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 d15N and d18O in the ocean interior. The d18O of nitrate is our focus because it has been explored less in previous work. Denitrification raises the d15N and d18O of mean ocean nitrate by equal amounts above their input values for N2 fixation (for d15N) and nitrification (for d18O), generating parallel gradients in the d15N and d18O of deep ocean nitrate. Partial nitrate assimilation in the photic zone also causes equivalent increases in the d15N and d18O 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 d15N of the regenerated nitrate is equivalent to that originally consumed, whereas the regeneration replaces nitrate previously elevated in d18O due to denitrification or nitrate assimilation with nitrate having the d18O of nitrification. This lowers the d18O of mean ocean nitrate and weakens nitrate d18O gradients in the interior relative to those in d15N. 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 d18O and d15N within the ocean interior and a mean ocean nitrate d18O 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 d15N and d18O of preformed nitrate and/or too little cycling of intermediate-depth nitrate through the low latitude photic zone. © 2009 Elsevier Ltd. All rights reserved.