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Sorting out possible scenarios about the future of oxygen minimum zone systems

Sorting out possible scenarios about the future of oxygen minimum zone systems. Andreas Oschlies GEOMAR, Kiel, Germany. SFB 754. Outline. Uncertainties about the future of OMZs associated with: Mixing & transport Temperature effects on metabolic rates CO 2 effects Anthropogenic N supply.

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Sorting out possible scenarios about the future of oxygen minimum zone systems

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  1. Sorting out possible scenarios about the future of oxygen minimum zone systems Andreas Oschlies GEOMAR, Kiel, Germany SFB 754

  2. Outline • Uncertainties about the future of OMZs associated with: • Mixing & transport • Temperature effects on metabolic rates • CO2 effects • Anthropogenic N supply

  3. (i) OMZs expand. Do they? south eq.Pac. OMZ expands obs. 300dbar O2 change 1960-2010 • Observations mostly suggest O2 decline • Particularly in the tropics, including the OMZs(?) • Trend? Oscillation? mmol kg-1yr-1 (Stramma et al., 2008) (Stramma et al., BGD 2012)

  4. What do the models say? • Global mean O2 declines (Bopp et al., GBC 2002) Average decline: few mM over 21st century

  5. What do the models say? • Global mean O2 declines (Bopp et al., GBC 2002) Average decline: few mM over 21st century

  6. What do the models say? • O2 declines (except in the tropics?) (Matear & Hirst, GBC 2003) Keeling et al.: Ocean deoxygenation in a warming world. (Ann.Rev.Mar.Sci.2010)  Tropical O2 increase likely an artifact caused by excessive mixing

  7. What do the models say? Global O2 decline is relatively insensitive to mixing SRES A2 emission scenario increasing kv rel. change (%) global ocean O2 0.5 cm2s-1 0.05 cm2s-1 TIME (Duteil & Oschlies, 2011)

  8. What do the models say? BUT: evolution of suboxia is sensitive to kv increasing kv SRES A2 CO2 emission scenario global ocean O2 0.5 cm2s-1 -2.5% MOC rel. change (%) subox. Vol. 0.15 cm2s-1 EP 0.10 cm2s-1 SFB 754 tracer release exp., N.Atl.OMZ 0.05 cm2s-1 TIME (Duteil & Oschlies, 2011)

  9. “Most” models predict decline of Vsubox (V.Cocco, pers.comm.)

  10. CO2 & ballast Higher CO2 • less CaCO3 ballast • shallower remineralization • enhanced OMZs without ballast effect Simulated A.D.3000 O2 with ballast effect (Hofmann & Schellnhuber, 2009)

  11. CO2 & stoichiometry Mesocosm results C:N=const. C:N=f(pCO2) 50% increase in suboxic volume (<5mmol/m3) (Riebesell et al., 2007) (Oschlies et al., 2008)

  12. Conclusions (i) • Evolution of suboxic volume is sensitive to • diapycnal mixing • zonal tropical mixing • CO2-dependent ballast effect • CO2-dependent C:N stoichiometry • ……

  13. Conclusions (i) So what? • Evolution of suboxic volume is sensitive to • diapycnal mixing • Zonal tropical mixing • CO2-dependent ballast effect • CO2-dependent C:N stoichiometry • ……

  14. Data!O2 changes 1960 – 2010,300dbar mmol kg-1yr-1 mmol kg-1yr-1 All models simulate O2 increase in tropical thermocline!

  15. Data!O2 changes 1960 – 2010,300dbar mmol kg-1yr-1 mmol kg-1yr-1 All models simulate O2 increase in tropical thermocline!

  16. Data!O2 changes 1960 – 2010,300dbar mmol kg-1yr-1 mmol kg-1yr-1 “All” models simulate O2 increase in tropical thermocline!

  17. Zonally averaged O2 change (1960-2010) IPSL UVic UBern MPI obs BCCR Simulated and observed O2 changes are anticorrelated!

  18. Conclusions (ii):OMZs – do they expand? • Observations suggest “yes”, most models say “no”. • Current models cannot reproduce observed tropical O2 changes very well.  Currently, I would bet on “yes”.

  19. (ii) Marine N2O emissions increase.Do they? • Extrapolated from past observations • Expected decrease of export production • decrease in nitrification & N2O production? • here: look at possible temperature effects

  20. Consensus on temperature effects? Primary Production “All” models show a decrease in primary production (Steinacher et al., 2010)

  21. PO4 in 2 specially designed models TEMP NOTEMP WOA Atlantic Pacific Indian O. RMS=0.138 mmol m-3 RMS=0.157 mmol m-3 skill not significantly different (Taucher & Oschlies, 2011)

  22. Simulated evolution of PP and EP PP EP NOTEMP TEMP pCO2 Vsubox • EP, Vsubox similar for TEMP and NOTEMP • PP increases in run TEMP (Taucher & Oschlies, 2011)

  23. Simulated evolution of PP N2O ? (Behrenfeld, 2011) • Faster remineralisation (more heterotrophic ocean) may support higher levels of PP! • What about N2O?

  24. What about N2O? TEMP NOTEMP N2O according to Suntharalingam et al. (2000) (here allow for N2O production below z=50m)

  25. Temperature effects on metabolic rates • Well known, in principle (van’t Hoff, 1884; Arhenius, 1889; Eppley, 1972) • Little attention wrt biogeochemical impacts • could change sign of predicted changes in • primary production • N2O, CH4, DMS,… fluxes

  26. (iii) More N supply (N2 fixation, dust) increases marine N inventory.Does it? • Something fishy is going on in modeled OMZs • Models generally have too large OMZs with too low NO3 levels • Often need “tricks” to avoid model OMZs running out of NO3

  27. N2 fix and N loss closely coupled? • Geochemical estimates and models say “yes” • Appealing: could support balanced N budget (Deutsch et al., 2007) (Landolfi et al., subm.)

  28. The more you fix the more you lose? Stoichiometry: each mole Norg denitrified uses up ~7 moles of NO3 (e.g., Paulmier et al., BG, 2009) (Landolfi et al., subm.)

  29. The more you fix the more you lose? Simulated N inventory, Starting from WOA, No N2 fix, only denitr. N2 fix control N2 fix DOM run (Landolfi et al., subm.)

  30. The more you fix the more you lose? Simulated N inventory, Starting from WOA, No N2 fix, only denitr. Control IRON N2 fix control N2 fix DOM run (Landolfi et al., subm.)

  31. The more you fix the more you lose? Stoichiometry: each mole Norg denitrified uses up ~7 moles of NO3 (e.g., Paulmier et al., BG, 2009) (Landolfi et al., subm.)

  32. The more you fix the more you lose? Simulated N inventory, Starting from WOA, No N2 fix, only denitr. Control IRON DOM N2 fix control N2 fix DOM run (Landolfi et al., subm.)

  33. The more you deposit the more you lose? Relative amount of N loss per N gain (WOA O2, Martin curve) Atmospheric N deposition (mmol m-2yr-1), A.D.2000 (59TgN/yr; Duce et al., 2008). 12% of atmospheric N supply may be lost via denitrification

  34. The more you deposit the more you lose? denitrification Cumulative N deposition (corresponding to A.D. 2000) Response of model’s diazotrophs realized N increase: ~30% of N supply N deposition reduces ecological niche of model’s diazotrophs.

  35. Conclusion (iii)More N supply increases N inventory. Does it? • Location, location, location… • Destabilizing effects of N2 fixation in/near OMZs? • Current models of N2 fixation seem to couple N2 fix and N loss too closely • vicious cycle and runaway N loss • Spatial decoupling of N sources and sinks is needed for balanced N inventory

  36. Multi-millennial response to business as usual SAT • IPCC business-as-usual (SRES A2) until year 2100 • Linear decrease to zero emissions in year 2300, zero emissions thereafter pCO2 ocean <T>

  37. More O2 in a warmer ocean!? O2 • abiotic O2 (~Ar) declines by ~6% (solubility) • O2 increases by ~8% • Biology must be main driver (even though EP increases)! abiotic O2

  38. What’s the source of the extra O2? • O2 increase AND continuous O2 outgassing! (~10Tmol/yr) DO2 O2 air-sea flux

  39. What’s the source of the extra O2? anaerobic remin.  H2S • O2 increase AND continuous O2 outgassing! (~10Tmol/yr) • Exact magnitude depends on where the H2S is oxidized. DO2 O2 air-sea flux

  40. Conclusions • Stay tuned for more surprises, better understanding and better models.

  41. Thank you!

  42. Conclusions • Current models cannot reliably reproduce observed patterns & past changes • Transport (mixing), direct temperature effects • Biogeochemical feedback processes • Modeling N2 fixation appears particularly challenging • Stay tuned for more surprises, better understanding and better models.

  43. Conundrum:More O2 in a future warmer ocean? • Biogeochemical models predict O2 decline • so far mostly for 21st century • idealised models for some 100,000 years

  44. Multi-millennia global warming Primary Production, + 60% Export Production, + 8% suboxic Volume, + 220% anoxic Volume, + 3300% mean O2, + 8%

  45. Conculsions Expect more surprises to come Thank you!

  46. WOA09 NO3 A B C D

  47. WOA09 NO3 A B C D

  48. Larger Vsubox by enhanced zonal mixing? • Alternating zonal jets – net effect similar to zonal mixing? • Sensitivity experiment with enhanced zonal mixing in the tropics kx=51,200 m2/s kx=1200 m2/s kx=21,200 m2/s (J. Getzlaff, pers.comm.) Change in simulated suboxic volume sensitive to tropical zonal mixing!

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