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Sorting out possible scenarios Sorting out possible scenarios about the future of oxygen about the future of oxygen
minimum zone systemsminimum zone systems
Andreas OschliesGEOMAR, Kiel, Germany
SFB 754
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Outline
• Uncertainties about the future of OMZs associated with:
– Mixing & transport
– Temperature effects on metabolic rates
– CO2 effects
– Anthropogenic N supply
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(i) OMZs expand. Do they?south eq.Pac. OMZ expands
(Stramma et al., 2008)
mol kg
-1yr -1
obs. 300dbar O2 change 1960-2010
(Stramma et al., BGD 2012)• Observations mostly suggest O2 decline
– Particularly in the tropics, including the OMZs(?)– Trend? Oscillation?
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What do the models say? • Global mean O2 declines
Average decline: few M over 21st century
(Bopp et al., GBC 2002)
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What do the models say? • Global mean O2 declines
Average decline: few M over 21st century
(Bopp et al., GBC 2002)
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What do the models say? • O2 declines (except in the tropics?)
Keeling et al.: Ocean deoxygenation in a warming world. (Ann.Rev.Mar.Sci.2010)
Tropical O2 increase likely an artifact caused by excessive mixing
(Matear & Hirst, GBC 2003)
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Global O2 decline is relatively insensitive to mixing
What do the models say?re
l. ch
ange
(%) g
loba
l oce
an O
2
TIME
increasing kv
SRES A2 emission scenario
0.05 cm2s-1
0.5 cm2s-1
(Duteil & Oschlies, 2011)
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What do the models say?
(Duteil & Oschlies, 2011)
rel.
chan
ge (%
) sub
ox. V
ol.
TIME
increasing kv SRES A2CO2 emissionscenario
0.05 cm2s-1
0.5 cm2s-1
SFB 754 tracer release exp., N.Atl.OMZ
0.15 cm2s-1
0.10 cm2s-1
global ocean O2
-2.5%
BUT: evolution of suboxia is sensitive to kv
EP
MOC
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“Most” models predict decline of Vsubox
(V.Cocco, pers.comm.)
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CO2 & ballast
Higher CO2
less CaCO3 ballast shallower
remineralization enhanced OMZs
(Hofmann & Schellnhuber, 2009)
without ballast effect
with ballast effect
Simulated A.D.3000 O2
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CO2 & stoichiometry
C:N=const.C:N=f(pCO2)
50% increasein suboxicvolume(<5mmol/m3)
Mesocosm results
(Riebesell et al., 2007)
(Oschlies et al., 2008)
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Conclusions (i)
• Evolution of suboxic volume is sensitive to
– diapycnal mixing
– zonal tropical mixing
– CO2-dependent ballast effect
– CO2-dependent C:N stoichiometry
– ……
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Conclusions (i)
So what?
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Data!O2 changes
1960 – 2010,300dbar
All models simulate O2 increase in tropical thermocline!
mol kg-1yr-1
mol kg-1yr-1
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Data!O2 changes
1960 – 2010,300dbar
All models simulate O2 increase in tropical thermocline!
mol kg-1yr-1
mol kg-1yr-1
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Data!O2 changes
1960 – 2010,300dbar
“All” models simulate O2 increase in tropical thermocline!
mol kg-1yr-1
mol kg-1yr-1
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Zonally averaged O2 change (1960-2010)
obs
BCCR
IPSL
UBern MPIUVic
Simulated and observed O2 changes are anticorrelated!
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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”.
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(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
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Consensus on temperature effects?
(Steinacher et al., 2010)
“All” models show a decrease in primary production
Primary Production
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PO4 in 2 specially designed modelsTEMP NOTEMP WOA
AtlanticPacific
Indian O.
RMS=0.138 mmol m-3 RMS=0.157 mmol m-3
(Taucher & Oschlies, 2011)skill not significantly different
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Simulated evolution of PP and EP
(Taucher & Oschlies, 2011)
EP NOTEMP
TEMP
• EP, Vsubox similar for TEMP and NOTEMP• PP increases in run TEMP
PP
pCO2 Vsubox
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Simulated evolution of PP
• Faster remineralisation (more heterotrophic ocean) may support higher levels of PP!
• What about N2O?
(Behrenfeld, 2011)
N2O ?
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What about N2O?
NOTEMP
TEMP
N2O according to Suntharalingam et al. (2000)
(here allow for N2O production below z=50m)
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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
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(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
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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.)
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The more you fix the more you lose?
(Landolfi et al., subm.)
Stoichiometry: each mole Norg denitrified uses up ~7 moles of NO3
(e.g., Paulmier et al., BG, 2009)
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The more you fix the more you lose?
N2 fix control N2 fix DOM run
(Landolfi et al., subm.)
Simulated N inventory,Starting from WOA,No N2 fix, only denitr.
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The more you fix the more you lose?
N2 fix control N2 fix DOM run
(Landolfi et al., subm.)
Simulated N inventory,Starting from WOA,No N2 fix, only denitr.ControlIRON
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The more you fix the more you lose?
(Landolfi et al., subm.)
Stoichiometry: each mole Norg denitrified uses up ~7 moles of NO3
(e.g., Paulmier et al., BG, 2009)
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The more you fix the more you lose?
N2 fix control N2 fix DOM run
(Landolfi et al., subm.)
Simulated N inventory,Starting from WOA,No N2 fix, only denitr.ControlIRONDOM
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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
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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.
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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
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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
pCO2
ocean <T>
SAT
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More O2 in a warmer ocean!?
• abiotic O2 (~Ar) declines by ~6% (solubility)
• O2 increases by ~8%
• Biology must be main driver (even though EP increases)!
abiotic O2
O2
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What’s the source of the extra O2?
O2 air-sea flux
O2
• O2 increase AND continuous O2 outgassing! (~10Tmol/yr)
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What’s the source of the extra O2?
• O2 increase AND continuous O2 outgassing! (~10Tmol/yr)
• Exact magnitude depends on where the H2S is oxidized.
O2 air-sea flux
O2
anaerobic remin. H2S
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Conclusions
• Stay tuned for more surprises, better understanding and better models.
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Thank you!
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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.
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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
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Multi-millennia global warming
Primary Production, + 60% Export Production, + 8%
suboxic Volume, + 220%
mean O2, + 8%anoxic Volume, + 3300%
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Conculsions
Expect more surprises to come
Thank you!
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NO3
A
C
B
D
WOA09
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NO3
A
C
B
D
WOA09
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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=1200 m2/s
kx=51,200 m2/s
kx=21,200 m2/s
Change in simulated suboxic volume sensitive to tropical zonal mixing!
(J. Getzlaff, pers.comm.)