Permafrost, tectonics, and past and future regional climate change ...

Permafrost, tectonics, and past and future regional climate change, Yukon and adjacent Northwest Territories C.R. BURN Geotechnical Science Laboratories, Department of Geography, and Ottawa - Carleton Centre for Geoscience Studies, Carleton University, 1125 Colonel By Drive, Ottawa, ON KIS 5B6, Canada

Received July 12, 1993 Revision accepted October 30, 1993 Late Tertiary changes in the general circulation of the atmosphere, regionally enhanced by uplift of the Wrangell - Saint Elias and Coast mountains, were sufficient to promote permafrost development in the western Arctic. Permafrost developed in Yukon Territory and adjacent Northwest Territories during early Pleistocene glacial periods, after continued tectonic activity led to further modification of regional climate, but degraded in the interglacials. Permafrost has been present in northe m parts of the region since the Illinoian glaciation, but most ground ice in central Yukon formed in the Late Wisconsinan. The present interglacial is the only one with widespread evidence of permafrost, which is maintained in the valleys of central and southern Yukon by the Saint Elias Mountains blocking continental penetration of maritime air from the Gulf of Alaska. This reduces snow depth in winter, while cold-air drainage in the dissected terrain of the Yukon Plateaus enhances the nearsurface inversion, leading to continental minimum temperatures. General circulation models used to simulate climate represent the physiography of northwest Canada crudely. As a result, the simulations are unable to reproduce conditions responsible for the development and preservation of permafrost in the region. Le soulkement des massifs Wrangell, Saint Elias et de la chaine CBt2re amplifice de facon regionale des changements dans la circulation atmosphCrique gCnCrale qui eurent lieu au Tertiaire supCrieur. Ces changements furent suffisants pour provoquer le dCveloppement du pergelis01 dans la partie ouest de 1'Arctique. Le pergClisol s'est form6 dans le territoire du Yukon et les territoires du Nord-Ouest adjacents, durant les periodes glaciaires du PlCistocbne infkrieur, lorsque le climat rtgional fut modifiC davantage par une activitk tectonique continue, mais il s'est dBgrad6 durant les periodes interglaciaires. Le pergCliso1 Ctait prCsent dans la partie nord de la region depuis la glaciation Illinoienne, mais la majeure partie de la glace de sol du Yukon central s'est formte durant le Wisconsinien supCrieur. Seul l'interglaciaire actuel prCsente beaucoup d'Cvidences de pergCliso1, qui est conserv6 dans les vall6es du centre et le sud du Yukon par la barribre que forme le massif Saint Elias, qui empeche la pCnCtration sur le continent de l'air maritime en provenance du golfe d'Alaska. Ceci rkduit I'kpaisseur de neige en hiver alors que la dispersion d'air froid dans les terrains dissCquCs du plateau du Yukon favorise l'inversion en surface menant B des temp6ratures continentales minimales. Les mo&les de circulation atmosphCrique gkntrale, simulant les rCgimes climatiques, reprksentent de facon incomplbte la physiographie nord-ouest du Canada. Par conskquent, les simulations ne peuvent reproduire les conditions nCcessaires au dCveloppement et B la prCservation du pergklisol. Can. J. Earth

Sci. 31,

182-191 (1994)

Introduction Permafrost is a geologic manifestation of climate. Thick permafrost, hundreds of metres in depth, may take millennia to adjust fully to changes in climate (Osterkarnp and Gosink 1991), but "warm," thin permafrost, tens of metres in depth, responds relatively rapidly (Burn 1992). Most of Yukon Territory and the upper Mackenzie Valley, Northwest Territories, are in the widespread and scattered permafrost zones where permafrost is thin (Fig. 1). The potential for climate warming to cause instability in permafrost terrain as the near-surface layers of ice-rich ground melt is well recognized (e.g., Woo et al. 1992). Additionally, frozen ground is a critical component of boreal forest ecosystems through its influence on drainage. Considerable reorganization of forest communities may occur over periods of decades to centuries as permafrost recedes. Currently there is considerable interest in the regional impact of climate warming. Such impact is often predicted on the basis of results from general circulation models (GCMs). In northwest Canada topographic influences on climate are substantial, but not well represented at the scale of most GCMs. The purposes of this paper are to (i) discuss physiographic influences on regional climate in Yukon and adjacent Northwest Territories that lead to the presence of permafrost; (ii) demonstrate their efficacy through the late Cenozoic tectonic and sedimentary records; and (iii) consider the implicaPrinted in Canada I Imprimt au Canada

tions for permafrost of future regional climate scenarios from GCMs that represent topography in a generalized manner.

Physiography of southern Yukon and adjacent Northwest Territories Physiographic units The physiography of the region is divided into three main systems (Bostock 1948; Mathews 1986) (see Fig. 2). The Saint Elias Mountains and Coast Mountains flank the Pacific margin of northwest Canada. The Saint Elias Mountains is the most tectonically active unit in the region and contains large areas over 3500 m as1 (Eisbacher and Hopkins 1977). The Yukon Plateaus, Liard Lowland, Kaska Mountains, Selwyn Mountains, and Ogilvie Mountains make up most of the southern half of Yukon. Unglaciated portions of the Yukon Plateaus are "a maze of deep, narrow valleys separated by long, smooth-topped ridges whose elevations are very uniform'' at about 1300 m (Bostock 1948, p. 69). In the glaciated part, "the larger valleys have been accentuated and form a network surrounding tablelands" (Bostock 1948, p. 64); the relief of these broad valleys is about 1000 m, similar to that of the unglaciated portions. The Mackenzie Mountains and Franklin Mountains represent the eastern physiographic system. Mackenzie Lowland, containing Mackenzie River, is a broad valley of little relief, bounded by the Mackenzie Mountains and Franklin Mountains.

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B e a u f o r t

Inuvik

C

O

N

*

T

I

N

U

O

U

S

Old Crow

Norman Wells 65"-

W

I

D

P

R

E

A

D

FIG. 1. Permafrost map of Yukon and adjacent Northwest Territories (after Brown 1978), showing location of sites in Table 1 .

Tectonism and evolution of physiography The eastern mountains formed first, in late Paleozoic Mesozoic time, and the system remains tectonically active, but not to the extent of the western mountains (Gabrielse 1967). The Yukon Plateaus are remnants of a Tertiary erosion surface that was levelled between 45 and 15 Ma (Templeman-Kluit 1980). Rudiments of the present drainage network were defined by the end of the Miocene (Templeman-Kluit 1980), but the Early Pleistocene relief was likely much less than today (Mathews 1989). The uplift history of the Saint Elias Mountains is not known precisely. Uplift has been continuous since Upper Oligocene time (30 Ma), but relatively rapid (0.3 mmla on average) since 16 Ma (Eisbacher and Hopkins 1977; Parrish 1981). The uplift is due to collision of the Pacific and America plates along the Queen Charlotte - Fairweather fault systems (Adarns and Clague 1993). The elevation was sufficient by Middle to Late Miocene time for glaciation of these mountains, leading to deposition of both tillites inland, and the glaciomarine facies of the Yakataga formation in the Gulf of Alaska (Denton and Armstrong 1969; Plafker and Addicott 1976). Miocene denudation by glacial and other forces reduced relief, for glaciation was limited in Pliocene time: a well-dated pollen record from Lost Chicken mine, east-central Alaska, indicates temperate vegetation at 2.19 Ma (Matthews and Ovenden 1990). Pleistocene ice emanating from the Saint Elias Mountains may have blocked the Tertiary drainage from the Yukon Plateaus into the Gulf of Alaska, reversed the flow, and created the present Yukon drainage system (Templeman-Kluit 1980). Much of the uplift has been tectonically driven, as indicated by active Holocene faults (Eisbacher and Hopkins 1977), but some

Elevation above: 0

1000 in

111

2000 m

300 km

FIG. 2. Regional topography of Yukon and adjacent Northwest Territories.

enhancement of uplift rates is likely due to rebound following denudation, with increased erosion rates accompanying glaciation (Molnar and England 1990). A minimum estimate for the rate of tectonic uplift along the Queen Charlotte Fault in Quaternary time is 2 mmla (Clague 1989), which is sufficient to have created the highest mountain range in Canada from its subdued configuration 2 Ma ago (Mathews 1989).

Physiographic influences on climate General circulation Continental-scale tectonism and uplift of mountain ranges have probably contributed to long-term climatic cooling since the Cretaceous (Ruddiman and Raymo 1988). At global scale, the principal climatic effects of uplift are to (i) raise zonal mean albedo via increased snow cover (Birchfield and Weertman 1983); (ii) alter the structure of planetary waves to cool the North American and European continents (Ruddiman and Raymo 1988); and (iii) lower the natural "greenhouse effect" by C02 sequestration, after increased erosion at higher elevations leads to an increase in total chemical weathering (Raymo et al. 1988). Ruddiman et al. (1989), Ruddiman and Kutzbach (1989), and Kutzbach et al. (1989) investigated the sensitivity of the atmospheric system to uplift of the Tibetan Plateau and of the North American Cordillera. Numerical simulations without topography, with "half ' mountains, and with "full' ' mountains generally reproduced Tertiary climatic changes for the Northern Hemisphere inferred from the geologic record. However, as the topography developed, the models

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TABLE1. Climate statistics from selected stations in Yukon and Alaska

Juneau Whitehorse Mayo KenoHill Juneau Whitehorse Mayo Keno Hill

Oct.

Nov.

Dec.

Annual mean

9.6 7.8 6.5 2.0

5.4 1.3 -2.3 -6.3

0.3 -9.4 -15.2 -12.3

-3.0 -17.1 -24.2 -16.9

4.5 -1.0 -4.0 -5.0

Monthly mean precipitation (mm) 71.6 88.1 73.9 106.7 129.5 159.8 35.2 25.1 12.2 11.1 33.9 33.6 30.3 41.5 8.6 19.5 35.3 51.7 76.7 51.3 28.8 21.9 59.0 55.9

188.7 17.5 28.3 61.9

128.3 23.6 24.4 46.0

Jan.

Feb.

Mar.

Apr.

-5.4 -20.9 -29.0 -19.6

-2.2 -12.9 -19.9 -13.9

-0.1 -7.4 -11.7 -13.7

Monthly mean air temperature ("C) 4.0 8.2 11.7 13.2 12.6 1.0 6.7 12.1 14.5 12.6 -0.4 7.5 13.4 15.2 12.6 -6.6 1.4 7.8 10.0 7.9

94.2 21.6 17.5 41.5

91.2 12.1 16.4 54.0

80.0 15.4 10.3 35.5

May

June

July

Aug.

Sept.

! 109.0 20.4 22.5 57.7

1321.3 261.7 306.3 590.2

NOTES:Data for Juneau Airport (5g022'N,134O35'W;elev. 6 m) from Arctic Environmental Information and Data Center (1986).Data for Whitehorse Riverdale (6Oo43'N, 135"011W; elev. 643 m),Mayo Airport (63"37'N,135"52'W;elev. 504 m), and Keno Hill (63"56'N,135"12'W; elev. 1472 m) from Environment Canada (1982).

result, the influence on regional climate of the emerging western Cordillera was not reproduced. Regional climate The continental climate of east-central Alaska, Yukon, and western Northwest Territories, with severe winter temperatures, is due to the mountains of the western Cordillera (Wahl et al. 1987). In southwest Yukon large portions of these ranges reach elevations over 3500 m, approximately the 65 kPa level, below which is one-third of the mass of the atmosphere. The mountains effectively block weather driven by westerlies off the Gulf of Alaska from most of the Yukon Territory. The climate statistics for Juneau and Whitehorse (Table 1) illustrate the remarkable difference in regime effected by the mountains. The influence is evident in isobaric charts of mean atmospheric pressure, illustrating the dominance of the arctic anticyclone over the interior during winter (Fig. 3). Similarly stable conditions are present in spring and early summer (Wahl et al. 1987, Figs 7.1 -7.3), but in late summer and autumn the northward extent of the jet stream regularly guides depressions into the Gulf of Alaska, and some penetrate the mountains (Wahl et al. 1987, Figs 7.4 and 12.1).

Interval

=

0.2 kPa

FIG. 3. Mean sea-level pressure (kPa) for January for Yukon and adjacent Northwest Territories (after Wahl et al. 1987, Fig. 7.1).

projected cooler summers and warmer winters in AlaskaYukon. Ruddiman and Kutzbach (1989, p. 18 41 8) recognized the discrepancy with the regional geologic record, which suggests only gradual cooling from the Eocene onwards, with a temperate, i.e., warm summer, vegetation, including Corylus, as late as the Pliocene (Wolfe 1981). They emphasized that the regional topography is not well represented in their models, with the physiography prescribed for interior Alaska and Yukon by a low dome of elevation less than 800 m. As a

Air temperature Continentality, i.e., the difference between winter and summer temperatures, increases with distance from the coast, as illustrated by the monthly air temperature data for Whitehorse and Mayo (Table 1). The coldest temperatures of the Canadian winter are usually recorded in the valleys of the Yukon Plateaus. The continental minimum for conventional observations (-62.8"C) was recorded at Snag, northwest of Kluane Lake (3 February 1947). Minimal solar heating and densitydriven cold-air drainage combine to reduce monthly mean temperatures in winter at valley-bottom sites (Fig. 4). Overall, there is a difference of 9°C in January daily mean temperature between Keno Hill and Mayo (Table I), separated by only 50 krn horizontally, but 1000 m vertically. The topographically enhanced inversion requires time to develop, and so relies on the maintenance of anticyclonic conditions facilitated by the Saint Elias barrier. Summers, although brief, are warm in valley bottoms (Table 1). Normal lapse conditions apply with ascending elevation. Valleys of the Yukon Plateaus are usually snow free

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Interval Colder than: -10 0

-20

-30

- 200

rnm

1

300 km

0

300 km

FIG.4. January daily mean air temperature ("C) for Yukon and adjacent Northwest Territories (after Wahl et al. 1987, Fig. 8.8).

FIG.5. Annual mean total precipitation (mm) for Yukon and adjacent Northwest Territories (after Wahl et al. 1987, Fig. 9.5).

by early May, whereas the tablelands are covered for a further month.

northern Norway less than 2000 m asl. In consequence, a maritime climatic regime, with abundant snowfall, prevails in Finnmark and northwest Russia. A south-north transect of the Yukon Territory, or Mackenzie Valley, encompasses the range of permafrost conditions encountered in Canada. Alpine permafrost is found at high elevations in the mountains, whereas perennially frozen ground is sporadic in the southern valleys. Permafrost thicknesses of < 2 m have been recorded during municipal excavations near Teslin, southern Yukon (EBA Engineering Consultants 1987). In the widespread permafrost of central Yukon, permafrost thicknesses of up to 40 m have been measured near Mayo and 60 m near Dawson (McConnell 1905; Burn 1991). The Yukon coastlands were unglaciated in the Late Wisconsinan and are underlain by continuous permafrost, likely well over 300 m thick (Rampton 1982). In south and central Yukon, both low winter temperatures and low snowfall contribute to the development and maintenance of permafrost in the valleys of the region. At higher elevations short, cool summers allow the ground to remain frozen at depth (Harris 1983).

Precipitation The dominant feature of the regional precipitation regime is the rain shadow to lee of the coastal mountains (Fig. 5). Data for Juneau and Whitehorse indicate the substantial depletion of moisture from maritime air masses effected by the mountains (Table 1). In summer interior portions of the region are also served by local convective activity and occasional storms from the Bering Sea or the Beaufort Sea, but in winter precipitation is solely derived from air masses that penetrate the mountains (Wahl et al. 1987). On the Yukon Plateaus, higher elevations receive greater precipitation than valleys because of local orographic effects, as illustrated by data for Mayo and Keno Hill (Table 1).

Permafrost distribution In general, permafrost is present at sites where, annually, the number of near-surface ground freezing degree-days exceeds the number of thawing degree-days (Nelson and Outcalt 1987). The influence of snow is critical in determining ground surface temperatures in winter. The principal difference between Eurasia and North America in distribution of permafrost is its occurrence, in Alaska and Yukon and adjacent Northwest Territories, on the leading, i.e., northwestern, edge of the American continent and its general absence from northwest Europe. The Scandinavian mountains are subdued in comparison with the western Cordillera, with summits in

Physiographic evolution and development of permafrost Late Cenozoic environmental change The general trend in late Cenozoic climate for northwest North America has been cooling from the Late Oligocene on (McNeil 1990). The Late Pliocene and Pleistocene contained at least four major Cordilleran glacial advances in Yukon

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(Bostock 1966). The last two, in Illinoian and Late Wisconsinan time, are spatially and temporally well defined (Bostock 1966; Matthews et al. 1 9 9 0 ~ ) Paleomagnetic . evidence presented by Jackson et al. (1990) from Fort Selkirk, central Yukon, indicates that the oldest, and most extensive, glacial event occurred > 1.87 Ma. This glaciation was marked in the Klondike District by piedmont glaciers extending into Tintina Trench from the Ogilvie Mountains, depositing the proglacial Klondike Gravel over the auriferous White Channel Gravel (Hughes et al. 1972). At this stage, the coastal mountains were lower, and the rain shadow would have been reduced. In central Yukon subsequent glaciations, both of the Cordilleran Ice Sheet and from alpine icefields, have been progressively restricted as the coastal mountains have risen and the rain shadow has been enhanced (Ward and Jackson 1992). Stratigraphic evidence for permafrost Most soil wedges in Cenozoic sequences in northwest Canada are interpreted as ice-wedge casts and have been used to infer permafrost conditions (e.g., Pearce et al. 1982; Tarnocai 1990). The earliest evidence of permafrost in northwest Canada appears in late Pliocene sediments, immediately below glacial drift. Structures interpreted as ice-wedge casts have been described from (i) White Channel gravels, of Pliocene early Pleistocene age, below the glacial Klondike gravel (Morison and Hein 1987); (ii) Unit 2 of Ch'ijee's Bluff, on the Porcupine River near Old Crow, which is possibly > 2.48 Ma and contains cryoturbated involutions and ice-wedge casts, and lies below magnetically reversed sediments ascribed to the Matuyama Chron (Pearce et al. 1982); and (iii) the Worth Point Formation on Banks Island, deposited in the Matuyama Chron (0.79 -2.48 Ma, Vincent 1990). In addition, the development of speleothems in caves south of Old Crow ceased during a period of magnetic reversal, tentatively identified as the Matuyama Chron, implying that groundwater circulation terminated due to development of permafrost (Lauriol et al. 1993). Climatic conditions before the Late Pliocene, as represented by the Tertiary pollen record (Matthews and Ovenden 1990), were not conducive to development of permafrost. Observations of magnetically normal speleothems in the north Yukon caves, tentatively ascribed to the Gauss Chron (2.48 3.40 Ma), suggest groundwater circulation was uninhibited by permafrost before the Late Pliocene (Lauriol et al. 1993). In this period, the coastal ranges did not affect the synoptic regime sufficiently to produce arctic conditions, and the region was colonized by a temperate flora. Tundra vegetation and permafrost were probably first present in latest Pliocene time, as the Pleistocene glacial climate developed. The first appearance of permafrost was likely associated with global climatic cooling enhanced locally by regional physiographic modification of synoptic conditions. Early Pleistocene permafrost was sufficiently thin in the southern portion of the region to be eradicated during interglacial~.In central Yukon, luvisolic soil profiles up to 2 m thick on glaciated surfaces suggest a wetter climate with deep infiltration during these intervals. Five paleosols, developed in montane tills, representing interglacials up to Illinoian time, are exposed in a stacked sequence on the Little Bear River in the Mackenzie Mountains (Hughes et al. 1993). There are no cryogenic features in any of these well-developed paleoluvisols, but pedologic indicators suggest cooling of successive interglacial climates. Indeed, the only indication of interglacial permafrost in pre-Illinoian time is a possible ice-wedge cast

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from the Liard Lowland, southwest Yukon, tentatively assigned to the Yarmouthian interglacial (