Early Holocene Glacial Lake Meltwater Injections Into the Labrador Sea and Ungava Bay Krister N

Home , Lake Minto, Lake Ojibway, Ungava Bay, Ungava Peninsula

PALEOCEANOGRAPHY, VOL. 19, PA1001, doi:10.1029/2003PA000943, 2004

Early Holocene glacial lake meltwater injections into the Labrador Sea and Ungava Bay Krister N. Jansson Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, UK

Johan Kleman Department of Physical Geography and Quaternary Geology, Stockholm University, Stockholm, Sweden Received 24 June 2003; revised 9 October 2003; accepted 30 October 2003; published 15 January 2004.

[1] In this paper we analyze drainage routes and estimate fluxes of meltwater released from Labrador-Ungava glacial lakes into the Labrador Sea, Ungava Bay, and Hudson Bay between 7.5 and 6.0 kyr BP (8.4–7.0 calendar (cal) years ka). The analysis and estimates are based on landform-based reconstructions of the Laurentide Ice Sheet (LIS) decay pattern and the associated glacial lake evolution. Geomorphological data constraining the spatial extent of glacial lakes are coupled to a digital terrain model for meltwater volume calculations. The LIS ice recession between 7.5 and 6.0 kyr BP led to the formation of a large number of glacial lakes, which drained in approximately 30 meltwater pulses, with fluxes exceeding 0.015 Sv (1 Sv = 106 m3 s1), into Labrador Sea, Ungava Bay, and Hudson Bay. The inferred rapid ice margin retreat during late stages of deglaciation indicates that these drainage events were relatively short-lived. The early Holocene glacial lakes of Labrador-Ungava released meltwater, resulting in a total inflow of 6000 km3 freshwater to the North Atlantic. The pulsed nature of meltwater release from the lakes is likely to have resulted in rapid repeated cooling of the Labrador Sea surface water. INDEX TERMS: 1824 Hydrology: Geomorphology (1625); 9315 Information Related to Geographic Region: Arctic region; 9325 Information Related to Geographic Region: Atlantic Ocean; 9345 Information Related to Geographic Region: Large bodies of water (e.g., lakes and inland seas); KEYWORDS: Laurentide Ice Sheet, glacial lake, Labrador Sea Citation: Jansson, K. N., and J. Kleman (2004), Early Holocene glacial lake meltwater injections into the Labrador Sea and Ungava Bay, Paleoceanography, 19, PA1001, doi:10.1029/2003PA000943.

1. Introduction existence of previously unknown glacial lakes [Jansson, 2002]. A reconstruction of the ice margin retreat pattern that [2] Gray and Lauriol [1985], Vincent [1989], and Jansson satisfies the occurrence of glacial lakes and ice flow et al. [2002] have recently detailed the generalized pattern indicators depicts a northward retreat of the ice margin to of Holocene glacial lakes in Labrador-Ungava. Although a final position of remnant ice over southern Ungava Bay traces of glacial lakes, formed at the margin of the Lauren- and the adjacent southern shore (Figure 1) [Ives, 1960a, tide Ice Sheet (LIS), are recognized over large areas of 1960b; Gray and Lauriol, 1985; Kleman et al., 1994; Clark Labrador-Ungava [Ives, 1957, 1960a, 1960b, 1960c; et al., 2000; Jansson et al., 2002]. This retreat pattern is in Henderson, 1959; Andrews, 1961; Matthew, 1961; Barnett, agreement with a recent marine investigation based on the 1963, 1967; Harrison, 1963; Barnett and Peterson, 1964; sedimentary record of Ungava Bay [MacLean et al., 2001]. Hughes, 1964; Peterson, 1965; Prest et al., 1968; Lauriol [4] The formation of North Atlantic Deep Water (NADW) and Gray, 1983, 1987; Gray and Lauriol, 1985; Gray et al., is controlled by thermohaline circulation (THC), which in 1993; Liverman and Vatcher, 1993; Jansson et al., 2002] turn is driven by density differences caused by sea surface they have previously not been synthesized into an evolu- salinity and temperature variations. Variability of NADW in tionary sequence. Hence the limited knowledge of the the North Atlantic region during the last glaciation has been outline, drainage routes, and the succession of the Labra- linked to abrupt century- to millennial-scale climate changes dor-Ungava lakes are probable reasons why they have not induced by meltwater discharge [Boyle and Keigwin, 1982; been systematically included in regional reconstructions and Broecker et al., 1985; Fairbanks, 1989; Keigwin et al., modeling experiments of deglacial meltwater events [e.g., 1991; Alley et al., 1997]. The drainage of glacial lakes Teller, 1987; Licciardi et al., 1999; Clark et al., 2001]. Agassiz and Ojibway/Agassiz through the Gulf of St. [3] A new detailed reconstruction of the glacial lakes in Lawrence and Hudson Strait, has been inferred to mark Labrador-Ungava, based on the first regional geomorphic the end of LIS meltwater events. These meltwater events are map that includes meltwater features such as glacial lake thought to have altered ocean surface salinity, circulation, shorelines, deltas and meltwater channels, documents the and are likely to have forced the Younger Dryas and the 7.7 kyr BP (10314C years before present [8200 calendar Copyright 2004 by the American Geophysical Union. years]) cooling events [Broecker et al., 1989; Barber et al., 0883-8305/04/2003PA000943 1999; Teller et al., 2002]. However, repeated changes in sea

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Figure 1. Study area is shown by black box. Watersheds and the general direction of drainage in Labrador-Ungava are marked by black lines and arrows, respectively. Drilling sites discussed in text are indicated by solid circles. Crosses, zones of deep water formation; NADW, North Atlantic Deep Water; LC, Labrador Current. surface temperature (SST), planktonic foraminiferal record, Canada, and the most probable location and outline of and salinity in the North Atlantic between 8.5 and 6.0 kyr the damming ice margins. The glacial lake reconstructions BP [e.g., Bond et al., 1997; Labeyrie et al., 1999; are based on detailed geomorphological mapping of shore- Waelbroeck et al., 2001] indicate a need to extend the lines, deltas, spillways, and drainage channels, using aerial deglacial history of meltwater events after 7.7 kyr BP. The photograph stereopairs at a scale of 1:30 000 and 1:60 000. 8.5–6.0 kyr BP time span also coincides with freshwater This data set is coupled to the Global Land One-km Base turbidity layers in the Labrador Sea [Fillon and Harmes, Elevation (GLOBE) digital elevation model, which was 1982], intense Ungava Bay sedimentation [Andrews et al., used for the analysis of drainage routes and calculation of 1995], and regional cooling outside Nova Scotia, eastern meltwater volumes. GLOBE comprises a 300 latitude- Canada [Keigwin and Jones, 1995]. The origin of these longitude array [Hastings et al., 1999]. For the volume events is, however, poorly understood. calculations, the northern margin of the glacial lakes was [5] Here we analyze drainage routes and estimate volume defined by the reconstructed outline of the ice margin. and discharge rates of meltwater released from the Labra- Islands in the glacial lakes were also accounted for during dor-Ungava glacial lakes into the Hudson Bay, Labrador the calculations of meltwater volumes. The lake volumes Sea and Ungava Bay between 7.5 and 6.0 kyr BP. The are calculated with the assumption that the Labrador- analyses and estimates are based on a reconstruction of the Ungava glacial lakes existed as open lakes [Jansson, LIS decay pattern and the associated glacial lake evolution 2003]. The contradicting results of studies dealing with in Labrador-Ungava [Jansson, 2003]. These are coupled glacioisostatic recovery patterns in Labrador-Ungava with a digital terrain model (DTM) for volume calculations. [Løken, 1962; Barnett and Peterson, 1964; Andrews and The results are tentatively correlated to terrestrial and Barnett, 1972; Allard et al., 1989], the sparseness of well- marine geomorphological and sedimentary evidence and dated shoreline tilt data and difficulties in correlating are compared to proxy data from offshore. fragmentary shoreline systems in the study area at present do not allow for corrections of nonuniform glacioisostaic recovery. 2. Method [7] Our estimates of meltwater fluxes during the drainage [6] This study considers the outlines of individual glacial of glacial lake substages are based on the calculation of lake lakes and glacial lake substages in Labrador-Ungava, volumes and the assumption that the average duration of

2of12 PA1001 JANSSON AND KLEMAN: EARLY HOLOCENE GLACIAL LAKE MELTWATER PA1001 these discharge events was 10, 30, or 100 days (for [Hillaire-Marcel, 1976; Hardy, 1977], close to the final motivation, see discussion). drainage of glacial lake Ojibway [Veillette, 1994]. The Nain area at Labrador coast (Table 3 and Figures 3 and 4) is 3. Results suggested to have been ice free at 7.9–7.6 kyr BP [Clark and Fitzhugh, 1990]. [8] During the last deglaciation of north-central Labrador- [12] The initiation of glacial lake Naskaupi at 7.5 kyr BP Ungava, low-lying terrain was successively exposed as the [Clark and Fitzhugh, 1990] started the Labrador-Ungava ice-margin retreated northward across the major watershed glacial lake era. This date, extracted by extrapolation of 14C- (Figure 1). The meltwater, impounded between the retreat- derived emergence curves, is supported by minimum ages ing ice margin and surrounding terrain formed glacial lakes for deglaciation in the Hopedale area [Awadallah and in large shallow basins in the interior of Labrador-Ungava Batterson, 1990]. The termination of the glacial lake era and smaller deeper lakes toward the watershed to the east coincides with the final deglaciation of southwestern and west. Ungava Bay at 6.2–6.0 kyr BP [Gray et al., 1980; Lauriol, [9] Figure 2 and Table 1 show the results of the 1982; Gray and Lauriol, 1985; Lauriol and Gray, 1987, reconstructed glacial lake outlines, calculations of meltwa- 1997]. Radiocarbon dates indicate that the southeastern part ter volumes, and estimated drainage discharges. The de- of Ungava Bay became ice-free earlier than the southwest- glaciation of Labrador-Ungava leads to the consecutive ern part (Table 3 and Figure 4), which supports the formation and lowering of approximately 16 glacial lakes interpretation of drainage from the Labrador-Ungava glacial (53 substages (Figure 2)). The early glacial lakes in central lakes toward the northeast during late stages of deglaciation. 14 Labrador-Ungava drained eastward along the ice margin [13] A few C dates bracketing the formation of glacial into the Labrador Sea, except for glacial lakes Boilay and lake Minto at 7.0 kyr BP and the final drainage of Naskaupi Gayot-Delay and high levels of glacial lake Caniapiscau, at 6.5 kyr BP allow some control on the timing of ice which drained westward into Hudson Bay. At a later stage recession of the western and eastern portions of the LIS during the deglaciation, drainage was rerouted northward [Lauriol and Gray, 1983; Clark and Fitzhugh, 1990]. The into Ungava Bay (Figure 2). The major drainage routes inferred timing of the end of the glacial lake era is also through which meltwater was evacuated into Labrador Sea supported by 14C dates of 6.0 kyr BP from Saglek Bay were the Kanairitok, Adlatok, Kogaluk, Anaktalik, Fraser, [Fillon and Harmes, 1982] and 6.2 kyr BP from eastern Kingurutik, Hebron Fiord and Nakvak valleys (Figure 2). Ungava Bay [Andrews et al., 1995], both indicating an The meltwater reaching Ungava Bay drained along the ice abrupt end of freshwater deposition of suspended material margin into eastern or western Ungava Bay. The total in those environments. volume of meltwater discharging from glacial lakes in [14] The suggested retreat pattern is in reasonable agree- Labrador-Ungava during the early Holocene was ment with 14C dates in the peripheral areas of Labrador- 3 3 6100 km of which 2900 km drained into the Labrador Ungava, but in the interior, nearly all dates, with the 3 3 Sea, 2100 km into Ungava Bay and 1100 km into exception of two adjacent dates of 5.8 and 7.3 kyr BP in Hudson Bay (Table 2). southwestern Ungava Bay, fall between 7.0 and 6.0 kyr BP [10] Estimated meltwater discharges show that several with no clear spatial pattern (Figure 4). No clear age releases of meltwater into the Labrador Sea, Ungava Bay gradient emerges from the 14C-data in this region, leaving and Hudson Bay, exceeded 0.015 Sv (Figure 2 and the glacial lake traces as the primary tools for constraining 6 3 1 Table 1; 1 Sv = 10 m s ). Estimates based on the final retreat pattern. 10 day duration of drainage events indicate that 39 events exceeded 0.015 Sv, whereas estimates based on 32 or 5. Discussion 100 day durations indicate 32 or 18 events in excess of 0.015 Sv, respectively (Tables 2 and 3). 5.1. Drainage Fluxes [15] Estimates of meltwater discharges may involve calculations based on channel cross-section areas, bed load 4. Chronology of Events size, or estimates of the duration of individual drainage [11] Figure 3 shows the inferred early Holocene LIS events. Most of the large lakes drained eastward across the retreat pattern in Labrador-Ungava between 8.0 and Labrador-Ungava/Labrador Sea watershed into preexisting 6.0 ka BP as six time slices. The first area to become river channels. Because these valleys have an erosional ice-free was probably the northern coast of St. Lawrence history that both predate and postdate the deglacial drainage (Table 3 and Figure 2). The outline of the LIS at 8.0 kyr BP period, cross sections cannot be related to any single shows the ice sheet margin across eastern Hudson Bay, drainage event. The cross-section approach is therefore of parallel to the Sakami moraine southeast of James Bay, and limited value for estimates of meltwater fluxes. Similar standing in the Nain area at the Labrador coast. This outline problems exist for a bed load size approach, leaving corresponds well with existing 14C dates from the Labrador duration time of drainage events as the most realistic coast (Table 3 and Figure 4) [Løken, 1962; Lowdon and approach for estimating meltwater discharge from the gla- Blake, 1980; Clark, 1988; Clark et al., 1989; Awadallah cial lakes of Labrador-Ungava. and Batterson, 1990; Clark and Fitzhugh, 1990], and the [16] Calculation of meltwater discharge is tentative be- pattern of end-moraines, till lineations, and eskers [Prest et cause the duration of drainage events is too short to be al., 1968; Vincent, 1989]. The Sakami moraine (Figures 2 resolved by current dating methods. We argue that the use and 3) is likely to have formed between 8.2 and 8.0 kyr BP of a drainage length of 10, 30, or 100 days for the

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Table 1. Estimated Drainage Fluxes, Calculated for a Range of Table 2. The Early Holocene Glacial Lake Drainage Events in Duration Times of Labrador-Ungava Glacial Lake Drainage Events Labrador-Ungavaa Fluxes (Sv) Calculated on Number 10, 30, and 100 Days of Numbera Glacial Lake Substage Duration of Drainage Events Drainage 1 Attikamagen 0.005 0.001 <0.001 Radiocarbon Calendar Destination Number of Events, Total Age, Age, of Meltwater >0.015 Volume, 2 Caniapiscau 1 0.096 0.032 0.001 b c d 3 2 0.042 0.014 0.004 kyr BP cal ka Meltwater Events Sv km 3 0.133 0.044 0.013 7.5–7.0 8.4–7.5 HB 2 2 1 0 119 4 0.095 0.032 0.009 LS 9 210 111 5 0.263 0.087 0.026 7.0–6.5 7.5–7.0 HB 4 4 4 2 638 6 0.248 0.083 0.025 LS 10 8 6 2 1031 7 0.403 0.134 0.041 6.5–6.0 7.0–6.4 HB 6 4 2 1 341 8 0.369 0.125 0.037 LS 12 9 9 7 1805 9 0.254 0.084 0.025 EUB 4 4 4 2 1061 10 0.359 0.120 0.036 SUB 2 2 1 1 306 11 0.619 0.206 0.062 6.0 6.4 SUB 1 1 1 1 207 12 0.233 0.078 0.023 WUB 3 332 486 13 0.093 0.031 0.009 aSuggested time periods are estimates based on existing 14C dates (see 3 Delornieu 1 0.010 0.003 <0.001 discussion of chronology). For details on individual glacial lakes, see 2 0.009 0.003 <0.001 Figure 2. Abbreviations are as follows: HB, Hudson Bay; LS, Labrador 4 Marble 0.005 0.001 <0.001 Sea; EUB, Eastern Ungava Bay; WUB, Western Ungava Bay. 5 Naskaupi 1 0.035 0.012 0.003 bFor details, see Table 3, Figure 4, and discussion of chronology. 2 0.052 0.017 0.005 cApproximate calibrations to calendar ages were made using the CALIB 3 0.348 0.116 0.035 4.3 software [Stuiver and Reimer, 1993; Stuiver et al., 1998a, 1998b]. 4 0.147 0.049 0.015 dCalculated on 10, 30, and 100 day duration of drainage events. 5 0.170 0.057 0.017 6 0.079 0.026 0.008 7 0.248 0.095 0.028 6 Petitsikapau 0.005 0.001 <0.001 calculations of meltwater fluxes is a plausible assumption. 7 Astray 0.005 0.001 <0.001 This is based on bracketing estimates for much larger events 8 Wapussakatoo 1 0.005 0.001 <0.001 and smaller events that occur today. Large-scale events, 2 0.008 0.003 <0.001 9 Chaigneau 1 0.022 0.007 0.002 such as the drainage of glacial lakes Agassiz and Ojibway in 2 0.245 0.082 0.025 North America and the Baltic Ice Lake in Scandinavia, were 10 McLean 0 0.080 0.027 0.008 modeled to have lasted approximately one year [Barber et 1 0.002 <0.001 <0.001 al., 1999; Lindeberg, 2002]. Studies from Iceland show that 2 0.056 0.019 0.006 4 0.046 0.015 0.005 jo¨kulhlaups often occur within a few days to a week 4 0.188 0.063 0.019 [Thorarinsson, 1939; Bjo¨rnsson, 1992]. The numerous 5 0.197 0.066 0.020 lateral meltwater channels associated with glacial lake out- 6 0.226 0.075 0.023 lines indicate that most lakes in Labrador-Ungava drained 7 0.076 0.025 0.008 along the ice margin. For a given lake volume, ice-marginal 11 Cambrien 1 0.336 0.105 0.032 2 0.240 0.080 0.024 drainage in open channels tends to have significantly higher 12 Boilay 1 0.005 0.001 <0.001 peak discharges than subglacial drainage events [Walder 2 0.001 <0.001 <0.001 and Costa, 1996]. Lateral drainage events for glaciers today 13 Gayot-Delay 1 0.001 <0.001 <0.001 occur generally within a week [Tweed and Russell, 1999]. 2 0.002 <0.001 <0.001 3 0.003 0.001 <0.001 The rapidly retreating ice margin and the thermal erosion of 4 0.079 0.015 0.008 ice walls by impounded water [Liestøl, 1956], most prob- 5 0.245 0.082 0.024 ably forced rapid glacial lake drainages during the early 6 0.042 0.014 0.004 Holocene in Labrador-Ungava. On the basis of the argu- 7 0.022 0.007 0.002 ments presented above, we prefer the assumption that the 14 Nachikapau 0.038 0.013 0.004 15 Me´le`zes 1 0.200 0.065 0.020 drainage events lasted 30 days. This assumption accounts 2 0.113 0.038 0.011 for the larger size of the Labrador-Ungava glacial lakes 16 Minto 0.254 0.084 0.025 compared with contemporary glacial lakes and the smaller aRefers to location in Figure 2. size compared to, for example, glacial lake Agassiz.

Figure 2. (a) The glacial lake systems, represented by encircled numbers 1 to 16. Substages within the glacial lake system are indicated by similar graded color. Each glacial lake level is coupled to a damming ice margin (in black) constructed to yield a best fit with drainage traces and the outline of the lake. Blue arrows indicate major drainage routes, thin black line eskers, and red lines major end moraines. Redrawn and modified from Jansson [2002], reprinted with permission from Elsevier. (b) The substages of the glacial lakes that existed in the area during the early Holocene are marked in colors corresponding to Figure 2a. Labels refer to substage and destination of the meltwater released from the glacial lakes. LS, Labrador Sea; HB, Hudson Bay; EUB, eastern Ungava Bay; SUB, southern Ungava Bay; WUB, western Ungava Bay. Some glacial lakes and/or substages exhibit an uncertain northerly extent (label on gray background), which may have resulted in an overestimated size and drainage rate for these lakes.

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Table 3. Details on 14C Dates From Labrador-Ungavaa Numberb Laboratory Radiocarbon Age, years ±1s Species Notes Reference St. Lawrence Northern Coast 1 Gif-3770 10190 ± 180c Ms minimum age deglaciation Dubois [1980] 2 QU-574 9930 ± 130c Ms minimum age deglaciation Dubois [1980] 3 GSC-1337 9100 ± 100c Ms minimum age deglaciation Dredge [1983]

Sakami Moraine 4 QU-122 7890 ± 160c Ms drainage lake Ojibway Hardy [1977] 5 I-8363 8230 ± 135 C maximum age of construction Hillaire-Marcel [1976]

Labrador Coast 6 AA-1825 7950 ± 100 Lc minimum age deglaciation Clark et al. [1989] 7 L-642 8960 ± 200c Ms minimum age deglaciation Løken [1962] 8NDd 8500 ± 200 ND ND Clark and Fitzhugh [1990] 9NDd 7600 ± 200 ND ND Clark and Fitzhugh [1990] 10 NDd 7500–7700 ND ND Clark and Fitzhugh [1990] 11 TO-1123 7690 ± 60 Ms minimum age delta Awadallah and Batterson [1990] 12 Beta-28885 7950 ± 95 Ms minimum age deglaciation Awadallah and Batterson [1990] 13 GX-9293 8700 ± 470c Ms ND Clark [1988] 14 GSC-2970 7560 ± 50c Ms minimum age deglaciation Lowdon and Blake [1980]

Central Labrador-Ungava 15 GSC-3094 6320 ± 90 Lc minimum age deglaciation Richard et al. [1982] 16 GSC-3241 6500 ± 50 G minimum age deglaciation Blake [1982] 17 GSC-3644 6200 ± 50 G minimum age deglaciation King [1985] 18 Beta-9516 6600 ± 100 G minimum age deglaciation Vincent [1989] 19 GSC-3252 6420 ± 75 G minimum age deglaciation Blake [1982] 20 GSC-3238 6120 ± 70 G minimum age deglaciation Blake [1982] 21 GSC-3154 5980 ± 120 Lc minimum age deglaciation Blake [1982] 22 GSC-1592 6460 ± 100 B minimum age deglaciation Richard et al. [1982]

Southeastern Ungava Bay 23 MBN-198 6680 ± 100c Ms ND Gangloff and Pissard [1983] 24 UL-358 7340 ± 90c Ms minimum age deglaciation Allard et al. [1989] 25 AA-9289 7575 ± 125 Bs minimum age deglaciation Manley and Jennings [1996] 26 SI-1959 6815 ± 125c G drainage of lake Naskaupi Short [1981] 27 QL-1002 6200 ± 200 ND base of sediment Allard et al. [1989]

Southwestern Ungava Bay 28 TO-5677 6600 ± 60c Ha ND Gray [2001] 29 DIC 1277 6260 ± 75c Mt ND Lauriol and Gray [1987] 30 GX-5091 5980 ± 225c GNDLauriol [1982] 31 I-9632 6950 ± 150c Mt ND Gray et al. [1980] 32 GX-5308 6920 ± 205 Me ND Gray et al. [1980] 33 GX-4740 5795 ± 185c Mt ND Lauriol and Gray [1987] 34 GX-5093 7350 ± 320c Mo ND Lauriol and Gray [1987] 35 GX-5083 6375 ± 160c Mt ND Lauriol and Gray [1987]

Central Ungava Bay 36 TO-2458 6750 ± 70 c Bi marine at 6500 BP Andrews et al. [1995] 37 TO-2440 7650 ± 70 c Mt minimum age deglaciation Gray et al. [1993] 38 AA-14687 8070 ± 70 c Mc deglaciation Lauriol and Gray [1987] 39 CAMS-18690 8180 ± 60 c Yf ND Manley and Jennings [1996] aFor position and spatial distribution, see Figure 4. Abbreviations are as follows: GSC, dates normalized to ± 1s; Ms, marine shells; C, concretion in lake Ojibway sediment; G, gyttja; Lc, lake clay; Bs, bulk sediment; Os, organic silt; B, bog; Mo, Marine organics; Bi, Bivalve; Ha, Hiatella arctica; Mt, Mya truncata;Mc,Macoma calcarea; Yf, Yoldiella fraternal; Me, Mytilus edulis; ND, no data. bNumber refers to location in Figure 4. cNormalized to 25% d13C (410 year) and corrected for 450 year reservoir effect. dExtrapolation of 14C-derived emergence curves.

[17] The following discussion is based on the assumption lowering of glacial lake Naskaupi. As ice retreated north- of 30 day durations for drainage events. ward it exposed cols along the eastern margin that were lower in elevation than those along the western margin. The 5.2. Glacial Lakes and Meltwater Drainage implication of this asymmetry is that most of the glacial 5.2.1. Between 7.5 and 6.5 kyr BP (8.4––7.0 cal ka) lakes in the interior of the area, existing between 7.5 and [18] Meltwater drainage routes toward the Labrador Sea 6.5 kyr BP, were successively drained eastward along the ice across the Labrador-Ungava/Labrador Sea watershed are margin into glacial lake Naskaupi, which overflowed the associated with glaciofluvial deposits [Klassen et al., 1992; Torngat Mountain watershed. Meltwater pulses from glacial Fulton, 1995], and were first exploited during the successive lake Naskaupi spilled into the Adlatok, Kogaluk, Anaktalik,

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Figure 3. The retreat pattern of the Laurentide Ice Sheet across Labrador-Ungava. Modified from Jansson [2003], reprinted with permission from Elsevier. Black areas show glacial lakes, heavy black lines show major end moraines, light gray areas represent the postglacial marine limit [Vincent, 1989], and eskers are marked with thin black lines [Prest et al., 1968]. Bold numbers show radiocarbon ages in kyr BP and number in brackets shows age in cal ka.

Fraser and Kinguritik valleys, respectively (Figure 2). Lakes along the western flank of the LIS drained through Rivie´re Laforge toward Hudson Bay. Seven releases of lake water exceeding a discharge of 0.015 Sv are inferred to have occurred into the Labrador Sea in the vicinity of Nain during this time period (Table 2 and Figure 2). 5.2.2. Between 6.5 and 6.0 kyr BP (7.0––6.4 cal ka) [19] Soon after 6.5 kyr BP the LIS retreat exposed drainage routes into Hebron Fiord and Nakvak Brook, respectively. Intermediate stages of glacial lake Naskaupi (substages 4–6) and late stages of glacial lakes Caniapiscau (substages 7–10) and McLean (substages 6–7) released 9 pulses of lake water, exceeding 0.015 Sv into the Labrador Sea in the Saglek Bay area (Table 2 and Figure 2). Fillon and Harmes [1982] suggested that deposition of large quantities of suspended sediment by turbid meltwater injections occurred between 8.4 and 6.1 kyr BP in Saglek Fiord. They estimated that the suspended sediments were trans- ported by rivers with a carrying capacity similar to that of the Mississippi River (0.018 Sv) [Baumgartner and Reichel, 1975]. It is possible that six pulses, released from glacial lakes Naskaupi and Caniapiscau, account for some of the meltwater identified by Fillon and Harmes [1982]. Figure 4. Location map for 14C dates in Labrador-Ungava [20] At the end of this time period, open drainage paths (cf. Table 3). The first number refers to the sample location were exposed along the western flank of the Torngat and the second number indicates its age in radiocarbon Mountains. This allowed for eastward drainage, along the years. The area outlined by the broken line is the region ice margin or subglacially, of glacial lake Caniapiscau where nearly all dates fall between 7.0 and 6.0 kyr BP. (substages 11–13) and Naskaupi (substage 7) into eastern However, no clear age gradient emerges from the given Ungava Bay and glacial lake Cambrien (substage 1) into dates in this region and they do not help constrain any southern Ungava Bay. Five pulses of lake water, exceeding particular final retreat pattern.

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0.015 Sv, reached Ungava Bay during this time (Table 2 and occurrence and the d18O[Labeyrie et al., 1999] reflect Figure 2). These events occurred subsequent to drainage variations in the amount of colder water brought to the core into the Saglek Bay area but prior to 6.0 kyr BP and may be site by the Labrador Current. relatedtorapidsedimentationineasternUngavaBay [24] Keigwin and Jones [1995] recognized a peak in the between 6.8 and 6.2 kyr BP [Andrews et al., 1995]). The abundance of N. pachyderma (s).and a minimum in N. last three glacial lakes, Cambrien, Me´le`zes and Minto, pachyderma (s) d18O at 7.1 kyr BP outside Nova Scotia released four meltwater pulses exceeding 0.015 Sv into (Figure 5). Their interpretation of these data as evidence for southern and western Ungava Bay around 6.0 kyr BP a meltwater event originating from the Great Lakes and/or (Table 2), close to the final deglaciation of southern Ungava Labrador-Ungava is supported by our study. The evidence Bay [Lauriol and Gray, 1987; Andrews et al., 1995]). The for increased meltwater discharge during this period is also meltwater release into Ungava Bay may be related to (1) a supported by reconstruction of SST and salinity based on 80 m thick sediment sequence in the marginal channel of dinoflagellate cysts outside the Scotian Shelf (core 91-039- the southern Ungava Bay [MacLean et al., 1991; Andrews 008P, Figure 1), which indicates that a last pulse of et al., 1995] and (2) foraminifera assemblages indicating meltwater affected the shelf between 8.5 and 6.5 kyr BP reduced salinity and high sedimentation rates during the [Levac, 2001]. This meltwater pulse was most probably deposition of these sediments [MacLean et al., 1991]. The transferred to the area of core 91-039-008P trough the outflow of 2000 km3 of water into Ungava Bay at 6.5– Labrador Current [Levac, 2001], whereas an analogous 6.0 kyr BP is likely to have caused early Holocene turbid decrease in SST lacks in the Cabot Strait (core 95-030-24, watershed deposition along with ice-rafted debris along the Figure 1) [de Vernal et al., 1993]. We argue that it is Labrador coast [Josenhans et al., 1986; Wang and Hesse, probable that repeated meltwater pulses released from the 1996]. Hall et al. [1999] suggested that sediment starvation Labrador-Ungava lakes forced at least some of the recorded occurred in the Karlsefni Trough after 6.0 kyr BP, which changes in the SST in the Labrador Sea between 7.5 and may indicate the end of the glacial lake era. 6.4 cal ka. We also note that evidence for ocean surface cooling, during the time of Labrador-Ungava glacial lake 5.3. Proxy Records Indicating Early Holocene drainage, are recorded from eastern North Atlantic [Bond et Meltwater Events al., 1997; Dolven et al., 2002], the area south of Greenland [21] The ice recession between 7.5 and 6.0 kyr BP (8.4– [Hillaire-Marcel et al., 2001], and the area north of Iceland 6.4 cal ka) was accompanied by 40 meltwater releases (Figure 5) [Andrews and Giraudeau, 2003]. It is unclear if from the Labrador-Ungava glacial lakes into the Labrador and how these more distant areas were affected by the Sea and Ungava Bay (Table 2 and Figure 2). The most pulses of meltwater from Labrador-Ungava. intense period of glacial lake drainage is inferred to have [25] Ice cores from the Agassiz Ice Cap, Arctic Canada occurred between 7.0 and 6.0 kyr BP (7.5–6.4 cal ka), [Fisher, 1992; Fisher and Koerner, 2003], show low values with 30 drainage events exceeding 0.015 Sv (calculated on for d18O, increased summer melting, and low methanesul- 30-day durations of discharge events). The majority of the fonate content during the 8.2 cal ka event (Figure 5). These drainage events (Table 2) occurred into the Labrador Sea data are interpreted to reflect a decrease in North Atlantic and Ungava Bay, where the meltwater was probably surface salinity and an increase in meltwater outflow rapidly transferred into the Labrador Current. [Fisher and Koerner, 2003]. Data indicating similar events [22] Results showing a stable surface reservoir age since between approximately 6.0 and 8.0 cal ka coincide with low approximately 9 cal ka in the North Atlantic indicate that a SST and sea-surface salinity in northern Baffin Bay after correction of 400 years for the reservoir effect is appropriate 7.0 cal ka (core 91-039-008P, Figure 1 [Levac et al., 2001], [Waelbroeck et al., 2001]. The lack of terrestrial dates that and may support the view that periodical episodes of narrowly constrain individual drainage events and sparsely meltwater outflow from the Labrador-Ungava glacial lakes dated offshore cores limit the possibility of detailed corre- were of regional climatic significance. This time period also lations between short-duration individual drainage events seems to coincide with repeated decrease in d18O recorded and fluctuations within offshore proxy data. It is, however, in the GISP2 ice core [Grootes and Stuiver, 1997; Grootes by focusing on the 8.4–6.4 cal ka interval, possible to et al., 1993; Meese et al., 1994; Steig et al., 1994; Stuiver et compare the timing of increased flooding from Labrador- al., 1995], and in high latitude temperatures reconstructed Ungava with variations in different proxy records. from the Greenland Summit ice cores (Figure 5) [Cuffey [23] The Labrador-Ungava drainage events seem to coin- and Cow, 1997; Andrews and Giraudeau, 2003]. The cide with a period of fluctuating SST in the southwestern association between the Labrador-Ungava meltwater events Labrador Sea. Labeyrie et al. [1999] and Waelbroeck et al. and variations in SST and ice core proxies seems to support [2001] have shown, by reconstructions from planktonic the view that the North Atlantic ocean surface and the foraminifera abundances, that repeated cooling of SST atmosphere above Greenland and possibly Arctic Canada occurred during the 7.5–6.4 cal ka time period, with a were a coupled system [Bond et al., 1997]. major decrease of between 8 and 15C at approximately 7 cal ka (Figure 5). Also the N. pachyderma sinistral (s) 5.4. Implications for NADW Formation 18 coiling d O record from core CH69-K09 (Figure 1) shows [26] The THC and formation of NADW are sensitive to notable fluctuations during the same time interval [Labeyrie freshwater inflow [Keigwin et al., 1991; Rahmstorf, 1995; et al., 1999]. As the N. pachyderma (s) is an exotic species Licciardi et al., 1999; Marshall and Clark, 1999]. For at the core site it is possible that the fluctuations in its example, several studies suggest that the Younger Dryas

8of12 PA1001 JANSSON AND KLEMAN: EARLY HOLOCENE GLACIAL LAKE MELTWATER PA1001 and the 8.2 cal ka events were induced by meltwater forcing 2002]. The meltwater volume released from the Labrador- [e.g., Broecker et al., 1988, 1989; Keigwin et al., 1991; Ungava glacial lakes, between 7.5 and 6.4 cal ka, compared Clark et al., 2001] corresponding to discharges of 0.3 Sv and to glacial lakes Agassiz and Ojibway at 8.5 cal ka was 0.06 Sv, respectively [Barber et al., 1999; Teller et al., considerable smaller (Figure 2). Here, we present new evidence that the 7.5 to 6.0 kyr BP (8.4–6.4 cal ka) time period, a period previously supposed to lack the imprint of significant meltwater events, and was characterized by multiple meltwater events, although of relatively small magnitude. [27] Models evaluating the sensitivity of the THC and the NADW formation to freshwater forcing indicate a high sensitivity to short duration freshwater events (10 years) with high fluxes (1 Sv) [Manabe and Stouffer, 1995], long duration events (200–500 years) with low fluxes (0.015 Sv) [Rahmstorf, 1995; Ganopolski and Rahmstorf, 2001] and intermediate-scale events (150–300 years 0.5 Sv) [Rind et al., 2001]. Model results have shown that relatively small and short-lived meltwater pulses in order of 0.3 Sv during 50 years were sufficient to cause weakening of the THC and to trigger the 8.2 cal ka cooling event [Renssen et al., 2002], but no modeling effort has yet explicitly considered the type of meltwater forcing we report here; up to 40 individual drainage events within a 1.8 kyr time span. It has been suggested that a pulsed nature of meltwater forcing [Fanning and Weaver, 1997], and vicinity of meltwater injections to NADW formation sites [Fanning and Weaver, 1997; Licciardi et al., 1999; Seidov and Maslin, 1999] may increase the responsiveness of the THC. We regard it as possible that the numerous short-duration events from the Labrador-Ungava lakes may have increased the sensitivity of NADW formation to

Figure 5. Comparison between the most intense period of glacial lake drainage from Labrador-Ungava (7.5–6.4 cal ka (in gray)) and different proxy data from North Atlantic Ocean. Broken line shows the 8.2 cal ka cooling event [Barber et al., 1999]. For location of core sites, see Figure 1. (a) Temperature reconstruction at Summit, Greenland [Cuffey and Cow, 1997; Andrews and Giraudeau, 2003]. (b) GISP2 data [Grootes and Stuiver, 1997; Grootes et al., 1993; Meese et al., 1994; Steig et al., 1994; Stuiver et al., 1995]. (c) Core A 84, Agassiz Ice Cap, Ellesmere Island, Arctic Canada [Fisher, 1982]. (d) Reconstruction of SST from core CH- 69-09 [Waelbroeck et al., 2001]. (e) Reconstruction of SST from core CH-69-K09 [Labeyrie et al., 1999]. (f) Relative abundance and d18O record of N. Pachyderma in core HU73- 031-7 [Keigwin and Jones, 1995]. Original timescale has been converted to calendar years. (g) The occurrence of coccolith species associated with Atlantic water [Andrews and Giraudeau, 2003] in core B-997-330. (h) The benthic d13C from core ODP 980. Low values indicate reduced NADW contribution [Oppo et al., 2003]. (i) EOF1 (polar circulation index) from GISP2. Positive values indicate an expansion of the north polar vortex [O’brien et al., 1995]. (j) Reconstructed SST from Norwegian Sea, core MD95- 2011 [Dolven et al., 2002]. (k) Relative abundance of G. quinqueloba (cores VM 29-191 and VM 28-14) is inferred to be the most consistent evidence for ocean surface cooling [Bond et al., 1997].

9of12 PA1001 JANSSON AND KLEMAN: EARLY HOLOCENE GLACIAL LAKE MELTWATER PA1001 freshwater forcing. The increased sensitivity of the THC these glacial lakes, and to frequent meltwater drainage into during the Labrador-Ungava drainage events may be the Labrador Sea, Ungava Bay and Hudson Bay (30 marked by a main trend of decrease in NADW contribution events >0.015 Sv). These drainage events were probably to deep water that began at 6.5 cal ka [Oppo et al., 2003], short-lived because of the inferred rapid ice margin retreat at changes in atmospheric circulation over Greenland this late stage of the deglaciation. Combined, these early [O’brien et al., 1995] between 6.5 and 5.0 kyr BP and Holocene glacial lakes of Labrador-Ungava released the establishment of colder bottom water conditions asso- 6000 km3 of meltwater. The pulsed nature of these ciated with Northwest Atlantic Deep Water as late as meltwater events is likely to have caused rapid and repeated 5.7 cal ka (Figure 5) [Bilodeau et al., 1994]. However, cooling of the Labrador Sea surface water. the effects of Labrador-Ungava drainage events on the THC and NADW formation need to be further evaluated by, for [29] Acknowledgments. Financial support for this work was provided example, new studies that strengthen the chronological through grants from the Swedish Research Council to J. Kleman. Support from the Royal Physiographic Society in Lund, the Royal Swedish Academy control on both the terrestrial and the marine record. of Sciences, and Carl Mannerfelts fund to K. Jansson is gratefully acknowl- edged. We also wish to thank Arjen Stroeven, Clas Ha¨ttestrand, and Gunhild Rosqvist at the Department of Physical Geography and Quaternary Geology, 6. Conclusions Stockholm University, and Neil Glasser and James Etienne at the Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, for [28] Numerous glacial lakes existed in Labrador-Ungava constructive comments on the manuscript. We are grateful to referee Bob during the last deglaciation. The LIS ice recession between Oglesby and two anonymous referees for their helpful comments that 7.5 and 6 kyr BP (8.4–6.4 cal ka) led to the formation of improved this paper.

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