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 –CO 2 effects –Anthropogenic N supply

3. (i) OMZs expand. Do they? south eq.Pac. OMZ expands (Stramma et al., 2008)  mol kg -1 yr -1 obs. 300dbar O 2 change 1960-2010 (Stramma et al., BGD 2012) Observations mostly suggest O 2 decline –Particularly in the tropics, including the OMZs(?) –Trend? Oscillation?

4. What do the models say? Global mean O 2 declines Average decline: few  M over 21 st century (Bopp et al., GBC 2002)

5. What do the models say? Global mean O 2 declines Average decline: few  M over 21 st century (Bopp et al., GBC 2002)

6. What do the models say? O 2 declines (except in the tropics?) Keeling et al.: Ocean deoxygenation in a warming world. (Ann.Rev.Mar.Sci.2010)  Tropical O 2 increase likely an artifact caused by excessive mixing (Matear & Hirst, GBC 2003)

7. Global O 2 decline is relatively insensitive to mixing What do the models say? rel. change (%) global ocean O 2 TIME increasing k v SRES A2 emission scenario 0.05 cm 2 s -1 0.5 cm 2 s -1 (Duteil & Oschlies, 2011)

8. What do the models say? (Duteil & Oschlies, 2011) rel. change (%) subox. Vol. TIME increasing k v SRES A2 CO 2 emission scenario 0.05 cm 2 s -1 0.5 cm 2 s -1 SFB 754 tracer release exp., N.Atl.OMZ 0.15 cm 2 s -1 0.10 cm 2 s -1 global ocean O 2 -2.5% BUT: evolution of suboxia is sensitive to k v EP MOC

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

10. CO 2 & ballast Higher CO 2  less CaCO 3 ballast  shallower remineralization  enhanced OMZs (Hofmann & Schellnhuber, 2009) without ballast effect with ballast effect Simulated A.D.3000 O 2

11. CO 2 & stoichiometry C:N=const. C:N=f(pCO 2 ) 50% increase in suboxic volume (<5mmol/m 3 ) Mesocosm results (Riebesell et al., 2007) (Oschlies et al., 2008)

12. Conclusions (i) Evolution of suboxic volume is sensitive to –diapycnal mixing –zonal tropical mixing –CO 2 -dependent ballast effect –CO 2 -dependent C:N stoichiometry –……

13. Conclusions (i) Evolution of suboxic volume is sensitive to –diapycnal mixing –Zonal tropical mixing –CO 2 -dependent ballast effect –CO 2 -dependent C:N stoichiometry –…… So what?

14. Data! O 2 changes 1960 – 2010, 300dbar All models simulate O 2 increase in tropical thermocline!  mol kg -1 yr -1

15. Data! O 2 changes 1960 – 2010, 300dbar All models simulate O 2 increase in tropical thermocline!  mol kg -1 yr -1

16. Data! O 2 changes 1960 – 2010, 300dbar “All” models simulate O 2 increase in tropical thermocline!  mol kg -1 yr -1

17. Zonally averaged O 2 change (1960-2010) obs BCCR IPSL UBernMPI UVic Simulated and observed O 2 changes are anticorrelated!

18. Conclusions (ii): OMZs – do they expand? Observations suggest “yes”, most models say “no”. Current models cannot reproduce observed tropical O 2 changes very well.  Currently, I would bet on “yes”.

19. (ii) Marine N 2 O emissions increase. Do they? Extrapolated from past observations Expected decrease of export production  decrease in nitrification & N 2 O production? here: look at possible temperature effects

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

21. PO 4 in 2 specially designed models TEMPNOTEMPWOA Atlantic Pacific Indian O. RMS=0.138 mmol m -3 RMS=0.157 mmol m -3 (Taucher & Oschlies, 2011) skill not significantly different

22. Simulated evolution of PP and EP (Taucher & Oschlies, 2011) EP NOTEMP TEMP EP, V subox similar for TEMP and NOTEMP PP increases in run TEMP PP pCO 2 V subox

23. Simulated evolution of PP Faster remineralisation (more heterotrophic ocean) may support higher levels of PP! What about N 2 O? (Behrenfeld, 2011) N 2 O ?

24. What about N 2 O? NOTEMP TEMP N 2 O according to Suntharalingam et al. (2000) (here allow for N 2 O 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 N 2 O, CH 4, DMS,… fluxes

26. (iii) More N supply (N 2 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 NO 3 levels –Often need “tricks” to avoid model OMZs running out of NO 3

27. N 2 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? (Landolfi et al., subm.) Stoichiometry: each mole N org denitrified uses up ~7 moles of NO 3 (e.g., Paulmier et al., BG, 2009)

29. The more you fix the more you lose? N 2 fix controlN 2 fix DOM run (Landolfi et al., subm.) Simulated N inventory, Starting from WOA, No N 2 fix, only denitr.

30. The more you fix the more you lose? N 2 fix controlN 2 fix DOM run (Landolfi et al., subm.) Simulated N inventory, Starting from WOA, No N 2 fix, only denitr. Control IRON

31. The more you fix the more you lose? (Landolfi et al., subm.) Stoichiometry: each mole N org denitrified uses up ~7 moles of NO 3 (e.g., Paulmier et al., BG, 2009)

32. The more you fix the more you lose? N 2 fix controlN 2 fix DOM run (Landolfi et al., subm.) Simulated N inventory, Starting from WOA, No N 2 fix, only denitr. Control IRON DOM

33. The more you deposit the more you lose? Relative amount of N loss per N gain (WOA O 2, Martin curve) Atmospheric N deposition (mmol m -2 yr -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? Cumulative N deposition (corresponding to A.D. 2000) realized N increase: ~30% of N supply Response of model’s diazotrophs denitrification 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 N 2 fixation in/near OMZs? Current models of N 2 fixation seem to couple N 2 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 IPCC business-as-usual (SRES A2) until year 2100 Linear decrease to zero emissions in year 2300, zero emissions thereafter pCO 2 ocean SAT

37. More O 2 in a warmer ocean!? abiotic O 2 (~Ar) declines by ~6% (solubility) O 2 increases by ~8% Biology must be main driver (even though EP increases)! abiotic O 2 O2O2

38. What’s the source of the extra O 2 ? O 2 air-sea flux O2O2 O 2 increase AND continuous O 2 outgassing! (~10Tmol/yr)

39. What’s the source of the extra O 2 ? O 2 increase AND continuous O 2 outgassing! (~10Tmol/yr) Exact magnitude depends on where the H 2 S is oxidized. O 2 air-sea flux O2O2 anaerobic remin.  H 2 S

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 N 2 fixation appears particularly challenging Stay tuned for more surprises, better understanding and better models.

43. Conundrum: More O 2 in a future warmer ocean? Biogeochemical models predict O 2 decline –so far mostly for 21 st century –idealised models for some 100,000 years

44. Multi-millennia global warming Primary Production, + 60%Export Production, + 8% suboxic Volume, + 220% mean O 2, + 8%anoxic Volume, + 3300%

45. Conculsions Expect more surprises to come Thank you!

46. NO 3 A C B D WOA09

47. NO 3 A C B D WOA09

48. Larger V subox by enhanced zonal mixing? Alternating zonal jets – net effect similar to zonal mixing?  Sensitivity experiment with enhanced zonal mixing in the tropics k x =1200 m 2 /s k x =51,200 m 2 /s k x =21,200 m 2 /s Change in simulated suboxic volume sensitive to tropical zonal mixing! (J. Getzlaff, pers.comm.)