Carbonate system buffering in the water masses of the Southwest Atlantic sector of the Southern Ocean during February-March 2008

BGD 8, 435–462, 2011 Carbonate system in the Southern Ocean in 2008 M. Gonz´ alez-D´ avila et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Biogeosciences Discuss., 8, 435–462, 2011 www.biogeosciences-discuss.net/8/435/2011/ doi:10.5194/bgd-8-435-2011 © Author(s) 2011. CC Attribution 3.0 License. Biogeosciences Discussions This discussion paper is/has been under review for the journal Biogeosciences (BG). Please refer to the corresponding final paper in BG if available. Carbonate system buffering in the water masses of the Southwest Atlantic sector of the Southern Ocean during February–March 2008 M. Gonz´ alez-D´ avila1, J. M. Santana-Casiano1, R. A. Fine2, J. Happell2, B. Delille3, and S. Speich4 1Departamento de Qu´ ımica, Facultad de Ciencias del Mar, Universidad de Las Palmas de Gran Canaria, 35017, Spain 2Rosenstiel School, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149-1098, USA 3Unit´ e d’Oceanographie Chimique, Astrophysics, Geophysics and Oceanography Department, University of Li` ege, All´ ee du 6 Aoˆ ut, 17 (Bˆ at B5), 4000 Li` ege, Belgium 4Laboratoire de Physique des Oceans (LPO), CNRS/IFREMER/UBO, Brest, France Received: 14 December 2010 – Accepted: 18 December 2010 – Published: 17 January 2011 Correspondence to: M. Gonz´ alez-D´ avila ([email protected]) Published by Copernicus Publications on behalf of the European Geosciences Union. 435 BGD 8, 435–462, 2011 Carbonate system in the Southern Ocean in 2008 M. Gonz´ alez-D´ avila et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Abstract Carbonate system variables were measured in the South Atlantic sector of the Southern Ocean along a transect from South Africa to the southern limit of the Antarctic Circumpolar Current (ACC) in February-March 2008. Eddies detach from retroflection of the Agulhas Current located north of the Subantarctic Front (SAF). The eddies in-5 crease the gradients observed at the fronts so that minima in fCO2and maxima in pH in situ on either side of the frontal zone are observed, while within the frontal zone fCO2 reached maximum values and pH in situ was a minimum. Mixing at the frontal zones, in particular where cyclonic rings were located, brought up CO2-rich water (low pH and high nutrient) that spread out the fronts where recent biological production favored by10 the nutrient input increases the pH in situ and decreases the fCO2levels. Vertical distributions of water masses were described by their carbonate system properties and their relationship to CFC concentrations. Upper Circumpolar Deep Water (UCDW) and Lower Circumpolar Deep Water (LCDW) had pHT,25 values of 7.56 and 7.61, respectively. UCDW also had higher concentrations of CFC-12 (>0.2 pmol kg−1)15 as compared to deeper waters, revealing the mixing with recently ventilated waters. Calcite and aragonite saturation states (Ω) were also affected by the presence of these two water masses with high carbonate concentration. Ωarag =1 was observed at 1000 m in the subtropical area and north of the SAF. At the position of the Polar front and under the influence of UCDW and LCDW Ωarag =1 deepen from 600 m to 1500 m20 at 50.37◦S, and it reaches to 700 m south of 57.5◦S. High latitudes are the most sensitive areas under future anthropogenic carbon increase. Buffer coefficients related to changes in [CO2], [H+] and Ωwith changes in CTand ATshowed the minimum values are found in the Antarctic Intermediate Water (AAIW), and UCDW layers. These coefficients suggest that a small increase in CTwill sharply decrease the pH and the25 carbonate saturation states. Here we present data that are used to suggest that south of 55◦S by the year 2045 surface water will be undersaturated in aragonite. 436 BGD 8, 435–462, 2011 Carbonate system in the Southern Ocean in 2008 M. Gonz´ alez-D´ avila et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 1 Introduction The Southern Ocean plays an important role in modulating the global climatic system by transporting and storing heat, fresh water, nutrients, and anthropogenic CO2(e.g., Lovenduski and Gruber, 2005). This region is predicted to be greatly influenced by global change, given that polar marine ecosystems are particularly sensitive to car-5 bonate change (Sarmiento et al., 1998; Orr et al., 2005). Since preindustrial times, ocean uptake of CO2have modified the chemistry of the ocean, lowered the pH and concentration of carbonate ion (CO2− 3) with the high latitudes one of the most affected areas (Caldeira and Wickett, 2003; Orr et al., 2005). Surface ocean pH levels have already been observed to have decreased by 0.1 units in the Southern Ocean (McNeil10 and Matear, 2007; Key et al., 2004) and are projected to decline to around 0.3 by the year 2100 (McNeil and Matear, 2008). Orr et al. (2005) predicted that the Southern Ocean will begin to experience aragonite under-saturation by the year 2050. On the other hand, a study based on a large-scale Southern Ocean observational analysis that considers the seasonal magnitude and variability of CO2− 3and pH, suggest that15 the Southern Ocean aragonite under-saturation will already happen by the year 2030 (McNeil and Matear, 2008). As the dissolution of anthropogenic carbon increases the total inorganic concentration of the surface waters, the buffer factors decrease resulting in a much greater sensitivity to local variations in total inorganic carbon and total alkalinity. The lowest buffer values have been observed in the Southern Ocean (Egleston20 et al., 2010), as this area is particularly sensitive to increasing CO2. The Southern Ocean is particularly efficient in ventilating deep and bottom waters (e.g., Toggweiler et al., 2006). Deep ventilation takes place south of the Polar Front (PF). There are clear links between the seasonal carbon dynamics and known areas of deep water ventilation and Antarctic Bottom Water (AABW) formation regions (Mc-25 Neil et al., 2007). These deep waters are rich in dissolved inorganic carbon, but are carbonate poor. The entrainment of these waters into the surface layers lowers the carbonate concentration considerably (McNeil and Matear, 2008). A recent study showed 437 BGD 8, 435–462, 2011 Carbonate system in the Southern Ocean in 2008 M. Gonz´ alez-D´ avila et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | the upwelling of carbonate depleted deep waters is the most dominant driver of wintertime carbon cycling, in comparison with the solubility or biological processes (McNeil et al., 2007). Physical processes such as deep water formation in the Weddell Sea, upwelling of deep water at the divergence zone and formation of intermediate water in the Antarctic Polar Zone (APZ) affect the carbon dioxide system parameters, which has5 consequences for the CO2air-sea fluxes. In this region where several frontal systems are observed, sharp gradients in temperature and salinity (Lutjeharms and Valentine, 1984; Belkin and Gordon, 1996) and important changes in the CO2air-sea exchange have been described (Bakker et al., 1997; Hoppema et al., 1995; Chierici et al., 2004; McNeil et al., 2007).10 In the framework of the BONUS-GoodHope project, the parameters of the carbonate system, pH, ATand CTwere measured in the Southwest Atlantic sector of the Southern Ocean (Fig. 1). The main objective of this work was to characterize the carbon system of the water masses, defining the buffer capacity and their sensitivity to the increase of CO2in the ocean.15 2 Data and methods The BONUS-GoodHope cruise took place on board of the French R/V Marion Dufresne in the Southwest Atlantic sector of the Southern Ocean in the region 33◦580S–57◦330S, 17◦130E–0◦E (Fig. 1). It started on 13 February 2008 in the shelf region of Cape Town, and was completed 17 March 2008. During the cruise full depth CTD data were done20 at 79 stations and samples were taken at 22 depths for the measurements of salinity, dissolved oxygen, nutrients, pH, ATand CT. Samples were collected for later laboratory analysis of two chlorofluorocarbons, CFC-11 and CFC-12. The three variables of the carbonate system were measured on board of the Marion Dufresne in order to achieve the highest level of data quality and resolution. The hydro-25 cast stations (78 stations plus station zero) were sampled for pH in total scale at 25 ◦C (pHT,25), total alkalinity (AT, in µmol kg−1) and total dissolved inorganic carbon (CT, in 438 BGD 8, 435–462, 2011 Carbonate system in the Southern Ocean in 2008 M. Gonz´ alez-D´ avila et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | µmol kg−1). There were a total of about 1639 bottles in hydrocast CTD stations at not repetitive depths, and in some cases samples were flagged. As a result, high quality data are available for pH of 1609 samples, 1559 for ATand 1504 for CT. 2.1 Sampling procedure 500 ml glass bottles were used for the analytical determination of both pH and AT.5 100 ml glass bottles were used to analyze CT. The bottles were rinsed twice with seawater and over-filled with seawater. Samples were shielded from the light and analysed between stations. In shallow stations and in case the samples could not be analyzed for CTin less than 5 h after sampling, they were poisoned with HgCl2(60 µl, saturated solution).10 2.2 pH measurements The pH was measured in total scale ([H+]T=[H+]F+[HSO− 4], where [H+]Fis the free proton concentration), pHTat a constant temperature of 25 ◦C. An automated system based on the spectrophotometric technique of Clayton and Byrne (1993) with m-cresol purple as indicator was used (Gonz´ alez-D´ avila et al., 2003).15 2.3 Total alkalinity measurements Samples for ATwere potentiometrically titrated with standarized 0.25 M HCl (0.45 M in NaCl) to the carbonic acid end point using a system described in detail in Mintrop et al. (2000). The titration of certified reference Material for Oceanic CO2, CRMs (#85) was used to test the performance of the titration system given values that were within20 ±1.1 µmol kg−1of the certified value. 439 BGD 8, 435–462, 2011 Carbonate system in the Southern Ocean in 2008 M. Gonz´ alez-D´ avila et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2.4 Total dissolved inorganic carbon measurements A VINDTA 3C system (Mintrop et al., 2000) (www.MARIANDA.com), with coulometer determination was used for the titration of the total dissolved inorganic carbon after phosphoric acid addition. The titration of CRMs (#85) was used to test the performance of the equipment after the preparation of each titration cell. A CRM was analysed ev-5 ery time a new titration cell for CTdetermination was prepared (1 a day), the total was 31. Results give a value of 1996.0 ±1.6 µmol kg−1for CT, while the certified value is 2000.4 ±0.4 µmol kg−1. A study done on board indicates that this difference is related to the temperature of determination of the CTthat in our case was 25 ◦C. Data have been corrected for this shift multiplying them by the factor 1.0022. Each CRM sample10 was also analysed for total alkalinity. The agreement between on board experimental data (NAT=2293.7 ±1.1) and the certified value (NAT=2293.7 ±0.8) indicates accurate HCl concentration and pipette volume for the titration system. 2.5 Calcite and aragonite saturation state The degree of saturation state of seawater with respect to calcite and aragonite was15 calculated as the ion product of the concentration of calcium and carbonate ions, at the in situ temperature, salinity and pressure divided by the stoichiometric solubility product (K∗ sp) for those conditions Ωcal =[Ca2+][CO2− 3]/K∗ sp,cal (1) Ωarg =[Ca2+][CO2− 3]/K∗ sp,arg (2)20 where the calcium concentration is estimated from the salinity, and the carbonate ion concentration is calculated from ATand CT, and computed by using CO2sys.xls v12 (Lewis and Wallace, 1998). 440 BGD 8, 435–462, 2011 Carbonate system in the Southern Ocean in 2008 M. Gonz´ alez-D´ avila et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2.6 CFC sampling and measurement 1191 samples (a mean of 18 samples per hydro casts) were collected from Niskin bottles. The samples (about 125 ml) were taken via Viton tubes connected to glass bottles with connectors. The bottles and caps were thoroughly rinsed with the water to be sampled. The bottles were filled and capped underwater in a 1 l beaker. At the5 University of Miami laboratory, water samples were analyzed for CFC-11 and CFC-12 using an extraction system and gas chromatograph following established procedures (Bullister and Weiss, 1988). Analytical uncertainties for CFC-11 and CFC-12 are each ±8%. Chemical structures of the two CFC gases are CCl3F for CFC-11, CCl2F2for CFC-12.10 3 Results and discussion 3.1 Surface distribution The region studied (Fig. 1) is divided in three main regimes, namely, the subtropical domain north of 40◦S–42◦S, the Antarctic Circumpolar Current (ACC) between 40◦S– 42◦S and 55◦S–57◦S, and the eastern part of the Weddell Sea gyre to the South15 (Park et al., 2001; Gladyshev et al., 2008). In this region several frontal systems have been described in a review by Orsi and Whitworth (2005), using as indicators potential temperature, θ, salinity and oxygen. These frontal zones are also defined by sharp changes in temperature and salinity, enhanced Chl-aconcentrations and reduced fCO2values (Smith and Nelson, 1986, 1990; Chierici et al., 2004; Laika et al.,20 2009). During the BONUS-GoodHope cruise, the expected trend of decreasing surface temperature towards the south was observed. This temperature gradient was correlated by a decrease in pHT,25, and an increase in the surface inorganic carbon total concentration CT(Fig. 2). 441 BGD 8, 435–462, 2011 Carbonate system in the Southern Ocean in 2008 M. Gonz´ alez-D´ avila et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | By using the data on Sea Surface Temperature (SST) and Sea Surface Salinity (SSS) from this work and the definitions of the characteristics of the major fronts South of Africa, the five major oceanic frontal structures have been identified and are marked in Fig. 2. In the subtropical domain, the Subtropical Front (STF) divides the warmer tropical waters and the colder subantarctic waters at 42◦20S. The area is then divided5 by the North Subtropical Front (N-STF) and the South Subtropical Front (S-STF). At the N-STF located at 38.1◦S, SST drops form 20.95 ◦C at 37.7◦S to 15.31 ◦C at 38.83◦S while SSS decreases from 35.52 to 34.6. Just North of the N-STF the cruise crossed a thin and narrow layer of warm and salty water from two Agulhas rings that were crossed at their boundaries covering 36◦S–38◦S (A2) and 35◦S (A1) and the influence10 of a cyclonic ring close to 36◦S (C1) which increased the gradient observed at the position and north of the N-STF. The fronts are thus strongly affected by the boundaries between the different eddies. The cyclonic structure C1 has been injected in the region from the African slope (from the Agulhas Banc) as it is proven by both by its tracking from satellite altimetry (Fig. 1) and its hydrologic characteristics (e.g., salinity and oxy-15 gen, Fig. 3). These features are common in the Cape Basin and come from the strong interaction between the Agulhas Current and slope and shelf waters in the Agulhas Banc (Boebel et al., 2003; Richardson, 2006). From 41.60◦S to 42.03◦S, at the S-STF, SST drops from 15.64 to 12.06 ◦C and SSS falls from 34.75 to 34.22. From 39.2◦S to 40.2◦S SST as high as 17 ◦C and SSS of20 35 are also found related to the influence of another Agulhas ring (A3), centered 40◦S, 14◦E (Fig. 1). The changes in both temperature and salinity also affected carbonate system variables that can also be used to clearly distinguish the presence of the fronts. At the N-STF, the pHTat 25 ◦C, pHT,25 shifted from 8.040 to 7.946. However, the presence of the Agulhas rings in the 36◦S–38◦S increased the pH from 7.97 at 36◦S25 to 8.040 all along 36.5◦S to 37.8◦S, at the position of the N-STF. At the S-STF, pHT,25 decreased from 7.948 to 7.887, a total change inside the STF of 0.15 pH units. Again, from 39.2◦S to 40.2◦S, the pH increased from 7.95 to 8.00, which follows the observed temperature increase. Strongly correlated with salinity is total alkalinity. ATdecreased 442 BGD 8, 435–462, 2011 Carbonate system in the Southern Ocean in 2008 M. Gonz´ alez-D´ avila et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | at the N-STF from 2334 to 2296 µmol kg−1. At the S-STF, ATdrops from 2294 µmol kg−1 at 41.60◦S to 2273 µmol kg−1at 42.03◦S. Also noticeable is the increase in CTat both locations with changes of 35 µmol kg−1at the N-STF area and around 20 µmol kg−1 at the southern front. After normalization to a constant salinity, NCTincreased by 90 µmol kg−1at the N-STF and 38 µmol kg−1at the S-STF.5 These important variations clearly indicate that mixing of deep rich CO2waters are taking place in this frontal area that, at least at the time of the cruise, overcompensating any reduction due to biological activity. An examination of Fig. 3, suggests a deepreaching character of the STF related to the presence of Agulhas rings detached from the retroflection of the Agulhas Current. Actually, it has been recently shown that these10 eddies define the position of the N-STF and S-STF (Dencausse et al., 2010). In the ACC domain four main fronts are identifiable. The Subantarctic Front (SAF) is located at 44◦20S. The SSS drops from 35.037 at 43◦190S to 33.93 at 44◦20S and SST falls from 13.74 to 9.74 ◦C, located just south of an old but still intense Agulhas Ring (M in Fig. 1). At these positions, pHT,25 sharply decreases 0.1 pH units15 from 7.938 to 7.839 (from 8.11 to 8.068 at in situ conditions), ATdrops from 2314 to 2265 µmol kg−1while CTincreases from 2070 to 2082 µmol kg−1. The presence of the old Agulhas Ring M, just north of the SAF, where strong mixing occurs, affected the surface inorganic carbon distribution. The PF was found at 50◦220S. There were not very pronounced surface temperature and salinity gradients. However, a significant20 pHT,25 gradient was observed at the front changing from 7.768 to 7.740. Total alkalinity increased by 7 µmol kg−1from 2280 µmol kg−1while CTincreased by 10 µmol kg−1 from 2130 µmol kg−1at 50◦220S to 2140 µmol kg−1at 50◦380S. A deep-reaching front observed to the south, the southern ACC front (SACCF), was located at 52◦390S. At this front, SST slightly decreases from 2.44 ◦C at 52◦360S to25 1.75 ◦C at 52◦550S while SSS increases from 33.705 to 33.742. The pHT,25 appears to show with higher definition the position of SACCF, the pH decreased from 7.716 to 7.696 as we moved southward. At these positions, CTincreases by 13 µmol kg−1from 443 BGD 8, 435–462, 2011 Carbonate system in the Southern Ocean in 2008 M. Gonz´ alez-D´ avila et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | are observed in the 1000–1500 m range north of the SAF, and approaching the 250 to 400 m range south of the PF. These minimum values are found in the layer where AAIW and UCDW are located. They clearly indicate that these water masses are particularly sensitive to increases in [CO2], and small increases of CTwill strongly decrease the pH and the carbonate saturation state.5 Mixing processes in the Southern Ocean bring up relatively low AT/CTwaters that mix with waters where biological production is not able to use up the macronutrients and to draw down the inorganic carbon. This results in low AT/CTratios. The pH and saturation state are also highly sensitive to changes in CTand ATin these waters, with values at the end of the austral summer of βH=0.36 mmol kg−1and Ω = 0.12 in10 surface waters. These are minimum values for both parameters in the ocean, and the chemistry of these surface waters becomes much more sensitive to local variations in both CTand AT. South of 55◦S, an increase in CTdue to the uptake of anthropogenic carbon of 10 µmol kg−1(∆AT=0) would increase the [CO2] by 7.1%, the [H+] by 6.4% (a pH15 decrease of 0.027 units) while the saturation state will decrease by 8.0%. Ωcal will decrease from an actual surface value of 2.35 ±0.06 to 2.16 ±0.07 while Ωarag will change from 1.47 ±0.04 to 1.35 ±0.05. These calculations clearly indicate that chemistry of surface water at high Southern Ocean latitudes will become highly sensitive to variations in CT(and in ATif a decrease in calcification takes place) due to increasing20 CO2under future climate change. A 10 µmol kg−1increase in CThas been reported for a period of only 10 years in the North Subtropical Atlantic at the ESTOC site (SantanaCasiano et al., 2007; Gonz´ alez-D´ avila et al., 2010). Assuming a similar rate of change south of Africa, by 2045 south of 55◦S surface water will be aragonite under-saturated, which is in line with model predictions (Orr et al., 2005; McNeil and Matear, 2008).25 These same surface waters will be transported equatorward and subduct into the thermocline near the SAF, forming the highly sensitive AAIW. Furthermore, the results presented in this work provide a basis for comparing the buffering capacity of the Southern Ocean to other oceans. 450 BGD 8, 435–462, 2011 Carbonate system in the Southern Ocean in 2008 M. Gonz´ alez-D´ avila et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 4 Conclusions The objective is to predict the evolution of the carbonate system in the ocean, to quantify the impact of high CO2on ocean chemistry and marine biology and to determine the consequences for our future climate. For this purpose, the distribution of carbonate system variables were measured in the Atlantic sector of the Southern Ocean during5 February 2008. The several frontal systems presented in the Southern Ocean have been characterized by considering surface pH variability. The frontal zones were defined by sharp changes in temperature and salinity, high Chl-aand accompanied by high pH in situ and minima in fCO2. These characteristics are the result of relatively recent biological activity favored by upwelling in the presence of cyclonic eddies de-10 tached from the Agulhas retroflection region. In other areas the pH and fCO2was controlled mainly by hydrography. Along the section, pHTin situ and fCO2presented two different mean values. North of the SAF a mean value of 8.102 ±0.014 with fCO2 of 335 ±5 µatm was observed while to the south pHTin situ was 8.069 ±0.008 and fCO2increased to 365 ±10 µatm.15 The carbonate properties are presented as a function of the different water masses found in the region. In the subtropical zone, the distribution of all properties is governed by deep anticyclonic and cyclonic features generated by the Agulhas Current System At the SAF, subduction of the South Atlantic variety of AAIW is well identified by the northward deepening of carbonate variables and elevated CFCs concentrations. At the20 Cape Basin area, it met the Indian AAIW injected with the Agulhas Rings, becoming saltier and warmer but also with higher content of inorganic carbon than that found at 45◦S. At greater depths the two NADW branches have been defined. The first one, corresponding to the eastern NADW pathway, with low CFC-12 concentration, <0.02 pmol kg−1. The second one, encompassing the APZ, is associated with NADW25 injected in the ACC in the south-western Argentine Basin with CFC-12 concentrations in the 0.08–0.1 pmol kg−1range. 451 BGD 8, 435–462, 2011 Carbonate system in the Southern Ocean in 2008 M. Gonz´ alez-D´ avila et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | AABW was also distinguished by slightly higher CFC-12 concentrations and low pHT,25 values, becoming older and diluted as it spreads northward to 36◦S. Two varieties of deep circumpolar waters were also distinguished. UCDW composed of low pHT,25 and oxygen; LCDW, characterized by high salinity and pHT,25. Both are characterized by maximum in CTconcentrations, attributed to the influence of old waters from5 the Indian and Pacific Oceans. The Southern Ocean is known to be very sensitive to climate change. Eight buffer indices related to changes in CTand ATon [CO2], [H+] and saturation state showed low values, thus is, low buffering capacity and highly sensitivity waters under future increase in CO2. The 1000–1500 m range north of the SAF, and the 250 to 400 m range10 south of the PF the lowest values. It corresponds to the water layer where AAIW and UCDW are located, being both of them and all along the section, particularly sensitive to increases in atmospheric CO2. Strong decreases in the pH and in the carbonate saturation state are shown under actual rate of change in the oceanic carbon dioxide scenario for the Southern Ocean surface seawater. It has being predicted that surface15 water south of 55◦S by the year 2045 will be undersaturated in aragonite. Acknowledgement. This research was carried out inside the French International Polar Year Program under the BONUS-GoodHope project. Carbon dioxide study was founded by the Spanish Ministry of Science under grant CGL2007-28899-E. We are grateful to the officers and crew on the R/V Marion Dufresne for making this experiment possible. The comments and20 helpful discussions to this paper by Michel Arhan are strongly acknowledged. 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L., and Forbes, A.: Ocean-ice shelf interaction and possible bottom15 water formation in Prydz Bay, Antarctica, Antarct. Res. Ser., 75, 173–187, 1998. 456 BGD 8, 435–462, 2011 Carbonate system in the Southern Ocean in 2008 M. Gonz´ alez-D´ avila et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Figure 1. 609 610 611 Fig. 1. Map showing the cruise track for the Southwest Atlantic sector of the Southern Ocean during the BONUS GoodHope 2008 cruise. The track is plotted over an altimetry image for the day 20 February and the fronts are identified: STF (Subtropical Front), SAF (Subantarctic Front), PF (Polar Front), SACCF (Southern ACC Front) and SBdy (Southern Boundary). Cyclonic (Ci) and anticyclonic (Aiand M) Agulhas rings are also marked. 457 BGD 8, 435–462, 2011 Carbonate system in the Southern Ocean in 2008 M. Gonz´ alez-D´ avila et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 612 613 614 Figure 2. 615 616 Fig. 2. (A) Sea surface temperature (SST), salinity (SSS) and pH in total scale at 25 ◦C, pHT,25, along the cruise track for samples analyzed in the upper 10 m. The figure shows the position of the major frontal zones during the BONUS GoodHope cruise. (B) Surface ocean Chlorophyll-a, partial pressure of CO2in seawater expressed as fugacity, fCO2,sw (µatm) and pH in total scale at in situ conditions, pHT,is. 458 BGD 8, 435–462, 2011 Carbonate system in the Southern Ocean in 2008 M. Gonz´ alez-D´ avila et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | (a) (b) (c) Figure 3 617 618 619 Fig. 3. Vertical distribution of (a) potential temperature ( ◦C), (b) salinity, (c) pH in total scale at 25 ◦C, pHT,25 (µmol kg−1), (d) AT(µmol kg−1), (e) CT(µmol kg−1) and (f) CFC-12 (pmol kg−1) along the Southwest Atlantic sector of the Southern Ocean during February–March 2008. Dots indicate locations of discrete samples. 459