Early Holocene Glacial Lake Meltwater Injections Into the Labrador Sea and Ungava Bay Krister N
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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 sÀ1), 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 PA1001 1of12 PA1001 JANSSON AND KLEMAN: EARLY HOLOCENE GLACIAL LAKE MELTWATER PA1001 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)).