Ž .Global and Planetary Change 28 2001 227–240www.elsevier.comrlocatergloplacha

A 340,000 year record of ice rafting, palaeoclimatic fluctuations,and shelf-crossing glacial advances in the southwestern

Labrador Sea

Richard N. Hiscott a,), Ali E. Aksu a, Peta J. Mudie b, David F. Parsons a

a Department of Earth Sciences, Memorial UniÕersity of Newfoundland, St. John’s, Newfoundland, Canada A1B 3X5b Geological SurÕey of Canada-Atlantic, P.O. Box 1006, Dartmouth, NoÕa Scotia, Canada B2Y 4A2

Received 16 March 1999; received in revised form 5 July 1999; accepted 25 October 1999

Abstract

Orphan Basin, southwestern Labrador Sea, is a strategic site for the study of Quaternary palaeoceanography andŽ . X Xpalaeoclimate. A 31.45-m-long piston core MD95-2025 was raised from 2925-m-depth at 49847.645 N, 46841.851 W, just

beyond the seaward limit of stacked debris-flow tongues derived from the Northeast Newfoundland Shelf. The core extendsŽ . Ž .to oxygen isotopic stage 9 ;340,000 years , and includes 13 prominent ice-rafted layers Heinrich events H1–H13 , many

of which are characterized by abundant detrital Palaeozoic limestone and dolomite. Warm peaks in sea-surface temperatureŽ . Ž .SST show poor correlation with accentuated ice rafting, except for 20–60 ka H3–H5 when the terminations of meltwater

Ž 18 .pulses d O minima lagged warm peaks in SST by ;1000 years, and peaks in ice rafting either coincided with peaks inŽ . Ž .SST H4, H5 , or lagged warmer peaks in SST by ;500 years H3 . These lags are attributed to the delayed response of ice

Ž . Žsheets e.g., iceberg and meltwater production rates to palaeoceanographic and palaeoclimatic forcing factors e.g.,.incursions of the warm North Atlantic Drift into the Labrador Sea; orbital-induced changes in insolation . The remarkable

covariance between SST and ice rafting from 20–60 ka is inconsistent with models for ice-stream surging through HudsonwStrait Marshall, S.J. and G.K.C. Clarke, 1997. A continuum mixture model of ice stream thermomechanics in the Laurentide

xIce Sheet 2: application to the Hudson Strait ice stream. J. Geophys. Res., B102, 20615–20637 , and instead suggests thatregional changes in ocean circulation played an important role in destabilizing icesheets. Heinrich layers H1, H3–H6, H9,

18 Ž .H11, and H13 formed during times of sharply decreasing d O values i.e., ice sheet melting . Heinrich layers H2, H7 andH12 formed at transitions from interglacialrinterstadial to glacial stages, coincident with both cool SST and low fluxes ofdetrital carbonate. They may represent the initiation of calving as growing ice sheets readvanced to coastal areas of theLabrador Sea and Baffin Bay. Carbonate-poor H8 and H10 developed during interglacial stages 5 and 7, and may have beenderived mainly from Greenland like the modern ice-rafted sediments of the Labrador Sea. q 2001 Elsevier Science B.V. Allrights reserved.

Keywords: ice rafting; palaeoclimate; glaciation; Quaternary; Labrador Sea

) Corresponding author. Tel.: q1-709-737-8394; fax: q1-709-737-2589.Ž .E-mail address: rhiscott@sparky2.esd.mun.ca R.N. Hiscott .

0921-8181r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.Ž .PII: S0921-8181 00 00075-8

( )R.N. Hiscott et al.rGlobal and Planetary Change 28 2001 227–240228

1. Introduction

Compositionally distinctive ice-rafted depositsŽ .Heinrich layers , widespread in the North Atlanticbasin, have been ascribed to surges of the Laurentide

Ice Sheet each ;7000–10,000 years through Hud-Žson Strait Heinrich, 1988; Andrews and Tedesco,

1992; Broecker et al., 1992; Bond et al., 1993;Andrews et al., 1994; Hillaire-Marcel et al., 1994;Bond and Lotti, 1995; Dowdeswell et al., 1995;

Ž .Fig. 1. Bathymetry of NW Atlantic Ocean, location of Orphan Basin rectangle north of Grand Banks and present-day surface waterŽ .circulation after Fairbridge, 1966 . Cores MD95-2025, 92045-9P, -10P, and -11P in Orphan Basin are shown in the inset; MD95-2025 and

92045-11P positions coincide at this scale. Other cores discussed in the text are labelled on the main map.

( )R.N. Hiscott et al.rGlobal and Planetary Change 28 2001 227–240 229

Johnson and Lauritzen, 1995; Gwiazda et al.,1996a,b,c; Hiscott and Aksu, 1996; van Kreveld et

.al., 1996 . The Labrador Sea is strategically locatedfor the study of abrupt climatic and palaeoceano-graphic changes associated with growth and decay ofQuaternary continental ice sheets because ice-rafted

ŽHeinrich layers are well-developed there Hiscott.and Aksu, 1996 . Sea-surface palaeotemperatures and

proxy climatic indicators show significant excursionsthat track the interplay of the cold Labrador Current

Žand the warm North Atlantic Drift Aksu et al.,.1992 , and debris-flow wedges along segments of the

Žcontinental margin e.g., Orphan Basin — Aksu and.Hiscott, 1992 record times when ice tongues reached

the edge of the shelf.Orphan Basin is located in the southern Labrador

Sea, north of the Grand Banks of NewfoundlandŽ .Fig. 1 . The western slope and rise of the basin areunderlain by as much as 1 km of alternating sheet-likeacoustically stratified units of hemipelagic sedimentsŽ .;1 s two-way travel in seismic profiles , includingice-rafted deposits, and wedge-shaped units formed

Žof debris-flow deposits Aksu and Hiscott, 1992;. Ž .Hiscott and Aksu, 1996 . Hiscott and Aksu 1996

described stratified glaciomarine sediments just be-yond the seaward limit of debris-flow wedges, andidentified nine Heinrich layers deposited since oxy-gen-isotopic substage 5e in 11.7-m-long piston core92045-11P. In an effort to better understand therecord of glacial–interglacial climate changes in this

Žstrategic area, a 31.45-m-long piston core MD95-.2025 was collected from the research vessel Marion

Dufresne II at position 49847.645XN, 46841.851XWŽ .260 m from coresite 92045-11P; 2925 m of water .Most 1995 Marion Dufresne II cores were stretchedby 50–100% during coring, apparently because of

Ženormous piston-suction in the Calypso corer Hil-.laire-Marcel et al., 1999, Section 5.8.2 . As a result,

11-m-depth in core 92045-11P correlates with ;

16.5-m-depth in core MD95-2025. This thicknessdifference is a coring artifact, not a primary feature.

2. Methods

Immediately after core recovery, P-wave velocitywas measured each 2 cm using Ocean Drilling Pro-gram procedures. On shore, the split core was de-

scribed and then systematically sampled each 10 cm.Ž .Procedures outlined by Aksu et al. 1992 were

followed for textural, foraminiferal, palynologicaland stable isotopic analyses, and for sea-surface-tem-

Ž .perature determinations. Following Heinrich 1988 ,a split of 200 loose grains in the size range 180–3000mm was counted using a binocular microscope andthe variables RMsundifferentiated rock fragmentsand silicate mineral grains, and Fs foraminifera

Ž . Žtests. A AHeinrich variableB, defined as RM r RM.qF , was computed for each of 315 samples as a

measure of intensity of ice rafting. Rock fragmentsand silicate mineral grains are mostly very angular tosubangular.

Forty-seven samples were taken from ice-raftedŽ .Heinrich layers for analysis of reworked paly-nomorphs as provenance indicators, following the

Ž .methods of Bischof et al. 1997 and McCarthy andŽ .Mudie 1998 . In the western Labrador Sea, detrital

carbonate beds contain few Quaternary pollen grainsand dinoflagellates, but reworked palynomorphs areoften abundant and serve as useful provenance indi-cators. For example, Ordovician–Silurian acritarchscharacterize the Palaeozoic carbonates of the Cana-

Ždian Arctic, while Mesozoic Cretaceous–early.Palaeogene dinoflagellates, pollen and spores char-

acterize black shales and volcanics of west Green-land.

Calcite and dolomite percentages were determinedfor 315 bulk samples by measuring Ca2q and Mg2q

concentrations using a Dionex DX-100 ion chro-matograph fitted with a cation-exchange column, insolutions obtained by digesting 0.5-g powdered sam-ple in 25 ml 1 M HCl for 30 min, then diluting up toa 1000-ml volume. The eluent was 22 mN H SO at2 4

a flow rate of 1 mlrmin. Analytical precision was;3–5% of the amount present. The weight percentof stoichiometric dolomite needed to account for allthe Mg2q ions was calculated. Excess Ca2q wasthen used to calculate a weight percent calcite. Three-63 mm fractions were powdered and analyzedfrom horizons rich in detrital carbonate to checkcalciterdolomite ratio in the mud fraction.

Thirteen thin sections were prepared from sandfractions of Heinrich layers and 300 grains werecounted along line traverses in each thin section.Carbonate grains were stained with Alizarin red S todistinguish calcite and dolomite.

( )R.N. Hiscott et al.rGlobal and Planetary Change 28 2001 227–240230

Four hand-picked planktonic foraminiferal sam-ples were submited to IsoTrace Radiocarbon Labora-

Ž .tory University of Toronto, Canada for AMS radio-carbon dating.

3. Core analysis

The core consists mainly of burrowed, slightly tomoderately calcareous andror dolomitic silty mudŽ .Fig. 2a with widely scattered plutonic, metamor-phic and carbonate pebbles. There are several slightlyburrowed intervals of sandy mud with gradationaltops, sharp unburrowed bases, and high carbonate

Ž .contents H1, H3–H6, H9, H11, H13, Fig. 2c and e .Carbonate abundance is tracked by peak values of

Ž .P-wave velocity Fig. 2b and e . Calcite and dolomiteŽ .abundances are similar Fig. 2e , except in biogenic-

carbonate-rich beds that accumulated during inter-glacial stages 1, 5, 7 and 9, where calciticforaminifera tests are visibly abundant and calcite

Žlocally exceeds dolomite e.g., Fig. 2d and e, at the.core top and a depth of 23.8 m . Thin sections show

that, except for the foraminifera, sand- and gravel-sized carbonate clasts are rock fragments of lime-stone and dolostone. Silt- and clay-sized calcite anddolomite in the carbonate-rich beds of sandy mud areinferred to be glacially ground limestone and dolo-stone.

The AHeinrich variableB exceeds 60% in 13 inter-Ž .vals Fig. 2g . The six youngest intervals have radio-

Ž .carbon ages Fig. 2h, Table 1 , or interpolated agesbased on dated oxygen-isotopic stage boundaries,that are consistent with the published ages of AHein-rich eventsB H1–H6 in other parts of the North

Ž .Atlantic Table 2 ; a minor peak in the MD95-2025data at ;35 ka has only been recognized elsewhere

Ž .by Andrews et al. 1994 and is designated as a1.Ž .Hiscott and Aksu 1996 recognized older Heinrich

events H7–H9, and the new MD95-2025 resultsextend this record of Heinrich events back to H13 atthe base of oxygen-isotopic stage 9. Van Kreveld et

Ž .al. 1996 also recognized older ice-rafted beds backŽ X X .to 200 ka in core T88-9P 48823.03 N, 25805.1 W ,

which they designated as h7–h13. Some of theirice-rafted units have an Icelandic provenance. Promi-nent ice-rafted layers with the ages of h8–h13 arenot present in core MD95-2025, although the authors

speculate that subdued peaks in the Heinrich variableŽ .from 18–25 m bsf Fig. 2g may correlate with

Ž .h8–h13 Table 2 .Heinrich events H1, H3–H6, H9, H11 and H13

occur at glacial–interglacial transitions and through-Ž .out interstadial stage 3 Fig. 2c and e . They are

slightly burrowed massive beds of sandy mud, mostwith gradational tops, sharp unburrowed bases, andhigh carbonate contents. In contrast, Heinrich eventsH2, H7, H8, H10 and H12 are horizons of sandymud within intervals of sand-bearing silty mud butthese do not occur as sharp-based beds, containrelatively little detrital carbonate and relatively few

Ž .foraminifera Fig. 2c and e . Of these, H8 and H10formed during cooler parts of interglacial stages 5and 7. The rest, H2, H7 and H12, occur at transitions

Ž .from interglacial to glacial stages H7, H12 andŽ .from interstadial stage 3 to glacial stage 2 H2 . All

of the Heinrich layers are interpreted as primarilyice-rafted deposits, augmented by variable amountsof fine-grained hemipelagic detritus.

Thin sections of Heinrich layers show a prepon-derance of quartz and variable quantities of detrital

Ž .carbonate Fig. 3 . Dolomite grains form 20–50% ofthe carbonate fraction; chemical analyses of bulksamples and -63 mm fractions from Heinrich lay-ers show a similar or slightly higher proportion of

Ž . Ždolomite Fig. 2e . Rounded quartz grains r)4,.Folk, 1974 constitute 8–10% of all quartz.

An unusual sandy and gravelly, normally gradedbed of olive black mud occurs at 9.95–10.40 m bsfŽ .H5, lower part . The sand fraction of this bed is28% basaltic rock fragments and 18% feldspar.Mesozoic palynomorphs predominate. Nearby cores

Ž .92045-9P, -10P and -11P Fig. 1 only contain thecarbonate-rich upper part of H5. The black mud, richin detrital basalt, likely represents a localized drop ofMesozoic mafic volcanic detritus from a single sedi-

Žment-laden iceberg — west Greenland Disko Is-.land; Henderson, 1973 or the Cape Dyer region

Ž .Baffin Island are probable sources for this detritus.There are a few horizons that are neither ice-rafted

nor hemipelagic in origin. H2 is overlain by a 40-cm-thick unit of sharp-based, thin beds that gradefrom fine or medium sand to mud. These are inter-preted as turbidites. This turbidite unit is also present

Ž .in cores 92045-9P, -10P and -11P Fig. 1 . Sand- andsilt-sized volcanic ash occurs in marked concentra-

()

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Ž . Ž .Fig. 2. Stratigraphy of core MD95-2025: visual description, occurrences of carbonate-rich sediments brick pattern , large dropstones diamond symbols and abundant visibleŽ . Ž . Ž . Ž . Ž .planktonic foraminifera circled F symbols column a ; P-wave velocity column b ; texture columns a and c ; concentrations of planktonic foraminifera column d ;

Ž .concentrations of calcite and dolomite from chemical analysis of acid-leach filtrates column e ; winter and summer sea-surface temperatures from foraminifera-based transferŽ . Ž . Ž . Ž . Ž . 18 Žfunctions column f ; Heinrich variable and Heinrich layers of this paper H- and van Kreveld et al. 1996 h- column g ; d O of planktonic foraminifera ‰ variation from

. Ž . Ž .PDB standard and AMS radiocarbon dates column h ; isotopic stages column i .

( )R.N. Hiscott et al.rGlobal and Planetary Change 28 2001 227–240232

Table 1Radiocarbon dates obtained on samples of mixed planktonic foraminifera

14 aŽ .Depth cm Sample location Lab number C age ErrorŽ ."68.3% confidence limits

60–62 Base foraminifera-rich TO-6537 6260 70Holocene

230–232 10 cm below H1 TO-6538 12,470 90420–422 10 cm below H2 TO-6539 22,510 180640–642 Base of H3 TO-6540 28,960 270

aAge based on 14C half life of 5568 years. No reservoir corrections have been applied.

tions at depths of 0.60 and 9.70 m bsf. The ash isdisseminated in muddy sediments in concentrations

Ž .of 5–19 grainsrg dry-weight . It consists primarilyof colourless, platy, bubble-wall shards, with rarelight-brown bubble-wall shards. Similar tephra havebeen identified in the upper Quaternary of deep-seacores from the North Atlantic, forming three discrete

Ž .layers Ruddiman and McIntyre, 1984 . The upperash in MD95-2025 occurs within sediment dated at

Ž .6260 year BP Table 1 so it cannot be the ;10.3-kaŽ .Ash 1 Vedde Ash; Birks et al., 1996 , but may

represent upward displacement due to biogenic orphysical reworking. The deepest ash is correlatedwith the 57.5-ka Ash 2 of Ruddiman and McIntyreŽ .1984 .

4. Abrupt palaeoclimatic and glaciogenic events

Key palaeoclimatic and palaeoceanographic vari-ables are plotted in a time domain in Fig. 4. Thetransformation to time is based on four AMS radio-

Table 2Ž .MD95-2025 and published ages ka of North Atlantic Heinrich layers

Heinrich MD95-2025 Heinrich van Kreveld Bond Grousset Broecker Andrewsa Ž . Ž . Ž . Ž . Ž . Ž .layers 1988 et al. 1996 et al. 1993 et al. 1993 et al. 1992 et al. 1994

Ž .H1 h1 11–12 12 13 14–15 15 14–15.5 13–14.5Ž .H2 h2 18–22 22 18 21 20 20–21 19.5–21Ž . w xH3 h3 28–29 33 25–27 27 27 25–28 34Ž .H4 h4 39–42 48 40–43 35–36 38 38.5–40.5Ž .H5 h5 50–53 59 52–54 50 52 49.5–51.5

w x w xH6 59–61 71 – 66Ž . w xH7 h6 69 71 64–67

H8 92–108 91 –Ž .H9 h7 121–126 128–131

Ž . w xh8 143 142Ž .h9 – 146–149Ž . w xh10 159 164–167Ž . w xh11 182 182–183Ž .h12 – 189Ž . w xh13 200 201H10 231–240H11 240–249H12 297–300H13 335–340

See each reference for core locations.a Ž .H entries from Fig. 2; h entries from van Kreveld et al. 1996 . There is general agreement on numbering of Heinrich layers back to

H5. Ages in square brackets are uncertain matches with layers in other cores. Data for some cores has been omitted if the abundance ofŽ .ice-rafted detritus was very low see text .

( )R.N. Hiscott et al.rGlobal and Planetary Change 28 2001 227–240 233

Ž . ŽFig. 3. Core MD95-2025 grain counts ;300rsample for thin sections of 63–400 mm separates from the base of Heinrich layers Parsons,. Ž .1997 and reworked palynomorphs countsrg sediment for events H1–H9, expressed as a Floral Index ratio of Palaeozoic acritarchs to

Ž .total Palaeozoic and Mesozoic palynomorphs. H5 is the upper carbonate part, and H5 the lower basaltic part, of event H5. a Variations inu lŽ .major mineral grains; b variations in minor mineral grains and corresponding changes in the Floral Index. Note that basaltic rock

Ž . Žfragments form 28% of the sand fraction in H5 , off scale for graph b . Ferromagnesian heavy minerals form 1–5% of all samples mainlyl.pyroxene, hornblende, biotite, chlorite, garnet; minor amounts of epidote, glauconite, pumpellyite, olivine .

Ž .carbon dates Table 1 and the ages of Imbrie et al.Ž .1984 for boundaries of oxygen-isotopic stages andsubstages, derived from orbital theory. The stage

boundaries are placed, by convention, at the mid-point between adjacent positive and negative peaksin d

18 O. Sharp increases or decreases in d18 O at

()

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Ž . Ž . Ž . Ž . Ž .Fig. 4. Chronology of sea-surface temperatures columns a, b , Heinrich variable and labelled Heinrich layers of this paper H- and van Kreveld et al. 1996 h- column c ,18 Ž . Ž . Ž . Ž .d O for planktonic foraminifera column d , oxygen-isotopic stages column e , and key reflections Blue and Green column f of Hiscott and Aksu 1996 . These reflections

underlie seaward-tapering wedges of glaciogenic debris-flow deposits that pinch out upslope of the coresite.

( )R.N. Hiscott et al.rGlobal and Planetary Change 28 2001 227–240 235

stage boundaries ensure little error in the placementof these boundaries. Other cores in the Labrador Seashow similar sharp boundaries between isotopicstages. Between stage boundaries, all ages are inter-polated based on an assumption of constant sedimen-tation rate between age picks. This procedure exag-gerates to 2–5 kyr the durations of Heinrich events,

Ž .although Veiga-Pires and Hillaire-Marcel 1999 havedemonstrated using 230 Th-excess measurements thatthey are probably more rapidly deposited over peri-ods of ;1–1.5 kyr.

Ž .Estimates of sea-surface temperatures SSTŽ .summer and winter were calculated by constructinga pseudo-factor matrix using the planktonicforaminiferal assemblage data and the varimax as-

Ž X. Ž .semblage description matrix F of Kipp 1976 ,then applying his transfer function F13B-4SE to thispseudo-factor matrix. The temperature estimates in-dicate a shift of about 68C between summer and

Ž .winter SSTs SST estimates throughout the last340,000 years, with highest temperatures during parts

Ž .of interglacial stages 1, 5, 7 and 9 Fig. 4 . Exceptfor stage 9, SST estimates are significantly belowmodern values, suggesting long-term cooling by theLabrador Current. This situation contrasts with themore northerly ODP Site 646 off south GreenlandŽ .Fig. 1 , where stage 5, 7 and 9 SST estimatesexceed modern values by ;48C and glacial values

Ž .by ;68C Aksu et al., 1992 . These warm waterincursions into the Labrador Sea during interglacialstages largely bypassed Orphan Basin by swinging

Žeast of the polar front compare modern circulation,.Fig. 1 . The North Atlantic Drift is the likely source

of this warm water. For comparison, SST south ofsŽ .Newfoundland at coresite 87008-003 Fig. 1 , under

the long-term influence of the North Atlantic Drift,exceeded 208C for most of the last 340,000 yearsŽ .Piper et al., 1994 .

Ž .Bond and Lotti 1995 stated that pulses of ice-Ž .berg calving and deposition of ice-rafted debris are

independent of SST. A plot of SST against thesŽ .Heinrich variable Fig. 5 appears to support this

observation. However, visual inspection of data forSST , the Heinrich variable, and d

18 O from 20–60sŽ .ka Fig. 4 shows a marked coincidence of most

peaks. An expanded plot of these three variables andŽ .lithology over this time interval Fig. 6 shows sev-

eral one-for-one matches of peak positions, although

with leads and lags of ;500–1000 years betweenŽpeak values actual lag times may be less if Heinrich

events were more rapidly emplaced in ;1–1.5 kyras proposed by Veiga-Pires and Hillaire-Marcel,

. 181999 . Weak correlation of SST, d O, and theŽ .Heinrich variable e.g., Fig. 5 is ascribed in part to

such peak offsets, and to cases where the Heinrichvariable significantly leads SST and d

18 O changesŽe.g., set of peaks including a1 at ;35–37 ka, Fig..6 . We are unable to explain these larger leads and

Ž .lags e.g., a1 , and focus our subsequent analysis onthe more strongly correlated peaks in Fig. 6.

The 20–60-ka data suggest that increased rates ofdeposition of ice-rafted detritus in H3–H5 were pro-moted by faster melting of icebergs in warmer sur-face waters that periodically characterized Orphan

Ž .Basin. Gwiazda et al. 1996a,c show, however, thatthe mineralogy and mineral ages of ice-rafted grainschange from Heinrich events to intervening deposits.Elevated SST alone could not account for such min-eralogical changes; instead, sharp variations in geo-graphic positions and rates of iceberg calving arerequired. Hypotheses for the origin of such sharp

Ž .variations include: a the suggestion by Bond andŽ .Lotti 1995 that sharp increases in the flux of

ice-rafted sediment resulted from prolific icebergŽ .calving following atmospheric cooling events; b

Ž .the proposal of Johnson and Lauritzen 1995 thatperiodic catastrophic emptying of an ice-dammedlake in what is now Hudson Bay might explain peaks

Ž . Ž .in ice rafting jokulhlaup model ; c the bingerpurge¨Ž .model of MacAyeal 1993 and Alley and MacAyeal

Ž .1994 that predicts ;7000-year cycles of buildupof geothermal heat beneath the central Laurentide IceSheet followed by its basal melting and surging of

Ž .glaciers to the coast; and d the sophisticated ther-Ž .momechanical model of Marshall and Clarke 1997

that predicts surging of ice streams on time scalesconsistent with Heinrich events, but independent ofmillennial-scale climate fluctuations.

The MD95-2025 data show that several Heinrichlayers formed during times of sharply decreasing

18 Žd O values H1, H3–H6, H9, H11, H13; Fig. 4c. 18and d . The sharp decreases of d O might record

meltwater pulses associated with times of rapidglacial surging and calving of icebergs. As eachpulse of meltwater discharge waned, d18 O eventuallystopped decreasing so that there is a peak on the

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Fig. 5. Sample-by-sample plot of SST vs. the Heinrich variable.s

18 Žd O curve Fig. 6e — peaks extend to the right and18 .mark minima in d O . Terminations of meltwater

pulses at 28, 39.5 and 51 ka lagged warmest valuesof SST by ;1000 years. Minima in d

18 O occurtoward the end of deposition of Heinrich layers, as

Ž .also noted by Hillaire-Marcel and Bilodeau 2000 .Peaks in the Heinrich variable either coincide with

Ž .peaks in SST H4, H5 , or lag the associated warmŽ .peak in SST by ;500 years H3 .

As demonstrated by Veiga-Pires and Hillaire-Ž .Marcel 1999 and Hillaire-Marcel and Bilodeau

Ž .2000 and confirmed by Fig. 6, the increased flux ofice-rafted debris leading to Heinrich events beganwell before the first arrival of abundant carbonatedetritus. Carbonate source rocks were apparentlytapped following a period of enhanced ocean warm-

Ž . Žing Fig. 6b and c and meltwater production Fig..6e . When this occurred, the switch to a carbonate-

Ž .rich source was abrupt Fig. 2a and e . Apparently,oceanic and climatic warming, leading to increasedglacial melting, acted as a trigger for glacier surgingfrom carbonate-floored bedrock terranes in Arctic

Ž .Canada e.g., through Hudson Strait , and icebergsladen with carbonate detritus subsequently meltedthroughout the Labrador Sea because of warmersurface waters. A similar temporal change in source

Ž .rocks was demonstrated by Hiscott et al. 1989 forŽ .ice-rafted units in Baffin Bay Fig. 1 , inferred to

result from surging of wet-based glaciers followingclimatic warming.

The remarkable covariance of SST and the Hein-Ž .rich variable during 20–60 ka Fig. 6 is inconsistent

with thermomechanical models that provide no link-age between ice-stream surges and regional changes

Žin climate or SST MacAyeal, 1993; Alley and.MacAyeal, 1994; Marshall and Clarke, 1997 . This

( )R.N. Hiscott et al.rGlobal and Planetary Change 28 2001 227–240 237

Ž .Fig. 6. Leads and lags of ;0.5 to ;1.0 kyr between peak values of winter and summer sea surface temperature SST and SST ,w s

the Heinrich variable, and d18 O minima over the time period 20–60 ka, all correlated to core lithology. All data are extracted from Figs. 2

and 4.

is because cycles of buildup of geothermal heatbeneath the Laurentide Ice Sheet ought to be unre-lated to advection of warmer waters into the LabradorSea from the North Atlantic Drift. Marshall and

Ž .Clarke 1997 hypothesize, however, thatmillennial-scale climate changes might influence hy-

Ž .drological conditions e.g., fluid pressure at the baseof an unstable ice sheet in the Hudson Strait area,sufficient to trigger surging and prolific iceberg dis-charges. At present, such fluid pressure changes arenot incorporated into quantitative glaciological mod-els, but might provide the best explanation for corre-lations and temporal lags in Fig. 6.

During cooler parts of glacial isotopic stages 5Ž .and 7, two peaks in the Heinrich variable H8, H10

show inverse correlation with peaks in SST. Thesedeveloped at times of reduced meltwater input intothe Labrador Sea or net transfer of water to slowlygrowing icesheets. Sand fraction is elevated, but notenriched in detrital carbonate. The origin of thesepeaks is poorly understood. The sandier sedimentsmay have been emplaced by local downslope trans-

Žport from the Northeast Newfoundland Shelf in.which case these would not be Heinrich events , or

may record sources unrelated to the Laurentide IceSheet because of its much reduced size followingstrong ablation during interglacial times. Instead, theGreenland icecap, as is the case today, may haveacted as the primary source for ice-rafted detritus inLabrador Sea sediments.

( )R.N. Hiscott et al.rGlobal and Planetary Change 28 2001 227–240238

H2, H7 and H12, also poor in detrital carbonate,formed at interglacialrinterstadial to glacial transi-tions during times of low SST. They may representthe initiation of calving as growing ice sheets read-vanced to coastal areas of the Labrador Sea andBaffin Bay, not just in the area of Hudson Strait.

Ž .Gwiazda et al. 1996c advocate mainly a ChurchillProvince source for H2.

Upslope from the MD95-2025 coresite, severallandward-thickening wedges of debris-flow deposits

Žpunctuate the upper Quaternary succession Aksu.and Hiscott, 1992; Hiscott and Aksu, 1996 . Hiscott

Ž .and Aksu 1996 dated the youngest debris-flowwedge at ;35.5–12 ka by reflection tracing to

Ž .coresite 92045-11P 260 m from MD95-2025 . Thisage suggests that glaciers from the Newfoundlandicecap did not reach the shelf edge until well after

Ž .the onset of the stages 4–2 glacial advance Fig. 4g .By this time, Heinrich events were already repeat-edly depositing ice-rafted layers on the continental

Ž .rise of Orphan Basin. Hiscott and Aksu 1996 fur-ther estimated the age of the next oldest debris-flowwedge as ;195–160 ka, based on linear extrapola-tion of the sedimentation rate in core 92045-11P to adepth of 6 m below the base of the core. Our resultsfrom core MD95-2025 show that sedimentation ratedecreases significantly below the base of stage 5Ž .compare Figs. 2 and 4 . A better tentative estimatefor the age of the second youngest debris-flow wedge,taking into account the stretching of core MD95-2025, is ;260–240 ka, late in oxygen isotopic stage8. This estimate is consistent with the suggestion by

Ž .Hiscott and Aksu 1996 that glaciers as far south asNewfoundland only reached the shelf edge to feeddebris flows late in glacial isotopic stages, but wouldimply that ice tongues did not reach the shelf edge

Ž . Ž .during stage 6 Fig. 4g . Piper et al. 1994 demon-strated that terrigenous flux to the J-Anomaly Ridge

Ž .south of Newfoundland coresite 87008-003; Fig. 1was much lower during stage 6 than during stages4–2 or 8, possibly as a result of less extensiveglacial advance onto the shelf at that time.

5. Conclusions

Piston core MD95-2025 provides a wealth of newŽdata on short-duration pulses of ice-rafting Heinrich

. Žlayers H1–H13 , water–mass interactions SST esti-.mates and local ice advances on the island of New-

Ž .foundland debris-flow wedges . There are complexand temporally varying relationships between the

Ž .intensity of ice-rafting Heinrich variable , global iceŽvolume or meltwater discharge fine structure of the

18 .d O curve , and the incursion of warm water fromŽthe North Atlantic Drift into the Labrador Sea SST

.estimates . At times these parameters are nearly inŽ .phase e.g., 20–60 ka ; at other times they seem to

vary independently or the Heinrich variable showsinverse correlations with SST estimates and d

18 OŽ .e.g., carbonate-poor H2, H7, H8, H10, H12 . Theinverse correlations are poorly understood, but mightindicate special emplacement processes for the sand

Ž .fraction e.g., local downslope transport or deriva-tion from sources dynamically uncoupled from theLaurentide Ice Sheet.

Glacial isotopic stages 2–4 are apparently uniqueŽ .at least in Orphan Basin with development of alarge number of Heinrich layers. The leads and lagsthat characterize palaeoceanographic, palaeoclimatic,and ice-sheet variables during the period 20–60 kaprovide fertile ground for future research and impor-tant boundary conditions for quantitative glaciologi-cal modelling. At other times in the last 340,000years, ice rafting generally peaked at the beginning

Ž .of warm interglacial stages H1, H9, H11, H13 .Glacial stage 6 shows a number of minor peaks inice rafting that we have not recognized as Heinrichlayers, although elsewhere in the North Atlantic

Žthese peaks have attracted greater attention van.Kreveld et al., 1996 . Except for H12 at its base,

glacial stage 8 lacks Heinrich layers in Orphan Basin,but was apparently a time when ice tongues crossedthe Northeast Newfoundland Shelf and fed numerousdebris flows to the continental slope.

Acknowledgements

Funding for this study was provided by a NaturalSciences and Engineering Research CouncilŽ .NSERC Research Networks Grant in support of the

Ž .Climate System History and Dynamics CHSD andIMAGES projects, and by A-base funding for Geo-

Ž .logical Survey of Canada-Altantic GSCA Project920063. We thank the captain, crew and technical

( )R.N. Hiscott et al.rGlobal and Planetary Change 28 2001 227–240 239

staff on Marion Dufresne II for acquiring samplesand data at sea. Shore-based laboratory assistancewas provided by H. Gillespie, P. King, and K.Jarrett. We also thank D.J.W. Piper and A. Rochon,GSCA, for critical reviews of the first draft of thispaper. This is GSC contribution 1999036.

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