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PALEOCEANOGRAPHY,
VOL. 6, NO. 1, PAGES 1-20, FEBRUARY
1991
BENTHIC FORAMINIF RAL also IN THE
OCEAN'S TEMPERATURE-SALINITY-DENSITY
FIELD: CONSTRAINTS
ON ICE AGE
THERMOHALINE
CIRCULATION
RainerZahnl
Departmentof Oceanography,University of British
Columbia, Vancouver, Canada
Alan C. Mix
Collegeof Oceanography,
OregonStateUniversity,
Corvallis
Abstract.Benthic15180
datafrom95 coresitesare
used to infer possible temperature-salinity(T-S)
fields of the Atlantic
and Pacific oceans at the Last
Glacial Maximum (LGM).
A constraint of stable
densitystratificationyieldslogically consistentscenarios
for both T and S.
The
solutions
are not
unique but are useful as a thinking tool. Using
(AT-3øC) than overlying deep waters,in conflict
with otherdata, suggestingice agedeepwater much
colder than at present. It is also possiblethat the
observed15cgradientsare an artifact of laboratory
intercalibration.If Atlanticdeepandbottomwater
values were similar to deep Pacific values, this
wouldbe consistent
with thehypothesis
of a stronger
GEOSECS data, we solve for the modem relation-
southern
shipbetween
1518Owate
r (15w)
andsalinity
inthedeep
deep-oceanventilation at the LGM. Taking the
observedgradientsat face value, however,a solution
couldbe thattheLGM bw-Sslopein deepandbottom
waters was higher than at present, conceivably
becauseof a strongercontributionof saltto the deep
oceanvia more intenseseaice freezing. This would
allow PacificdeepwatersandAtlantic bottomwaters
to have a common source,again in the Antarctic.
Both wouldbe moredensethanArianticdeepwaters,
even thoughthe deepwaterswere much colderthan
at present. To better constrain these inferences
sea:15w(SMOW) = 1.529 * S - 53.18. As a starting
point, we assumethat the slope of this equation
applies
toLGMconditions
andpredict
1518Ocalcit
e(•c)
gradientsin equilibriumwith probableT-S fieldsof
LGM deepandbottomwaters. Benthicforaminiferal
15180
datafromthedeep
Pacific
(2-4kmdepth),
and
the bottom Atlantic (> 4 km depth), are 0.1-0.2%o
lower than from the deepAtlantic (2-4 km depth) at
the LGM. If the modem15w-S
slopeapplies,Atlantic
deep and bottom waters were more dense than
Pacific deepwaters. This assumptionwould imply
bottom waters both fresher (AS >0.5) and colder
•Now at GEOMAR, Kiel, Federal
Republic of Germany.
Copyright 1991
by theAmericanGeophysical
Union.
Papernumber90PA01882.
0883- 8305/91/90PA-01882510.00
ocean versus North
Atlantic
source for
drawn
fromthespatial
distribution
ofbenthic
15180,
we mustreducescatterin the 15180datawith more
high-qualitymeasurements
in high sedimentation
rate
cores.This is especiallytrue at bottom water sites.
Also, we must intercalibratemassspectrometers
at
different isotope laboratories more accurately, to
insureour isotopedataarecompatible.
INTRODUCTION
Rhythmic
variations
offoraminifera115•80
indeepsea sediments,first detected by Emiliani [1955],
ZahnandMix: BenthicForaminiferal
15180
primarily
reflect
changes
inglobal
ice
volume
Shackleton,
1967].
Intercore
differences
in the
variationsare commonlyattributedto spatialvariations of deep water temperature [Chappell and
Shackleton, 1986; Labeyrie et al., 1987]. Yet we
know from work on the modernocean [e.g., Craig
andGordon,
1965]thattheb•80 composition
of
differentwatermasses(bw)in the oceanis quitevariable. Thus changesin water massare anothervariable that must be considered.Broecker [ 1986, 1989]
discusses
possiblevariationsin the distributionof
in the glacial oceans. He emphasizeswater mass
changesin surface waters relative to mean deep
the same vessel. Memory between samples,less
than 1% of the isotopeoffset between samples,is
undetectablein normalmarinesamples.Calibration
to the international Pee Dee Belemnite (PDB) scale
was done primarily through the U.S. National
Institute of Standardsand Technology carbonate
stan,,dard
"NBS-20"
and
secondarily
through
"NBS19. Long-term
reproducibility
(+1(5)for15180
and
15•3C
over1year(1989)is0.09and0.04%0,
respectively, for a local calcitestandard(n=229), and0.04
and0.03%0,respectively,for NBS-20 (n=71).
The15•80
dataused
herearefromthetwowidely
usedbenthicgeneraUvigerinaandCibicidoides.We
waters,whichwouldaffectb•80 in planktonic corrected
all15•80
datatotheUvigerina
scale,
which
foraminifera.
isthought
tobeclose
to15•80
equilibrium
withsea
water[Shackleton,
1974].Thecorrection
for15180
Here we take the next stepand explorethe useof
spatial
distributions
ofbenthic
foraminiferal
b•80as
data from Cibicidoides to Uvigerina is +0.64%0
a tracerfor both temperatureand salinityin the deep
sea. We start by evaluatingthe equilibrium frac-
tionation
of 15ISOcalcite
(15c)
in theocean's
temperature-salinity-density
field. Next we derivebw-Srelationshipsfor the modernocean. To interpretthe ice
agedatain termsof T and S, we fin:stapplythe slope
of the moderndeep-seabw-Srelationship. We later
relax this restriction and consider how the bw-S
patternmay have changedin the past. Finally, we
infer the range of possibleice age distributionsof
temperature,salinity,anddensityin thedeepsea. A
uniquesolutionto two variables(T and S) cannotbe
found from measurementsof one variable (bc). In
spiteof this, the constraintof densitystratification
gives insight into possibleT and S distributionsin
the deepocean,which hasimplicationsfor the mode
of deep and bottomwater formationin the ice age
ocean.
[Shackletonand Opdyke, 1973; Shackleton,1974].
The error introducedhere may be as large as 0.2%0
for individual samples[Mix and Fairbanks,1985].
On average,though,the adjustmentappearsto be
quitestablein boththeAtlanticandPacific.
We haveusedcoretopdatato examinethemodem
distribution
pattern
ofbenthic
b•80.Coretops
with
anomalously
lowb•3Cwererejected.
Wesuspect
theseare contaminatedwith glacial material. The
LGM is deftnedasthemostrecentforaminiferal
b•80
maximumbelow the last glacial-interglacialtransition. This may differ slightly from published
chronologies,
but given the potentialage errorsin
someof the datausedhere,we preferthe objective
criterion
ofusing
the15•80
maximum.
FORAMINIFERAL 15•80AND
PALEOTEMPER AT[ IRE EQUATIONS
METHODS
AND DATA
BASE
Glacial-interglacialfluctuationsof foraminiferal
Wecompile
benthic
15180
datafrom58northeast I5•80 combine
the
of changing
temperature
•8 signals
Ariantic and 39 Pacific core sites for the modern and
are listed in Table 1. Someof the isotopedata used
here are new. Thesewere measuredat OregonState
University, on a Finnigan MAT 251 mass spectrometer.This facility is equippedwith microvolume
inlet and an AutoprepSystemsautomatedcarbonate
device. Sample preparation follows the standard
techniques[Shackletonand Opdyke, 1973], except
and changing150water(15w).TO solvefor temperature,we mustchooseamongavailableisotopicequilibrium equations. Figure la showsempirical 15c
equilibriumpredictions,defined by sevendifferent
equations(see review by Mix [1987]). The equationspredictshiftsin 15c
of 0.21-0.27%oper 1øC temperaturechange(Table 2). At temperaturesabove
10øC,all but the earliestequationsyield nearlythe
same isotopevalues. However, at typical abyssal
temperatures,
i.e., below5øC,theequationsdiverge.
To test the palcotemperatureequations,we com-
that reactions occur at 90øC.
parecoretop15180
datawithdepth
profiles
ofpre-
the Last Glacial Maximum (LGM, approximately
18,000 years ago) (Table 1). Most of the data used
here were available
from the literature.
References
It is a "common acid
bath" system,in which up to 40 reactionsoccur in
dictedequilibrium15c
(Figure lb). The equilibrium
ZahnandMix: BenthicForaminiferal•80
o
•5o
oO 0
,-• t.• oo oo .q- 0
i
i
i
i
•2uo
i
i
i
oo (•,1
i
i
ZahnandMix: BenthicForaminiferal
b•80
oo
o•
•
o
•uOUOuuu
o
uu•u•uuuu
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
OuuuuuOOUO•
uuuu
ZahnandMix: BenthicForaminiferal
•0
oou
UUU
••U
uuoo
UU
UU•U•UU
ou
•2ouuuuo
U
U
U•UUUUU
ZahnandMix: BenthicForaminiferal
•5180
•5'80,•z( %oPDB)
1
2
3
4
0
18
2
P_•T.•.OTEMPERATURE
EOUATIONS
AFTER:
14
a) MCCREA [1950]
12
b) EREZANDLUZ [1983]
10
c) O'NEmETAL. [ 1969]
8
d) SHACKLETON
[1974]
6
e) EeST•INETAL. [1953]
4
2
0 C•dO [1965]
0
g) HORmE
•
-2-2 -1 0
1 2
3 4
++ 4•
O•^ [1972]
5
5
a 1:
•te•[
6
/5•SOeee
• minusõ•Owater
A
B
Fig.1. (a)Empirical
relationships
between
•518Ocalcite
(•) andwater
temperature.
(b)Vertical
profries
of•5c,
using
temperature
and•S18Owater
profiles
from
GEOSECS
stations
29and115(data
from(Sstlund
etal.[1987a]).
Crosses
arebenthic
•5180
values
(Uvigerina
scale)
fromNortheast
Arianticcoretops.Bestfit is withequation
b.
profilesfornortheast
Atlanticcoresitesarecomputed
from vertical temperature and •Swsectionsof
GEOSECSstations29 and 115 [Ostlundet al.,
1987a]. The bwvalueswere transformedfrom the
StandardMean Ocean Water (SMOW) scaleto the
PDB carbonatescaleby subtracting
0.27%0fromthe
data tabulatedon the SMOW scale[Hut, 1987].
Offsetsbetweenpredicted
valuesandcoretopdata
(Figurelb) pointeitherto errorin calibratingthe
equations
at lowtemperatures
or to isotopic
disequilibriumin Uvigerina.The bestfit to thecoretopdata
below2 km depthis with theequationof Erez and
Luz [1983]. This equationwas calibratedbetween
14øC and 30øC, with cultured planktonic
foraminifera. We will usethisequationto estimate
equilibrium•5c.Our conclusions
wouldnot change
had we chosenoneof the otherequations,however.
The fit of the Erez and Luz equationto core top
does not indicate failure of the Erez and Luz [ 1983]
equationbutrathera watermasseffect.
For the Pacific, we use T and •Sw data from
GEOSECS stations 306, 322, and 345 [Ostlund et
.o
al., 1987a]. Here, the Erez and Luz equationpredicts •5clower than some of the core-topdata (see
Figure 5 below). We cannotprovewhy this mismatchoccurs.We speculate,
however,thatbiological mixingof foraminiferalshellsfrom late glacial
coresectionsto the coretopsis more severeat some
of the Pacific core sites we have used. In the Pacific,
foraminiferal
shells are most common in carbonate-
rich glacialsediments
andlesscommonin coretops
wherecarbonatecontentsare stronglyreduced.The
opposite
is u'ueof manyAtlanticsites.Thusthereis
a higherchancefor Pacificcoretopsto be contami-
nated
withhigh•5•80
glacial
material
duetobioturbation. Birchfield [ 1987] reacheda similarconclusion.
benthic
•5180
values
atwaterdepths
< 2 kmispoor.
Predictions
aresystematically
lessthanthemeasured
values(Figurelb). At thesedepths,mostof thecore
sites used here are near the Strait of Gibraltar and are
influenced
by180-enriched
Mediterranean
outflow
waters. The hydrographicprofilesof GEOSECS
stations29 and 115, which we use to computethe
vertical•5cprofilein Figure1, arefar fromGibraltar.
They arenot stronglyinfluenced
by Mediterranean
waters. Thusthe mismatchof coretop dataandthe
predictedwatercolumnvaluesabove2 km depth
TABLE 2. SlopeandTemperature
Predictions
Using
Different
Paleotemperature
Equations
Equation
Slope
T øCa
McCrea [1950]
0.21
-0.75
Epsteinet al. [1953]
Craig[1965]
0.26
0.27
3.17
3.79
O'Neil et al. [1969]
0.25
2.80
Horibe andOba [ 1972]
0.27
3.81
Shackleton[1974]
0.25
2.90
Erez andLuz [1983]
0.23
1.52
a calculatedfor 15w
of +0.27%o(SMOW) and 15c
of
+3.5%o (PDB).
ZahnandMix: BenthicForaminiferal
8]80
WATER MASS PATI'E•S
OF 8wAND 8cIN
(Abw/AS = -1.5 in Figure 2) thus yielding a
"fictitious"
zero-salinity
intercept
of 8180,much
THE DEEP SEA
lowerthanthe8180valueof thetruefreshwater
dilu-
Thedistributions
of b]80in seawater
(bw)and
ent.
Water
salinity(S) arebothcontrolledmostlyby evaporation
and precipitation [Dansgaard, 1964; Craig and
masses from
the southern ocean and the
North Atlanticaretheprincipalsources
of ventilation
in the modem deep ocean. This was probablyalso
true (at varying mixing ratios) in the glacial ocean
eachregion8wis linearlyrelatedto watermasssalin[Curry and Lohmann, 1985; Curry et al., 1988;
ity (Figure2). The slopeof the 8w-S relationship,
Duplessy et al., 1988; Keigwin, 1987; Oppo and
however, varies between 0.1 (for tropical surface
Fairbanks,1987]. Mixing betweentheseend-memwaters) and 0.6 (for high-latitude surfacewaters)
ber water massesis most easily seenin the Atlantic
[Craig and Gordon, 1965]. This reflects greater
Oceanwhich representsthe geographicconnection
Rayleighdistillationof precipitation
in highlatitudes,
between
the deep and bottomwater sourceareasin
which
preferentially
removes
•80fromhigh-latitude
fresh waters. Without additional mechanisms such
the high-latitude North Atlantic and the southern
ocean. Using GEOSECS 8w and salinity data
asfreezingof seawater,
thezero-salinity
intercept
in
Gordon, 1965; Joussaume et al., 1984].
Within
the8w-Splotgivesthe8•80 valueof themean
[0stlundet al., 1987a],we evaluate8w-Srelation-
regionalfreshwater diluentof seawater.
shipsfor theAtlanticmodemdeepandbottomwaters
(Figure 3). Becauseof their close similarity, we
combinethe 8w-Sequationsfrom Figure 3 into one
singledeep-oceanequation:
I
1
8w(SMOW)=l.529*S-53.18
(r2=0.95,
n=110) (1)
.
it,
J
•
[SURFACE
/
7
Again, note the relatively negativezero-salinity
intercept, below -50%0 (SMOW). In contrast,the
average
8•80composition
ofprecipitation
andfiver
NORTHATLANTIC_DEEP
WATER runoff in the high-latitudeNorth Atlantic is about-
0 /WEDDELL
/l[ PACIF[•_.AN_._.D,,,I
,ND.I_.•_,•_OCF_•
'
21%o(SMOW) [CraigandGordon,1965;0stlund
]
-11
33
DEEP
WATER
ANTARCTIC
.
0.4
BOTTOM
WATER
,
34
.
,
35
.
,
36
.
i
37
.
0.3
38
SALINITY
Fig.2. Regression
linesandslopes(s)for 8wand
0.1
salinity. Line A, NorthAtlanticsurfacewaters.Line
B, tropicalsurface
waters[afterCraigandGordon,
1965]. Line C, Mediterranean
Waters[Stahland
Rinow,1973].Dashed
lineisfordeepandbottom
watersamples
in theAtlantic(seealsoFigure3).
0.0
-0.1
This is not true in polar regions,where salinityis
influencedby freezing of sea ice. Freezingrejects
34.6
34.7
34.8
34.9
35.0
35.1
SALINITY
A: $•,= 1.45* S- 50.55(r2 = 0.95)
Thus southern ocean waters
havea relativelywide rangeof salinities(33.5-34.5)
even thoughthey havevirtually the same8w(about0.2%0 versus SMOW) (Figure 2) [Craig and
2-4kin (A)
>4kin (B)
-0.3
salt,butcauses
almost
no8•80fractionation
[Craig
and Gordon, 1965].
[]
A
-0.2
B: $w= 1.68* s - 58.36 (r2 = 0.96)
Gordon, 1965; Weiss et al., 1979; Jacobs et al.,
Fig.3. Relationships
between
8]SOwater
andsalinity
1985]. For deep water, mixing of water masses
derived from sea ice freezing with other water
massesresultsin high slopesof the8w-Srelationship
in Atlanticdeepwaters(opensquares,line A) and
bottomwaters(opentriangles,line B). Data from
..
Ostlund et al. [ 1987a].
ZahnandMix: BenthicForaminiferal
•80
andHut. 1984;Osttundet at., 1987b]. The esti-
mated
•f80ofprecipitation
andmeltwaters
inthe
SouthernOcean rangesfrom about -13%oto -50%0
(SMOW) [Weiss et al., 1979; Jacobset at., 1985].
At its most negativeextreme,this approachesequation (1)'s interceptvalue of-53.18%o. The contribu-
tionof180-depleted
Antarctic
icesheet
meltwaters
to
•80 waterbelow4 km depth(Figure4a). This
patternis similarin the westernandeasternbasinsof
the North Atlantic. The isotopicoffsetbetweendeep
andbottomwatersis slightlylessin the east,because
theMid-AtlanticRidgeformsa topographic
barrierto
AABW [Metcalf et al., 1964].
dituent[Craig andGordon,1965].
We will usethe slopeof equation(1) as a starting
pointto relatesalinityto f•win the glacialocean. It is
importantto notethe limits in this assumption,
however. Using equation (1) for the past oceans
assumes
thattherole of freezingandsaltrejection(as
Using publisheddata setsof •w and temperature
[Ostlundet al., 1987a] and applyingthe paleotemperatureequationof Erez and Luz [1983], we esti18
mate fi Ocalcite
(•5c)valuesfor the deepAtlantic and
Pacific oceans (Figure 4b and Table 3). In the
Atlantic, the decreaseof fiwfrom NADW to AABW
goesalongwith a decreasein temperature.The net
result is similar tic values for Atlantic deep and
bottom water masses. In the Pacific, a slight
(-•0.1%o)increasein predictedfieoccursin bottom
waters,due mostlyto lower temperatures.
Consistentwith equilibriumpredictions,below-•2
a fractional contributionto water masses)is the same
kmwaterdepth,
theAtlantic
coretopbenthic
fi180
as at present. We relax this constraintlater in the
paper, to explore the possibilityof varying slopeof
the deep-seaf•w-Srelationship.
valuesreach a roughly constantlevel of 3.1-3.3%o
(Figure 5 andTables 1 and 3). At the LGM, benthic
Antarctic Bottom Water, however, is less than 0.1%
of the total [Weiss et al., 1979]. Thus the low inter-
ceptof•180in equation
(1)mustreflect
mixing
of
North Atlantic Deep Water with a freeze-enhanced
Antarctic end-member, rather than a true freshwater
In addition, the role of intermediate waters of
different •w is neglected. For example, Craig and
Gordon[ 1965] notethatmodemPacificdeepwaters
have slightly lower salinitiesthan expectedfrom
simplemixing betweenNADW and AABW (Figure
2). They suggestthat this offset may be explained
by an admixture of lower salinity intermediate
waters. Birchfield [1987] disagreed. Arguing that
Craig and Gordon'sAntarctic end-memberwas too
oo
fi180is higher
everywhere.
Thisreflects
thecombined effects of increasedglobal ice volume and
decreased
watertemperature(Figure6 andTable4).
The meanAtlanticincreaseof 1.7%ois largerthanthe
mean Pacific increaseof 1.5%•. In Figure 6, hypotheticalverticalticprofilesfor the LGM assumethat
modemwater masspatternsapply. We accountfor
icevolume
byshifting
themodem
equilibrium
fi•80
profiles by +1.3%o,consistentwith recent data on
sea level [Fairbanks, 1989]. Deviations of the data
lowin •180,heinferred
thatonlytwoend-members from theselinesreflectchangesnot directlyrelatedto
are needed. The general point that intermediate
storage
of •80-depleted
iceonland,i.e.,duetolocal
waters could have had different •w-S patternsthan
modem deep water, however,remains. We cannot
yet quantifytheimportanceof intermediatewatersin
the glacialocean,but it appearsthat they were better
ventilated than now [Boyle, 1988; Oppo and
Fairbanks, 1987; Zahn et al., 1987; Kallel et al.,
changesin temperatureor f•wof the watermass.
Most of the measured benthic tic data from the
LGM are significantlyheavier than would be predicted by a 1.3%oice volume effect (Figure 6). In
the Atlantic, 15cvalues below 4 km approachthose
predictedand are on average-0.2%0 lower than the
deepwatertic mean between2 and 4 km. A cross
section (Figure 7) shows that the deep water tic
1988]. Thus it is probablethat the deepwater•w-S
relationshipwas different in the glacial ocean. We
will illustratethe effectsof varyingf•w-Srelationships
maximum
on the reconstruction
the Pacific, the data do not reach true bottom water
of T and S from foraminiferal
fi•80inthefinalsection
ofthisstudy.
is most obvious north of about 10øN.
In
depths.The latitudinalcrosssectionhintsat a deepwater maximum of benthic tic to the north of 40øN
DISTRIBUTION
OF BENTHIC
FORAMINIFERAL b•80:MODERNAND LGM
In the modemAtlantic,North AtlanticDeepWater
(NADW)formsa fi180maximum
between
2 and3
km depth with •w values >+0.2%0 (SMOW).
Antarctic Bottom Water (AABW) is evident in low
(Figure 7). At present,however, we cannot con-
clude
thatanystatistically
significant
fi•80gradients
existbetweenglacialPacificdeepandbottomwaters.
The Atlantic offsetbetweendeepandbottomwaters
is statisticallysignificantat the 99% level (t testfor
differencebetweenmeans)if thereareno systematic
measurementerrors between depth intervals. The
ZahnandMix: BenthicForaminifera115
•80
LATITUDE
0ø
10ø
20ø
30ø
40ø
50ø
60øN
A
0o
10o
20ø
30ø
400
50ø
60ON
B
Fig. 4. Modemdistributions
of (a) •, and(b) t•½
in theNorthAtlantic(datafromOstlundet al.
[1987a];equation
of ErezandLuz [1983]).NADW andAABW canbeseenin hwbutnotin hc.
Thedecrease
of f• is offsetby lowertemperatures
atdepth.
offsetbetweenAtlanticandPacificdeepwaters,with
Pacific valueslower than Atlantic valuesby 0.10.2%0at the LGM, are also significantat the 99%
level. Beforewe interpretthe isotopedatain terms
of water masschanges,however,we mustevaluate
potentialgeological
andanalyticalproblems
thatmay
limit our conclusions.
INTEGRITY OF BENTHIC•5180:
BIOTURBATION AND MASS SPECTROMETER
CALIBRATION
A possibleexplanationfor intercoredifferencesof
stratigraphicrecordsis to assumesmoothingof
signalsby bioturbation[Bergerand Heath, 1968;
TABLE3. Hydrography
andOxygenIsotope
Signalof ModemWaterMasses
Depth
Level,
km
2-4
>4
2-4
>4
InSireTemperature,
øC
Salinity
15w,
%0SMOW
NorthAtlantic: (Geosecs
Stations29, 115)
2.9
34.92
0.26
2.4
34.89
0.21
15c,
%0PDBa
3.17
3.25
CentralPacific: (Geosecs
Stations
306,322, 345)
1.7
1.1
34.67
34.70
0.00
-0.01
a calculated
usingtheseawater-•equilibrium
equation
of ErezandLuz [1983].
3.19
3.32
ZahnandMix: BenthicForaminiferal
•j180
1o
IMODERNI
NE-ATLANTIC
PACIFIC
•80 (%.PDB)
6 180 (%oPDB)
1.5
2.5
3.5
1.5
0
2.5
i
ß
3.5
i
.
i
0
1
•
•'
1
©
2
[]CAMBRIDGE
• 2 OGIF
S.
YVETTE
+ KIEL
ß LDGO
3
• 3 $OSU
•x WHOI
4
4
6
6
Fig.5. Modem
depth
distributions
ofbenthic
foraminifera16180
(PDB
scale,
normalized
to
Uvigerina).Symbols
identifylaboratories.
Solidcurvesarepredicted
equilibrium
6c(equation
of
ErezandLuz [1983]). Valuesfor temperature
and6warefromGEOSECSstations
29 and115in
theAtlanticandstations
306,322and345in thePacific[•stlundet al., 1987a].
ILASTGLACIALMAXIMUMi
NE-ATLANTIC
PACIFIC
•5'80 (%oPDB)
•5'SO (%oPDB)
3
4
5
3
4
i
i
5
.
i
0
[21
2
N• 000
[] CAMBRIDGE
O GIF S. YVETTE
+ KIEL
ß LDGO
$ OSU
o
4
a WHOI
5
6
Fig.6. Depth
distributions
ofbenthic
foraminiferal
•j180
(PDB
scale,
normalized
toUvigerina)
at
theLastGlacialMaximum.Symbols
identifylaboratories.
Solidcurvesarethesameasin Figure
5 except
increased
by 1.3%o
to account
foricevolumechange.
ZahnandMix: BenthicForaminiferal
•5180
TABLE 4. Benthic•5180(%0PDB) at Northeast
AtlanticandPacificCoreSites
M o d ½r n
518
0 a
st
WaterDepth,km
_Last GlacialMaximum
A•j180
5180a
St
n LGM-Rec
n
(cm1000yr4)
Northeast Atlantic
2-4
>4
Whole-basin mean c
3.21 (b)
0.1
15
4.99
0.2
35
1.78
3.22
3.22
0.1
0.1
5
20
4.82
4.97
0.1
0.2
5
40
1.60
1.75
2-4
> 4
Whole-basin mean c
3.41
3.14
3.39
0.1
0.1
10
1
11
0.2
0.3
0.2
29
2
31
1.46
1.50
1.47
5.8
1.9
Pacific
4.88
4.64
4.86
5.2
st,lo standarddeviation;n, numberof coresitesper depthhorizon.
a Uvigerina
scaleof •5•80.
bWithout
valueof 2.89%0
fromcoreEND066-10GGC
(seeTable1).
c Weightedmean.
CLIMAP Project Members, 1984]. Bioturbation
worksmostefficientlywheresedimentation
ratesare
kyr-1 (Tables1 and4). Thuswemighthypothesize
low.
from differentialsmoothingasa functionof sedimen-
Sedimentation
thatthescatter
inour•180profiles
maycome
inpart
rates at the core sites from > 2
tation rate.
km water depthusedherevary between1 and 8 cm
ILASTGLACIALMAXIMUM I
S
LATITUDE
0
10
20
30
N
40
50
60
0
4.83
[] CAMBRIDGE
o GIF s. YVETTE
+ KIEL
-10
0
10
20
30
40
50
60
0
6
ß LDGO
ß OSU
t, WHOI
6
!.'."2•
4.755.0o/o0
(PDB)
"'::•
..... >5.0
o/o0
(PDB)
Fig.7. Meridional
cross
sections
ofbenthic
foraminiferal
•5•80
attheLastGlacial
Maximum
(PDB
scale,normalizedto Uvigerina). Symbolsidentifylaboratories.Relativelylow valuesarefoundat
depthsbelow 3.5-4.0 km.
ZahnandMix: Benthic
Foraminifera115180
12
from six isotopelaboratories
(Table 1). To checkfor
consistency
betweenlaboratories,
we averagedLGM
15180
valuesfrom the northeastAtlanticandPacific
between2 and 4 km depth. In this interval thereis
A numericalbioturbationmodel (similar to that of
Penget al. [ 1979])predictsthatsmoothing
becomes
important
at sedimentation
ratesbelow4 cmkyr-1
(Figure 8). We assumea 10 cm mixed layer
(probablyan overestimate),
with diffusivityof 60
no15180
trendwithwaterdepth,
andfortheAtlantic,
cm2 kyr-1, andconstant
downcore
abundances
of
data are available from all six laboratories.
benthicforaminifera. The modelpredictsthat core
Atlantic, four of the laboratoriesgive mean values
within one standard deviation (+ 0.17%o)of the
mean. At the extremes,two laboratorymeansdiffer
by nearly 0.4%0from each other (Table 5). The
Pacificdeepwatermeanvalue (Table4) is dominated
tops are affected more than glacial maximum
samples.
Upward
mixing
ofhigh-15180
glacial
shells
ismoreeffective
thandownward
mixing
oflow4180
Holocene shells. This is becausedownwardmixing
is limited to one mixing depth. There is no clear
correlationbetweenthe model predictionsand the
For the
b[8
data
from
only
twolaboratories
(Table
5).The
15 O gradientbetweenthe oceans,if calculatedon
with increasingsedimentationrates (Figure 8).
Changes
in theabundance
of benthicforaminifera
in
glacialandinterglacial
sediments
wouldmodifythe
the basisof datafrom thesetwo laboratories,is only
0.05%0, i.e., at the border of analytical detection.
Thus, we cannot exclude the possibility that the
observedintraoceanicandinteroceanicgradientsare,
results shown here. Based on the available data,
at least to some extent, an artifact of calibration off-
however,we concludethat biomrbationis unlikely to
setsbetween the laboratories. This does not prove
thattrue calibrationoffsetsexist. Our comparisonis
basedon relativelyfew samples,and it is not based
on analysesof aliquotsof the samecarbonatesample
core data. The scatter of the data does not decrease
bethesolecauseof scatter
in the15180
data.
Anotherpotentialbiasmay resultfrom improper
intercalibration
of massspectrometers.
We usedata
ß
i
-
i
-
i
-
i
-
ß
-
i
.
w
-
l.•
[
LGM
m/nu•
MODERN
I
0.0
0
•.•
12
2
ß
i
4
-
i
6
-
i
8
10
12
14
16
-
[] CAMBRIDGE
o Gig s. YVETTE
5.0
o
+
E 4.5[
•o
¸ 4.0[
35
0
•
4.5
12
4.0
....
2
ß
KIEL
ALDGO
ß OSU
AWHOI
LAST
GLACIAL
MAXIMUM
4
6
8
10
8
10
12
14
...............
•O 3.0
'• +
•
MODERN
2.5
0
2
4
6
SEDIMENTATION
12
14
16
RATES
( cm 1000yr 4)
Fig.
8.Modem
and
Last
Glacial
Maximum
benthic
foraminiferal
15180
and
glacial-interglacial
1518
O
shifts
atdepths
> 2kmcompared
tosedimentation
rates.
Solid
curves
represent
numerically
predicted
values
if bioturbated
layer
is10cmthick,
withdiffusivity
of60cm
2ky-1.
ZahnandMix: BenthicForaminiferal
•j180
13
Table5. MeanBenthic
•j180attheLGM FromCoreSitesBetween
2 and4 kmWaterDepth
Northeast
Laboratory
a
Atlantic
•518
Ob
st
Cambridge
4.95
0.22
6
Gif sur Yvette
Kiel
•
OSU
WHOI
Mean
5.17
5.08
4.81
4.96
4.85
4.99
0.01
0.10
0.07
0.18
0.16
0.18
3
12
3
7
5
36
Pacific
n
Ac
•5180
b
st
n
A c
-0.04
4.85
0.18
16
-0.03
4.73
4.97
4.85
4.88
0.27
0.11
0.29
0.18
2
9
2
29
+0.18
+0.11
-0.18
-0.03
-0.14
.
-0.15
.
+0.09
-0.03
st, 1 o standarddeviation;n, numberof datapoints.
a For explanationof laboratorycodesseeTable 1.
b Uvigerina
scaleof •5••0 (%.PDB).
c Deviation
from mean.
measuredat differentlaboratories.A more thorough
intercalibration
between
the laboratories
is needed
before corrections to the data can be made.
It appearsthat at least some of the variability
observedin Figures 5, 6, and 7 comes from true
differencesin the core samplesanalyzed. Given the
noise level in individual measurements, our discus-
sionwill emphasizeaveragevalueswithindeepwater
(2-4 km) and bottomwater (> 4 km) core sites(Table
4). Because of questionsabout intercalibration,
however, the difference in mean •5cbetween the
glacialAtlantic andPacificdeepwatersandAtlantic
deep and bottomwatersis questionable.We will
interpretthe •5½
patternsconsideringtwo options:that
themeasured
gradients
arereal,or thatthereis no
gradient.
(4) the global salt budgetmustbalance;and (5) any
verticalprofile musthave eitherconstantor increasing densitywith increasingwaterdepth.
The first and second points are physical constraints. Note that the freezingconstraintappliesto
sourcewaters at the sea surface. By the time these
waters mix into the deep-sea, ambient heat fluxes
require that they are well abovethe freezingpoint
[Mix and Pisias, 1988]. The third point is an
assumption. It is not foolproof, as warm salty
bottomwatersare conceivable,but it appearsto be
reasonablefor the LGM. The fourthandfifth points
arephysicalconstraints
whichcannotbe violated,but
they do not yield unique solutions. Becausewe do
not have full coverageof the glacial oceans,one
could always argue that the salt budget balances
somewhere
FORAMINIFERAL •5180IN THE OCEAN'S
TEMPERATURE-SALINITY-DENSITY
FIELD
For
the densityconstraint,stablewater massstratification
may come from a wide rangeof T-S combinations.
The major purposeof points4 and 5 is to eliminate
unrealistic
Foraminifera115•80
monitorsthecombinedvariationsof watertemperature
andbw. Unfortunately,bw
is not uniquelylinked to salinity. Thus, measure-
outside our field of measurements.
scenarios that would create an unstable
water column,or make it impossibleto balancethe
saltbudget.
Several water masspatternsare possible. Each
ments
of benthic
•5180
ontheirowncannot
uniquely
would be consistent with the vertical and horizontal
reconstructbothtemperatureandsalinityin thepast.
distributions
of benthic•5•80at theLGM. We show
The•5•80
gradients
inbenthic
foraminifera,
however, theseon traditionaltemperature-salinity(T-S) diacan narrow the field of acceptablesolutions,if we
includethe followingrequirements
andassumptions:
(1) temperatures
of the sourcewatersmustnot fall
belowthe freezingpointof seawater;(2) in situtemperaturesare usedto calculate•5c(not potentialtemperatures as there are no pressure effects on
foraminiferal•5c);(3) the deepestwater massesform
at high latitudesand have the lowest temperatures;
grams. Also plotted are •5cequilibrium isolines on
the diagrams.To do thiswe initially convertsalinity
to •Swusingequation(1) and usethe seawater-calcite
equilibrium equation of Erez and Luz [1983].
Densityis computedfrom the internationalequation
of stateof seawater[UNESCO, 1981] andexpressed
in conventionalsigma(o) units. We calculatedensity at the 4 dbarlevel (04, approximately4 km water
ZahnandMix: Benthic
Foraminiferal
$•80
14
depth) to eliminatepressureeffectson seawater
densityat depth.
Note thatthediagramaxesareAT andAS, rather
than T and S. We avoid absolute T-S estimates,
becausethesedependon knowingthe ice volume
effect well. In eachdiagram,T, S, 15c,
and o4 are
givenin comparison
to a reference
watermass.We
wishto emphasize
the spatialvariationsin the signals, rather than the absoluteglacial-interglacial
changes.The differencecalculations
arerelatively
insensitive to small errors in the choice of a reference
water mass. We use a hypotheticalmeanAtlantic
deepwaterasthereference.ThemodemT-S values
of this reference water mass are T=2.5øC
S=34.9.
M0DERN
,•,
•
o
and
For the glacial maximum, we set the
deepwater
reference
temperature
to 0øC[Duplessy
et
al., 1980;ChappellandShackleton,1986;Labeyrie
et al., 1987], and salinityto 35.9 to accommodate
lower glacialsealevels. PositiveAT, AS, Abc,or
Ao4 numbersindicatehigherT, S, bc,or o4 values
with respectto thereferencewatermass.
TheAtlantic-Pacific
Gradient
ofBenthic
i•180
at Depths2-4 Ion
Pacificdeepwaterstodayarecolder(AT = -1.2),
lesssaline(AS = -0.2) andslightlymoredense(Ao4
= +0.05) than deep watersof the North Atlantic
(Figure9a andTable3). At thesametime,benthic
15•80
obtained
fromcoretopsamples
is essentially
LAST
GLACIAL
MAXIMUM
LAST
GLACIAL
option
1
optionMAXIMUM
2
+1
I ATLANTtC= ....
+1
o
I
t , :--..-•.--•/.
ß
,+1
Co ..•5"'
ß'
["'PACIFIC.•:."'-/f.."
t
'
.-' PACIFIC.-'"
... '
'-'
A' '"''"".'•
9."/..."
B'1
-1t"
-'"
.?""-"
5'"'
• t.."•"P"
V•
..,•.5".
i"
...,.,
.1117
'11'
Ct
•!
- 0.2
0
+0.2
- 0.2
•
+0.2
- 0.2
0
+0.2
A SALINITY
EQUILIBRIUM
FRACTIONATION
LINES
FOR•180calcite
(ASA•18Ocalcit
½)
........
DELTADENSITY
ISOLINES
((h)
Fig.9. Temperature,
salinity,
andwatermass
density
(Ao4,dotted
lines)anomalies
of deep
watersrelativeto NADW reference.Solidlinesshowequilibrium
bcanomalies
(asA&). Positive
numbers
forallproperties
indicate
higher
values
thantheAtlantic
deepwaterreference.
(a)
Modem:NorthAtlanticdeepwater(solidsquare)
andNorthPacificdeepwater(solidtriangle).
ThePacificis colder(AT=-1.2),lesssaline(AS=-0.2),andslightlymoredense(Ao4=+0.05)than
theAtlantic.The•cvaluesaresimilarin bothoceans
(Abc=0).(b)LastGlacialMaximum,option
1:Benthic
foraminiferal
•80 inthedeep
NorthPacific
islowerthaninthedeepNoahAtlantic
(A•c=-0.2).Opentriangle
shows
anomalies
if ASstayed
atitsmodem
value(AT=-0.4,Ao4= 0.09). Opencircleshows
anomalies
if salinity
wasthesamein AtlanticandPacificdeepwaters
(AT=+0.8,Ao4=-0.13).Bothscenarios
predict
thatAtlanticdeepwaters
weremoredense
than
Pacific
deep
waters
attheLGM.(c)Last
Glacial
Maximum,
option
2: Benthic
foraminiferal
•80
inthedeepNorthPacific
isthesame
asinthedeepNorthAtlantic
(Abc=0).
Solidpentagon
shows
anomalies
if ASwasthesameastoday(AT=-0.4,Ao4=-0.03).Dashedarrowshowsshiftof
Pacificwatersif theirsalinity
wasthesameasin theAtlantic(openpentagon).
In thiscasethe
Atlantic
andPacificproperty
fieldswereidentical
(AT=O,
Ao4---0).
Scenarios
forLGM assume
•salinityslopesameastoday.
ZahnandMix: BenthicForaminiferal
•j180
the samein both oceans(3.2 to 3.3%0;Tables 3 and
4). This is illustratedin Figure 9a by the •5cequilibrium fractionationline, which passesthroughboth
the Pacific and Atlantic water masses.
At theLGM,the•5•80excursion
relativetomod-
15
bothoceanshad identicaltemperatures
anddensities
(openpentagonin Figure9c).
Other scenariosare also possible. If inferred
Pacific waterswere falling alongthe A•5cisolineof0.29'00relative to the Atlantic reference water mass, a
em is largestin benthicforaminiferafrom the North
Atlantic (averageof 1.8%o). This exceedschanges
observedat Pacific core sitesby about0.2%0(Table
4; seealsoDuplessyet al. [1980], Duplessy[1982],
and Shackletonet al. [1983]), yielding an interoceanicA•5cvalue of-0.2%0 at the LGM. This gradient may be questionabledue to inaccurateintercalibrationof massspectrometers
(seeabove).
Figure 9b showsthe line of acceptablesolutions
for T-S of Pacific deep watersfor a 15c
value 0.2%0
solutionwith deep Pacific densitiesas great as or
greaterthanAtlantic densitieswould not be possible
without changingthe slopeof the •5w-Srelationship
as it would requiredeepPacific temperaturesbelow
freezing. The inferenceof higher-densitywatersin
the Atlantic at the LGM would suggesta separate
source of cold and/or salty deep waters to the
Atlantic. Thisproblemdisappears
if the observed•5c
gradient between both oceanswere an artifact of
mass spectrometerintercalibration.If Pacific and
lower than the Atlantic.
Atlantic •5cvalues were similar, this would be more
If we maintain the modem
salinity contrastbetweenoceans(AS = -0.2), then
the Pacific-Atlantictemperatureoffset decreasedto
AT =-0.4 (opentrianglein Figure 9b). This is less
than half of the modern interoceanictemperature
difference. If this scenariois correct, it reversed the
densitycontrastbetweenthe deepPacific and deep
Atlantic, with Ate4= -0.09. The deepAtlanticwould
have been denser that the Pacific, because of the
extra glacial cooling of the Atlantic coupledto its
highersalinity. If howeverPacific•Jcvalueswerethe
same as Atlantic 15c
values (Abc= 0), then Pacific
waterwouldlie alongthe same•5cisolineasAtlantic
waters. If the modem salinityand temperaturecon-
consistent
with recentinferencesfrom carbonisotope
andcadmiumdataof strongersouthernoceanbottom
water sources, and weaker NADW
sources at the
LGM [Boyle and Keigwin, 1982; Broecker,1986;
Oppo and Fairbanks, 1987], and perhapsreplacement of NADW
with a North Atlantic
intermediate
water source [Boyle, 1988]. However, we will
return to this questionfollowing discussionof isotopicvariabilitywithin the North Atlanticand show
that if Pacific •5cvalues were to be 0.2%0 lower than
Atlantic values,a reasonablechangein the •5w-Srelationshipcanalsogive an acceptable
solution.
trast between both oceans were maintained, the
TheVertical
Distribution
ofBenthic
i5180
densityof PacificandAtlanticwaterswouldbe more
similarthantoday(Ate4= +0.03; closedpentagonin
Figure 9b), becauseof the steeper slopesof the
isopycnalsunderlow, glacialtemperatures.
This is not the only acceptablescenario,however.
in the North Atlantic
Stablewatermassstratification
betweendeep(2-4
km) and bottom (>4 km) waters in the modem
The measuredPacific 15c
valueswould requiredeep
Atlanticis dueprimarilyto a decrease
of temperature
with depth. As shown in Figure 10a, AT = -0.3
(bottom-deepAtlantic temperatures).This temperature decreasewould increase•Jc,but it is offsetby a
slightdecreasein salinitywith depth(AS = -0.03).
The net effect is a small density increase(Ate4 =
water in the Pacific to be warmer than in the Atlantic
+0.03)withdepthandbenthic
•180essentially
the
(AT = +0.8, opencirclein Figure9b). Note thatthe
increasein deepPacificsalinityrelativeto the mean
oceanwould require lower salinitieselsewherefor
the salt budgetto balance. As in the previouscase,
deep Pacific water would be lessdensethan deep
North Atlantic water. Here, the interoceanicdensity
contrastwouldbe evenlarger(Ate4=-0.13) thanthat
predictedin the first scenario.If, however,Abc= 0
and AS = 0, then AT = 0 and A(•4 = 0. That is, if
therewasno 15c
gradientbetweenbothoceans,andif
salinitywasthe samefor PacificandAtlanticwaters,
same at deep and bottom water sites in today's
Atlantic(Figure10aandTable 3).
In this first scenario, we assumed that the intero-
ceanicsalinitygradientwasthe sameastoday. What
if we assumeinsteadthatLGM salinitywasthe same
in the Atlantic and Pacific Oceans (i.e., AS = 0)?
At theLGM,Atlantic
benthic
15•80
isincreased
by
1.6-1.89'oo
relativeto themodem. The verticalprof'fie
in thenortheast
Atlanticindicates
a slight151•O
decreaseof about0.2%0from deep to bottomwater
sites (Figures 6 and 7, and Table 4). As noted
above,this gradientcan be questioned,due to the
low numberof analysesin the bottomwatersand
possible
problems
withinterlaboratory
calibration
of
themassspectrometers.
Giventhisuncertainty,
we
ZahnandMix: BenthicForaminiferal
8180
l0
LAST GLACIAL MAXIMUM
option 1
MODERN
+1
LAST GLACIAL MAXIMUM
option 2
I'''•AN¾I'C
./
.'!
" .....
ATLAN+),_
'• BOTToMIC
I3
½.
- 0.2
0
+ 0.2
.......
DE•.WA• 1
WA ' _.-'
'"'"
t
-0.2
.]
.t
•...'
......
/ o'.:.ATLANTI•"q
I.. -...
-1
C........
.........ß
.......
.......
1
XTLANTf/::
/Lff'"
/
0
1
ß .•.o....•.WAT
.E•
+0.2•0
0
+ 0.2
A SALINITY
EQummanm FRACnOSAZ•OS
Lmms
FOR
815OCalci•
(ASA815
Ocaici•)
......
D•.LTADENSITYISOL•ES ( o4 )
Fig. 10. SameasFigure9, but for Atlanticdeepandbottomwaters. (a) Modem: North Atlantic
deepwater(solidsquare)andbottomwater(solidtriangle).The bottomwatersarecolder(AT=
-0.3), lesssaline(AS=-0.03) andslightlymoredense(Ac•4=+0.03)thandeepwaters. The 8cvaluesare similarat bothdepths(ASc=0).(b) LastGlacialMaximum,option1: Assumes8w-Sslope
sameastoday,andno differencebetween8cin deepandbottomwaters. If the deep-bottom
water
salinitygradientwas the sameas at present(opentriangle),the verticaldensitygradientwould be
lower (Ac•4= +0.01), dueto the lowerreferencetemperature.To maintainverticalstabilityequal
to today,T and S gradientsmustincrease(opencircle,AT=-0.9, AS=-0.12, Ac•4=+0.03). (c) Last
Glacial Maximum, option2: Assumesbottomwaterbenthic8cis 0.2%0lessthanin deepwater. If
8w-Sslopewasthe sameastoday,neutralstabilityrequiresbottomwatersto be colderandless
salinethanoverlyingdeepwaters(AT=-2.9, AS=-0.56). Stablestratification
wouldrequireeven
largergradients.Alternatively,a steeper8w-Sslope(bolddashedlines)wouldallow for morereasonablehydrographicpropertiesof bottomwater (opencircle, AT=-0.9, AS=-I.2, Ac•4=+0.03),
while maintaininglower 8cin bottomwater. In thiscase,PacificdeepwaterscouldhaveT-S
propertiescloseto Atlanticbottomwaters.
discussthe implicationsof two possiblescenarios:
first one with no 8c gradientbetweenAtlantic deep
today. In contrast, the well-documented benthic
and bottom waters, and secondone with the 0.2%08c
LGM [Curry and Lohmann, 1985; Curry et al.,
1988] is generallythoughtto reflectlessactivemixing (hencegreaterdensitycontrast)
betweendeepand
gradientasmeasured.
In the first scenario(Figure 10b), we assumethat
bcis the sameat the deepandbottomwatersites. If
so, the T-S gradientsbetween deep and bottom
waters in the LGM
North Atlantic could have been
essentiallythe sameastoday(opentrianglein Figure
10b). In this scenariothe densitygradientbetween
deepandbottomwaterswouldhavebeenlowerthan
today (Ac•4= +0.01). This resultsfrom changesin
the slopeof isopycnalsunderthe glacialconditions
of lowertemperatures.
This lower-density
stratification, if correct,would presumablyallow for more
verticalmixingbetweendeepandbottomwatersthan
813Cdecrease
fromdeeptobottom
watersitesatthe
bottom waters in this area.
To havea verticaldensitycontrast
atLGM equalto
today's(stillassuming
no•c offsetbetweendeepand
bottom water), bottom water must have been both
freshet(AS = -0.12) and colder (AT = -0.8) than the
deep water (open circle in Figure 10b). This is
possible,givenreconstructions
of strongerinfluence
of southernoceanwaterin the deepglacialAtlantic
[e.g. Boyle and Keigwin 1982, 1985/1986; Oppo
and_
Fairbanks,1988]. It doesrequire,however,that
thiswater masswas muchcolderthanat present,at
ZahnandMix: BenthicForaminiferal
•5•80
least-0.8øC if our assumedtemperatureof 0øC for
the NorthAtlanticdeepwatertemperature
at LGM is
correct.This may be an acceptable
scenario.
In the second scenario, we take the measured
benthicforaminiferal
•5•80gradient
of -0.2%0
betweendeepand bottomwater sitesat face value.
This would requirea thermohalinestructureof the
deepAtlanticoceanquitedifferentthanat present.If
the •5w-Sslope was 1.5, as it is now, to maintain
neutral buoyancy bottom waters must have been
much colder (AT = -2.9) and fresher (AS = -0.57)
than the overlying deep waters (open triangle in
Figure 10c). Even larger temperatureand salinity
differences
would
be needed to achieve the stable
stratification
neexled
topreserve
abenthic
•5•80
gradient. This scenario,however,would either put bottom waters below the freezing point (which is
impossible)or requiredeepwatersto approachmodem temperatures,
thusrequiringan ice volumeeffect
on•5•80
ofnearly
1.7%o
(which
violates
thesealevel
constraintof Fairbanks[1989]). This appearsto be
an unacceptablesolution. Either the Atlantic deepbottomwater • gradientat LGM or the assumptions
goinginto theT-S-o4-•cplot arewrong.
A solutioncouldbe foundif the slopeof the •5w-S
relationship was greater at LGM than at present.
This would be possibleif the contributionof salt
from seaice freezingto the deepoceanwatermasses
was more important at the LGM. Source water
temperaturesin today'ssouthernoceanare close to
the freezing point and could not have been much
colder at the LGM. A plausible way to maintain
higher bottom water densitiesover the cold glacial
deepwaterswould be to increasethe southernocean
salt contentrelative to its •5w.More freezingwould
inject more salt into the source waters, and thus
increasetheir density, without changingthe end-
member
•Sw.
Thisrelatively
salty,
•80-depleted
bottom water, after mixing with deep waters, would
increasethe slopeof the deep ocean•5w-Srelationship. If this slopedoubled,to a glacialvalue of 3.0
(dashedlines in Figure 10c), water mass stability
couldoccurwith reasonableT-S propertiesin bottom
waters(AT = -0.9, AS = -0.15; opencircle in Figure
10c).
A steeper•5w-Srelationshipwould also solve a
problemwith thepossibleAtlantic-Pacificdeepwater
•5cgradientdiscussedaboveand illustratedin Figure
9. Using the modem •5w-Sslope, we inferred that
Atlantic deep and bottom waterswere much more
densethanPacificdeepwatersat the LGM (if the •5c
offset between both oceans is real). This would
17
require different sourcesof Pacific deep water and
Atlantic bottom water. If the •Sw-Sslope was 3.0
instead of 1.5, this problem goes away. Bottom
watersin the Atlantic couldhavehad T-S properties
similarto the deepPacific. Thesewaterswouldhave
beenslightlyfresherandcolderthanthe diminished
NADW sources,but more dense consistentwith their
positionas bottom water in the Atlantic. A likely
source for both water masses would be the Antarctic.
SUMMARY
AND
CONCLUSIONS
Craig [1965, p. 173] stated,"It is obviousthat a
detailedknowledgeof Pleistocenevariationsin isotopic compositionof the oceanis very importantfor
the understanding of the causes of glaciation".
Broecker[1986, p. 133] repeatedthis statementand
added
that"unfortunately
aknowledge
of•80distribution in the glacial oceanis currentlybeyondour
grasp". We pick up the task here, and attemptto
constrain
thedistribution
of •5•80withinthedeep
sea. A completepictureremainsbeyondour grasp,
but with new datawe have madesomeprogress.
We do not arrive at a uniquesolutionto the distri-
bution
of•5•80
intheglacial
ocean.
Instead,
wehave
usedhypotheticaldistributionsof •Swater
(•Sw)in the
ocean'stemperatureand salinityfields as a thinking
tool to examinethe logicalconsequences
of possible
circulationschemesin termsof densitydistributions.
Finding true relationshipsbetween•5wand S in the
pastwill be very difficult. It dependson ratesand
patternsof evaporation,precipitation,runoff, the
ratio of seaice freezingto deepwaterformationrates,
and the mixing patternsof water masseswithin the
deep sea. For simplicity, we have not considered
intermediate
water masses.
Some of these water
massesat presenthave slightlydifferent•5w-Srelationshipsthandeepwaters(Figure2), sovariations
in theirisotopebudgetscouldaddcomplexity.
We findthatglacialmaximum
•5180
values
of
benthicforaminifers(•5c)fromdeepAtlanticsites(2-4
km depth) are on averagehigher than thosefrom
Atlanticbottomwatersites(>4 km depth)or from the
deepPacific. This finding, thoughconsistentwith
other studies,may in part reflectproblemsof interlaboratory calibration. It can, however, be interpretedin termsof reasonable
watermasscharacteristics.
Consideringthe constraintaddedby densitystratification, it appearsthat changesin the slopeof the
•5w-Srelationshipare critical to making senseof ice
age •5cpatterns. If we assumethat the measured•5c
ZahnandMix: BenthicForaminiferal
•5180
18
gradientsin the glacialoceanare real andif themodem relationshipof 15wand salinityin the deep sea
appliesto thelastglacialmaximum,Atlanticdeepand
bottom waters must have been more dense than those
in the Pacific. This densitydistributionwould seem
to requirea majorsourceof highdensityglacialdeep
water in the Atlantic and a separatesourceof deep
water somewhere in the Pacific.
This is inconsistent
with circulationpatternsinferredfromcarbonisotope
distributions.If we assumethat ice age 15c
was the
same at Atlantic and Pacific core sitesor if the •5w-S
slopewas higher than it is now, this conflict goes
away. In both cases, Pacific deep waters and
Atlantic
bottom waters could have had a common
source in the southern ocean.
A weak northern
sourceof glacialAtlanticdeepwaters(or mixingwith
intermediatewaters) is permitted. We tentatively
conclude that sea ice formation contributed more salt
to the glacial deep oceanthan today. If true, this
would change the salinity distribution within the
ocean[BroeckerandPeng, 1987], whichhasimplicationsfor paleo-CO2and other geochemicalbudgets. At present, however, the interpretationsare
limitedbothby precisionandaccuracyin theisotopic
measurements.More analysesare neededat bottom
water sites. Different
laboratories must intercalibrate
theirmassspectrometers
moreaccurately
by sharing
standardsand techniques.In principle,theseproblemscanbe solved,anda betterview of ice agecirculation can be achieved.
Acknowledgments.We thankMichaela Knoll for
her advice on physical oceanographicprocedures.
R.Z. appreciatesdiscussions
with LaurentLabeyrie
during the Third International Conference on
Paleoceanographyin Cambridge,UK, September
1989, where parts of this study were presented.
Reviews by Christina Ravelo and an anonymous
reviewer were very helpful. Supportfor this study
came from NSERC-Canada
and the National Science
Foundation(OCE87-16856 and ATM88-12640).
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anhand stabiler
(ReceivedOctober23, 1989;
revisedAugust16, 1990;
acceptedAugust 17, 1990.)