PALEOCEANOGRAPHY,VOL. 10, NO. 3, PAGES 527-544, JUNE 1995
Surface water temperature, salinity, and density changes
in the northeast Atlantic during the last 45,000 years:
Heinrich events, deep water formation, and climatic
rebounds
M. A. Maslin1 and N.J. Shackleton
GodwinLaboratory,Universityof Cambridge,Cambridge,England
U. Pfiaumann
Geologisch
Paliiontologisches
Institut, UniversitiitKiel, Kiel, Germany
Abstract. We developeda new methodto calculateseasurfacesalinities(SSS) anddensities
(SSD)fromplanktonic
foraminiferal
õ18
0 andseasurface
temperatures
(SST)asdetermined
from
planktonicforaminiferalspeciesabundances.
SST, SSS,andSSD recordswerecalculatedfor the
last45,000yearsfor Biogeochemical
OceanicFlux Study(BOFS)cores5K and8K recovered
from
thenortheast
Atlantic. The strongest
featureis thedramaticdropin all threeparameters
duringthe
Heinrich"ce-rafting"events.We modelledthepossibilityof deepwaterformationin the northeast
Atlantic from the SSD records,by assuming
thatthe surfacewatersat our sitescooledasthey
flowedfurthernorth. Comparison
with modelledNorthAtlanticdeepwater
densitiesindicatesthat
there
could
havebeen
periods
ofdeepwater
formation
between
45,000
and30,000
14C years
B.P.
(interrupted
byiceberg
meltwater
input
ofHeinrich
event
3 and4,at27,000
and38,00014Cyears
B.P.) andduringtheHolocene. No amountof coolingin the northeastAtlanticbetween30,000
and13,000yearscouldcausedeepwaterto form,because
of thelow salinitiesresultingfromthe
highmeltwaterinputsfromicebergs.Ourrecordsindicatethatai•ereachHeinricheventtherewere
periodsof climaticrebound,withmilderconditions
persisting
for up to 2000years,asindicatedby
thepresence
of warmerandmoresalinewatermasses.At•er thesewarmperiodsconditions
returned
to averageglaciallevels. Theseshorttermcoldandwarmepisodes
in thenortheast
Atlanticare
superimposed
on thegeneraltrendtowardscolderconditions
of theLastGlacialMaximum(LGM).
Heinrichevent 1 appearsto be uniqueasit occursas insolationroseandwascoevalwith the initial
meltingof the Fennoscandian
ice sheet. We proposethatmeltwaterinputof Heinrichevent 1
significantly
reducedNorthAtlanticDeepWaterformation,reducingtheheatexchange
betweenthe
low andhighlatitudes,
thusdelayingdeglaciation
by about1500radiocarbon
years(2000calendar
years).
Introduction
During the last glacial, surfacewater circulationin the
At present,the surfacewatersin the North East Atlantic are
NorthAtlanticwasdominated
by ananticlockwise
polargyre
dominatedby the Gulf Stream. Its warm, salinewatersflow
[Ruddiman
and Mcintyre,1976;Grousset
et al. 1993;
from the Gulf of Mexico acrossthe North Atlantic and into the
Robinsonet al, 1995]. The dominantsurfacewater currentin
AtlanticduringtheLastGlacialMaximum(LGM)
NorwegianSea, while the cold, low-salinitypolar surface thenortheast
waters are restricted to the far northwest Atlantic.
The
northward
flow of warmsalinewatersfromthetropicaland
subtropicalAtlantic to higher latitudesis instrumentalin the
formation
of NorthAtlanticDeepWater(NADW). Cooling
of
wasthe lateralwestto eastpolarcurrentat 40'-45'N,
originating
in the Labrador
andGreenland
Seas,andthe
northward
progression
of thiscurrentbetween15' and20'W
[Ruddirnan
andMcintyre,
1976;Ruddirnan,
1977;
Robinson
et
was,however,
significant
iceberg
debris
the surfacewatersduringnorthwardtransportation
increases al., 1995].Them
south
of Iceland
(63'N),during
theLGM[Fillon
et
their densityuntil a criticalvalue;then they sink in the deposition
Greenland Sea and form the NADW.
a/,.1981;
Fillon,1985]suggesting
thatsouthern
subtropical
surfacewaterfrequently
intruded
intothenortheast
Atlantic
and
promoted
iceberg
melting.
•Nowat Geologisch
Pal•ontologisches
Institut,Universit•t
polargymtransported
icebergs
fromthe
Kiel,Kiel,Germany,
andDepartment
ofGeography,
University Thisanticlockwise
CollegeLondon,London,England.
Copyright1995by theAmericanGeophysical
Union.
Papernumber94PA03040.
0883-8305/95/94PA03040510.00
Laurentide
icesheet
intothenortheast
Atlantic
throughout
the
lastglacial,
asshown
by records
of theshortperiods
of
intense
icebergs
production
(socalled
Heinrich
events)
in the
northeast
Atlantic[Ruddiman,1977;Heinrich, 1988;
Broecker
et al., 1992;Bondet al., 1992;Grousset
et al.,
1993].Wedocument
seasurface
temperature
(SST),
salinity
528
MASLINETAL.'HEINRICH
EVENTS
ANDDEEPWATER
FORMATION
Table 1. Site Data for the NortheastAtlanticCoresSelectedfrom RRS DiscoveryCruise184,
Biogeochemical
OceanFluxStudy(BOFS)1989Leg3 for thisStudy
Position
Station
Core
Number
Latitude,
'N
Longitude,
'W
Water
Lengthof Core
Depth,m
Recovered,
m
11882
5K
50'41.3'
21'51.9'
3547
2.63
11886
8K
52'30.1'
22'04.2'
4045
2.46
11891
11K
55'11.5
20'21.1'
2004
0.96
11896
14K
58'37.2'
19'26.2'
1756
2.28
11905
17K
58'00.2'
16'30.3'
1150
2.44
(SSS), and density(SSD) changesat the northernBOFS sites
(50'-60'N, 20'W; Table 1 and Figure 1) in order to determine
the varyinginfluenceof warm and surfacecold currentsand the
effectsof the Heinrich eventson deepwatercirculation.
_
.
using the CLIMAP group taxonomy [Kipp, 1976]. Any
specimens
whichwerenot part of the 16 selectedspecieswere
classifiedas "other". Figure2 showsthe relativeabundance
of
planktonic
foraminifera
species
versus
14Cage[Maslin,
1993; Manighetti et al., this issue]for BOFS 5K. BOFS 5K
Sea Surface Temperature
SST of the North Atlantic was first comprehensively
reconstructed
by the CLIMAP group [Mcintyre et al., 1976;
Climate: Long-RangeInvestigationand Prediction(CLIMAP)
Project Members 1976 & 1981] with the aim of reconstructing
the surfacewater temperatures
of the worldsoceansat the LGM
(18,000 years B.P.), and to compare these to present day
conditions.The SST was estimatedusingthe lmbrie and Kipp
(1971) transfer function method on different microfossil
groups. In the North Atlantic, only the foraminiferal
assemblageswere used for palcotemperaturereconstruction,
because of the rarity of other microfossils and/or the
domination of their assemblageby one species[CLIMAP
Project Members, 1976]. CLIMAP resultsindicateda strong
north-southtemperaturegradientin an east-westbandbetween
40' and 45'N in summerand winter at 18,000 yearsB.P. In
contrast,at presentin the North Atlantic, the cold polar waters
(<4'C) are restrictedto a narrow bandin the northwestAtlantic
wasselected
forAMS14Cdating
asit hadthebest
oxygen
isotope stratigraphyindicating high sedimentationrates and
no hiatusis for the last 45,000 years [Maslin, 1993]. The
relative abundance data were used to calculate SST.
Quantitative methodshave been widely used to analyze
mieropaleontologieal data to derive estimates of
environmentalparameters. The most widely used are those
estimating the SSTs from planktonic foraminifera and other
microfossilassemblage.A major assumptionis that species
abundance distributions are related systematically to one
parameterof the environmentin which they live [lmbrie and
Kipp, 1971; Birks et al., 1990]. In most studies,faunal data
are calibrated to only one environmental parameterin one
equation. In doingso, one assumesthat the speciesabundance
of the planktonic foraminifera is solely, or dominantly,
controlledby that one parameter. These two assumptions,
a
systematic relationship and dominant control by a single
parameter, are conceptually different. In reality, every
assemblageis jointly influenced linearly and/or nonlinearly
because the warm Gulf Stream influences much of the North
by many factorssuch as nutrient availability, light intensity,
Atlantic and the southern parts of the Norwegian Sea.
interspecific competition, temperature, salinity, etc. [e.g.,
CLIMAP resultslacked fine-scaleregional detail, and we aim
Bd, 1977]. Different combinations of these controlling
to estimate SSTs at high spatial and temporal resolution,to
factors may lead to the same faunal composition. Another
build up a derailedregionalpictureof the water masschanges
complicationof quantitativemethodsis that the fossildata do
in the northeastAtlantic over the last 45,000 years.
not necessarilyreflect the true living floating foraminiferal
assemblage. Changesfrom the "true" assemblagemay be
Statistical Estimation of Temperature
caused by: differential transportation to the sediment,
BOFS cores 5K, 8K, 11K, 14K, and 17K were selectedas
differential dissolution of species, bioturbation, sample
splitting errors, and errors in speciesidentification. lrnbrie
having the best preservation,and were sampledevery 2 cm.
The samples were disaggregatedby soaking and gentle and Kipp (1971)regarded the chronologicalheterogeneityof
of years,as
shakingin distilledwater overnight,and then washedthrough core top samples,which couldrepresentthousands
the largestsourceof error in their transfermethod. The final
a 63-tamsieve.After washing,the coarsefractionwas dried in
an ovenat 60'C and weighed. The sizefraction>150 tamwas problem associatedwith the methods of estimating SSTs
quantitativelyis that the data are usually in a percentageand
split using Soiltest CL-242A, as many times as required to
obtain a subsampleof approximately300 whole planktonic not an absolute form. This means that common species
covary inversely, even without any inverse relationshipin
foraminifera.
The final .split was placed on a
micropaleontologicalpicking tray. All whole, or nearly their absoluteabundances,the "fixed sum" problem. These
whole planktonicforaminifera,were identifiedand counted problemswere monitoredusing the fragmentationratio and
MASLIN ET AL.: HEINRICH EVENTS AND DEEPWATER FORMATION
65°(
2
:;}
529
117 ;.;;;
60·
55·
50·
20·
25·
30·
15°
5·
10·
Figure 1. General location of the BOFS cores taken on RRS Discovery cruise Leg 3 (1989). The
numbered locations refer to the Kastern core taken at that site. Water depth given in meters.
o
g
o
Olher species
G.calida
G. lruncatulinoides
rJ G.ruber
8 poD inJergrade
<I>
<oJ
=
'C
~
•
O. wUversa
G. hirsuta (L)
G. hirsu/a (R)
G. scilula
• G·fa/conensis
• G. quinque/oba
N. pachyderma (s)
~ G. inflata
II G. glutinata
II G. bulloides
• N. pachyderma (d)
o
ca
=
:s
~
~
rn
<I>
.~
B
<I>
o
cz::
--.---.---.---,
o
20
40
60
80
100
120
140
160
180
200
220
240
260
Depth (em)
Figure 2. Relative abundance plot of the planktonic foraminifera species found in core BOFS 5K.
530
MASLIN ET AL.' HEINRICH EVENTS AND DEEPWATER FORMATION
the planktonic to benthie foraminifera ratio and were calibratedfor the entire Atlantic Ocean [Molfino et al., 1982].
The MAT calculatesthe similarity coefficientsbetweena
considered
wheninterpreting
the SSTsestimates.In thisstudy
two approaches
were used to estimateSST, the Transfer downcoresampleand eachof the core top samples[Hutson,
Function (TF) method and the Modern Analog Technique 1979; Overpecket al., 1985; Prell, 1985;Le, 1991]. From
(MAT).
Themostextensively
applied
method
of estimating
SSTs
has been the TF method as initiated by Itnbrie and Kipp
[1971]. An empiricalequationis developedfrom the coretop
data set, which can then be applied downcore. The most
successful
applicationof this methodhasbeenin the Atlantic
for whichImbrie and Kipp [1971] first developedthe TF [Mix
et al., 1986; Ruddimanet al., 1987; Mcintyre et al., 1989].
Ruddirnanand Esrnay[1987) attempted
to calibratea TF with
only four major temperature-sensitive
speciesin the North
Atlantic,to permitfastercounting.Extensiv.,..
reviewsof the
development
of TF equationscan be foundin the worksby
Imbrieand Kipp [1971],Molfinoet al. [1982],andLe [1991].
For the Biogeochemieal
OceanicFlux Study(BOFS)coreswe
used the CLIMAP FA20 TF equationocean-widedata set,
among all the core top samples,it selectsthen a subsetof
sampleswhich are most similar to that downcoresampleas
modem analogues. The SST estimateof the core sampleis
obtainedby averagingthe SSTs of its modemanaloguesby
either a weightedor unweightedapproach. In the weighted
solution, a sample with a larger similarity coefficient
contributesmore to the SST estimate. The advantagesand
disadvantages
of MAT andTF methodare discused
at lengthby
Prell [1985] and Le [1991]. To make "correct"SST estimates,
a wide distribution of core-top samples is needed to
sufficiently representall possible conditions;otherwisesome
core samplesmay have no close modem analogues. For the
BOFS cores,two different MATs were used:(1) MAT basedon
the originalCLIMAP coretop globaldataset, calculatedby J.
Le and (2) MAT usingthe EPOCH coretop regionaldataset,
'
20
SUMMERSEASURFACE
TEMPERATURES
18
16
•
14
o
o
10
'.'
CLIMAP
.:..
Transfer Function FA20
..............
CLIMAP MAT (Global data se0
.......
EPOCH (SIMMAX)
MAT (N. Atlantic data se0
XVINTER
SEA SURFACE
TEMPERATURES
13
11
:.3'1
0
5000
10000
;
15000
20000
25000
30000
35000
40000
45000
AGE / C14 years BP
Figure3. BOFS5K: Comparison
of thedifferent
estimates
of seasurface
temperatures
usingthesame
species
counts.Theestimates
include
CLIMAPtransfer
function
equation
FA20,CLIMAPmodern
analog
technique
(MAT) usinga globaldatabase,
andEPOCH(SIMMAX)MAT usinga NorthAtlanticdatabase
[Pflaumann
et al., 1992alsosubmitted
manuscript,
1995].
MASLIN ET AL.: HEINRICH EVENTS AND DEEPWATER FORMATION
calculatedusing the SIMMAX approach[Pflaumannet al.,
1992, also SIMMAX, a transferfunctiontechniqueto deduce
Atlantic sea surface temperatures from planktonic
foraminifera: the EPOCH approach, submitted to
Paleoceanography,1995 (hereinafterreferredto as submitted
manuscript,1995)].
The three SST estimates for BOFS 5K show a maximum
differenceof more than 2'C (Figure 3). This is within the
errors associatedwith the different estimates, which is about
:kl.2'C at the 80% confidence limit.
Of the three estimates the
CLIMAP MAT has the most variability, especially during
531
SST Changes in the BOFS Sites during the Last
45,000
Years
Figure 4 showsthe summerand winter SST estimatesfor
several
BOFS
cores
versus
14C age.Theagemodels
used
in
thisstudyweredeveloped
by Maslin [1993] andare differentto
those of Manighetti et al. [this issue]. The reason for this
difference
is thegreatemphasis
ontheplanktonic
•jl80 and
•j13Ccompared
toothersedimentary
records
tomake
them
more compatiblewith other stable isotopesstratigraphies
[e.g. Jansenand Veum,1990;Duples•yet al., 1991, 1992].
Whole Core MagneticSusceptibility(WCMS) documents
the
warmerperiods. This couldbe causedby the useof a small
global databasecomparedwith the much larger regional
EPOCH data base; or by the use of the P-D intergrade(an
relativeamountof ice-rafteddebrisin the BOFS cores[Maslin,
1993; Robinson et al., 1995] and was used to define the
intermediateform betweenN. dutertreiandN. pachyderma(d)
definedby Kipp [1976] to reduce inconsistencies
between
differentworkers). The EPOCH MAT estimatedoesnot usethe
All the BOFS coresshowa pronounced
increasein SST at
the last termination;in the coreswith the high sedimentation
P-D intergradedefinition,and thosespecimens
are combined
with N. pachyderma(d) total [Pflaumannet al., 1992]. The
glacial-Holocene
SSTrangeof theEPOCH(0' to 12'C) andthe
CLIMAP FA20 (1.5' to 11'C) estimatesalso differs, due to the
increasing inaccuracies of all SST estimates as the
foraminiferal
assemblagse become increasingly
monospecific.The largestSST differences
occurbetweenthe
CLIMAP FA20 and the CLIMAP MAT in the summerglacial
but not in the winter estimates.
location of Heinrich zones 1 to 4.
rates we see the two step deglaeiation. SSTs at 5K and 8K
droppedsignificantlyduringHeinrichevent3 and 4. Between
H4 andH3 (35-36kyr B.P.)thereis a peakin theSSTin all the
cores,but mostintensein 5K, wherethe SST was6'C higher
thanduringH4 and 4'C higherthanthe averagetemperature
between H3 and H4.
Sea Surface Salinity
strikinglysimilar, the minor variationsbeing causedby the
Initial attemptsto estimatesalinity of surfacewaterswere
madeusingthe transferfunctionapproach[Imbrieand Kipp,
1971; Kipp 1976;Cullen, 1981]. This approachwasfoundto
difference
be successfulonly in restrictedregions,suchas the Arabian
The results
of the three
different
SST
estimates
are
in modern data sets as well as in methods of
estimation. In this study,we usedCLIMAP equationFA20
Sea,with a highgradientpatternof salinitywhichwashighly
estimates because its estimates had the smallest range,
correlatedto one of the foraminiferalfactors[Cullen, 1981].
There are two major reasonsfor the generalfailure of the
miniraisingits effect on the salinityestimate. In addition,the
resultsof this studyare compareddirectlywith CLIMAP and
work based on it.
transferfunctionmethodof estimatingsea surfacesalinity
(SSS): 1. both in laboratorycultureand in the open ocean
BOFS5K WCMS
øo
,,oo
5OOO
10000
.
55OOO
Figure 4. Summerandwinterseasurfacetemperatures
for the BOFScoresestimatedusingthe CLIMAP TF
FA20. Age modelfromMaslin [1993]. Dottedlinesrepresent
theHeinricheventsas definedby the whole
core magneticsusceptibility(WCMS) of BOFS 5K [Maslin, 1993].
532
MASLIN ET AL.: HEINRICH EVENTS AND DEEPWATER FORMATION
salinityhas little relative effect on the growthor distribution To calculatethe •Jwof surfacewatersthe equationcan be
of planktonicforaminifera. It has even been demonstrated rewritten:
thatN. pachyderma(s) can surviveandflourishwithin seaice
(4)
and in the surroundingextremelylow salinity watersof the
5w = {(1000+ 5c)/ øtc-w} - 1000
Antarctic [Spindlerand Dieckmann,1986;Dieckmannet al.,
1991]. 2. Salinity is usuallyrelatedto SST (Duplessyet al.
where
•cisthe• 180 oftheplanktonic
foraminifera
in$MOW
[1991] usingthe datafrom Gex•hemiealOceanSectionsStudy (seeequation5) and 0re_
w is the isotopicfraetionation
factor
(GEOSECS)[1987]). It is thusvery difficultto reconstruct
SSS of oxygenbetweencalciteand watercalculatedin equation(2).
by the transferfunctionmethodin the midlatitutes,because Theconversion
of theplanktonic
foraminifera
5180records
SST
is the dominant
control
on the distribution
of the
from the PDB (belemnite from the Peedee Formation, South
Carolina)isotopicscaleto that of V-SMOW (Vienna-Standard
Mean Ocean Water) was done usingthe Coplenet al. [1983]
extract
theinformation
from5180ofplanktonic
foraminifera,equation:
as farsttriedby Fillon and Williams [1984], who estimatedthe
meltwater discharge of the Laurentide icesheet into the
fi18OV_SMOW
= 1.03091
fi18OpD
B + 30.91
(5)
foraminifera,and salinity is not independent.
Another approachto estimatingsea surfacesalinity is to
Labrador
SeaandtheGulfof Mexico.Duplessy
etal. [1991,
1992]used
a different
approach,
alsobased
onthe51øOof
planktonicforaminifera,to estimatesurfacewatersalinityfor
the LGM for the North Atlantic. They relied heavily on
calibrationusing the GEOSECS [1987] and core top data sets.
GEOSECS data are limited becausethere was only one northsouth transectin the North Ariantie (March - October). Core
We used the North Atlantic temperaturecalibrationsof
Duplessyet al. [1991, 1992] to estimatethe ambientsurface
watertemperature
(T). For eachspecies,
Duplessy
et al. [1991,
1992] calibratedthe summerSST from the CLIMAP core top
data
setwiththeisotopic
temperature
calculated
from
the5180
recordsfor the whole of the North Atlantic. They were thus
top datahaveinherentinaccuracies
dueto the assumption
that
the core top representsthe presentday. The BOFS coresin
this studyshow that the age of the core topscan vary from
between0 and 6(X• yearsB.P. (Manighettiet al., this issue),
able to correct the summer SST and obtained the effective
due to bioturbation and the relative successof recovering
G. bulloides
temperature
of ealeifieation(T*) for eachspecies(equations6
and 7).
T* = summer$ST - IøC
7ø-22øC
(6)
3'-10'C
(7)
intaetedcore tops. In this study we thereforepresentan
alternative method which relies less on such calibrations.
N. pachyderma
(s) T* = summerSST - 2.5'C
Estimating
Sea Surface Salinity
Downcore
The effective temperatureof calcification(T*), basedon the
CLIMAP FA20 temperatureestimates,for each specieswas
controlled
bythe5180ofthesurroundi,nag
water,
andthe then used as the ambient surface water temperature(T) to
temperature
atwhich
it is formed.
The51øOof thesurface calculate•w (equation2). To estimatea completedowncore
water (•w) is controlledby the salinityof the surfacewaters recordof summersalinity,we combinedthe •Jwrecordsof G.
bulloides and N. pachyderma (s) based on the optimum
which is influenced by (1) global ice volume, (2)
temperature
rangessuggested
by Duplessyet al. [1991]. At
precipitation/evaporation
regime of the area, and (3) river or
The5180of thecalcite
testof planktonic
foraminifera
is
summer•w recordof G.
meltwater
input.Wetriedtoremove
thetemperature
effect
and SSTsgreaterthan10øCthecalculated
theglobal
icevolume
effectfromthe51øOof planktonicbulloideswasused,at SSTslessthan7'C thesummer•Jwrecord
foraminiferato estimatelocalized salinity changes.
The isotopicfraetionation
factorof oxygenbetweenCaCO3
- H20 wascalculated
usingequation(1) proposed
by OWed et
al. [1969] and Shackleton [1974].
1000
In•c_w=2.78(106T
•2)- 3.39
(1)
of N. pachyderma
(s), andbetween7' and10'C theaverageof
theG. hulloides
andN. pachyderrna
(s) •Jwrecord.
The next stagein the calculationof SSS is the removalof
the globalsalinityeffect causedby changesin the globalice
volume. The bestestimateof the changesin the volumeof the
continental icesheets is derived from the sea level curve based
on dated coral reefs drilled off-shore of Barbados[Fairbanks,
or
C•-w= exp[0.00278(106T
'2)- 0.00339]
mass
(2) 1989;Bardet al., 1990a, b], with14C accelerator
where •-w is the isotopicfractionationfactor of oxygen
betweenCaCO3 andH20, T is the temperature
of the ambiant
surfacewatersin Kelvin (T* + 273.16), and T* is the effective
temperatureof ealeifieation defined for each planktonic
foraminiferalspecies[Duplessyet al., 1991, 1992].
spectrmeter
(AMS) datesgo back to 18,200-t400years B.P.
with a resolutionof 100 to 1300 y_ears.Recentestimatesfor
theconversion
of sealevelto 5180 changes
in thesurface
watershaverangedfrom 0.011%dmto 0.013%rim[Labeyrieet
al., 1987; Shackleton, 1987; Fairbanks, 1989]. Duplessy et
al. [1991] adopteda meanvalue 0.012%rim,whichwe also
used.
The isotopicfractionationfactorbetweencalcite(c) and water
(w) is definedas
ac-w= (1+ 10-3•Se)/(1
+ 10-3•Sw)
where
•Se
is•5180
ofcalcite,
and•Sw
is•5180
ofwater.
(3)
The sea level record only goes back to 18,200 years,
whereasthe lowestsamplesfrom the BOFS coreshavean age
of58,000
years.
Toextend
theglobal
icevolume
5180record
back in time, we selectedtwo oceaniccoresfrom the Pacific,
TR163-31B (3'37.2'S, 83'58'W, in a water depthof 3210 m)
and V19-30 (3ø21'S, 83ø21'W, in a water depth 3091 m).
MASLIN ET AL.' HEINRICH EVENTS AND DEEPWATER FORMATION
These coresare in very close proximity and in the tropics,so
we expect that the local changesin SST and salinity due to
changes in the precipitation/evaporation regimes were
minimal. Core TR163-31B has a reliable age model basedon
533
oscillatingsystem'[MacAyealand Wang,1992;MacAyeal,
1993 a&b], and periodicmeltingof the soft, unconsolidated
basalmaterialoccurredas a resultof the temperature
increase
fromgeothermal
heat,eausingthe ice sheetto 'fail' andto start
2314CAMSdates
onplanktonic
foraminifera
[Shackleton
et surging on a massive scale [Andrews and Tedesco, 1992;
al.,1988].Torepresent
the•global
15
w changes
in theoceans MacAyealandWang,1992;MacAyeal,1993a, b]. Icedumped
in thepast,weusedtheõ1øOrecord
of thebenthie
genus into the Labrador Sea via the Hudson Strait would have
Uvigerina.
Thisrecord
wasadded
totheBarbados
õ18
0 curve
extendingit to 31,360 years B.P. Core TR163-31B doesnot
extend back as far as Heinrich
event 4.
To extend the record
originatedfrom both the centreand the marginsof the ice
sheet,thusits averageisotopiccomposition
wouldhavebeen
muchlighterthan-20%0. Estimates
of the averageisotopic
further
back,theUvigerinaõ180 (minus
thepresent
day compositionof the Laurentideicesheethave rangedfrom
õ180)record
fromcoreV19-30
wasused.Theagemodel
for -30%0to -40%0[e.g.,Shackleton,1967b(-309'00);
Wely, 1968
V19-30 is orbitally calibrated to calendaryears [Shackleton
(-40•), Dansgaardand Tauber,1969 (_<-30%0);Broecker,
andPisias,
1985].Asthe14C-calendar
years
calibration
does 1978 (-359'00)].Mix and Ruddiman(1984) statedthat the
not extend further than 25 kyrs [Bard et al., 19901,b], it was
assumedthat calendaryears from 32 to 58 kyr B.P. were the
same
as14Cyears
B.P.[Stuiver
andReimer,
1993].
presentconsensuson the mean isotopiccompositionof the
Laurentideice sheetduringtheLGM was-35%0.
A second model was constructed to estimate the surface
To correct the 15wrecord for ice volume variations, the
ehronologiesof all recordswere made compatible:a Gaussian
interpolation using weighted duplicates was applied to the
recordswith a samplespacingof 200 yearsand a window size
of 1198 years. The õw was correctedby subtractingthe ice
volume isotopic effect from the raw õw- To convertthe
correctedõw to salinity(%o),we assumedthat the freshwater
input (zero salinity) had an oxygen isotopiccompositionof
-20•, the isotopiccompositionof snowenteringand leaving
the present Arctic ice sheet, assuming mass balance
equilibrium[Craig and Gordon,1965;Shackleton,19671;Mix
water salinity for the Heinrich eventsusing the -35%0endmember.
S = Sm+ Sv+ 1.120•w*
The errorof the salinityestimatewascalculated,
combining
the assumederrors of the SST estimates(one standard
deviation
is _+I'C (Pflaumann
et al., submitted
manuscript,
1995) and the oxygenisotopemeasurements
(one standard
deviation is _+0.1%o
(M. Hall, personalcommunication,
1993). The standard
deviationin the salinityestimateis at
least
andRuddiman,
1984].The15180
ofmean
annual
precipitation
at Barfin Island and the Labrador Sea (believed to be indicative
of conditions in the North Ariantie during the last glacial
[InternationalAtomic Energy Agency (IAEA), 1981]) varies
between -15 and -20900 in isotopic composition. The
calibration of Duplessy et al. [1991] for the modern
relationshipbetween [w to salinity in the North Ariantie
from the GEOSECS [1987] datagivesa similarend-memberof
(10)
+0.659'00.
Salinity Estimate Sensitivity to SST
To investigatethe sensitivityof the salinityestimatesto
the SST estimates
two differentapproaches
wereused:
1. We first used the Duplessy et al. [1991, 1992]
calibration method in conjunctionwith our own method
(described
above)to calculatesummersurfacewater•w. We
-19.2649oo.
The equation,assuming
an end-member
of -20%0, calculated the 15
w with two different SST estimates:the
is
CLIMAP transfer function equationFA20 and SIMMAX
S = Sm + Sv+ 1.735
r•W*
(8)
EPOCH
MAT(Figure
5). During
theLGMthe15180
offset
between the two estimatesis up to 0.5%0, a ditTereneeof
whereS is Salinity%0,Sm is Presentmeanoceansalinity,
between0.6-0.9ø2'00
in surfacewatersalinity,depending
on
whetherequation(9) or (10) is used. This hasimplications
34.74%0
[GEOSECS,
1987],
bw*=bwanomaly
(bw[sample]
- bw[modem]
) / %oSMOWandSv isthemean
ocean
salinity concerningcompatibilityof salinity estimatesmade with
changesdueto changesin thevolumeof the ice sheets(%0)see
equation(9) below.
Sv = 34.74 x SL/(3900- SL)
(9)
different SST estimates. We used the CLIMAP FA20 estimate
in this paper so that the resultswould be compatiblewith
previoussalinityestimatesof Duplessyet al. [1991, 1992].
2. We alsoattempted
to calculate
summer
andwinterSSSby
using the oxygen isotopic records of G. bulloides and N.
pachyderma(s) separately(Figure 6). It hasbeenshownthat
0.012%dm,34.74 is the presentmeanoceansalinityin %0 peak fluxes of G. bulloidesin the North Atlantic are associated
(GEOSECS,1987),and3900 is thepresentmeanoceandepth with phytoplanktonbloomsin the summermonths[Ganssen,
in meters.
1983]. During glacial period thesephytoplanktonblooms
were restrictedto a muchshorterseasonaloccurrence
[Thomas
whereSL is the changesin sealevel assuminga conversion
of
The source area within the Laurentide ice sheet which fed the
increasediceberg flux during the Heinrich events is not
known. If icebergscamefrom massivecalvingat the marine
marginsdue to externalclimate forcing [Heinrich, 1988;Bond
et al., 1992] the isotopic compositionof the ice would be
close to-20%0 [Mix and Ruddiman, 1984]. An alternative
theory is that the Laurentideice sheetwas part of a 'free
et al., thisissue];thusduringthelastglacialG. bulloidesmay
have only occurred during "high" summer [Ottens, 1991,
1992; Tolderlund and BE, 1971; Ganssen and Sarnthein,
1983]. On the basis of these facts the alternative "summer"
salinity calculationof the BOFS coresused the G. bulloides
isotopic record and the summer SST as an estimate of the
ambientsurfacewater temperature(T). N. pachyderma(s)
534
MASLIN
A/
ET AL.: HEINRICH
EVENTS AND DEEPWATER
FORMATION
G. bulloides
ß, !
ß •
.
..................
'
B/
N. pachyderma
..'
,..
FA20
EPOCH
.'.r
.•VV
%•
sunnncr
...'
SST
summer
- 1 øC
SST
- 1 øC
(L)
..................
FA20
EPOCH
summer
SST
summer
- 2.5øC
SST
- 2.5øC
Age/14C
yrs BP
Figure 5. BOFS 5K: Comparison
of the calculatedsurfacewaterisotopiccompositionusingtwo different
$ST estimates;CLIMAP FA20 and EPOCH (SIMMAX), for both the G. bulloidesand N. pachyderma(s)
records. The SSTs were correctedaccordingto the calibrationof isotopictemperaturesof Duplessyet al.
[1991, 1992] see text.
present temperaturerange in the northeastAtlantic suggests
that its annualoccurrencemay havebeen expandedduringthe
last glacial to include some of the spring [Ottens,
1991&1992; Tolderlund and Bd, 1971;Reynoldsand Thunell,
1986]. For the alternativeBOFS "winter" salinity calculation
the N. pachyderma(s) oxygen isotoperecord and winter SST
were used to estimatethe ambiant surfacewater temperature
(T). The recordsof G. bulloidesandN. pachyderma(s) could
thus provide a meansof estimatingthe salinity during the late
winter to early spring,and the summer.
Figure 6 shows these two alternativesalinity calculations
for summer and winter, comparedwith the variations in the
periodsvariedon an annualbasisßDowdeswelland Murray
[1990] have suggestedthat the maximum speedof floating
icebergis 0.5m/s, thereforeit would take a minimumof 100
days for an iceberg to cross from Newfoundlandinto the
northeast Atlantic. Fillon [1985] has shown that the overall
supplyof icebergsto the North EastAtlanticcamefrom both
the east and west of the Labrador Sea and east of Greenland.
As
icebergsfollow the prevailing surface water currents,they
would have had to travel up the eastand then downthe west of
the Labrador Sea, and this would take at least another 100
days, while those icebergsproducedon the east coast of
Greenlandwouldtake evenlonger. Thusevenif theproduction
globalmeanoceansalinity(34.749'oo
+ Sv ). Duringthe last of icebergswere restrictedto a particularseason,the different
glacial the salinityof the northeastAtlantic surfacewaterswas time lags involved in the transportationof icebergsto the
much lower than the global mean,presumablydue to the large northeast Atlantic would mean that the seasonal"production"
flux from melting icebergs. The Heinrich events are signalwould have been obliterated(J. Dowdeswell,personal
1992). The major controlon the meltingof
significant,especiallyin the N. pachyderma(s) record,even communications,
the icebergsis surfacewater temperature[Dowdeswell and
assuming
a meltwaterinputof-355roo.
Calculatedseasonaldifferencesin the estimatedsalinity of
Murray, 1990],and thusthe lowestsalinitiesshouldbe in the
BOFS 5K (summer minus winter salinity), shows that there summer. This is oppositeto what is shownby the seasonality
were major shifts in the seasonalityat each Heinrich event of thesalinityresultsof BOFS 5K and8K at theLGM andH2-4
during the last 45,000 years (Figure 7). During Heinrich ice where the lowest-salinityestimatesare found in the winter
rafting events2-4, more meltwaterwas producedduring the record. This suggests
that the G. hulloides"summer"andN.
winter than during the summer. This could be correctonly if
pachyderma(s) "winter"definitionsmaybe incorrect.
the supplyof icebergsto the northeast
Ariantleduringtlies6
There are three possibleexpalanationsfor the anomalous
MASLIN
A/Summer
ET AL.: HEINRICH
EVENTS AND DEEPWATER
FORMATION
535
37
LGM summer
salinityestimated
by Duplessy
et al. (1992)
36
35-
33
1s.d.
varibility
in
salinityestimate
32
B/Winter
32
31 I salinity
1s.d.
varibility
in
estimate
30
,',
0
ß , i ....
5000
i ....
10000
i ....
i ....
15000
20000
i ....
25000
i ....
30000
i ....
35000
i
40000
45000
Age / 14C yrs BP
Figure 6. BOFS 5K: Solid line representthe sea surfacesalinity(SSS) calculatedusingequation8,
assuminga meltwaterinputof-20%o. Dottedlinesrepresents
the reconstruction
of the $SS duringthe
Heinricheventsassuming
a meltwaterinputof -35%o(equation10). Solidsquares
represents
thevariationin
theglobalmeanoceansalinity.MWM, modemwintermean;MSM, modemsummer
meansalinityat 50øN,
20'W [Ottens,1991]. Errorbar wascalculated
assuming
an errorof + I'C in the$$T estimates
and+ 0.1%o
in the oxygen isotopemeasurements.
results.First,the winterSST usedfor theN. pachyderma(s)
In conclusion,we believe at presentthat the best method
recordmay havebeena significantunder-estimate,
suggesting for calculatingSSS is usingthe methodwe have suggested,
thatN. pachyderma
(s) occurred
through
outtheyearduringthe with the T* calibrationsand temperaturerangesof G. bulloides
lastglacial.The underestimate
wouldhavehadto havebeenup and N. pachyderma (s) estimatedby Duplessyet al. [ 1991,
to 4•C, i.e., the difference between the summer and winter
1992], but there are, however, still many problemswith the
estimatedSST. This is not large enoughto explain the reliable calculationof sea surfacesalinity:
discrepancy
at H4. Second,the G. bulloides abundances
were
1. The relianceon the accuracyof the SST estimates:an
verylow (0-1%)duringtheHeinrichevents(seeFigure7), and errorof I'C altersthe •w estimateby over 0.5%o(a salinity
the G. bulloidesin the samplesmay only representa small errorof between0.6-0.9%•;Figure5).
proportionof the 150-300yearsthat eachsamplerepresents.
2. The sensitivityof the salinity estimateto temperature
The G. bulloidesthusmayrecordsetsof yearsonlywhenthe means that the assumedtemperatureof ealeifieation (T*) of
conditionsare closer to its optimum,i.e., with higher sea each speciesis extremely important. In this study, two
surfacetemperatures
dueto fewericebergs.In contrast,
theN.
different assumptionswere made about the seasonal,and thus
pachyderma(s) recordrepresents
the wholeperiod,sincethe SST occurrenceof the foraminiferalspecies.Limitationof the
species
is unaffected
by low temperatures
or salinity[Spindler combinedsummersalinity approachis that the assumptionof
and Dieckmann, 1986; Dieckmann et al., 1991]. Or third, T* is basedon the calibrationof a coretop datasetincluding
there may have been no G. bulloides presentduring the the wholeNorth Ariantie. With suchmajorchangesin elimate
Heinrich events and those found in the sediment are from
in the recent past it is unlikely that the T* has remained
bioturbation[Manigehetti et al., this issue].
constant.
536
MASLIN ET AL.: HEINRICHEVENTSAND DEEPWATERFORMATION
A / Abundance
of G. bulloides
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
B / Salinity seasonality
4
3
2
1 -
H1
-2
........
O
i
....
i
H2
....
i
....
H3
!
....
H4
!
....
!
....
!
....
50•O 1OOOO
15000 20000 25000 30000 35000 40000 45000
Age / 14C yrs BP
Figure 7. Comparisonbetween (a) the relative percentageabundanceof G. bulloides and (b) the
reconstructed
salinityseasonalityin core BOFS 5K for the last 45,000 years.
3. To calculatesalinity,a 8w - salinityrelationshipmust
temperature
recorded
in NADW at present[GEOSECS,1987],
be assumed, and the salinity estimate is sensitive to the
assumedisotopiccompositionof the meltwaterinput (Figures
6 and 7). This relationshipis unlikely to be constantwith
time, as the evaporation-preeepitationregime will have
varied, and the sourceof the icebergsand thus their isotopic
compositionmay not have been constant.
and I'C is the lowest temperatureof NADW duringthe last
Sea Surface Density
Temperature and Density
Using the summerSST and SSS (Figure8) it is possibleto
calculatethe summersea surfacedensity,using the equation
calibrated by Cox et al. [1970]. For comparison,we also
calculatedthe densityof NADW for the last 45,000 yearsby
making assumptionsabout its temperatureand salinity. The
first assumptionwas the base salinity from which the global
mean salinity variations due to ice volume (Sv) occurred.
Labeyrie et al. [1992] suggestedtwo different models, (1)
assumingthat the Sv changesoccurredfrom the modem mean
ocean salinity of 34.74%0 [GEOSECS, 1987], and (2)
assumingthat the Sv changesoccurredfrom the salinity of
NADW at the present, 34.959'00[GEOSECS, 1987]. To
calculatethe densityof NADW, temperatureis required. Two
temperatureswere used, I'C and 3'C: 3'C is the highest
glacial
according
tobenthie
8180records
[Labeyrie
et al.,
1992]. The most extreme estimatesof NADW densities
(34.74%oat 3'C, 34.95%oat I'C) are plottedon Figures9 and
10 to comparewith resultsfromBOFS5K and8K.
Discussion of Summer Sea Surface Salinity,
Figures9 and 10 showthe Heinricheventsdefinedbothby
the WCMS and the % coarse fraction lithie debris of BOFS 5K
and 8K, compared with the summer insolation at 60'N
calculatedby Berger and Loutre (1988): Heinrich events
occurred at different insolation levels.
H2 and H4 occurred
while insolationwas decreasing,H3 while the insolationwas
relatively stable and H1 when insolation increased,
suggesting
thatthe lastfour Heinricheventswerenot directly
orbitallyforced. This is in contrastto the suggestion
of Bond
et al. [1992], that the Heinricheventswere due to repeated
advancesof the Laurentideicesheetafter short 3000 year
cooling episodes. There is no evidencein the BOFS coresfor
coolingperiodsbefore the Heinrichevents,and clearly the
major drop in SST occurred during the Heinrich events.
MASLIN ET AL.: HEINRICH EVENTS AND DEEPWATER FORMATION
A/ Surface
waterisotopic
-2.5
-1.5
-0.5
lOOOO
20000
1.5
30
31
32
33
34
35
Temperature
/ øC
36
37
4
6
8
10
12
14
16
18
..;.......
½
m .....
15000
0.5
C/Summer Sea Surface
B/Summer
SeaSurface
salinity
composition
(V.SMOW)
537
H2
""-__
.............
25000
H3
............
30000
35000
.....
.•..'e:.-•2•_?.•
................
45000
Figure 8. (a) BaFS 5K estimatedsurfacewaterisotopiccomposition
correctedfor variationsin globalice
volume. Dottedlinesrepresents
G. bulloidesresults,solidN. pachyderma(s) andopencirclesan averageof
theG. bulloidesandN. pachyderrna
(s) results;definedby the isotopictemperature
rangesof Duplessyet al.
[1991, 1992], seetext. (b) saps 5K: Summerseasurfacesalinitycalculatedfrom the composite
recordin
Pigure8a. Solid line showssalinitycalculatedwith a freshwaterisotopiccomposition
of-20%0, dottedline
showssalinity calculationsfor the Heinrich eventsassumingfreshwatercompositionof -35%0. (c) saps
5K: Summerseasurfacetemperatures
estimated
usingtheCLIMAP PA 20 equation.
Oerlemans [1993] modelled the possibilities of repeat
advances of the Laurentide
ice sheet as a cause of the Heinrich
events, both with and without prior cooling episodes. He
found that neither model could generate an appreciable
increasein the calvingrate duringH2 or H3, and only a small
increaseduringH1 which would not be sufficientto sustainthe
magnitudeof the eventobservedin deep-seasediments.
These results add support to the free-oscillating system
theoryof MacAyeal and Wang [1992] and MacAyeal [1993
a,b], who suggest that periodic melting of the soft
unconsolidated
basal sediment of the Laurentide
ice sheet due
to the build up of geothermalheat causedthe ice sheetto fail
and surgeon a massivescale. Once enoughice had beencalved
out into the North
Atlantic
the blanket
effect
of the ice was
lower, dueto the inputof near-freezing
surfacewater. Despite
the lower SST the surfacewaterdensitydroppeddramatically
duringHeinrichevents. Our recordsshowthat the temperature
dropoccurredduringand not beforethe Heinrichevents. This
is clearly shownin the extremelysharpboundariesat the base
of the Heinrich
events
shown
in the WCMS
and Lithie
content. X ray photographs
of the saps coresalso showthis
very sharpbasalboundaryand beneaththe coarseice-rafting
materiala "trichichnuslayer" (a layer wheremicroburrows
are
preserved,[Manighetti et al., this issue]). The presentsof
theseburrows suggeststhat the initialisation of the Heinrich
eventsoccurredvery rapidlyon the orderof years,thusadding
supportto the catastrophicfailure theory.
At present,warm salinesurfacewatersflow northwardin the
northeastAtlantic, and during this flow cooling increases
their density. At a critical temperature,the surface water
sinks, forming 'new' deep water in the Norwegian Sea.
removed and the sedimentm-froze, halting the outpouringof
ice. MacAyeal [1993 a,b] calculatedthat this free-oscillating
systemwould causethe Laurentideice sheetto fail every 7000
to 10,000 years, in agreementwith the deep-seasediment
Robinsonet al. [1995] from magneticsusceptibilityof over
record.
80 cores in the North Atlantic reconstructed-thesurface water
During each of the Heinrich eventsthem was a significant
(1 to 45'00)
dropin theSSS,presumably
dueto theinputof fresh
circulation
water from melting icebergs. During H2-4 SSTs were also
of the North Atlantic.
Their results show that
duringthe last glacial them was still northwardtransportation
of surfacewater in the northeast Atlantic. The possibility
538
MASLIN ET AL.: HEINRICH EVENTS AND DEEPWATER FORMATION
BOFS5K WCMS
Coarse
Lithic
Debris
(>150gm)
(c.g.s.
/ le-6G/Oe)
Summer
Salinity
0
20
•
I
•0
m
I
m
I
Summer
Density
(sigma
O)
100 30 31 32 33 34 35 36 37 24.0
O)
ß
25.0
26,0
27.0
28,0
I
o
'i
5000
......
15000
zoooo
4-.
35O00
40O00
45000'
ß.... ',:"."•
',ß
TemperatheSalinity
s
410 420 430 440 450 460 470 480
Insolation
60øN
7
9
11
13
15
BOFS
5K
NADW
17
Summer
Sea
Surface
(watts
per
squ.
meter)
Temperature/øC
(Berger
&Loutre,
1988)
Figure 9. BOFS5K: WCMS, percentcoarsefractionlithic debris,summerseasurfacetemperature,
salinityanddensity,
compared
withtheinsolation
at 60øN[BergerandLoutre,1988]andthecalculated
NADW density(seetext).
that surfacewater which passedthe site of 5K could become
denseenoughto form NADW further north, was modelledby
assumingthat this surfacewater cooledas it flowed northward.
Two temperatures
to which the surfacewater may havecooled
were selected and the surface water densities re-calculated.
The
farstwas the averageAugust SST in the NorwegianSea (6'C)
where deepwaterforms at the presentday [CLIMAP Project
Members, 1976, 1982]. This allows reconstruction of the
likely cooling duringthe Holoceneand the warmerperiodsin
the last glacial. The secondtemperatureselectedwas the most
extreme August SST in the far northeastAtlantic during the
LGM,I'C [CLIMAP Project Members, 1976, 1982],
reconstructingmaximum cooling during the last glacial.
Figure 11 shows the density record at 5K, the estimated
range of the NADW densities, and the range of densities
assumingthat 5K surfacewater flowed northwardand cooledto
between 6'C and I'C. This does not take into accountany
variationsin the salinity due to meltwaterinput or changesin
the evaporation/precipitation
regime, as there are no derailed
down core salinity recordsfor the far north North Atlantic.
Duplessy et al. [1991], however, suggestedthat during the
LGM the summer salinity in the far northeastAtlantic and
NorwegianSeawaslessthan 1%olower thanthatat 50'N. The
salinitydropwould haveopposedthe coolingeffect by reduced
the density and would have made deepwaterformationeven
less likely.
Our model suggeststhat there may have been deepwater
formation
during
thefollowing
periods'
41 14Ckato3914C
ka,which
washalted
byH4,3514C kato3014C kawhich
was haltedby H3, and present levelsof deepwater formation
during the Holocene. This model agrees with the work of
Labeyrie et al. [1992] who suggestedthat NADW was
significantlyreducedduringmuchof the last glacial, from the
salinity calculationsof Duplessyet al. [1991]. They also
suggested
that NADW was fed only by smallmounts of salty
subtropical waters penetrating into the central northern
Atlantic. Deepwaterformationthatcouldhaveoccurredduring
the beginningof the lastglacialwashaltedby Heinrichevents
H3 and H4. Broecker and Bond [1992] and Broecker [ 1994]
have suggestedthat the reductionin the NADW during the
Heinrich eventsmay have pushedthe climate systemtowards
maximumglaciation. We agreebut suggestthat the climate
responsewas morecomplicated;
after eachHeinricheventthe
climate overcompensated
and them was a strongrise in both
SST (1' to 6'C) and SSS (1 to 49'00).Only after a maximumof
2000 years did thesemilder episodescease. We suggestthat
MASLIN ET AL.' HEINRICH
EVENTS AND DEEPWATER FORMATION
BOFS8K WCMS
10
.
i
20
.
i
.
30
i
.
40
i
.
Summer
salinity
Coarse
Lithie
Debris
(>150gm)
(c.g.s.
/ le-6G/Oe)
•
,
.
60
0
20
40
60
80
10032.5
33.5
,
34.5
I
,
Summer
density
(sigma
O)
35.5
I
,
539
3•5
I
ß
i
25.0
....
26.0
i
,
27.0
,
i
ß
i
....
28,0
i
5000
10000
150O0
25O00
300OO
35000
400OO
.,Temperature
s•iimty
I
BOFS
8K
ß
450OO
ß
i
ß
I
ß
I
ß
i
ß g
ß i
410 420 430 440 450 '460 470 '480
Insolation
60øN
(watts
persqu.
meter)
l'
I
2
4
'
I
'
6
I
8
'
I
'
I
'
I
'
I
'
I
NADW
'
10 12 14 16 18 20
Summer
SeaSurface
Temperature/øC
(Berger
& Loutre,
1988)
Figure10. BOFS8K: WCMS,percent
coarse
fraction
lithiedebris,
summer
seasurface
temperature,
salinityanddensity,compared
with theInsolation
at 60'N [BergerandLoutre,1988]andthecalculated
NADW density(seetext).
with the collapseof the Laurentideicesheetthe predominant
cold winds from the North Americancontinentabated(D.
Seidovet al., Towardsa betterunderstanding
of themeltwater
event near 13.6 kyr B.P.: A numericalmodellingapproach,
submitted
to Paleoceanography
1995);thusreducingboththe
coolingof and the strengthof the polar gyre, so allowing
warmer southernwater to penetratefurther into the northeast
Ariantie. Figure12 showsourtheoretical
interpretation
of the
changesin the elimate systemover the last 45,000 years,
includingtheelimarierebounds
causedby the Heinrichevents.
[Weinelt et al., 1991; Sarntheinet al., 1992;Lehman and
Keigwin, 1992]. This initial melting of the northern
hemisphereice sheets occurred as the summer insolation
initiallyrose,suggesting
thatat leasta partof thismelting
phasewasclimaticallydriven. This meltingbeganat least
150014Cyears
before
thefirstmajor
meltwater
discharge
fromtheLaurentide
icesheet[Fairbanks,
1989]. Though
the
inertia of ice sheetdynamicsmay have contributedto this
delayit is unlikelythatit is thewholecause.We suggest
that
themassive
meltwater
effectof iceraftingduringH1 wasable
This short-term
eyelieityhasalsobeenshowneonvineently to reduceNADW andthe heatexchangebetweenthe low and
in the Greenland ice core and the North Ariantie sediment
recordfrom OceanDrilling Program(ODP) Site 609 [Bondet
high latitudes,delayingthe disintegration
of the continental
ice sheetsby 1500 yearsafter the initial meltingof their
al., 1993].
margins.
Of the first four Heinrichzones,H1 (14.8 - 13 kyrs) is
uniqueas it occursafter the LGM, as both insolationand SST
Benthie
foraminifera
/513
C records
fromAMS 14C dated
coresfrom both the North and SouthAtlantic [Jansenand
increased.Salinityestimates
showthatthe icebergmeltwater Veum, 1990;Charlesand Fairbanks,1992;Keigwin and
effectduringH1 wasmorewidespreadthanduringtheother Lehman,1994]suggested
thattherewasindeedreducedNADW
Heinrichevents,spanningat least23' latitudein the northeast between 14.6 and 13 kyr. From time slice reconstructions
over95 AMS14Cdated
benthie
/513Crecords
in the
Ariantie[Duplessyet al., 1992;Marin, 1993;Keigwinand using
Lehman, 1994] as well as affectingthe northwestAriantie northeastAtlantic, Sarntheinet al. [1994]. have confumed
[KeigwinandLehman,1994]. H1 wascoevalwiththemelting that the ventilation of the NADW was at its lowest for the
of the Fennoseandian
ice sheetmargins,whichhasbeenAMS
wholeof thelastglacialbetween
14.6and13 kyr. Thisdrop
14C dated
asoccurring
between
15,000
and13,000
years
B.P. inbenthie/513C
at 14.6kyriscoeval
withthelowest
SSDof
540
MASLINETAL.:HEINRICH
EVENTS
AND DEEPWATER
FORMATION
29.0
NADW
28.0
../
s• Northward
a
.....
27.0
26.0
H1
25.0
H3
H4
24.0
ß
'
'
'
I
'
'
5000
'
'
I
'
'
10000
'
'
I
'
'
15000
'
'
I
'
'
20000
'
'
I
'
'
25000
'
'
I
'
'
30000
'
'
I
'
'
'
35000
I
'
29.0
' •....--.-----:/ • .,-,,,•\
.............................
,,,,
/
28,0
I
27,0
.--'-'_
•..•r
'
40000
'
'
45000
"''"'-..'•'.1•S½!•-•...,....
-..
ß
.........
•.s,,'%,.;
A /_;;.'/,V
/i[f
•
....
,,
!
,
,.
--./
,",, ,
"
26.0
."!
,•: 1
,.a
,-•
'/
'-,"V
6øC
I1
25.0
24.0
....
I ....
.5000
I ....
10000
I ....
1.5000
I ....
20000
I ....
2:5000
I ....
:30000
! ' ß 1• ß I ....
:3`5000
40000
4`5000
Age / 14C yrs BP
Figure 11. (a) The possiblerange of densitiesof the NADW (see text) comparedwith surface water
densitiesat BOFS 5K for the last 45,000 years. (b) Modelled densitiesassumingnorthwardtransportof
surfacewater from 5K and cooling to either 6'C or I'C. Arrows indicate when there could have been
deepwaterformationin the far northeastAtlantic.
BOFS 5K and other publishedcores [Duplessyet al., 1992;
Keigwin and Lehman, 1994, Figure 13]. This reductionin
NADW production is also confirmed by the high benthie
foraminiferaCd/Ca resultsat •,14.5 kyr, from the Bermuda
Rise [Keigwin et al., 1991]. When the meltwatereffect at
BOFS 5K ceased at 13 kyr there is a dramatic rise in the
and was thus instrumentalin delaying deglaciationby up to
benthie
813C,which
Sarnthein
et al. [1994]interpreted
asa
surface
waterof thenortheast
Atlantichavebeencontrolled
by
the relative influencesof the polar and subtropical
gyres.
Duringthelastglacialthenortheast
Atlanticwasperiodically
150014C years
(2000calendar
year).
Conclusions
During the last 45,000 years the characteristicsof the
rapidswitching"on"of theNADW. At 13 kyr the discharge
of
the Laurentideice sheetalso rose sharply,and this marks the
flux of icebergsfrom the Laurenfide
beginning
of thefirstmajorretreatof theice sheet[Fairbanks, affectedby an enhanced
icesheet,
duringthe socalled"Heinrichevents".We presenta
1989]. We suggestthat with the switching'on' of the NADW
increased the transfer of heat from the lower latitudes,
newmethod
ofcalculating
SSSfromtheSSTand8180 records
acceleratingthe disintegration
of the northernhemisphere
ice
of planktonicforaminifera. Our results show that there was a
significant
dropin the SST, SSS,andSSD duringtheHeinrich
We proposethatthe meltwaterinputfrom the increased
flux events,probablydue to the input of meltwater.
ApartfromtheHeinricheventstherewasverylittlechange
of melting icebergsduring H1 reduceddeepwaterformation,
reducingthe heatexchange
betweenthe highandlow latitudes in $SD between the glacial and Holocene in the northeast
sheets.
MASLIN ET AL.: HEINRICH EVENTS AND DEEPWATER FORMATION
Warmer &
Higher salinity
GLACIAL
HOLOCENE
NEADW
NEADW
Average
Climate
•Global
NEAD
Allerod
&
\
Bolling
N.E. Atlantic
Climate
B
Colder &
Lower salinity
H1
H3
H4
TIME
Figure 12. Theoreticalreconstructionof the climate of the northeastAtlantic comparedwith the global
elimatefor the last 45,000 years. Heinrich(H) 1-4 representperiodswhen the Laurentideice sheetis failing
calving out huge quantitiesof icebergsinto the North Atlantic causingvery cold and low-salinitysurface
waters. PeaksA-D representclimatic reboundsafter the Heinrich eventswith much warmer and higher
salinity conditionswhich last a maximumof two thousandyears. NEADW are periodswhere there was
NADW
formation
the far northeast Ariantie.
Insolation
60'N
(watts
per
squ.
meter)
(Berger
& Loutre,
1988)
BOFS
5K
WCMS
Benthk
Delta
(c.g.s./le-6
G/Oe)
410 420 430 440 450 460 470 0 10 20 30 40 50 60 70 25.5
....
8000
S.
Atlantic
RCII-83
Density
(sigma
O)
26.5
!
....
27.5
!
....
28.5
1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2
- . - i . i . i _I
9000
1000O
11000
12000
13000
14o00
15000
16000
17000
RCII-83
18000
ß
0
i
1oooo
Discharge
(Km3/yr)
Fairbanks
(1989)
20000
-0.2-10-012-1416'0'.8-1.0
BmthicDelta13C/ %0
N. AtlanticV2•I
Figure 13. Theinsolation
for 60'N [BergerandLoutre,1988]andthe Laurentide
meltwater
discharge
[Fairbanks,1989]arecompared
withtheBOFS5K wholecoremagnetic
susceptibility
andsummer
$SD
records.BOF$ 5K $$D showsa majordropbetween15,000and 13,200yearsB.P. thisis coevalwith a
decline
inbenthie/j13C
records
ofV23-81
[Jansen
andVeum,
1990]
in theNorth
Atlantic
andRCll-83
[Charlesand Fairbanks,1992] in the SouthAtlanticindicatinga reducedventilationof the NADW between
14,600and 13,000. ThisreducedNADW ventilationmayhavebeenresponsible
for the 1500yearsdelay
betweenthe farstinitial meltingof the northernhemisphere
ice sheetsduringHeinrichevent 1 and the first
deglaeiationstepindicatedby the Laurentidedischargecurve.
541
542
MASLIN ET AL.: HEINRICH EVENTS AND DEEPWATER FORMATION
Atlantic becausethe increasedlocal meltwaterinput duringthe
last glacialcounteredthe globalrise in seawatersalinitydue to
the growthof the ice sheets. Modellingcoolingcausedby the
northward transportationof surface water in the northeast
Atlantic, indicates that NADW formation could have occurred
during the Holocene and at periods between 45,000 and
30,000 years. Deepwaterformationwas haltedby H3 and H4,
and no deepwater formation occurred between 30,000 and
13,000 years in the northeastAtlantic. We suggestthat there
is no evidencefor cooling prior to the Heinrich events. We do
observe
climatic
rebounds
after
each
Heinrich
event
consistingof milder climate (with strong rises in both sea
surfacetemperatureand salinity) lastinga maximumof 2000
years, after which conditionsreturnedto more averageglacial
levels. These cold and warm episodesin the northeastAtlantic
are superimposedon the general trend during the last glacial
towards colder condition of the LGM and the rapid
deglaciation. We suggestthat the Heinrich eventsand their
associated
reductionin NADW productionwere instrumentalin
enhancing glacial conditions. The exception to this is H1
which
occurred
while
insolation
rose.
The
reduction
in
deepwaterformation during this period becauseof the great
meltwater input from the inceasediceberg flux may have
caused
deglaciation
tobedelayed
by 150014C years(2000
calendaryears).
Furtherwork is requiredto investigatethe links betweenthe
Heinrich events,deep water formationand global climate. If
this is conf'mned
then we must consider
that the Heinrich
events(and their assoeaitedwarm periods)may havebeenpart
of a non-Milankovitch free-oscillating system, externally
forced only by the presenceor absenceof an icesheeton the
North American
continent.
Acknowledgments. We wouldlike to thankthe following:Ellen
Thomas, Mike Hall, Simon Crowhurst,J. Le, Julian Dowdeswell,
Andrew Richards,Neil Loader, Linda Booth, JenniToken, Lorraine
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