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 (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

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

'

'

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'

20000

'

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25000

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30000

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45000

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ß ......... •.s,,'%,.; A /_;;.'/,V /i[f • .... ,,

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6øC

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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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M. A. Maslin and U. PfiaumannGeologisch-Paliiontologisches Institut,UniversitiitKiel, OlshausenstraBe 40, D-24098Kiel, Germany M. A. MaslinDepartment of Geography, UniversityCollegeLondon (Email [email protected]) N.J. Shackleton, GodwinLaboratory,Universityof Cambridge,Free SchoolLane,Cambridge,CB 3RS, England.

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