 15N and δ 18O of deep ocean nitrate. Partial nitrate assimilation in the photic zone also causes equivalent increases in the δ 15N and δ 18O of the residual nitrate that can be transported into the interior. However, the regeneration and nitrification of sinking N can be said to decouple the N and O isotopes of deep ocean nitrate, especially when the sinking N is produced in a low latitude region, where nitrate consumption is effectively complete. The δ 15N of the regenerated nitrate is equivalent to that originally consumed, whereas the regeneration replaces nitrate previously elevated in δ 18O due to denitrification or nitrate assimilation with nitrate having the δ 18O of nitrification. This lowers the δ 18O of mean ocean nitrate and weakens nitrate δ 18O gradients in the interior relative to those in δ 15N. This decoupling is characterized and quantified in the box model, and agreement with data shows its clear importance in the real ocean. At the same time, the model appears to generate overly strong gradients in both δ 18O and δ 15N within the ocean interior and a mean ocean nitrate δ 18O that is higher than measured. This may be due to, in the model, too strong an impact of partial nitrate assimilation in the Southern Ocean on the δ 15N and δ 18O of preformed nitrate and/or too little cycling of intermediate-depth nitrate through the low latitude photic zone.