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Nitrate isotopes as a constraint on the ocean's fixed N budget

Nitrate isotopes as a constraint on the ocean's fixed N budget. Daniel Sigman, Princeton University. Sargasso Sea nitrate N isotopes, GCM experiments, and Atlantic N 2 fixation. Angela Knapp, University of Southern California Peter DiFiore, Princeton University Curtis Deutsch, UCLA

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Nitrate isotopes as a constraint on the ocean's fixed N budget

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  1. Nitrate isotopes as a constraint on the ocean's fixed N budget Daniel Sigman, Princeton University Sargasso Sea nitrate N isotopes, GCM experiments, and Atlantic N2 fixation Angela Knapp, University of Southern California Peter DiFiore, Princeton University Curtis Deutsch, UCLA Fred Lipschultz,BIOS

  2. Talk outline • Nitrate N isotopes from a Sargasso Sea transect: estimate of “recently” fixed nitrate in Sargasso Sea water column • MOM3 simulations forced with various Atlantic N2 fixation fields predict recently fixed nitrate in the Sargasso Sea • Uncertainties: impact of other processes on the N isotopes of nitrate • Nitrate O isotopes: correct nitrate N isotopes for nitrate assimilation?

  3. N and O isotopes N : 14N 99.6337% 15N 0.3663% O : 16O 99.7630% 17O 0.0375%18O 0.1995% d15N (vs. atm. N2) = (((15N/14N)sample/(15N/14N)air) - 1)*1000‰ d18O (vs. VSMOW) = (((18O/16O)sample/(18O/16O)VSMOW) - 1)*1000‰

  4. Kinetic isotope effect 14k 15k 16k 18k

  5. N* Complementary constraints from[NO3-], N* and d15N

  6. BATS Validation Cruise 32 (October, 2002): Stations sampled for nitrate and DON isotopes Depth Increasing N2 fixation? ~2 yr vent. age increase

  7. Nitrate data NO3- assim.? N2 fix.?

  8. Nitrate d15N in the Sargasso Sea water column [NO3-] (µM) • No evidence of southward N2 fixation increase or a southward accumulation of newly fixed nitrate in the thermocline • Basis for average Sargasso Sea profile 15N of nitrate (‰ vs. air)

  9. Estimating ‘recently’ fixed nitrate (d15N - d15Nimported) (5.3‰ - d15N) f = = (d15Nnew - d15Nimported) 6.3‰

  10. From a recently fixed nitrate pool to aN2 fixation rate S N Atlantic “Plan A” “Plan B” S Atlantic N

  11. MOM4 simulations of the recently fixed nitrate field Deutsch et al. 2007 Atlantic field; 45°N-S; 27.85 Tg N yr-1 Gruber and Sarmiento 1997; 45°N-0°; 28.1 Tg N yr-1 GS ‘97 N. Atl. rate extended to 45°N-45°S; 56.1 Tg N yr-1

  12. Column inventories 0-600 m 0-1200 m

  13. Interhemispheric exchange of recently fixed nitrate Hemispheric asymmetry:North Atlantic tends to collect recently fixed nitrate

  14. 15N enrichment in the nitrate entering the Atlantic?Southern Ocean data Sigman et al. 1999

  15. Uniform nitrate d18O below 300 min this region 15N of nitrate (‰ vs. air) 18O of nitrate (‰ vs. SMOW) “18O-corrected” 15N of nitrate

  16. Low latitude N cycling and the loss of nitrate O isotope signals The O isotopes may not record the N and O isotope enrichment of imported nitrate.

  17. 18e/15e = 1.0 High nitrate d18Oin theshallow Sargasso Sea 15N of nitrate (‰ vs. air) 18O of nitrate (‰ vs. SMOW) “18O-corrected” 15N of nitrate

  18. Decoupling of nitrate N and O isotopes bysimultaneous nitrate assimilation and nitrification Monterey Bay: Wankel et al., 2007

  19. Conclusions • Nitrate N isotopes indicate 2 µM or more of recently fixed nitrate in the Sargasso Sea thermocline. • Coherent spatial trends are not apparent in the region studied. This suggests that gradients in N2 fixation are too weak to imprint nitrate N isotope gradients on the circulating thermocline. We did not sample an adequately wide range in ventilation age to follow the ‘isopycnal’ approach used by Gruber and Sarmiento (1997) for N*. • We essentially follow a diapycnal approach by our use of MOM3 with high latitude sponge walls. In this context, the isotope data suggest a relatively low rate for N2 fixation in the Atlantic (30 Tg N yr-1 or less, a la Deutsch et al. 1997). • Uncertainties include the possible impacts of nitrate assimilation inside and outside the Atlantic on the d15N of nitrate in the interior. The nitrate O isotopes were discussed in this context. • GEOTRACES should provide the opportunity to do this for real.

  20. The End

  21. What if N2 fixation aboundswhere denitrification is prevalent? Deutsch et al. 2004

  22. Low nitrate d15N in the Sargasso Sea thermocline:Consistent with N2 fixation input Knapp et al. 2005

  23. Conclusions • Nitrate N and O isotopes have fundamentally different behavior, making them powerful complements. • Coupled N and O isotope systematics: • Newly produced nitrate: d18O ~ 0‰ vs. SMOW • Nitrate assimilation: 18e = 15e (~ 5-10‰) • Denitrification: 18e = 15e (20-30‰) • Application to the eastern North Pacific margin: • d18O:d15N anomaly (D(15,18) min.) in the thermocline • Interpretations: • addition of low-d15N N from N2 fixation • an active NO3-/NO2- redox cycle in or near the suboxic zone

  24. SAMW ~ 7.2‰ 15N enrichment in the nitrate entering the Atlantic?Southern Ocean data DiFiore et al. 2006

  25. Algal nitrate assimilation: 18e~15e  18e/15e = 1.0 Progressive nitrate consumption Granger et al., 2004

  26. The cycle and budget of N in the ocean N2Norg (NO3-) NO3-  N2

  27. Complementary constraints from[NO3-], N* and d15N

  28. N isotopes in the whole ocean N budget Brandes and Devol 2002

  29. Denitritification: 18e~15e  18e/15e = 0.95 Granger et al., in review

  30. Decoupling of nitrate N and O isotopes bysimultaneous nitrate assimilation and nitrification Monterey Bay: Wankel et al., 2007

  31. The nitrate O isotopes are usefulbecause ofwhat they don’t record

  32. The nitrate O isotopes are usefulbecause ofwhat they don’t record

  33. Summary of systematics

  34. Hemispheric asymmetry

  35. Stations on the Baja California margin [O2]<5 µM Contour every 750 m van Geen cruise, Nov. 1999

  36. All stations Red = N (~Point Conception) Blue = S (~S tip of Baja)

  37. D(15,18) = d15N - d18O - 5.5‰ Deviations from 1:1 variation innitrate d18O and d15N Southern Baja stations All stations

  38. a. b. c. d. Figure 3

  39. Nitrate isotopes near southern tip of Baja

  40. Cause of the D(15,18) minimum?

  41. NO3- NO2-  … Cause of the D(15,18) minimum?

  42. For 200-800 m: N2 fix. ~ 0.65*denit. N* minimum: -12 µM  -24 µM At 200 m: NO2- ox. > 0.7*NO2- red. Quantifying putative processes from the D(15,18) anomaly

  43. Bering Sea shelf:O/N isotope decoupling by in situ nitrification M.B.: Wankel et al. B. Brunelle

  44. Polar Antarcticnitrate isotope fractionation P. Difiore

  45. Southern Baja station results compared to stations further North

  46. HOT Station ALOHA Is the open tropical Pacific the source of the D(15,18) minimum ?

  47. ENP vs. HOT

  48. observations (200-800 m) N2 fixation = 0.65*denitrification [NO3-]B - [NO3-]M Nitrate d15N Nitrate D(15,18)

  49. NO2- oxidation > 0.7*NO2- reduction at 200 m Assumptions: (1) 15eNiO = 15eNiR, (2) 18e/15eNiO = 18e/15eNiR

  50. Why no anomaly in the Santa Barbara Basin?

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