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

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.)