JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 105, NO. C5, PAGES 11,337-11,346, MAY 15, 2000
Deep and bottom water of the Bransfield Strait eastern
and central
Arnold
basins
L. Gordon
and Manfred
Mensch •
Lamont-DohertyEarth Observatory,ColumbiaUniversity,Palisades,New York
Zhaoqian Dong
Polar ResearchInstitute of China, Shanghai
William
M. Smethie
Jr. and Jose de Bettencourt
Lamont-DohertyEarth Observatory,Columbia University,Palisades,New York
Abstract. Temperature,salinity,oxygen,and chlorofluorocarbon
(CFC) data obtainedin
September1997 define the water typesventilatingthe deep easternand centralbasinsof
the BransfieldStrait. Thesewater typesare observedadjacentto JoinvilleIsland and are
clearly derivedfrom the Weddell Sea. The easternbasinbottom water characteristics
closelymatch that of the bottom water at 310 dbar depth -50 km east of Joinville Island.
The easternbasinbottomwater is a simplemixtureof low-salinityfreezingpoint shelf
water (65%) and relativelywarm salineWeddell Deep Water (35%), with a CFC-113:
CFC-11 ratio age of 8.5 years.The easternbasinbottom water sharesa commonorigin
with a weak salinityminimumobservedat 1000 dbar within the centralbasin,though
overflowfrom the central basinto the easternbasinfloor may also occur.The bottom
water within the centralbasinis colderand saltierwith higher concentrations
of oxygen
and CFC-11
than that of the eastern basin. The central basin bottom
water with a CFC
ratio age of 7.5 yearsis composedof sameWeddell water typesthat form the eastern
basinbottomwater plus a large contributionof high-salinityfreezingpoint shelfwater of
the characteristics
of water observed
10 km northeast
of Joinville
Island and a small
amountof Pacificpycnoclinewater. The components(and their percentage)are Weddell
Deep Water (11%), low-salinityshelf(24%), high-salinityshelf(60%), to Pacific
pycnocline(5%).
1.
bottom
Introduction
waters in the strait are different
from those at corre-
spondingdepthin adjacentareas.He speculatedthat the thermohalinecharacteristics
of the deepwater fillingthe Bransfield
are the three smalldeepbasinsof the BransfieldStrait (Figure Strait basinsare derivedat leastin part from winter water mass
1), eachwith uniquebottomwater propertiesrelativeto those conversionof Weddell shelfwater alongthe southernboundof the open SouthernOcean. Oceanographers
havelong been ary of the BransfieldStrait.
curiousas to the origin and fate of thesewaters.To the north
The Bransfield Strait stratification is similar to that of the
and west of the basins are topographicbarriers that isolate Antarctic continentalmargin,with a muchsubduedexpression
BransfieldStrait water from the southernDrake Passageat of the warm salinedeepwater characteristic
of the neighboring
depths>600 dbar [Smithand Sandwell,1997]. To the east, openocean[Hofmannet al., 1996;Hofmannand Klinck, 1998].
BransfieldStrait is separatedfrom the Weddell Sea by a shal- No evidence of dense shelf water formation is observed within
low plateau (labeled "plateau"on Figure 1), with a deepest the Bransfield Strait; rather the water over the western Antpassageof 750 dbar (at station79), stretchingfrom Joinville arcticPeninsulashelfis modified(cooledand freshened)PaIsland to -61øS and 52øW.A 1000 dbar passagebetweenthe cific Ocean derivedCircumpolarDeep Water (CDW) [Hofplateau and Elephant Island is referred to as the Northeast mann and Klinck, 1998;Smithet al., 1999] or drawnfrom the
Channel (Figure 1). The Bransfield eastern basin depth Weddell Sea along the southernboundary of the Bransfield
reachesa depth of 2700 dbar; the central basin reaches1700
Strait [Clowes,1934;Gordonand Nowlin, 1978].An eastward
dbar; and the westernbasin reaches1200 dbar [Wilsonet al.,
flowing current at the southernDrake PassageintroducesPa1999].
cific water into the northern fringe of the BransfieldStrait
Clowes[1934], usingdata from the Discovery,DiscoveryH,
within the 500-600 dbar deep channelseparatingSmith and
and William Scoresbyexpeditions,noted that the deep and
SnowIslandsof the SouthShetlandIslandchain[Capellaet al.,
1992;Hofmann et al., 1996;Hofmann and Klinck, 1998]. Ad1Nowat Institutffir Umweltphysik,
Universitaet
Heidelberg,
Hei- ditional CDW is introduced,perhapsin episodicevents,over
delberg,Germany.
the shallowershelf (400-500 dbar sill) betweenSmith Island
Copyright2000 by the American GeophysicalUnion.
and Brabant Island (Brabant Island is situatedsouthof 64øS
near 63øW,not includedwithin the area coveredby Figure 1
Paper number 2000JC900030.
0148- 0227/00/2000JC900030509.00
[Hofmannand Klinck, 1998;Yangand Zhao, 1989;Smithet al.,
Between the South Shetland Islands and Antarctic
Peninsula
11,337
11,338
GORDON ET AL.: DEEP AND BOTTOM WATER OF BRANSFIELD
STRAIT
Northeast Channel
g,')' 61' 60'
59' 58'' 57'' 56' 55''154'v..•,
53' 52' 5•.... 50'W
I b
63' '-'"'
61'
61'S
62'
62'
63'
64'
64'
Figure 1. Region of the BransfieldStrait. DOVETAIL stations68-97 used in this studyare shown.The
three deepbasinsare denotedby E, eastern;C, central;andW, western."Ant Sound"is AntarcticSound.The
isobaths[Smithgng $gngw½ll,1997] includedare 100, 200, and 500 dbar and at intervalsof 500 dbar to the
deepestvalues.The shadedarea denotesoceandepths<500 dbar.
1999].CDW may alsoenter the BransfieldStrait betweenthe source of the Bransfield basin water is confirmed, and water
massmixingrecipesconsistent
in temperature,salinity,oxygen,
King George and Elephant Islands,with a sill depth of 400600 dbar [Yangand Zhao, 1989]thoughHofrnannet al. [1996] and CFC are presented.
find mainly export of Bransfieldwatersthroughthat passage.
Smith et al. [1999] find that the relativelystronggeostrophic
flow toward
the northeast
at 200 dbar relative
the western
end of Bransfield
Strait
coincides
to 400 dbar at
with
a front
markingthe polewardlimit of >0øC CDW. This may be considered the BransfieldCurrent and BransfieldFront, respectively [L6pezet al., 1999].There is little exchangebetweenthe
BransfieldStrait and west Antarctic Peninsula[Smith et al.,
1999].
Gordonand Nowlin [1978] studiedthe origin of the Bransfield basinwater usinga few deep-reachingstationsobtainedin
1975. From the potentialtemperature-salinity
(0/S) relationship they find that the Bransfieldeasternbasin and adjacent
northwest
Weddell
Sea share a common
cold end-member:
2.
DOVETAIL
Data
As part of the Deep Ocean Ventilation Through Antarctic
IntermediateLayers(DOVETAIL) programthe northwestern
Weddell, southern Scotia Sea, and Bransfield Strait were sam-
pled from the polar researchvesselNathanielB. Palmer for
temperature, salinity, oxygen,and CFCs during the austral
winter seasonof August-September1997 [Gordonet al., 1998;
Menschet al., 1998a].
Drift betweenthe precruiseand postcruisetemperaturecalibrationswas <0.0004øC,implyinga temperatureaccuracyof
better
than the manufacturer's
stated value
of 0.001øC.
The
averagedifference in readingsbetween the two conductivityfreezingpoint water with a salinityof 34.62. They suggestthat temperature-depth
(CTD) temperaturesensorswas0.0005øC.
this water is derived from the continental shelf at the tip of The CTD conductivitywas adjustedto conductivitiesderived
Antarctic Peninsula.They surmisethat it is advectedfrom the from water samplesanalyzedfor salinityduringthe cruiseon a
Weddell Sea by the northwarddrift along Antarctic Peninsu- Guildline 8400B laboratory salinometer.The same batch of
la's easternmargin.The lack of a significantpresenceof warm, International Associationfor PhysicalSciencesof the Oceans
saline deep water, in the form of CDW or of Weddell Deep (IAPSO) standardwaterwasusedthroughout(P131). A CTD
Water (WDW) within the BransfieldStrait,explainsthe colder conductivitysensordrift correctionof •0.001 mmho/cmper
form of the Bransfieldbasindeep and bottom watersrelative month was applied prior to adjustmentto the bottle data.
to the neighboringopenocean.The centralbasindeepwater is Replicatesalinometersalinitymeasurements
agreedto within
substantiallycolder, more saline, and higher in oxygenthan _+0.002(two standarddeviations).Dissolvedoxygenconcenobservedin the easternbasin.They find that the westernbasin- tration was rrfeasffred
with an automatedtitrator utilizingan
bottom water is warmer than that of the other two basins, optical endpoint detector.Replicatevaluesagreed to within
denoting much reduced accessof the cold shelf water end- 0.01 mL/L (two standarddeviations)or 0.2%. Measurement
memberor, more likely, increasedmodifiedCDW input from accuracyof CFC is estimatedto be 0.005 pmol/kgor 1% with
the west.
a detectionlimit of 0.005 pmol/kg.
Potentialtemperature,salinity,oxygen,and CFC-11 sections
Whitworthet al. [1994]view both the Weddell-ScotiaConfluence and Bransfieldbasin renewal not as a convectivepro- composedof stations82 and 86-96 characterizethe waters
cessbut rather asan isopycnalprocess.Mixturesof CDW with along the BransfieldStrait eastern and central basins,across
somewhatcooler Weddell Deep Water (WDW) and near- the plateau,and into the northwestedgeof PowellBasin(FigfreezingpointAntarcticshelfwater spreadinto the deepwater ure 2). Stations68-84 samplethe watersover the crestof the
along densitysurfaces.
JoinvilleRidge (stations68-75) whichdefinethe WeddellSea
In the present study the Weddell Sea shelf origin for the input, acrossthe plateauand NortheastChannel(stations75-
GORDON
ET AL.: DEEP AND BOTTOM
96
95
(a) o
94
500
93
92
....0.1-"
WATER OF BRANSFIELD
91
90
82
89
88
87
STRAIT
86
o
D'
500
1000
1000
\
•
11,339
\
1500
•
2000
13_2500
2500 13_
3000
3000
3500
POT. TEMP. (øC)
Bransfield
3500
Strait
4000
4000
400
350
300
250
200
150
100
50
0
Distance (km)
(b)
96
95
94
93
92
91
90
82
89
88
87
86
0
0
500
......
•
500
lOOO
•.5•
•1500
•
1500
2000
2000
13_2500
•
•
2500 Q.
3000
3000
3500
SALINITY
3500
Id Strait
4000
4000
400
350
300
250
200
150
100
50
0
Distance (km)
Figure2. (a) Potentialtemperature
(øC),(b) salinity,(c) oxygen(mL/L), and(d) CFC-11(pmol/kg)along
the central axisof the BransfieldStrait, composedof DOVETAIL stations86-89, 82, and 90-96. The
referenceletterscorrespond
to thosegivenon thescatterplots
(Figure4): D denotestheWeddellDeepWater;
D' andD" indicatethepositionof weakwarmdeepcorelayer(500-750dbar)in the easternandcentralbasin,
respectively.
Be andBc represent
bottomwaterof the easternandcentralBransfield
basins,
respectively.
Bc'
is the weak salinityminimumnear 1000dbarwithin the centralbasin.
84) of the BransfieldStraiteasternboundary(Figure3). The
potential temperatureversussalinity,oxygen,and CFC-11
scatterplots
(Figure4) arereferenced
to the sections
byS,shelf
water; D, deep water; and B, bottomwater.
basin,the WDW 0ma
x at 1000 dbar is only slightlyabove0øC
(station86).At stations
87-89 (Figure2) and78-81 (Figure3)
the warmest, most saline water is in contact with or close
proximityto the shallowseafloor.Thiswater spansa temperature range of -0.4 ø to -0.7øC and salinityof 34.54-34.56,
(D). The WDW potentialtemperaturemaximum0ma
x core with oxygenlevelsof 6.0-6.3 mL/L and CFC-11 of 2.5-3.3
attenuatesas the continentalmarginis approached,as typical pmol/kg,and may be consideredas modifiedWDW.
A weak0ma
x (colderthan -0.6øC) near500 dbarwithinthe
of Antarctic margins.At the northwesternfringe of Powell
The warmest water in the Weddell
Sea is that of the WDW
11,340
GORDON ET AL.: DEEP AND BOTTOM WATER OF BRANSFIELD STRAIT
96
(c) o
500
95
94
93
- -- _,
.
•),r' 6.3--.
92
91
90
82
89
88
87
86
o
500
ß
"'--- '--'- -- '- ...'902'MIN
ß
ß
1000
.
lOOO
ß
1500
1500
2000
2000
2500
2500
3000
3000
OXYGEN ( mL/L )
3500
3500
Bransfield Strait
4OOO
4OOO
4OO
350
300
250
200
150
100
0
50
Distance (km)
(d) 096'
95
94
93
92
91
90
82
89
88
87
86
0
500
500
lOOO
lOOO
1500
•1500
'0
'-
2000
2000
2500
o_ 2500
3000
3000
3500
3500
4OO0
4OOO
4OO
350
300
250
200
150
1O0
50
Distance (km)
Figure 2. (continued)
easternbasin(D'), closerto 700dbarin thecentralbasin(D"),
are both coolerandfresherthanWDW (D). The easternbasin
0max
(D') maybe explained
bywestwardspreading
of thewater
observedslightlyoff the plateauseafloor to the east.The 0ma
x
at station90 is too warm for a perfectfit to D'. Thereforeit is
likelythat southernDrake Passage
pycnocline
water,whichis
warmerthanthat of the WeddellSeaat anygivensalinity(e.g.,
station84, Figure4a), infiltratesthe easternbasin.
Over the central Bransfieldbasin the 0ma
x (D") is deeper
than that in the easternbasinwith highersalinity(34.570) and
lower oxygenand CFC-11 concentrations
(6.3 mL/L and 3.2
pmol/kg,respectively).
The main differenceof D" from the
near bottomwater of the plateauis that D" is saltier.While it
is temptingto believethisis a reflectionof CDW enteringthe
BransfieldStrait betweenSnowand Smith Islands,thismaynot
be the situation.The Snow/SmithCDW intrusionis expected
to advectalongthe northernboundaryof the BransfieldStrait,
followingthe regionalcycloniccirculation,probablyexitingthe
BransfieldStrait without entering the central basin [Clowes,
1934;Smithet al., 1999].It is suggested
that the centralbasin
0max(D") is a simpletwo-pointmixtureof low-salinity
shelf
water, S1, which is observed east of Joinville Island, with
WDW (D; along the dashedline connectingS1 and D on
Figure4a). A slightlywarmerform of sucha mixtureis ob-
GORDON ET AL.: DEEP AND BOTTOM WATER OF BRANSFIELD STRAIT
(qp) e•nsse•cl
(qp) o•nsso•cl
(qp) e•nsse•cl
m
(qp) o•nsse•cl
(qp)oJnssoJd
(qp) oJnsso•cl
(qp) e•nsse•cl
•c•
(qp)oJnssoJd
11,341
11,342
GORDON
ET AL.: DEEP AND BOTTOM
WATER OF BRANSFIELD
STRAIT
(b) 0.8
(a) 0.8
0.6
,.,.,-73
--
/4t++?=
.
' '•+.+D
+ 68-73
0.4
77-81
85
0.2
•
82,
86-89
• 90-93
94-96
• -o.4
(D
/
l;:-o.6
I- -o.8
/
..•.•'• /
•x••,
x 77-81,85
•oO.' +++x
• .
/
•
•;C(•,•;/.
•.++.•
] o•0.0ß ' -5?•'
/p•• ••f•? • •-0.2
'
*•'..••'
••- ß•• •(•;t • -4
++•+
• • :$•;•/• •:• 1
• .•;,-
i
[-•. .
ß 83,84
082,
86-89
• 90•93
c94-96
.o
•-0.8
-1.0
-,-' -1.2
o
-1.4
-1.4
-1.6
..,••
•-•••.t:.
+ •?t ,{
/ ]
-•.•
-1.8
[,34.3
-2.o
34.2
,
,/ ",
34.4
34.5
34.6
l
1
34.7
-2.o 5.0
Salinity
5.5
6.0
6.5
7.0
7.5
8.0
Oxygen (ml/I)
(C) 0.8
0.6
+%++
+ 68-73
0.4
J 74-76
+
+
+
x 77-81, 85
++ xx xo0
+
x ß
0.2
++
x
ß 83, 84
x•
.,
*•,
.•,p
+
o82,
86-89
ß
E 90-93
x
• -0.4
(D
c 94-96
+++
++,it
ßß
ßxxøX•
E -0.6
++lit ß •EE
,
i-- -0.8
-,-' -1.2
0
-1.4
-1.6
ß x•_o
-1.8
I
-2.0
1
,
I
2
,
I
3
,
I
,
4
rS'•, S21
5
6
CFC-11 (pmol/kg)
Figure 4. (a) Potential temperature(øC) versussalinity,(b) potential temperature(øC) versusoxygen
(mL/L), (c) potentialtemperature(øC) versuschlorofluorocarbon
CFC-11 (pmol/kg)relationshipsfor the
DOVETAIL stationsshownin Figure 1. The referenceletters correspondto those given on the sections
(Figures2 and 3): D for deepwater; and Be and Bc for bottomwater of the easternand centralBransfield
basins,respectively.Bc' is the weak salinityminimum within the deep water of the central basin. S1 and S2
are the freezingpoint shelfwatersfound adjacentto JoinvilleIsland. P is pacificpycnoclinewater observed
at station 84 that may be form minor constituentof Bc.
served at the 0ma
x of station 79 near 600 dbar, the deepest minimum near 1000 dbar (Bc'), which may sharea common
of the Weddell Sea to the Bransfield Strait within
originwith the easternbasinbottomwater,discussed
in section3.
the plateauregion.
Bottom water in the easternbasin (Be) is about -1.01øC,
with salinity slightlyless than 34.558, oxygenof 6.55 mL/L 3. Discussion
(81% of full saturation),and CFC-11 of nearly 3.67 pmol/kg.
Bottom water within the central basin (Bc) is substantially 3.1. Bransfield Basin Deep Basin Source Waters
With the eastern and central basin water characteristics in
colder (-1.62øC) and saltier (34.580) with elevated oxygen
concentrations
(nearly7.08mL/L; 86% of full saturation),and mind, we can inspectthe Weddell water off the tip of Antarctic
CFC-11 of 4.79 pmol/kg.The centralbasinhasa broad salinity Peninsula(Figure3) to locatethe potentialsourcewatersand
connection
GORDON ET AL.: DEEP AND BOTTOM WATER OF BRANSFIELD
Table
la.
Characteristics
of Bottom
Waters
of the Eastern
and Central
STRAIT
Bransfield
11,343
Strait
Basinsand of ContributingSourceWater Types
DOVETAIL
Potential
Station
Water Type
Identifier
Temperature
Salinity
Oxygen
CFC-11
74 100 m
lower-salinityshelf
S1
-1.894
34.500
7.47
5.28
75 bottom
92
95
saline shelf
eastern basin
central basin
S2
Be
Bc
-1.914
- 1.007
- 1.615
34.599
34.558
34.580
7.69
6.55
7.08
5.60
3.67
4.79
70 0max
96 1000 m
Weddell Deep Water
Sminin central basin
D
Bc'
0.564
-1.200
34.680
34.550
4.92
6.76
0.67
4.13
derivequantitative
mixingrecipesconsistent
with the DOVE-
Remarkably,the O/S characteristicsof the bottom water observedat station 73 matchesthe temperature and salinity of
the easternbasinbottomwater (Be; station73 bottomwater is
0.008øCcolder and 0.003 saltier than Be). Station 73 bottom
water is slightlylower in oxygenconcentration(6.44 at station
73 versus6.54 mL/L of Be) and CFC-11 values(3.4 versus3.7
pmol/kg) than that of the easternbasin,but the oxygenand
CFC-11 gradientsbetweenstations73 and 74 are quite large
(Figure 3), and one can envisionwater slightlywestof station
73 more closelymatchingBe dissolvedgasconcentrationwithout producinga significantmismatchwith O/S characteristics.
Comparing O/S properties east of Joinville Island with those
north of the island(Figure 3) revealscontinuityof properties
alongisobaths,implyingan isobathfollowinggeostrophicflow
path aroundJoinvilleIsland into the easternbasin.
The source of the eastern basin bottom water is clearly
drawn from the inshore edge of the shelf-slopefront of the
bottom water portraysa more complexrecipe than that of the
easternbasin.The centralbasinsalinebottom water (Bc) requiresshelfwater componentmore salinethan that of S1. The
water within the central basindeep water followsalong a O/S
line connectingBc' and Bc. Extrapolatingthis O/S structureto
the freezing line suggestsa shelf water componentof salinity
34.59 (S2), aboutwhat is neededas the cold end-memberfor
the bottomwater of the westernWeddell Sea [Gordon,1998].
The salineshelfwater, S2, requiredby the centralbasinbottom
water is observedadjacentto Joinville Island. Stations74-76
show near-freezing point shelf water, reaching a salinity of
34.60 at the sea floor. This water, proposedto be the cold
end-membersourcefor Bc, spreadsinto the BransfieldStrait
alongwith the over the Antarctic Peninsulaside of the Bransfield Strait, passingto the southof the easternbasin.
Freezingpoint shelfwater is expectedto be high in oxygen
and CFC owing to recent interaction with the atmosphere
acrossthe sea-air-iceinterface. The Joinville Island regional
shelfwater is indeed rich in theseproperties.The oxygenand
CFC-11 of S1 are -7.5 mL/L and 5.3 pmol/kg, respectively,
and thoseof S2 are slightlyhigherat 7.7 mL/L and 5.6 pmol/kg,
respectively.The S2 propertiesare closeto that of the Weddell
Sea high-salinityWestern Shelf Water, believed to be a product of the coastalpolynyas[GammelsrOd
et al., 1994].That S2
is slightlymore ventilated than St may be explainedby its
greater contactwith the atmosphereafforded by the polynya
western Weddell
environment.
TAIL temperature,salinity, oxygen,and CFC data.
Stations68-75 (Figure1) depictthe typicalcontinentalmargin stratificationof the westernWeddell Sea [Fahrbachet al.,
1995;Gordon,1998].The WDW 0max
core(D) isnear600dbar
at stations68-70. The shelf-slopefrontal transition occurs
between stations 70 and 73. Within this zone, traces of WDW
form a weak temperature maximum at middepth over the
continental
shelf and at the 300 dbar sea floor at station 73.
Sea off Joinville Island. The water mass mix-
ture that eventuallyfills the easternBasinis alreadyachieved
east of Joinville
Island near 300 dbar. This water sinks into the
eastern basin without further dilution, in what Whitworthet al.
[1994]refer to asisopycnalspreading.A mixtureof WDW and
low-salinityWeddell shelfwater rendersthe simplestexplanation for the eastern basin bottom
further
water mass modification
water.
within
No need is seen for
the Bransfield
Strait.
The centralbasin salinityminimum (Smin near 1000 dbar;
Bc') is basicallythe same as the easternbasinbottom water
(Be). The bottomwater of the easternbasinmay either share
a commonorigin with the central basinSminof may be derived
from overflowof the Sminwater into the easternbasin.As the
match between Bc' and Be is best at station 96, at the western
end of the central basin, while at station 94, located closer to
the controllingsill is slightlytoo salineto match the Be water,
the sharedcommonsourceis a favored explanation(though
the two scenariosare not mutually exclusive).L6pez et al.
[1999] present50 daysof direct current meter data collected
during the austral summer of 1992-1993 at 700 dbar depth
near the sill dividingthe easternand centralbasin.The current
is weak and directed toward the south,with no evidenceof flow
towardthe east,aswouldbe expectedif overflowwere present.
The nonlinearO/Srelationshipof the centralbasindeep and
3.2.
Bransfield Basin Deep Basin Mixing Recipes
The water typeblendsinferred from propertycharacteristics
are used to find a mixing recipe consistentwith the DOVETAIL temperature,salinity,oxygen,and CFC data. The percentagesof the Be buildingblocks,St, and D (Table la) are
determinedfor each parameter (O/S, 02, and CFC-tt), assuming a simple linear mixture. The suggestedrecipe is the
averageof the components(Table lb). The bottom water of
the eastern basin, Be, is approximatelya 35:65 mixture of
WDW (D) to low-salinityshelfwater (St). Temperature,salinity, oxygen,and CFC-11 all give essentiallythe samemixing
ratios. That the time-dependent parameters of oxygen and
CFC-11 yield the same ratio as temperature, and salinity implies a rapid ventilation timescale,10 years or less.
As mentionedabove,the recipe of the centralbasinbottom
water, Bc, is more complex than that of the eastern basin,
clearly involvingmore than two end-members.We first investigate the possibilitythat the formation of Bc involvesonly
threewater types:WDW (D), the low-salinityshelfwater (S1),
and the higher-salinityshelfwater (S2). The percentageof the
buildingblocksare calculatedas follows:first a linear mixture
of D and S1 is used to produce the warm end-memberof the
11,344
GORDON
ET AL.: DEEP AND BOTTOM WATER OF BRANSFIELD
Table lb. Mixing Recipesfor Productionof Observed
BottomWaters of Eastern(Be) and CentralBasin(Bc)
Potential
Identifier Temperature Salinity Oxygen CFC-11
Eastern
Recipe for Be
Recipe for Bc
(Weddell
Only)
Basin
S1
36
64
32
68
36
64
35
65
D
S1
S2
Central Basin
12
30
58
11
28
61
18
47
35
14
41
45
D
STRAIT
agesfor D, S1, and S2 asP is increasedfrom 1% by increments
of 1% (the aim was to minimizethe need for pycnoclinewater). The mostprobablerecipeis determinedwhen the resulting value for Bc is closestto the observedvalue for Bc. The
resultrecipe leadsto more consistentinput of eachparameter
than afforded by only a three componentmixture. A ratio
D:Si:S2:P of 11:24:60:5yields a Bc of nearly the observed
propertiesof Bc (Table lc). It appearsthat a smallamountof
Pacificpycnoclineinput to Bc is likely, effectivelyreplacing
someS1 water of a "Weddell-only"blend.
Wilsonet al. [1999] present temperature and salinity data
obtained
in November
1995 from Nathaniel
B. Palmer
within
Values are given in percentage.
the BransfieldStrait.They investigatethewater massblendsof
the bottomwater of the easternbasin,but lackingoxygenand
CFC data, they had lesscontrolof the recipepercentagesthan
mixturewith S2 formingBc (see the dashedline betweenS2 affordedby the 1997DOVETAIL data set.Direct comparison
and its intersectionwith the dashedline denotingthe S1 and D of their recipe to ours is complicatedas they did not use the
linear mixture producingBe, Figure 4a). This warm end- primarywater typesthat have accessto the BransfieldStrait.
member is approximatelythe same as Bc', the Smin at 1000 They useWeddell Sea BottomWater (WSBW) as one of the
dbarswithin the centralbasin.The percentagesof S1 and D to ingredientsrather than its buildingblocksof WDW and freezproducethe warm end-memberfor Bc are determinedfor each ing point shelfwater. WSBW is clearlyblockedby topography
parameter. Calculation of the mixture of the warm end- from enteringthe BransfieldStrait,whereasthe buildingblock
member and S2 to form Bc are determined in similar manner.
water types have access.They do note this and say that the
The averageof the mixturesindicatedby the four parameters convection of surface waters that form WSBW have access to
are usedfor the final suggested
recipe.
the BransfieldStrait, but they do not explorethis quantitaThe percentagecomponentsof D:SI:S2 derived from tem- tively. Furthermore, their 1995 data do not include synoptic
perature and salinityare 11:29:60(Tables la and lb). How- observationsat the eastern boundary of BransfieldStrait as
ever, the ratios found from oxygenand CFC-11 are different obtained during the DOVETAIL 1997 expedition;instead,
from thosebasedon thermohalineparameters,which are 18: they use 1976 data to representone of their three ingredients,
47:35 for oxygen and 14:41:45 for CFC-11. This implies a the Weddell Sea sill water (Weddell waterspassingover the
slightlyslowerrate of ventilationfor the centralbasinthan for easternboundarysills).They saythat this representsthe largthe easternbasin,but the CFC ratios do not supportthis. The est uncertaintyin their calculations.
bottom water in the central and eastern basin are estimated to
Wilsonet al. [1999, p. 471] find that easternbasinbottom
have CFC-113:CFC-11ratiosagesof 7.5 years(basedon five water is a mixture of "central basin sill water, Weddell Sea sill
samplesfrom the lower 500 dbar at station 95, CFC-113: water, and Weddell Sea Bottom Water." The Weddell Sea sill
CFC-11 ratio rangeof 0.084-0.086) and 8.5 years (basedon water composes40-60% of the easternbasin bottom water.
the four samplesfrom the lower 500 dbar at station92, CFC- We agreethat the sill watersof the centralbasinmay overflow
113:CFC-11 ratio range of 0.077-0.078) years, respectively. into the easternbasin,but we alternativelystatethat they may
This is similarto the ageof Weddell Sea Bottom Water, 7 +_2 share common origin involvinglow-salinityshelf water from
years,observedin the northwestWeddell Sea [Menschet al., the Weddell Sea. In the eastern basin this water type can
descend to the sea floor, but in the central basin, denser bot1998b].
We next explore a recipe for Bc, which requires a small tom water, enriched in a high-salinityshelf water from the
contributionof Pacificpycnoclinewater (P). The Pacificpyc- Weddell Sea, prohibitsdescentto the seafloor.
noclineis of lower oxygenand CFC-11 concentrationthan the
Perhapsthe most significantcontrastin the 1995 and 1997
Weddellpycnocline.Thiswater mayenterthe BransfieldStrait
from the west between
Smith
Island
and Antarctic
Peninsula
alongwith PacificCDW [Hofmannand Klinck, 1998].Tracing Table lc. Mixing Recipe for Central Basin Bottom Water
a straight line from S2 along the O/S scatter of the bottom InvolvingPacificPycnoclineContribution
water in the centralbasinwould meet the pycnoclineof station
Potential
84 near -0.3øC (point P on Figure 4). Specificpropertiesof
Identifier
Temperature
Salinity
Oxygen
CFC-11
point P are given in Table l c, basedon the averageof water
samplesat 200 and 250 dbar. Station 84 displaysthe warmest P
-0.35
34.48
6.14
3.09
pycnoclinewater of the DOVETAIL data setwithin the Brans- Bc observed
- 1.615
34.580
7.08
4.79
- 1.556
34.579
7.26
4.86
field Strait environs,reflectinginfluenceof Pacificwater. Using Bc calculated
station84 pycnoclinepropertiesasthe warm end-membermixRecipe
ing partnerwith S2 yieldsa P:S2of 19:81for temperatureand
Components
Percentage
salinity but 40:60 for oxygenand 33:67 for CFC-11, no improvement over mixtures that involve only Weddell water D (Table la)
11
types.
We find that mixturesinvolvingall four water types(D, S1,
S2, and P) provide a more convincingrecipe.The four-point
mixture involvingPacificpycnoclinewater to derive a recipe
for Bc involvesa trial and error processof adjustingpercent-
P (Table lc)
S1 (Table la)
S2 (Table la)
5
24
60
The DOVETAIL stationfor Pacificpycnoclinewater (P) is station
84 between
200 and 250 dbar.
GORDON
ET AL.: DEEP AND BOTTOM
WATER OF BRANSFIELD
Northeast
STRAIT
11,345
Channel
--,o
60ø 59ø 580 570 560 550 •54ø 53ø 5'"o
•
,"
630 62
ø t•" ""o'• 3,'/r• I'"•^•-•,..•
I • I --•TM
••
....
ß oDra,•ke•Passage/.•/'
. , ,•:':•;:•
•
o
•1 50ow
61
1øS
o
Figure 5. Schematicof the flow pathsinto the BransfieldStrait'seasternand centralbasins.Solid arrows
denotemovementof Weddellwaters.Thesewatersare a combinationof freezingpoint low-salinity(S1) and
high-salinity(S2) shelfwaterwith smalleramountsof WeddellDeep Water. The centralbasinreceivesa more
salineform of shelf water. Dashed arrowstrace the pathwaysof CircumpolarDeep Water and pycnocline
water from the Pacificsectorinto the BransfieldStrait. The pycnoclinewater may be a minor constituentof
the central basinbottom water. The solid arrow drawn along the NortheastChannel is the expectedexport
path for Bransfieldbasinwaters.The open arrow throughAntarcticSoundis pure conjecture,as no data are
availablein that area,but it is deepenoughto providea pathwayfor shelfwater and henceis includedasonly
a possibility.
data setsis the warming seen in the bottom waters of the
easternand centralbasinsduring the intervening2 years.The
Wilsonetal. [1999]station7, whichcoincides
with DOVETAIL
station92, showsthat the bottom water in 1995 is nearly 0.2
colderthan that of 1997.Approximatelythe samedifferenceis
the westernrim of the Weddell Sea composes--•65% of the
bottom
waters within
the eastern
basin and between
80 and
90% of the central basin bottom waters.
On the basisof the distributionof water typesover Joinville
Ridge and on the mixing recipesfor Be and Bc a flow path
seen for the other stations in the eastern and central basins.
schematic(Figure 5) is proposed.A mixtureof Weddell shelf
Using a coldervalue for Be basedon the 1995 data requiresa water and deep water advectsnorthwardpast JoinvilleIsland
WDW:S1 ratio of 28:72 rather than 36:64 (both based on to enter the BransfieldStrait. The saltierfreezingpoint shelf
temperature)asfoundwith the 1997data. Salinitydifferences water (S2) turns into the Bransfield Strait very close to
betweenthe 1995 and 1997 data are slight,as expectedfrom Joinville Island contributingto the central basin, bypassing
the small salinity range of the componentwater types. It is the eastern basin. While a flow path of S2 water into the
suggested
that a slightreductionin shelfwater componenthas central basin passesaround the eastern edge of Joinville
occurredin the intervening2 years.
Island, the possibilitymustbe reservedfor an additionalflow
Wilsonet al. [1999] investigatethe temporalchangesin the path passingwest of JoinvilleIsland, within Antarctic Sound
BransfieldStrait deep and bottomwatersfor the last 30 years.
(Figure 1).
They find the changesin the easternbasinbottom water has
The lower-salinityshelfwaters may enter the easternbasin
been sporadicwith the warmest,saltiestwatersfound in 1963
directly or first enter the central basin forming a 1000 dbar
(-0.84øC; 34.60), while the 1995 valuesare the coldestand
deep salinityminimumwith subsequentoverflowinto the eastfreshest.The 1997 bottom water propertiesreversethe 1963- ern basin. Pacific water enters the Bransfield Strait from the
1995 trend (thoughwe do not have a completetime series
west and throughpassages
in the South ShetlandIslands.Enbetween1963 and 1995 to fully resolvehigher-frequency
varitering from the west contributesto the bottom water of the
ability);the 1997bottomwater wasstill muchcoolerthan the
1963 conditions.
central
basin.
Where does the basin residencewater displacedby newly
formed deep water go? Station 83 (the cooler of the two
4.
Conclusions
stationsshownas solid circlesin Figure 4, the other being
station 84) within the Northeast Channel is the most likely
The bottom waters of the Bransfield Strait's eastern and
central basinsare primarily derived from the Weddell Sea candidate marking an export pathway. There is no obvious
watersenteringthe BransfieldStrait from the region off Join- exportof concentrateddeepbasinwater. Rather it is suspected
ville Island. The bottom waters of the central basin contain a
that the basinwater is lifted into the pycnoclinelayer of the
Weddell-Scotia
Confluence,where it is rapidly exportedeastsmallamountof pycnoclinewater from the Pacificwater column. All of the deep water ingredientsare imported; there ward. The fate of this pycnoclinewater, as it spreadsinto the
does not appear to be a need for water massmodification Scotia Sea, requirescareful study.Its densitymay allow it to
within the BransfieldStrait. Freezing point shelfwater along eventuallyventilate the deep ocean.
11,346
GORDON ET AL.: DEEP AND BOTTOM WATER OF BRANSFIELDSTRAIT
Acknowledgments.
The researchis supported
by NationalScience Hofmann,E. E., J. M. Klinck, C. M. Lascara,and D. A. Smith,Water
mass distribution and circulation west of the Antarctic Peninsula and
Foundation
grantOPP95-28807andbya grant/cooperative
agreement
including
Bransfield
Strait,Foundations
for Ecological
Research
west
from NOAA (UCSIO P.O. 10075411).The tracerchemistry
is supof theAntarcticPeninsula,
Antarct.Res.Set.,vol.70, editedby R. M.
portedbyNSFgrantOPP95-28806.
ZhaoqianDongwasonleavefrom
Polar Research Institute of China. His residenceat Lamont was sup-
Ross,E. E. Hofmann, and L. B. Quentin, pp. 61-80, AGU, Wash-
portedin partbytheChinese
Commission
onScience
andTechnology ington,D.C., 1996.
grant98-927-0101
and in part by OPP 95-28807.The DOVETAIL L6pez, O., M. Garcia,D. Gomis,P. Rojas,J. Sospedra,and A.
CTD data collectiongroupwere B. Huber, P. Mele, A. Orsi, S. Ma,
and R. Iannuzzi. S. Green and S. Peacockperformed the oxygen
analysis.
G. MathieuandS.MathieuperformedtheCFC analysis.
The
Palmerofficersand crewand the ASA groupprovidedexcellentsup-
Sfinchez-Arcilla,
Hydrographic
andhydrodynamic
characteristics
of
the easternbasinof BransfieldStrait (Antarctica),DeepSeaRes.,
Part I, 46, 1755-1778, 1999.
Mensch, M., W. M. SmethieJr., and P. Schlosser,Tracer oceanogra-
phyin the WeddellScotiaConfluence
duringNBP 97-5, U.S.Antport. Particularthanksis givento the anonymous
reviewers
of this
paper,whosedetailedcomments
greatlyimprovedthe presentation. arct.J., in press,1998a.
The viewsexpressed
hereinare thoseof the author(s)and do not Mensch, M., W. Smethie Jr., P. Schlosser,R. Weppernig, and
R. Bayer,Transienttracerobservations
from the westernWeddell
necessarily
reflecttheviewsof NOAA or anyof itssubagencies.
LDEO
Contribution
5895.
Seaduringthe drift andrecovery
of ice stationWeddell,in Oceans,
Ice,andAtmosphere:
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