ARTICLE IN PRESS

0277-3791/$ - se

doi:10.1016/j.qu

�CorrespondE-mail addr

Quaternary Science Reviews 25 (2006) 1542–1551

Seasonality of the late Pleistocene Dawson tephra andexceptional preservation of a buried riparian surface

in central Yukon Territory, Canada

Duane G. Froesea,�, Grant D. Zazulab, Alberto V. Reyesa

aDepartment of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, T6G 2E3, CanadabDepartment of Biological Sciences, Simon Fraser University, Burnaby, British Columbia, V5A 1S6, Canada

Received 7 October 2005; accepted 20 January 2006

Abstract

The late Pleistocene Dawson tephra was deposited by one of the largest Quaternary eruptions in northwestern North America. Its

distribution is known sparsely from sites near the source area in southwestern Alaska and central Yukon Territory, where more than 20

occurrences are documented in the Klondike region. Dawson tephra erupted about 25; 300 14C yrBP, near the onset of the last glaciation,

and provides a stratigraphic marker across Eastern Beringia. We report radiocarbon ages, paleobotanical data, and cryostratigraphic

observations from a new Dawson tephra locale at Goldbottom Creek, in the Klondike region of Yukon Territory, which collectively

indicate that the eruption occurred in the late winter or early spring. Multiple, fining-upward tephra-rich ice beds are interpreted as

remnants of surface icings, which presently are common in the region during spring. A buried in situ riparian meadow, preserved below

the icing and tephra, consists of abundant tufted hair grass (Deschampsia caespitosa), with interspersed horsetails (Equisetum cf. palustre)

and mosses. Detrital plant remains and preserved in situ grass inflorescences entombed in the icing had expelled their fruits, consistent

with a late season surface when the icing was active. The extraordinary thickness of Dawson tephra in central Yukon likely reflects

reworking of a winter-deposited tephra by snow melt in the spring following the eruption, indicating that the primary thickness may be

overestimated at valley-bottom sites. Winter deposition of the tephra may have, in part, minimized the terrestrial ecological impacts of

the eruption on zonal ‘‘steppe-tundra’’ vegetation through the retransportation of tephra from hillslopes to the riparian areas, where the

tephra became incorporated into local fluvial systems.

r 2006 Elsevier Ltd. All rights reserved.

1. Introduction

Tephra beds derived from source volcanoes in theAleutian Arc-Alaska Peninsula and Wrangell Volcanicfields cover large areas of Alaska and adjacent YukonTerritory. The application of tephrochronologic methods todistal deposits of these eruptions has enabled theircorrelation and an assessment of the magnitude of pasteruptions by more accurate understanding of their distribu-tion and volume (e.g., Westgate et al., 1983; Beget,2001; Froese et al., 2002; Mangan et al., 2003). Paststudies of these eruptions have focused on their use aschronostratigraphic markers, while acknowledging that the

e front matter r 2006 Elsevier Ltd. All rights reserved.

ascirev.2006.01.028

ing author. Tel.: +780 492 1968; fax: 780 492 2030.

ess: duane.froese@ualberta.ca (D.G. Froese).

environmental impact of these eruptions may have beenconsiderable, either indirectly through atmospheric impactsor through direct deposition of tephra on the landscape. Inthis paper, we document cryostratigraphic and paleobota-nical evidence from a new locality in central YukonTerritory, which indicates winter or early spring depositionof the late Pleistocene Dawson tephra. The timing of theeruption and influence of snow and ice in remobilizingtephra may have decreased the impacts of the eruption onregional late Pleistocene terrestrial ecosystems. We alsopresent a revised eruption chronology and document anexceptional permafrost-preserved, in situ, vegetated surfaceburied beneath the tephra. These observations highlight theimportance of this regional chronostratigraphic marker andits significance to understanding paleoenvironments ofeastern Beringia during the last glaciation.

ARTICLE IN PRESSD.G. Froese et al. / Quaternary Science Reviews 25 (2006) 1542–1551 1543

2. Dawson tephra

The late Pleistocene Dawson tephra is the mostprominent tephra bed in the Klondike region of centralYukon Territory (Westgate et al., 2000; Froese et al., 2002).It consists mainly of thin, bubble-wall glass shards ofrhyolitic composition with a crystal component comprisingorthopyroxene, plagioclase, magnetite and ilmenite withminor clinopyroxene, apatite and zircon (Westgate et al.,2000). Its mineralogy and chemistry indicate a sourceregion in the Aleutian-Arc Alaska Peninsula of south-western Alaska (Westgate et al., 2000). Later studies of theEmmons Lake Volcanic centre on the Alaska Peninsula,1700 km southwest of the Klondike region, correlate alarge (450 km3) caldera-forming event at Emmons to theDawson tephra on the basis of compositional data andradiocarbon ages (Mangan et al., 2003).

Dawson tephra has been identified at more than 20 sitesin the Klondike and Sixtymile Placer districts of westernYukon (Fig. 1) (Froese et al., 2002). In this area it ispresent within ‘muck’ deposits, a regional term for valley-bottom accumulations of ice-rich loess and retransported

Fig. 1. Location of Klondike area study site and probable distribution of D

loess and colluvium overlying gold-bearing gravel inmining areas of Yukon and Alaska (Fraser and Burn,1997; Kotler and Burn, 2000). It has been assumed, becauseof the extraordinary thickness of the tephra, rangingbetween 15 and 30 cm in the region, that the primarythickness of the bed was similarly large and likely 415 cm(Westgate et al., 2000; Froese et al., 2002). The probableminimum distribution of Dawson tephra is shown in Fig. 1based on the source area at Emmons Lake. Beget et al.(2005) suggest a more complex trajectory to explain its lackof prominence in Alaska, based on sediment cores takenfrom the Gulf of Alaska and a thin bed in the Fairbanksarea. However, loess deposits dating to marine isotopestage 2 are rare in the Fairbanks area (Muhs et al., 2003),and the lack of prominent beds of Dawson tephra in theFairbanks stratigraphic sequences may, in part, reflect itslack of burial and preservation.

3. Methods

Mining exposures at Goldbottom Creek, 30km southeastof Dawson City (Fig. 1), were examined during May and

awson tephra in Yukon and Alaska (modified from Froese et al., 2002).

ARTICLE IN PRESSD.G. Froese et al. / Quaternary Science Reviews 25 (2006) 1542–15511544

July, 2004. Several exposures were logged and sampled withspecial emphasis on the interval associated with Dawsontephra. Elevations between sites were measured with aLasertech 200XL laser range finder, which measures verticaldistances to 0.1m vertical precision; however with operatormovement, we estimate overall vertical accuracy of about0.2m. Ground ice descriptions follow the cryostructures ofMurton and French (1994), but since ice volume was notdetermined, facies distinctions between ice-rich sedimentand sediment-rich ice are based on whether sediment or iceappeared more dominant in the field.

Pore and massive ice samples were collected for stableisotope analyses. Surface ice was cleared and trenches etchedinto the frozen surface to channel meltwater away from thesampling area. Double-bagged ice samples were allowed tocompletely melt before decanting water into glass vials thatwere promptly sealed. Oxygen and hydrogen isotopiccomposition was determined at G.G. Hatch IsotopeLaboratories, University of Ottawa, Canada. Results areexpressed in standard d-notation, with routine d18O and dD2s analytical uncertainty of 0:10% and 1:5%, respectively.

A 25 cm� 30 cm portion of the buried vegetation (sampleno. GZ.04.29) was removed from the site intact. In thelaboratory, the entire sample was soaked in water to dispersethe sediment and screened through a 250mm sieve. Theretained plant material was scanned wet in petri dishes using adissecting microscope and identified with the aid of herbariumreference material. Paleobotanical samples were radiocarbondated at the Christian Albrechts University and calibratedusing the 50,000yr calibration dataset of Hughen et al. (2004).

4. Goldbottom site

Mining at Goldbottom Creek has produced a series ofmining cuts ranging from 25 to 50m in length, exposing

Fig. 2. Positions of Dawson tephra locales a

2–3m of gravel overlain by up to 5m of massive to crudelystratified silt. The silt material is a regional late Pleistoceneunit found in the Klondike area, locally ice-rich to ice-poor, interpreted as primary and retransported loess or‘muck’ (Fraser and Burn, 1997). We excavated exposures atfour sites (Fig. 2) that highlight the variable preservation ofthe tephra and associated ice bodies and vegetation.

4.1. Stratigraphy

4.1.1. Site 1

Site 1 is on the northern (downstream) end of theexposure �5m above the projected gravel-silt contact,which is covered (Fig. 2 inset). Dawson tephra is preservedas a concave-up bed at least 5m across within crudelystratified-to-massive silt with rare graminoid rhizomes. Thetephra bed is not exposed along the upvalley extension ofthe bed and pinches out downvalley from a maximumthickness of 35 cm (Fig. 3A). Interbeds of sandy silt arepresent within the tephra, along with at least one large(15 cm diameter) rodent burrow trace that is comparable tofossil arctic ground squirrel burrows known in the area(Zazula et al., 2005). Silts underlying and overlying thetephra were frozen at depth, but ice-poor, and thusrepresent dry permafrost.

4.1.2. Site 2

Site 2 is 45m south of site 1 in the centre of the lower pitand marks the thickest bed of Dawson tephra observed atGoldbottom Creek (Fig. 3B) Basal gravel at the site isexposed over the length of the mining cut up to 2.5m abovebedrock. The lower gravel (0–1.5m) is imbricate, domi-nated by coarse, sandy-matrix-filled, angular-to-subangu-lar clasts. The lower gravel grades to planar–tabular cross-bed sets up to 1.0m thick. The upper surface of the

t the Goldbottom site described in text.

ARTICLE IN PRESS

Fig. 3. (A) Tephra and ground squirrel burrow trace (circled) at site 1. Knife handle (arrow) is 10 cm long; (B) Site 2 exposure of Dawson tephra as a

continuous bed up to 80 cm thick; (C) Fluvial gravel at site 2. Ice axe is 80 cm long; (D) Vegetated surface underlying Dawson tephra adjacent to site 2.

Folding knife is 9 cm long.

D.G. Froese et al. / Quaternary Science Reviews 25 (2006) 1542–1551 1545

planar–tabular cross-bed set is capped by a laterallycontinuous bed of fibrous organic material over at least10m (Fig. 3C). The organic bed is locally eroded by theoverlying 50–85 cm of crudely imbricate matrix-filledcobble gravel. The upper surface of the gravel is overlainby a 20–35 cm-thick densely fibrous organic silt horizon(described as vegetated surface below) (Fig. 3D), andincludes discontinuous beds of sandy granules up to 1 cmthick. The vegetated surface can be traced laterally abovethe gravel along the mining cut exposed at site 2 (40mlaterally) and along the perpendicular cut separating thelower and upper pits (Fig. 2 inset map).

Tephra in the lower pit is up to 80 cm thick and locallyoverlies a 30–40 cm thick ice complex of wavy-parallel-to-planar-parallel layered ice and sediment/tephra-rich ice. Atsite 2, the lowest ice (10–15 cm) directly overlies thevegetation surface and undulates slightly with the topo-graphy of the underlying vegetation surface (Fig. 4A). Theice is translucent with near-spherical bubbles 2–3mm

across and includes grass stems in growth positionextending into the ice (Fig. 4C, D). Grass florets were notattached to in situ grass panicles recovered from within theice. The lowermost 10 cm of ice includes minor silt and raregrass leaves, but lacks tephra. Sharply overlying the tephra-free ice is 10 cm of tephra-rich ice (Fig. 4A). The tephra-rich ice is sharply overlain by 15 cm of ice-rich tephra withrare interbedded silt, which grades to 45 cm of weaklystratified ice-rich tephra with silt intrabeds. The uppersurface of the tephra is diffuse and grades to 3m of massivegrey-brown silt terminating at an upper surface disturbedby mining at the site.

4.1.3. Site 3

A third site was investigated in the upper pit where thebase of Dawson tephra is 3.9m higher than at site 2(Fig. 2). The basal gravel was not exposed by mining whenthe site was described. The base of the exposure was 0.5mbelow the base of Dawson tephra, with massive ice-rich silt

ARTICLE IN PRESS

Fig. 4. (A) Tephra-free and tephra-rich ice overlying vegetated surface at Site 2. Knife handle is 10 cm long; (B) Tephra-free ice overlain sharply by ice-rich

tephra at site 3. Arrow points to grass stem in ice; (C) Tephra-free ice with grass stems entombed in ice near site 2 with surface drape of Dawson tephra.

Folding knife is 9 cm long; (D) Grass stems entombed in tephra-free ice at site 2; (E) Stratified tephra-rich ice with detrital plant material (circles) sharply

overlain by ice-rich tephra at site 4. Scale bar marks 10 cm; (F) Fining-upward lenses of tephra-rich ice. Note thin occurrence of tephra at base of ice lens

grading to tephra-poor ice.

D.G. Froese et al. / Quaternary Science Reviews 25 (2006) 1542–15511546

ARTICLE IN PRESSD.G. Froese et al. / Quaternary Science Reviews 25 (2006) 1542–1551 1547

with rare granules exposed over that interval. Undulatingwavy horizontally layered tephra-free ice, consisting of3–8 cm thick beds, sharply overlies the massive silt. Airbubbles within the ice are up to 2.5mm in diameter andweakly horizontal to elongate. Detrital plant remains arepresent in the lowermost 3–5 cm of ice (Fig. 4B). This lowerice lens is sharply overlain by 2–3mm diameter granularice, which is similarly free of tephra, but lacks visible plantremains. Ice-rich Dawson tephra (12–30 cm thick) sharplyoverlies the tephra-free ice (Fig. 4B). The upper surface ofthe tephra grades to silt-rich tephra and eventually, overabout 1m, into massive ice-rich silt.

4.1.4. Site 4

A fourth site was investigated in the upper pit, about25m west of site 3 (Fig. 2). The basal gravel was notexposed when the site was described, but a thick bed ofDawson tephra was present with prominent stratified iceunderlying the tephra (Fig. 4E). Stratified lenses of tephra-rich ice grading to translucent-to-milky ice (2–4 cm thickbeds) sharply overlie massive ice-rich silt (Fig. 4F). Thesethree beds, totalling 12–15 cm, are flat-lying to wavy andinfill the topography of the underlying ice-rich silt. Theisotopic composition of three samples taken from thestratified ice lenses was relatively constant, with d18O anddD values ranging from �26:4% to �26:9%, and �213%to �218%, respectively. Detrital plant remains, includinggrass stems and leaves, are present within some of thestratified ice lenses (Fig. 4E). The stratified tephra-rich iceis sharply overlain by 50 cm of finely laminated to massivetephra. A pore ice sample recovered from the tephra hadlow d18O (�28:3%) and dDð�225% values, relative to theunderlying stratified ice. The upper surface of the tephragrades to �1:5m of massive, ice-rich silt, the upper surfaceof which is disturbed by mining at the site.

5. Vegetation surface

The exceptional permafrost-preservation of plant re-mains enables us to reconstruct vegetation that was presenton the buried surface immediately before deposition ofDawson tephra at Goldbottom Creek (Table 1). Plantremains are dominated by graminoid (grass and/or sedge)stems, leaves, roots, and rhizomes, empty grass panicles,stems, rhizomes and roots of Equisetum cf. palustre andvarious mosses. Abundant grass florets were recovered,dominated by Deschampsia caespitosa, with rare specimensof Elymus sp., Festuca sp., Poa sp. and Phalaris arundina-

ceae. The dominance of D. caespitosa florets and theirexceptional preservation of awns and hairs suggest thatmuch of the in situ vegetative graminoid material alsobelongs to this taxon. Empty panicles in which the floretshad dehisced were also consistent with D. caespitosa. Thesemacrofossils indicate that the buried surface represents amesic grass-dominated riparian meadow. Seeds and fruitsfrom a wide variety of dicotyledonous forbs and othergraminoids are rare and probably allocthonous in the

assemblage because no intact stems with leaves orinflorescences of these plants were recovered. The lack ofmacrofossils from arboreal taxa suggests trees were locallyrare or absent. Rare Salix twigs suggest that dwarf willowsmay have been the only shrubs present. Some herbs, forexample Chenopodium, probably inhabited well-drained,gravel and sand bars in that floodplain that were frequentlydisturbed through fluvial activity and other well drainedareas in the watershed. Specimens of other grasses such asFestuca and Elymus, and forbs including Phlox hoodii,Draba and Bistorta vivipara were probably components ofupland steppe-tundra vegetation that inhabited the localvalley slopes, and are similar to assemblages proposed forother sites in interior and northern Yukon (Zazula et al.,2003a, b; 2005, 2006).

6. Age of the Dawson tephra

Two samples of individual graminoid leaves from thevegetated surface directly underlying Dawson tephra at site2 were sampled for radiocarbon dating. Each sampleyielded in excess of 1mg of carbon for precise measurement(P. Grootes, written communication, 2005). The two agesoverlap at the 1s confidence interval and provide anew maximum eruption age of about 25; 300 14C yrBP(Table 2). The new maximum age is consistent, althoughsomewhat older than, a previous minimum age estimate ofabout 24; 000 14C yrBP (Froese et al., 2002). Thus, deposi-tion of Dawson tephra can be constrained between 25,300and 24; 000 14C yrBP. Given the close association betweenthe vegetated surface and deposition of Dawson tephra, itsage is likely closer to the age of the vegetated surface, orabout 25; 300 14C yrBP. Calibration of this age withthe Cariaco Basin 14C dataset provided by Hughen et al.(2004) indicates the eruption occurred at about 28,700–29,500 cal yr BP, somewhat older than the estimate ofFroese et al. (2002).

7. Stratigraphic, cryostratigraphic and paleobotanical

interpretation

The gravel at the Goldbottom site is characteristic of latePleistocene fluvial deposits in the Klondike area (Froeseet al., 2001; Froese and Jackson, 2005a, b). Crude,imbricate cobble gravel with sandy-matrix fill indicates anabundance of coarse bedload addition to the creek bottomand reorientation of clasts by traction currents. Well-developed planar–tabular cross-bed sets, grading frompebble gravel to sand and silt is diagnostic of the presenceof transverse bars, indicating multiple channels or braiding,in paleo-Goldbottom Creek. Relief of cross-bed sets andthe presence of discrete beds of sand and silt along planar–tabular cross-beds suggest multi-year bars undergoingmigration, in contrast to more proximal braided systemsundergoing rapid bar changes (Miall, 1977; Smith, 1985).Field relations at Goldbottom Creek allow us to

constrain the origin of the wavy-to-parallel-bedded ice

ARTICLE IN PRESS

Table 1

Plant macrofossils recovered from buried vegetation surface (sample no. GZ.04.29). Nomenclature follows Cody (2000) and Flora of North America

(1993+)

Taxon Abundancea Ecology

Equisetaceae

Equisetum cf. palustre (stems, roots) *** Wet woods and meadows, shores and shallow water

Poaceae

Deschampsia caespitosa (stems, leaves, *** Wet meadows, lakeshores, gravel bars

roots, florets, empty panicles)

Elymus sp. (florets) * Sandy, gravelly riverbanks, hillsides, open, well-drained slopes

Festuca sp. (florets) * Grassland, tundra, sandy, gravelly riverbanks, lakeshores

Phalaris arundinaceae (florets) * Moist shores of riverbanks and sandy lakes

Poa sp. (florets) * Tundra, grassland, meadows, streambanks, open well-drained slopes

Cyperaceae

Carex sp. (trigonous achenes) * Mesic to xeric, grassland to tundra, often riparian

Carex sp. (lenticular achenes) *

Juncaceae

Juncus sp. (seeds) * Wet, riverbanks, tundra, gravel bars, lakeshores

Salicaceae

Salix sp. (twigs) * Probably dwarf shrub, tundra, riverbanks

Polygonaceae

Bistorta vivipara (bulbils) * Turfy places, rocky barrens, moist grassy herbmats, favors manured places

Chenopodiaceae

Chenopodium sp. (seeds) * Weedy, disturbed, often halophytic

Caryophyllaceae

Silene taimyrensis (capsule fragments, seeds) * Sandy and rocky open slopes and cliffs

Brassicaceae

Draba sp. (seeds) ** Arctic-alpine tundra, often calcareous, well-drained, rocky and turfy places

Erysimum cf. cheiranthoides (seeds) ** Rocky outcrops, dry slopes, river terraces

Rosaceae

Potentilla sp. (achenes, herb type: not * Grassy riverbanks, dry slopes, alpine tundra, open

P. palustris, P. fruiticosa or P. norvegica) meadows

Polemoniaceae

Phlox hoodii (leaves, capsule fragments) * Grasslands and foothills

Asteraceae

Artemisia sp. (flowers) *

Artemisia frigida (leaves) * Steep open slopes and sandy riverterraces

Bryophytes ***

a� ¼ rare; �� ¼ moderate; � � � ¼ abundant.

Table 2

Radiocarbon ages associated with Dawson tephra in the Klondike area

Age (14C yrBP) d13C Dated material Laboratory no. Context Source

23; 520� 210 �25.0a Small twigs and grass TO-6968 Above tephra Westgate et al., 2000

23; 990� 130 �26.3 Grass Beta-161238 Above tephra Froese et al., 2002

24; 280� 130 �25.0 Draba sp. seed capsule Beta-161239 Above tephra Froese et al., 2002

25; 410� 160 �20.0 Grass leaf KIA25702 In situ surface below tephra This paper

25; 210� 260 �24.6 Grass leaf KIA25703 In situ surface below tephra This paper

25; 700� 400 Carex achenes Beta-171748 Below tephra Zazula et al., 2003b

21; 880� 180 �25.0a Peat TO-8304 Below tephra Froese et al., 2002

22; 300� 190 �25.0a Small twigs and grass TO-6967 Below tephra Westgate et al., 2000

24; 025� 550 �25.9 Grass bed BGS-1755 Below tephra, in situ grass bed Fraser and Burn, 1997

25; 240� 140b �25.9 Peat Beta-133410 Below tephra Froese et al., 2002

aEstimated.bSame sample as TO-8304.

D.G. Froese et al. / Quaternary Science Reviews 25 (2006) 1542–15511548

ARTICLE IN PRESSD.G. Froese et al. / Quaternary Science Reviews 25 (2006) 1542–1551 1549

underlying Dawson tephra. If it were intrusive ice, commonwithin the discontinuous permafrost zone, disruption of thehost sediments typically occurs (e.g., Mackay, 1972) and isinconsistent with interbedding of the tephra and detritalplant remains observed at Goldbottom Creek. Thisinterbedding of tephra and detrital plant remains, togetherwith the presence of horizontally oriented bubbles at site 3,likewise precludes a segregated origin. French and Pollard(1986) hypothesized a buried snowbank origin for massiveground ice in the Klondike area near the base of the mucks,which includes minor sediment concentrations, detritalplant remains, and sub-horizontal bubble orientations.However, a buried snowbank origin is not supported by thewell-developed stratification and bedding present withinburied ice at sites 2 and 4.

The presence of graminoid stems extending into the iceat sites 2 and 3 (Fig. 4B, C, D) indicates that vegetation wasstanding in growth position under ponded water. The lackof florets within grass panicles indicates that the grasseshad expelled their fruits prior to being entombed, and issuggestive of a post-growing season surface (fall to earlyspring) at the time of water mobility. The sharp contactbetween beds of tephra-free ice and overlying tephra-richice (Fig. 4A) suggests that the ice originated as surfacewater rather than buried snow because differential infiltra-tion of tephra-laden meltwater through a melting snow-pack would have resulted in an irregular contact.

The well-developed stratification at site 4, in particularthe presence of multiple, aggrading, fining-upward beds oftephra-rich ice (Fig. 4E, F), strongly suggests that waterflowing along the surface of the ice-rich silt mobilizedtephra during a period of surface water mobility andfreezing. The fining-upward beds grade from tephra-richice at the base to clear-to-translucent ice near the surface,and include detrital plant remains (Fig. 4E). Importantly,the fining-upward horizons predate deposition of the majorthickness of Dawson tephra at the site, measured at 50 cm,suggesting that diurnal surface melting of snow and icemobilized tephra prior to deposition of the main thicknessof tephra being transported from hillslopes to the valleybottom. It is possible that flowing surface waters couldhave originated from a perennial spring, but our isotopicdata do not allow us to unambiguously distinguish thewater source.

The cryostratigraphic and paleobotanical observationscollectively suggest that snow and ice were present on thelandscape and had entombed surface vegetation prior tothe eruption that deposited Dawson tephra. The presenceof multiple-layered tephra-rich ice horizons overlyingtephra-free ice suggests that surface waters mobilizedtephra on the landscape prior to the main thickness ofDawson tephra being emplaced in the valley bottoms, andsecondly, that these surface water processes were activeprior to, and following, the eruption of Dawson tephra.The presence of these tephra-rich ice horizons suggests thatthe eruption occurred while the diurnal temperature rangewas sufficient to allow surface water transport and freezing

on the landscape. Our cryostratigraphic observations pointto an icing (also known as naled or aufeis) origin for thebedded ice at Goldbottom Creek. Stacked beds of ice aretypically observed in field and laboratory investigations oficings (Carey, 1973; Schohl and Ettema, 1986). In interiorYukon today, ground icings, locally termed ‘glaciers’, aremost common in the late spring (March–May), when thediurnal temperature range allows daytime melting andevening freezing, and commonly affect roads in theKlondike region.Our observations at Goldbottom Creek suggest a two-

stage mobilization that led to local overthickening ofDawson tephra, and prompts some critical reappraisal ofthe primary thickness of Dawson tephra. Previous esti-mates of415 cm (Westgate et al., 2000; Froese et al., 2002)were based on measured occurrences, which are preferen-tially located along valley margins as a result of the generaldistribution of mining activities in the region. In a generalsense, earlier mining in the Klondike region focused onvalley-centre deposits, which were in many cases of highergrade and more suitable to dredging and mining. Based onour experience in the region, the Goldbottom exposure istruly exceptional in that a single tephra bed could beexamined in several positions on the landscape. The muchthinner Dawson tephra bed at Last Chance Creek, in asimilar valley-bottom setting, ranged from 5 to 10 cmthickness (Zazula et al., 2003b) and may be a closerestimate to the primary airfall thickness. However, even atthis reduced thickness, the magnitude of the Dawsontephra eruption is still considerable given the �1700 kmdistance between the Klondike area and the eruptive sourceat Emmons Lake (Mangan et al., 2003). Dawson tephrathickness is comparable to or thicker than that of theregionally prominent Old Crow tephra (Westgate et al.,1983) and, furthermore, is the thickest of the Aleutian-sourced tephra beds ever recovered in Yukon Territory.

7.1. Paleobotanical significance of vegetated surface

Much of the discussion on last-glacial vegetation ineastern Beringia has been based on the interpretation ofregionally biased paleoecological proxies, such as lacus-trine-derived pollen records (e.g., Anderson and Brubaker,1994), or inferences based on fossil mammals (Guthrie,1990). Plant macrofossil records have provided data onlocal vegetation at some sites, though they too have severaltaphonomic issues that affect their interpretations (Birksand Birks, 2000; Zazula et al., 2006). Buried in situ surfacesoffer rare opportunities to unequivocally determine floristiccomposition and structure at a particular site. The onlyother site in eastern Beringia with in situ vegetation is the18; 000 14C yrBP upland tundra community buried underDevil Mountain Lake tephra on the Seward Peninsula(Goetcheus and Birks, 2001). Although these two buriedsurfaces are ca 1200 km apart and differ in age by severalthousands of years, they offer a stark contrast of mesicriparian and xeric upland habitats during last-glacial

ARTICLE IN PRESSD.G. Froese et al. / Quaternary Science Reviews 25 (2006) 1542–15511550

conditions and point to the importance of locally derivedpaleobotanical data. Elsewhere in the Klondike, plantmacrofossils recovered from arctic ground squirrel mid-dens associated with Dawson tephra contrast with theriparian grassy meadow vegetation at Goldbottom Creek(Zazula et al., 2005). Midden records are dominated byupland graminoids and forbs that probably inhabited thelocal hillslopes and are present as rare detrital macrofossilsassociated with the buried vegetation at GoldbottomCreek (Table 1). The marked habitat and vegetationdifferences between upland and lowland sites may havebeen a key factor that enabled diverse mammals to meetnutritional requirements throughout the year during thelast glaciation.

A new generation of paleoecological approaches basedon ancient genetic analyses from plants (Willerslev et al.,2003) and, more commonly, the exceptional large mammalrecords in the region (Shapiro et al., 2004), show potentialfor detailed examination of the response of specificpopulations to environmental stresses. These approachesare still in their infancy as paleoecological tools, requiringlarger datasets and integration with other paleoenviron-mental proxies. Dawson tephra is ideally situated withinpermafrost-preserved sedimentary sequences to examinethe potential local and regional impacts of this largeeruption on last-glacial Beringian ecosystems. Our obser-vations at Goldbottom Creek, which suggest that theeruption occurred in the winter or spring when surfaceicings were active, lead us to consider that the environ-mental impacts of this eruption were likely lessened by itsseasonality. If melting snow and ice were the mainmechanisms by which the tephra was transported to valleybottom sites in the spring, tephra-laden hillslopes may havebeen opened to new vegetation and grazing opportunities.

8. Conclusions

The Goldbottom Creek record is the first documented insitu vegetated surface from the interior Yukon during thelast glaciation. Field investigations at the GoldbottomCreek site provide new insight into the environmentalsetting of interior Yukon at about 25; 000 14C yrBP, nearthe start of the last glaciation. Fluvial systems draining theunglaciated uplands of the Klondike region began toaggrade as hillslope materials, accumulated during the mid-Wisconsinan interstadial, were transferred to valley bot-toms. Loess aggradation at about this time occurredregionally within west-central Yukon Territory. Dawsontephra erupted in the late winter or spring as evidenced bythe interbedding of tephra with thin surface icing horizons,while the bulk of the tephra was mobilized later into thevalley bottoms, likely in the same season. The extraordin-ary thickness of Dawson tephra at sites in the Klondikearea thus includes significant reworking of the tephra, witha probable primary thickness of less than 15 cm. The buriedmesic riparian meadow, preserved at Goldbottom Creek,strongly contrasts with xeric upland vegetation recon-

structed from fossil ground squirrel middens in the area.Winter deposition of the tephra may have, in part,minimized the terrestrial ecological impacts of the eruptionon zonal ‘‘Steppe-tundra’’ vegetation through the retran-sportation of tephra from hillslopes to the riparian areas,where the tephra became incorporated into local fluvialsystems.

Acknowledgements

This study would not have been possible without thesupport and logistical aid provided by David Millar andfamily, operators of the Goldbottom mine. Discussions inthe field with Scott Smith, Paul Sanborn, and JohnWestgate are gratefully acknowledged, in particularSmith’s insights into icing processes. Able field assistancewas provided by Britta Jensen, John Storer, Elizabeth Hall,Victoria Castillo, Eske Willerslev, and Alan Cooper.Financial support for this study was provided by a NaturalSciences and Engineering Research Council DiscoveryGrant to DF, and Northern Scientific Training Programgrants (administered through Department of IndianAffairs and Northern Development) to AVR and GDZ.

References

Anderson, P.M., Brubaker, L.B., 1994. Vegetation history of northcentral

Alaska: a mapped summary of late-Quaternary pollen data. Quatern-

ary Science Reviews 13, 71–92.

Beget, J.E., 2001. Continuous late Quaternary proxy climate records from

loess in Beringia. Quaternary Science Reviews 20, 499–507.

Beget, J.E., Pedersen, T.F., Muhs, D., 2005. Terrestrial-marine correlation

of the Dawson tephra. In: Alloway, B.V., et al. (Eds.), Proceedings of

the International Field Conference and Workshop on Tephrochronol-

ogy and Volcanism. Institute of Geological and Nuclear Sciences,

Lower Hutt, New Zealand, 58 pp.

Birks, H.H., Birks, H.J.B., 2000. Future uses of pollen analysis must

include plant macrofossils. Journal of Biogeography 27, 31–35.

Carey, K.L., 1973. Icings developed from surface water and ground water.

Cold Regions Science and Engineering Monograph III-D3.

Cody, W.J., 2000. Flora the Yukon Territory – 2nd. edition. National

Research Council of Canada Press, Ottawa, Canada.

Flora of North America Editorial Committee (Eds.), 1993+. Flora of

North America North of Mexico. 7+ vols. Oxford University Press,

New York and Oxford.

Fraser, T.A., Burn, C.R., 1997. On the nature and origin of ‘‘muck’’

deposits in the Klondike area Yukon Territory. Canadian Journal of

Earth Sciences 34, 1333–1344.

French, H.M., Pollard, W.H., 1986. Ground-ice investigations, Klondike

District, Yukon Territory. Canadian Journal of Earth Sciences 23,

550–560.

Froese, D.G., Jackson, Jr., L.E., 2005a. Surficial geology, Granville,

Yukon Territory (NTS 115-O/10). Geological Survey of Canada, Open

File 4587, scale 1:50,000.

Froese, D.G., Jackson, Jr., L.E., 2005b. Surficial geology, Grand Forks,

Yukon Territory (NTS 115-O/14). Geological Survey of Canada, Open

File 4591, scale 1:50,000.

Froese, D.G., Enkin, R.J., Smith, D.G., 2001. Placer depositional settings

and their ages along Dominion Creek, Klondike area, Yukon. In:

Emond, D.S., Weston, L.H. (Eds.), Yukon Exploration and Geology

2000. Exploration and Geological Services Division, Yukon Region,

Indian and Northern Affairs, Canada, pp. 159–169.

ARTICLE IN PRESSD.G. Froese et al. / Quaternary Science Reviews 25 (2006) 1542–1551 1551

Froese, D.G., Westgate, J.A., Preece, S.J., Storer, J., 2002. Age and

significance of the late Pleistocene Dawson tephra in eastern Beringia.

Quaternary Science Reviews 21, 2137–2142.

Goetcheus, V.G., Birks, H.H., 2001. Full-glacial upland tundra vegetation

preserved under tephra in the Beringia National Park, Seward

Peninsula, Alaska. Quaternary Science Reviews 20, 135–147.

Guthrie, R.D., 1990. Frozen Fauna of the Mammoth Steppe: The Story of

Blue Babe. University of Chicago Press, Chicago.

Hughen, K., Lehman, S., Southon, J., Overpeck, J., Marchal, O., Herring,

C., Turnbull, J., 2004. 14C activity and global carbon cycle changes

over the past 50,000 years. Science 303, 202–207.

Kotler, E.T., Burn, C.R., 2000. Cryostratigraphy of the Klondike ‘‘muck’’

deposits west-central Yukon Territory. Canadian Journal of Earth

Sciences 37, 849–861.

Mackay, J.R., 1972. The world of underground ice. Annals of the

Association of American Geographers 62, 1–22.

Mangan, M.T., Waythomas, C.F., Miller, T.P., Trusdell, F.A., 2003.

Emmons Lake Volcanic Center, Alaska Peninsula: source of the Late

Wisconsin Dawson tephra, Yukon Territory, Canada. Canadian

Journal of Earth Sciences 40, 925–936.

Miall, A.D., 1977. A review of the braided river depositional environment.

Earth Science Reviews 13, 1–62.

Muhs, D.R., Ager, T.A., Bettis III, E.A., McGeehin, J., Been, J.M., Beget,

J.E., Pavich, M.J., Stafford Jr., T.W., Stevens, D.S.P., 2003.

Stratigraphy and palaeoclimatic significance of Late Quaternary

loess-palaeosol sequences of the Last Interglacial-Glacial cycle in

central Alaska. Quaternary Science Reviews 22, 1947–1986.

Murton, J.B., French, H.M., 1994. Cryostructures in permafrost,

Tuktoyaktuk coastlands, western Arctic, Canada. Canadian Journal

of Earth Sciences 31, 737–747.

Schohl, G.A., Ettema, R., 1986. Theory and laboratory observations of

naled ice growth. Journal of Glaciology 32, 168–177.

Shapiro, B., Drummond, A.J., Rambaut, A., Wilson, M.C., Matheus,

P.E., Sher, A.V., Pybus, O.G., Gilbert, M.T.P., Barnes, I., Binladen, J.,

Willerslev, E., Hansen, A.J., Baryshnikov, G.F., Burns, J.A.,

Davydov, S., Driver, J.C., Froese, D.G., Harington, C.R., Keddie,

G., Kosintsev, P., Kunz, M.L., Martin, L.D., Stephenson, R.O.,

Storer, J., Tedford, R., Zimov, S., Cooper, A., 2004. Rise and fall of

the Beringian steppe bison. Science 306, 1561–1565.

Smith, N.D., 1985. Proglacial fluvial environment. In: Ashley, G.M.,

et al. (Eds.), Glacial Sedimentary Environments. Society of

Economic Paleontologists and Mineralogists, Short Course no. 16,

pp. 85–134.

Westgate, J.A., Hamilton, T.D., Gorton, M.P., 1983. Old Crow tephra:

a new late Pleistocene stratigraphic marker across north-central Alaska

and western Yukon Territory. Quaternary Research 19, 38–54.

Westgate, J.A., Preece, S.J., Kotler, E.C., Hall, S., 2000. Dawson Tephra:

a prominent stratigraphic marker of late Wisconsinan age in west-

central Yukon, Canada. Canadian Journal of Earth Sciences 37,

621–627.

Willerslev, E., Hansen, A.J., Binladen, J., Brand, T.B., Gilbert, M.T.P.,

Shapiro, B., Bunce, M., Wiuf, C., Gilichinsky, D.A., Cooper, A., 2003.

Diverse plant and animal genetic records from Holocene and

Pleistocene sediments. Science 300, 791–795.

Zazula, G.D., Froese, D.G., Schweger, C.E., Mathewes, R.W., Beaudoin,

A.B., Telka, A.M., Harington, C.R., Westgate, J.A., 2003a. Ice age

steppe vegetation in east Beringia. Nature 423, 603.

Zazula, G.D., Froese, D.G., Telka, A.M., Harington, C.R., Mathewes,

R.W., Westgate, J.A., 2003b. Plants, bugs and a giant mammoth tusk:

paleoecology of Last Chance Creek, Yukon Territory. In: Emond,

D.S., Lewis, L.L. (Eds.), Yukon Exploration and Geology 2002.

Exploration and Geological Services Division, Yukon Region, Indian

and Northern Affairs, Canada, pp. 251–258.

Zazula, G.D., Froese, D.G., Westgate, J.A., La Farge, C., Mathewes,

R.W., 2005. Paleoecology of Beringian ‘‘packrat’’ middens from

central Yukon Territory, Canada. Quaternary Research 63, 189–198.

Zazula, G.D., Schweger, C.E., Beaudoin, A.B., McCourt, G.H., 2006.

Macrofossil and pollen evidence for full-glacial steppe within

an ecological mosaic along the Bluefish River, eastern Beringia.

Quaternary International 142–143, 2–16.