Dune stabilization in central and southern Yukon in relation to early Holocene environmental change, northwestern North America

Quaternary Science Reviews 30 (2011) 324e334

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Dune stabilization in central and southern Yukon in relation to early Holocene environmental change, northwestern North America Stephen Wolfe a, *, Jeffrey Bond b, Michel Lamothe c a

Geological Survey of Canada, Natural Resources Canada, 601 Booth St., Ottawa, ON K1A 0E8, Canada Yukon Geological Survey, Yukon Government, Box 2703, Whitehorse, YT Y1A 2C6, Canada c Département des sciences de la terre et de l’atmosphere, Université du Québec à Montréal, C.P. 8888, Succursale Centre-Ville, Montréal, QC H3C 3P8, Canada b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 May 2010 Received in revised form 10 November 2010 Accepted 15 November 2010 Available online 10 December 2010

Eolian deposits of central and southern Yukon, northwestern Canada, consist of loess mantles, small areas of active dunes, and larger stabilized dune fields. Dune fields in valley settings within the region are situated both within and beyond the limit of the last glaciation. Infrared stimulation luminescence (IRSL) dating in central and southern Yukon reveals that these dune fields stabilized as late as 9e8.5 ka, well after the retreat of Cordilleran glaciers. These findings are comparable to other valley-setting dune fields and loess from central Alaska, which record activity during the period from the Lateglacial to the Holocene Thermal Maximum (HTM), and reduced or altered activity after 9e8 ka. Post-glacial dune activity was most likely related to warm, dry conditions during the HTM, under predominantly shrubtundra vegetation. Early Holocene stabilization of these dunes probably occurred in response to cooler, moister conditions, and replacement of predominantly tundra by boreal forest cover, dominated by spruce. Stabilization of dune fields in southern Yukon and Alaska most likely represented an extension of the time-transgressive stabilization of dune fields that occurred across northwestern North America with the post-glacial expansion of the boreal forest. Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved.

1. Introduction Quaternary dune fields are widespread throughout northern hemisphere high latitudes including Europe, Russia, Alaska and Canada (Wolfe, 2006). Much of North America north of 45
N, and Europe north of 50
N, was glaciated during the late Quaternary, and dune fields residing along former margins of these continental ice sheets represent potentially informative sedimentary environments with respect to understanding Lateglacial and Holocene paleoclimates. In Europe, despite numerous chronostratigraphic studies, a firm chronology of the phases of dune activity is still lacking (Koster, 2005). In northwestern North America, although chronostratigraphic studies of some high-latitude dune fields have been undertaken (e.g. Mann et al., 2002; Bateman and Murton, 2006) a broader understanding of the Quaternary history of these dune fields is still needed. Eolian deposits in the northwestern North America represent important sedimentary archives of past high-latitude, cold-climate environments. Morphologic and stratigraphic characteristics of

* Corresponding author. Tel.: þ1 613 992 7670; fax: þ1 613 992 0190. E-mail address: [email protected] (S. Wolfe).

these deposits can be used to infer past localized wind regimes and regional to continental circulation patterns (Lea and Waythomas, 1990). More detailed chronostratigraphic sequences may also provide important information about changes in vegetative, niveoeolian or periglacial processes through time. Given the significant Lateglacial and Holocene environmental changes that have occurred in northwestern North America (Kaufman et al., 2004; Kokorowski et al., 2008), knowledge of past eolian systems may significantly aid in the interpretation of associated environmental changes. Nevertheless, an absence of fundamental information about eolian deposits remains. To date, for example, the extent of eolian deposits in Yukon has not been recognized and the significance of eolian processes largely under-rated. Similarly, the stratigraphic and chronologic significance of dune fields in Yukon has not been studied. Finally, there remains little integration of evidence from different eolian deposits, such as dunes and loess, which has inhibited a more comprehensive understanding of past environmental history. This study attempts to address these shortcomings. This paper provides the first compilation of the distribution of eolian deposits in central and southern Yukon. The objective of this study is to (1) document the timing of dune stabilization in this area for two study sites using luminescence dating; (2) compare these

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results to dune and loess activity in central Alaska; (3) discuss the evidence and causes of post-glacial eolian activity through to about 9e8 ka together with the changing environmental conditions that resulted in dune stabilization. 2. Regional setting 2.1. Eolian deposits in northwestern North America Much of the Yukon landscape has been repeatedly glaciated in the Quaternary period. During each of these glaciations, lobes of ice converged to form the northern extent of the Cordilleran ice sheet. The ice sheet was significant, covering much of southern and eastern Yukon, however large areas of unglaciated terrain persisted in the western and northern part of the territory (Figs. 1 and 2). Drainage patterns reflect this history with most large rivers converging on the unglaciated terrain of central Yukon before draining northwestward into Alaska (Fig. 2). Valley bottoms within the glacial limits are flanked with either broad outwash or glaciolacustrine plains remnant from glacial and deglacial sedimentation. These sediment sources, coupled with semi-arid climate conditions are responsible for abundant eolian activity during past transitions from glacial to interglacial periods. These processes persist in southwest Yukon where the landscape is dominated by the Saint Elias Mountains, its ice cap and a rain-shadow effect (Fig. 2). Preservation of these eolian deposits is facilitated by the presence of sporadic to widespread permafrost. Most notably in unglaciated central Yukon significant paleoenvironmental records containing past glacial and interglacial vegetation and soils are preserved in perennially frozen loess (Sanborn et al., 2006). These deposits are supported by an extraordinary tephra chronology record related to the volcanic history of the Wrangell volcanic field and Alaska Peninsula (Preece et al., 2000). In southern and central Yukon, recent evidence of this activity is visible in the form of the White River tephra (eastern lobe) that was deposited ca 1.15 ka (Froese and Jensen, 2005). This tephra originated from the volcano, Mount

325

Bona-Churchill (Saint Elias Mountains) near the AlaskaeYukon border and forms a conspicuous marker in modern soil profiles. Eolian deposits in the western arctic and sub-arctic regions of North America are widespread, occurring extensively in Alaska and extending into the Yukon and western Northwest Territories (Fig. 1). Eolian sand activity is most prevalent today in the Kobuk and Lower Koyukuk valley dunes in Alaska (Fig. 2; Mann et al., 2002), with smaller active dunes in the Carcross and Champagne areas of the southern Yukon (Fig. 2). Modern mineral dust (i.e. loess) is deflated from glacially-fed streams such as the Tanana River in central Alaska (Muhs et al., 2003) and the Slims River in southwestern Yukon (Nickling, 1978). Eolian sand is deposited in coastal and fluvial settings along eroding shorelines and cliff-tops (Ruz, 1993; Lauriol et al., 2002a). In contrast to these small areas of present eolian activity are the vast areas indicative of past activity. Loess is the most extensive surface deposit within the unglaciated regions of Alaska (Muhs et al., 2003) and west-central Yukon (Fig. 1) and may be several tens of metres thick. Deposits in the Klondike valley area extend back to 1.5 Ma (Froese et al., 2000; Westgate et al., 2001), though most deposits are Late Pleistocene in age (Fraser and Burn, 1997). Dune fields and sand sheets are also widespread, covering over 12,000 km2 of the Alaskan Arctic Coastal Plain and 2500 km2 of the Tuktoyaktuk coastlands (Fig. 1). Dune and loess deposits are mapped in Alaska but are still largely under-represented on geological maps of western Canada. Although eolian deposits are widespread in the Yukon, they are thinner and less continuous than those in Alaska for several reasons. First, in contrast to Alaska, more of Yukon was glaciated during the Wisconsinan, limiting the age and extent of eolian deposits. Second, many glaciated valleys were occupied by proglacial lakes in Yukon rather than extensive outwash plains, which may have acted as “sinks” for silt-sized sediments (Flint, 1971; Hopkins, 1982). Lastly, the rugged relief within Yukon promotes colluviation, resulting in discontinuous and variable thicknesses of surficial sediment (Smith et al., 2009), limiting accumulation and preservation of primary air-fall loess.

Fig. 1. Eolian deposits of northwestern North America. Derived from various sources as compiled by Wolfe et al. (2009) and glacial limit from Dyke et al. (2003).

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Fig. 2. Eolian deposits in central and southern Yukon Territory in relation to Quaternary glacial limits. Active dune areas and dune transport directions were derived from aerial photographs and satellite imagery. On original maps, loess is mapped as loess sheets, except for that south of Dawson which is mapped as colluvial eolian apron (muck) deposits on lower valley slopes. Loess and cliff-top eolian deposits have, at times, also been identified in stratigraphic sections (e.g. Jackson, 1993; Ward and Jackson, 1993a,b) and have also been studied in some detail at a few sites (Foscolos et al., 1977; Sanborn and Jull, 2010) but have not been included in the figure. Note, only the present extent of known mapped deposits is shown. Other eolian deposits are known to occur (e.g. loess in the Kluane Lake region), but have not been mapped. Note that “ka” in the legend and throughout the paper signifies thousands of calendar years ago.

2.2. Central and southern Yukon eolian deposits Surficial eolian sediments in Yukon consist of loess, inland sand dunes, sand sheets, lakeshore and riverside dunes, and cliff-top eolian deposits. In addition, ventifacts and buried sand wedges at the interface between loess and paleosols further attest to the effect of past eolian processes (Foscolos et al., 1977). These eolian deposits have their origins in the complex glacialeinterglacial history of the region (Fig. 2). Central and southern Yukon are dominated by rolling upland plateaus ranging between 1000 and 2000 m in elevation. Valley systems eroded during preglacial and glacial times incise the plateaus 500 m or more (Jackson, 1989). Eolian deposits are typically located on sides and bottoms of valleys. Loess occurs in most valleys in southern and central Yukon, though the deposits have no distinctive surficial expression and thus loess extent is probably under-represented relative to its true extent. Thin loess deposits occur widely on flat and gently sloping surfaces in glaciated areas. These layers extend to elevations of about 300 m above valley floors. On till surfaces of Late Wisconsinan age (McConnell glaciation), loess thickness is typically 30 cm or less, and on outwash of the same age, 15 cm or less (Foscolos et al., 1977). The thickest and most extensive loess deposits in Yukon Territory occur in the Yukon River valley (Fig. 2), primarily in unglaciated terrain and extending onto Reid (penultimate glaciation, before w130 ka) or pre-Reid terrain. Loess thickness here ranges mainly from 10 to 60 cm, and locally upward to 120 cm. Organic silt deposits 3 m thick or more are common on valley floors and low terraces in the Klondike, and are considered to be re-deposited loess derived from adjacent slopes (Bond and Sanborn, 2006; Jackson et al., 2009). These extensive organic loessic units,

modified by colluvial processes, are mapped in the pre-Reid and unglaciated terrains of west-central Yukon and Dawson (Fig. 1), where they are also known colloquially as “muck” deposits. Radiocarbon ages indicate that thick loess deposition occurred in association with the McConnell glaciation (Fraser and Burn, 1997; Kotler and Burn, 2000; Froese et al., 2002), and older tephra beds indicate several intervals of loess accumulation associated with previous glacial intervals (Westgate et al., 2001). Along rivers in southwestern Yukon such as the Alsek and White (Fig. 2), which today are fed by glacial meltwater, the diurnal rise and fall of outflows in summer create ideal conditions for deposition of fine glacial silts over the wide river flats near the junctions of the streams (Kindle, 1952; Nickling, 1978). Modern eolian silts are generated and deposited in this region of Yukon (Laxton et al., 1996; Sanborn and Jull, 2010). In contrast to loess, dune fields have distinctive surficial expressions on air photos and on the ground (Fig. 3), and thus are typically identified on surficial maps. Most dunes occur in areas of the McConnell or the Reid glaciations (Fig. 2). A cluster of dune fields occurs in southwestern Yukon between the Yukon River and Haines Junction, entirely within the limits of the McConnell glaciation. The largest of these fields covers the valley floors between the mountains north of Champagne. Other dunes are found on high bluffs over the Dezadeash River, and on lower valley slopes at the north end of Dezadeash and Kusawa Lake, and the east end of Bennett Lake at Carcross, where strong winds have blown across the lakes and mobilized the sand. Another small dune field occurs in southeastern Yukon also within the McConnell glacial limit, near Watson Lake. A second large dune field cluster occurs in the central Yukon, within the limit of the Reid glaciation. On a more localized

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Fig. 3. Whitehorse dune field. A) Air photo of portion of dune field showing location of optical sample SAW10-03. Parabolic dunes outlined for clarity; B) Stabilized dune ridge vegetated by lodgepole forest; C) Sample pit showing tephra layer, eolian stratigraphy, and optical dating sample location for SAW10-03.

scale, small, active dunes consisting of remobilized White River tephra have also been noted north of Kluane Lake. These dunes occur within proximal areas of the eruption plume that consist of sand-sized tephra particles (Bond and Lipovsky, 2009). Dune orientations (Fig. 2) typically depict past or present dominant sediment-transporting wind directions, which may reflect regionalscale surface winds and localized topography. Most Yukon dunes are parabolic and are stabilized, except for some active dunes and blowouts in the southwest (Fig. 2). Active parabolic dunes occur at Carcross on the windy eastern shores of Lake Bennett, where sediment is supplied from both the sandy lakeshore and re-activation of stabilized dunes due to human disturbance. Active dunes associated with river sources also occur north of Kusawa Lake on the Takhini River, west of Kluane Lake at Bullion Creek on the Slims River, and on the Alsek River between the Alsek Glacier and Haines Junction. Activity is primarily due to sand availability from localized fluvial deposition, and locally is due to strong winds that mobilize the sand into dunes. Active blowouts also occur extensively on ridges of older stabilized dunes north of the town of Champagne, and in dune fields to the south near the Dezadeash River. These active blowouts probably resulted from the loss of forest cover from fires in the 1940se50s. Most other southwestern Yukon dunes are fully stabilized. Dune fields in southwestern and southeastern Yukon post-date the McConnell glaciation, as they are contained entirely within this glacial limit (Fig. 2). Consequently, the maximum-limiting age of southwestern Yukon dunes is about 14e12 ka, and those at Watson Lake are no older than 13 ka (Dyke, 2004). In contrast, most central Yukon dunes reside beyond the limit of the McConnell glaciation (Fig. 2) and could potentially be older than the southern dunes. At present, however, there are no chronological data from stabilized dune fields in the Yukon. In this study, two dune fields of unknown age were selected for investigation to determine the timing of stabilization using

luminescence dating. The first area includes the Whitehorse dune field and relict shoreline dunes along Lake Laberge, within the limits of the McConnell glaciation. The second area, along the Stewart River west of Mayo, is outside the McConnell limit but within the Reid limit. 3. Study sites The present climate of central and southern Yukon is sub-arctic continental, with long cold winters and short mild summers. As most moisture is derived from the Pacific Ocean, the regional climate is significantly influenced by the mountains of the western Cordillera that effectively block weather driven by westerlies off the Gulf of Alaska (Wahl et al., 1987). Whereas mountain regions such as the Saint Elias Range in southwestern Yukon receive total annual precipitation exceeding 2000 mm, valleys within the interior plateau regions in southern Yukon, such as at Whitehorse, receive 260 mm or less creating dry sub-humid conditions. This effect is felt less in central Yukon, where the annual precipitation at Mayo is about 300 mm, creating more mesic conditions (Burn, 1994). The mean annual air temperature at Whitehorse is about 1
C, whereas at Mayo it is about 4
C (Burn, 1994). Monthly temperatures are similar throughout the region in summer, ranging from 12 to 15
C in July. In winter, however, temperature inversions in the central Yukon create colder conditions in valleys, such that the January mean at Mayo of 29
C is about 9
C colder than in Whitehorse (Burn, 1994). Most valley areas are vegetated by closed coniferous forests dominated by white spruce (Picea glauca) or lodgepole pine (Pinus contorta ssp. latifolia) and aspen (Populus tremuloides) on welldrained sites, black spruce (Picea mariana) on lowland sites and north-facing slopes, and balsam poplar (Populus balsamifera) and white spruce on alluvial sites. Fossil pollen data indicate that lodgepole pine migrated northward into Canada from refugia

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located south of the continental glacial limits, and arrived in the southern Yukon only in the late Holocene (MacDonald and Cwynar, 1985). Tundra cover at higher elevations includes dwarf shrub, dominated by white birch (Betula papyrifera) and willow (Salix ssp.), or lichen on rock with scattered herbs. 3.1. Whitehorse dune field and Lake Laberge shorelines The Whitehorse area was most recently ice covered during the McConnell glaciation (Fig. 2; Bond, 2004), which retreated from the area between 12 and 11.5 ka (Dyke, 2004). During retreat, water levels in glacial Lake Laberge were elevated by an outlet dam formed by a large recessional moraine at the north end of the lake. Elevations of a fluvial terrace at the outlet of Lake Laberge, and an uppermost strandline near Fox Creek (Fig. 3a) indicate that the highest outlet was some 84 m above the modern outlet (Horton, 2007). Flights of strandlines at various intermediate elevations around Lake Laberge indicate that erosion of the outlet may have been relatively constant. Numerous raised sandy shorelines occur at Fox Creek (Fig. 4a) between 700 m and the modern shoreline (Horton, 2007). As the ice retreated, glaciolacustrine sediments were deposited along the ice front, leaving locally thick deposits in the Whitehorse vicinity. Fluvial incision of the sediment dam on

Lake Laberge caused lake levels to fall well after the ice had retreated south of the Whitehorse area. As the lake level lowered, the southern shoreline of Lake Laberge and the Yukon River delta migrated northward depositing deltaic sands over the glaciolacustrine sediments. Once subaerially exposed, the sands were reworked by eolian processes to form the Whitehorse dune field, immediately north of the city (Bond, 2004). The Whitehorse dune field is identified on several surficial geology and soils maps (Morison et al., 1982; Morison and Klassen, 1991; Bond et al., 2005). The dune field is situated in a valley setting on either side of the Yukon River north of Whitehorse, though most of it is on the eastern side. The field is about 9 km long and 8 km wide, and is bounded to the west by the Yukon River, to the north by Swan Lake, and on the remaining sides by glaciofluvial and glaciolacustrine sediments (Fig. 4b). It lies at an elevation of about 680 m asl, and consists of parabolic dunes trending toward 335
, roughly paralleling the trend of the valley. The sandy soils within the Whitehorse dune field are classified as Orthic Dystric to Orthic Eutric Brunisols within the Carcross soil association complex (Morison et al.,1982), though most soils on elevated dune surfaces are regosols. A series of raised sandy shorelines above Swan Lake also indicate former levels of glacial Lake Laberge (Fig. 4b). The highest shoreline at 650 m asl borders the northern edge of the Whitehorse

Fig. 4. Simplified surficial geology, topography and optical ages. A) Fox Creek; B) Whitehorse dune field; C) Rusty Creek dune field. Surficial geology modified from Bond et al. (2005), Klassen and Morison (1987), and Bond (1997), respectively.

S. Wolfe et al. / Quaternary Science Reviews 30 (2011) 324e334

dune field. The abruptness of this shoreline indicates that the shoreline dunes were active when the lake was at this level, and at least one dune appears to have migrated over this eroded shoreline, indicating that it was active after the lake fell below this level. 3.2. Rusty Creek dune field The Rusty Creek dune field, mapped by Bond (1997), is one of many small stabilized dune fields in the central Yukon that lie beyond the McConnell glacial limit and within the limits of the Reid glaciation (Fig. 2). Thus, the source deposits for these dunes may have originated during the Reid deglaciation (ca >124 ka; Preece et al., in press; Ward et al., 2008) or, more likely, from McConnell outwash (ca >29.6 ka; Matthews et al., 1990) from meltwater drainage occupying tributaries of the Yukon River. The sediments were likely reworked into dunes by winds during and after the McConnell glaciation. Rusty Creek is a tributary of the Stewart River, about 14 km northeast of Stewart Crossing. At this location, the Stewart River valley is nearly 4 km wide and drains southwestward at an elevation of about 485 m. Rusty Creek valley and the associated dune field are on the north side of the Stewart River, nearly perpendicular to the orientation of the main valley. The dune field is 8.5 km long and trends toward 295
(Fig. 4c). The dunes have apparently migrated as much as 6.5 km up valley from their originating source along Stewart Valley, through Rusty Creek valley, and descended an additional 2 km into the upper reaches of Reversing Creek. The total elevation gain of the dune field is about 200 m. 4. Field and laboratory methods Seven sediment samples were obtained for luminescence dating from stabilized parabolic dunes and raised shorelines (Table 1) that were recognizable on aerial photographs and on the ground (Fig. 4). Two samples were collected from parabolic dunes in the Whitehorse dune field at a local elevation of about 680 m, and a third from a shoreline dune representing a former level of Lake Laberge at 650 m, just below the elevation of the dune field (Fig. 4b). Two other samples were collected from shoreline dunes related to former levels of Lake Laberge at Fox Creek; one at the same elevation as first shoreline sample, and a second about 25 m lower, 4 m above the modern lake level (Fig. 4a). Two other samples were collected from dunes in the Rusty Creek area; one from a parabolic dune near the summit of the field, and another from a dune near the southeastern edge of the field (Fig. 4c). The elevations of the Whitehorse-area sites were measured using a differential global positioning system by Horton (2007) in his analyses of the paleogeography of glacial Lake Table 1 Sample locations and attributes. Location

Sample type

Latitude

Longitude

Elev. (m)

Orient. (
)

SAW0310 SAW0311 SAW0312 SAW0315 SAW0316 SAW0317 SAW0318

Whitehorse dune field Whitehorse dune field Swan Lake

Parabolic dune Parabolic dune Beach dune Beach dune Beach dune Parabolic dune Cliff-top dune

60
510 0500

135
070 0300

680

335

60
520 2000

135
060 4400

680

337

60
530 2000

135
050 5800

650

342

61
070 3700

135
120 3200

651

337

61
070 1600

135
120 1900

634

328

63
290 1000

136
310 4900

640

295

63
270 4900

136
280 1800

610

295

Fox Creek/ Lake Laberge Fox Creek/ Lake Laberge Rusty Creek dune field Rusty Creek dune field

Laberge. As no natural exposures exist at these sites, all samples were collected from a depth of 1 m, in pits excavated to a depth of about 120 cm in well-defined stabilized dunes or shorelines to avoid potential mixing of the parent dune sands with overlying pedogenically-altered sediment. The sampling strategy was meant to focus on the most recent episodes of deposition that can be correlated to the existing surface morphology. Stratigraphy was described at each sample site, noting depth of pedogenesis and orientation of bedding planes within eolian sediments. Dune and shoreline orientations, reflecting the dominant transporting directions during formation, were determined in the field. The morphology and orientation of dunes were also derived from aerial photographs. Luminescence dating, specifically infrared stimulation luminescence (IRSL) was used to determine the timing of eolian activity. This technique measures the time elapsed since mineral grains were last exposed to sunlight, which usually corresponds to the time since the grains were buried. By assessing the radiation dose received by the grains, termed the paleodose (P in Gy), and the radiation dose _ in Gy/ka), one can obtain received by year, termed the dose rate (D _ Paleodoses were measured on sand-sized Ka burial age using P=D. feldspar grains (180e250 mm diameter) based on burial depth, mineralogy and past water content (Table 2) using the single aliquot regeneration (SAR) protocol with infrared stimulation. The range in uncertainty of water contents is considered adequate to account for past variations in average water content at 1 m depth, and is within the range of water contents for dunes dated elsewhere in northern Canada (Wolfe et al., 2004). Details of the experimental procedures are similar to those described in Auclair et al. (2003) but in addition, bleaching of the aliquots was carried out before each radiation dose steps in the SAR routine. IRSL ages were corrected for anomalous fading following the Huntley and Lamothe (2001) method. Fading rates were measured on every sample using SAR (Auclair et al., 2003) and were calculated from data collected up to 3 months after laboratory irradiation. The luminescence ages are approximately equivalent to calibrated (i.e. calendric) radiocarbon years. The average analytical uncertainty in the optical ages is about 10% of the reported age at 1s, with about half of this uncertainty being derived from uncertainty in the fading rates (Table 3). 5. Results 5.1. Whitehorse dune field and Lake Laberge shorelines The Whitehorse dune field consists of stabilized parabolic dunes, ranging from 100 to 500 m long, 100 to 200 m wide and up to 25 m high, residing on a relatively flat inter-dune surface of Table 2 Sample depth, K, Th and U concentrations and water contents of samples used for dosimetry. Sample #

Sample number

329

SAW03-10 SAW03-11 SAW03-12 SAW03-15 SAW03-16 SAW03-17 SAW01-18

Depth (cm)a

K (% 3.2%) Sample

Surroundingsb

100 100 100 100 100 100 100

2.01 2.28 2.21 1.72 1.84 0.60 0.55

2.22 2.44 2.06 1.79 1.90 0.58 0.54

Th (ppm) (10%)

U (ppm) (6%)

Water contentc

7.0 5.1 3.5 3.7 2.8 3.5 4.7

2.5 1.7 1.4 1.3 1.2 1.1 1.6

5.0 5.0 7.5 7.5 7.5 5.0 5.0

      

2.5 2.5 2.5 2.5 2.5 2.5 2.5

Notes: U, Th and K contents are from neutron activation analyses. K and Rb contents of the K-feldspar grains are assumed to be 12.5% and 350 ppm respectively. a Sample depth beneath the ground surface. b Average of K content of samples collected 30 cm below and above the main sample. c Water content ¼ [(mass water)/(dry mineral mass)  100], and correspond to best estimates of the average moisture content of the sediment during period of burial.

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Table 3 _ equivalent doses (De), and optical ages. Total dose rates (D), Sample number

D_ a (Gy/ka)

SAW03-10 SAW03-11 SAW03-12 SAW03-15 SAW03-16 SAW03-17 SAW03-18

3.95 3.74 3.38 3.18 3.18 2.05 2.30

 0.22  0.19  0.16  0.14  0.14  0.11  0.12

De b (Gy) 20.8 21.7 19.6 19.5 5.8 13.0 13.5

Uncorrected Fading ratec Corrected optical age (%/decade) optical aged (ka) (ka)  0.4  0.4  0.7  0.5  0.3  0.4  0.3

5.3 5.8 5.8 6.1 1.8 6.3 5.9

 0.3  0.4  0.4  0.3  0.1  0.4  0.3

5.7 5.7 5.7 5.7 5.7 4.1 4.1

 0.6  0.6  0.6  0.6  0.6  0.5  0.5

9.1 10.0 10.0 10.6 3.1 9.3 8.6

 1.0  1.1  1.1  1.1  0.3  0.8  0.7

a _ ¼ Dc þ D Total dose rate, D a;b;g , where Da,b,g is the dose rate due to a,b, and g radiation and Dc is the cosmic-ray dose rate. Dc is evaluated at 0.20  0.02 Gy/ka. b Equivalent dose measured on K-feldspar using the single aliquot regeneration (SAR) protocol with infrared stimulation. The delay between laboratory irradiation and equivalent dose measurement is in the order of a few minutes. De value measured using SAR for sample SAW03-10 is consistent with that determined with multiple aliquots using the regeneration method, the latter being measured two days after irradiation. c Fading rates were measured on every sample using SAR (Auclair et al., 2003) and are calculated from data collected up to 3 months after laboratory irradiation. An average value is used for feldspars derived from the same geological source. d Optical ages corrected for anomalous fading following the Huntley and Lamothe (2001) method.

eolian sand. The dunes are well-defined, with little evidence of blowouts that would signify subsequent partial reworking. The dunes are forested by open-canopy lodgepole pine, and a sparse understory of aspen and willow trees, rose and bearberry (Arctostaphylos uva-ursi) shrubs, and a ground cover of lichen and grass. The organic surface litter ranges in thickness from 2 to 4 cm, and is primarily comprised of pine needles. The White River Ash, being the only near surface tephra in the Whitehorse area (Richter et al., 1995), occurs at both dune sites at 4e10 cm depth (Fig. 5), indicating only minor deposition in the last 1200 years. The upper 45e50 cm of sediment is massive, light brown, mottled and oxidized medium sand. Distinctive eolian stratification is visible in both sections below this depth, primarily parallel, sub-horizontal lamination. Dip-angles increase to between 8 and 15
, below a depth of about 90 cm, and were indicative of dune topsets (Fig. 5). The two age results from the Whitehorse dunes indicate that the dunes were active at about 10.0  1.1 and 9.1  1.0 ka (Table 3). The dunes trend in a direction of about 335
, similar to other dunes in this field (Fig. 4b). Because the ages are from the heads of the dunes and at relatively shallow depths of about 1.0 m, they provide a maximum-limiting age on stabilization of about 9 ka. The Lake Laberge shoreline samples include one from Swan Lake, immediately north of the Whitehorse dune field (Fig. 4b) and two from Fox Creek (Fig. 4a) about 25 km north of Whitehorse. The shoreline ridges range in height from 1.5 to 4 m, and represent

sandy beaches built, in part, by wind action. The fact that most of the well-developed ridges are oriented approximately perpendicular to the local dune transport direction of 335
suggests that a combination of wind-driven wave action and onshore eolian sediment transport helped form the ridges. The shoreline ridges are presently stabilized with similar vegetation cover as the dunes, though white spruce dominates the forest cover at Fox Creek. A tephra, again probably White River Ash, occurs in all ridges at 4e9 cm depth, indicating little deposition in the last 1200 years (Fig. 5). The shoreline samples provide age control on glacial Lake Laberge levels and the Whitehorse dune field. The higher shoreline samples returned ages of 10.0  1.1 ka at Swan Lake, and 10.6  1.1 ka at Fox Creek, constraining the elevation of Lake Laberge at about 650 m, between 10.6 and 10.0 ka. The ages from the Whitehorse dune field indicate the dunes were active when the lake was at this elevation and shortly thereafter. The lowest elevation sample at 634 m at Fox Creek returned an age of 3.1  0.3 ka. The series of shorelines between 651 and 634 m suggests gradual lowering of Lake Laberge between about 10 and 3 ka (Fig. 4a). 5.2. Rusty Creek dune field The Rusty Creek dune field consists of large stabilized parabolic dunes exceeding 40 m in height, with broad frontal ridges ranging from 200 to 2000 m wide. They are transverse dune ridges but with distinctive parabolic curvature (Fig. 4c). As noted earlier, the dunes appear to have migrated up this tributary valley of the Stewart River. The headwaters of Rusty Creek reside within a 70 m-high, amphitheatre-shaped, sand-walled valley at the southern end of the dune field. As in the Whitehorse dune field, the dunes are welldefined, without blowouts or other evidence of reworking, indicating that they may have been mostly stable since their initial stabilization. The dune ridges are covered by an open forest of white spruce and lodgepole pine, with poplar in low-lying areas between the dune ridges. Lodgepole pine is near its northern range limit at this location. The organic surface litter ranged from 1 to 2 cm thick, and no tephra was observed below the surface. The higher elevation sample site (640 m in Fig. 4c) is located on the head of a large stabilized dune. The upper 30 cm of sand is oxidized and massive with some reddening. Distinctive, parallel, sub-horizontal laminations occur between 30 and 120 cm depth, which appear to represent eolian deposits on the relatively flat surface of the dune (Fig. 5). The lower site is on a small parabolic dune along the upper edge of the sand-filled valley near the headwaters of Rusty Creek. The upper 30 cm of the sampling section is massive, fine, wellsorted sand containing a buried Ah-horizon at 4e7 cm depth.

Fig. 5. Near-surface stratigraphy and luminescence ages from eolian sections.

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Between 30 and 75 cm depth, the sand is mottled and oxidized. Below this, the sand is laminated and dipping at about 8
(Fig. 5). The Ah-horizon is buried by eolian sand that had probably blown onto the dune surface from the adjacent steep valley sides. The dune itself, however, does not appear to have been re-activated. The ages from the Rusty Creek dune field indicate dune activity at about 9.3  0.8 and 8.6  0.7 ka (Table 3). We interpret these ages to indicate a maximum-limiting age of stabilization of the dune field, at about 9.0e8.5 ka. 6. Discussion The dunes in the Whitehorse area of southern Yukon and Rusty Creek area of central Yukon were active during the early Holocene until about 10e8.5 ka, well after the 12.5e11.0 ka retreat of the latest (McConnell) ice sheet. Wheeler (1961) proposed that the Whitehorse dune field had probably formed when winds were stronger than today, at a time when glacial ice lay toward the south in the Yukon River valley, with katabatic winds blowing off the ice in a northerly direction and dunes forming on a vegetation-free outwash plain or dried-up lake bed in front of the ice sheet. Our results suggest that this mode of ice-proximal dune formation, though possibly applicable to the onset of dune activity, cannot be applied to these dune fields for this later time period. Rather, luminescence ages are more closely align with the termination of the Holocene Thermal Maximum (HTM), and onset of cooler and moister climatic conditions after 10 ka. In central Alberta, Wolfe et al. (2004) found that dune activity occurred between 16 and 13 ka under ice-proximal tundra setting, and continued between 13 and 11 ka under parkland and grassland settings as the Laurentide Ice Sheet retreated to the northeast. Throughout central Alberta and Saskatchewan, dune fields stabilized in a time-transgressive manner from the southwest, at about 11 ka, to the northeast, at about 9 ka, with the establishment of boreal forest vegetation and reduce wind strength (Wolfe et al., 2004). And in central and northern Alaska Mason and Bigelow (2008) propose that the development of the Northern Archaic culture in this region was, similarly, accompanied by the spread of boreal forest into the area ca 7 ka. In that study, the authors draw upon a suite of proxy records including loess and dune stratigraphy, in support of their hypothesis. We propose that dune activity in central and southern Yukon after deglaciation was primarily sustained by a sparse vegetation cover of predominantly shrub-tundra vegetation cover, perhaps with open stands of poplar (Dyke, 2005), and that stabilization of these valley dune fields was primarily a result of the introduction of northern boreal forest cover, including spruce, between 9 and 10 ka (Ager and Brubaker, 1985; Dyke, 2005). In support of this hypothesis, we briefly discuss the climatic, vegetation, and eolian evidence. 6.1. Late-glacial and Holocene climate and biomes During the Lateglacial period, unglaciated portions of the northwestern Canada and interior Alaska were colder and drier than present (Barnosky et al., 1987), being in close proximity with Laurentide and Cordilleran ice sheets. Much of the western arctic was occupied by herb/steppe-tundra, consisting of grass, sage and sedge (Dyke, 2005; Zazula et al., 2007). Subsequently, the Younger Dryas stade (13e11.6 ka) in Alaska was characterized by cooling in the south, and uniform to warmer-than-present conditions in central Alaska, including warm, dry conditions in the Alaskan interior. Kokorowski et al. (2008) suggest that the period from 16 to 13 ka witnessed the introduction and expansion of shrub tundra

331

into eastern Alaska and the western arctic, and restriction of herb tundra nearer to receding ice sheets (Dyke, 2005). Northwestern North America experienced the HTM peak between ca 11 and 9 ka, when summer temperatures were about 1e3
C warmer than present (Kaufman et al., 2004). This period corresponds to the Holocene peak summer (June) insolation at 60
N (Berger and Loutre, 1991). In Alaska, warmer-than-present temperature occurred between 11.5 and 9 ka, although evidence of HTM here is inconsistent compared to other regions of the western arctic, and less pronounced than simulated by general circulation models (Bartlein et al., 1998). In northwest Canada summer temperatures were warmer than present beginning 11.6 ka (Kaufman et al., 2004; Cwynar, pers. comm. 2009). At that time, Yukon was dominated by birch shrub tundra, with central Alaska into west-central Yukon vegetated by forest tundra (i.e. herb and shrub tundra with open stands of poplar; Dyke, 2005). In southern Yukon, Cwynar (1988) notes a dominance of Artemisia, with woodland areas occupied by Populus and Shepherdia canadensis between 13 and 10.4, and a subsequent increase in P. glauca indicating the introduction of mesic forest communities in the area. A thaw unconformity including truncated ice wedges, near Mayo, central Yukon indicates greater active-layer depth, consistent with the HTM, ca 9.9 cal ka BP (Burn et al., 1986). An increase in effective moisture in the western arctic appears between 10 and 9 ka, with climatic conditions after ca 9 ka being cooler and moister (Kaufman et al., 2004). At 9e10 ka there was a rapid increase in lake level in the southern Yukon, suggesting a shift in the precipitation regime (Anderson et al., 2005). Rising water levels after this time, corresponding with appearance of P. glauca (white spruce), are evidences of increasing effective moisture (Anderson et al., 2005). The western boreal forest was expanding considerably by 10 ka, advancing into central Yukon and Alaska (Dyke, 2005), with the treeline advancing toward the Arctic Ocean coastline in the western NWT (Ritchie, 1984). By about 9 ka boreal forest biome was well-established in Yukon (Dyke, 2005). 6.2. Tanana River basin dunes and loess As no other chronological studies of dune fields exist in Yukon, we compare our results to valley dune fields and loess in central and southeastern Alaska with reference to changing Lateglacial and early Holocene environmental conditions. Most dune fields in the Alaskan and Yukon interior reside in valley settings like the Whitehorse and Rusty Creek dune fields (Fig. 6). In eastern Alaska, sand dunes occur along the Upper Tanana River, near the Yukon border. Radiocarbon dates from organic beds within the uppermost parts of the dunes indicate that they were active between about 14.3 and 9 ka (Fernald, 1965), and Carter and Galloway (1984) obtained similar ages from interdune pond deposits in the dune sections (Fig. 6). Overlying dated organic materials indicate that these dunes were completely stabilized sometime between 9 and 7 ka. Unfortunately, environmental conditions related to these valley dunes are not wellestablished. However, in additional support of our hypothesis, we can turn to loess chronologies along the Tanana River. Muhs et al. (2003) report up to 3 m of loess deposition occurred between ca 17 and 8 ka in the Fairbanks region of the Tanana River valley. They note, however, that only moderate rates of loess deposition can be attributed to the last glacial period, with much higher rates of deposition occurring during the early Holocene, including episodic deposition in the late Holocene, after ca 2.8 ka (Fig. 6). Similarly, archeological sites in the Tanana valley, southeast of Fairbanks record between 0.5 and 1.5 m of loess deposition with thin paleosols between ca 14 and 8 ka, the majority of which may have

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Fig. 6. Documented time range of eolian deposition between 18 and 5 ka in for selected areas in southwestern Yukon and eastern Alaska. A) Documented eolian deposition, with dashed lines indicating episodic or uncertain activity. B) Map showing locations of sites referenced in A with approximate maximum late Wisconsin glacial limit after Dyke et al. (2003).

been deposited between 10.5 and 8.5 ka (Holmes, 1996; Holmes et al., 1996). In the upper Tanana River basin in southwestern Yukon, just beyond the last glacial limit, Easton et al. (2008) identified a loess and paleosol sequence indicating loess deposition around 14.0 ka, followed by continued loess deposition and A-horizon development from 11.7 to 9.8 ka, with some (still undated) loess deposition after 9.8 ka (Fig. 6). Muhs et al. (2003) have addressed the environmental conditions during loess deposition in central Alaska. Climatic conditions during the last glacial period there were colder, at least in summer, and drier than present; and vegetation cover was dominated by herb tundra (Ager and Brubaker, 1985; Dyke, 2005). Muhs et al. (2003) note, however, only moderate loess deposition rates during the last glacial period, and much higher rates during the early Holocene. In a conceptual model, they proposed that central Alaskan loess deposits reflected changes in local vegetation cover, rather than changes in the rate of loess production. Thus, whereas loess production may have been very high during full glacial conditions, accumulation rates were very low prior to 13.0 ka predominantly due to herb and shrub-tundra vegetation (Dyke, 2005) that did not trap loess effectively, thus allowing regional export of most of the dust. In contrast, although loess production rates may have decreased during Lateglacial and Holocene times, the migration of forests into the area increased local trapping of loess, and thus accumulation rates. Ager and Brubaker (1985) indicate that spruce arrived in central Alaska around 9e10 ka, and Dyke (2005) suggests that much of southern Yukon and southeastern Alaska was occupied by spruce forest by 9.0 ka. The introduction of forest cover that enhanced loess accumulation in the central Tanana River valley supports our interpretation dune stabilization in this region. In this regard, the stabilization of valley dune fields in southern Yukon, and likely those in southern Alaska including the Upper Tanana River, may represent an extension of the time-transgressive stabilization of dune fields that occurred across northwestern North America with the post-glacial expansion of the boreal forest.

7. Conclusions Our results suggest that reduced vegetation cover and sufficient supply of sediment and transporting winds acted to create an environment conducive to eolian processes until the introduction of boreal forest vegetation. Valley dune fields in the southern and central Yukon, residing within and beyond the limit of the last glacial (McConnell) ice limit, were active well after the retreat of glacial ice. Our maximum-limiting ages of stabilization for these dune fields (between 9 and 8.5 ka) coincide with the establishment of boreal forest cover from earlier shrub and forest-tundra vegetation. We suggest that the introduction of coniferous forest vegetation, including P. glauca and P. mariana, effectively stabilized these dunes, as occurred throughout western Canada following the retreat of the Cordilleran and Laurentide ice sheets (Wolfe et al., 2004). Stabilization of these dune fields also coincides with increased loess accumulation observed in the Tanana River valley, which may be similarly attributed to the expansion of spruce forest cover into Alaska (Ager and Brubaker, 1985; Muhs et al., 2003). Similarly, stabilization of dune fields on the Upper Tanana River observed by Fernald (1965) and Carter and Galloway (1984) might further be attributed to this expansion. Stabilization of these valley dune fields in Yukon and Alaska following the Holocene Thermal Maximum is probably tied to broader changes in eolian environmental systems in northwestern North America. Along the Porcupine River of northern Yukon (Fig. 1), for example, Lauriol et al. (2002a) found initial eolian activity in cliff-top eolian deposits occurring around 18 ka, with high activity between 13.5 and 9.0 ka, and lesser activity from 5.3 ka to present. High eolian activity coincided with warm, dry conditions, evidenced by various land mollusks and presence of S. canadensis pollen, between 13.5 and 9.1 ka, and reportedly xeric conditions marked by gypsum concretions between about 11.2 and 10.4 ka. Lauriol et al. (2002b) further found that major aggradation of the Porcupine and Old Crow rivers took place between 9 and 4.5 ka, and suggested that the absence of cliff-top eolian deposition

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between this time was indicative of moister conditions under climatic conditions cooler and moister than today. Reduced cliff-top eolian activity corresponded with the arrival of spruce forest in the region and the beginning of a maximum expansion of the boreal forest cover in northern Yukon. Furthermore, on the Alaskan Arctic Coastal Plain and Tuktoyaktuk Coastlands (Fig. 1), active sand dunes occurring between ca 40 and 14e13 ka occurred under polar desert climatic conditions, which likely extended across northeast Beringia at that time (Bateman and Murton, 2006). A transition from dunes to sandsheet formation after ca 13 ka, followed by stabilization after ca 8 ka signifies amelioration from cold arid conditions to a less severe periglacial climate. Across this region, a climatic transition to cooler and moister conditions was accompanied by a change in vegetation biomes from herb tundra to shrub tundra (Dyke, 2005) with an increased maritime influence of the Arctic Ocean (Carter, 1993; Burn, 1997; Bateman and Murton, 2006). Overall, our results from stabilized valley dunes fields, coupled with those from other eolian systems in northwestern North America, support other regional evidence for environmental and landscape process changes that occurred with the termination of the Holocene Thermal Maximum (Kaufman et al., 2004; Mason and Bigelow, 2008). Acknowledgements The authors acknowledge GIS assistance by Louis Robertson, field assistance by Kristen Kennedy, and optical dating laboratory assistance by Marie Auclair. Draft versions of this paper benefitted significantly from comments by Dan Muhs, Art Dyke, Kristen Kennedy and Duane Froese. Reviews by Chris Burn and an anonymous reviewer further improved this paper. Funding for this work was supported in part by the Yukon Geological Survey and NSERC. This paper is a contribution to Natural Resources Canada’s Climate Change Program, and is Geological Survey of Canada contribution number 20090434 and Yukon Geological Survey contribution 009. References Ager, T.A., Brubaker, L., 1985. Quaternary palynology and vegetation history of Alaska. In: Bryant Jr., V.M., Holloway, R.G. (Eds.), Pollen Records of LateQuaternary North American Sediments. American Association of Stratigraphic Palynologists Foundation, Dallas, Texas, pp. 353e383. Anderson, L., Abbott, M., Finney, B., Edwards, M.E., 2005. Palaeohydrology of the southwest Yukon Territory, Canada, based on multiproxy analyses of lake sediment cores from a depth transect. The Holocene 15, 1172e1183. Auclair, M., Lamothe, M., Huot, S., 2003. The measurement of anomalous fading for feldspar RSL using SAR. Radiation Measurements 37, 487e492. Barnosky, C.W., Anderson, P.M., Bartlein, P.J., 1987. The northwestern US during deglaciation: vegetational history and paleoclimatic implications. In: Ruddiman, W.F., Wright Jr., H.E. (Eds.), North America and Adjacent Oceans During the Last Deglaciation. The Geology of North America. Geological Society of North America. Colorado, Boulder, pp. 289e321. Bartlein, P.J., Anderson, K.H., Anderson, P.M., Edwards, M.E., Mock, C.J., Thompson, R.S., Webb, R.S., Whitlock, C., 1998. Paleoclimate simulations for North America over the past 21,000 years: features of the simulated climate and comparisons with the paleoenvironmental data. Quaternary Science Reviews 17, 549e585. Bateman, M.D., Murton, J.B., 2006. The chronostratigraphy of Late Pleistocene glacial and periglacial aeolian activity in the Tuktoyaktuk Coastlands, NWT, Canada. Quaternary Science Reviews 25, 2552e2568. Berger, A., Loutre, M.F., 1991. Insolation values for the climate of the last 10 million years. Quaternary Science Reviews 10, 297e317. Bond, J.D., 1997. Late Cenozoic history of McQuesten (115P), Yukon Territory. Unpublished Masters of Science thesis, University of Alberta, pp. 1e161. Bond, J.D., 2004. Late Wisconsinan McConnell glaciation of the Whitehorse map area (105D), Yukon. In: Emond, D.S., Lewis, L.L. (Eds.), Yukon Exploration and Geology. Yukon Geological Survey, pp. 73e88. Bond, J.D., Morison, S.R., McKenna, K., 2005. Surficial Geology of Upper Laberge (1:50 000 Scale). Yukon Geological Survey. Geoscience Map 2005-8. Bond, J.D., Sanborn, P.T., 2006. Morphology and Geochemistry of Soils Formed on Colluviated Weathered Bedrock: Case Studies from Unglaciated Upland Slopes in West-central Yukon. Yukon Geological Survey, Open File 2006-19, pp. 1e70.

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