@article{SigmanDiFioreHainetal.2009,
  author    = {Sigman, Daniel M. and DiFiore, Peter J. and Hain, Mathis P. and Deutsch, Curtis and Wang, Yi and Karl, David M. and Knapp, Angela N. and Lehmann, Moritz F. and Pantoja, Silvio},
  title     = {The dual isotopes of deep nitrate as a constraint on the cycle and budget of oceanic fixed nitrogen},
  issn      = {0967-0637},
  doi       = {10.1016/j.dsr.2009.04.007},
  year      = {2009},
  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 delta N-15 and delta O-18 in the ocean interior. The delta O-18 of nitrate is our focus because it has been explored less in previous work. Denitrification raises the delta N-15 and delta O-18 of mean ocean nitrate by equal amounts above their input values for N-2 fixation (for delta N-15) and nitrification (for delta O-18), generating parallel gradients in the delta N-15 and delta O-18 of deep ocean nitrate. Partial nitrate assimilation in the photic zone also causes equivalent increases in the delta N-15 and delta O-18 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 delta N-15 of the regenerated nitrate is equivalent to that originally consumed, whereas the regeneration replaces nitrate previously elevated in delta O-18 due to denitrification or nitrate assimilation with nitrate having the delta O-18 of nitrification. This lowers the delta O-18 of mean ocean nitrate and weakens nitrate delta O-18 gradients in the interior relative to those in delta N-15. 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 delta O-18 and delta N-15 within the ocean interior and a mean ocean nitrate delta O-18 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 delta N-15 and delta O-18 of preformed nitrate and/or too little cycling of intermediate-depth nitrate through the low latitude photic zone.},
  language  = {en}
}
@article{SigmanDiFioreHainetal.2009,
  author    = {Sigman, Daniel M. and DiFiore, Peter J. and Hain, Mathis P. and Deutsch, Curtis and Karl, David M.},
  title     = {Sinking organic matter spreads the nitrogen isotope signal of pelagic denitrification in the North Pacific},
  issn      = {0094-8276},
  doi       = {10.1029/2008gl035784},
  year      = {2009},
  abstract  = {Culture studies of denitrifying bacteria predict that denitrification will generate equivalent gradients in the delta N-15 and delta O-18 of deep ocean nitrate. A depth profile of nitrate isotopes from the Hawaii Ocean Time-series Station ALOHA shows less of an increase in delta O-18 than in delta N-15 as one ascends from abyssal waters into the denitrification-impacted mid-depth waters. A box model of the ocean nitrate N and O isotopes indicates that this is the effect of the low latitude nitrate assimilation/regeneration cycle: organic N sinking out of the surface spreads the high-delta N-15 signal of pelagic denitrification into waters well below and beyond the suboxic zone, whereas the nitrate delta O-18 signal of denitrification can only be transmitted by circulation in the interior.},
  language  = {en}
}