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

We compare the output of an 18-box geochemical model of the ocean withmeasurements to investigate the controls on both the mean values and variation ofnitrate d15N and d18O in the ocean interior. The d18O of nitrate is our focus because it hasbeen explored less in previous work. Denitrification raises the d15N and d18O of meanocean nitrate by equal amounts above their input values for N2 fixation (for d15N) andnitrification (for d18O), generating parallel gradients in the d15N and d18O of deep oceannitrate. Partial nitrate assimilation in the photic zone also causes equivalent increases inthe 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 Nand O isotopes of deep ocean nitrate, especially when the sinking N is produced in a lowlatitude region, where nitrate consumption is effectively complete. The d15N of theregenerated nitrate is equivalent to that originally consumed, whereas the regenerationreplaces nitrate previously elevated in d18O due to denitrification or nitrate assimilationwith nitrate having the d18O of nitrification. This lowers the d18O of mean ocean nitrateand weakens nitrate d18O gradients in the interior relative to those in d15N. Thisdecoupling is characterized and quantified in the box model, and agreement with datashows its clear importance in the real ocean. At the same time, the model appears togenerate overly strong gradients in both d18O and d15N within the ocean interior and amean 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 d15Nand d18O of preformed nitrate and/or too little cycling of intermediate-depth nitratethrough the low latitude photic zone.