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CO2 in the Weddell Gyre and Antarctic Circumpolar Current: austral autumn and early winter
Marine Chemistry 72 Ž2000. 203–220 www.elsevier.nlrlocatermarchem
CO 2 in the Weddell Gyre and Antarctic Circumpolar Current: austral autumn and early winter Mario Hoppema a,) , Michel H.C. Stoll b,1, Hein J.W. de Baar b,1 a
Department of Tracer Oceanography, UniÕersity of Bremen, FB1, IUP, P.O. Box 330 440, D-28334 Bremen, Germany b Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Texel, Netherlands Received 1 June 1999; received in revised form 24 December 1999; accepted 7 August 2000
Abstract Quasi-continuous fugacity of CO 2 ŽfCO 2 . data were collected in the eastern Weddell Gyre and southern Antarctic Circumpolar Current ŽACC. of the Southern Ocean during austral autumn 1996. Full depth Total CO 2 ŽTCO 2 . sections are presented for austral autumn and winter Ž1992. cruises. Pronounced fCO 2 gradients were observed at the Southern Ocean fronts. In the Weddell Gyre, fCO 2 regimes appeared to coincide with surface and subsurface hydrographic regimes. The southern ACC was supersaturated with respect to CO 2 , as was part of the northern Weddell Gyre. The southern Weddell Gyre was markedly undersaturated. The great potential of autumn cooling for generating undersaturation and CO 2 uptake from the atmosphere was demonstrated. In the northeastern Weddell Gyre, upwelling of CO 2- and salt-rich deep water was shown to play a role as the horizontal fCO 2 distribution closely resembled that of the surface salinity. The total uptake of atmospheric CO 2 by the Weddell Gyre in autumn Ž45 days. was calculated to be 7 P 10 12 g C. The deep TCO 2 distribution noticeably reflected the different water masses in the region. A new deep TCO 2 maximum was detected in the ACC, which apparently characterizes the boundary between the equatorward flowing Antarctic Bottom Water ŽAABW. and the Circumpolar Deep Water ŽCDW.. East of the Weddell Gyre, the AABW stratum is much thicker Ž) 2000 m. than more to the west, on the prime meridian Ž- 300 m.. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Weddell Sea; Autumn; Carbon dioxide; Air–sea exchange; Antarctic Bottom Water
1. Introduction Exchange of carbon dioxide between the oceans and the atmosphere is a pivotal process within the
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Corresponding author. Tel.: q49-421-2182931; fax: q49421-2187018. E-mail addresses: [email protected] ŽM. Hoppema., [email protected] ŽM.H.C. Stoll., [email protected] ŽH.J.W. de Baar.. 1 Fax: q31-222-319674.
Žanthropogenically perturbed. global carbon cycle, and a major forcing of the atmospheric CO 2 content. Unfortunately, there is much confusion pertaining to this issue. This ranges from the actual parameterization of the process of gas exchange across the air–sea interface for calculating fluxes ŽLiss and Merlivat, 1986; Wanninkhof, 1992., to the overall shortness of data for reliably calculating the annual CO 2 flux for the world ocean. In temperate and tropical regions, some basic level of spatial and temporal coverage of the partial pressure or fugacity of CO 2 ŽfCO 2 . in the
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surface has been achieved. However, for the Southern Ocean, such a basic level is not even feasible, data outside the summer period being particularly scarce. Recently, the seasonal coverage of surface CO 2 in parts of the Southern Ocean has somewhat improved ŽPoisson et al., 1993; Takahashi et al., 1993; Metzl et al., 1995; Hoppema et al., 1995., but due to the large spatial and short-term temporal variability, which is not yet sufficiently understood, it remains difficult to produce annual estimates of CO 2 exchange between the ocean and the atmosphere. Another approach for obtaining air–sea CO 2 exchange is constructing a surface layer budget using Total CO 2 ŽTCO 2 . ŽHoppema et al., 1999. or fCO 2 ŽPoisson et al., 1993; Bakker at al., 1997.. Under certain conditions, this method enables to calculate annual fluxes based on only few cruises. Applied to the offshore Weddell Sea, this region was found to be an annual sink for atmospheric CO 2 ŽHoppema et al., 1999.. Budgetting CO 2 in the surface layer and utilizing fCO 2 measurements to calculate CO 2 fluxes are both valuable approaches to the issue of air–sea exchange. The magnitude of the CO 2 flux as obtained from a budget is a useful quantity for constraining global carbon cycle models. On the other hand, fCO 2 distributions combined with other surface layer properties are useful for studying processes and variability in the oceanic surface layer, which need to be understood to improve the predictive ability of biogeochemical processes and their modelling. It should be added that the surface layer fCO 2 distribution is to some smaller or larger extent dependent on subsurface processes as well, particularly as the Southern Ocean is a known upwelling region for deep and bottom waters from the other ocean basins. The state of the Southern Ocean in autumn is largely unknown ŽStoll et al., 1999a.. In autumn, the surface layer is preconditioned for the subsequent winter, when large parts become ice covered. It is anticipated that physical processes such as surface water cooling and entrainment of subsurface water generate a significant portion of the CO 2 variability, but biological processes could also play a part. Here we present novel observations of the fCO 2 collected during cruise ANT XIIIr4 Ž1996.. These are austral autumn data for the eastern Weddell Gyre, a region that has been poorly investigated. We explore the
state of the surface layer and calculate fluxes between the ocean and the atmosphere. Accompanying thermosalinograph data are used to explain variations of the fCO 2 distribution. Because most of the deep water of the Antarctic Ocean has recently been ventilated, the deep water distribution has a high relevance with regard to CO 2 . On one hand, large-scale upwelling occurs in the Southern Ocean, while on the other hand, new deep and bottom waters are generated, suggesting a close coupling between surface and subsurface waters. Moreover, the Antarctic Ocean provides the preformed TCO 2 values for mixing in other ocean basins. Thus, besides surface fCO 2 data, we also present for the eastern Weddell Gyre and adjacent circumpolar current new TCO 2 sections, which were collected during cruises ANT Xr4 Ž1992. and ANT XIIIr4 Ž1996.. This work is part of a study on carbon cycling in the Atlantic sector of the Southern Ocean. We took part in several cruises. Previous publications arising from this study ŽHoppema et al., 1995, 1998, 1999; Stoll et al., 1999a. focussed on the Weddell Sea proper, i.e. west of about 108W. Some other TCO 2 data for the eastern Weddell Gyre from cruise ANT Xr4 were already presented by Hoppema et al. Ž1997.. In the relevant Section 3.1, the difference with the present data is indicated and when appropriate we refer to these data.
2. Methods Data are presented from two cruises with the German ice-breaker AFS PolarsternB ŽFig. 1., one in MayrJuly 1992 ŽANT Xr4; Lemke, 1994. and one in MarchrMay 1996 ŽANT XIIIr4; Fahrbach, 1997.. Water samples for the determination of TCO 2 were collected with a 24-place General Oceanics rosette sampler that was coupled to a Conductivity–Temperature–Depth ŽCTD. instrument. These samples were distributed over the whole water column, with bias towards the upper 1000 m. Underway surface temperature and salinity were measured with the ship’s thermosalinograph, which is mounted 8 m below the sea surface. The fCO 2 was measured underway in surface seawater quasi-continuously only on autumn cruise ANT XIIIr4. fCO 2 is equivalent to the partial pressure of CO 2 , but corrected for the non-ideal
M. Hoppema et al.r Marine Chemistry 72 (2000) 203–220 Fig. 1. Map of the southern South Atlantic Ocean including the Atlantic sector of the Southern Ocean. Positions of stations where full-depth profiles for vertical sections were obtained are indicated. Stations during austral winter cruise ANT Xr4 represented by diamonds and during austral autumn cruise ANT XIIIr4 by stars. In between ANT XIIIr4 stations fCO 2 , salinity and temperature were measured quasi-continuously. Map created with Generic Mapping Tools ŽWessel and Smith, 1995.. 205
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behaviour of CO 2 in a gas mixture. Both quantities are nominally only slightly different from each other ŽWeiss, 1974.. TCO 2 was determined with the coulometric method after Johnson et al. Ž1987. as slightly modified by Robinson and Williams Ž1992. and Stoll Ž1994., using the same equipment on both cruises. The TCO 2 concentration is the sum of all carbonate species dissolved in seawater, i.e. TCO 2 s wCO 2 x q wH 2 CO 3 x q wHCOy x w 2y x. Subsamples were 3 q CO 3 collected in glass bottles Ž0.5 l. with flexible screw caps. All analyses were performed within 24 h, but most within 12 h of sampling. Accuracy on cruise ANT XIIIr4 Ž1996. was set by certified reference material ŽDOE, 1994. made available by Prof. A. Dickson of the Scripps Institution of Oceanography ŽUSA.. At cruise ANT Xr4 these standards were not yet available to us. Standards were measured for each cell prepared for the coulometer and the cells were changed about once a day. The measured standards Žbatch number 30. were on average lower than the certified value by 10.7 " 2.2 mmol kgy1 Ž n s 52.. However, data collected during the utilization of one cell were corrected using the standard measured for this cell. During the first part of cruise ANT XIIIr4, for which data are presented here, 68 duplicates were measured which on average differed by 1.4 " 0.9 mmol kgy1 . This is worse than for the second part of the cruise Ždifference 1.0 mmol kgy1 ., due to suboptimal performance of the coulometer liquid. During cruise ANT Xr4, 83 duplicates were measured, differing 0.9 " 0.6 mmol kgy1 . The ANT Xr4 data were corrected using standardized data of another cruise; for details see Hoppema et al. Ž1995.. fCO 2 was measured on-line in surface seawater and in the overlying atmosphere using a fully automated sampling system, designed after Wanninkhof and Thoning Ž1993. including a Li-Cor ŽModel 6252. infrared analyzer. A continuous supply of marine air from the crow’s nest was maintained which was subsampled at regular intervals. For determination of fCO 2 in the oceanic surface layer, seawater was drawn off the ship’s continuous water supply and sprayed into the continuous-flow equilibrator ŽRobertson et al., 1993; Bakker et al., 1997. at a rate of 40–60 cm3 sy1 . A slight modification to the equilibrator was made so that ice blockage would hardly occur, thus enabling us to analyze water from under
the sea ice. The inlet in the hydro-well of APolarsternB allowed a fairly uninterrupted water supply in spite of sea ice. Temperature of the seawater inlet and equilibrator were recorded simultaneously with each measurement. The headspace gas was circulated and subsampled at regular intervals. The equilibrator was vented to the atmosphere Žmarine air.. Sample air was dried with Aquasorb ŽMerck. before being flushed through the infrared cell of the Li-Cor. The response time of the system was less than 2 min. During the transport of seawater from the ship’s inlet to the equilibrator a slight heating occurs which was generally less than 0.18C, but in some cases 0.3– 0.48C. As fCO 2 is dependent on temperature, it was corrected for this temperature increase using the equations of Copin-Montegut Ž1988, 1989.. Obvious outliers and datapoints with too large a temperature difference Žindicating blockage in the seawater supply. were deleted from the dataset. An analysis cycle of fCO 2 measurements, which takes less than 1 h, consists of a calibration Žusing three reference gases of 266.95, 367.76 and 443.66 mmol moly1 CO 2 in air, respectively., air, seawater Ž5 = ., air, seawater Ž5 = . and a calibration. For each sample, 10 readings Ževery 0.5 s. were taken and averaged during post-processing. Final data were obtained by interpolating between two consecutive calibration cycles. The reference gases used on board were recalibrated in the home laboratory against National Oceanic and Atmospheric Administration ŽNOAA. certified standard gas mixtures Žaccurate to "0.01 mmol moly1 . before and after the cruise. For all reference gases, the precision during the calibration was better than 0.07 mmol moly1 . No significant difference was observed between the pre-cruise and post-cruise calibrations. It is not possible to give a robust estimate of the precision and accuracy of underway oceanic fCO 2 measurements. However, during the cruise fCO 2 was measured as if underway while the ship was stationary for about one and a half days. The mean fCO 2 was 346.5 " 1.2 matm Ž n s 326.. As the real fCO 2 is subject to some natural variability on this position Žthe ship also drifted somewhat., this precision should be considered an upper bound. As regards the atmospheric CO 2 measurements, these can be compared with other data collected in the region. The mean atmospheric mixing ratio Žmole fraction. measured
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during cruise ANT XIIIr4 is 360.7 mmol moly1 , standard deviation 0.6 mmol moly1 and range 359.5–362.5 Ž n s 10,331.. This compares well with extrapolated data from measurements at Halley Bay, Weddell Sea Ž361.3 mmol moly1 . and Palmer Station, western Antarctic Peninsula, amounting to 359.9 mmol moly1 ŽConway et al., 1994.. The CO 2 mixing ratio measured at the South Pole in April–May 1996 is somewhat lower than our value, i.e. 359.4 mmol moly1 ŽKeeling and Whorf, 1999., but it is also further away than the other stations. From the difference between fCO 2 in seawater and in the atmosphere, the net gas exchange of CO 2 across the air–sea interface was computed according to the method and parameterizations of Wanninkhof Ž1992.. In situ wind speed and sea surface temperature ŽSST., which go into this calculation, were obtained from the APolarsternB meteorological data acquisition system.
3. Meridional section along the prime meridian 3.1. TCO2 distribution across the ACC Fig. 2 shows a TCO 2 section of the Antarctic Circumpolar Current ŽACC. and the northernmost part of the Weddell Gyre along the prime meridian obtained during the austral winter cruise. Upper water column TCO 2 data from the same cruise ANT Xr4, but for the Weddell Gyre Ži.e. south of the transect shown here., were previously shown by Hoppema et al. Ž1997.. In the north, the section starts just south of the Subtropical Front. The Weddell Front ŽWF. at about 55.58S constitutes the boundary between the ACC and the Weddell Gyre ŽOrsi et al., 1995.. The positions of the fronts as shown in Fig. 2 were fixed according to the simultaneously measured, corresponding salinity and potential temperature sections ŽSchroder and Fahrbach, ¨ 1999; Lemke, 1994.. The surface TCO 2 transect exhibits maximal horizontal gradients across the Subantarctic and Weddell Fronts ŽFig. 2., and generally, a large gradient in the region of the Polar Front ŽPF. and Southern ACC Front ŽSACCF.. The increase of TCO 2 in the surface layer from north to south is mainly due to the increasing CO 2 solubility with decreasing temperature. In the Weddell regime
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among the highest surface layer TCO 2 concentrations in the world oceans are found Žsee also Hoppema et al., 1995.. The large-scale TCO 2 distribution reflects the known distribution of water masses in the region ŽFig. 2.. At about 508S, i.e. near the PF, the downward sloping to the north of TCO 2 isolines from the Žsub.surface layer ŽFig. 2. points to Antarctic Intermediate Water ŽAAIW.. As the isolines are steepest in this area, this is designated a subsurface expression of the PF. Underneath the AAIW and between 508S and 55.58S directly below the surface layer, lies the Circumpolar Deep Water ŽCDW., extending over almost all of the rest of the water column. Upper CDW features a TCO 2 maximum Žcentered around 1500 m between 418S and 458S. and Lower CDW a TCO 2 minimum Žat about 3000 m near 428S.. Lower CDW originates from the relatively young North Atlantic Deep Water ŽNADW. with its accordingly low TCO 2 concentration. Characteristics of the Upper CDW derive from the older deep water of the Pacific and Indian Oceans, which have a higher TCO 2 concentration. In southward direction, the CDW shoals considerably, for example, the TCO 2 minimum of the Lower CDW moves from 3000 m depth at 428S to 800 m depth in the southernmost ACC at 558S ŽFig. 2.. Simultaneously, the TCO 2 value in the TCO 2 minimum increases markedly by about 40 mmol kgy1 , which reflects the decreasing influence of the NADW and increasing influence of the deep waters of the Pacific and the Weddell Sea. Near the bottom of the ACC, Antarctic Bottom Water ŽAABW. is found. The high TCO 2 of AABW derives from the subpolar region south of the ACC, such as the Weddell Sea. On its way northwards, the AABW mixes with the overlying CDW, which continuously changes its composition ŽOrsi et al., 1999.. On close inspection, a weak TCO 2 maximum can be discerned 100–300 m above the bottom Žindicated by crosses in Fig. 2.. This TCO 2 maximum constitutes the upper boundary of the AABW. In Section 4.1, this issue is discussed in more detail. 3.2. fCO2 surface transect through the southern ACC and Weddell Gyre in austral autumn Fig. 3 portrays the autumn transect ŽApril 1996. of fCO 2 across the southern ACC and Weddell Gyre along the prime meridian, including the correspond-
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Fig. 2. Vertical TCO 2 section Žbottom. along the prime meridian across the ACC into the northern Weddell Gyre obtained during cruise ANT Xr4 ŽMayrJune 1992.. Sampling depths shown by dots. Crosses in the contour plot display the location of near-bottom TCO 2 maxima. Positions of hydrographic fronts and water masses are indicated: SAF, Subantarctic Front; PF, Polar Front; SACCF, Southern ACC Front; WF, Weddell Front; AAIW, Antarctic Intermediate Water; UCDW, Upper Circumpolar Deep Water; LCDW, Lower Circumpolar Deep Water; AABW, Antarctic Bottom Water. The distribution of TCO 2 in the near-surface water of the section is shown separately Žtop..
ing SST and surface salinity. In this figure, the measured atmospheric fCO 2 is displayed, which varies between 343 and 358 matm. Note that the atmospheric fCO 2 is a partial pressure equivalent indeed, hence depending on the varying total atmospheric pressure. At 55–568S, the marked salinity gradient is a fine indicator of the WF. The SST signal across this front is weak, whereas the fCO 2 gradient is pronounced with about 10 matm ŽFig. 3.. Around 52.58S, the strong SST gradient appears to be an expression of
the SACCF — the PF in this region occurs between 498S and 508S ŽSchroder and Fahrbach, 1999.. An ¨ elevated fCO 2 gradient in this area occurs somewhat to the north of the maximal SST gradient, but it coincides with another elevated SST gradient centred at 51.58S. In the northernmost part of the transect, the water is undersaturated with respect to CO 2 ŽFig. 3.. This surface water tends to flow northwards to participate in AAIW formation Žsee Fig. 2 and Section 3.1., the latter process thus constituting a conduit for CO 2 uptake from the atmosphere.
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Fig. 3. Distribution of fCO 2 in the surface water and the overlying atmosphere along the prime meridian in autumn Ž9–22 April 1996. during cruise ANT XIIIr4 Žtop. and the corresponding sea surface temperature and salinity Žbottom.. Locations of major fronts and subareas are indicated: ASF, Antarctic Slope Front; WF, Weddell Front; SACCF, Southern ACC Front; ACC, Antarctic Circumpolar Current; WG, Weddell Gyre.
Within the Weddell Gyre, the northern part Ž55– 608S. was clearly supersaturated with fCO 2 . This fCO 2 supersaturation appears to exactly coincide with the northern rim current of the Weddell Gyre ŽSchroder and Fahrbach, 1999.. Note that the rim ¨ current is a feature pertinent to the whole water column. Thus, this CO 2 supersaturation appears to originate west of our transect. Advection of surface layer properties is a major reason why transect-wide
relationships between fCO 2 on one hand and SST and surface salinity on the other hand are scattered ŽFig. 4.. In contrast to the northern area with eastward flow and supersaturation, the area between 628S and the Antarctic coastline reveals CO 2 undersaturation ŽFig. 3. and has principally westward flow. Thus, within the Weddell Gyre, the CO 2 under- or supersaturation is closely coupled to the origin of the
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surface water. Additionally, this subdivision of relatively high and low CO 2 saturation ŽFig. 3. is roughly coincident with the areas where relatively shallow and deep lying TCO 2 maxima occur, respectively ŽHoppema et al., 1997.. This is an indication that upwelling of deep water plays a role in the regional distribution of fCO 2 in the surface layer as well. In autumn, surface water cooling occurs, which reduces the fCO 2 by increasing the CO 2 solubility. We can tentatively assess the effects of cooling on the fCO 2 . In this region, the summer surface temperature is 0.5–18C ŽStoll et al., 1999b.. Using the thermodynamic relationships of the CO 2 system, we can compute fCO 2 at these summer temperatures, assuming constant TCO 2 and alkalinity, the latter property being conservative in the Weddell Sea ŽAnderson et al., 1991.. From Fig. 3, it can be deduced that most cooling has occurred in the south and least in the north. We distinguish regions with different temperatures and thus different cooling. In the region abutting the coastline Žabout 69.5–68.58S., the surface layer is approximately at freezing point, indicating that at summer temperature the fCO 2 could have been 40–50 matm higher. In the region between 68.58S and 66.58S, where the autumn SST is about y18C, the potential fCO 2 difference with summer is 30–40 matm. For the region 64.5–61.58S with a SST of about y0.38C, the summer fCO 2 might be higher by 15–25 matm. Finally, for the northern limb of the Weddell Gyre, which is supersaturated in autumn ŽFig. 3., this supersaturation could have been even higher by 5–15 matm in summer. Our calculations suggest that the entire Weddell Gyre may have been supersaturated before the onset of autumn cooling, the northern limb indeed still is. Supersaturation could be caused by remineralization of organic matter after the cessation of phytoplankton blooms in summer. Particularly in the region south of 608S extensive summer blooms were indeed observed ŽStoll et al., 1999b.. An alternative explanation for the observed CO 2 undersaturation in the south ŽFig. 3. is an autumn phytoplankton bloom.
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However, the contribution of active phytoplankton growth in autumn to CO 2 drawdown was minor as evidenced by the low level of chlorophyll a at this transect ŽC. Dubischar, Alfred-Wegener-Institut, personal communication, 1996.. A general tendency of a higher SST corresponding to a higher fCO 2 is revealed in Fig. 4. In essence, this is caused by the difference between the Weddell Gyre and the ACC. Within these subregions, there is no relationship ŽWeddell Gyre., or maybe a slight negative correlation ŽACC.. Within the southern and central Weddell Gyre, there is a similarity between the distributions of fCO 2 and surface salinity ŽFig. 3., which translates into a weak positive correlation for this subregion ŽFig. 4.. This may be explained by upwelling of deep water. However, the scattering and the only weak correlation hint that differences in upwelling activity cannot be the main explanation for the horizontal distributions. The northern Weddell Gyre and ACC appear as separate clusters in Fig 4. In the Maud Rise region Ž64–678S., circulation– topography interactions lead to an enhancement of upwelling of deep water, which causes a locally high salt content of the surface layer ŽGordon and Huber, 1990.. Our data around 66.58S Žover the crest of Maud Rise at this longitude. confirm this ŽFig. 3.. The salinity maximum is accompanied by a faint, local fCO 2 maximum ŽFig. 3.. However, the whole area contains water that is undersaturated in CO 2 , indicating that seasonal cooling is dominant over upwelling.
4. Quasi-zonal section along 0–398E 4.1. TCO2 distribution Fig. 5 shows a TCO 2 section of the easternmost Weddell Gyre and adjacent ACC. These data were collected during cruise ANT XIIIr4 ŽMarchrApril 1996. and represent the first survey of this part of the Weddell Gyre and adjacent ACC. The SACCF is
Fig. 4. fCO 2 in surface water vs. sea surface temperature Žtop. and vs. surface salinity Žbottom. for the transect along the prime meridian in austral autumn 1996. Three different sub-regions of the transect are indicated in the plots Žsee text.: WG, Weddell Gyre; ACC, Antarctic Circumpolar Current.
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Fig. 5. Quasi-zonal vertical TCO 2 section between the Conrad Rise Žright. and the prime meridian across the eastern Weddell Gyre and adjacent ACC. Data from cruise ANT XIIIr4, 23 March–5 April 1996. Sampling depths shown by dots. Positions of hydrographic fronts, and water masses are indicated: WF, Weddell Front; SACCF, Southern ACC Front; WDW, Warm Deep Water; WSDW, Weddell Sea Deep Water; WSBW, Weddell Sea Bottom Water; AABW, Antarctic Bottom Water.
a very sharp feature all through the water column below the surface layer ŽFig. 5.. In contrast, no clear expression can be discerned for the deep WF. In the Weddell Gyre part, west of 25–268E ŽFig. . 5 , the Warm Deep Water at 300–700 m depth features TCO 2 concentrations above 2270 mmol kgy1 , i.e. much higher than in the deep water entering the Weddell Gyre ŽHoppema et al., 1997.. This is indicative of the presence of an older type of WDW. Underneath the WDW, the Weddell Sea Deep Water and Weddell Sea Bottom Water are found with monotonically decreasing TCO 2 concentrations. An upward hump of the TCO 2 isolines is
discernible for the Weddell Gyre part ŽFig. 5.. This phenomenon was also observed in the corresponding temperature and salinity sections, in which it points to a northward water flow component west of about 138E and a southward flow component east of 138E ŽSchroder ¨ and Fahrbach, 1999.. The 138W meridian also appears to be the division line between near bottom water with relatively low TCO 2 Ž- 2250 mmol kgy1 . west of it and bottom water with higher TCO 2 Ž) 2250 mmol kgy1 . east of it ŽFig. 5.. In the ACC part of the section east of the SACCF, the vertical structure and water mass characteristics ŽFig. 5. seem rather different from those in the ACC
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on the prime meridian Žcompare with Fig. 2.. However, only the mutual proportions of the water masses are different. The TCO 2 minimum at about 1200– 1400 m is pertinent to the Lower CDW, while the TCO 2 maximum at about 600 m is characteristic of the Upper CDW ŽFig. 5.. The marked TCO 2 maximum at about 3000 m ŽFig. 5. corresponds to the weak TCO 2 maximum, not far from the sea floor ŽFig. 2; Section 3.1. on the prime meridian. The TCO 2 maximum at about 3000 m depth marks the upper boundary of the AABW that has left the subpolar Žhere: Weddell. regime. The origin of this TCO 2 maximum is the following: In the ACC below the TCO 2 minimum, TCO 2 increases with depth. In contrast, in the subpolar regime a decrease of TCO 2 with depth is found below the WDW stratum because of the TCO 2 minimum in the bottom water. Hence, when the water of the ACC and the subpolar regime are superimposed, a TCO 2 maximum is induced at the interface. It is important to appreciate that it is a unique feature of the TCO 2 distribution to reveal the upper boundary of the AABW. The temperature and salinity distributions do not show an extremum at this boundary ŽSchroder ¨ and Fahrbach, 1999., because the vertical gradients of temperature and salinity in the deep ACC and the deep Weddell Gyre have the same sign. 4.2. fCO2 surface transect in austral autumn On the quasi-zonal transect between the Conrad Rise and the prime meridian, both undersaturation and supersaturation of CO 2 were observed in austral autumn ŽFig. 6.. fCO 2 values in the southern ACC were generally above equilibrium with the atmosphere ŽFig. 6., like at the prime meridian ŽFig. 3.. The SACCF, which is distinguishable near 288E by a westward decrease of SST by more than 18C and increase of the surface salinity ŽFig. 6., also appears to separate waters with different fCO 2 levels. The large supersaturation could be caused by a combination of intense upwelling in the southernmost ACC ŽComiso et al., 1993., and remineralization of organic matter, as biological activity may be relatively high there ŽHoppema et al., 2000.. Only the southern ACC appears to be supersaturated with CO 2 in this time of the year, as more northern parts Žnear the Polar and Subantarctic Fronts. were markedly under-
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saturated ŽFig. 3; and other unpublished data.. The WF is not very pronounced in the temperature and salinity distributions, but appears to be associated with a dramatic fCO 2 change of about 30 matm. The distinct fCO 2 signature of the WF stands in contrast to the deep TCO 2 expression of the front which is only weak. Dissimilar biological conditions on both sides of the front ŽSpiridonov et al., 1996. may be responsible for the observed fCO 2 distribution. There is a high degree of conformity in the horizontal distributions of fCO 2 and surface salinity ŽFig. 6.. A transect-wide positive relationship exists between fCO 2 and surface salinity ŽFig. 7., which underscores the importance of large-scale upwelling for explaining these distributions. Upwelling occurs both in the southern ACC and in the Weddell Gyre ŽGordon and Huber, 1990; Comiso et al., 1993. and transports TCO 2-rich Žhigh-fCO 2 ., saline water into the surface layer. Ice formation or wet precipitation from the atmosphere could frustrate the correlation of fCO 2 and salinity due to upwelling, because it changes the salinity in the surface layer. However, no ice had yet been formed in the area before the time of our observations. It should be realized that supersaturation of fCO 2 can, besides by remineralization of organic matter, only be caused by upwelling. Between fCO 2 and SST, a transect-wide relationship due to upwelling is not observed ŽFig. 7.. This is because in the western part of the transect, where the SST is low, upwelling would increase the SST, whereas in the eastern part, where the SST is high, it would decrease the SST. Besides that, cooling of the surface layer, which occurs at its annual maximum rate in autumn, could substantially alter the SST Žand also fCO 2 . additionally. Fig. 7 shows that an anticorrelation exists of fCO 2 and SST for temperatures below about 18C, and a positive correlation for the temperature range 1–2.58C. A positive correlation may be the consequence of surface water cooling, but the fCO 2 range of about 35 matm is too large to be caused by cooling of only 18C Žsee Fig. 7.. This observation agrees with the above suggestion that upwelling plays a part in this area. An anti-correlation of fCO 2 and the SST could arise when surface water with a temperature above 18C is subject to upwelling of WDW with a lower temperature. In conjunction with the fact that 138W is the division
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Fig. 6. Distribution of fCO 2 in the surface water and the overlying atmosphere on a quasi-zonal transect through the eastern Weddell Gyre and southern ACC in austral autumn Ž25 March–4 April 1996. during cruise ANT XIIIr4 Žtop. and the corresponding sea surface temperature and salinity Žbottom.. Locations of major fronts are indicated: WF, Weddell Front; SACCF, Southern ACC Front.
between water with northward flow and water with southward flow ŽSchroder and Fahrbach, 1999; see ¨ also Section 4.1., we note that the horizontal fCO 2 , salinity and SST variations around 138E are relatively large, while this is also the location where the SST reaches the value of about 18C ŽFig. 6.. This strongly suggests that it is the water with the northward flow component Žwest of 138W. that has an anti-correlation of fCO 2 and SST ŽFig. 7., whereas
in the water with the southward flow component a positive correlation of fCO 2 and SST exists. In the Weddell regime, fCO 2 is largely below equilibrium with the atmosphere ŽFig. 6.. Undersaturation of fCO 2 in autumn is likely to be caused by cooling of the surface layer. Photosynthetic drawdown of CO 2 was hardly active here, as evidenced by the low level of chlorophyll a ŽC. Dubischar, personal communication, 1996.. Only in the western
M. Hoppema et al.r Marine Chemistry 72 (2000) 203–220
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Fig. 7. fCO 2 in surface water vs. sea surface temperature Žtop. and vs. surface salinity Žbottom. for the quasi-zonal transect through the eastern Weddell Gyre and southern ACC in austral autumn 1996.
part of the transect, supersaturation was observed. The latter area is spatially connected with the high-
fCO 2 region at the prime meridian Žsee Figs. 1 and 3..
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5. Air–sea fluxes of CO 2 CO 2 fluxes were calculated for different sub-regions of our area of investigation using in situ auxiliary data Žwind speed, surface water temperature.. We distinguish the southern ACC as the area between the SACCF and the WF Žsee also Fig. 1.. Within the Weddell Gyre, the northern part, in essence corresponding to the cold regime deep water of Gordon and Huber Ž1990., is situated between 61.48S and the WF. This includes our quasi-zonal transect from 0–25.98E ŽSection 4.2. and part of the prime meridian transect ŽSection 3.2.. The southern part of the Weddell Gyre Žanalogously the warm regime deep water of Gordon and Huber, 1990. lies between 61.48S and the Antarctic coastline. Results appear in Table 1. The southern ACC in autumn is a source of CO 2 to the atmosphere, east of the Weddell Gyre distinctly more than at the prime meridian. Within the northern Weddell Gyre, both sources and sinks are found, approximately equal in magnitude. The large southern Weddell Gyre is a pronounced sink in autumn with almost 6 mmol my2 dayy1 . The air–sea CO 2 flux for the entire Weddell Gyre was computed as the data of cruise ANT XIIIr4 cover almost the whole gyre area. This includes data from the western Weddell Sea between Kapp Norvegia and the tip of the Antarctic Peninsula as reported by Stoll et al. Ž1999a.. We calculated a mean flux of y2.52 mmol m2 dayy1 for the entire Weddell Gyre, that is, the Weddell Gyre is a sink for atmospheric CO 2 in autumn. A total air to sea flux of 7 P 10 12 g C Table 1 Calculated mean air–sea CO 2 fluxes Žmmol my2 dayy1 . for different subregions of the Southern Ocean Žin brackets the assumed boundaries. and for the Weddell Gyre as a whole Žincluding data from the western Weddell Sea; Stoll et al., 1999a.. Negative flux indicates uptake by the sea
Southern ACC ŽSACCF–WF. Northern Weddell Gyre ŽWF–61.48S. Southern Weddell Gyre Ž61.48S–coast. Total mean flux of Weddell Gyre
Prime meridian
Eastern Weddell
q1.75 q2.33
q5.47 y2.74
y5.89
y2.52
SACCF: Southern ACC Front; WF: Weddell Front.
Žs 0.007 Gton C. is obtained over the time period of our autumn cruise Ž45 days. for the entire area of the Weddell Gyre Žapproximately 5 P 10 12 m2 .. An annual total uptake of 8 P 10 12 g C was obtained for the offshore western Weddell Sea in a budget study ŽHoppema et al., 1999.. The annual physically mediated CO 2 uptake in that same area amounts to about 2.4 P 10 12 g C ŽStoll et al., 1999a.. Accounting for the larger surface area Žabout 3 times the western Weddell Sea., the figure for the entire Weddell Gyre as obtained in the present study fits well into this. Our data are consistent with the notion that physically mediated CO 2 uptake largely occurs in autumn. Robertson and Watson Ž1995. calculated a CO 2 uptake of 1 P 10 14 g C over a 4-month summer period for an area including most of the Weddell Gyre, which translates into approximately 5 P 10 13 g C for the Weddell Gyre. Thus, CO 2 uptake by the Weddell Gyre in summer is much larger than in autumn. Though in the other seasons some outgassing may occur as a consequence of upwelling, all data support the conclusion that the western Weddell Sea is an annual CO 2 sink ŽHoppema et al., 1999. and this is likely for the greater Weddell Gyre as well.
6. General discussion fCO 2 in the surface layer of the Southern Ocean is determined by a combination of processes, notably upwelling, biological activity and temperaturersalinity changes. Upwelling constitutes the link between the deep water TCO 2 and the surface fCO 2 . In fact, at the prime meridian different fCO 2 regimes can be discerned which correspond to different deep water regimes. Also at the major frontal systems there is a covariation in the deep and surface distributions of CO 2 . Though resemblance of surface and deep CO 2 distributions can be observed, its full extent can only be visualized by means of a modelling effort. The CO 2 supersaturation at the western end of the quasi-zonal transect ŽFig. 6. is spatially connected with a similar area on the prime meridian ŽFig. 3., the latter in turn corresponding to the northern rim current of the Weddell Gyre ŽSection 3.2.. As the water with CO 2 supersaturation appears to be pinched
M. Hoppema et al.r Marine Chemistry 72 (2000) 203–220
off at about 88E ŽFig. 6., this would imply that the rim current is pinched off as well, or at least is diverted. As mentioned before, the hydrographic observations in this area do indeed hint at a northward deflection ŽSchroder and Fahrbach, 1999.. East of ¨ 138E on the same transect, the water column has a southward flow component Žsee also Sections 4.1 and 4.2.. The surface salinity here is very low ŽFig. 6. and moreover similar to that in the southern ACC ŽFig. 3., suggesting that this is an intrusion of surface water stemming from the southern ACC. Indeed, Bagriantsev et al. Ž1989: their Fig. 2. suggest intrusion of subsurface water from the ACC in this region. As regards CO 2 , while the surface water of the southern ACC is supersaturated with CO 2 ŽFig. 3., the possibly intruded surface water in the eastern Weddell Gyre is distinctly undersaturated ŽFig. 6.. This difference cannot be caused by differences in SST. Thus, the lower CO 2 saturation in the eastern Weddell Gyre could be due to the fact that once in the Weddell Gyre, the surface water is subject to less upwelling activity. During our study, fCO 2 exhibits a large spatial variation Ž318–375 matm., which has been established by other investigations in the Southern Ocean as well. The seasonal fCO 2 variation appears to be considerable ŽPoisson et al., 1993; Takahashi et al., 1993.. Biological processes, with their high propensity for patchiness, cause large variations mainly in summer. During our cruise in autumn, biological activity was relatively low, suggesting that fCO 2 variations were predominantly caused by physical factors. Poisson et al. Ž1993. did not find significant correlations between fCO 2 and SSTrsurface salinity for data collected in the January–April period, concluding that the mesoscale variability is large. Our analysis of the fCO 2 data with the aid of surface hydrographic parameters demonstrates that the fCO 2 distribution can be fairly well understood when the spatial scale considered is relatively small. Our autumn data supplement the few existing data of this area to enhance the seasonal coverage. Comparing data from different seasons allows us to extend the interpretation of fCO 2 variations. In the southern ACC at the prime meridian, the slight supersaturation ŽFig. 3. appears to be in line with fCO 2 data from the other seasons ŽWeiss et al., 1992: their Figs. 55, 63 and 65; Bakker et al., 1997.,
217
which are all close to full saturation. This region is subject to intensive upwelling of CO 2-rich Upper CDW ŽHoppema et al., 2000., the CO 2-elevating effect of which is counteracted by biological drawdown of CO 2 . South of this, the northern limb of the Weddell Gyre features CO 2 supersaturation not only in autumn ŽFig. 3., but also in winter ŽWeiss et al., 1992: their Fig. 63., spring ŽBakker et al., 1997. and summer ŽWeiss et al., 1992: their Fig. 55.. Stoll et al. Ž1999b. report a slight undersaturation for the summer season. This area is not biologically productive, the often-observed CO 2 supersaturation being caused by upwelling. Still further south in the southern limb of the Weddell Gyre, the CO 2 undersaturation in autumn ŽFig. 3. stands in contrast to the slight winter and higher spring supersaturation as observed by Weiss et al. Ž1992.. A dramatic undersaturation is perceived in summer ŽStoll et al., 1999b; Weiss et al., 1992: their Fig. 55.. The interpretation is as follows: In summer CO 2 undersaturation is shaped due to intensive CO 2 fixation by photosynthesizing plankton. In autumn, remineralization of organic material elevates fCO 2 but this is counteracted by surface water cooling leading to undersaturation. Towards winter, entrainment of CO 2-rich deep water causes supersaturation, which extends into the next spring. It should be realized that during winter the CO 2 supersaturation does not lead to outgassing to the atmosphere, because the region is largely ice-covered then. Some outgassing is anticipated during early spring when the ice starts to melt, but a new cycle of photosynthetic CO 2 fixation, which may even benefit by the ice melt, will rapidly bring down the fCO 2 below the atmospheric level. We thus expect this region to be a sound sink for atmospheric CO 2 just like the western Weddell Gyre ŽHoppema et al., 1999; Stoll et al., 1999a.. The seasonal cycle of fCO 2 for the easternmost Weddell Gyre and adjacent ACC is not known. The zonal distribution of our fCO 2 data, with undersaturation in the Weddell Gyre and supersaturation in the ACC ŽFig. 6., is in conformance with that presented by Robertson and Watson Ž1995. for Februaryr March. For the period of JanuaryrFebruary, Metzl et al. Ž1991. found supersaturation, whereas Poisson et al. Ž1994. observed a slight undersaturation near 308E. Albeit some summertime variation of fCO 2
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exists, the similarity of the summer and autumn spatial fCO 2 distributions points to the relative importance of abiotic factors for explaining it. We conjecture that a similar fCO 2 distribution will also be prevalent during the less variable winter and spring periods. This would make the eastern Weddell Gyre a sink for atmospheric CO 2 on an annual basis. In the ACC east of the Weddell Gyre, the AABW stratum is up to ) 2000 m thick ŽFig. 5.. This AABW, which derives from the Weddell Sea Deep Water, has recently left the Weddell Gyre, probably from the southwest of its location on the transect. The large vertical extent of the layer indicates that it is a major outflow of bottom water. Mantyla and Reid Ž1995. also reported outflow of bottom water in this region. As AABW contains anthropogenic CO 2 absorbed by the Weddell Sea ŽAnderson et al., 1991., this outflowing AABW could be a major conduit for sequestering anthropogenic CO 2 in the world oceans, but this requires further investigation. At the prime meridian, the AABW manifests itself as a relatively thin layer. It is not clear what the origin of this AABW is. One possibility is the Drake Passage, and thus the Ross Sea in the Pacific sector. Another possible source region of it is the western Weddell Sea from where it could be transported through the South Sandwich Trench or through smaller, shallower passages in the South Scotia Ridge ŽLocarnini et al., 1993.. In both cases, it is not surprising that the layer thickness is much smaller at the prime meridian than east of the Weddell Gyre. First, at the prime meridian, the AABW is farther from its source area, making it plausible that more water mass spreading has occurred, over a larger surface area. Second, part of the AABW spreads northwards along the western South Atlantic ŽOrsi et al., 1999. and thus never reaches the prime meridian. As based on the layer thickness, it is impossible to decide which source of AABW, the western one or the one from the eastern Weddell Gyre, is more important.
7. Conclusions The horizontal distribution of fCO 2 is strongly influenced by the flow of surface water masses with different fCO 2 levels, especially when the distribu-
tion perpendicular to the main circulation features is considered Žfor example, across the ACC and Weddell Gyre.. This is also evident from fCO 2 gradients across the major fronts. Along the main direction of the circulation, upwelling was shown to play a role. Its effect for elevating the fCO 2 level is probably modest, because even over Maud Rise, a region with known enhanced upwelling activity, a clear CO 2 undersaturation was observed. Superimposed is cooling as the main factor for reducing fCO 2 in autumn. Because of this, the Weddell Gyre in autumn is a sink for atmospheric CO 2 . Before the onset of autumn cooling, though, it has probably been supersaturated in CO 2 . The deep TCO 2 distribution reflects all different water masses and some Žsurface. fronts. In the ACC, a deep TCO 2 maximum indicates the upper boundary of the AABW. This may be a convenient aid to trace AABW through the ocean basins, at least in the ACC, because a definition of AABW based on fixed property values is impossible ŽOrsi et al., 1999.. Acknowledgements We are grateful to the Alfred-Wegener-Institut fur ¨ Polar- und Meeresforschung ŽBremerhaven, Germany. for making available AFS PolarsternB. This research was in part supported by the Dutch–German cooperation in marine sciences ŽNEBROC. and by the EC Environment and Climate Research Programme Žcontract: ENV4-CT97-0472, climate and natural hazards.. References Anderson, L.G., Holby, O., Lindegren, R., Ohlson, M., 1991. The transport of anthropogenic carbon dioxide into the Weddell Sea C. J. Geophys. Res. 96, 16679–16687. Bagriantsev, N.V., Gordon, A.L., Huber, B.A., 1989. Weddell Gyre: temperature maximum stratum C. J. Geophys. Res. 94, 8331–8334. Bakker, D.C.E., De Baar, H.J.W., Bathmann, U.V., 1997. Changes of carbon dioxide in surface waters during spring in the Southern Ocean. Deep-Sea Res., Part II 44, 91–127. Comiso, J.C., McClain, C.R., Sullivan, C.W., Ryan, J.P., Leonard, C.L., 1993. Coastal Zone Color Scanner pigment concentrations in the Southern Ocean and relationships to geophysical surface features. J. Geophys. Res. 98, 2419–2451. Conway, T.J., Tans, P.P., Waterman, L.S., 1994. Atmospheric
M. Hoppema et al.r Marine Chemistry 72 (2000) 203–220 CO 2 records from sites in the NOAArCMDL air sampling network. In: Boden, T.A., Kaiser, D.P., Sepanski, R.J., Stoss, F.W. ŽEds.., Trends ’93: A Compendium of Data on Global Change. ORNLrCDIAC-65. pp. 41–119. Copin-Montegut, C., 1988. A new formula for the effect of temperature on the partial pressure of CO 2 in seawater. Mar. Chem. 25, 29–37. Copin-Montegut, C., 1989. Corrigendum. A new formula for the effect of temperature on the partial pressure of CO 2 in seawater. Mar. Chem. 27, 143–144. DOE, 1994. In: Dickson, A.G., Goyet, C. ŽEds.., Handbook of Methods for the Analysis of the Various Parameters of the Carbon Dioxide System in Sea Water. version 2 ORNLr CDIAC-74. Fahrbach, E. ŽEd.., Cruise Report Polarstern Expedition ANT XIIIr4, 1996. Ber. Polarforsch., vol. 239, pp. 71–97. Gordon, A.L., Huber, B.A., 1990. Southern Ocean winter mixed layer. J. Geophys. Res. 95, 11655–11672. Hoppema, M., Fahrbach, E., Schroder, M., Wisotzki, A., De Baar, ¨ H.J.W., 1995. Winter–summer differences of carbon dioxide and oxygen in the Weddell Sea surface layer. Mar. Chem. 51, 177–192. Hoppema, M., Fahrbach, E., Schroder, M., 1997. On the total ¨ carbon dioxide and oxygen signature of the Circumpolar Deep Water in the Weddell Gyre. Oceanol. Acta 20, 783–798. Hoppema, M., Fahrbach, E., Stoll, M.H.C., De Baar, H.J.W., 1998. Increase of carbon dioxide in the bottom water of the Weddell Sea, Antarctica. Mar. Chem. 59, 201–210. Hoppema, M., Fahrbach, E., Stoll, M.H.C., De Baar, H.J.W., 1999. Annual uptake of atmospheric CO 2 by the Weddell Sea derived from a surface layer balance, including estimations of entrainment and new production. J. Mar. Syst. 19, 219–233. Hoppema, M., Fahrbach, E., De Baar, H.J.W., 2000. Surface layer balance of the southern Antarctic Circumpolar Current Žprime meridian. to derive carbon and silicate consumptions and annual air–sea exchange for CO 2 and oxygen. J. Geophys. Res. 105, 11359–11371. Johnson, K.M., Sieburth, J.M., Williams, P.J.leB., Brandstrom, ¨ L., 1987. Coulometric total carbon dioxide analysis for marine studies: automation and calibration. Mar. Chem. 21, 117–133. Keeling, C.D., Whorf, T.P., 1999. Atmospheric CO 2 records from sites in the SIO air sampling network. Carbon Dioxide Information Analysis Center, Trends: A Compendium of Data on Change ORNL U.S. DoE, Oak Ridge, Tenn., U.S.A. http:rr cdiac.esd.ornl.govrtrendsrco2rsio-keel.htm. Lemke, P. ŽEd.., 1994. The Expedition ANTARKTIS Xr4 of RV AolarsternB in 1992. Ber. Polarforsch., vol. 140, pp. 1–90. Liss, P.S., Merlivat, L., 1986. Air–sea exchange rates: Introduction and synthesis. In: Buat-Menard, P. ŽEd.., The Role of ´ Air–Sea Exchange in Geochemical Cycling. NATOrASI series. Reidel, Dordrecht, The Netherlands, pp. 113–127. Locarnini, R.A., Whitworth, T. III, Nowlin, T. Jr., 1993. The importance of the Scotia Sea on the outflow of Weddell Sea Deep Water. J. Mar. Res. 51, 135–153. Mantyla, A.W., Reid, J.L., 1995. On the origins of deep and bottom waters of the Indian Ocean. J. Geophys. Res. 100, 2417–2439.
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Metzl, N., Beauverger, C., Brunet, C., Goyet, C., Poisson, A., 1991. Surface water carbon dioxide in the southwest Indian sector of the Southern Ocean: a highly variable CO 2 sourcer sink region in summer. Mar. Chem. 35, 85–95. Metzl, N., Poisson, A., Louanchi, F., Brunet, C., Schauer, B., Bres, B., 1995. Spatio-temporal distributions of air–sea fluxes of CO 2 in the Indian and Antarctic Oceans: a first step. Tellus 47B, 56–69. Orsi, A.H., Whitworth, T. III, Nowlin, T. Jr., 1995. On the meridional extent and fronts of the Antarctic Circumpolar Current. Deep-Sea Res., Part I 42, 641–673. Orsi, A.H., Johnson, G.C., Bullister, J.L., 1999. Circulation, mixing, and production of Antarctic Bottom Water. Prog. Oceanogr. 43, 55–109. Poisson, A., Metzl, N., Brunet, C., Schauer, B., Bres, B., RuizPino, D., Louanchi, F., 1993. Variability of sources and sinks of CO 2 in the western Indian and Southern Oceans during the year 1991. J. Geophys. Res. 98, 22759–22778. Poisson, A., Metzl, N., Danet, X., Louanchi, F., Brunet, C., Schauer, B., Bres, B., Ruiz-Pino, D., 1994. Air–sea CO 2 fluxes in the Southern Ocean between 258E and 858E. In: Johannessen, O.M., Muench, R.D., Overland, J.E. ŽEds.., The Polar Oceans and their Role in Shaping the Global Environment. Geophysical Monograph 85. American Geophysical Union, Washington, DC, pp. 273–284. Robertson, J.E., Watson, A.J., 1995. A summer-time sink for atmospheric carbon dioxide in the Southern Ocean between 888W and 808E. Deep-Sea Res., Part II 42, 1081–1091. Robertson, J.E., Watson, A.J., Langdon, C., Ling, R.D., Wood, J.W., 1993. Diurnal variation in surface pCO 2 and O 2 at 608N, 208W in the North Atlantic. Deep-Sea Res., Part II 40, 409–422. Robinson, C., Williams, P.J.leB., 1992. Development and assessment of an analytical system for the accurate and continual measurement of total dissolved inorganic carbon. Mar. Chem. 34, 157–175. Schroder, M., Fahrbach, E., 1999. On the structure and the ¨ transport of the eastern Weddell Gyre. Deep-Sea Res., Part II 46, 501–527. Spiridonov, V.A., Nothig, E.-M., Schroder, M., Wisotzki, A., ¨ ¨ 1996. The onset of biological winter in the eastern Weddell Gyre ŽAntarctica. planktonic community. J. Mar. Syst. 9, 211–230. Stoll, M.H.C., 1994. Inorganic Carbon Behaviour in the North Atlantic Ocean. Rijksuniversiteit Groningen, Groningen, The Netherlands, 193 pp. Stoll, M.H.C., De Baar, H.J.W., Hoppema, M., Fahrbach, E., 1999a. New early winter fCO 2 data reveal continuous uptake of CO 2 by the Weddell Sea. Tellus 51B, 679–687. Stoll, M.H.C., De Baar, H.J.W., Zondervan, I., De Jong, E., Bathmann, U.V., Fahrbach, E., 1999b. Carbon Dioxide Uptake and Release by Phytoplankton Blooms at the Antarctic Marginal Ice Zone. Submitted for publication. Takahashi, T., Olafsson, J., Goddard, J.G., Chipman, D.W., Sutherland, S.C., 1993. Seasonal variation of CO 2 and nutrients in the high-latitude surface oceans: a comparative study. Global Biogeochem. Cycles 7, 843–878.
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Wanninkhof, R., 1992. Relationship between wind speed and gas exchange over the ocean. J. Geophys. Res. 97, 7373–7382. Wanninkhof, R., Thoning, K., 1993. Measurement of fugacity of CO 2 in surface water using continuous and discrete sampling methods. Mar. Chem. 44, 189–205. Weiss, R.F., 1974. Carbon dioxide in water and seawater: the solubility of a non-ideal gas. Mar. Chem. 2, 203–215. Weiss, R.F., Van Woy, F.A., Salameh, P.K., 1992. Surface water
and atmospheric carbon dioxide and nitrous oxide observations by shipboard automated gas chromatography: Results from expeditions between 1977 and 1990. SIO 92-11 and ORNLrCDIAC-59, Scripps Institution of Oceanography, Carbon Dioxide Information Analysis Center, Oak Ridge, TN. Wessel, P., Smith, W.H.F., 1995. New version of the Generic Mapping Tools released. EOS Trans. AGU 76, 329.
CO 2 in the Weddell Gyre and Antarctic Circumpolar Current: austral autumn and early winter Mario Hoppema a,) , Michel H.C. Stoll b,1, Hein J.W. de Baar b,1 a
Department of Tracer Oceanography, UniÕersity of Bremen, FB1, IUP, P.O. Box 330 440, D-28334 Bremen, Germany b Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Texel, Netherlands Received 1 June 1999; received in revised form 24 December 1999; accepted 7 August 2000
Abstract Quasi-continuous fugacity of CO 2 ŽfCO 2 . data were collected in the eastern Weddell Gyre and southern Antarctic Circumpolar Current ŽACC. of the Southern Ocean during austral autumn 1996. Full depth Total CO 2 ŽTCO 2 . sections are presented for austral autumn and winter Ž1992. cruises. Pronounced fCO 2 gradients were observed at the Southern Ocean fronts. In the Weddell Gyre, fCO 2 regimes appeared to coincide with surface and subsurface hydrographic regimes. The southern ACC was supersaturated with respect to CO 2 , as was part of the northern Weddell Gyre. The southern Weddell Gyre was markedly undersaturated. The great potential of autumn cooling for generating undersaturation and CO 2 uptake from the atmosphere was demonstrated. In the northeastern Weddell Gyre, upwelling of CO 2- and salt-rich deep water was shown to play a role as the horizontal fCO 2 distribution closely resembled that of the surface salinity. The total uptake of atmospheric CO 2 by the Weddell Gyre in autumn Ž45 days. was calculated to be 7 P 10 12 g C. The deep TCO 2 distribution noticeably reflected the different water masses in the region. A new deep TCO 2 maximum was detected in the ACC, which apparently characterizes the boundary between the equatorward flowing Antarctic Bottom Water ŽAABW. and the Circumpolar Deep Water ŽCDW.. East of the Weddell Gyre, the AABW stratum is much thicker Ž) 2000 m. than more to the west, on the prime meridian Ž- 300 m.. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Weddell Sea; Autumn; Carbon dioxide; Air–sea exchange; Antarctic Bottom Water
1. Introduction Exchange of carbon dioxide between the oceans and the atmosphere is a pivotal process within the
)
Corresponding author. Tel.: q49-421-2182931; fax: q49421-2187018. E-mail addresses: [email protected] ŽM. Hoppema., [email protected] ŽM.H.C. Stoll., [email protected] ŽH.J.W. de Baar.. 1 Fax: q31-222-319674.
Žanthropogenically perturbed. global carbon cycle, and a major forcing of the atmospheric CO 2 content. Unfortunately, there is much confusion pertaining to this issue. This ranges from the actual parameterization of the process of gas exchange across the air–sea interface for calculating fluxes ŽLiss and Merlivat, 1986; Wanninkhof, 1992., to the overall shortness of data for reliably calculating the annual CO 2 flux for the world ocean. In temperate and tropical regions, some basic level of spatial and temporal coverage of the partial pressure or fugacity of CO 2 ŽfCO 2 . in the
0304-4203r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 4 2 0 3 Ž 0 0 . 0 0 0 8 2 - 7
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surface has been achieved. However, for the Southern Ocean, such a basic level is not even feasible, data outside the summer period being particularly scarce. Recently, the seasonal coverage of surface CO 2 in parts of the Southern Ocean has somewhat improved ŽPoisson et al., 1993; Takahashi et al., 1993; Metzl et al., 1995; Hoppema et al., 1995., but due to the large spatial and short-term temporal variability, which is not yet sufficiently understood, it remains difficult to produce annual estimates of CO 2 exchange between the ocean and the atmosphere. Another approach for obtaining air–sea CO 2 exchange is constructing a surface layer budget using Total CO 2 ŽTCO 2 . ŽHoppema et al., 1999. or fCO 2 ŽPoisson et al., 1993; Bakker at al., 1997.. Under certain conditions, this method enables to calculate annual fluxes based on only few cruises. Applied to the offshore Weddell Sea, this region was found to be an annual sink for atmospheric CO 2 ŽHoppema et al., 1999.. Budgetting CO 2 in the surface layer and utilizing fCO 2 measurements to calculate CO 2 fluxes are both valuable approaches to the issue of air–sea exchange. The magnitude of the CO 2 flux as obtained from a budget is a useful quantity for constraining global carbon cycle models. On the other hand, fCO 2 distributions combined with other surface layer properties are useful for studying processes and variability in the oceanic surface layer, which need to be understood to improve the predictive ability of biogeochemical processes and their modelling. It should be added that the surface layer fCO 2 distribution is to some smaller or larger extent dependent on subsurface processes as well, particularly as the Southern Ocean is a known upwelling region for deep and bottom waters from the other ocean basins. The state of the Southern Ocean in autumn is largely unknown ŽStoll et al., 1999a.. In autumn, the surface layer is preconditioned for the subsequent winter, when large parts become ice covered. It is anticipated that physical processes such as surface water cooling and entrainment of subsurface water generate a significant portion of the CO 2 variability, but biological processes could also play a part. Here we present novel observations of the fCO 2 collected during cruise ANT XIIIr4 Ž1996.. These are austral autumn data for the eastern Weddell Gyre, a region that has been poorly investigated. We explore the
state of the surface layer and calculate fluxes between the ocean and the atmosphere. Accompanying thermosalinograph data are used to explain variations of the fCO 2 distribution. Because most of the deep water of the Antarctic Ocean has recently been ventilated, the deep water distribution has a high relevance with regard to CO 2 . On one hand, large-scale upwelling occurs in the Southern Ocean, while on the other hand, new deep and bottom waters are generated, suggesting a close coupling between surface and subsurface waters. Moreover, the Antarctic Ocean provides the preformed TCO 2 values for mixing in other ocean basins. Thus, besides surface fCO 2 data, we also present for the eastern Weddell Gyre and adjacent circumpolar current new TCO 2 sections, which were collected during cruises ANT Xr4 Ž1992. and ANT XIIIr4 Ž1996.. This work is part of a study on carbon cycling in the Atlantic sector of the Southern Ocean. We took part in several cruises. Previous publications arising from this study ŽHoppema et al., 1995, 1998, 1999; Stoll et al., 1999a. focussed on the Weddell Sea proper, i.e. west of about 108W. Some other TCO 2 data for the eastern Weddell Gyre from cruise ANT Xr4 were already presented by Hoppema et al. Ž1997.. In the relevant Section 3.1, the difference with the present data is indicated and when appropriate we refer to these data.
2. Methods Data are presented from two cruises with the German ice-breaker AFS PolarsternB ŽFig. 1., one in MayrJuly 1992 ŽANT Xr4; Lemke, 1994. and one in MarchrMay 1996 ŽANT XIIIr4; Fahrbach, 1997.. Water samples for the determination of TCO 2 were collected with a 24-place General Oceanics rosette sampler that was coupled to a Conductivity–Temperature–Depth ŽCTD. instrument. These samples were distributed over the whole water column, with bias towards the upper 1000 m. Underway surface temperature and salinity were measured with the ship’s thermosalinograph, which is mounted 8 m below the sea surface. The fCO 2 was measured underway in surface seawater quasi-continuously only on autumn cruise ANT XIIIr4. fCO 2 is equivalent to the partial pressure of CO 2 , but corrected for the non-ideal
M. Hoppema et al.r Marine Chemistry 72 (2000) 203–220 Fig. 1. Map of the southern South Atlantic Ocean including the Atlantic sector of the Southern Ocean. Positions of stations where full-depth profiles for vertical sections were obtained are indicated. Stations during austral winter cruise ANT Xr4 represented by diamonds and during austral autumn cruise ANT XIIIr4 by stars. In between ANT XIIIr4 stations fCO 2 , salinity and temperature were measured quasi-continuously. Map created with Generic Mapping Tools ŽWessel and Smith, 1995.. 205
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behaviour of CO 2 in a gas mixture. Both quantities are nominally only slightly different from each other ŽWeiss, 1974.. TCO 2 was determined with the coulometric method after Johnson et al. Ž1987. as slightly modified by Robinson and Williams Ž1992. and Stoll Ž1994., using the same equipment on both cruises. The TCO 2 concentration is the sum of all carbonate species dissolved in seawater, i.e. TCO 2 s wCO 2 x q wH 2 CO 3 x q wHCOy x w 2y x. Subsamples were 3 q CO 3 collected in glass bottles Ž0.5 l. with flexible screw caps. All analyses were performed within 24 h, but most within 12 h of sampling. Accuracy on cruise ANT XIIIr4 Ž1996. was set by certified reference material ŽDOE, 1994. made available by Prof. A. Dickson of the Scripps Institution of Oceanography ŽUSA.. At cruise ANT Xr4 these standards were not yet available to us. Standards were measured for each cell prepared for the coulometer and the cells were changed about once a day. The measured standards Žbatch number 30. were on average lower than the certified value by 10.7 " 2.2 mmol kgy1 Ž n s 52.. However, data collected during the utilization of one cell were corrected using the standard measured for this cell. During the first part of cruise ANT XIIIr4, for which data are presented here, 68 duplicates were measured which on average differed by 1.4 " 0.9 mmol kgy1 . This is worse than for the second part of the cruise Ždifference 1.0 mmol kgy1 ., due to suboptimal performance of the coulometer liquid. During cruise ANT Xr4, 83 duplicates were measured, differing 0.9 " 0.6 mmol kgy1 . The ANT Xr4 data were corrected using standardized data of another cruise; for details see Hoppema et al. Ž1995.. fCO 2 was measured on-line in surface seawater and in the overlying atmosphere using a fully automated sampling system, designed after Wanninkhof and Thoning Ž1993. including a Li-Cor ŽModel 6252. infrared analyzer. A continuous supply of marine air from the crow’s nest was maintained which was subsampled at regular intervals. For determination of fCO 2 in the oceanic surface layer, seawater was drawn off the ship’s continuous water supply and sprayed into the continuous-flow equilibrator ŽRobertson et al., 1993; Bakker et al., 1997. at a rate of 40–60 cm3 sy1 . A slight modification to the equilibrator was made so that ice blockage would hardly occur, thus enabling us to analyze water from under
the sea ice. The inlet in the hydro-well of APolarsternB allowed a fairly uninterrupted water supply in spite of sea ice. Temperature of the seawater inlet and equilibrator were recorded simultaneously with each measurement. The headspace gas was circulated and subsampled at regular intervals. The equilibrator was vented to the atmosphere Žmarine air.. Sample air was dried with Aquasorb ŽMerck. before being flushed through the infrared cell of the Li-Cor. The response time of the system was less than 2 min. During the transport of seawater from the ship’s inlet to the equilibrator a slight heating occurs which was generally less than 0.18C, but in some cases 0.3– 0.48C. As fCO 2 is dependent on temperature, it was corrected for this temperature increase using the equations of Copin-Montegut Ž1988, 1989.. Obvious outliers and datapoints with too large a temperature difference Žindicating blockage in the seawater supply. were deleted from the dataset. An analysis cycle of fCO 2 measurements, which takes less than 1 h, consists of a calibration Žusing three reference gases of 266.95, 367.76 and 443.66 mmol moly1 CO 2 in air, respectively., air, seawater Ž5 = ., air, seawater Ž5 = . and a calibration. For each sample, 10 readings Ževery 0.5 s. were taken and averaged during post-processing. Final data were obtained by interpolating between two consecutive calibration cycles. The reference gases used on board were recalibrated in the home laboratory against National Oceanic and Atmospheric Administration ŽNOAA. certified standard gas mixtures Žaccurate to "0.01 mmol moly1 . before and after the cruise. For all reference gases, the precision during the calibration was better than 0.07 mmol moly1 . No significant difference was observed between the pre-cruise and post-cruise calibrations. It is not possible to give a robust estimate of the precision and accuracy of underway oceanic fCO 2 measurements. However, during the cruise fCO 2 was measured as if underway while the ship was stationary for about one and a half days. The mean fCO 2 was 346.5 " 1.2 matm Ž n s 326.. As the real fCO 2 is subject to some natural variability on this position Žthe ship also drifted somewhat., this precision should be considered an upper bound. As regards the atmospheric CO 2 measurements, these can be compared with other data collected in the region. The mean atmospheric mixing ratio Žmole fraction. measured
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during cruise ANT XIIIr4 is 360.7 mmol moly1 , standard deviation 0.6 mmol moly1 and range 359.5–362.5 Ž n s 10,331.. This compares well with extrapolated data from measurements at Halley Bay, Weddell Sea Ž361.3 mmol moly1 . and Palmer Station, western Antarctic Peninsula, amounting to 359.9 mmol moly1 ŽConway et al., 1994.. The CO 2 mixing ratio measured at the South Pole in April–May 1996 is somewhat lower than our value, i.e. 359.4 mmol moly1 ŽKeeling and Whorf, 1999., but it is also further away than the other stations. From the difference between fCO 2 in seawater and in the atmosphere, the net gas exchange of CO 2 across the air–sea interface was computed according to the method and parameterizations of Wanninkhof Ž1992.. In situ wind speed and sea surface temperature ŽSST., which go into this calculation, were obtained from the APolarsternB meteorological data acquisition system.
3. Meridional section along the prime meridian 3.1. TCO2 distribution across the ACC Fig. 2 shows a TCO 2 section of the Antarctic Circumpolar Current ŽACC. and the northernmost part of the Weddell Gyre along the prime meridian obtained during the austral winter cruise. Upper water column TCO 2 data from the same cruise ANT Xr4, but for the Weddell Gyre Ži.e. south of the transect shown here., were previously shown by Hoppema et al. Ž1997.. In the north, the section starts just south of the Subtropical Front. The Weddell Front ŽWF. at about 55.58S constitutes the boundary between the ACC and the Weddell Gyre ŽOrsi et al., 1995.. The positions of the fronts as shown in Fig. 2 were fixed according to the simultaneously measured, corresponding salinity and potential temperature sections ŽSchroder and Fahrbach, ¨ 1999; Lemke, 1994.. The surface TCO 2 transect exhibits maximal horizontal gradients across the Subantarctic and Weddell Fronts ŽFig. 2., and generally, a large gradient in the region of the Polar Front ŽPF. and Southern ACC Front ŽSACCF.. The increase of TCO 2 in the surface layer from north to south is mainly due to the increasing CO 2 solubility with decreasing temperature. In the Weddell regime
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among the highest surface layer TCO 2 concentrations in the world oceans are found Žsee also Hoppema et al., 1995.. The large-scale TCO 2 distribution reflects the known distribution of water masses in the region ŽFig. 2.. At about 508S, i.e. near the PF, the downward sloping to the north of TCO 2 isolines from the Žsub.surface layer ŽFig. 2. points to Antarctic Intermediate Water ŽAAIW.. As the isolines are steepest in this area, this is designated a subsurface expression of the PF. Underneath the AAIW and between 508S and 55.58S directly below the surface layer, lies the Circumpolar Deep Water ŽCDW., extending over almost all of the rest of the water column. Upper CDW features a TCO 2 maximum Žcentered around 1500 m between 418S and 458S. and Lower CDW a TCO 2 minimum Žat about 3000 m near 428S.. Lower CDW originates from the relatively young North Atlantic Deep Water ŽNADW. with its accordingly low TCO 2 concentration. Characteristics of the Upper CDW derive from the older deep water of the Pacific and Indian Oceans, which have a higher TCO 2 concentration. In southward direction, the CDW shoals considerably, for example, the TCO 2 minimum of the Lower CDW moves from 3000 m depth at 428S to 800 m depth in the southernmost ACC at 558S ŽFig. 2.. Simultaneously, the TCO 2 value in the TCO 2 minimum increases markedly by about 40 mmol kgy1 , which reflects the decreasing influence of the NADW and increasing influence of the deep waters of the Pacific and the Weddell Sea. Near the bottom of the ACC, Antarctic Bottom Water ŽAABW. is found. The high TCO 2 of AABW derives from the subpolar region south of the ACC, such as the Weddell Sea. On its way northwards, the AABW mixes with the overlying CDW, which continuously changes its composition ŽOrsi et al., 1999.. On close inspection, a weak TCO 2 maximum can be discerned 100–300 m above the bottom Žindicated by crosses in Fig. 2.. This TCO 2 maximum constitutes the upper boundary of the AABW. In Section 4.1, this issue is discussed in more detail. 3.2. fCO2 surface transect through the southern ACC and Weddell Gyre in austral autumn Fig. 3 portrays the autumn transect ŽApril 1996. of fCO 2 across the southern ACC and Weddell Gyre along the prime meridian, including the correspond-
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Fig. 2. Vertical TCO 2 section Žbottom. along the prime meridian across the ACC into the northern Weddell Gyre obtained during cruise ANT Xr4 ŽMayrJune 1992.. Sampling depths shown by dots. Crosses in the contour plot display the location of near-bottom TCO 2 maxima. Positions of hydrographic fronts and water masses are indicated: SAF, Subantarctic Front; PF, Polar Front; SACCF, Southern ACC Front; WF, Weddell Front; AAIW, Antarctic Intermediate Water; UCDW, Upper Circumpolar Deep Water; LCDW, Lower Circumpolar Deep Water; AABW, Antarctic Bottom Water. The distribution of TCO 2 in the near-surface water of the section is shown separately Žtop..
ing SST and surface salinity. In this figure, the measured atmospheric fCO 2 is displayed, which varies between 343 and 358 matm. Note that the atmospheric fCO 2 is a partial pressure equivalent indeed, hence depending on the varying total atmospheric pressure. At 55–568S, the marked salinity gradient is a fine indicator of the WF. The SST signal across this front is weak, whereas the fCO 2 gradient is pronounced with about 10 matm ŽFig. 3.. Around 52.58S, the strong SST gradient appears to be an expression of
the SACCF — the PF in this region occurs between 498S and 508S ŽSchroder and Fahrbach, 1999.. An ¨ elevated fCO 2 gradient in this area occurs somewhat to the north of the maximal SST gradient, but it coincides with another elevated SST gradient centred at 51.58S. In the northernmost part of the transect, the water is undersaturated with respect to CO 2 ŽFig. 3.. This surface water tends to flow northwards to participate in AAIW formation Žsee Fig. 2 and Section 3.1., the latter process thus constituting a conduit for CO 2 uptake from the atmosphere.
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Fig. 3. Distribution of fCO 2 in the surface water and the overlying atmosphere along the prime meridian in autumn Ž9–22 April 1996. during cruise ANT XIIIr4 Žtop. and the corresponding sea surface temperature and salinity Žbottom.. Locations of major fronts and subareas are indicated: ASF, Antarctic Slope Front; WF, Weddell Front; SACCF, Southern ACC Front; ACC, Antarctic Circumpolar Current; WG, Weddell Gyre.
Within the Weddell Gyre, the northern part Ž55– 608S. was clearly supersaturated with fCO 2 . This fCO 2 supersaturation appears to exactly coincide with the northern rim current of the Weddell Gyre ŽSchroder and Fahrbach, 1999.. Note that the rim ¨ current is a feature pertinent to the whole water column. Thus, this CO 2 supersaturation appears to originate west of our transect. Advection of surface layer properties is a major reason why transect-wide
relationships between fCO 2 on one hand and SST and surface salinity on the other hand are scattered ŽFig. 4.. In contrast to the northern area with eastward flow and supersaturation, the area between 628S and the Antarctic coastline reveals CO 2 undersaturation ŽFig. 3. and has principally westward flow. Thus, within the Weddell Gyre, the CO 2 under- or supersaturation is closely coupled to the origin of the
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surface water. Additionally, this subdivision of relatively high and low CO 2 saturation ŽFig. 3. is roughly coincident with the areas where relatively shallow and deep lying TCO 2 maxima occur, respectively ŽHoppema et al., 1997.. This is an indication that upwelling of deep water plays a role in the regional distribution of fCO 2 in the surface layer as well. In autumn, surface water cooling occurs, which reduces the fCO 2 by increasing the CO 2 solubility. We can tentatively assess the effects of cooling on the fCO 2 . In this region, the summer surface temperature is 0.5–18C ŽStoll et al., 1999b.. Using the thermodynamic relationships of the CO 2 system, we can compute fCO 2 at these summer temperatures, assuming constant TCO 2 and alkalinity, the latter property being conservative in the Weddell Sea ŽAnderson et al., 1991.. From Fig. 3, it can be deduced that most cooling has occurred in the south and least in the north. We distinguish regions with different temperatures and thus different cooling. In the region abutting the coastline Žabout 69.5–68.58S., the surface layer is approximately at freezing point, indicating that at summer temperature the fCO 2 could have been 40–50 matm higher. In the region between 68.58S and 66.58S, where the autumn SST is about y18C, the potential fCO 2 difference with summer is 30–40 matm. For the region 64.5–61.58S with a SST of about y0.38C, the summer fCO 2 might be higher by 15–25 matm. Finally, for the northern limb of the Weddell Gyre, which is supersaturated in autumn ŽFig. 3., this supersaturation could have been even higher by 5–15 matm in summer. Our calculations suggest that the entire Weddell Gyre may have been supersaturated before the onset of autumn cooling, the northern limb indeed still is. Supersaturation could be caused by remineralization of organic matter after the cessation of phytoplankton blooms in summer. Particularly in the region south of 608S extensive summer blooms were indeed observed ŽStoll et al., 1999b.. An alternative explanation for the observed CO 2 undersaturation in the south ŽFig. 3. is an autumn phytoplankton bloom.
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However, the contribution of active phytoplankton growth in autumn to CO 2 drawdown was minor as evidenced by the low level of chlorophyll a at this transect ŽC. Dubischar, Alfred-Wegener-Institut, personal communication, 1996.. A general tendency of a higher SST corresponding to a higher fCO 2 is revealed in Fig. 4. In essence, this is caused by the difference between the Weddell Gyre and the ACC. Within these subregions, there is no relationship ŽWeddell Gyre., or maybe a slight negative correlation ŽACC.. Within the southern and central Weddell Gyre, there is a similarity between the distributions of fCO 2 and surface salinity ŽFig. 3., which translates into a weak positive correlation for this subregion ŽFig. 4.. This may be explained by upwelling of deep water. However, the scattering and the only weak correlation hint that differences in upwelling activity cannot be the main explanation for the horizontal distributions. The northern Weddell Gyre and ACC appear as separate clusters in Fig 4. In the Maud Rise region Ž64–678S., circulation– topography interactions lead to an enhancement of upwelling of deep water, which causes a locally high salt content of the surface layer ŽGordon and Huber, 1990.. Our data around 66.58S Žover the crest of Maud Rise at this longitude. confirm this ŽFig. 3.. The salinity maximum is accompanied by a faint, local fCO 2 maximum ŽFig. 3.. However, the whole area contains water that is undersaturated in CO 2 , indicating that seasonal cooling is dominant over upwelling.
4. Quasi-zonal section along 0–398E 4.1. TCO2 distribution Fig. 5 shows a TCO 2 section of the easternmost Weddell Gyre and adjacent ACC. These data were collected during cruise ANT XIIIr4 ŽMarchrApril 1996. and represent the first survey of this part of the Weddell Gyre and adjacent ACC. The SACCF is
Fig. 4. fCO 2 in surface water vs. sea surface temperature Žtop. and vs. surface salinity Žbottom. for the transect along the prime meridian in austral autumn 1996. Three different sub-regions of the transect are indicated in the plots Žsee text.: WG, Weddell Gyre; ACC, Antarctic Circumpolar Current.
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Fig. 5. Quasi-zonal vertical TCO 2 section between the Conrad Rise Žright. and the prime meridian across the eastern Weddell Gyre and adjacent ACC. Data from cruise ANT XIIIr4, 23 March–5 April 1996. Sampling depths shown by dots. Positions of hydrographic fronts, and water masses are indicated: WF, Weddell Front; SACCF, Southern ACC Front; WDW, Warm Deep Water; WSDW, Weddell Sea Deep Water; WSBW, Weddell Sea Bottom Water; AABW, Antarctic Bottom Water.
a very sharp feature all through the water column below the surface layer ŽFig. 5.. In contrast, no clear expression can be discerned for the deep WF. In the Weddell Gyre part, west of 25–268E ŽFig. . 5 , the Warm Deep Water at 300–700 m depth features TCO 2 concentrations above 2270 mmol kgy1 , i.e. much higher than in the deep water entering the Weddell Gyre ŽHoppema et al., 1997.. This is indicative of the presence of an older type of WDW. Underneath the WDW, the Weddell Sea Deep Water and Weddell Sea Bottom Water are found with monotonically decreasing TCO 2 concentrations. An upward hump of the TCO 2 isolines is
discernible for the Weddell Gyre part ŽFig. 5.. This phenomenon was also observed in the corresponding temperature and salinity sections, in which it points to a northward water flow component west of about 138E and a southward flow component east of 138E ŽSchroder ¨ and Fahrbach, 1999.. The 138W meridian also appears to be the division line between near bottom water with relatively low TCO 2 Ž- 2250 mmol kgy1 . west of it and bottom water with higher TCO 2 Ž) 2250 mmol kgy1 . east of it ŽFig. 5.. In the ACC part of the section east of the SACCF, the vertical structure and water mass characteristics ŽFig. 5. seem rather different from those in the ACC
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on the prime meridian Žcompare with Fig. 2.. However, only the mutual proportions of the water masses are different. The TCO 2 minimum at about 1200– 1400 m is pertinent to the Lower CDW, while the TCO 2 maximum at about 600 m is characteristic of the Upper CDW ŽFig. 5.. The marked TCO 2 maximum at about 3000 m ŽFig. 5. corresponds to the weak TCO 2 maximum, not far from the sea floor ŽFig. 2; Section 3.1. on the prime meridian. The TCO 2 maximum at about 3000 m depth marks the upper boundary of the AABW that has left the subpolar Žhere: Weddell. regime. The origin of this TCO 2 maximum is the following: In the ACC below the TCO 2 minimum, TCO 2 increases with depth. In contrast, in the subpolar regime a decrease of TCO 2 with depth is found below the WDW stratum because of the TCO 2 minimum in the bottom water. Hence, when the water of the ACC and the subpolar regime are superimposed, a TCO 2 maximum is induced at the interface. It is important to appreciate that it is a unique feature of the TCO 2 distribution to reveal the upper boundary of the AABW. The temperature and salinity distributions do not show an extremum at this boundary ŽSchroder ¨ and Fahrbach, 1999., because the vertical gradients of temperature and salinity in the deep ACC and the deep Weddell Gyre have the same sign. 4.2. fCO2 surface transect in austral autumn On the quasi-zonal transect between the Conrad Rise and the prime meridian, both undersaturation and supersaturation of CO 2 were observed in austral autumn ŽFig. 6.. fCO 2 values in the southern ACC were generally above equilibrium with the atmosphere ŽFig. 6., like at the prime meridian ŽFig. 3.. The SACCF, which is distinguishable near 288E by a westward decrease of SST by more than 18C and increase of the surface salinity ŽFig. 6., also appears to separate waters with different fCO 2 levels. The large supersaturation could be caused by a combination of intense upwelling in the southernmost ACC ŽComiso et al., 1993., and remineralization of organic matter, as biological activity may be relatively high there ŽHoppema et al., 2000.. Only the southern ACC appears to be supersaturated with CO 2 in this time of the year, as more northern parts Žnear the Polar and Subantarctic Fronts. were markedly under-
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saturated ŽFig. 3; and other unpublished data.. The WF is not very pronounced in the temperature and salinity distributions, but appears to be associated with a dramatic fCO 2 change of about 30 matm. The distinct fCO 2 signature of the WF stands in contrast to the deep TCO 2 expression of the front which is only weak. Dissimilar biological conditions on both sides of the front ŽSpiridonov et al., 1996. may be responsible for the observed fCO 2 distribution. There is a high degree of conformity in the horizontal distributions of fCO 2 and surface salinity ŽFig. 6.. A transect-wide positive relationship exists between fCO 2 and surface salinity ŽFig. 7., which underscores the importance of large-scale upwelling for explaining these distributions. Upwelling occurs both in the southern ACC and in the Weddell Gyre ŽGordon and Huber, 1990; Comiso et al., 1993. and transports TCO 2-rich Žhigh-fCO 2 ., saline water into the surface layer. Ice formation or wet precipitation from the atmosphere could frustrate the correlation of fCO 2 and salinity due to upwelling, because it changes the salinity in the surface layer. However, no ice had yet been formed in the area before the time of our observations. It should be realized that supersaturation of fCO 2 can, besides by remineralization of organic matter, only be caused by upwelling. Between fCO 2 and SST, a transect-wide relationship due to upwelling is not observed ŽFig. 7.. This is because in the western part of the transect, where the SST is low, upwelling would increase the SST, whereas in the eastern part, where the SST is high, it would decrease the SST. Besides that, cooling of the surface layer, which occurs at its annual maximum rate in autumn, could substantially alter the SST Žand also fCO 2 . additionally. Fig. 7 shows that an anticorrelation exists of fCO 2 and SST for temperatures below about 18C, and a positive correlation for the temperature range 1–2.58C. A positive correlation may be the consequence of surface water cooling, but the fCO 2 range of about 35 matm is too large to be caused by cooling of only 18C Žsee Fig. 7.. This observation agrees with the above suggestion that upwelling plays a part in this area. An anti-correlation of fCO 2 and the SST could arise when surface water with a temperature above 18C is subject to upwelling of WDW with a lower temperature. In conjunction with the fact that 138W is the division
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Fig. 6. Distribution of fCO 2 in the surface water and the overlying atmosphere on a quasi-zonal transect through the eastern Weddell Gyre and southern ACC in austral autumn Ž25 March–4 April 1996. during cruise ANT XIIIr4 Žtop. and the corresponding sea surface temperature and salinity Žbottom.. Locations of major fronts are indicated: WF, Weddell Front; SACCF, Southern ACC Front.
between water with northward flow and water with southward flow ŽSchroder and Fahrbach, 1999; see ¨ also Section 4.1., we note that the horizontal fCO 2 , salinity and SST variations around 138E are relatively large, while this is also the location where the SST reaches the value of about 18C ŽFig. 6.. This strongly suggests that it is the water with the northward flow component Žwest of 138W. that has an anti-correlation of fCO 2 and SST ŽFig. 7., whereas
in the water with the southward flow component a positive correlation of fCO 2 and SST exists. In the Weddell regime, fCO 2 is largely below equilibrium with the atmosphere ŽFig. 6.. Undersaturation of fCO 2 in autumn is likely to be caused by cooling of the surface layer. Photosynthetic drawdown of CO 2 was hardly active here, as evidenced by the low level of chlorophyll a ŽC. Dubischar, personal communication, 1996.. Only in the western
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Fig. 7. fCO 2 in surface water vs. sea surface temperature Žtop. and vs. surface salinity Žbottom. for the quasi-zonal transect through the eastern Weddell Gyre and southern ACC in austral autumn 1996.
part of the transect, supersaturation was observed. The latter area is spatially connected with the high-
fCO 2 region at the prime meridian Žsee Figs. 1 and 3..
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5. Air–sea fluxes of CO 2 CO 2 fluxes were calculated for different sub-regions of our area of investigation using in situ auxiliary data Žwind speed, surface water temperature.. We distinguish the southern ACC as the area between the SACCF and the WF Žsee also Fig. 1.. Within the Weddell Gyre, the northern part, in essence corresponding to the cold regime deep water of Gordon and Huber Ž1990., is situated between 61.48S and the WF. This includes our quasi-zonal transect from 0–25.98E ŽSection 4.2. and part of the prime meridian transect ŽSection 3.2.. The southern part of the Weddell Gyre Žanalogously the warm regime deep water of Gordon and Huber, 1990. lies between 61.48S and the Antarctic coastline. Results appear in Table 1. The southern ACC in autumn is a source of CO 2 to the atmosphere, east of the Weddell Gyre distinctly more than at the prime meridian. Within the northern Weddell Gyre, both sources and sinks are found, approximately equal in magnitude. The large southern Weddell Gyre is a pronounced sink in autumn with almost 6 mmol my2 dayy1 . The air–sea CO 2 flux for the entire Weddell Gyre was computed as the data of cruise ANT XIIIr4 cover almost the whole gyre area. This includes data from the western Weddell Sea between Kapp Norvegia and the tip of the Antarctic Peninsula as reported by Stoll et al. Ž1999a.. We calculated a mean flux of y2.52 mmol m2 dayy1 for the entire Weddell Gyre, that is, the Weddell Gyre is a sink for atmospheric CO 2 in autumn. A total air to sea flux of 7 P 10 12 g C Table 1 Calculated mean air–sea CO 2 fluxes Žmmol my2 dayy1 . for different subregions of the Southern Ocean Žin brackets the assumed boundaries. and for the Weddell Gyre as a whole Žincluding data from the western Weddell Sea; Stoll et al., 1999a.. Negative flux indicates uptake by the sea
Southern ACC ŽSACCF–WF. Northern Weddell Gyre ŽWF–61.48S. Southern Weddell Gyre Ž61.48S–coast. Total mean flux of Weddell Gyre
Prime meridian
Eastern Weddell
q1.75 q2.33
q5.47 y2.74
y5.89
y2.52
SACCF: Southern ACC Front; WF: Weddell Front.
Žs 0.007 Gton C. is obtained over the time period of our autumn cruise Ž45 days. for the entire area of the Weddell Gyre Žapproximately 5 P 10 12 m2 .. An annual total uptake of 8 P 10 12 g C was obtained for the offshore western Weddell Sea in a budget study ŽHoppema et al., 1999.. The annual physically mediated CO 2 uptake in that same area amounts to about 2.4 P 10 12 g C ŽStoll et al., 1999a.. Accounting for the larger surface area Žabout 3 times the western Weddell Sea., the figure for the entire Weddell Gyre as obtained in the present study fits well into this. Our data are consistent with the notion that physically mediated CO 2 uptake largely occurs in autumn. Robertson and Watson Ž1995. calculated a CO 2 uptake of 1 P 10 14 g C over a 4-month summer period for an area including most of the Weddell Gyre, which translates into approximately 5 P 10 13 g C for the Weddell Gyre. Thus, CO 2 uptake by the Weddell Gyre in summer is much larger than in autumn. Though in the other seasons some outgassing may occur as a consequence of upwelling, all data support the conclusion that the western Weddell Sea is an annual CO 2 sink ŽHoppema et al., 1999. and this is likely for the greater Weddell Gyre as well.
6. General discussion fCO 2 in the surface layer of the Southern Ocean is determined by a combination of processes, notably upwelling, biological activity and temperaturersalinity changes. Upwelling constitutes the link between the deep water TCO 2 and the surface fCO 2 . In fact, at the prime meridian different fCO 2 regimes can be discerned which correspond to different deep water regimes. Also at the major frontal systems there is a covariation in the deep and surface distributions of CO 2 . Though resemblance of surface and deep CO 2 distributions can be observed, its full extent can only be visualized by means of a modelling effort. The CO 2 supersaturation at the western end of the quasi-zonal transect ŽFig. 6. is spatially connected with a similar area on the prime meridian ŽFig. 3., the latter in turn corresponding to the northern rim current of the Weddell Gyre ŽSection 3.2.. As the water with CO 2 supersaturation appears to be pinched
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off at about 88E ŽFig. 6., this would imply that the rim current is pinched off as well, or at least is diverted. As mentioned before, the hydrographic observations in this area do indeed hint at a northward deflection ŽSchroder and Fahrbach, 1999.. East of ¨ 138E on the same transect, the water column has a southward flow component Žsee also Sections 4.1 and 4.2.. The surface salinity here is very low ŽFig. 6. and moreover similar to that in the southern ACC ŽFig. 3., suggesting that this is an intrusion of surface water stemming from the southern ACC. Indeed, Bagriantsev et al. Ž1989: their Fig. 2. suggest intrusion of subsurface water from the ACC in this region. As regards CO 2 , while the surface water of the southern ACC is supersaturated with CO 2 ŽFig. 3., the possibly intruded surface water in the eastern Weddell Gyre is distinctly undersaturated ŽFig. 6.. This difference cannot be caused by differences in SST. Thus, the lower CO 2 saturation in the eastern Weddell Gyre could be due to the fact that once in the Weddell Gyre, the surface water is subject to less upwelling activity. During our study, fCO 2 exhibits a large spatial variation Ž318–375 matm., which has been established by other investigations in the Southern Ocean as well. The seasonal fCO 2 variation appears to be considerable ŽPoisson et al., 1993; Takahashi et al., 1993.. Biological processes, with their high propensity for patchiness, cause large variations mainly in summer. During our cruise in autumn, biological activity was relatively low, suggesting that fCO 2 variations were predominantly caused by physical factors. Poisson et al. Ž1993. did not find significant correlations between fCO 2 and SSTrsurface salinity for data collected in the January–April period, concluding that the mesoscale variability is large. Our analysis of the fCO 2 data with the aid of surface hydrographic parameters demonstrates that the fCO 2 distribution can be fairly well understood when the spatial scale considered is relatively small. Our autumn data supplement the few existing data of this area to enhance the seasonal coverage. Comparing data from different seasons allows us to extend the interpretation of fCO 2 variations. In the southern ACC at the prime meridian, the slight supersaturation ŽFig. 3. appears to be in line with fCO 2 data from the other seasons ŽWeiss et al., 1992: their Figs. 55, 63 and 65; Bakker et al., 1997.,
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which are all close to full saturation. This region is subject to intensive upwelling of CO 2-rich Upper CDW ŽHoppema et al., 2000., the CO 2-elevating effect of which is counteracted by biological drawdown of CO 2 . South of this, the northern limb of the Weddell Gyre features CO 2 supersaturation not only in autumn ŽFig. 3., but also in winter ŽWeiss et al., 1992: their Fig. 63., spring ŽBakker et al., 1997. and summer ŽWeiss et al., 1992: their Fig. 55.. Stoll et al. Ž1999b. report a slight undersaturation for the summer season. This area is not biologically productive, the often-observed CO 2 supersaturation being caused by upwelling. Still further south in the southern limb of the Weddell Gyre, the CO 2 undersaturation in autumn ŽFig. 3. stands in contrast to the slight winter and higher spring supersaturation as observed by Weiss et al. Ž1992.. A dramatic undersaturation is perceived in summer ŽStoll et al., 1999b; Weiss et al., 1992: their Fig. 55.. The interpretation is as follows: In summer CO 2 undersaturation is shaped due to intensive CO 2 fixation by photosynthesizing plankton. In autumn, remineralization of organic material elevates fCO 2 but this is counteracted by surface water cooling leading to undersaturation. Towards winter, entrainment of CO 2-rich deep water causes supersaturation, which extends into the next spring. It should be realized that during winter the CO 2 supersaturation does not lead to outgassing to the atmosphere, because the region is largely ice-covered then. Some outgassing is anticipated during early spring when the ice starts to melt, but a new cycle of photosynthetic CO 2 fixation, which may even benefit by the ice melt, will rapidly bring down the fCO 2 below the atmospheric level. We thus expect this region to be a sound sink for atmospheric CO 2 just like the western Weddell Gyre ŽHoppema et al., 1999; Stoll et al., 1999a.. The seasonal cycle of fCO 2 for the easternmost Weddell Gyre and adjacent ACC is not known. The zonal distribution of our fCO 2 data, with undersaturation in the Weddell Gyre and supersaturation in the ACC ŽFig. 6., is in conformance with that presented by Robertson and Watson Ž1995. for Februaryr March. For the period of JanuaryrFebruary, Metzl et al. Ž1991. found supersaturation, whereas Poisson et al. Ž1994. observed a slight undersaturation near 308E. Albeit some summertime variation of fCO 2
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exists, the similarity of the summer and autumn spatial fCO 2 distributions points to the relative importance of abiotic factors for explaining it. We conjecture that a similar fCO 2 distribution will also be prevalent during the less variable winter and spring periods. This would make the eastern Weddell Gyre a sink for atmospheric CO 2 on an annual basis. In the ACC east of the Weddell Gyre, the AABW stratum is up to ) 2000 m thick ŽFig. 5.. This AABW, which derives from the Weddell Sea Deep Water, has recently left the Weddell Gyre, probably from the southwest of its location on the transect. The large vertical extent of the layer indicates that it is a major outflow of bottom water. Mantyla and Reid Ž1995. also reported outflow of bottom water in this region. As AABW contains anthropogenic CO 2 absorbed by the Weddell Sea ŽAnderson et al., 1991., this outflowing AABW could be a major conduit for sequestering anthropogenic CO 2 in the world oceans, but this requires further investigation. At the prime meridian, the AABW manifests itself as a relatively thin layer. It is not clear what the origin of this AABW is. One possibility is the Drake Passage, and thus the Ross Sea in the Pacific sector. Another possible source region of it is the western Weddell Sea from where it could be transported through the South Sandwich Trench or through smaller, shallower passages in the South Scotia Ridge ŽLocarnini et al., 1993.. In both cases, it is not surprising that the layer thickness is much smaller at the prime meridian than east of the Weddell Gyre. First, at the prime meridian, the AABW is farther from its source area, making it plausible that more water mass spreading has occurred, over a larger surface area. Second, part of the AABW spreads northwards along the western South Atlantic ŽOrsi et al., 1999. and thus never reaches the prime meridian. As based on the layer thickness, it is impossible to decide which source of AABW, the western one or the one from the eastern Weddell Gyre, is more important.
7. Conclusions The horizontal distribution of fCO 2 is strongly influenced by the flow of surface water masses with different fCO 2 levels, especially when the distribu-
tion perpendicular to the main circulation features is considered Žfor example, across the ACC and Weddell Gyre.. This is also evident from fCO 2 gradients across the major fronts. Along the main direction of the circulation, upwelling was shown to play a role. Its effect for elevating the fCO 2 level is probably modest, because even over Maud Rise, a region with known enhanced upwelling activity, a clear CO 2 undersaturation was observed. Superimposed is cooling as the main factor for reducing fCO 2 in autumn. Because of this, the Weddell Gyre in autumn is a sink for atmospheric CO 2 . Before the onset of autumn cooling, though, it has probably been supersaturated in CO 2 . The deep TCO 2 distribution reflects all different water masses and some Žsurface. fronts. In the ACC, a deep TCO 2 maximum indicates the upper boundary of the AABW. This may be a convenient aid to trace AABW through the ocean basins, at least in the ACC, because a definition of AABW based on fixed property values is impossible ŽOrsi et al., 1999.. Acknowledgements We are grateful to the Alfred-Wegener-Institut fur ¨ Polar- und Meeresforschung ŽBremerhaven, Germany. for making available AFS PolarsternB. This research was in part supported by the Dutch–German cooperation in marine sciences ŽNEBROC. and by the EC Environment and Climate Research Programme Žcontract: ENV4-CT97-0472, climate and natural hazards.. References Anderson, L.G., Holby, O., Lindegren, R., Ohlson, M., 1991. The transport of anthropogenic carbon dioxide into the Weddell Sea C. J. Geophys. Res. 96, 16679–16687. Bagriantsev, N.V., Gordon, A.L., Huber, B.A., 1989. Weddell Gyre: temperature maximum stratum C. J. Geophys. Res. 94, 8331–8334. Bakker, D.C.E., De Baar, H.J.W., Bathmann, U.V., 1997. Changes of carbon dioxide in surface waters during spring in the Southern Ocean. Deep-Sea Res., Part II 44, 91–127. Comiso, J.C., McClain, C.R., Sullivan, C.W., Ryan, J.P., Leonard, C.L., 1993. Coastal Zone Color Scanner pigment concentrations in the Southern Ocean and relationships to geophysical surface features. J. Geophys. Res. 98, 2419–2451. Conway, T.J., Tans, P.P., Waterman, L.S., 1994. Atmospheric
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