Distribution, thickness and origin of Heinrich layer 3 inthe Labrador Sea

Harunur Rashid a;�, Reinhard Hesse a, David J.W. Piper b

a Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, QC, Canada H3A 2A7b Geological Survey of Canada (Atlantic), Bedford Institute of Oceanography, 1 Challenger Drive, Dartmouth, NS, Canada B2Y 4A2

Received 25 February 2002; received in revised form 8 October 2002; accepted 14 October 2002

Abstract

The presence of Heinrich layer 3 (HL-3) in the northwest Labrador Sea has been debated in the literature. Calypsogiant piston core MD99-2233, five new standard piston cores, and re-interpretation of 34 cores from previous cruisesconfirm the presence of HL-3 in the Labrador Sea. It is identified by high total carbonate concentration (up to 45%),an increase in coarse fraction content, and lighter N

18O values in polar species planktonic foraminiferNeogloboquadrina pachyderma (left-coiling) as low as 3.1x. The age of HL-3 of V27 ka was bracketed in thevarious cores by about 50 14C-accelerator mass spectrometer dates. Where it is present in ice-proximal regions, itconsists of nepheloid-flow deposits at the base and mud turbidites at the top. The thickness of HL-3 varies between4.8 m (proximal to Hudson Strait) and 0.9 m (distal), decreasing rapidly seaward. On the upper continental slope,HL-3 was too deeply buried to be sampled. Elsewhere, HL-3 is absent in some cores, probably due to slumping orerosion associated with sandy turbidity currents or debris flows.7 2002 Elsevier Science B.V. All rights reserved.

Keywords: Heinrich layers; nepheloid-£ow deposits; Labrador Sea; Hudson Strait

1. Introduction

Late Pleistocene sediments from the North At-lantic Ocean contain a sequence of layers of ice-rafted debris (IRD) known as Heinrich layers

[1,2], which are interlayered with hemipelagic andpelagic sediments. The Heinrich layers documentepisodes of massive iceberg discharge from theLaurentide Ice-Sheet. Ages of Heinrich eventshave been determined in the North AtlanticOcean by radiocarbon dating and extrapolationof sedimentation rates and are summarized byBond et al. [3]. Heinrich layers 1 and 2 were ¢rstsystematically described in the Labrador Sea byAndrews and Tedesco [4] and deeper Heinrichlayers were reported by Hillaire-Marcel et al. [5],Hiscott et al. [6], and Andrews et al. [7]. Theorigin, presence, and distribution of Heinrichlayer 3 (HL-3) in the Labrador Sea has been de-

0012-821X / 02 / $ ^ see front matter 7 2002 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 2 ) 0 1 0 4 7 - 6

* Corresponding author. Present address: Department ofEarth, Atmospheric, and Planetary Sciences, MassachusettsInstitute of Technology, 77 Massachusetts Avenue, Cam-bridge, MA 02139, USA. Tel.: +1-617-253-5725; Fax:+1-617-253-8630.E-mail address: hrashid@mit.edu (H. Rashid).

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bated in the past. For example, Grousset et al. [8]stated that HL-3 is restricted to the central andeastern part of the northern North AtlanticOcean. Based on a limited number of cores, An-drews et al. [7] and Kirby and Andrews [9] sug-gested that HL-3 is absent in the northwest Lab-rador Sea. In a re-examination of the cores fromthe North Atlantic studied by Bond et al. [10],Bond and Lotti [11] identi¢ed a narrow peak ofcarbonate grains at the level of HL-3 in Orphan

Knoll core EW9303-GGC31 and subsequentlyfound equivalent narrow detrital carbonate peaksin DSDP Site 609 and core V23-81 in the easternNorth Atlantic [3] not seen in earlier work onthose cores [10]. Hillaire-Marcel et al. [5] also no-ticed the presence of HL-3 in two cores (Hu90-13-12 and 13) from the Greenland slope.

In the northwest Labrador Sea, eight Heinrichevents, H0 to H6 (including H5a [12]), have beenidenti¢ed between 10 and 60 ka by their known

97-14 97-9

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Fig. 1. Location map of cores used in this study and their relation to the down-slope converging tributary system of the North-west Atlantic Mid-Ocean Channel (NAMOC). Tributaries are labeled in counter-clockwise squence and according to headwardbranching pattern.

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ages, distinct nepheloid-£ow deposit structure,high total carbonate concentration, increasedcoarse fraction content, mostly light oxygen iso-tope ratios, and low magnetic susceptibility values[13]. Using 40 cores (Fig. 1; Table 1), 34 of whichare from our core collection, this study attemptsto delineate the regional extent of HL-3 and mapits thickness. The signi¢cance of establishing the

distribution and thickness of HL-3 in the Labra-dor Sea is that it would demonstrate the extent towhich iceberg discharge was similar and from thesame sources during di¡erent Heinrich events,and in particular whether Hudson Strait was aprimary source also for HL-3. For the absenceof HL-3 in certain cores, alternative causes ratherthan the lack of iceberg discharge from the Hud-

Table 1List of cores used in this study

Core ID Latitude Longitude Water depth Core length Presence/absence of HL-3 Reference(m) (cm)

Hu75-55 61‡30.30N 58‡38.60W 2440 724 [9]Hu75-56 61‡26.90N 58‡33.40W 2464 386 [9]Hu77-14-4 48‡43.60N 47‡32.50W 2359 1062 6Hu87-033-09 62‡30.99N 59‡26.82W 1437 1101 6(?) [7,9]88-024-08 59‡15.55N 54‡18.24W 3522 102388-024-10 59‡12.47N 55‡51.31W 3131 110988-024-11 58‡47.70N 56‡21.51W 3027 108188-024-13 57‡29.10N 56‡21.54W 2798 1074 6Hu90-13-13 58‡12.59N 48‡22.40W 3380 V1740 6 [5]Hu90-13-25 59‡53.32N 58‡51.39W 2450 725Hu90-13-26 59‡31.34N 57‡16.64W 2895 1105Hu90-13-28 58‡45.80N 57‡06.75W 2913 1175 6Hu90-13-29 58‡23.61N 56‡45.76W 2918 1402 6Hu90-13-33 57‡16.84N 56‡50.70W 2577 770 6Hu91-45-94 50‡12.26N 45‡41.14W 3448 V1090 6 [5]Hu92-45-01 56‡09.61N 57‡07.28W 1469 1250Hu92-45-02 57‡00.47N 58‡10.69W 1602 1236Hu92-45-04 59‡48.62N 58‡08.00W 2761 1350Hu92-45-05 59‡49.14N 56‡27.38W 2932 970Hu92-45-06 58‡30.69N 55‡48.38W 3135 1149 6EW9303-GGC31 50‡57.00N 46‡35.00W 1796 120 6 [11]MD95-2019 51‡05.68N 43‡13.27W 262 128.6MD95-2020 51‡02.41N 43‡00.62W 4289 86.3MD95-2022 50‡34.38N 43‡03.51W 4245 183.7 6MD95-2023 50‡58.32N 43‡13.04W 4198 346.8 6MD95-2024 50‡12.40N 45‡41.21W 3536 261.5 6 [31]MD95-2025 49‡47.64N 46‡41.85W 3009 351.2 6 [6]MD95-2026 48‡13.61N 46‡39.39W 878 279.1 6Hu96-018-05 47‡61.24N 46‡09.92W 827 566 6Hu96-018-06 47‡74.54N 46‡20.31W 1050 789 6Hu96-018-07 47‡58.74N 46‡18.26W 984 598 6Hu96-018-08 46‡61.30N 46‡62.45W 933 598 6Hu97-048-07 62‡40.38N 60‡34.38W 938 1225 6Hu97-048-09 60‡32.82N 60‡26.54W 1575 1116Hu97-048-10 61‡13.62N 57‡33.14W 2725 1098Hu97-048-13 61‡00.71N 58‡21.34W 2725 1098Hu97-048-14 60‡22.07N 60‡47.02W 961 982Hu97-048-16 59‡42.02N 60‡14.30W 1085 942MD99-2233 59‡49.46N 59‡09.35W 2350 2462 6MD99-2237 50‡11.93N 45‡41.03W 3530 4400 6

6 indicates the presence of HL-3.

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Fig. 2. Downcore plots of concentration of total carbonate of cores Hu87-033-09, Hu75-55, 56, and 54 from the northern Labra-dor Slope and Slope o¡ Hudson Strait as a function of depth. The 14C-AMS dates are from (1) Andrews and Tedesco [4],(2) Andrews et al. [7,16], (3) Kirby and Andrews [9], and Barber [28]. A reservoir age of 450 years was subtracted from the mea-sured ages. Note the presence of a debris-£ow interval between 230 and 450 cm in core Hu87-033-09 [28]. Correlation of cores ison the base of H2.

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son Strait have been considered, which are dis-cussed in this contribution.

HL-3 is identi¢ed by bracketing 14C-acceleratormass spectrometer (AMS) dates of 26 and 28 ka,by high total carbonate content (up to 55%) andhigh L-values of re£ectance, low magnetic suscep-tibility, N18O values mostly lighter than 3.1x inpolar species Neogloboquadrina pachyderma (left-coiling; hereafter N. pachyderma (l)), and an in-crease in IRD [4,13]. Sedimentologically, it ischaracterized by typical nepheloid-£ow depositstructure [14], consisting of graded mud layersthat contain dispersed IRD and are occasionallyinterrupted by thin laminae of IRD. The basalboundary with the underlying hemipelagic sedi-ment is sharp (in some cores erosional). Theupper boundary is either gradual to overlyinghemipelagic sediments or in sharp contact withoverlying parallel laminated, carbonate-rich, mudturbidites devoid of IRD.

2. Cores from which HL-3 has been reported asabsent in the Labrador Sea

Andrews et al. [7] and Kirby and Andrews [9]in their study of four cores, Hu75-54, Hu75-55,Hu75-56, and Hu87-033-09 (from water depths of1169, 2410, 2434, and 1120 m, respectively), fromthe northern Labrador slope suggested that HL-3was absent in these cores. In all four cores, HL-0,HL-1, and HL-2, where present, were identi¢edby these authors by their prominent peaks in totalcarbonate content, increase in coarse fraction con-tent, low magnetic susceptibility, lighter N18O val-ues in N. pachyderma (l), and bracketing radio-carbon ages (Fig. 2).

In core Hu75-56, a high total-carbonate contentpeak is identi¢ed between 238 and 305 cm and anage of 23 878Q 238 14C yr at a depth of 228 cmsuggests that the carbonate event is older thanH2 [15]. However, two radiocarbon ages of37 935Q 1020 and 33 615Q 600 14C yr, at depthsof 350 and 375 cm, respectively, suggested to Kir-by and Andrews [9] that this carbonate-rich inter-val represented H4.

Core Hu75-55 shows a thick carbonate-rich in-terval below HL-1 and HL-2 at a depth between

320 and 700 cm. An AMS date of 45 500Q 55014C yr at 306 cm depth is too old for HL-4. The3.8 m thick carbonate layer (up to 40% carbonate)observed in this core below 320 cm was inter-preted by Andrews et al. [16] as not related toany speci¢c Heinrich event, but rather as £ow-induring coring or more likely as a sediment grav-ity-£ow deposit.

In core Hu75-54, retrieved from the south sideof Davis Strait on the Greenland slope [17], thetotal-carbonate content (%) pro¢le remains closeto the Labrador Sea background level concentra-tion of 6 5^10% throughout most of the core(Fig. 2). However, a few peaks exceeding 10, 15or 20% total carbonate were identi¢ed by Kirbyand Andrews [9] (their ¢gure 2) as HL-2, HL-4,HL-5(?), and HL-6(?), respectively.

The downcore total carbonate content pro¢le incore Hu87-033-009 shows HL-0, HL-1, and HL-2followed by a fourth high carbonate layer at adepth from 950 to 1020 cm, near the bottom ofthe core. Kirby and Andrews [9] identi¢ed this asHL-4 on the basis of a radiocarbon age of33 560Q 675 yr at 973.5 cm from the upper partof this layer, implying that HL-3 was absent inthis core.

3. Records of HL-3 in the Labrador Sea

From the 40 cores used in this study, threecores are chosen for graphic presentation of avail-able data based on the location, quality, and in-tegrity of the stratigraphic record, and the avail-ability of 14C-AMS dates. Cores MD99-2233 andHu90-29 were retrieved from the Labrador riseat water depths of 2350 and 2895 m and are2462 and 1100 cm long, respectively (Figs. 1 and3). Core Pa96-06 was collected from FlemishPass in the southern Labrador Sea at a waterdepth of 1085 m and is 789 cm long (Figs. 1and 3). A full description and evaluation ofthe data sets for core Hu90-29 is given in [12].Depth pro¢les showing the correlation of totalcarbonate concentration (%), coarse-fraction con-tent (%), and oxygen isotope records of thesethree cores are plotted in Fig. 3. Based on theserecords, individual Heinrich events were identi¢ed

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Fig. 3. Depth pro¢les showing the concentration of total carbonate (%), coarse-fraction content (%), and oxygen isotopes from cores MD99-2233, Hu90-29, and96-06. Oxygen isotopes were obtained from N. pachyderma (l). The top four Heinrich events, H1 to H4, were constrained by 14C-AMS dates obtained from theplanktonic foraminifer N. pachyderma (l). A reservoir correction of 450 and 400 years was applied to ages obtained from cores from the northwest Labrador Seaand Orphan Knoll, respectively.

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by the following speci¢c characteristics of theirdeposits :

HL-1 shows light N18O and N

13C spikes in N.pachyderma (l) with minima ranging from 3.06 to3.3x (N18O) and 30.12 to 0.01x (N13C), respec-tively, an interval of high carbonate concentrationof up to 57% and a peak in coarse-fraction con-tent up to 19%. A sharp erosional basal contactwith the underlying hemipelagic sediments with-out IRD and a gradual transition to the overlyingHolocene sediments at the top is characteristic ofHL-1. 14C-AMS dates of 15 100Q 55, 14 816Q 75,and 14 320Q 170 14C yr at the base of this layer inthree cores con¢rm the interpretation as HL-1(Fig. 3). The thickness of HL-1 varies from 40to 155 cm.

Like HL-1, HL-2 has a sharp erosional basalcontact, while the transition at the top into theoverlying hemipelagic sediments with IRD isgradual. HL-2 is identi¢ed by an interval of highcarbonate concentration of up to 50%, coarse-fraction content of up to 9%, and light N

18O (aslow as 2.86x) and N

13C values (as low as30.25x) in N. pachyderma (l). 14C-AMS datesof 22 000Q 190 and 21 450Q 120 at the base, and20 040Q 130 and 19 500Q 110 14C yr at the topbracket the age of HL-2 (Fig. 3). The thicknessof HL-2 ranges from 50 to 200 cm.

The thickest Heinrich layer in core MD99-2233was identi¢ed as HL-3, consisting of nepheloid-£ow deposits, which are underlain and cappedby 50 and 110 cm respectively of thick parallel-laminated, carbonate-rich mud turbidites, whichare devoid of IRD. HL-3 in cores Hu90-29 andPa96-06 has a sharp basal erosional contact,whereas the top grades into hemipelagic sedi-ments. High carbonate concentration (45%), anincrease in coarse-fraction content (up to 29.5%)in conjunction with light N18O and N

13C values, aswell as the presence of HL-1 and -2 in the strati-graphic column above were the criteria for iden-ti¢cation of HL-3. Three 14C-AMS dates wereobtained at the base (27 500Q 130 14C yr) andat the top of HL-3 (26 250Q 370, 26 620Q 27014C yr), respectively (Fig. 3).

HL-4 consists of a stack of thin nepheloid-£owdeposits with a sharp erosional basal contact anda gradual transition into the overlying hemipela-

gic sediments with IRD at the top in the threecores. HL-4 also exhibits a high concentrationof total carbonate (up to 43%), a peak incoarse-fraction content (26%) and light values ofN18O (as low as 2.81x) and N

13C (as low as30.06x) in N. pachyderma (l). Four 14C-AMSdates of 35 350Q 300, 32 650Q 330, 33 300Q 290,and 33 250Q 290 14C yr, constrain the basal ageof HL-4 (Fig. 3). The thickness of HL-4 decreasesfrom 110 cm in the proximal region o¡ the Hud-son Strait to 50 cm in the southern Labrador Sea.

Heinrich layers in ice-proximal cores (e.g.,MD99-2233 and Hu90-29) are characterized bythe typical sedimentary structure of nepheloid-£ow deposits. Nepheloid-£ow deposits are absent,however, in the Flemish Pass core Pa96-06, whichconsists of clayey silt (5Y 5/2 Munsell color) [18].

4. Distribution and thickness of HL-3 in theLabrador Sea

Data presented here (Fig. 4) clearly show thatHL-3 is present in 24 out of 40 cores. Eight coreshad insu⁄cient penetration to reach HL-3 and ineight cores HL-3 appeared to be missing. Radio-carbon dates (14C-AMS) were obtained whereverpossible from both the base and the top to iden-tify HL-3 and bracket its duration. With the aidof this tool it can be shown that HL-3 is absent inthree areas: in cores Hu92-01 and 02 on the upperLabrador Slope, in cores Hu92-04 and 05, Hu90-25 and 26, Hu88-10 and 08 (Fig. 1) in an areaimmediately south of the NAMOC and its NCand DAB tributaries (see [19], their ¢gure 1) be-low the 2500 m isobath, and in core Hu75-54 onthe southern slope of Davis Strait.

The maximum thickness of HL-3 is observed ina core just south of the mouth of the HudsonStrait, i.e., core MD99-2233, where it is 4.8 mthick. Southeastward, HL-3 abruptly thins to-wards the central and southern Labrador Sea.Data are not available from within the HudsonStrait to the north and in Ba⁄n Bay and thesouthern slope of Davis Strait. On Orphan Knoll,HL-3 is 20 cm thick (core EW93-03/GGC3) [11]and to the south in Flemish Passitis 50 cm thick(core Pa96-06). In the North Atlantic, HL-3

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reaches a maximum thickness of 40 cm at 45^49‡N and 48^30‡E and decreases to 5 cm at 42^52‡N and 30^18‡E [8]. However, only a part ofHL-3 at these locations contains detrital carbon-ate, which is 2^10 cm thick at the most.

5. Re-interpretation of previously studied coresfrom the northwest Labrador Sea

Of the cores previously studied by Andrews etal. [7] and Kirby and Andrews [9], the Heinrichlayer identi¢ed as HL-4 in two cores (i.e., theHu75-56 and Hu87-033-09), could be either HL-3 or HL-4. In the remaining cores, HL-3 may infact be missing. In core Hu75-56, the two radio-carbon ages of 37 935Q 1020 and 33 615Q 600 atdepths of 350 and 375 cm, respectively, show a

stratigraphic reversal (Fig. 2), implying that atleast one of them is unreliable. If the youngerage were reliable, the detrital carbonate event be-tween 238 and 305 cm would most likely be HL-3.The sample with the older age may have beencontaminated. If this was the true age, the layercould correspond to either HL-3 or HL-4.

For the carbonate-rich layer between 950 and1010 cm at the bottom of core Hu87-09 thatwas identi¢ed to correspond to HL-4, Kirby andAndrews [9] reported a radiocarbon age of33 560Q 675 14C yr at 973.5 cm (Fig. 2). Sincethis date is from a level 27 cm below the top ofthe carbonate interval, the age could re£ect con-tamination by reworked older foraminifera, sothat this high carbonate-content layer could cor-respond to either HL-3 or HL-4.

Re-examination of the X-radiographs from

2

2

Fig. 4. Isopach map of HL-3 in the Labrador Sea and North Atlantic. Schematic position of NAMOC is shown by continuousblack line. Open circle: cores lacking HL-3. Data for the North Atlantic east of 40‡W (lower right corner) are from [9]. Datafrom cores MD99-2025, MD90-2024 and EW9303-GGC31 are from [20,31] and Bond (2002, personal communication).

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core Hu75-55 showed no £ow-in structure at thebottom of the core as proposed by Andrews et al.[16]. From the study of the X-radiographs, how-ever, it cannot be excluded that the high carbon-ate-content layer between 320 and 700 cm mightbe a debris-£ow deposit as suggested by Andrewset al. [16] such as that identi¢ed in nearby coreHu97-10 [12] and those that have been reportedby Hiscott and Aksu [20] and Wang and Hesse[21] from this general area. As far as the identi-¢cation of HL-3 is concerned, this core has insuf-¢cient penetration.

In core Hu75-54, small carbonate peaks above10% could represent Heinrich events as suggestedby Kirby and Andrews [9]. The ¢rst carbonatepeak at 40 cm, which, if not due to biogenicCaCO3, would be equivalent to HL-2, and couldbe derived from icebergs which re-entered theLabrador Sea via the West Greenland currentfrom the southern tip of Greenland. These ice-bergs would have lost much of their IRD andtherefore leave only a weak provenance signal.However, there are no elevated carbonate peaksbetween 100 and 200 cm and it is most likely thatHL-3 is absent in this core.

6. Why is HL-3 absent in some cores and why wasit not found in some of the earlier NW LabradorSea cores?

HL-3 is absent in a number of cores for variousreasons.1. In high sedimentation rate cores, Heinrich

layers older than HL-2 were not penetrated.This is the case for the upper Labrador slopecores Hu97-16, 14, 13, 10 and 09 but also forlower slope and rise cores Hu75-55, 56, 60, 61,and 62 (Fig. 2).

2. Heinrich layers are absent from cores Hu92-01and -02 from the upper slope, perhaps becausethese locations were landward of the LabradorSea iceberg trajectory or may have experiencedpermanent sea-ice cover and hence the cores donot record ice-rafting.

3. Processes responsible for the removal of ice-rafted sediment of HL-3 include slumping onthe slope and rise as well as erosion caused by

mass-£ow events associated with the North-west Atlantic Mid-Ocean Channel (NAMOC)and its tributaries [19,21]. In some cores, HL-3is probably missing due to slumping. In others,parallel-laminated, carbonate-rich mud turbi-dites are found in the stratigraphic positionof HL-3. This is the case, for example, incore Hu88-08 which has an 8 m thick layerof stacked thin, carbonate-rich mud turbiditesthat resulted from spill-over from the NAMOCand could either be time-equivalent of or im-mediately post-date HL-3. Core Hu92-02 fromthe upper Labrador Slope o¡ Nain Bank prob-ably has also been a¡ected by slumping at thistime ([22], p. 29) although Heinrich layers seemnot to have been deposited at this site.

4. Cores Hu92-04, -05, Hu90-25, -26 and Hu88-10 are located in a narrow, elongated down-slope corridor south of and parallel to theNC and DAB tributaries of the NAMOCand the NAMOC itself (Fig. 1). The strati-graphic position of HL-3 in these cores ap-pears to be occupied by thin-bedded, carbon-ate-rich, spill-over mud turbidites whereas incore MD99-33 (30, 100, 140 and 230 kmaway from cores Hu90-25, 92-04, 90-26 and92-05, respectively) HL-3 is present (see Sec-tion 3 above) and is 4.8 m thick. The lack ofHL-3 in the ¢ve cores contrasts with its pres-ence and maximum thickness in nearby coreMD99-33 and suggests that HL-3 was not pri-marily absent in this region but has been re-moved by erosion.It is possible that a high-density turbidity cur-

rent, which may have breached the right levee ofthe NC tributary as well as that of the DAB trib-utary, may have eroded HL-3 but not have left adeposit of its own in this region. High-density,sandy turbidity currents are required for erosionof the thick HL-3. Dilute muddy spill-over cur-rents, whose deposits ¢ll the stratigraphic positionof HL-3 in cores where HL-3 is absent, would notbe capable of removing a layer of this thickness.The occurrence of sandy £ows in the NAMOCtributaries can be deduced from the existence ofa sand plain adjacent to the western NAMOClevee south of 57.30‡N where it resulted fromthe spill-over of sandy £ows from the E/F tribu-

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tary [23]. Cores situated in the pathways of tur-bidity currents may thus not show HL-3. Debris£ows could have had the same erosive e¡ect,although in the case of debris £ows one wouldexpect a deposit to be left in place.

7. Discussion

The detrital carbonate content identi¢ed in theLabrador Sea and North Atlantic Heinrich layersoriginated largely from the Hudson Strait outletof the Laurentide Ice Sheet, which acted as a ma-jor conduit for the subglacial drainage of theHudson Bay, Foxe Basin, Ba⁄n Island and sur-rounding areas. These areas are mostly underlainby Paleozoic carbonates. However, petrographicand geochemical studies [11,24,25] have impli-cated East Greenland as another source in addi-tion to the Paleozoic limestones of eastern Cana-da.

In the North Atlantic, in most Heinrich layersthe increase in the proportion of IRD is due toboth a higher lithic content and a lower concen-tration of foraminifera. In contrast, in HL-3 inthe North Atlantic, the increase in the IRD frac-tion has been attributed to the depletion in theabundance of foraminifera, while the coarse lithiccontent remains constant or increases only slightlywith respect to the concentration in the sedimentbetween Heinrich events. For example, in DSDPSite 609 and core V23-81, the concentration offoraminifera in HL-3 is lower than in HL-1 andHL-2 [10,11] and this was originally thought to bethe principal cause of the increased proportion ofIRD. However, re-examination of these cores [3]showed that detrital carbonate makes up 10^20%of the IRD in HL-3, requiring a contributionfrom the Labrador Sea source. In the LabradorSea cores, the concentration of IRD in HL-3 ismore or less equal to, if not higher than that inHL-1 or HL-2. Only in cores close to HudsonStrait does HL-3 show a thick interval wherethe IRD content is high but variable (Fig. 3d).

Sr and Nd isotopic analyses of the bulk silicatefractions indicate that HL-3 along the band ofmaximum IRD accumulation in the North Atlan-tic between 40‡ and 50‡N [26] originated from a

di¡erent source than the other Heinrich layers [8].Grousset et al. [8] and Gwiazda et al. [24] deter-mined the provenance of HL-3 in cores from thewestern and central North Atlantic and showedthat HL-3 is di¡erent, i.e., they suggested it hadno detrital carbonate compared to HL-1, HL-2,and HL-4. These authors postulated a Greenlan-dian source for HL-3. Alternatively, it may havehad a dominant western European source, sinceRevel et al. [27] showed similar Sr isotopic ratiosfor HL-3 and ambient sediments from easternAtlantic cores. However, in a recent analysis ofSr and Nd isotopes from the Labrador Sea cores,Barber [28] demonstrated that isotopic valueswithin the HL-3 did not show any dissimilarityin their pattern in comparison to HL-1 and 2.This ¢nding complements the earlier ¢ndings ofBond et al. [3] in the North Atlantic and Rashidet al. [29] in the Labrador Sea which suggest thatHL-3 shares the same source with HL-1 and HL-2. There may be components from Greenland orwestern European sources in the North Atlantic,but these are not present in signi¢cant amounts inthe Labrador Sea. In the northwestern LabradorSea, the high detrital carbonate content of HL-3indicates a source similar to that for HL-1, 2 and4, i.e., the Hudson Strait. Our results thus matchprevious ¢ndings [3,29], which suggested that theHudson Strait was an active supplier of icebergsduring the deposition of HL-3, and show that forthe Labrador Sea it was in fact the dominantsource for HL-3. However, for various possiblereasons the contribution of Hudson Strait materi-al to H3 in the North Atlantic appears to be sub-dued compared to HL-1, HL-2 or HL-4.

Since HL-3 in all cores in the Labrador Sea andwestern North Atlantic shows an increase in de-trital carbonate and coarse fraction content, andlighter N

18O values, massive iceberg dischargefrom the Laurentide Ice Sheet through the Hud-son Strait also took place during HL-3. It is pos-sible that the amount of icebergs discharged var-ied compared to other Heinrich events or the seasurface temperature (SST) was di¡erent duringthe deposition of HL-3 causing more rapid melt-ing of icebergs. A lesser iceberg discharge rateduring HL-3 as favored by Gwiazda et al. [24] isnot very likely because our data from the Labra-

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dor Sea, which cover the latitudes from 62‡ to46‡N including the Orphan Knoll, clearly demon-strate the presence of a HL-3 that is of compara-ble thickness to other Heinrich layers in theregion. Although IRD is interbedded with neph-eloid layer deposits, integrating the abundance ofIRD measured in individual samples over the to-tal thickness of the layer in the cores in Fig. 3shows that the total amount of IRD in HL-3 iseither greater than (core 99-33) or similar to(cores 96-06, 90-29) that in HL-1 and HL-2.These data also show that the overall thicknessof a Heinrich layer is an approximate proxy fortotal IRD content, although closer to the ice mar-gin, a greater proportion of the thickness is madeup of nepheloid layer deposits. It thus appearsthat an equivalent amount of icebergs was re-leased from the Laurentide Ice Sheet source dur-ing HL-3 as during the deposition of other Hein-rich layers.

An alternative scenario, which could explainthe lack of a prominent HL-3 in the eastern andcentral Atlantic, is relatively warm SSTs. It ispossible that during the deposition of HL-3, theNorth Atlantic SSTs were warmer than duringother Heinrich events (especially HL-1, HL-2,and HL-4). As a result, icebergs entering theNorth Atlantic may have lost most of the IRDin the western North Atlantic before reaching thecentral and eastern Atlantic. So detritus-poor ice-bergs deposited whatever debris was left but thiswas enough to still give a weak detrital carbonatepeak observed in DSDP Site 609 and core V23-81by Bond et al. [10]. Recent Mg/Ca and N

18O de-terminations on N. pachyderma (l) and Globiger-ina bulloides from DSDP Site 609 by Dave¤ ([30] ;his ¢gure 4-3, p. 44) lend support to the proposedhypothesis. According to this study, HL-3 is asso-ciated with a two-fold increase in freshwater sup-ply compared to HL-1, HL-2, and HL-4, whichcould be due to increased melting of icebergs as afunction of higher SST or increased meltwaterdischarge from the ice stream outlets.

8. Conclusions

The debate about HL-3, caused by suggestions

concerning its absence in some Labrador Seacores, has led to the identi¢cation of HL-3 in alarge number of additional cores. The provenanceof HL-3 in the Labrador Sea is from the sameHudson Strait source as that of the other Hein-rich layers. It is identi¢ed based on age, its typicalnepheloid-£ow deposit sedimentary structure to-gether with high total carbonate concentration,high coarse fraction content and lowered N

18Ovalues. On the upper continental slope, it hasnot been penetrated in numerous cores due tohigh sedimentation rates. HL-3 is also absent ina narrow corridor on the slope south of the NA-MOC and its NC and DAB tributaries, whereHL-3 has probably been eroded by sandy turbid-ity currents. Moreover, as the carbonate-rich HL-3 in the Labrador Sea is the equivalent of HL-3 inthe North Atlantic, the conclusions of Grousset etal. [8] that HL-3 had a source o¡ east Greenlandand of Kirby and Andrews [9] that HL-3 resultedfrom a northward ice advance in the HudsonStrait from Ungava Bay like during H0 have tobe modi¢ed for the Labrador Sea, where HL-3had the same main Hudson Strait source as,e.g., HL-1, HL-2, and HL-4.

Acknowledgements

This research was supported by grants from theNatural Sciences and Engineering ResearchCouncil of Canada (NSERC), and the NationalScience Foundation (NSF) of the United States toR.H., D.J.W.P., and H.R. H.R. thanks NSERC,NSF, Fonds pour la Formation de Chercheurs etl’Aide a' la Recherche (FCAR), Que¤bec, andMcGill University (Carl Reinhardt major fellow-ship), for ¢nancially supporting his researchthrough doctoral scholarships and funding his re-search. Piston cores were obtained on the 1997CCGS Hudson and 1999 Marion Dufresne cruisesfunded by the Geological Survey of Canada,NSERC, and l’Institut FrancYais pour la Re-cherche et la Technologie Polaires. We thank cap-tains, o⁄cers, and crews for their excellent coop-eration at sea. We are grateful to William Ryan(LDEO of Columbia University), and TimothyJull (NSF-AMS Facility of the University of Ari-

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zona) for providing assistance to obtain 14C-AMSdates.[SK]

References

[1] W.S. Broecker, G.C. Bond, M. Klas, E. Clark, J. McMa-nus, Origin of the North Atlantic’s Heinrich events, Clim.Dyn. 6 (1992) 265^273.

[2] H. Heinrich, Origin and consequences of cyclic ice raftingin the northeast Atlantic Ocean during the past 130,000years, Quat. Res. 29 (1988) 143^152.

[3] G.C. Bond, W. Showers, M. Elliot, M. Evans, R. Lotti, I.Hajdas, G. Bonani, S. Johnson, The North Atlantic’s 1^2kyr climate rhythm: relation to Heinrich Events, Dans-gaard/Oeschger cycles and the Little Ice Age, in: P.U.Clark, R.S. Webb, L.D. Keigwin (Eds.), Mechanisms ofGlobal Climate Change at Millennial Time Scales, AGU,Washington, DC, 1999, pp. 35^58.

[4] J.T. Andrews, K. Tedesco, Detrital carbonate-rich sedi-ments, northwestern Labrador Sea Implications for ice-sheet dynamics and iceberg rafting (Heinrich) events inthe North Atlantic, Geology 20 (1992) 1087^1090.

[5] C. Hillaire-Marcel, A. de Vernal, G. Bilodeau, G. Wu,Isotope stratigraphy, sedimentation rates, deep circula-tion, and carbonate events in the Labrador Sea duringthe last 200 kyr, Can. J. Earth Sci. 31 (1994) 63^89.

[6] R.N. Hiscott, A.E. Aksu, P.J. Mudie, D.F. Parsons, A340,000 year record of ice-rafting, paleoclimatic £uctua-tions, and shelf-crossing glacial advances in the south-western Labrador Sea, Global Planet. Change 28 (2001)227^240.

[7] J.T. Andrews, H. Erlenkeuser, K. Tedesco, A.E. Aksu,A.J.T. Jull, Late Quaternary (Stage 2 and 3) meltwaterand Heinrich events, Northwest Labrador Sea, Quat.Res. 41 (1994) 26^34.

[8] F.E. Grousset, L. Labeyrie, J.A. Sinko, M. Cremer, G.C.Bond, J. Dupart, E. Cortijo, S. Huon, Patterns of ice-rafted debris in the glacial North Atlantic, 40‡^55‡N, Pa-leoceanography 8 (1993) 175^192.

[9] M.E. Kirby, J.T. Andrews, Mid-Wisconsin Laurentide IceSheet growth and decay implications for Heinrich events-3 and 4, Paleoceanography 14 (1999) 211^223.

[10] G.C. Bond, H. Heinrich, W.S. Broecker, L. Labeyrie, J.McManus, J.T. Andrews, S. Huon, R. Jantschik, S. Cla-sen, C. Simet, K. Tedesco, M. Klas, G. Bonani, S. Ivy,Evidence for massive discharges of icebergs into theNorth Atlantic Ocean during the last glacial period, Na-ture 360 (1992) 245^261.

[11] G.C. Bond, R. Lotti, Iceberg discharges into the NorthAtlantic on millennial time scales during the last glacia-tion, Science 267 (1995) 1005^1010.

[12] H. Rashid, R. Hesse, D.J.W. Piper, Evidence for an addi-tional Heinrich event between H5 and H6: global impli-cation, Paleoceanography (2002) submitted.

[13] H. Rashid, R. Hesse, D.J.W. Piper, Origin of unusually

thick ice-proximal Heinrich layers H-1 to H-3 in thenorthwest Labrador Sea, Earth Plant. Sci. Lett. (2003)submitted.

[14] R. Hesse, S. Khodabakhsh, Depositional facies of latePleistocene Heinrich events in the Labrador Sea, Geology26 (1998) 103^106.

[15] M.E. Kirby, Mid to Late Wisconsin Ice Sheet/Ocean In-teractions in the Northwest Labrador Sea: Sedimentol-ogy, Provenance, Processes, and Chronology, Unpubl.M.Sc. Thesis, University of Colorado at Boulder, CO,1996.

[16] J.T. Andrews, K. Tedesco, A.E. Jennings, Heinrichevents: Chronology and processes, east-central LaurentideIce Sheet and NW Labrador Sea, in: R. Peltier (Ed.), Icein the Climatic System, NATO ASI Series, vol. 12,Springer-Verlag, Berlin, 1993, pp. 167^186.

[17] D. Fillon, J.-C. Duplessey, Labrador Sea bio-, tephro-,oxygen isotopic stratigraphic and late Quaternary pale-oceanographic trends, Can. J. Earth Sci. 17 (1980) 831^854.

[18] D.C. Campbell, Report on data from cruise Parizeau 96-018 to Flemish Pass, Geological Survey of Canada (At-lantic), unpub. internal report, 1997.

[19] R. Hesse, I. Klaucke, W.B.F. Ryan, D.J.W. Piper, Ice-sheet sourced juxtaposed turbidite systems in LabradorSea, Geosci. Can. 24 (1997) 3^12.

[20] R.N. Hiscott, A.E. Aksu, Submarine debris-£ows andcontinental slope evolution in front of Quaternary icesheets, Ba⁄n Bay, Canadian Arctic, Am. Assoc. Pet.Geol. 78 (1994) 445^460.

[21] D. Wang, R. Hesse, Continental slope sedimentation ad-jacent to an ice-margin. II. Glaciomarine depositional fa-cies on Labrador Slope and glacial cycles, Mar. Geol. 135(1996) 65^96.

[22] S. Khodabakhsh, Pleistocene Laurentide Ice Sheet Drain-age into the Labrador Sea: Sedimentary Facies, Deposi-tional Mechanisms, Stratigraphy and Signi¢cance ofHeinrich Events, Unpubl. Ph.D. Thesis, McGill Univer-sity, Montreal, QC, 1997, 266 pp.

[23] R. Hesse, I. Klaucke, S. Khodabakhsh, D.J.W. Piper,W.B.F. Ryan, the NAMOC Study Group, Sandy subma-rine braid plains ^ potential deep-water reservoirs, Am.Assoc. Petrol. Geol. 85 (2001) 1499^1521.

[24] R.H. Gwiazda, S.R. Hemming, W.S. Broecker, Prove-nance of icebergs during Heinrich Event 3 and the con-trast to their sources during other Heinrich episodes, Pa-leoceanography 11 (1996) 371^378.

[25] S.R. Hemming, R.H. Gwiazda, J.T. Andrews, W.S.Broecker, A.E. Jennings, T.C. Onstott, 40Ar/39Ar andPb-Pb study of individual hornblende and feldspar grainsfrom southeastern Ba⁄n Island glacial sediments: impli-cations for the provenance of the Heinrich layers, Can. J.Earth Sci. 37 (2000) 879^890.

[26] W.F. Ruddiman, Late Quaternary deposition of ice-raftedsand in the subpolar North Atlantic (lat 40‡ to 65‡N),Geol. Soc. Am. Bull. 88 (1977) 1813^1827.

[27] M. Revel, J.A. Sinko, F.E. Grousset, P.E. Biscaye, Sr and

EPSL 6469 3-1-03

H. Rashid et al. / Earth and Planetary Science Letters 205 (2003) 281^293292

Nd isotopes as tracers of North Atlantic lithic particlespaleoclimatic implications, Paleoceanography 11 (1996)95^113.

[28] D.C. Barber, Laurentide Ice Sheet Dynamics from 35 to7 ka: Sr-Nd-Pb Isotopic Provenance of NW North Atlan-tic Margin Sediments, Unpubl. Ph.D. Thesis, Universityof Colorado at Boulder, CO, 2001, 142 pp.

[29] H. Rashid, R. Hesse, D.J.W. Piper, Origin of unusuallythick ice-proximal Heinrich layers H-1 to H-3 in thenorthwest Labrador Sea, EOS Trans. AGU 81 (2000)F617.

[30] A. Dave¤, Reconstructing Climate-Based HydrographicVariations in the Eastern North Atlantic over the Last37,000 Years Using Foraminiferal Magnesium, Unpubl.M.Sc. Thesis, University of California at Santa Barbara,CA, 2000, 70 pp.

[31] J.S. Stoner, J.E.T. Channell, C. Hillaire-Marcel, C. Kissel,Geomagnetic paleointensity and environmental recordfrom Labrador Sea core MD95-2024: global marine sedi-ment and ice core chronostratigraphy for the last 110 kyr,Earth Planet. Sci. Lett. 183 (2000) 161^177.

EPSL 6469 3-1-03

H. Rashid et al. / Earth and Planetary Science Letters 205 (2003) 281^293 293