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

Outline

• Uncertainties about the future of OMZs associated with:

– Mixing & transport

– Temperature effects on metabolic rates

– CO2 effects

– Anthropogenic N supply

(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?

What do the models say? • Global mean O2 declines

Average decline: few M over 21st century

(Bopp et al., GBC 2002)

What do the models say? • Global mean O2 declines

Average decline: few M over 21st century

(Bopp et al., GBC 2002)

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)

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)

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

“Most” models predict decline of Vsubox

(V.Cocco, pers.comm.)

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

CO2 & stoichiometry

C:N=const.C:N=f(pCO2)

50% increasein suboxicvolume(<5mmol/m3)

Mesocosm results

(Riebesell et al., 2007)

(Oschlies et al., 2008)

Conclusions (i)

• Evolution of suboxic volume is sensitive to

– diapycnal mixing

– zonal tropical mixing

– CO2-dependent ballast effect

– CO2-dependent C:N stoichiometry

– ……

Conclusions (i)

So what?

Data!O2 changes

1960 – 2010,300dbar

All models simulate O2 increase in tropical thermocline!

mol kg-1yr-1

mol kg-1yr-1

Data!O2 changes

1960 – 2010,300dbar

All models simulate O2 increase in tropical thermocline!

mol kg-1yr-1

mol kg-1yr-1

Data!O2 changes

1960 – 2010,300dbar

“All” models simulate O2 increase in tropical thermocline!

mol kg-1yr-1

mol kg-1yr-1

Zonally averaged O2 change (1960-2010)

obs

BCCR

IPSL

UBern MPIUVic

Simulated and observed O2 changes are anticorrelated!

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”.

(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

Consensus on temperature effects?

(Steinacher et al., 2010)

“All” models show a decrease in primary production

Primary Production

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

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

Simulated evolution of PP

• Faster remineralisation (more heterotrophic ocean) may support higher levels of PP!

• What about N2O?

(Behrenfeld, 2011)

N2O ?

What about N2O?

NOTEMP

TEMP

N2O according to Suntharalingam et al. (2000)

(here allow for N2O 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• N2O, CH4, DMS,… fluxes

(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

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.)

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)

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.

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

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)

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

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

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 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

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

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

What’s the source of the extra O2?

O2 air-sea flux

O2

• O2 increase AND continuous O2 outgassing! (~10Tmol/yr)

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

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 N2 fixation appears particularly challenging

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

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

Multi-millennia global warming

Primary Production, + 60% Export Production, + 8%

suboxic Volume, + 220%

mean O2, + 8%anoxic Volume, + 3300%

Conculsions

Expect more surprises to come

Thank you!

NO3

A

C

B

D

WOA09

NO3

A

C

B

D

WOA09

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.)