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Abby Ren, Princeton Daniel Sigman, Princeton Nele Meckler, Caltech Rebecca Robinson, URI

Foraminifera-bound nitrogen isotopes evidence for reduced nitrogen fixation in the Atlantic Ocean during the last ice age. Abby Ren, Princeton Daniel Sigman, Princeton Nele Meckler, Caltech Rebecca Robinson, URI Yair Rosenthal, Rutgers Gerald Haug, ETH.

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Abby Ren, Princeton Daniel Sigman, Princeton Nele Meckler, Caltech Rebecca Robinson, URI

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  1. Foraminifera-bound nitrogen isotopes evidence for reduced nitrogen fixation in the Atlantic Ocean during the last ice age Abby Ren, Princeton Daniel Sigman, Princeton Nele Meckler, Caltech Rebecca Robinson, URI Yair Rosenthal, Rutgers Gerald Haug, ETH

  2. Pleistocene glacial/interglacial cycles

  3. Biological pump

  4. Low vs. High Latitude Low Latitude: Change nutrient inventory High Latitude: Change nutrient consumption

  5. ~3 kyr ~15-80 kyr Low-latitude biological pump N2 N fixation denitrification N2 Broecker, 1982

  6. N isotope terms

  7. Effects of major N fluxes on nitrate 15N

  8. Lower water column denitrification during LGM Chile: de Pol-Holz et al, 2006 SBB: Emmer and Thunell, 2000 AS: Altabet et al., 2002

  9. Low d15N of nitrate in the North Atlantic thermocline: Paleoceanographic utility [NO3-] (µM) 15N of nitrate (‰ vs. air) Knapp et al., 2005

  10. Planktonic foraminifera Orbulina universa photo: H. Spero Montoya et al., 2002

  11. Study sites

  12. Thermocline nitrate vs. coretop foraminifera Ren et al., 2009

  13. ODP site 999A: Caribbean Sea Holocene LGM Ren et al., 2009 18O, age: Schmidt et al., 2004

  14. N fixation rate: LGM/Holocene ~ 20% Ren et al., 2009 POM: Altabet, 1988 NO3- : Knapp et al., 2005

  15. Deglacial increases in both denitrification and N fixation Chile: de Pol-Holz et al, 2006 SBB: Emmer and Thunell, 2000 AS: Altabet et al., 2002 Cariaco: Haug et al., 1998

  16. A stable N inventory regulated by N/P ratio? ~1P:16N    Fix! Don’t Fix! Gruber and Sarmiento, 1997

  17. Deglacial scenario:N fixation feedback through N/P sensitivity

  18. Summary • In Caribbean Sea sediments, planktonic foraminiferal 15N/14N decreases from the last ice age to the current interglacial. • The foraminiferal change is best explained by less N fixation in the Atlantic during the last ice age, leading to higher nitrate 15N/14N in the Caribbean thermocline. • The reconstructed increase in N fixation at the end of the last ice age is most likely a response to the previously recognized deglacial increase in global denitrification. • As with our findings regarding the modern Atlantic N fixation rate, this is consistent with a strong P control on N fixation. • A significant glacial/interglacial change in the nitrate reservoir has not been ruled out. However, our evidence that Atlantic N fixation acts as a negative feedback argues qualitatively for limits to such a change.

  19. Acknowledgement • Princeton University • Consortium of Ocean Leadership • Sigman Group: Franky Wang, Brigitte Brunelle, Julie Granger, Peter Difore. • J. Bernhard and D. McCorkle

  20. Nitrogen cycle

  21. Minimal Holocene 15N decrease in bulk sediment Ren et al., 2009 18O, age: Schmidt et al., 2004

  22. Foraminifera-bound N isotope analysis • Sieve, pick, crush • Clean fragments with wet oxidation • Acid dissolution to release internal Norg • Norg NO3- (persulfate oxidation) • NO3- N2O (denitrifier method) • N2O isotopic analysis (continuous flow, purge/trap, gas chromatography, gas-source magnetic sector mass spectrometry)

  23. Carbon inventory

  24. Fig. S1

  25. Fig. S2

  26. Thermocline nitrate/surface sediment comparison Altabet, 2005

  27. Downcore changes in 13Corg and Corg/TN

  28. Evidence for terrestrial and shelf material in ODP 999A sediments C/N~15 (by weight)  terrestrial organics

  29. Fig. S4

  30. Bulk sediment 15N records from the North Pacific Bulk sediment records from diverse settings: Mean ocean nitrate 15N did not decrease (much) into the Holocene. Kao et al., 2008

  31. Estimating N fixation change

  32. Incomplete nutrient consumption in the polar ocean Surface nitrate Surface chlorophyll

  33. Rapid surface-deep exchange in the polar ocean releases deeply sequestered CO2 to the atmosphere Eq. S

  34. Mechanisms for reducing the Southern Ocean CO2 leak

  35. The link between the N isotopes and nitrate consumption

  36. Subantarctic (E11-2) diatom-bound 15N/14N: Link to glacial/interglacial cycles • More complete nitrate consumption in glacial Subantarctic d18O + age: Ninneman and Charles, 1997

  37. Subantarctic is well situated fordust-driven iron fertilization during the ice ages modern annual average dust deposition (g m-2 yr-1) Mahowald et al., 2005

  38. Antarctic 15N/14N:higher during the last glacial maximum LGM

  39. Polar AA nutrient drawdown: More CO2 without complete deepwater formation shutdown Subantarctic nutrient drawdown: CO2  ~40 ppm AA deep water formation “off”: CO2 ~35 ppm Glacial/interglacial changes in the Southern Ocean inferred from the N isotopes

  40. Low and High latitude connection Lower rates of N fixation

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