Achievements and challenges in Southern Ocean CO 2 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 CO 2 sink # Deep carbon inventories # Air-sea CO 2 fluxes # Evolution of the Southern Ocean CO 2 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 Anthropogenic carbon in AABW (Antarctic Bottom Water)? Cant (mol m -2 )

How much C ant in AAIW and AABW? (Lo Monaco et al., JGR, 2005) Differences in C ant (µmol/kg) from the C 0 and  C* methods along 30°E CDW AAIW AABW

High C ant at the surface, especially north of 60°S and at the shelf where no sea-ice hampers the gas-exchange. Low C ant in deep and bottom water - close to the error of the method. (Hauck, Hoppema et al., in preparation) Accumulation of C ant in the Weddell Sea between 1992 and 2008 C ant accumulated in the Weddell Sea (1992 – 2008) - C T 2008 fitted as a function of θ, S, O 2 and p C T 1992 fitted as a function of θ, S, O 2 and p = Extended Multiple Linear Regression (eMLR)  Cant along 0°W (µmol/kg)

1.2) Air-sea CO 2 fluxes of surface water pCO 2 Poor seasonal coverage in surface water fCO 2 (Takahashi et al., 2009) Palmer PolarsternOISO

S.O. CO 2 sink (Pg C /yr) Global oceans (1990s, ) 0 2.2±0.5 Surface pCO 2 SAZ (STF-SAF, ~40-50°S) 2, PZ (SAF-PF) 2 <0.1 South of 50°S Atmospheric + ocean models South of ~45°S Pg = 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 pCO 2 at ice edge (Takahashi et al., 2009)

Monitoring fCO 2 with CARIOCA drifters Ocean CO 2 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 fCO 2 in the South Pacific Ocean from CARIOCA and shipboard data (Barbero et al., in preparation, 2009). Future: Quantify the effect of mesoscale activity on fCO 2 and DIC from CARIOCA and satellite data. SAF PF 6 source nk  fCO 2 (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 fCO 2 and DIC for all CARIOCA drifters. Sunset ~ ~ 0. 3  mol/kg/day 9 days (Nov-Dec 2006) in the Polar Zone; high fluorescence ~Gross Community Production-Respiration fCO 2 (  atm) DIC (  mol/kg) (Boutin, Merlivat et al., in revision, GRL, 2008)

Atmospheric CO 2 data and an ocean model suggest a reduction in the efficiency of the Southern Ocean CO 2 sink since Changes have been ascribed to an increase in wind speed. 1.3) Evolution of the Southern Ocean CO 2 sink Sea-air CO 2 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: µatm/yr Trend ocean: µatm/yr Decrease of ocean sink? -0.4 µatm/yr OISO Cruises (Metzl, 2009, DSR SOCOVV, in press) All data 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 Decrease in ”natural” carbon Total Carbon change (µmol/kg) Section around 70E: Comparing

What drives the observed variability of the carbon cycle in the Southern Ocean ? (Lenton et al., sub., 2008)

Oceanic CO 2 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 CO 2 Atlas) NEW surface pCO 2 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. Pfeil (

2) Process studies on interactions between physics, biology and the CO 2 sink Diffusivity (log (m 2 /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 CO 2 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

C11 A03 Natural iron fertilisation at the Kerguelen Plateau, (Blain et al., 2007; Jouandet et al., 2008) KEOPS/OISO-12, January 2005 fCO 2 (µatm)

Natural iron fertilisation at Crozet 8 November – 8 December 2004  fCO 2 (w-a) (µatm) SAF Crozet Plateau Upstream (South): Little effect of marine biota on surface water fCO 2. Downstream (North): Large phytoplankton blooms lower fCO 2 by 70 µatm. Chlorophyll (mg/m 3 ) November 2004 (Bakker et al., 2007)

1 M-P. Jouandet et al., Deep-Sea Res. II, 2008, 55, D. Bakker et al., Deep-Sea Res. II, 2007, 54, 2174 SGeorgia Crozet Kerguelen NASA SeaWiFS project, DAAC/GSFC, ORBIMAGE Island blooms vs HNLC bloom stations HNLC stations 1 2 Blooms are 2-3 times as productive as HNLC waters and are large CO 2 sinks. (Jones et al., see poster)

Entrainment creates high fCO 2 and DIC below sea ice in the Weddell Gyre Below sea ice:  fCO 2 (w-a) 0 to +40 µatm in December Upward movement of Warm Deep Water in the Weddell Gyre creates high fCO 2 and DIC below the winter ice. The ice prevents outgassing of CO 2 (Bakker et al., 2008, Biogeosciences). Dissolved inorganic carbon (µmol/kg), 17-23°E WDW

Rapid reduction of surface water fCO 2 during and upon ice melt Brown ice, 17-20/12/02 Below ice:  fCO 2 (w-a) 0 to 40 µatm Upon melt:  fCO 2 (w-a) -50 to 0 µatm Biological carbon uptake rapidly creates a CO 2 sink during and upon ice melt. The importance of ice-related 08-10/12/ /12/2004 0°W Surface fCO 2 decrease during ice melt 17/12/2004 (%) Sea ice cover CaCO 3 processes is not clear. The Weddell Gyre may be an annual CO 2 sink. (Bakker et al., 2008) This supports the role of Antarctic sea ice on glacial-interglacial CO 2 variations (Stephens and Keeling, 2000). (Bakker et al., 2008, Biogeosciences)

Role of ikaite in sea ice Ikaite CaCO 3.6H 2 O in sea ice (Dieckman et al., 2008) Ikaite precipitates along brine channels during ice formation, thus increasing fCO 2. Ikaite dissolves during/upon ice melt, thus reducing fCO 2.

Turbulent CO 2 fluxes (g CO 2 m -2 d -1 ) by eddy correlation in December 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). Sink Source CO 2 uptake by multi-year sea ice in the western Weddell Sea

-8°C > T° -8°C < T° < -5°CT° < -5°C CO 2(g) CO 2(aq) Brine sinking entrain produced CO 2 below the pycnocline while CaCO 3 remain trapped within sea ice CaCO 3 + H 2 O + CO 2 CO 2(g) 2 HCO Ca 2 2+ CaCO 3 + H 2 O + CO 2 2 HCO Ca 2 2+ CO 2 uptake by biology at both top and bottom of sea ice Semiletov et al. 2004, 2007 Zemmelink et al Zemmelink et al Delille et al Rysgaard et al Papadimitriou et al Dieckmann et al.2008 Graphics by Bruno Delille Sea ice: Ikaite and biological carbon uptake

CO 2(g) CaCO 3 + H 2 O + CO 2 2 HCO Ca ) Measurement of air-ice CO 2 fluxes by micro-meterological methods 2) Sea ice processes should be addressed by ice-coring and related analysis 3) Impact of precipitation of CaCO 3 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 CO 2 chemistry

Effect of ocean acidification on the CO 2 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)

Achievements Significant progress has been made on quantifying Southern Ocean CO 2 uptake in the CarboOcean era. New topics have emerged, notably the evolution of the Southern Ocean CO 2 sink and the role of sea ice. Challenges: I) Quantify the evolution of the Southern Oceanic CO 2 sink Sustained observations of surface fCO 2, deep carbon transport and atmospheric CO 2 Identify the best method(s) for quantification of anthropogenic carbon Quantify C ant in Antarctic Bottom and Intermediate Water II) Assess the processes driving (changes in) oceanic CO 2 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 pCO 2 w Evolution of the oceanic CO 2 sink with / without an O 3 hole in a coupled carbon climate model

CO 2 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 CO 2 gradients in snow on top of sea ice in the western Weddell Sea Over slushOver solid ice } In snow