Water Masses and Distribution of Physico-Chemical Properties in The

Deep-Sea Research II 49 (2002) 585–602

Water masses and distribution ofphysico-chemical properties in the Western Bransﬁeld Strait and Gerlache Strait during Austral summer 1995/96

M.A. Garc!ıaa,b,*, C.G. Castroc, A.F. R!ıosc, M.D. Dovalc, G. Roson! d, D. Gomise,f, O. Lopez! g a Laboratori d’Enginyeria Mar!ıtima, ETS d’Enginyers de Camins, Canals i Ports, Universitat Politecnica" de Catalunya, Barcelona, Spain b Direccio! General de Ports i Transports, Generalitat de Catalunya, Av.Josep Tarradellas 2-6, 08029 Barcelona, Spain c Instituto de Investigacions Marinas,* Consejo Superior de Investigaciones Cient!ıﬁcas, c/Eduardo Cabello 6, 36208 Vigo, Spain d Departamento de F!ısica, Facultad de Ciencias del Mar,Universidad de Vigo, Apdo.874, 36200 Vi go, Spain e Departament de F!ısica, Universitat de les Illes Balears, Spain f Institut Mediterrani d’Estudis Avancats,- IMEDEA (CSIC-UIB), Carretera de Valldemossa, km 7.5, 07071 Palma de Mallorca, Spain g In.no va Oceanograﬁa Litoral, c/Independencia" 361, ent.3er, 08024 Barcelona, Spain

Received 10 May 1999; received in revised form 16 January 2001; accepted 15 June 2001

Abstract

In the framework of the FRUELA project, two oceanographic surveys were conducted by R/V Hesperides! in the eastern Bellingshausen Sea, western basin ofthe Bransfield Strait and Gerlache Strait area during December 1995 and January 1996. The main hydrographic structures ofthe study domain were the Southern Boundary ofthe ACC and the Bransfield Front. The characteristics and zonation oflocal water masses are discussed in terms oftemperature, salinity, dissolved oxygen, nutrient and inorganic carbon concentrations. Concentration intervals for water mass labelling, on the basis ofchemical parameters in addition to the common y=S-based classification, are defined. Silicate seems to be a very good discriminator for local water masses. r 2001 Elsevier Science Ltd. All rights reserved.

1. Introduction can be divided into three major basins which are separated from each other by sills shallower than The Bransﬁeld Strait is a semi-enclosed Antarc- 1000 m. The western basin ofthe Strait is tic sea located between the South Shetlands connected to the neighbouring Bellingshausen archipelago and the Antarctic Peninsula coast. Sea through passages existing between the wes- The extent ofthe Strait is about 50,000 km 2 and it ternmost South Shetland islands and also through the Gerlache Strait, and to the Drake Passage via the Boyd Strait principally. The Gerlache Strait is *Corresponding author. Current address: Direccio! General ﬂanked by the western Antarctic Peninsula coast de Ports i Transports, Generalitat de Catalunya, Av. Josep and the Palmer archipelago. The sill connecting Tarradellas 2-6, 08029 Barcelona, Spain. Tel.: +34-93-495- 8070; fax: +34-93-495-8196. the Gerlache Strait to the Bellingshausen Sea is E-mail address: [email protected] (M.A. Garc!ıa). about 350 m deep.

In the framework of the FRUELA carbon flux very useful reference for the oceanographic aspects study funded by the Spanish National Programme ofthe FRUELA project, it was clear fromthe on Antarctic Research, two oceanographic surveys beginning that additional hydrographic observa- were conducted by R/V Hesperides! in the eastern tions ofthe area ofinterest should be a key Bellingshausen Sea, western basin ofthe Bransfield component ofthe Spanish study. On one hand, the Strait and Gerlache Strait area in December 1995 flow variability documented by Niiler et al. (1991) and in January 1996. From the point ofview of implied that ad hoc physical measurements were physical oceanography, the aims ofthe surveys needed to compute fluxes ofbiogeochemical were to gain insight into the characterization ofthe variables contemporary to data acquisition. On local water masses including the details oftheir the other hand, Niiler et al. (1991) could not intraseasonal variability during Austral summer extend their measurements deeper than 200 m due and to establish the local mesoscale circulation in to limitations ofresources and shiptime imposed the study region with a resolution which should be by the funding programme. This resulted in an adequate to meet the project needs. This paper underestimation ofthe magnitude ofthe major deals with the distribution ofwater masses as local circulation featureFthe Bransfield Curren- observed during the FRUELA 1995/96 cruises and tFpossibly by one half(Gomis et al., 2002). Given the companion article by Gomis et al. (2002) the presumed importance ofthe advective trans- discusses the 3D circulation and mass transport port term in the local carbon budget (see Huntley patterns. et al., 1991), reassessing the geostrophic transport The Bransfield Strait is one ofthe most densely with a deeper reference layer was regarded as a observed Antarctic seas due to its geographical must. accessibility, its favourable sea ice conditions and the existence ofmultiple logistic resources in the area. Modern oceanographic research in the zone 2. Material and methods can be dated back to the Discovery expeditions in the late 20s (Clowes, 1934). The international R/V Hesperides! conducted two cruises in the ISOS and BIOMASS programmes, which were FRUELA study area in December 1995 and in carried out during the late 70s and the early 80s January 1996, respectively. The December 1995 respectively, produced a basic understanding of cruise comprised two hydrographic legs. In leg 1 the local water mass structure and circulation (hereafter referred to as MACRO ‘95), a series of 5 patterns on the basis ofcoarse-resolution surveys hydrographic transects were visited across the (see e.g. Gordon and Nowlin, 1978; Grelowski major hydrographic fronts of the project study et al., 1986; Tokarczyk, 1987). Hofmann et al. area. This leg was carried out between 3rd and (1996) provided descriptions ofthe large scale 11th December and was composed of33 stations water mass distribution and circulation patterns in (Fig. 1a). The characteristic spacing between ad- the Bransfield Strait on the basis ofhistorical jacent stations was about 17 nautical miles on each hydrographic data. transect and the spacing between adjacent trans- A number ofeddy-resolving field studies aus- ects was twice as long. Leg 2 (named MESO ‘95 in piced by the US and Spanish national Antarctic what follows) consisted of a higher-resolution programmes targeted the mesoscale circulation at survey ofthe western Bransfield basin and the the different Bransfield Strait basins in the late 80s Gerlache Strait carried out between 12th and 19th and early 90s (Niiler et al., 1991; Garc!ıa et al., December. In this leg, 107 hydrographic stations 1994; Lopez! et al., 1994; Lopez! et al., 1999). In were visited and the typical spacing between particular, Niiler et al. (1991) carried out a detailed adjacent stations was about 8 nautical miles. As analysis ofthe geostrophic circulation in the for the January 1996 cruise, it included one single western Bransfield Strait region with a focus on hydrographic leg (MACRO ‘96) which replicated the circulation variability at seasonal scale. the MACRO ‘95 survey except for the western- Although Niiler et al.’s (1991) work provided a most transect in the Bellingshausen Sea (Fig. 1b). M.A. Garc!ıa et al./ Deep-Sea Research II 49 (2002) 585–602 587

Fig. 1. (a) Map ofhydrographic stations where water samples were obtained during the MACRO ‘95 leg, December 1995 cruise. (b) Same for the MACRO ‘96 leg, January 1996 cruise. The CTD station coverage of each survey is shown in Figs. 2a and 4a. The dashed straight lines indicate the locations ofthe vertical sections represented in Figs. 6 and 7. The 0-m, 200-m, 500-m and 1000-m depth contours are depicted.

Twenty seven stations were occupied in the AAII autoanalyzers, following Hansen and MACRO ‘96 leg from 21st to 27th January Grasshoff (1983) with some improvements 1996. During both MACRO ‘95 and MACRO (Mourino* and Fraga, 1985; Alvarez-Salgado! ‘96, a section along the Gerlache Strait et al., 1992). Alkalinity and pH(NBS) were was sampled. As the main aim ofthis paper is to determined by potentiometric methods according explore the intraseasonal variability ofthe to Perez! and Fraga (1987a, b). The accuracy ofpH local hydrography and to discuss concen- and alkalinity, estimated using CO2 certified tration thresholds ofthe local water mass label- reference material, was 70.004 and 71.4 mmol/kg, ling, we shall only focus on the MACRO ‘95 respectively. Total inorganic carbon (TIC) con- and MACRO ‘96 data sets. Both sets and the centrations were computed from pH and alkalinity MESO ‘95 data are used in the companion data by using the thermodynamic equations ofthe paper by Gomis et al. (2002) to compute the carbonate system (Dickson, 1981) and the con- local 3D mesoscale circulation and related trans- stants determined by Mehrbach et al. (1973) with ports. an accuracy of 74 mmol/kg (Millero, 1995; Lee On each hydrographic station, a surface-to- et al., 1996). TIC data were normalized at salinity bottom CTD cast was performed with a GO 35 psu by means ofthe formalism NTIC=TIC*35/ MkIIIC WOCE probe provided with extra dis- S. Water sample analyses were carried out on solved oxygen, fluorescence and light transmission board. sensors. Water samples were obtained routinely at The contouring package used to map the 24 levels with a GO Rosette equipped with 10 l distributions is the Surface Mapping System v. Niskin bottles. Some ofthe Niskin bottles carried 6.03 from Golden Software Inc. The y and S RTM SiS 4002 digital reversible thermometers. distributions have been derived from CTD profiles Salinity was obtained from water samples by whereas water sample data (available on every means ofa Guildline Autosal 8600 B. Dissolved other CTD station) have been used for the spatial oxygen concentrations were measured following interpolation ofchemical properties. The land Winkler’s method, with an estimated error of areas have been masked after the interpolation, so 71 mmol/kg. Nutrient concentrations were deter- details ofthe interpolated fields near coastal mined by segmented flow analysis with Technicon boundaries may not be properly assessed. 588 M.A. Garc!ıa et al./ Deep-Sea Research II 49 (2002) 585–602

3. Local water masses and fronts separates the ACC waters from an area of relatively vertical hydrographic homogeneity ex- The water mass zonation ofthe FRUELA study tending on the South Shetlands continental shelf. area in terms ofphysical water properties is quite In our case, the locus where the 01C isotherm well established in the literature. A summary ofthe intersects the surface layer can be roughly con- local water mass classification used follows for sidered as the position ofthe SbyACC. Eastward- reference. The conventions are similar to those flowing geostrophic jets are related to both the used by Garc!ıa et al. (1994), Garc!ıa (1996) and SACCF and the SbyACC in the southern Drake Lopez! et al. (1999), which were distilled from Passage, although the SbyACC jet is not a literature and were applied to analyses undertaken circumpolar feature like the SbyACC itself (the in the framework of previous Spanish Antarctic hydrographic expression ofthe SbyACC is not a reserach projects. For the sake ofclarity, the density front everywhere). description has been splitted into three parts: (i) From the hydrographic point ofview, the eastern Bellingshausen Sea and southern Drake Bransfield Strait is best defined as a transition Passage, (ii) Bransfield Strait and (iii) Gerlache zone between the Bellingshausen Sea and the Strait. Weddell Sea. The Strait basins are mostly occupied The typical water masses sequence in the eastern by water masses whose properties are controlled Bellingshausen Sea and southern Drake Passage by the characteristics ofthe inflows fromthe sector consists offourmain water masses: adjacent seasFa warm and relatively fresh inflow Antarctic Surface Water (AASW), Upper Circum- from the Bellingshausen Sea (typically 0.5–3.01C polar Deep Water (UCDW), Lower Circumpolar and 33.1–33.9 psu in summer) and a cool and Deep Water (LCDW) and Antarctic Bottom relatively salty inflow ofWeddell Sea surface and Water (AABW). AASW is a cold water mass deep waters (typically with negative temperatures originated all around Antarctica in early winter and salinity ranging from 34.1 to 34.6 psu in and typically extending down to 200 m. The summer) -. The main local water masses can then hydrographic signature ofthe AASW core is a be referred to as Transitional Zonal Waters with subsurface temperature minimum (typically Bellingshausen Sea influence (TBW) and Transi- yo01C) embedded in a strong halocline. Below tional Zonal Waters with Weddell Sea influence AASW, a two-layer system ofCircumpolar Deep (TWW) depending on the dominant source waters. Water (CDW) occupies most ofthe water column. Both water masses are separated by a shallow The difference in physical properties between both hydrographic front which has a clear surface CDW cores, upper CDW (UCDW) and lower thermal signatureFseveral summer cruises per- CDW (LCDW) corresponds to their different formed in the area suggest that the 1.01C isotherm extra-Antarctic origin and is subtle. The deep is a good discriminator between TBW and TWW y ¼ 01C isoline is considered to be the boundary in mid summer. TBW appears confined to a between LCDW and AABW. In the sector, narrow stretch ofthe mixed layer extending along AABW mostly consists ofWeddell Sea Deep the northern halfofthe Strait whereas TWW Water which is advected westward after leaving occupies almost the entire volume ofthe Bransfield the Weddell basin through deep gaps across the basins. As mentioned before, modified LCDW South Scotia Ridge and also further east. may flow into the Bransfield Strait through the The Southern ACC Front (SACCF) and the Boyd Strait and other passages. When it occurs, it Southern Boundary ofthe ACC (SbyACC) as they replaces TWW and produces a characteristic deep were defined by Orsi et al. (1995) are the main temperature maximum (y > 01C). Still another hydrographic features in the eastern Bellingshau- water mass can be traced below the TWW sen Sea and southern Drake Passage. The SACCF layerFthe Bransfield Deep Water (BDW). It is is the only ACC front that does not separate believed that this water mass is an isopycnal distinct water masses. In the FRUELA study mixture ofWeddell Sea shelfwaters and Warm domain, the SbyACC shows as a structure which Deep Water which is advected into the eastern part M.A. Garc!ıa et al./ Deep-Sea Research II 49 (2002) 585–602 589 ofthe Bransfield Strait and may be further temperature and salinity values with respect to the modified in situ by winter convection. The canonical local summer range of y and S: Contrary 1.01C and 34.5 psu y and salinity values define to this, the sea surface distributions of temperature the shallowest extent limit ofBDW under TWW. and salinity in late January 1996 (Figs. 4a and b) Two hydrographic fronts have been described in match better with the paradigm for local summer the Bransfield Strait: the aforementioned TBW/ conditionsFin our case, the BF front can be TWW surface front and the so-called Bransfield traced through the 34.0 psu isohaline. A compar- Front (BF)Fthe density front extending along the able situation is found, e.g., when comparing data southern South Shetlands continental slope, which from the previous BRANSFIELD 9112 cruise separates TWW from waters sitting on the carried out in December 1991 (Rojas et al., 1996) archipelago’s shelf. Of the two, only the BF is with the distributions ofthe BIOANTAR 93 cruise relevant from the dynamical point of view. As performed in January 1993 (Garc!ıa et al., 1994). A both fronts occupied the same geographic position similar but less pronounced intraseasonal change during the FRUELA cruises, the combined frontal oftemperature Fofabout 0.2 1C between Decem- zone will be referred to as BF. ber 1995 and January 1996Fcan be noticed in the The hydrography ofthe Gerlache Strait has 300 m distributions of y in the southern halfofthe been little discussed in the open literature, to the western Bransfield Strait. The signature ofthe BF authors’ knowledge. As a first approach, the is visible in both deep distributions of y: The Gerlache Strait can be understood as a westward comparison ofthe temperature distributions sug- extension ofthe western basin ofthe Bransfield gests a limited variability ofthe BF between the Strait. Given the depth range ofthe Gerlache two study periods. The deep horizontal salinity Strait and the shallow sill existing on its western distributions show very little structure (Figs. 3b end, no BDW should be present in the area. The and 5b). typical Gerlache Strait water column should then The sea surface distribution of oxygen for consist ofan upper layer ofTBW and an under- MACRO ‘95 shows a zone ofhigh oxygen lying, bottom-reaching layer ofTWW. Limited concentrations (O2>390 mmol/kg) coinciding with intermediate intrusions ofCDW could be expected low nitrate (NO3o20 mmol/kg) and low NTIC to penetrate either from the west or from the east. (NTICo2150 mmol/kg) levels in the northeastern Local TBW should be fresher and cooler than in Bellingshausen Sea (Figs. 2c–e), in particular the Bransfield Strait due to the influence of where a meandering jet associated with the freshwater inputs from local glaciers. SbyACC was observed (Gomis et al., 2002). Strong phytoplankton accumulation in this region caused this signal in the chemical field (Varela 4. Results et al., 2002). No similar feature was observed during MACRO ‘96 one month later. In this 4.1. Horizontal zonation second cruise, the highest oxygen (O2>380 mmol/ kg), lowest nitrate (NO3o16 mmol/kg) and lowest Figs. 2 and 3 show the horizontal distributions NTIC (NTICo2170 mmol/kg) values were regis- of y; S; oxygen, nitrate, silicate and NTIC at sea tered in the Gerlache Strait related to phyto- surface and at 300 m depth in the western basin of plankton nutrient consumption/oxygen produc- the Bransfield Strait and eastern Bellingshausen tionFhigh chlorophyll concentrations were Sea as deduced from the MACRO ‘95 data set. measured at these stations (see Varela et al., ! Figs. 4 and 5 show similar distributions for the 2002; Alvarez et al. 2002; Castro et al., 2002). At MACRO ‘96 data. 300 m depth, the distributions ofthe chemical The MACRO ‘95 leg was carried out in early properties clearly follow the thermohaline distri- December shortly after sea ice retreat in the butions and a high percentage ofthe chemical Bransfield Strait. This is clearly noticeable in variability can be explained by variations in the Figs. 2a and b, which display low sea surface thermohaline properties. No significant variability 590 M.A. Garc!ıa et al./ Deep-Sea Research II 49 (2002) 585–602

Fig. 2. MACRO ‘95 leg. Surface horizontal distributions of (a) potential temperature (1C), (b) S (psu), (c) O2 (mmol/kg), (d) nitrate (mmol/kg), (e) NTIC (=TIC 35/S; mmol/kg) and (f) silicate (mmol/kg) in the western basin ofthe Bransﬁeld Strait and Bellingshausen Sea. M.A. Garc!ıa et al./ Deep-Sea Research II 49 (2002) 585–602 591

Fig. 3. MACRO ‘95 leg. Horizontal distributions at 300 m of(a) potential temperature ( 1C), (b) S (psu), (c) O2 (mmol/kg), (d) nitrate (mmol/kg), (e) NTIC (=TIC 35/S; mmol/kg) and (f) silicate (mmol/kg) in the western basin ofthe Bransﬁeld Strait and Bellingshausen Sea. 592 M.A. Garc!ıa et al./ Deep-Sea Research II 49 (2002) 585–602

Fig. 4. MACRO ‘96 leg. Surface horizontal distributions of (a) potential temperature (1C), (b) S (psu), (c) O2 (mmol/kg), (d) nitrate (mmol/kg), (e) NTIC (mmol/kg) and (f) silicate (mmol/kg) in the western basin ofthe Bransﬁeld Strait and Bellingshausen Sea. M.A. Garc!ıa et al./ Deep-Sea Research II 49 (2002) 585–602 593

Fig. 5. MACRO ‘96 leg. Horizontal distributions at 300 m of(a) potential temperature ( 1C), (b) S (psu), (c) O2 (mmol/kg), (d) nitrate (mmol/kg), (e) NTIC (mmol/kg) and (f) silicate (mmol/kg) in the western basin ofthe Bransfield Strait and Bellingshausen Sea. 594 M.A. Garc!ıa et al./ Deep-Sea Research II 49 (2002) 585–602 was observed between the two surveys. The intraseasonal variability in the study area (the distributions ofall the chemical properties MACRO ‘95 y and S values were influenced by (Figs. 3c–e, 5c–e), show strong gradients in the melting ofthe sea ice cover occurred shortly before SbyACC and BF regions. The low oxygen the cruise whereas the MACRO ‘96 distributions (O2o180 mmol/kg), high nitrate (NO3>35 mmol/ display full summer conditions). On the other kg) levels north ofthe SbyACC reflect the south- hand, the intrusion ofmodified LCDW into the ward extension ofthe LCDW. On the other hand, Bransfield Strait was larger in January 1996 than waters with Weddell Sea influence can be easily in December 1995, as the temperature distribuions traced by high oxygen (O2>250 mmol/kg) and low reveal (see Figs. 6a and 7a). Moreover, a careful nitrate (NO3o32 mmol/kg) and NTIC (NTIC scrutiny ofthe details ofthe y distributions shows o2270 mmol/kg) in the southeastern region ofthe that this feature is not an interpolation arti- Bransfield Strait. factFthe area covered by the 1.01C isotherm does The distributions ofsilicate behave in a more not include any ofthe data points ofstation 8 in conservative way than the other chemical proper- the MACRO ‘95 distribution shown in Fig. 6a, ties (i.e. nitrate distributions) in the study area. whereas it includes all points below 300 m in the Figs. 2fand 4fshow a consistent picture ofthe corresponding station (stn 215) ofthe MACRO silicate distribution at sea surface which appar- ‘96 distribution depicted in Fig. 7a. We think that ently is not affected by the December 1995 differential intrusion ofCDW is related to under- phytoplankton bloom nor by any other source of sampled deep mesoscale variability ofthe LCDW intraseasonal variability. The surface waters of the inflowFGarc!ıa et al. (1994) reported time series Bransfield Strait have silicate concentrations ex- observations oftemperature at 400 m acquired ceeding 65 mmol/kg whereas the corresponding south ofDeception Island during Austral summer values are o60 mmol/kg in the eastern Belling- 1992/93 which displayed transient temperature shausen Sea. Moreover, the position ofthe increases which could only be explained by SbyACC can be easily traced via the horizontal variability ofthe intrusion ofCDW at mesoscale. gradient ofthe sea surfacesilicate concentrations. On the other hand, there was no signature of This is not so clear, however, for the BF. The BDW on the transect in any ofthe two occupa- position ofthe SbyACC is also noticeable in the tions during the FRUELA study. silicate distribution at 300 m depth, although at As in the horizontal distributions shown before, this level waters pertaining to the ACC region do the vertical distributions ofoxygen, nitrate and not have significantly different silicate concentra- NTIC (Figs. 6c–e, 7c–e) closely mirror the tem- tions from Bransfield Strait waters. perature distributions. However, the temperature minimum ofAASW, at about 80 m depth, is not 4.2. Vertical distribution of water masses traced in the chemical fields. At this depth, strong gradients are observed in the chemical properties. Figs. 6 and 7 display the vertical distributions Below AASW, the poleward extension ofCDW is (up to 1200 m) ofthe thermohaline and chemical clearly discerned by minimum oxygen values and properties along the eastern Bellingshausen Sea– maximum concentrations ofnitrate and NTIC. Bransfield Strait transect through the Boyd Strait However, the chemical properties extrema are not corresponding to the MACRO ‘95 and MACRO located at the same depth. For the two cruises, we ‘96 surveys. Despite the y and the S distributions found the nitrate and NTIC maxima at about exhibit the same overall pattern in both legs, a 250 m depth and the oxygen minimum a little closer view reveals two noteworthy differences deeper (B350 m depth). In addition, the differen- (Figs. 6a and b, 7a and b). One is that sea surface tial intrusion ofCDW into the Bransfield Strait is temperature and salinity ofthe MACRO ‘95 leg also evident in these vertical distributions. During are lowerFby roughly 11C and 0.1 psu, respecti- MACRO ‘96, lower oxygen (O2o200 mmol/kg), velyFthan in the MACRO ‘96 leg (Figs. 2a and b, higher nitrate (NO3>33 mmol/kg) and higher 4a and b), which is a clear signal ofhydrographic NTIC (NTIC>2280 mmol/kg) values were reached M.A. Garc!ıa et al./ Deep-Sea Research II 49 (2002) 585–602 595

Station Station 5 6 7 8 9 10 11 12 5 6 7 8 9 10 11 12 0

400

Depth (m)Depth 800 MACRO'95 MACRO'95 θ S 1200 0

400

Depth (m) 800 MACRO'95 MACRO'95 O2 NO3 1200 0

400

Depth (m) 800 MACRO'95 MACRO'95 NTIC SiO2 1200

Fig. 6. MACRO ‘95 leg. Vertical distributions of(a) potential temperature ( 1C), (b) S (psu), (c) O2 (mmol/kg), (d) nitrate (mmol/kg), (e) NTIC (mmol/kg) and (f) silicate (mmol/kg) along the Bellingshausen Sea–Bransﬁeld Strait transect through the Boyd Strait. The Drake Passage is on the right and the Bransﬁeld Strait is on the left.

at stn 215 in comparison with concentrations vertical distributions ofsilicate are very similar. At measured at stn 8 during the December 1995 depth, the maximum silicate concentration values cruise. On the other hand, the strong vertical were attained just south ofthe Boyd Strait during chemical gradients between stns 6–8 for MACRO both cruises. This fact and the shape of the silicate ‘95 and stns 217–215 for MACRO ‘96 mark the concentration isolines suggest that these maxima position ofthe BF closely following the tempera- were related to the inflow ofLCDW into the ture distributions (Figs. 6a and 7a). South ofthe Bransfield Strait. With respect to this, the increase front, the domain of Weddell Sea water influence ofthe deep silicate concentrations in about is defined by high oxygen (O2>280 mmol/kg), 10 mmol/kg between December 1995 and January low nitrate (NO3o32 mmol/kg) and low NTIC 1996 would be an expression oflarger LCDW (NTICo2260 mmol/kg) levels. inflow. As regards to silicate, it is seen again to be a Figs. 8 and 9 depict interpolated vertical dis- better tracer oflocal water masses than nitrate tributions ofpotential temperature, salinity, oxy- (Figs. 6fand 7f).This is indeed clear in the upper gen, nitrate, NTIC and silicate along the Gerlache 100 m, where the MACRO ‘95 and MACRO ‘96 Strait obtained from the MACRO ‘95 and 596 M.A. Garc!ıa et al./ Deep-Sea Research II 49 (2002) 585–602

Station Station 217 216 215 214 213 212 211 217 216 215 214 213 212 211 0

400

Depth (m) 800 MACRO'96 MACRO'96 θ S 1200 0

400

Depth (m) 800 MACRO'96 MACRO'96 O2 NO3 1200 0

400

Depth (m) 800

MACRO'96 MACRO'96 NTIC SiO2 1200

Fig. 7. MACRO ‘96 leg. Vertical distributions of(a) potential temperature ( 1C), (b) S (psu), (c) O2 (mmol/kg), (d) nitrate (mmol/kg), (e) NTIC (mmol/kg) and (f) silicate (mmol/kg) along the Bellingshausen Sea–Bransﬁeld Strait transect through the Boyd Strait. The Drake Passage is on the right and the Bransﬁeld Strait is on the left.

MACRO ‘96 data sets. The y and S sections Bransfield Strait as the ACC (and UCDW) shifts (Figs. 8a and b, 9a and b) show a consistent offshore. Ifthis was the case during the FRUELA pattern whose overall features are repeated in both cruises, CDW flowing from the Bellingshausen Sea legs. The vertical distributions oftemperature into the Gerlache Strait would be LCDW. But it (Figs. 8a and 9a) are similar to that obtained by might also be that the CDW entering the Gerlache Roese and Speroni (1995) using XBTs in February Strait is uplifted UCDW. In any case, the warm 1991. The water sequence west ofthe Bismarck water found in the Gerlache Strait at depth is StraitFi.e., the western end ofthe Gerlache certainly some sort ofCDW modified through StraitFconsists ofa surface layer ofAASW heat exchange with the surface and mixing with including a temperature minimum and a deep coastal waters. layer ofCDW which hosts both the temperature Water flows from the Bellingshausen Sea into and the salinity maxima. In the eastern Belling- the Gerlache Strait through multiple paths like via shausen Sea, UCDW reaches the shelfbreak and is Bismarck Strait and through Dallman Bay and the observed near 300 m (Klinck, pers. comm.). It may Scholaert Channel (Klinck, pers. comm.). The be that LCDW has risen to 300 m northwest ofthe Bellingshausen Sea inflow has to pass over sills M.A. Garc!ıa et al./ Deep-Sea Research II 49 (2002) 585–602 597

Station Station 34 37 39 40 4142 43 47 34 37 39 40 4142 43 47 0

400

Depth (m) 800

MACRO'95 MACRO'95 θ S 1200 0

400

Depth (m) 800 MACRO'95 MACRO'95 O2 NO3 1200 0

400

Depth (m) 800 MACRO'95 MACRO'95 NTIC SiO2 1200

Fig. 8. MACRO ‘95 leg. Vertical distributions of(a) potential temperature ( 1C), (b) S (psu), (c) O2 (mmol/kg), (d) nitrate (mmol/kg), (e) NTIC (mmol/kg) and (f) silicate (mmol/kg) along the Gerlache Strait. The Bellingshausen Sea is on the left and the Bransﬁeld Strait is on the right.

whose depths are less than 400 m and this places oxygen concentration is higher and its silicate an important constraint to CDW. Most ofthe levels are lower, and also taking the local bathy- CDW layer is ‘‘lost’’ and is replaced by TWW to metric setting ofthe Gerlache Strait and surround- the extent that the deep temperature maximum ings into account, we believe that this feature may which characterizes CDW is totally eroded at the correspond to a filament ofmodified CDW centre ofthe Gerlache Strait. At surface, a sharper advected from the western Bransfield basin. transition from AASW to TBW is observed. As Another possible explanation is that this expected, TBW in the Gerlache Strait is somewhat water body consists ofBellingshausen Sea CDW cooler and fresher than in the Bransfield Strait. flown into the Gerlache Strait through the Back to deep layers, an isolated temperature Scholaert Channel or other passage further maximum (y > 0:21C) is observed near the eastern east. The differences in properties with respect end ofthe Gerlache Strait at depths ofthe order of to CDW found in the western part of the 200 m. Given the fact that the salinity correspond- Gerlache Strait would then result from mixing ing to this maximum is lower than the salinity of with waters adjacent to the western flanks ofthe the CDW core in the western part ofthe Strait, its Strait. 598 M.A. Garc!ıa et al./ Deep-Sea Research II 49 (2002) 585–602

Station Station 187 189 191 192193 195 186 187 189 191 192193 195 186 0

400

Depth (m) 800 MACRO'96 MACRO'96 θ S 1200 0

400

Depth (m) 800 MACRO'96 MACRO'96 O2 NO3 1200 0

400

Depth (m) 800 MACRO'96 MACRO'96 NTIC SiO2 1200

Fig. 9. MACRO ‘96 leg. Vertical distributions of(a) potential temperature ( 1C), (b) S (psu), (c) O2 (mmol/kg), (d) nitrate (mmol/kg), (e) NTIC (mmol/kg) and (f) silicate (mmol/kg) along the Gerlache Strait. The Bellingshausen Sea is on the left and the Bransﬁeld Strait is on the right.

The only remarkable differences between the inputs must have a significant effect on surface MACRO ‘95 and MACRO ‘96 y and S distribu- water properties. tions along the Gerlache Strait are constrained to The distributions ofoxygen, nitrate and NTIC the upper layers. As was the case in the western along the Gerlache for the two cruises are strongly basin ofthe Bransfield Strait, the MACRO ‘96 sea determined by phytoplankton consumption in the surface temperatures were higher in the Gerlache upper meters ofthe water column. High oxygen Strait than the corresponding MACRO ‘95 concentrations, low nitrate and low NTIC values values (see Figs. 8a and 9a) due to seasonal were obtained on both sides ofthe Gerlache Strait warming, but unlike in the Bransfield, the surface due to local high phytoplankton accumulation salinity was lower in January 1996 than in (Varela et al., 2002; Alvarez! et al., 2002; Castro December 1995. This probably reflects the increase et al., 2002). In fact, the lowest nitrate (NO3 ofthe freshwater supply related to melting o12 mmol/kg) and NTIC (NTICo2195 mmol/kg) processes at local glaciers and passing icebergs concentrations for the two surveys were registered as Austral summer progresses. In a limited-area near the southwestern end ofthe Gerlache Strait basin as the Gerlache Strait, such freshwater during MACRO’ 96. In the central sector ofthe M.A. Garc!ıa et al./ Deep-Sea Research II 49 (2002) 585–602 599

Gerlache Strait, a deep mixed layer probably The UCDW maximum temperature is associated prevented phytoplankton accumulation (Castro to an oxygen minimum and both nitrate and et al., 2002) and consequently lower oxygen and NTIC maxima (Table 1). UCDW’s average higher nitrate and NTIC levels were observed. temperature in the area ofthe Bellingshausen Sea The vertical distributions ofoxygen, nitrate and sampled by R/V Hesperides! was higher during NTIC in the Gerlache Strait precisely mimic the MACRO ‘95 than in MACRO ‘96 whereas temperature distributions at layers deeper than LCDW presented the highest average tempera- 100 m (Figs. 8c–e, 9c–e). The deep oxygen mini- tures in MACRO ‘96. The temperature changes in mum (O2o220 mmol/kg), nitrate maximum the UCDW and LCDW cores were correlated to (NO3>32 mmol/kg) and NTIC maximum the changing levels ofnitrate. On the other hand, (NTIC>2280 mmol/kg) are related to the above strong gradients in sea surface silicate distributions cited inflows ofLCDW. Again as in the Bransfield were observed across the SbyACC in the two Strait and in the eastern Bellingshausen Sea, surveys. Veth et al. (1997) also described a strong silicate seems to be a good tracer ofLCDW (see silicate surface gradient associated with this Figs. 8fand 9f).The silicate concentration range structure at 61W. within the LCDW body in the Gerlache Strait In the southwestern part ofthe Gerlache Strait, matches the values displayed by LCDW in the rest we found similar thermohaline and chemical ofthe FRUELA study area. properties ofAASW forthe two cruises (Table 1). AASW in the Gerlache Strait was less saline than at the eastern Bellingshausen Sea. Also 5. Discussion during the two cruises, the layer ofLCDW presented lower temperature and salinity values In Table 1, we show the average values ofthe with respect to the Bellingshausen Sea. Dilution of thermohaline and chemical properties for the the LCDW signal was also observed in oxygen, different water masses sampled in the study area. probably due to mixing with highly oxygenated In the eastern Bellingshausen Sea, the presence of TWW. However, nitrate and NTIC levels were not AASW is clearly noticeable through the tempera- significantly different from the concentrations ture minimum at about 80 m depth. The average found in the Bellingshausen area and silicate levels thermohaline properties ofthis minimum were were even higher, which might be a product of warmer and saltier during the MACRO’ 96 due to sediments supplied by local glaciers. In the north- intraseasonal variability (Table 1). The average east part ofthe Gerlache Strait, the water column concentrations ofoxygen, nitrate and NTIC were was occupied by TBW over TWW, though a little not very different between the two cruises and no intrusion ofLCDW was registered. Regarding correlation with the thermohaline properties was TBW, we found similar thermohaline properties found. However, about 52% and 68% of the for the two FRUELA cruises. Oxygen, nitrate and observed variability ofsilicate concentration NTIC levels were influenced by phytoplankton in the MACRO ‘95 and MACRO ‘96 data consumption, being stronger during the MACRO sets, respectively, can be explained by temper- ‘96 survey. However, silicate was not affected by ature and salinity. Whitehouse et al. (1995), phytoplankton activity as the region was domi- using data from a transect sampled in the nated by Cryptomonas populations (Varela et al., Belllingshausen Sea west ofour study area during 2002). In fact, silicate levels were very similar for the UK STERNA project, showed that silicate the two cruises and higher than silicate concentra- concentration in the core ofAASW was deter- tions for TBW in the western Bransfield region. mined by the hydrographic variability in contrast TWW was found between 300–700 m depth in the with nitrate, which presented an homogeneous Gerlache Strait, while in the Bransfield Strait it distribution. encompasses the whole water column at some North ofthe SbyACC, we can distinguish the areasFin particular, on the Antarctic Peninsula cores ofUCDW and LCDW in the chemical field. continental shelf. Consequently, we cannot 600 M.A. Garc!ıa et al./ Deep-Sea Research II 49 (2002) 585–602

Table 1 Average and standard deviation of the thermohaline and chemical properties for the different waters masses found in the eastern Bellingshausen Sea (EBELLINGS), the Gerlache Strait (GERLACHE) and the western Bransfield Strait (BRANSFIELD) during the MACRO ‘95 and MACRO ‘96 surveys. We also have introduced the maximum and minimum values in the western Bransfield region during the MACRO’96 for comparison with Tokarczyk’s water masses classification (see text and Table 2)a

Area WT y S O2 NO3 SiO2 NTIC MACRO ‘95 EBELLINGS AASW 0.94l70.24 33.97l70.05 309l716 29l71 66l711 2253l79 UCDW 2.15l70.06 34.55l70.06 180l74 35.1l70.4 77l73 2279l74 LCDW 1.6l70.2 34.69l70.02 188l77 32.9l70.3 97l76 2272l73 BRANSFIELD TBW 0.38l70.25 33.95l70.05 343l711 27l71 72l72 2239l710 TWW 0.80l70.15 34.18l70.08 330l716 27l71 81l72 2249l78 LCDW 0.57l70.41 34.51l70.11 228l717 32.4l70.3 92l73 2273l71 GERLACHE AASW 0.68l70.47 33.92l70.07 278l72 29l71 88.2l70.2 2262l72 CDW 0.96l70.30 34.57l70.07 195l713 32.9l70.1 102l73 2276l74 TBW 0.44l70.53 33.88l70.13 321l724 26l72 87l71 2247l714 TWW 0.56l70.19 34.54l70.02 251l74 31.84l70.06 87l71 2266l72

MACRO ‘96 EBELLINGS AASW 0.69l70.40 34.02l70.08 292l721 29l71 68l714 2258l712 UCDW 2.07l70.04 34.635l70.003 175l71 34.2l70.1 87l75 2285l71 LCDW 1.82l70.17 34.66l70.02 181l74 33.2l70.5 90l76 2280l73 BRANSFIELD TBW 0.84l70.51 33.93l70.11 322l711 25l72 75l76 2240l714 TBW max 1.65 34.16 335 29.0 84 2268 TBW min 0.02 33.72 291 21.3 68 2214 TWW 0.3l70.5 34.28l70.13 318l722 29l72 83l74 2252l712 TWW max 0.66 34.53 351 32.7 91 2273 TWW min 0.92 34.08 271 25.2 76 2222 LCDW 0.63l70.45 34.48l70.16 226l723 33.1l70.4 94l710 2279l72 LCDW max 1.15 34.66 261 33.7 107.1 2283 LCDW min 0.02 34.27 203 32.4 81 2275 GERLACHE AASW 0.71l70.21 33.79l70.01 285l712 29.4l70.6 91l74 2272l73 CDW 1.15l70.20 34.61l70.06 195l78 33.4l70.2 107l72 2281l74 TBW 0.47l70.19 33.71l70.25 328l747 20l76 86l76 2217l739 TWW 0.35l70.07 34.54l70.01 252l73 31.8l70.3 94l72 2276l72

a y in 1C, salinity (S) in psu and chemical property concentrations in mmol/kg. WT: water type. directly compare the average property values from ofa larger intrusion ofthis water mass in January the two regions. 1996. Also the higher nutrient levels and lower In the Bransfield Strait, the average temperature oxygen values support this idea. values for the upper water masses, i.e. TBW and To our knowledge, the paper by Tokarczyk TWW, reflect the seasonal heating (Table 1). For (1987) is the only paper published in the open the two water masses, the low oxygen values for literature in which an objective classification of MACRO ‘96, associated with no significant water masses has been proposed for the Bransfield changes in nitrate and NTIC concentrations Strait on the basis ofa statistical multidimensional between the two cruises, suggest the influence of analysis method involving both physical and remineralization processes during MACRO ‘96. chemical water properties. This author suggested During the two surveys, TBW was characterized a classification ofBransfield Strait waters in six by lower silicate values than TWW. The average groups to which the four local water masses temperature for LCDW was higher during discussed in Section 3 roughly correspondFTBW MACRO ‘96, which we intrepret as the expression approximately corresponds to Tokarczyk’s water M.A. Garc!ıa et al./ Deep-Sea Research II 49 (2002) 585–602 601

Table 2 Range ofvalues ofwater properties measured during the FIBEX cruise, February–March 1981 (afterTokarczyk, 1987) a,b

Type y S O2 NO3 SiO2 NTIC 1a 0.5–3.3 33.1–33.9 322–357 9–25 2–26 n.d. 1 0.1–0.8 32.9–33.9 327–379 2–16 20–60 n.d. 3 0.9–2.1 34.3–34.8 174–244 22–27 24–102 n.d. 4a 1.3–1.5 33.6–34.4 261–353 10–24 11–73 n.d. 4b 1.5–1.0 33.8–34.6 218–335 13–26 39–100 n.d.

a Note: Correspondence between water types from Tables 1 and 2: TBWB1, 1a; TWWB4a, 4b; LCDW B3; BDW included in 4b. See text for more details. b y in 1C, salinity (S) in psu, O2 in mmol/kg, NO3 in mmol/kg, SiO2 in mmol/kg and NTIC in mmol/kg. types 1 and 1a; TWW involves Tokarczyk’s types seasonal scale), so it is a robust water mass 4a and 4b; modified LCDW is Tokarczyk’s water indicator in the Gerlache Strait area. No interval type 3; BDW was not individualized by Tokarczyk can be assessed for BDW as it was not found in but included in type 4b. our study area. Another choice would be labelling Table 1 shows the range ofvalues ofwater water masses by using oxygen and nitrate con- properties measured in the western Bransfield centrations jointly, but it has the obvious draw- Strait and Gerlache Strait during the MACRO back that values in the euphotic zone are modified ‘96 cruise and Table 2 displays equivalent values by non-physical causes. obtained by Tokarczyk for his water classes using the FIBEX 1981 data set (FIBEX 1981 was one of the cruises auspiced by the international BIO- Acknowledgements MASS programme and roughly covered the same study area as the FRUELA cruises in February– We acknowledge the help and the support ofthe March 1981. Here we use the MACRO ‘96 data set crew ofR/V Hesperides and ofall colleagues who for comparison because it was obtained under made possible data acquisition during the FRUE- seasonal conditions which are closest to those of LA December 1995 and January 1996 cruises. FIBEX 1981). The comparison ofboth tables We are specially grateful to Julia Figa, Manuel suggests that Tokarczyk’s (1987) water types are Gonzalez,! Joan Puigdefabregas," Maribel Lloret, excessively broad in hydrographic terms. They are Pilar Rojas, Mario Manr!ıquez, Marcel. l!ı Farran,! best discriminated in y S space but the ranges Jorge Guillen! and Pere Masque! (hope to meet on for oxygen, nitrate and silicate are too wide and board again!). Our special thanks to T. Rellan! and ! overlap, so assigment ofwater samples to a M.V. Gonzalez for the oxygen and CO2 analyses particular water type on the basis ofa single during the two cruises and to M.J. Pazo! for the chemical property does not seem feasible. On the nutrient analyses during the MACRO ‘96 cruise. contrary, Bransfield Strait local water masses This study has been funded by the Spanish defined according to the conventions exposed in Comision! Interministerial de Ciencia y Tecnolo- Section 3 do have the same distinct character in gia, contract no. ANT94-1010. y S space but have much narrower ranges of chemical properties and little intersection among them. Table 1 suggests that silicate is a very good References discriminator ofwater masses in the Bransfield Alvarez,! M., R!ıos, A.F., Roson,! G., 2002. Spatio-temporal Strait. Moreover, the distributions shown in variability ofair-sea fluxes ofcarbon dioxide and oxygen in Figs. 8fand 9fevidence that silicate concentration the Bransfield and Gerlache Straits during Austral summer has very little variability (at least, on a sub- 1995–96. Deep-Sea Research II 49, 643–662. 602 M.A. Garc!ıa et al./ Deep-Sea Research II 49 (2002) 585–602

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