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
(i) OMZs expand. Do they? south eq.Pac. OMZ expands (Stramma et al., 2008) mol kg -1 yr -1 obs. 300dbar O 2 change (Stramma et al., BGD 2012) Observations mostly suggest O 2 decline –Particularly in the tropics, including the OMZs(?) –Trend? Oscillation?
What do the models say? Global mean O 2 declines Average decline: few M over 21 st century (Bopp et al., GBC 2002)
What do the models say? Global mean O 2 declines Average decline: few M over 21 st century (Bopp et al., GBC 2002)
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)
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 cm 2 s -1 (Duteil & Oschlies, 2011)
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 cm 2 s -1 SFB 754 tracer release exp., N.Atl.OMZ 0.15 cm 2 s cm 2 s -1 global ocean O % BUT: evolution of suboxia is sensitive to k v EP MOC
“Most” models predict decline of V subox (V.Cocco, pers.comm.)
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
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)
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 –……
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?
Data! O 2 changes 1960 – 2010, 300dbar All models simulate O 2 increase in tropical thermocline! mol kg -1 yr -1
Data! O 2 changes 1960 – 2010, 300dbar All models simulate O 2 increase in tropical thermocline! mol kg -1 yr -1
Data! O 2 changes 1960 – 2010, 300dbar “All” models simulate O 2 increase in tropical thermocline! mol kg -1 yr -1
Zonally averaged O 2 change ( ) obs BCCR IPSL UBernMPI UVic Simulated and observed O 2 changes are anticorrelated!
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”.
(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
Consensus on temperature effects? (Steinacher et al., 2010) “All” models show a decrease in primary production Primary Production
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
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
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 ?
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)
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
(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
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.)
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)
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.
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
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)
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
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
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.
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
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
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
What’s the source of the extra O 2 ? O 2 air-sea flux O2O2 O 2 increase AND continuous O 2 outgassing! (~10Tmol/yr)
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 O2O2 anaerobic remin. H 2 S
Conclusions Stay tuned for more surprises, better understanding and better models.
Thank you!
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.
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
Multi-millennia global warming Primary Production, + 60%Export Production, + 8% suboxic Volume, + 220% mean O 2, + 8%anoxic Volume, %
Conculsions Expect more surprises to come Thank you!
NO 3 A C B D WOA09
NO 3 A C B D WOA09
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.)