Deglaciation ages and meltwater routing in the Fort McMurray region, northeastern Alberta and northwestern Saskatchewan, Canada

Quaternary Science Reviews 28 (2009) 1608–1624

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Deglaciation ages and meltwater routing in the Fort McMurray region, northeastern Alberta and northwestern Saskatchewan, Canada Timothy G. Fisher a, *, Nickolas Waterson b, Thomas V. Lowell b, Irka Hajdas c a

Department of Environmental Sciences, MS #604, University of Toledo, 2801 W. Bancroft Street, Toledo, OH 43606, USA Department of Geology, University of Cincinnati, Cincinnati, OH 45226, USA c Ion Beam Physics, Paul Scherrer Institute and ETH Zurich, 8093 Zurich, Switzerland b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 October 2008 Received in revised form 27 January 2009 Accepted 4 February 2009

A field-based reconstruction of the deglacial paleogeography in the Fort McMurray area permits: 1) constraining the timing of meltwater routing to the Arctic from the present Hudson Bay drainage basin; and 2) minimum-age estimates for ice-margin positions that can be used to constrain ice-sheet modeling results. A downslope recession of the Laurentide Ice Sheet resulted in a series of proglacial lakes forming between the ice margin and higher land to the southwest. The paleogeography of these lakes is poorly constrained in part from the masking effect of boreal forest vegetation and map-scale issues. However, recent space-shuttle based DEMs increase the number and spatial extent of moraines identified within the study area resulting in a coherent pattern of ice margin retreat focused on the Athabasca River valley. An intensive lake-coring program resulted in a minimum ten-fold increase in the radiocarbon database used to limit moraine ages. Results indicate that deglaciation in this region was younger than previously reported, and it is likely that the meltwater could not drain northward to the Arctic Ocean from any source southeast of the Fort McMurray area until approximately 9850–9660 14C BP. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction The spatial and temporal patterns of deglaciation of the Laurentide Ice Sheet (LIS) in the mid-continent directly controlled the ‘where and when’ of freshwater routing to the oceans surrounding North America. Such meltwater fluxes had impacts on the ocean including the rise of sea level and perhaps even altered ocean circulation patterns. It has been proposed that changes in meltwater routing influenced past climate change (e.g., Rooth, 1982; Broecker et al., 1989; Clark et al., 2001; Teller and Leverington, 2004; Tarasov and Peltier, 2005). Glacial Lake Agassiz is the often implicated agent because from its location in the middle of North America water could potentially be delivered to the Arctic Ocean, Hudson Bay, North Atlantic via the St. Lawrence, and the Gulf of Mexico (Fig. 1). All of these possibilities depend upon the timing of ice sheet recession. Past studies have focused on the synchronicity between the timing of abrupt climate change, as recorded in glacier and ocean cores, and suggested drainage timing from Lake Agassiz (e.g., Broecker et al., 1989; Fisher et al., 2002; Teller et al., 2002; Teller and Leverington, 2004; Fisher, 2007). However, for many areas the chronology controlling the ice sheet margin position and

* Corresponding author. Tel.: þ1 419 530 2883; fax: þ1 419 530 4421. E-mail address: timothy.fi[email protected] (T.G. Fisher). 0277-3791/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2009.02.003

outlet history are often only based on a few data points. Recent results from studies directly addressing ice margin and outlet history by Fisher (2003), Lowell et al. (2005), Teller et al. (2005), and Fisher et al. (2008) questioned the timing and location of the routing from glacial Lake Agassiz during the beginning of the Younger Dryas event. To date, a meltwater outlet from Lake Agassiz open at the beginning of the Younger Dryas event has not been documented. For over a century it had been assumed to drain eastward to explain a low-stand in the main Agassiz basin (cf., Upham, 1895; Johnston, 1946; Elson, 1967; Teller and Thorleifson, 1983; Teller et al., 2005). Preliminary results constraining the age of deglaciation in the eastern outlets region, which would allow meltwater drainage eastward at the onset of the Younger Dryas cold event, were described by Teller et al. (2005) and Lowell et al. (2005), and now with greater detail presented in Lowell et al. (2009). These results suggest any such drainage eastward this early is problematic. Here we use a similar approach of constraining the age of deglaciation in the Fort McMurray, Alberta, region to track ice recession to a key chokepoint in the Athabasca River valley. This chokepoint would have controlled drainage of glacial Lake Agassiz or any meltwater from the present Hudson Bay drainage basin northwest into the Arctic Ocean. A major geomorphic feature in northwest Saskatchewan known as Clearwater Lower Athabasca Spillway (CLAS; Fig. 1A) records meltwater passage at some time through the area. Fisher (2007)

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Fig. 1. Location maps with former glacial lake outlines. A) Old ‘bulk’ radiocarbon ages expressed in radiocarbon years are from the Birch Hills (Vance, 1986) Mariana Lake on southwest Stoney Mountain (Hutton et al., 1994), Lofty Lake southeast of Pelican Mountain (Lichti-Federovich, 1970), Long Lake in the head of the Clearwater Lower Athabasca spillway (CLAS) along the Clearwater River (Anderson and Lewis, 1992), and Nipawin Bay (Anderson and Lewis, 1992). The presence of an extensive glacial lake in the Churchill River valley is show here at a maximum level as recorded by strandlines and glaciolacustrine sediment. Once drainage of the CLAS was initiated, water level dropped to its minimum level until controlled by sills by Wycherely Lake supplying water to Lake Wagtufro (Fisher and Souch, 1998). B) Generalized location map of glacial Lake Agassiz (A) dammed by the Laurentide Ice Sheet (LIS). Note also location of the lakes southern outlet (SO), eastern outlet (EO), and northwestern outlet (NWO).

used minimum ages from scour lakes within the CLAS and adjacent landscape to conclude that the CLAS was abandoned between 9.6 and 9.1 14C ka BP (10.81–10.22 cal ka BP), which can be considered a minimum estimate for deglaciation to locations near the spillway’s head. The strategy employed in this paper is to improve regional deglaciation chronology by dating cored lakes on either side of moraines recording former ice sheet margins. Sites were chosen along transects. To the east of the Fort McMurray area lies the present day drainage divide between Hudson Bay and the Arctic Ocean, in an area that must have been deglaciated before any water body in the upper Churchill River valley could gain access to the

Athabasca River valley (Fig. 1A). During deglaciation the topography allowed meltwater and surface water to pond and form proglacial lakes on either side of the Alberta–Saskatchewan Provincial boundary as evidenced by high elevation strandlines and glaciolacustrine deposits (Fisher and Smith, 1994). The full extent of these lakes and any connection to the main Lake Agassiz basin to the southeast remains under investigation. Over the broader regional area of northern Alberta, British Columbia and southwestern Northwest Territories, outlets for proglacial lakes predating lakes in the Fort McMurray, Alberta, region are not well understood. This includes the chronology and paleogeography of drainage

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northwards to the Mackenzie River and southwards to the Missouri River from glacial lakes west of the study area (Mathews, 1980; Lemmen et al., 1994). Similarly, drainage from ice dammed lakes associated with the Athabasca River before the formation of glacial Lake McMurray (Fig. 1) are unknown, but possibly was northwestwards through the gap at the southwest end of the Birch Mountains into glacial Lake Peace. Lakes in western Saskatchewan may have drained south through a large spillway near Big River (Fig. 1). The objectives of this paper are: 1) to determine when proglacial lakes in the Fort McMurray, Alberta, area could have drained north, and briefly allow drainage from the present Hudson River drainage basin to the northwest; and 2) to determine when the present northerly drainage of the Athabasca and Clearwater River was established. 2. Regional setting 2.1. Study area Quaternary sediments in the study area range in thickness from 0 to 720 m and unconformably overlie Phanerzoic rocks resting unconformably on granites and gneisses of the Archean-aged Precambrian Shield (McConville, 1975). To the northeast and southeast of the head of the CLAS (Fig. 1), the Quaternary sediment is thin to non-existent directly over shield rock (Schreiner, 1984). The Phanerozoic rocks are composed chiefly of Paleozoic carbonates and Mesozoic sandstones, siltstones, and shales, and the Athabasca tar sands are within these sandstones (Tremblay, 1960; Norris, 1963; Flach, 1984). The Athabasca River and Clearwater River have eroded through the Quaternary sediments exposing the McMurray Formation tar sands for well over 100 miles. Physiographically, the study area consists of numerous uplands surrounded by the Athabasca and Clearwater lowlands (Fig. 1). The uplands (e.g., Grizzly Bear Hills, Stoney, Birch, and Muskeg Mountains) are eroded bedrock remnants composed of Cretaceous shales and sandstones (Fenton et al., 2004) in places overlain by thick Quaternary-aged sediment (McPherson and Kathol, 1977). On top of the uplands Paulen et al. (2005), Fenton et al. (2004), and Pawlowicz and Fenton (1995) used dated sediment cores and geophysical profiles to describe preserved sediments from multiple glaciations lying partly in buried valleys. The lowlands are flat to rolling plains with a veneer of glaciolacustrine sediment, cut by the northward trending Athabasca valley, and westward trending Clearwater valley. These valleys were likely partially eroded prior to the first glacial advance to the region (Fenton et al., 2004). Furthermore, some of the irregular topography, both depressions and hills with linear edges, are likely glaciotectonic in origin (cf. Fenton and Pawlowicz, 2000). The Fort McMurray, Alberta, townsite is located at the confluence of the Athabasca and Clearwater Rivers in northeast Alberta (Fig. 1). One predominate landform is the 170 km long channel locally known as the CLAS. At its head, numerous streamlined hills are separated by channels cut into sedimentary and shield rock at multiple elevations. From its head the spillway extends west to the confluence with the Athabasca River then heads north for 75 km ending as wide channels cut across the Fort Hills (Fig. 2B). Sediment was deposited into glacial Lake McConnell forming a delta (Rhine and Smith, 1988). 2.2. Existing glacial chronology Numerous past studies and coring by the Alberta Geological Survey has resulted in a number of dated sites in the study area. Older ages come from Middle-Wisconsin interstadial materials consisting of organic sediment between tills from a borehole in the Birch Mountains. Dated sediment includes pine wood fragments

with an AMS age of 32.69  0.34 14C ka BP (TO-10545), and a >50.0 14C ka BP from organic detritus (BGS-2585) (Paulen et al., 2005) indicating ice free conditions at that time. The deglacial chronology in the Fort McMurray, Alberta area has been based on five radiocarbon dates of bulk-sediment samples shown in Fig. 1 (Lichti-Federovich, 1970; Vance, 1986; Anderson and Lewis, 1992; Hutton et al., 1994). The deglacial chronology reconstruction by Dyke et al. (2003) updating the original work by Dyke and Prest (1987) suggests that the western half of the study area was ice free between 12 and 11 14C ka BP with the Cree Lake Moraine dating to 10 14C ka BP. It is important to note that Dyke et al. (2003) developed isochrones in this region based upon minimum dates listed here within the context of regional considerations, many of which were not collected for the purpose of constraining ice-margin chronology (e.g., Fig. 1A). A younger deglaciation was more recently suggested from younger ages summarized in Fisher (2007). Because accurate ice-margin chronologies are necessary for constraining meltwater routing and timing, and are primary data for ice sheet models, we present 72 new radiocarbon dates from 31 different core sites to further constrain the ice-margin chronology. 3. Materials and methodology The minimum-age problem (Clayton and Moran, 1982) has long been recognized. It is cast as the poorly known lag time between retreat of the ice margin and vegetative colonization adjacent to and within a water body. It is thought to be exacerbated within kettles (e.g., Florin and Wright, 1969). A second complication in assigning chronologies is ‘bulk’ dates, artificially older than deglaciation if there is incorporation of radiometrically dead carbon (or a hard-water effect; e.g., Teller, 1989; MacDonald et al., 1991; Birks, 2001), or artificially younger than deglaciation if bulk sediment is collected over decimeters of core to accumulate sufficient carbon. To address the ‘minimum-age’ problem, we mostly targeted small, non-kettle lakes in bedrock basins or channels. At those locations we cored near their edges to improve chances of recovering terrestrial macrofossils. Where possible, core sites are adjacent to moraines for the specific purpose of generating chronological constraints on the ages of moraines. Moraines were mapped using the new SRTM digital elevation models (DEM) available through the United States Geological Survey (URL: http://seamless.usgs.gov/). The new DEM offered greater vertical resolution than the existing 1:50,000 topographic maps. All sites (Fig. 2) were accessed by helicopter in December/ January 2004 and March 2005. A hydraulically assisted, modified square-rod Livingston corer was used from lake-ice platforms. The hydraulic assist permits core recovery directly from target depths determined by probing with field-tile probe rods. Consequently, the uppermost sediment from each core site was not recovered, but instead, and depending upon distance between sites, 2–4 sites could be cored each day. In practice at each site we recovered one longer core to establish the site stratigraphy, and then 2–4 additional cores just across critical contacts to provide sufficient material for radiocarbon analysis. Five sites were revisited and cored with a piston-equipped vibracorer (Fisher, 2004) to penetrate further into the basal sediment. Core sites are listed in Table 1, and core stratigraphy is shown in Fig. 3. Laboratory analysis included loss-on-ignition (LOI) and volume magnetic susceptibility (VMS) in 105 S.I. units. Procedures for determining percent organic matter (550  C) and percent carbonate (1000  C) were from Heiri et al. (2001). Magnetic susceptibility was determined with a Bartington MS2E surface scanning sensor at 2 cm intervals, and used as a proxy for allocthonous terrigenous clastic material in the lake sediment.

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Fig. 2. A) Numbered coring sites and newly mapped moraines from a digital elevation model based on SRTM data. Most of these moraines were not previously mapped. B) Inset map of the region around the Firebag Moraine. Lighter tones represent higher elevations.

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Table 1 Coring sites. Numbers correspond to locations in Fig. 2. Latitude ( N)

Site 401 402 403 404 405 406 407 408 410 411 413 417 418 419 536 538 539 540 541 542 543 544 545 546 548 549 551 552 553 554 555

Hanging Stone Cabin Lake Crescent Lake Suncor Shield Site 1 Shield Site 2 Lower Grizzly Upper Grizzly Firebag 1 Inner Clearwater Vogulsang Lake Christina Spillway Hook Lake Deep Hole Lake Mariana Lake Sick Hill South Lake Don’s Lake Richardson Lake Esker Kettle Lake Johnson Lake Schwan Lake Dipper Lake White Cow Lake Survive Lake Twin Kettle Lake Seven Lake Long Lake Kearl Lake Round Lake Sandy Bog Lake Sweetheart Lake



56.2079 56.1272 57.4152 57.287 56.9275 56.8227 55.9307 55.9829 57.4205 57.1796 57.159 56.3829 56.3255 56.2953 55.94913 57.1315 57.2662 57.89328 57.9546 57.57592 56.36338 56.43922 56.66225 57.26107 57.42262 57.47378 57.0671 57.29007 56.78945 56.3178 56.23683

Longitude ( W)

Elevation m a.s.l.

111.4276 111.3557 111.0258 110.926 107.9701 107.9386 109.135 109.135 110.7753 110.7828 108.7518 110.286 110.8572 111.0422 112.02582 113.38413 112.59705 110.70977 110.45588 110.3397 113.33825 113.35017 113.33605 112.28315 110.94837 110.0613 110.8291 111.2466 111.86098 111.92933 111.43388

706 729 338 562 511 509 421 610 376 453 517 419 498 567 691 645 562 318 349 553 589 569 514 539 368 522 574 327 526 557 695

Most organic samples recovered from cores consisted of fragments. C3 land plants have d13C value of 26 to 28& (Meyers, 1997) and here are considered terrigenous whether they are terrestrial plants or emergent, aquatic plants that derive their carbon from the atmosphere. Samples with d13C values greater than 20& (e.g., 18&) are considered aquatic with their carbon sourced from lake water and potentially exposed to hard-water effects. The radiocarbon dates were determined at ETH Zurich (Swiss Federal Institute of Technology) except for a few dates from BETA Analytic in Florida. Radiocarbon dates were calibrated using Calib V5.0 (Stuiver and Reimer, 1993) with the IntCal 04 terrestrial calibration curve (Reimer et al., 2004). Probability plots of the resulting calendar ages are used in the time–distance figures. Construction of the time–distance diagram assumed all ages are minimum, thus a constraining envelope was placed where the individual site ages dropped to zero probability. 4. Results 4.1. Newly mapped moraines The geometry of ice-margin recession in the study area varied with overall ice recession from the southwest to the northeast. From analysis of the new SRTM DEM many new moraines were identified, but the lack of road access and winter coring did not permit direct ground inspection (Fig. 2), however most were verified with aerial reconnaissance. The description here focuses on the lower elevation moraines where ice margins would have dammed drainage ways to form proglacial lakes. The geometry of nested and looped moraines along the Athabasca valley suggests that during retreat, the ice formed lobate margins between uplands in the Athabasca lowland. Note the left-lateral moraines on the northwest side of Muskeg Mountain and Birch Mountain and the large kame moraine in the Athabasca River valley associated with the Firebag Moraine (cf.,

McPherson and Kathol, 1977) (Figs. 1 and 2). The distal end of the CLAS cuts this moraine recording drainage of a glacial lake. The ice margin in the Clearwater lowland was not as lobate. The Beaver River Moraine was mapped by Christiansen (1979) and Schreiner (1984) on the east side of glacial Meadow Lake (Fig. 1). In Fig. 2 it is shown only as a dashed line as it is not topographically well expressed in the study area. Further east, the Cree lake Moraine trends northwest-southeast and is a large sand and gravel (Jol et al., 1996) complex consisting of many parallel ridges, with large kames fed by eskers. Thus, in Saskatchewan the ice sheet maintained its broad front, while in northeastern Alberta, the greater topographic relief presumably resulted in a more lobate ice margin during recession. 4.2. Deglaciation chronology Minimum ages are assigned to moraines using basal dates from cores immediately adjacent to the moraine. Minimum-age estimates distal to a given moraine can also serve as a maximum age for the next younger moraine in the sequence. Because selected sites are small basins and dated wood is of terrestrial origin (in most cases), the ages of the oldest material are taken to represent the arrival of plants at the site after deglaciation. In some cases the enclosing sediment indicates mass movement, which could potentially produce ages not in strict stratigraphic order. Thus the oldest age, taken to represent the tightest bracket on deglacation, may not be in the very lowest stratigraphic position. The calibrated assigned radiocarbon ages for moraines are shown on a time– distance diagram (Fig. 4). This plot defines an envelope that must be younger than deglaciation and with that caveat a minimum rate of retreat. The location of coring sites is in Table 1. Table 2 lists the new radiocarbon ages, and Fig. 3 contains the lithostratigraphic logs of the cores. Below we describe the basis for this chronology in detail starting with the older moraines. 4.2.1. Athabasca lowland 4.2.1.1. Don’s Moraine. On the distal side of Don’s Moraine are seven sites used to estimate the maximum age of the moraine (Fig. 2). From south to north in a clockwise direction they are: sites 536, 554, 543, 544, 545, 538, and 539 (Figs. 2, 3, Table 1). Site 536 is Mariana Lake with a basal stratigraphy of silty clay changing upwards to silt and then gyttja, with a thin bed of sand within each unit. One date on wood from the lower silty clay is 10.31  0.08 14C ka BP. Site 554 is Sandy Bog Lake with a stratigraphy of thick basal sand overlain by laminated silt and organics with a thin sand horizon near the top of the core. There is one age on peat fragments dated at 37.82  0.52 14C ka BP from the middle of the basal sand unit. This and other ‘older’ ages are discussed below in Section 5.1. At site 543 (Schwan Lake), basal sand is overlain by silty clay, and then marl alternating with peat near the top of the core. A date from the basal silty clay of 8.2  0.07 14C ka BP is a poor constraint on the age of the moraine. Site 544 (Dipper Lake) contains sand and laminated silt overlying basal silty clay. An age of 12.78  0.09 14C ka BP is from woody organic fragments in laminated silts. At site 545 (White Cow Lake), diamicton is overlain by laminated silt and clay, massive silt and marl. There are two ages from just above the laminated sediment: 10.15  0.07 and 10.14  0.07 14C ka BP but the d13C values (Table 2) suggest that aquatic macrofossils were dated. The laminated sediment contains sand lenses, pebbles, intraformational conglomerates (‘rip-up’ clasts) and rhythmically laminated light and dark couplets, in places tilted, which could be explained as deposits from sediment gravity flows (cf. Lawson, 1979), proximal glacioclacustrine sediments, or both. Laminations vary in thickness from 1 to 8 mm and number approximately 250. If many of these are varves,

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Fig. 3. Lithostratigraphic logs of coring sites shown in Fig. 2. LOI refers to loss-on-ignition of sample at 550  C and 1000  C. VMS refers to volume magnetic susceptibility, a measure of clastic sediment.

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Fig. 3. (continued).

T.G. Fisher et al. / Quaternary Science Reviews 28 (2009) 1608–1624

Fig. 3. (continued).

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Fig. 4. Time–distance diagram for ice retreat in (A) the Athabasca lowland, and (B) Clearwater lowland. The connection line of the major ice margins represents a minimum age for recession. The dates shown are only the closest constraining ages, not every date from Table 2. The individual probability plots reflect conversion of radiocarbon years to calendar years.

then there are approximately 250 years of sedimentation once ice retreated from this lake basin. Site 538 (Sick Hill South Lake) stratigraphy consists of basal sand overlain by gyttja and peat with thin interbeds of sand. One age of 7.91  0.06 14C ka BP is too young to constrain the age of the moraine. The last site distal to Don’s Moraine is site 539 (Don’s Lake) just south of Birch Mountain. The core stratigraphy consists of laminated sandy silt overlying a grey, silt-clay diamicton, and underlying a transition zone of alternating gyttja and silt. From the base of the mostly inorganic laminated sandy silt are two ages on woody material of 10.21  0.07 and 10.46  0.07 14C ka BP. Assuming the diamicton from Don’s Lake is till or flow tills from the ice margin deposited during retreat, these ages record the presence of terrestrial vegetation on the adjacent landscape during deglaciation. From these seven sites the oldest age from Dipper Lake is on higher topography and is considerably older than the other sites. Here we consider the Dipper Lake age of 12.78 14C ka BP as

a maximum age for Don’s Moraine. The two best limiting ages for Don’s Moraine are from Mariana Lake and Don’s Lake, since the Don’s Lake site is slightly older we pick it as the closest limiting age. Here we suggest Don’s moraine formed about 10.46  0.07 14C ka BP. 4.2.1.2. Survive Moraine. Survive Moraine is 25–50 km proximal (inside) to Don’s Moraine; closer to the Athabasca River. Its age is constrained at two cores sites 55 km apart, both distal to the moraine, with site 546 (Survive Lake) located at the north end and site 553 (Round Lake) at the south end. The stratigraphy at Survive Lake is grey sand overlying diamicton, in turn overlain by organicrich silt then gyttja. Two ages from the base of the sand immediately above the diamicton are 9.85  0.07 14C ka BP on charcoal and 9.7  0.07 14C ka BP on wood. At Round Lake the stratigraphy is quite different; consisting of interbedded, rhythmically laminated, organic-rich, silty-clay units and peaty-gyttja units. Using

Table 2 Radiocarbon ages used to establish the deglaciation chronology of the Fort McMurray Area. Site numbers correspond to those in Table 1 and Fig. 2. Site

Lab #

14

error ()

d13C (&)

error ()

Material (cm)

Depth (cm)

Calendar age (2s range)

Prob.

401

ETH-30183 ETH-28524 ETH-29216 ETH-29215 ETH-29207 ETH-29206 ETH-28526 ETH-28525 ETH-30172 ETH-30173 ETH-30185 Beta-200071 Beta-200072 ETH-29208 ETH-29210 ETH-29209 ETH-28528 ETH-28527 ETH-30174 Beta-194056 ETH-30186 ETH-28530 Beta-194057 ETH-28529

8705 9635 34,980 42,540 9705 9710 9730 9795 9835 9940 9985 10,030 10,040 9090 9180 9290 9295 9335 9595 9660 8365 8660 8850 8975

70 75 390 780 75 70 65 75 75 70 110 50 50 70 70 70 70 70 70 40 75 70 40 60

17.5 26.8 23.1 25.3 28.1 28 25.9 26.3 28.9 32.8 18.6 28.1 27.6 27.7 30.1 29.5 26 23.1 25.9 28.5 22.2 24.2 27.1 26.6

1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.1 1.2 1.2 1.2

plant charcoal wood wood wood wood wood wood wood plant wood wood wood wood wood wood wood wood wood wood wood wood wood wood

280 280 280 280 965 895 895 965 895 895 895 895 895 428 428 435 435 428 450 435 267 820 820 825

9914–9534 11200–10750

0.99 1.0

ETH-29211 Beta-194055

9000 9520

70 40

27.3 26.5

1.2

wood wood

825 836

ETH-30175 ETH-29223 ETH-29222 ETH-29220 ETH-29217 ETH-29218 Beta-194060

9665 7730 7800 8345 9485 9745 8960

75 65 65 70 70 70 40

31.5 23.7 26.3 22.9 27 29.1 27.1

1.2 1.7 1.2 1.2 1.2 1.2

wood wood wood wood wood wood wood

860 327 329 340 905 918 732

ETH-29212 ETH-28532 ETH-29213 ETH-28531 ETH-29214 ETH-28533 Beta-194061 ETH-28534 ETH-29221 ETH-29219 ETH-30176 ETH-30177 ETH-28536 ETH-28535 ETH-30187 Beta-194059 Beta-194058 ETH-32165 ETH-30584 ETH-30585 ETH-30586 ETH-30587 ETH-30588 ETH-30589 ETH-32322 ETH-32166 ETH-32323 ETH-32324 ETH-30590 ETH-30591 ETH-30592 ETH-32325 ETH-30593 ETH-32326 ETH-30594 ETH-30595 ETH-32327 ETH-32167 ETH-30596

7340 7445 7450 7465 3755 3905 19,250 7760 8895 10,020 9375 10,030 9795 9820 9910 10,080 10,270 10,310 7910 10,210 10,460 9510 8330 8770 8205 12,780 10,150 10,140 9850 9730 8885 4545 9535 9395 9860 9730 8675 37,820 9625

65 65 65 60 55 50 80 65 70 75 70 75 70 70 75 40 50 75 55 65 65 70 60 60 65 95 70 70 65 65 60 55 65 75 65 65 110 520 60

24.7 26.2 24.8 23.9 26.1 26.2 22.6 29 20.9 26.7 23.6 23.2 26.9 23.1 25.8 25.5 25.6 27 18.5 30.4 26.4 22.3 23.5 26.4 15.9 23.5 17.4 13.9 25.8 26.9 29.9 18.9 28.5 20.3 30 31.7 18.1 22.1 25.9

1.2 1.2 1.2 1.2 1.2 1.2

wood plant wood wood wood charcoal wood wood wood wood wood wood wood wood wood wood wood wood seed pods wood wood wood wood wood seed pods organic pcs seed pods seed pods charcoal wood wood wood wood seed pods wood moss plant frags peaty frags wood

620 620 620 620 610 605 258 397 404 471 432 490 1038 1018 1093 1038 1038 1091 280 786 781 984 1057 439 343–349 334–338 257 257 1205–1207 1205–1207 1048 235 386 320 1273 1272 1225 396–398 696

11250–11060 11250–11060 11260–11070 11400–11070 11410–11100 11640–11220 11830–11220 11760–11300 11770–11310 10440–10150 10520–10230 10610–10260 10660–10270 10720–10370 11170–10730 11200–11070 9520–9230 9830–9530 10160–9760 10100–9910 10240–10110 10270–9900 10880–10680 11080–10930 11220–10770 8610–8400 8780–8420 9490–9140 10900–10570 11270–11070 10070–9920 10230–10120 8320–8020 8390–8160 8400–8160 8390–8180 4300–3960 4440–4220 23290–22530 8650–8410 10200–9740 11820–11250 10780–10370 11830–11260 11370–11080 11410–11090 11620–11200 11830–11400 12240–11820 12400–11820 8810–8600 12160–11690 12700–12120 11100–10640 9480–9200 9950–9550 9320–9010 15440–14730 12060–11590 12050–11590 11410–11160 11250–11070 10190–9770 5330–5040 11130–10660 10800–10400 11410–11170 11250–11070 9950–9480

0.61 0.67 0.8 0.95 0.94 0.98 0.95 0.99 0.99 0.96 0.99 0.94 1.0 0.94 1.0 0.59 0.93 0.96 0.99 0.47 0.53 1.0 0.56 0.44 1.0 0.98 0.99 1.0 0.70 0.81 0.46 0.54 1.0 0.98 0.99 1.0 0.98 0.91 1.0 0.99 1.0 0.99 0.99 0.98 0.96 0.99 0.99 0.97 0.97 0.97 0.67 0.97 1.0 0.95 0.96 0.91 0.95 1.0 0.90 0.87 0.96 0.79 0.99 0.94 0.99 0.95 0.94 0.79 0.93

11180–10760

1.0

402

403

404 405

406

407 408 410

411 413 417 418

419

536 538 539 540 541 542 543 544 545 546 548 549 551 552 553

554 555

C age (yr BP)

1.2 1.2 1.2 1.2 1.1 1.2 1.2 1.2 1.2

1.2 1.2 1.2 1.2 1.2 1.2 1.1 1.2

1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2

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Fig. 5. A) Low altitude, oblique aerial photograph of site 403 Crescent Lake towards the east. B) Coring site with western meltwater channel wall behind smoke from the fire.

a microscope, over 500 laminations were counted with no discontinuities observed. At 1225 cm depth a break in grain size, from sandy-silt laminations below, to clayey silt above was dated at 8.68  0.11 14C ka BP. Two additional ages from deeper in the core are 9.73  0.07 14C ka BP on a moss bed, and 9.86  0.07 14C ka BP on wood. The oldest ages from both sites are statistically the same, thus the minimum-age assignment for Survive Moraine is suggested to be 9.86  0.07 14C ka BP, and the maximum age from site 539 (Don’s Lake) is 10.5  0.07 14C ka BP. 4.2.1.3. Firebag Moraine. The Firebag Moraine is located east of the Athabasca River, just north of the Fort Hills. Based on the east-west trend of the moraine, the ice margin extended across the Athabasca River valley and abutted against Birch Mountain. Thus, only after retreat from this ice-marginal position could any proglacially dammed water drain to the north along the present waterway. Large fluvial channels that dissect the Fort Hills are the distal end of the CLAS and are the evidence for a flood following glacier recession from the Firebag Moraine. There is a coring transect of 11 sites from the northwest slope of Muskeg Mountain downslope and northwards across the Firebag Moraine. Because some sites could not be dated their lithostratigraphy is not presented here. Of the remaining sites distal to the Firebag Moraine, five of them (sites 404, 410, 542, 548, 549) have basal dates younger than site 403 (Crescent Lake) on the proximal slope of the Firebag Moraine, and are not further discussed, but the lithostratigraphic columns are shown in Fig. 3. The stratigraphy and chronology of the remaining three sites (551, 552, 403), from south to north, are discussed next. Site 551 (Long Lake) is on the distal side of an unnamed leftlateral moraine on the northwest side of Muskeg Mountain and distal to the Firebag Moraine. The stratigraphy consists of basal silty clay overlain by sandy-laminated silt, sand, and peaty gyttja. A date of 9.54  0.07 14C ka BP on wood is from the silty-clay unit and serves as a minimum age for the unnamed left-lateral moraine. Site 552 is Kearl Lake located lower on the landscape, south, and proximal to the Firebag Moraine; south and above the CLAS. The core bottomed in diamicton, which is overlain by sand, laminated silt and gyjtta. One age is 9.4  0.08 14C ka BP from laminated silt, just above the sand. Site 403 (Crescent Lake) is one of the bestdated sites in the study area. It is located part way up the proximal side of the moraine within a crescent-shaped channel interpreted as a meltwater channel (Fig. 5) presumably cut by escaping water trapped between the ice and moraine. Consequently, lag times from melt-out of buried ice delaying lake formation are not expected at this site. The site was revisited and vibracored resulting in the

recovery of more clastic basal sand, presumably of the moraine itself. From the Livingston core, basal sand is overlain by a thin peat and gyttja before passing upwards into marl. The marl is overlain by peat and gyttja interbedded with thin sand deposits. Wood ages from three depths show an orderly progression of younger ages up the core (Fig. 3). The lowest age from within sand is 9.6  0.08 14C ka BP. Three dates from within the peat directly over the sand gave dates of 9.66  0.04 14C ka BP, and two ages of 9.3  0.07 14C ka BP. And three dates from within the gyttja over the peat are 9.1  0.07, 9.3  0.07, and 9.2  0.07 14C ka BP. As can be seen from Fig. 5, Crescent Lake is narrow with steep slopes from which woody material can enter the lake. The two oldest ages overlap at two sigma error bars, thus the age of the lake and the Firebag Moraine are assigned a minimum age of 9.66  0.04 14C ka BP., and a maximum age of 9.85  0.07 14C ka BP from site 546 (Survive Lake). 4.2.1.4. Richardson Moraine. Richardson Moraine is located w50 km north-northeast of the Firebag Moraine (Fig. 2). Two sites were cored; 541 (Esker Kettle Lake) and 540 (Richardson Lake) which lie on the moraine. The stratigraphy at Esker Kettle Lake is silty clay overlain by a thin sand unit, followed by organic and marl units. From near the base of the organic sediment we obtained an age of 8.33  0.06 14C ka BP on wood. At Richardson Lake sand at the base of the core is overlain by silt and then interbedded marl and peat. An age of 9.51  0.07 14C ka BP from wood in sand is used as the best minimum age for this moraine, while the maximum age of 9.66  0.04 14C ka BP is from Crescent Lake on the Firebag Moraine. 4.2.2. Clearwater lowland A large area of the Clearwater lowland extending from Stoney Mountain eastwards onto the Canadian Shield (Fig. 1) is covered with glaciolacustrine sediment and few moraines. Because our sampling strategy required staying above any lake level, this transect has fewer coring sites. There is a cluster of core sites on and peripheral to Stoney Mountain discussed first, then two sites at the Grizzly Bear Hills, followed by four sites along the Cree Lake Moraine. 4.2.2.1. Stoney Mountain. Cabin Moraine is a small moraine on the uplands of Stoney Mountain and the lower, partially encircling Stoney Mountain Moraine appears to record the same ice margin as Don’s Moraine to the northwest (Fig. 2). Together they record thinning at the ice margin. Core site 402 is distal to Cabin Moraine while cores site 401 and 555 are on its proximal side. The stratigraphy at site 402 (Cabin Lake) is diamicton overlain by silty clay, silt

T.G. Fisher et al. / Quaternary Science Reviews 28 (2009) 1608–1624

and then alternating beds of peat and gyttja with laminated silts and a thin marl bed (Fig. 3). There are nine ages from the lower two peat units, all from wood, although one (ETH-30185) may be from aquatic plant material based on a higher 13C value. The ages (listed in Table 1) all overlap each other ranging between 9.71 and 10.04 14C ka BP. One piece of wood was split four-ways and dated at both ETH and BETA Analytic. The results ETH-30172, -30173, BETA200071, -200072 (Table 1) all overlap at the 2 sigma level. The stratigraphy at site 401 (Hanging Stone Lake, which lies in a channel) consists of nearly 1 m of diamicton overlain by a thinly laminated, silty-sandy clay unit, and capped with peat then gyttja. Four dates from one horizon provide a wide range of ages. The age of plant material is 8.7  0.07 14C ka BP, and a piece of charcoal is 9.6  0.08 14C ka BP, with two other pieces of wood dated at 35.9  0.39 and 42.5  0.78 14C ka BP. The Middle-Wisconsin ages are further discussed in Section 5.1. The most useful age from this site is from the charcoal, which is a little younger than from site 402 on the distal side of the moraine. Basal silt at site 555 (Sweetheart Lake) is overlain by a peaty gyttja and capped by sand. One age within the organic sediment is 9.6  0.06 14C ka BP, here considered a minimum age because of the thick sequence of organic sediment below it. Note the much higher percentage of carbonate in the organic sediment which negated getting a ‘bulk sample’ age. Because the older wood from site 401 is likely reworked from an older deposit, the Cabin Moraine is assigned a minimum age of 10.0  0.1 14C ka BP based on the oldest wood age from site 402. The lower encircling moraine here called Stoney Mountain Moraine (Fig. 2) has three coring sites proximal to it (417–419). At site 417 (Christina Spillway) the core site is a lake within a small spillway system (Fisher, 1993). The stratigraphy consists of an organic-rich, basal, silty clay overlying a peat interbedded with sand and sandy silt. Dates of 8.9  0.07 14C ka BP from the silty clay and 7.8  0.07 14C ka BP from the base of the peat are young compared with ages on Stoney Mountain. The nearly 20% organic material in the lower mud suggests that the base of the lake sediment was not reached. Site 418 (Hook Lake) is a fishhook-shaped lake wrapping around a hill. The stratigraphy includes a basal siltyclay unit overlain by thin silt and sand units before passing upwards into a thicker mottled silt unit. Above this, the organic content increases sharply within the silt, overlain by a peat and capped by marl. The mottled silt indicates past water table fluctuations. This lake was also vibracored, resulting in 1.5 m further penetration into the basal mud with no further organic recovery. From the Livingston cores, there are three dates all from wood, with the lowest two, 10.03  0.08 and 10.02  0.08 14C ka BP from the basal mud, and the third, 9.4  0.07 14C ka BP from the base of the organic-rich silt. Hook Lake is assigned a minimum age of 10.03  0.08 14C ka BP. Site 419 (Deep Hole Lake) is at the base of Stoney Mountain and its relationship to the Stoney Mountain Moraine is uncertain here. The bottom of the core is diamicton overlain with pebbly gravel, silty clay, organic-rich sand bounded by sandy-laminated silt, and capped with peat. Three horizons were dated. One date from within the lowermost diamicton is 9.9  0.8 14C ka BP. From the base of the organic-rich sand are three dates: 10.27  0.5, 10.1  0.04, and 9.8  0.7 14C ka BP, and the top of the organic-rich sand has a single date of 9.8  0.7 14C ka BP. Following arguments outlined above, a minimum age of 10.27  0.05 14C ka BP is assigned to Deep Lake and this is the oldest Late-Wisconsin age at this end of the transect at Stoney Mountain. The age of the Stoney Mountain and Don’s Moraines implies that subsequent proglacial lakes in the Clearwater lowland are younger than 10.5 14C ka BP and proglacial lakes west of Stoney Mountain would be older than 10.5 14C ka BP. 4.2.2.2. Grizzly Bear Hills. The two sites at the Grizzly Bear Hills lie between major moraines. Site 408 is on the top of the Grizzly Bear

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Hills while site 407 is below strandlines cut into the base of the hill. The stratigraphy at site 408 (Upper Grizzly Lake) consists of a basal diamicton overlain by a thin sand unit, followed by peat then gyttja. There is a single age from the peat of 9.0  0.04 14C ka BP. The strandlines above site 407 have been associated with glacial Meadow Lake (Christiansen, 1979; Schreiner, 1984) and Lake Agassiz (Fisher and Smith, 1994). At site 407 (Lower Grizzly Lake), thin basal sand is overlain again by a thin peat and topped with gyttja. An age from the basal sand is 9.8  0.07 14C ka BP and there is an age from the base of the peat of 9.5  0.07 14C ka BP. Both these sites are far from a mapped moraine, but help constrain ice recession along a time–distance transect between Stoney Mountain and the Cree Lake Moraine. Moreover, the oldest date of 9.8  0.07 14C ka BP on peat from site 407 indicates lake drainage at this location prior to that time. 4.2.2.3. Cree Lake Moraine. Two sites were cored on the proximal side of the moraine (405, 411) and of the three sites cored immediately distal to the moraine, one did not recover datable organics and so is not reported here. Of the distal sites, the stratigraphy at site 413 (Vogelsang Lake) consists of sandy silt overlain by laminated silt and then peat. One age of 19.25  0.08 14C ka BP is from the base of the laminated silt. This age is considered to be too old to use for deglaciation in this sector of the LIS, and is discussed briefly in Section 5.1. Site 406 (Shield Site 2) is a lake in a channel cut into bedrock immediately distal to the Cree Lake Moraine. Basal sand with a thick lamination of organic-rich laminated silt is overlain by peat and gyttja interbedded with sand. A date of 8.4  0.07 14C ka BP is from sand and two dates from the base of the peat are 7.7  0.07 and 7.8  0.07 14C ka BP. Of the two sites on the proximal side of the moraine, site 411 (Inner Clearwater) is too young, with basal dates of 3.8  0.06 and 3.9  0.5 14C ka BP (Fig. 3) and is not further discussed. Site 405 (Shield Site 1) is the important limiting site for the Cree Lake Moraine. Here within a small bedrock depression the core stratigraphy consists of basal sand, laminated sandy silt, and organic-rich silts, all overlain by organic units. Samples from four depths were dated, with two of them duplicates. From the laminated sandy silt there is an age of 9.7  0.08 14C ka BP, similar to an age of 9.5  0.04 14C ka BP from the base of organic-rich silt. At 5 cm from the top of the organic-rich silt duplicate wood dates of 9.0  0.07 and 8.98  0.06 14C ka BP overlap at the 1 sigma level. From the top of the organic-rich silt another duplicate date, this time between two different labs also overlap at the 1 sigma level: 8.66  0.07 and 8.85  0.04 14C ka BP. The age assignment for this lake is 9.7  0.08 14C ka BP, which is also used as a minimum for the age of the Cree Lake Moraine. 5. Discussion 5.1. Old wood ages and previously reported bulk ages The discovery of old Mid-Wisconsin-aged wood in the cores is an important finding for guiding sample selection for radiocarbon dating. Older wood was found at three core sites (401, 413, 554). In each case wood fragments were dated, and at the same stratigraphic level at site 401 (Hanging Stone Lake) the old wood was mixed with younger wood. A simple explanation for this is the erosion and retransportation of older wood entering the lake through mass wasting, fluvial or littoral processes. The older wood is known to be present in pre Late-Wisconsin-aged sediment from borehole data in the area (Paulen et al., 2005). If Mid-Wisconsinaged wood were inadvertently included in a sample of woody organic fragments, or within a bulk-sediment sample, an older age would result. For example, if a sample is contaminated by 15%

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30,000 year old wood, it would introduce an error of 1200 years too old (Bradley, 1999: fig. 3.5, based on Olsson, 1974). The basal date from site 544 (Dipper Lake) at 12.78  0.095 14C ka BP is our oldest Late-Wisconsin age date from organic fragments, which is considerably older than all other dates in the area. While it is possible that Mid-Wisconsin-aged wood fragments were included in the sample dated, the pieces had intact bark making a reworking, retransporting hypothesis of Mid-Wisconsin-age wood possible, but not favored. Nevertheless, additional cores and radiocarbon ages of individual wood fragments from this lake would be able to test an early deglacial hypothesis for this area. Perhaps our most unusual age is 19.25  0.08 14C ka BP from site 413 (Vogelsang Lake). By all accounts the ice margin should be close to its maximum extent at this time about 1100 km to the south (e.g., Dyke et al., 2003). Thus, this age does not represent the age when the lake formed. Another possible issue is potential contamination by hydrocarbons from Athabasca oil sands. It should be noted that Vance (1986) went to great lengths to remove this contamination. In Fig. 1, the five bulk ages previously used for mapping isochrones are plotted. Sites range geographically from the top of the Birch Mountains (11,280  275 14C ka BP, GX8910; Vance, 1986) to Long Lake, a scour lake in the head of the CLAS (11,100  150 14C ka BP, GSC-4807; Anderson and Lewis, 1992). Other than the Nipawin Bay site, which is a little younger (10,600  120 14C ka BP, GSC-4821; Anderson and Lewis, 1992) in the former Wagtufro Lake basin, the deglacial ages are the same across a distance of w500 km. One way to explain this pattern is that all these dates suffer from the hardwater effect where aquatic plant material takes in old carbon dissolved within the water, but this would require that the hardwater effect increases in the direction of ice retreat to make the younger ages even older, which is difficult to explain. The Long Lake site was recored and a new age from wood fragments resulted in a younger date (9120  50, Beta-104544: Fisher and Souch, 1998). Dated wood from Site 536 (Mariana Lake) of 10.31  0.08 is w1000 14C years younger than the 5 cm-thick bulk date (11.31  0.11, GSC-2038) reported from Hutton et al. (1994). Hutton et al. (1994) recognized the possibility of contamination by the presence of pre-Quaternary palynomorphs, and our core analysis shows equal percentages of carbonate and organic matter from the base of the core indicating the possibility that there was a hardwater effect from dating aquatic plant material, or older reworked carbon. Dateable organic material was not recovered from our cores from Eaglenest Lake on the Birch Mountains to redate that site. 5.2. The minimum-age problem Reconstructions of deglacial chronology using basal dates from lake cores are hampered by the minimum-age problem. Specifically, this problem consists of an unknown time period recorded by sediment deposited after deglaciation but before the lowermost dated sediment, and the time between deglaciation and deposition of macrofossils into the lake. First we address sediment lags then macrofossil lags. Sedimentation rates into lake basins upon deglaciation can be high. For example in Sunwapta Lake, Alberta, as of 1974 between 2 and 11 m of sediment was deposited in the 36–14 years following deglaciation (Gilbert and Shaw, 1981). In our study area, nine of the 31 cores ended in diamicton and the transition to organic-rich sediment was in all cases <1 m, except for in White Cow Lake where there was also w2 m of laminated sediment and at Hook Lake. Twelve cores bottomed in sand with w0–2% organic material, including sites with limiting ages including Crescent Lake, Shield Site 1, Richardson Lake, and Sandy Bog Lake with the Mid-

Wisconsin-aged wood. Although our cores did not penetrate to the base of the sand, for the lakes just mentioned, the low percentage of organic material in the sand, some of which may be Mid-Wisconsin-aged detritus (e.g., Sandy Bog Lake), is interpreted to represent the sand comprising the moraine, or high energy conditions adjacent to the glacier and high sedimentation rates. Other cores with basal sand from lakes too young to track deglaciation have higher percentages of organic carbon, with more variability, except for Shield Site 2 within a meltwater channel. Here the sand may be a fluvial deposit predating formation of the small lake. The other nine cores end in mud, either laminated or massive with varying amounts of sand, which like the sediment in Sunwapta Lake, could be deposited over a very short period of time. Cabin, Mariana, and Sweetheart Lakes all have w0.5 m of mud beneath the basal dated horizon, but the low percentage of organic material makes it unlikely that this sediment represents a significant time interval. The low pollen accumulation rate from the base of Mariana Lake was interpreted by Hutton et al. (1994) as a consequence of higher sedimentation rates, with Salix being the first colonizer. From this analysis of the sediment we would conclude that the basal dates obtained are good estimates for the age of their basins. Because we are unaware of a way to precisely determine the age of the lacustrine sediment deposited between deglaciation and the first arrival of macrofossils at a specific site, we use modern analogues of vegetation colonization from retreating glaciers in the Arctic to estimate time lags between deglaciation and colonization by vegetation. Jones and Henry (2003) determined that the woody shrub Salix arctica and the mosses Polytrichum-Pogonatum spp. dominated on terrain in as little as 33 years after deglaciation on Ellesmere Island. In addition, Moreau et al. (2008) sampled 300 plots in the 1 km long forefield of the glacier Midtre Love´nbreen in Spitsbergen deglaciated within the past 100 years. They found 16 species of vascular plants on outwash deposits and 15 species on moraines, with 12 of those species colonizing moraines in less than 30 years. The climate in this high Arctic setting with 6.3  C mean annual air temperature (Moreau et al., 2008) can be considered similar to the paleoclimate of our study area where permafrost conditions were reconstructed from periglacial sand wedge polygons following lake lowering (Fisher, 1996). Southeast prevailing winds at the time of deglaciation in our study area is evidenced by stabilized parabolic sand dunes (Fisher, 1996) that could have aided plant migration northward. Other studies from a variety of other deglacial environments (Mathews, 1992) show similar rapid rates of plant colonization, thus we would argue that the time lag between deglaciation, vegetation colonization, and terrestrial macrofossil delivery to the lake is short, and may be within the dating error. Although the stratigraphic relationships will not permit an exact dating of the deglacation, we argue these data represent a significant step forward for four reasons. First, we have removed uncertainties associated with bulk ages. Second, we have dated the same or older stratigraphic levels. Bulk ages can only be applied where there’s sufficient organic content. Several of our samples came from sediments below the stratigraphic levels of high organic content. Third, we have secured multiple sites for the same landform. From the above discussion it can be seen that for any particular moraine we systematically eliminated the younger ages to yield the oldest sample recovered. Whereas it might be argued additional sampling could recover yet older ages, we cannot presently assess the number of samples that would completely remove this uncertainty. Fourth, because of our sampling density both inside and outside of moraines we have a stratigraphic geomorphic order that must be adhered to. Thus the assignment in several cases of both a minimum and maximum for the same landform form a consistency check and produce the likely constraining limits for those moraines. Moreover, our reconstruction is internally consistent.

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5.3. A new deglacial chronology and implications for northwest drainage The sampling site density and availability of radiocarbon ages to reconstruct the deglacial chronology for this area of the LIS, like our parallel work in the Thunder Bay area (Lowell et al., 2009), significantly increases the number of sites available to track deglaciation. The nested moraines in the Athabasca lowland show an orderly retreat from 10.5 to 9.5 14C ka BP and the retreat in the Clearwater lowland, while not as well documented because of the low sampling density, shows a similar pattern. The actual retreat patterns in the study area may have been more complex. The LIS could have waxed and waned between moraine formation but without field-based evidence suggesting otherwise, we present the simplest reconstruction. Within this framework the time for any waxing and waning is limited. One site merits additional discussion. The relatively old age of 12.78  0.1 14C ka BP at the Dipper Lake site (544) implies a time gap between the formation of Dipper Lake and Don’s Moraine. Because pieces of organic matter were dated rather than a single piece (Table 2), inclusion of Mid-Wisconsin-aged wood may have contaminated the sample, in which case the age is spuriously old. Alternatively, if the age is correct, it opens the possibility for an earlier retreat and readvance to Don’s Moraine. Additional dated cores from Dipper Lake are required to confirm this age. Constraints on a possible earlier Younger Dryas aged retreat and readvance cycle are discussed below in Section 5.5. From assigning ages to moraines, provisional isochrones reflecting the geomorphology across the study area were drawn (Fig. 6). The isochrones are based on the minimum radiocarbon age of the moraine, and where a dashed line is used, the ice-margin position is less certain. The connection of the Cree Lake Moraine

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with either the Firebag or Richardson Moraines remains problematic. For the west and central study area, comparison with isochrones from Dyke et al. (2003) in Fig. 1 reveals that our reconstruction is more than 1000 years younger. However, for the eastern study area, the isochrone for the the Cree Lake Moraine is different by only 300 years, thus similar to previous reconstructions (e.g., Dyke et al., 2003). The general view emerging here is a millennium of ice-margin retreat, stepwise in the Athabasca lowland and leaving rare ice-margin positions in the Clearwater lowland. The geometry of the ice margin in the Athabasca lowland controls meltwater drainage. Only after the ice-margin retreated from the Firebag Moraine could any meltwater drain northwards into the Arctic Ocean via its present pathway. Here we consider the implications for the proglacial lakes in the Athabasca and Clearwater lowlands. First recall that the Firebag Moraine is suggested to form between 9.85 and 9.66 14C ka BP. This age is similar to the age assignment of site 407 (Lower Grizzly Lake), below a series of strandlines (not evident on the DEM [Fig. 2]) and yields a basal age of 9.75  0.07 14C ka BP from wood. The context of this sample indicates that lake lowering in northwestern Saskatchewan preceded that age. The similar dates of drawdown of a water body in Saskatchewan with the opening and drainage of water impounded behind the Firebag Moraine ice margin in Alberta allow the possibility that the water body extending between the two areas, known locally as glacial lakes McMurray and Meadow (Fig. 1), were continuous across the Athabasca and Clearwater lowlands. While the details of the ice dam failure at, or immediately following recession of the ice margin from the Firebag Moraine, can only be inferred, the dissection and overtopping of the Fort Hills by the flood indicates that the drainage of the proglacial lake began with lowering of the lake and dissection of a large kame deposit in the middle of the valley. Deltaic sediment north of the Firebag Moraine records deposition into glacial Lake McConnell postdating initial lake drainage. Duration of steady-state flow through the CLAS from proglacial lakes in the upper Churchill valley (Fig. 1) once the ice dam gave way north of the Firebag Moraine has been estimated. A basal date from Klap Lake, a scour lake in the head of the CLAS in the bottom of the spillway is 9.45  0.15 14C ka BP (AA-50805, Fisher and Souch, 1998), and there is a 9.38–8.59 14C ka BP extrapolated basal age based on sedimentation rates in Wycherely Lake (Fisher, 2007). The younger age of these dates compared with the older age range for the demise of the Firebag ice dam suggests that flow through the CLAS from northwestern Saskatchewan continued for several hundred radiocarbon years after northward drainage in the Athabasca River valley began (Fig. 7). 5.4. Northwest drainage of meltwater

Fig. 6. Isochrones from this study that are based on 31 coring sites supported by 72 new AMS radiocarbon dates.

It is worth considering the potential northwest drainage of glacial Lake Agassiz with these new chronology constraints. Previously, a northwest outlet was proposed by Smith and Fisher (1993) and Fisher and Smith (1994) based on flood and delta ages associated with the CLAS and high elevation strandlines and sediments in the Churchill valley. Cessation of northwest flow was based on ages from scour lakes at the head of the CLAS and at the southern end of Lake Wagtufro with a best estimate age of 9.45 14C ka BP (Fisher, 2007). From the results in this paper a northwest outlet of Lake Agassiz could not open until 9.85–9.66 14C ka BP. And once this drainage to the north began, there was an estimated drop in water level of at least 50 m as recorded by strandlines near the head of the CLAS spillway (Fisher and Smith, 1994). If Lake Agassiz was the lake in the upper Churchill River valley, then throughout its basin there was a rapid drop of at

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Fig. 7. Summary diagram of meltwater routing from the Clearwater Lower Athabasca spillway (CLAS) and glacial Lake Agassiz outlets. Greenland 18O record is from Alley (2000) and age of the Preboreal Oscillation (PBO) is from Rasmussen et al. (2007). The symbol ‘A?’ for the northwest outlet reflects the possibility that Lake Agassiz had merged with Lake Churchill and drained to the northwest. With this assumption, abandonment ages from scour lakes in the head of the northwest outlet (Fisher, 2007), overlap with ages dating a reoccupation of the southern outlet spillway channel (Fisher, 2003), and overlap with the oldest minimum age from an eastern outlet (Teller et al., 2005; Lowell et al., 2009, their fig. 7), all of which may be explained by a brief reactivation of the southern outlet before opening an eastern outlet.

least 50 m at 9.85–9.66 14C ka BP before stabilizing at a series of bedrock sills known as the Wycherely channels at the south end of Lake Wagtufro in Fig. 1 (Fisher, 2007). However, the only known drop in the Agassiz basin for which there is stratigraphic evidence is to the low-water Moorhead Phase at 10.65 14C ka BP, which is too old (Fig. 7). A rise in lake level from the Ojata to Upper Campbell beaches could be accommodated by the rebound of the Wycherely channels. However, there is no field data supporting the 50 m drop in lake level after the Ojata Beach was transgressed at 10.0 14C ka BP by rising lake water at the Redwood Loop site in North Dakota (Fisher et al., 2008). Thus, based on the timing of meltwater drainage events in the Fort McMurray region, and the stratigraphy at the Redwood Loop site it is very unlikely that Lake Agassiz drained northwards when the ice dam gave way and the CLAS was active at 9.8–9.66 14C ka BP. The water body at the maximum extent in the upper Churchill River valley (Fig. 1) is now interpreted as a regional lake, here termed glacial Lake Churchill assumed to be an expansion of glacial lakes Meadow and McMurray. It was likely separated from Lake Agassiz by ice or higher topography further to the southeast, probably in the La Ronge, Saskatchewan area. After northwestward drainage of glacial Lake Churchill was initiated, and with further ice-margin recession after w9.7 14C ka BP in the La Ronge area, it is still conceivable that the transgressive Lake Agassiz drained to the northwest after merging with the lowered Lake Churchill in eastern Saskatchewan (Fig. 1). Tentative support for this reconstruction is the tracing of strandlines from the Agassiz basin into the upper Churchill River valley by Fisher and Smith (1994) and Rayburn and Teller (2007), but strandlines and ice margins in east central Saskatchewan need to be dated to fully test this hypothesis. 5.5. The question of an earlier retreat and readvance cycle Here we discuss the possibility that meltwater might have drained via the Athabasca River prior to the deglaciation reported here. The notion of a major retreatdreadvance cycle in the Fort McMurray area was proposed by Teller et al. (2005). This stems

from the hypothesis of a lower Lake Agassiz at the beginning of the Younger Dryas that assumes an open outlet during the lowwater Moorhead phase. The southern outlet is unlikely (Fisher and Lowell, 2006) and eastern outlets are now problematic (Lowell et al., 2009), hence the argument that the lake lowered by draining to the northwest. Since the final deglaciation occurred later, an earlier open pathway requires a readvance (Teller et al., 2005; Teller and Boyd, 2006). The primary evidence to support this is an age of 10,310  290 (GX-5301-II) with an unclear stratigraphic context. It was originally reported as being within flood gravel by a personal communication by Neil O’Donnell in Smith and Fisher (1993). If so, it would imply flood gravels that surround the Fort Hills are younger than that. O’Donnell’s more recent personal communication with Teller et al. (2005) suggests that the wood is instead from a peat deposit on top of the flood gravel but within a depression. In this case the flood gravel would be older than 10.3  0.3 14C ka BP. This implies that the final deglaciation in the Fort McMurray region be much older than the reconstruction presented in this paper. Thus the context and significance of this sample merit discussion. Unfortunately, the information forming the basis for the personal communication is in a 1977 unpublished report that is unavailable for industry proprietary reasons, and the boulder gravel has long since been mined. Nevertheless, assuming the sample stems from the peat there are two reasonable interpretations: 1) the upper flood terrace was exposed prior to 10.3 14C ka BP, based on the age of the oldest piece of wood within peat (Teller and Boyd, 2006); or 2) that the wood was transported from sources upstream and deposited within the depression as the flood waned with subsequent peat growth around it. However, since the stratigraphy cannot be recovered, and it is possible to argue for either interpretation, we await additional reports on the nature of the gravels. 6. Conclusions A regional investigation into the age of deglaciation in the broader Fort McMurray, Alberta, region resulted in the mapping of new moraines and assigning isochrones more than 1000 years younger than previous reconstructions. Earlier reconstructions relied on bulk-sediment dates susceptible to contamination by older carbon from hard-water effects, possibly reworked interstadial-aged wood, and in this area, residue from the tar sands. Redating some of these lakes with AMS ages of woody material resulted in younger ages. Like all other reconstructions of the Laurentide Ice Sheet, the minimum-age problem persists. Here we used more than an order-of-magnitude increase in AMS dates, mostly on wood from basal lake sediments, at sites targeted to date the age of a succession of moraines. Not all core sites or radiocarbon dates proved useful, however. This reconnaissance-style sampling methodology could now serve as a foundation for focusing on intensive coring of a few key lakes in an effort to find the oldest age within each basin by dating more delicate macrofossils rather than robust wood fragments. Our preliminary results indicate that the Firebag Moraine was the chokepoint for drainage to the Arctic Ocean from central North America. Based on the oldest radiocarbon age from Crescent Lake, and opening of the Clearwater Lower Athabasca spillway occurred between 9.85 and 9.66  0.04 14C ka BP. Regional considerations suggest that this drainage involved local proglacial lakes McMurray and Meadow merged with a regional lake in the upper Churchill River valley. The drainage coincides in time with the brief Preboreal Oscillation (Fig. 7) and the delivery of additional freshwater to the Arctic Ocean may have increased sea-ice flux to the North Atlantic (Fisher et al., 2002).

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Acknowledgments Without the generous support of the Comer Science and Education Foundation to Fisher and Lowell this work could not have been done. Conversations with both Wally Broecker and George Denton proved particularly useful. Field assistance was provided by Katie Glover, Henry Loope, and briefly by Robert Kunzig and John Johnson. We especially wish to thank our helicopter pilot Don Cleveland of Canadian Helicopters for his stalwart service both on and off the ground. Dr. Michael Lewis and one anonymous referee are thanked for their thorough reviews.

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