Achievements and challenges in Southern Ocean CO2 research

Dorothee Bakker, Mario Hoppema, Marta Alvarez, Leticia Barbero, Nina Bednarsek, Richard Bellerby, Jacqueline Boutin, Melissa Chierici, Bruno Delille, Judith Hauck, Oliver Huhn, Elisabeth Jones, Andrew Lenton, Nicolas Metzl, Claire Lo Monaco, Benjamin Pfeil, Aida Riós, Henk Zemmelink, ....

Funded by EU CarboOcean and / or national funding bodies

Achievements and Challenges in:

The Southern Ocean CO2 sink# Deep carbon inventories# Air-sea CO2 fluxes# Evolution of the Southern Ocean CO2 sink

Process studies # Upwelling, subduction, mixing# Iron supply# Sea ice# Ocean acidification

Southern Ocean: part of the Meridional Overturning Circulation

(Open University)

North Atlantic Southern Ocean

Exchange of heat, elements and momentum between the deep ocean and the atmosphere.

1.1) Inventory of anthropogenic carbon

(Hanawa and Talley, 2001; Sabine et al., 2004)

Antarctic Intermediate Water (AAIW)

Subantarctic Mode Water (SAMW)

• 20 Pg C or 5% of anthropogenic carbon in SAMW and AAIW in 1994.• Anthropogenic carbon in AABW (Antarctic Bottom Water)?

Cant (mol m-2)

How much Cant in AAIW and AABW?

(Lo Monaco et al., JGR, 2005)

Differences in Cant (µmol/kg) from the C0 and C* methods along 30°E

CDW

AAIW

AABW

• High Cant at the surface, especially north of 60°S and at the shelf where no sea-ice hampers the gas-exchange.

• Low Cant in deep and bottom water - close to the error of the method.

(Hauck, Hoppema et al., in preparation)

Accumulation of Cant in the Weddell Sea between 1992 and 2008

Cant accumulated in the Weddell Sea

(1992 – 2008)-

CT2008 fitted as a

function of θ, S, O2 and p

CT1992 fitted as a

function of θ, S , O2 and p

= Extended Multiple Linear Regression (eMLR)

Cant2008-1992 along 0°W (µmol/kg)

1.2) Air-sea CO2 fluxes

of surface water pCO2

Poor seasonal coverage in surface water fCO2 (Takahashi et al., 2009)

PalmerPolarsternOISO

S.O. CO2 sink (Pg C /yr)Global oceans (1990s, 2000-05)0 2.2±0.5

Surface pCO2

SAZ (STF-SAF,~40-50°S) 2, 4 0.8-1.1PZ (SAF-PF)2 <0.1South of 50°S 0.063-0.44 Atmospheric + ocean modelsSouth of ~45°S5 0.3-0.6

Pg = 1015 g (0 – Denman et al., 2007; 2- Metzl et al., 1999; Boutin et al., 2008; 3 - Takahashi et al., 2009; 5 – Baker et al., 2006; Gruber et al., 2003 at ICDC7; 4 -McNeil et al., 2007)

(Takahashi et al., 2009)

• The ’circumpolar sink zone’ in the Subantarctic Zone (SAZ).• High pCO2 at ice edge

(Takahashi et al., 2009)

Monitoring fCO2 with CARIOCA drifters

• Ocean CO2 sinks of 0.8 Pg C / yr in the SAZ and <0.1 Pg C / yr in the PZ from CARIOCA data since 2001 (Boutin et al., L&O, 2008).

• Assess the effect of SAMW formation on fCO2 in the South Pacific Ocean from CARIOCA and shipboard data (Barbero et al., in preparation, 2009).

• Future: Quantify the effect of mesoscale activity on fCO2 and DIC from CARIOCA and satellite data.

SAF

PF

6

sourcenkfCO2(water -air) (µatm)

oceanic sink

Estimating NCP with CARIOCA drifters

Strong diurnal cycle allows estimation of net community production (NCP) from CARIOCA data.

Future: Provide estimates of NCP from the diurnal cycle in fCO2 and DIC for all CARIOCA drifters.

CARIOCA - 29/11/06 to 8/12/06

365

370

375

380

385

390

395

400

405

410

29/11 30/11 1/12 2/12 3/12 4/12 5/12 6/12 7/12 8/12

local time

fCO

2 (m

icro

atm

)

2110

2112

2114

2116

2118

2120

2122

2124

DIC

(m

icro

mo

l/kg

)

fCO2

fCO2_atm

DICSunset

~ <Net community production>~ 0. 3 mol/kg/day

9 days (Nov-Dec 2006) in the Polar Zone; high fluorescence

~Gross Community Production-Respiration

fCO

2 (

atm

)

DIC

(m

ol/

kg)

(Boutin, Merlivat et al., in revision, GRL, 2008)

Atmospheric CO2 dataand an ocean modelsuggest a reduction in the efficiency of the Southern OceanCO2 sink since 1980.

Changes have been ascribed to an increase in wind speed.

1.3) Evolution of the Southern Ocean CO2 sink

Sea-air CO2 flux anomaly (Pg C/yr)

Le Quéré et al., Science, 2007

More upta-ke

Less upta-ke

+ pulse model

Model, constant winds

Model, observed winds

Trend atmosphere: + 1.7 µatm/yrTrend ocean: + 2.1 µatm/yr

Decrease of ocean sink? -0.4 µatm/yr

OISO Cruises

trend = + 2,11 (0.07) µatm/yr

250

270

290

310

330

350

370

390

410

430

450

1990 1992 1994 1996 1998 2000 2002 2004 2006 2008

Year

fCO

2s (µ

atm

)

All 1°x1° average data in the South Indian (20°S-69°S, 30°E-90°E)

(Metzl, 2009, DSR SOCOVV, in press)All data 1991-2008 in SOCAT and CDIAC

A decrease of the S.O. sink?

Decadal changes of natural and anthropogenic carbon

Anthropogenic carbon at 500m in the late 1990’s

(Lo Monaco et al., sub., 2008)

Anthropogenic Carbon change (µmol/kg)

DIC < 0, Cant > 0Decrease in ”natural” carbon

Total Carbon change (µmol/kg)

Section around 70E: Comparing 2000-1985

What drives the observed variability of the carbon cycle in the Southern Ocean ?

(Lenton et al., sub., 2008)

Oceanic CO2 sink - ongoing

Falkland Islands

South Georgia

SCOTIA SEA

Cruises in Scotia Sea on JCR since 2006 (Poster Jones et al.)

• Data synthesis in CARINA and SOCAT (Surface Ocean CO2 Atlas)

• NEW surface pCO2 VOS on RRS James Clark Ross (Hardman-Mountford, Jones, et al.) and FS Polarstern (Hoppema, Neil et al.)

• Hydrographic sections with carbon and tracers

SOCAT version 1, d.d. 21/11/2008, @Benjamin Pfeil

(http://www.ioccp.org/)

2) Process studies on interactions between physics, biology and the CO2 sink

Diffusivity (log (m2/s))Scotia Sea (Naveira-Garabato et al., 200x)

• Enhanced mixing and upwelling over steep topography, iron supply, and occurrence of blooms. • Mesoscale dynamics and eddies;• Entrainment of CDW below ice;• Preconditioning of CO2 before subduction;• Sea ice dynamics.

(Naveira-Garabato et al., 2007; Solokov and Rintoul, 2007; Blain et al., 2007; Bakker et al., 2007, 2008; Boutin et al., 2008)

Marine productivity and sea ice

NASA SeaWiFS project, DAAC/GSFC, ©ORBIMAGE.

Winter Summer

SGeorgia

Crozet

Kerguelen

0

50

100

150

200

250

300

350

400

450

500

0 0.1 0.2 0.3 0.4 0.5 0.6

DFe (nmol L-1)

Dep

th (m

)

A3

C11

C11

A03

Natural iron fertilisation at the Kerguelen Plateau,

(Blain et al., 2007; Jouandet et al., 2008)

KEOPS/OISO-12, January 2005

fCO2 (µatm)

Natural iron fertilisation at Crozet

8 November – 8 December 2004

fCO2(w-a) (µatm)

SAF

Crozet Plateau

Upstream (South): Little effect of marine biota on surface water fCO2.

Downstream (North): Large phytoplankton blooms lower fCO2 by 70 µatm.

Chlorophyll (mg/m3)

6040 5045 55

14 - 18 November 2004

(Bakker et al., 2007)

1 M-P. Jouandet et al., Deep-Sea Res. II, 2008, 55, 856 2 D. Bakker et al., Deep-Sea Res. II, 2007, 54, 2174

SGeorgia Crozet

Kerguelen

NA

SA

Sea

WiF

S p

roje

ct,

DA

AC

/GS

FC

, O

RB

IMA

GE

Island blooms vs HNLC

bloom stations

HNLC stations

12

Blooms are 2-3 times as productive as HNLC waters and are large CO2 sinks.

(Jones et al., see poster)

Entrainment creates high fCO2 and DIC below sea ice in the Weddell Gyre

Below sea ice: fCO2(w-a) 0 to +40 µatm in December 2002.

Upward movement of Warm Deep Water in the Weddell Gyre creates high fCO2 and DIC below the winter ice. The ice prevents outgassing of CO2 (Bakker et al., 2008, Biogeosciences).

Dissolved inorganic carbon (µmol/kg), 17-23°E

WDW

Rapid reduction of surface water fCO2 during and upon ice melt

Brown ice, 17-20/12/02

Below ice: fCO2(w-a) 0 to 40 µatmUpon melt: fCO2(w-a) -50 to 0 µatm

Biological carbon uptake rapidly creates a CO2 sink during and upon ice melt. The importance of ice-related

08-10/12/2004

20/12/2004

0°W Surface fCO2 decrease during ice melt

17/12/2004

(%)

Sea ice cover

CaCO3 processes is not clear. The Weddell Gyre may be an annual CO2 sink. (Bakker et al., 2008)

This supports the role of Antarctic sea ice on glacial-interglacial CO2 variations (Stephens and Keeling, 2000).

(Bakker et al., 2008,Biogeosciences)

Role of ikaite in sea ice

Ikaite CaCO3.6H2O in sea ice (Dieckman et al., 2008)

Ikaite precipitates along brine channels during ice formation, thus increasing fCO2. Ikaite dissolves during/upon ice melt, thus reducing fCO2.

Turbulent CO2 fluxes (g CO2 m-2 d-1) by eddy correlation in December 2004.

Total carbon uptake by the multi-year ice zone of the western Weddell Sea could have been 6.6 Tg C y-1 in December 2004 (Zemmelink, 2005).

Day of the year

335 340 345 350 355 360 365

CO

2 f

lux

(g

m-2

d-1

)

-1,4

-1,2

-1,0

-0,8

-0,6

-0,4

-0,2

0,0

0,2

0,4

Sink

Source

CO2 uptake by multi-year sea ice in the western Weddell Sea

-8°C > T° -8°C < T° < -5°C T° < -5°C

CO2(g)

CO2(aq)

Brine sinking entrain produced CO2 below the pycnocline while

CaCO3 remain trapped within sea ice

CaCO3 + H2O + CO2

CO2(g)

2 HCO3- + Ca2

2+

CaCO3 + H2O + CO2

2 HCO3- + Ca2

2+

CO2 uptake by biology

at both top and bottom of sea ice

Semiletov et al. 2004, 2007Zemmelink et al. 2006

Zemmelink et al. 2006

Delille et al. 2007

Rysgaard et al. 2007

Papadimitriou et al. 2004Dieckmann et al.2008

Graphics by Bruno Delille

Sea ice: Ikaite and biological carbon uptake

CO2(g)

CaCO3 + H2O + CO2

2 HCO3- + Ca2

2+

1) Measurement of air-ice CO2 fluxes by micro-meterological

methods

2) Sea ice processes should be addressed by ice-coring and

related analysis

3) Impact of precipitation of CaCO3 to the water column can be addressed by TA profiles and

specific reanalysis of TA/DIC profiles

Slide by Bruno Delille

Future studies of the role of sea ice in CO2 chemistry

Effect of ocean acidification on the CO2 sink?• A more acid ocean reduces the carbonate concentration and calcification.

• Models predict that the Southern Ocean will become undersaturated for aragonite by 2050 in the IS92a scenario (Orr et al., 2005).

• The importance of calcifying organisms for the Southern Ocean carbon cycle is poorly known.

Abundance of the pteropod Limacina helicina in the Scotia Sea (Nina Bednarsek et al., 2008; poster)

AchievementsSignificant progress has been made on quantifying Southern Ocean CO2 uptake in the CarboOcean era. New topics have emerged, notably the evolution of the Southern Ocean CO2 sink and the role of sea ice.

Challenges:I) Quantify the evolution of the Southern Oceanic CO2 sink• Sustained observations of surface fCO2, deep carbon transport and atmospheric CO2

• Identify the best method(s) for quantification of anthropogenic carbon• Quantify Cant in Antarctic Bottom and Intermediate Water

II) Assess the processes driving (changes in) oceanic CO2 uptake:• Iron supply, • Sea ice,• Entrainment, mixing, subduction, pre-conditioning,• Marine productivity,• Ocean acidification.

Conclusions

(Lenton et al., sub., 2008)Model: IPSL-LOOP-CM4

DpCO2 --

CO2uptake --

(more wind)

Less uptake

Higher pCO2w

Evolution of the oceanic CO2 sink with / without an O3 hole

in a coupled carbon climate model

Day of the year335 340 345 350 355

pCO

2 (p

pmv)

364

366

368

370

372

374

376

378

Atmosphere0.45 m0.35 m0.25 m0.15 m

Day of the year340 345 350 355 360

A B

CO2 concentrations (ppmv) • in the atmosphere at 0.85 m from the ice and • in snow, as a function of distance from the ice surface. (Zemmelink)

Vertical CO2 gradients in snow on top of sea ice in the western Weddell Sea

Over slush Over solid ice

} In snow