4.1. eMLR Results: Anthropogenic Carbon
 Cant1992–2008 calculated by eMLR with the variables θ, S, O2, and p is exhibited for the “backward case” (eMLR with 2008 data, Figure 4a) and the “forward case” (eMLR with 1992 data, Figure 4b). There are two regions with high Cant1992–2008: at the shelf, penetrating down to about 1000 m, and between 56.5°S and 60°S. The backward and forward cases differ only by the maximum surface Cant1992–2008 concentrations. In the surface layer Cant1992–2008 concentrations vary between 2 and 11 μmol kg−1 for the backward case and between 5 and 8 μmol kg−1 for the forward case. At the shelf, between 5 and 8 μmol kg−1Cant1992–2008 are dissolved in the ocean. The vertical distribution shows a distinct Cant1992–2008 minimum between 58°S and 63.5°S in the WDW around 500 m. This represents WDW that has been circulating within the Weddell Gyre and is characterized by lower temperature and salinity due to mixing with waters above and below. WDW that has more recently entered the Weddell Gyre in the east is found in the southern part of the gyre (64–69°S) at a depth of 300–750 m and carries Cant1992–2008 of about 2 μmol kg−1. The source water of this recent WDW in the ACC can be distinguished at the northern end of the section between about 300 and 1000 m.
Figure 4. Cant1992–2008 (μmol kg−1) eMLR results: (a) “backward case” with ANT XXIV/2 data and variables θ, S, O2, and p and (b) “forward case” with ANT X/4 data and variables θ, S, O2, and p.
Download figure to PowerPoint
 Only very little Cant1992–2008 was found to be stored in the deep and bottom layers of the Weddell Sea (Figures 4a and 4b). In the central Weddell Sea, Cant1992–2008 concentrations ranged between 1 and 2 μmol kg−1 between 1500 and 4000 m with values below 1 μmol kg−1 near the bottom. These values are so low that they are close to the uncertainty of the method. Low values in deep Weddell waters have been reported earlier by Poisson and Chen , Hoppema et al. [2001b], and Sabine et al. , which support our results. Despite the low level of Cant1992–2008, a well-structured distribution of it can be discerned even in the intermediate and deep water (Figures 4a and 4b) which complies with the current knowledge of the hydrography. This is an additional indication that the level of Cant1992–2008 and its distribution, though very low, is significant.
 It is somewhat surprising that the Cant1992–2008 concentration in the WSDW is higher than that in the bottom water. Traditionally, the main ventilation pathway of deep Weddell water has been thought to be the formation of WSBW and its subsequent mixing up into the deep water body. In that case, a Cant1992–2008 maximum would be expected in the WSBW. The fact that Cant1992–2008 in the WSDW is higher suggests that the WSDW is also ventilated by a mechanism not involving the WSBW. Hoppema et al. [2001a] have reported significant ventilation of the deep Weddell Gyre by a core of water with a CFC maximum at 3000–4000 m, entering the gyre from east along the continental slope. Further to the west in the Weddell Sea, traces of this water mass were found in most parts of the basin. Via this mechanism Cant1992–2008 is also transferred into the deep gyre. At about 69°S, a Cant1992–2008 maximum is found, hugging the continental slope at about 3000 m depth, the same location where the CFC maximum is usually observed [Klatt et al., 2002].
 At about 59°S, there is a core of WSBW which has been relatively recently ventilated in the western Weddell Sea [Klatt et al., 2002], which does have a Cant1992–2008 maximum of about 1–2 μmol kg−1 (Figure 4b), that is, higher than Cant1992–2008 of the WSBW found in the central and southern basins. The explanation for this difference is as follows. Significant Cant uptake takes place by recently formed WSBW. WSBW, which has been circulating in the bottom layer of the gyre for longer times already, as found in the central and southern basins, did not yet absorb that much Cant because of the lower pCO2 in the atmosphere at the time of formation.
 Between June 1992 and December 2007 the partial pressure of CO2 in the atmosphere at the South Pole rose by 26.8 μatm (Table 4) [Keeling et al., 2001]. Using typical AT, S, T, PO4, and SiO4 values from the uppermost layer of ANT XXIV/2 data to convert pCO2 to CT reveals that this atmospheric pCO2 increase should result in a CT increase of 10.3–10.9 μmol kg−1 assuming complete CO2 equilibration and no mixing. This is consistent with the Cant1992–2008 surface values, which are about 11 μmol kg−1. However, these high values occur only between 56.5°S and 60°S. This southern boundary exactly reflects the extent of the sea ice in the Weddell Sea, which reached as far as 60°S during ANT XXIV/2, curtailing equilibration with the atmosphere. High Cant1992–2008 in shelf water is transported to the Prime Meridian as part of the coastal current. The occurrence of polynyas along the coast allow for a better air-sea equilibration [Hoppema and Anderson, 2007], which leads to a higher Cant1992–2008 indeed (Figures 4a and 4b).
 On average, 0.06 μmol kg−1 of Cant1992–2008 have been accumulated per year in WSBW (below 4000 m) between 1992 and 2008. This agrees fairly well with Hoppema et al. [2001b] who found 0.02–0.04 μmol kg−1 a−1 for the period 1973–1998. As regards to the latter estimate it should be realized that during the 1970s and 1980s the Cant uptake has been less than that in more recent years because of the lower level of Cant in the atmosphere.
 The results of this study do not agree with Cant results by Lo Monaco et al. [2005a, 2005b]. As Lo Monaco et al. [2005b] focus on Cant at the Indian-Atlantic boundary of the Southern Ocean, AABW outflow of the Weddell Sea is taken into account. On the basis of a back-calculation technique [Körtzinger et al., 1998], they provided two estimates: one assuming that oxygen in surface waters is equilibrated with the atmosphere and the other one presuming that due to sea ice, surface waters are mostly undersaturated with respect to atmospheric oxygen. Although the former option yields Cant values of 8–10 μmol kg−1 in the deep water of the Weddell Sea, the latter results in Cant accumulation of 22–23 μmol kg−1, prompting questions about the role of the oxygen disequilibrium. High Cant values for WSDW were also found by the “Tracer Combining Oxygen, Inorganic Carbon and Total Alkalinity” (TrOCA) method [Lo Monaco et al., 2005a]. These values appear to be too high in comparison with the low Cant1992–2008 concentrations we found in the deep Weddell Sea. As our method gives a more direct estimate of Cant using data, opposite to back-calculating techniques, we think our results are more reliable. This holds true also for the high estimation of carbon inventories [Vázquez-Rodríguez et al., 2009] on the same transect as Lo Monaco et al. [2005a]. Rather high inventories might be forced by back-calculation techniques, which introduce errors by using a poorly known air-sea CO2 equilibrium [Matsumoto and Gruber, 2005].
 The amount of anthropogenic CO2 stored per kg of seawater is, in any case, less than that in the North Atlantic. Friis et al.  reported an increase in Cant of 1–20 μmol kg−1 below 100 m in the subpolar North Atlantic between 1981 and 1997. Even below 2000 m they estimated excess CO2 of up to 15 μmol kg−1. This is anticipated as extensive deep and bottom water formation processes are known to occur in the North Atlantic in ice-free regions. About 17 Sv of deep water are formed in the North Atlantic, a similar amount as in the entire Southern Ocean (14 Sv) of which only a part is ventilated in the Weddell Sea [Orsi et al., 2002]. Additionally, the large extent of sea ice in the Weddell Sea hampers air-sea CO2 exchange, surface water CO2 concentrations get diluted by mixing with intermediate and deep waters with little anthropogenic CO2, and the residence time for nascent bottom water is too short for atmospheric CO2 to penetrate into the water as extensively as in the North Atlantic [Poisson and Chen, 1987; Hoppema et al., 2001b; Lee et al., 2003].
4.2. Acidification of the Weddell Sea
 Acidification is expressed by oceanic pH decrease. Alternatively, this is done by the decreasing CO32− concentration and saturation states of calcite (ΩC) and aragonite (ΩA). The change in pH (ΔpH) between 1992 and 2008 based on the eMLR Cant1992–2008 calculation is shown in Figure 5a. The same patterns are seen as in Cant1992–2008, reflecting water mass structures and the remote source from the east. In the core of the WDW and in WSBW, minimal pH change with a decrease of only 0.002 units is observed. In WSDW, a decrease of up to 0.005 pH units is found. Near the shelf, ΔpH exceeds −0.02 units, even down to 500 m, and in the uppermost layer north of 60°S a drop of 0.03 pH units occurs.
 The theoretical Weddell Sea surface layer pH change due to atmospheric pCO2 increase is about −0.03 in the period considered, again assuming complete CO2 equilibration and no mixing. Values in this range are only found north of 60°S. The theoretical values were not reached in areas where sea ice inhibits gas exchange.
4.2.1. Change in Carbonate Ion Concentration
 If the CO2 concentration changes, the carbonate ion concentration changes as well as a result of shifts in the equilibrium (see equation (1)). There is a decrease in [CO32−] between 3 and 4.5 μmol kg−1 at the shelf, up to 7 μmol kg−1 at the surface north of 60°S and below 1 μmol kg−1 in the deep sea (Figure 5b). The average decrease of carbonate ions is 2.1 μmol kg−1, representing a reduction of 2.5% compared to 1992 in the entire water column.
 In the Southern Ocean, surface layer [CO32−] concentrations are naturally low compared with global concentrations. Current average [CO32−] varies between 105 μmol kg−1 in the Southern Ocean and 240 μmol kg−1 in tropical regions [Orr et al., 2005]. The ANT XXIV/2 data from 2008, however, show even lower values with an average of 95.9 μmol kg−1 in the upper 100 m (range: 80.6–126.0 μmol kg−1). On top of that, Southern Ocean [CO32−] decreases seasonally by about 15 μmol kg−1 in winter as a result of lower temperatures [Orr et al., 2005]. We find a decrease at the surface of 3.9 μmol kg−1 or 4.0% due to anthropogenic CO2 invasion. Seasonal variations may occur in addition to our estimate. The annual rate of [CO32−] reduction in the surface layer is 0.25 μmol kg−1. By comparison, according to Orr et al.  modern surface [CO32−] in the Southern Ocean has decreased by 18 μmol kg−1 from preindustrial levels, which is converted to 0.07 μmol kg−1 between 1750 and 1994. As expected, because of nonlinear CO2 increase during this period, this demonstrates that the rate of [CO32−] decline has significantly accelerated.
 The annual rate of decline that we calculated is at the lower end of Orr et al.'s  predictions for the next decades. In their prediction with the IS92a scenario (“business-as-usual,” atmospheric CO2 reaches 550 ppm in 2050), the Southern Ocean surface [CO32−] becomes about 70 μmol kg−1 in 2050, i.e., a mean reduction of approximately 0.5 μmol kg−1 per year between 2000 and 2050. This is twice the annual reduction observed in our study. If we extrapolate the annual rate of 0.25 μmol kg−1 a−1 to the year 2050 linearly, a carbonate concentration of 85.4 μmol kg−1 would be reached. This is higher than all predictions in all scenarios used by Orr et al. , even the moderate scenario. The linear extrapolation may be problematic because it is not constrained, but it is worthwhile noting that also in the paper of Orr et al.  all predictions (except for scenario A1FI) between 2000 and 2050 are approximately linear. Also, Zeebe and Wolf-Gladrow  (their Figure 1.6.27) demonstrate that a decline in [CO32−] based on the IS92a scenario is almost linear. Therefore, the calculations made above are not significantly biased by the assumption of a linear reduction of [CO32−] based on IS92a. It should be noted that Raupach et al.  and Canadell et al. (http://www.globalcarbonproject.org/carbonbudget/index.htm, 2007) report that CO2 emissions are already higher than the worst-case scenario A1FI. A steepening of the [CO32−] reduction is hence likely to happen around the year 2030 [Orr et al., 2005], but the estimates of the [CO32−] decrease of IS92a and A1FI do not start to deviate significantly from each other before 2050.
 The reduction of [CO32−] in Weddell Sea surface waters is lower than that in the averaged Southern Ocean as estimated from global biogeochemical circulation models [Orr et al., 2005]. The Weddell Sea seems to react differently to increasing atmospheric CO2 than the mean Southern Ocean surface waters. This discrepancy may be explained by averaging over the entire Southern Ocean, by insufficient spatial resolution of global models in the Southern Ocean, by underestimating the large effect of sea-ice in the Weddell Sea or by neglecting biological feedback mechanisms in the models. We underline the importance of regional studies in the Southern Ocean to determine spatial deviations of the mean response of the Southern Ocean to increasing atmospheric CO2.
4.2.2. Changes in the Saturation States of Calcite and Aragonite
 The spatial distribution of changes in ΩC (Figure 6a) and ΩA (Figure 6b) corresponds closely to that of the parameters discussed in sections 4.1 and 4.2.1. The decrease in ΩA amounts to about two thirds of the decrease in ΩC. Note that ΩC dropped by up to 0.18 at the surface north of 60°S, 0.1 at the shelf and 0.01 in WSDW. Near the seafloor and in the WDW the reduction was weaker, between 0 and 0.01. Likewise, ΩA was reduced by 0.09 at the surface north of 60°S and 0.06 at the shelf, whereas reduction was below 0.01 in WSDW and below 0.005 in WDW and WSBW. On average, ΔΩC was −0.05 and ΔΩA −0.03.
Figure 6. Acidification of the Weddell Gyre based on eMLR calculations: (a) ΔΩC and (b) ΔΩA. White isolines show saturation index during ANT XXIV/2.
Download figure to PowerPoint
 The future projection of ΩA was estimated by an extrapolation, which was conducted as follows: ANT XXIV/2 data were used as starting point and to each data point the product of the annual ΔΩA (as calculated at this latitude, longitude, and depth) and the time difference in years to 2008 was added. This leads to
 The number of data points of ΩA < 1 in the upper 20 m divided by the total number of data points in the upper 20 m gives the percentage of surface area undersaturated with respect to aragonite. This extrapolation assumes a constant linear decrease of [CO32−] (discussed above), and constant temperature and salinity distributions. This is a qualitative estimate about the change of ΩA as a consequence of the accumulation of Cant; seasonal variations will occur on top of that. It is therefore only a rough estimate, but it shows that average Weddell Sea surface waters will not be completely undersaturated (90%) with aragonite at the end of the 21st century. Although a few single spots of permanent aragonite undersaturation in the surface layer may occur earlier, only a small part of the surface will be constantly undersaturated by 2100 (Figure 7). Temporary undersaturation in winter may occur earlier [McNeil and Matear, 2008].
Figure 7. Acidification of the Weddell Gyre based on eMLR calculations: qualitative estimation of percentage of Weddell Sea surface waters undersaturated with respect to aragonite until 2100.
Download figure to PowerPoint
 Nowadays ΩA is largest at the surface and decreases with depth (Figure 8a). An interesting feature is that the reduction of ΩA is strongest at the surface. Hence, ΩA could fall below 1 in the surface layer whereas the WDW below is still saturated with aragonite (Figure 8b). This is different from other regions of the world oceans.
Figure 8. Acidification of the Weddell Gyre based on eMLR calculations: (a) ΩA in 2008 and (b) possible scenario for calculated ΩA in 2100 showing that undersaturation may appear at the surface while intermediate water masses are still saturated with aragonite.
Download figure to PowerPoint
4.3. The CO2 Saturation State of the Weddell Sea
 ΔpCO2 was calculated for 2008 from CT and alkalinity, while for 1992 it necessarily had to be obtained from the hydrographic parameters for 2008 and CT1992 = CT2008 − Cant1992–2008. Thus, changes in ΔpCO2, i.e., ΔΔpCO2, are the result only of Cant1992–2008 and changes in atmospheric pCO2. In 1992, the Weddell Gyre was undersaturated with CO2 in almost all areas, except in the 3°W transect south of 67.5°S (Figure 9a). It should be pointed out again that ΔpCO21992 shows how the situation would have been in 1992 if CT changed as calculated based on the eMLR analysis. ΔpCO22008 varied between −103 and +9 μatm, with a mean value of −42.5 μatm (Figure 9b). Thus, there was undersaturation of CO2 in almost the entire Weddell Sea, also reported for the summertime Weddell Sea by Hoppema et al. . Saturation was observed in the western transect near the shelf. As discussed in section 3.1, despite the generally robust eMLR estimates, errors tend to be largest at the surface. Furthermore, variations in pCO2 can be high indeed at the surface and even higher at the shelf. Both, undersaturation and local supersaturation have been reported previously for the Weddell Gyre [Hoppema et al., 1995, 2000; Bellerby et al., 2004]. Gibson and Trull  mentioned whole year undersaturation for another coastal region, Prydz Bay in East Antarctica.
Figure 9. The difference between surface ocean pCO2 and atmospheric pCO2, ΔpCO2 (μatm): (a) in 1992 (calculated with eMLR), (b) in 2008 (calculated from measured data), (c) the change in ΔpCO2 between 1992 and 2008 ΔΔpCO21992–2008 (μatm), and (d) the change in ΔpCO2 between 1992 and 2008 ΔΔpCO21992–2008 (μatm) as obtained by using the data of Le Quéré et al. . Only the top data point of each station is used.
Download figure to PowerPoint
 If no change in the oceanic uptake rate for CO2 would occur, ΔΔpCO21992–2008 would be zero. Note that ΔΔpCO2 shows mostly negative values, i.e., a trend to stronger undersaturation from 1992 to 2008 (Figure 9c). Highest negative values are found on the Prime Meridian near the shelf. Some positive values are observed in the central Weddell Gyre. However, these are isolated data points which confirm the pCO2 variability. The mean ΔΔpCO2 is −5.4 μatm.
 The overall trend to negative values of ΔΔpCO2 indicates that between 1992 and 2008 oceanic pCO2 did not rise as fast as atmospheric pCO2. This is confirmed by the mean surface layer (100 m) pCO2 rise of 16.2 μatm as calculated using eMLR in comparison to the 26.8 μatm increase of atmospheric pCO2. Hence, the Weddell Sea still has a large uptake capacity and absorbs CO2 without a proportional increase of seawater pCO2. This leads to a decreasing buffer capacity and increasing Revelle factor. In the long term, the reduced buffer capacity will cause surface pCO2 to rise proportionally faster than CT, thereby diminishing ΔpCO2, the driving force of CO2 exchange between ocean and atmosphere [Völker et al., 2002; Sabine et al., 2004; Thomas et al., 2007]. The reduction of buffer capacity due to the CO2 uptake is, though slightly, noticeable: the Revelle factor in surface waters (upper 100 m) has increased from an average of 15.1 in 1992 to 15.5 in 2008.
 Recently, the possibility of a declining capacity of the Southern Ocean as CO2 sink has been discussed in the literature [e.g., Le Quéré et al., 2007; Zickfeld et al., 2008; Law et al., 2008; Le Quéré et al., 2008; Canadell et al., 2007]. Le Quéré et al.  argue that due to human activities Southern Ocean winds have increased, leading to intensified upwelling, thereby bringing water with high natural CO2 concentrations to the surface; consequently enhanced outgasing would reduce the oceanic CO2 sink capacity. If this mechanism were active, an increase of ΔpCO2 (from negative values toward zero) would be expected. Our results point to opposite changes in ΔpCO2.
 From a model run for the time period 1981–2004, Le Quéré et al.  conclude that the Southern Ocean sink was not further increasing as expected, but rather remained constant. We conducted the same eMLR analysis with the model output as described by Le Quéré et al.  for May/June 1992 and December 2007/January 2008. The model was subsampled for the ANT XXIV/2 stations. This resulted in a similar increase of undersaturation as found with the data based eMLR (see above), i.e., strengthening of the oceanic sink in this region between 1992 and 2008 (Figure 9d). The mean ΔΔpCO2 as obtained by data from Le Quéré et al.  appeared to be −3.8 μatm. Although it does not reproduce the strong increase of the sink near the shelf as does our data, the overall findings match very well. This emphasizes that regionality is high in the Southern Ocean. Although the Southern Ocean CO2 sink as a whole could be decreasing, the Weddell Sea could be an important and increasing CO2 sink. The apparent contrast between Le Quéré's model results for 1980–2004 (entire Southern Ocean) and 1992–2008 (Weddell Sea) also indicates that because of an accelerating atmospheric CO2 increase in recent years, the Southern Ocean oceanic sink is reinforced.