Deep and Bottom Water of the WeddellSea's Western Rim

Arnold L. Gordon, Bruce A. Huber, Hartmut H. Hellmer,Amy Ffield

Oceanographic observations from the Ice Station Weddell 1 show that the western rim ofthe Weddell Gyre contributes to Weddell Sea Bottom Water. A thin (<300 meters), highlyoxygenated benthic layer is composed of a low-salinity type of bottom water overlying a

high-salinity component. This complex layering disappears near 66°S because of verticalmixing and further inflow from the continental margin. The bottom water flowing out of thewestern rim is a blend of the two types. Additionally, the data show that a narrow band ofwarmer Weddell Deep Water hugged the continental margin as it flowed into the westernrim, providing the continental margin with the salt required for bottom-water production.

The South Atlantic's Weddell Gyre is thelargest cyclonic gyre poleward of the Ant-arctic Circumpolar Current (1, 2), coveringwith its western part the Weddell Sea.Vigorous vertical fluxes of heat and mois-ture in the presence of a variable sea-icecover couple Weddell waters with the coldpolar atmosphere, making the WeddellGyre a prime source of Antarctic BottomWater (AABW). Sea ice covers nearly theentire Weddell Gyre in winter, retreatingclose to the Antarctic continent in sum-

mer, except in the western Weddell. Win-ter expeditions have examined the condi-tions in the central and eastern portions ofthe gyre (3), but its western rim, between65° and 70°S and west of 50°W, had notbeen explored with modem field methodsbecause of the difficulties of navigating thethick perennial ice cover there.An effective way to gather extensive

observations in the ice-cluttered westernWeddell is to borrow a successful methodfrom the Arctic: a scientific station on a

drifting ice floe. The United States-Rus-sian Ice Station Weddell 1 (ISW) was

deployed on 11 February 1992 at 71°48'Sand 51°43'W from the ice-breaking re-

search vessel Akademik Fedorov. After a

northward drift of 3.5 months and almost700 km, the station and its occupants were

recovered by Akademik Fedorov andRVIB Nathaniel B. Palmer on 9 June 1992at 65°38'S and 52°25'W (Fig. 1). Thedrift of ISW followed closely the track ofHMS Endurance, which was beset in theWeddell Sea ice on 18 January 1915 andsank on 21 Noven-ber 1915 at 68°38'S and52°26'W (4).

Oceanographic data obtained during theoperation of ISW included 137 conductiv-ity-temperature-depth (CTD) profiles fromthe ships during ISW deployment, recov-

ery, and personnel-rotation cruises; at theice camp along the drift track; and from

helicopters deployed along sections approx-imately normal to the drift track (hCTDs).Water samples were drawn at most of thesites for immediate analysis of salinity anddissolved-oxygen concentration. Addition-al samples were drawn for later analysis ofH3, He, 180, and chlorofluorocarbons.

The thermohaline stratification in thewestern Weddell has the basic character of

the interior Weddell Gyre: a cold, low-salinity, surface-mixed layer separated by a

rather weak pycnocline from a thick layer ofrelatively warm, salty deep water referred toas Weddell Deep Water (WDW) (Fig. 2).The WDW is characterized by core layers inwhich the temperature and salinity profilesexhibit maxima (Tmax and Soax layers).Below this typical stratification, the ISWprofiles display a thin, cold, oxygenatedbenthic layer (Fig. 2). In the region of thewestern Weddell sampled during ISW, thetemperature-salinity (T-S) relation fromthe WDW Tmax to about 0°C is linear withlittle change between stations (Fig. 3). Thissection of the T-S relation represents thebulk of the water column below the WDWand is characteristic of the entire WeddellGyre. The WDW Tmax and Smax core wateris drawn from the Circumpolar Deep Waterinto the Weddell Gyre at its eastern marginnear 30°E (2, 5, 6), providing heat and saltto balance ocean and atmosphere exchangeswithin the Weddell Gyre. The lower, cold-er section of the nearly linear T-S curve

represents the mixing of WDW withAABW which has been spatially and tem-

Fig. 1. Station map showing CTD profile locations from ships (diamonds), helicopters (+), and the icestation (shaded circles). Numbers identify ice-station CTD profiles whose data are shown in Figs. 2and 3. Because existing bathymetric charts are unavailable for the ISW area, isobaths at 500, 1000,2000, 3000, and 4000 m were constructed from recent aero-gravity and magnetics survey data (21).SCIENCE * VOL. 262 · 1 OCTOBER 1993

Lamont-Doherty Earth Observatory, Palisades, NY10964.

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porally homogenized over the gyre region.The primary structure revealed in the

ISW data that sets the western Weddellapart from the rest of the gyre is the thinbenthic layer, which, by virtue of its coldtemperature (<-0.8°C), may be referred toas Weddell Sea Bottom Water (WSBW)(7). The benthic layer is manifested in theT-S relation as a concavity below the linearportion, beginning at about -0.4°C (Fig.3). Although this part of the T-S curve is avisually dominant feature, it represents aboundary layer only about 200 m thick (Fig.2). Minimum salinities in the benthic layersobserved during ISW [about 34.60 practicalsalinity units (psu)] were lower than thosereported for layers observed previously in thesouthwestern comer of the gyre (6). Northof 69°40'S (Fig. 1, CTD 19), the low-salinity benthic layer is often underlain by ahigher salinity bottom-water type (S >34.62 psu), which produces a local salinityminimum only 30 m above the sea floor.The low-salinity bottom-water type seems tobear a greater concentration of glacial melt(8). Along the ISW track to 66026'S, thehigh-salinity type is present in varyingamounts that show no correlation with bot-tom depth. The T-S structure indicates anorigin from the salty shelf water of thesouthwestern Weddell (9), part of which hasbeen observed at the southernmost hCTD(Fig. 3A), but hCTD data from farthernorth also indicate that salty contributionsfrom that shelf are likely as well.

Potential temperature (°C)-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0

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Density (a0)I.........r.........l.........l............,.!,.?)....................0 1 2 3 4 5 6 7 8 9 10

Oxygen (ml/L)Fig. 2. Data from CTD profile 35. Includespotential temperature (PT), salinity (Sa), density(De) (referenced to density at surface), andoxygen concentration (Ox). This profile is typi-cal of the ISW profiles along the continentalslope. The depth of WDW is marked. (Inset)Enlargement of benthic layer. It shows stablestratification with potential density referencedto 3000 m (to3).96

The benthic-layer water can be traced toslope plumes forced by export of variousshelf-water types, with a likely role forcabbeling and thermobaric processes (9-11). A mixing line drawn from the base ofthe linear part of the T-S curve through thelow-salinity WSBW end-member passesthrough T-S points of near-bottom shelfwater (Fig. 3B). The cold shelf water be-comes unstable to convection as pressure isexerted by downwelling over the continen-tal slope. Downwelling may be initiated byEkman veering and cabbeling, with ther-mobaric effects providing the positive feed-back that allows slope plumes to reach thebottom (9, 10).

The top of the benthic layer is markedby a weak pycnocline, which amounts to a

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34.3 34.4 34.5 34.6 34.7Salinity (psu)

Fig. 3. Composite T-S relation. (A) hCTD datafor stations west of the drift track. (B) Ice-stationdata from locations along the track chosen toexemplify development of the benthic bound-ary layer; the station numbers match those inFig. 1. The data are superimposed on theenvelope of all ISW data (shaded). The WDWcore is marked at a Tma and Sr, near 0.5°Cand 34.68 psu found near 500 m, below whichthe T-S curve is nearly a straight line. For deepwater colder than about -0.4°C, the curve isconcave, depicting the thin benthic layer. Thedashed line (shaded) in (B) is a possible mixingcurve indicating near-bottom shelf water as a

possible source for the low-salinity WSBW type.The thin, solid line in (B) represents the freezingpoint for surface waters.

SCIENCE * VOL. 262 * 1 OCTOBER 1993

density gradient of only 0.1 kg m-3 over

300 m. Near 66°S, the thin benthic layerabruptly thickens to as much as 800 m. Inthe T-S relation, the bottom part of thecurve moved toward warmer saltier water tooccupy the T-S cavity formed by the doublewater-type arc (Fig. 3B, stations 68 and70), whereas the portion of the curve be-tween -0.6°C and 0.1°C moved towardlower salinity. The mean potential temper-ature and salinity of the water columndeeper than the 0.2°C isotherm north of66°S is colder and fresher than that to thesouth (12). Thus, the increase in benthic-layer thickness is the result of further con-tribution of low-salinity WSBW from thewestern edge of the Weddell, rather than asimple (one-dimensional) mixing phenom-ena. What finally emerges from the westernWeddell is AABW of the type observed inarchived data (13). It gives little hint of thecomplex layering and multiple water typesinvolved in the origin of this globally im-portant water mass.

The contribution from the shelf to new-ly formed bottom water colder than -0.8°C(14) is estimated to be less than 3 x 106 m3s-~1. This estimate was derived with theassumption of a spatially continuous down-slope flow that crosses the ISW track at anunknown angle in a layer whose thickness isthe average thickness of the benthic layerobserved in the ISW data. Because direct

Fig. 4. Temperature (in degrees celsius) at thecore of the WDW (Tma). The narrow band ofTma water warmer than 0.5°C overlying thecontinental slope (depths between 2000 and3000 m) may be a continuous feature from theGreenwich Meridian, advected into the westernWeddell by the outer-limb or rim-current com-

ponent of the western boundary current. Heavydots represent a combination of ISW stationsand historical stations from 1986 to 1989. Con-tours of depth in thousands of meters aremarked by light dotted lines.

41.7° '. 4. ...

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measurements of bottom-current speed areunavailable in the ISW region, a flow speedof 7 cm s-1 was assumed, on the basis ofmeasurements of outflow from the FilchnerDepression, to the southeast of the ISWregion (15). To account for the observedincrease in the thickness of the benthiclayer, we used average thickness values of70 and 240 m for the regions south andnorth of 66°S, respectively. As a conse-quence of this subdivision, 30% of theestimated shelf-water contribution origi-nates from the area north of 66°S.

Data collected from the upper watercolumn during ISW reveal that a narrowband of Tmax water >0.5°C flowed alongthe western edge of the Weddell Sea overthe continental slope (Fig. 4). This featuremay extend from its first appearance nearthe Greenwich Meridian (16) covering adistance of 2000 km. Cooling of the Tmaxlayer en route from near 1.0°C at theGreenwich Meridian to 0.5°C at the north-ern end of the ISW data array representsattenuation by vertical and isopycnal pro-cesses that expose the WDW core to coldsurface and continental-shelf water masses.In the western Weddell, a more stable layerwithin the pycnocline is observed in the200- to 300-m interval, with the WDWTmax at 500 to 600 m. This stands in sharpcontrast to the shallow (at the base of thewinter mixed layer near 110 m) pycnoclineand the warmer Tmx (>0.8°C within the150- to 250-m interval) of the easternsection of the Weddell Gyre (3). Thisstratification difference may account fordifferent vertical heat fluxes observed in thetwo regimes (3, 17) and associated peren-nial ice cover of the western Weddell.

With the Weddell Gyre's center welleast of the ISW survey, generally between20° and 40°W (2), the baroclinic expres-sion of the western boundary current ap-pears to be over 400 km wide. However,the distribution of the Tmax along the west-ern rim of the Weddell Gyre distinguishestwo components of the western boundarycurrent: A rim current that advects thewarmest water from the eastern margins ofthe gyre to the western edge and a longerresidence interior component, basically arecirculation of the inner hub of the gyre(16, 18). The rim current is likely to bemore important to the overall water-massformation processes, as it provides the saltrequired for bottom-water formation to thecontinental margins. Determination of geo-strophic shear with the thermal wind equa-tion, with adjustment to the order of thecurrent speed observed in the near-surfacelayer at ISW (19), indicates that the rim-current transport is about 10 x 106 to 15 x106 m3 s-2. The interior recirculation flowis unknown, but it may be larger than 10 x106 m3 s-1: with a mean bottom-current

speed of 1.3 cm s-1 as a reference, asmeasured at 66°30'S and 41°W (20), trans-port of over 30 x 106 m3 s-1 is likely,producing a total western boundary trans-port of about 40 X 106 m3 s-1.

1.2.

3.

4.

5.

6.

7.8.9.

10.11.

REFERENCES AND NOTES

G. E. R. Deacon, Deep-Sea Res. 26, 981 (1979).A. Orsi, W. Nowlin, T. Whitworth, ibid. 40, 169(1993).A. Gordon and B. Huber, J. Geophys. Res. 95,11655 (1990).E. Shackleton, South (MacMillan, New York,1920).N. Bagriantsev, A. Gordon, B. Huber, J. Geophys.Res. 94, 8331 (1989).V. V. Gouretski and A. I. Danilov, Deep-Sea Res.40, 561 (1993).T. Foster and E. Carmack, ibid. 23, 301 (1976).P. Schlosser, personal communication.A. Foldvik, T. Gammelsred, T. Terresen, Antarct.Res. Ser. 43, 5 (1985).A. Gill, Deep-Sea Res. 20, 111 (1973).Cabbeling refers to the mixing of two parcels ofseawater with different temperature and salinitybut similar densities. Because of the nonlinearequation of state for seawater, such mixtures canproduce a parcel that is denser than the twoconstituent parcels. Thermobaricity is a propertyof seawater at temperatures below which thecompressibility of seawater is significant in thedetermination of its density. As a parcel of waterat sufficiently low temperature is subject to in-

creasing pressure, it is compressed and cantherefore sink.

12. This isotherm falls well within the linear T-S segmentand hence is above the influence of the local benthiclayer. The average potential temperature of thedeep and bottom water below the 0.20C isothermdecreases by 0.25°C from station 64 to station 70,coinciding with a thickening of the benthic layer. Thecorresponding salinity is reduced by 0.015 psu. Thissuggests additional injection of WSBW into thebenthic layer north of 66°S.

13. E. Carmack, in A Voyage of Discovery (Perga-mon, New York, 1977), pp. 15-42.

14. ___, Deep-Sea Res. 21,431 (1974).15. A. Foldvik, T. Kvinge, T. Terresen, Antarct. Res.

Ser. 43, 21 (1985).16. T. Whitworth and W. Nowlin, J. Geophys. Res. 92,

6462 (1987).17. M. McPhee, J. Morison, D. Martinson, Eos 73, 308

(1992).18. A. Gordon, in Antarctic Ocean and Resources

Variability (Springer-Verlag, Berlin, 1988).19. R. Muench, J. Gunn, M. Morehead, Eos 73, 301

(1992).20. T. Foster and J. Middleton, Deep-Sea Res. 26,

743 (1979).21. J. Labrecque and M. Ghidella, J. Geophys. Res.,

in press.22. Funded by NSF grant DPP 90-24577. We thank J.

Ardai, R. Guerrero, G. Mathieu, S. O'Hara, and R.Weppernig for help collecting the CTD data, andW. Haines and P. Mele for data processing. This iscontribution 5101 of Lamont-Doherty Earth Obser-vatory.

28 April 1993; accepted 15 July 1993

Pressure-Induced Enhancement of T7 Above150 K in Hg-1223

M. Nunez-Regueiro,* J.-L. Tholence, E. V. Antipov,J.-J. Capponi, M. Mareziot

The recently discovered homologous series HgBa2CaCuO,,+2 + possesses remark-able properties. A superconducting transition temperature, Tc, as high as 133 kelvin hasbeen measured in a multiphase Hg-Ba-Ca-Cu-O sample and found to be attributable totheHg-1223 compound. Temperature-dependent electrical resistivity measurements underpressure on a (>95%) pure Hg-1223 phase are reported. These data show that 7iincreases steadily with pressure at a rate of about 1 kelvin per gigapascal up to 15gigapascals, then more slowly and reaches a Tc = 150 kelvin, with the onset of thetransition at 157 kelvin, for 23.5 gigapascals. This large pressure variation (as comparedto the small effects observed in similar compounds with the optimal T) strongly suggeststhat higher critical temperatures could be obtained at atmospheric pressure.

In analogy with the Bi- and Tl-based su-perconducting copper oxides, the structuresof the HgBa2Ca~.CunO,+28 series areconstructed with n CuO2 layers sandwichedbetween rock-salt (BaO) (HgO) (BaO)

M. Nuiez-Regueiro and J.-L. Tholence, Centre deRecherches sur les Tres Basses Temperatures,CNRS-UJF, BP 166 Cedex 9,38042 Grenoble, France.E. V. Antipov, Department of Chemistry, Moscow StateUniversity, 119899 Moscow, Russia.J.-J. Capponi and M. Marezio, Laboratoire de Cristal-lographie, CNRS-UJF, BP 166 Cedex 9, 38042 Greno-ble, France.*On leave of absence from Centro At6mico Bariloche,8400 Bariloche, Argentina.tAlso at AT&T Bell Laboratories, Murray Hill, NJ07974.

blocks. In comparison with the former, theHg compounds yield noticeably higher con-ducting/superconducting transition temper-atures, namely 94, 129, 133, and 126 K forthe n = 1, 2, 3, and 4 compounds, respec-tively (1-3). The synthesis of the com-pounds is complicated by the low decompo-sition temperature of HgO. High pressurehinders this decomposition and facilitatethe formation reactions. However, it is stillnot clear if the measured values of T7 areoptimal. Besides the empirical method ofchanging the preparation conditions andstoichiometry, there is another way ofchecking this. It is now generally accepted(4-6) that application of high pressures up

SCIENCE * VOL. 262 · 1 OCTOBER 1993 97

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