Introduction

Information Sources

Physiography

Quaternary History and Paleoclimate

Geomorphic Processes and Landforms 

Ecoregions

Outstanding Features

Terrain Sensitivity

Resource Evaluation

Bibliography

QUATERNARY HISTORY AND PALEOCLIMATE

i) Introduction

In the study of glacial history and paleoclimate the scientist tries to unravel the mysteries of the timing, extent and characteristics of past glaciations and the environmental conditions which they produced in both glaciated and unglaciated areas. To a glacial geologist time is recorded in years "BP,", before present. The sources for this research are the landforms and surficial materials (materials covering the surface) which exist today. Bluffs, which have been created by rivers as they cut down through the sediments deposited on top of the bedrock, provide an important source of the natural records of changing conditions in the past.

Over time, sediments washed into the Old Crow and Bluefish Basins built up into deep deposits that buried and preserved the remains of a vast number of plants and animals. The Porcupine and Old Crow Rivers gradually cut deeper into the fossil-bearing deposits, creating steep bluffs some 30 m high. Erosion of the bluffs has shifted many fossils onto the newly formed bars and riverbanks, where they first attracted the attention of the Vuntut Gwitchin many years ago.

One of the most-studied locations in the vicinity of Vuntut National Park (but not in the Park or Special Management Area) is Ch'ijee Bluff (also called Twelve Mile Bluff and Big Bluff, or site HH228), located about 8 km southwest of Old Crow on the south bank of the Porcupine River in the Bluefish Basin (Figure 4.4 ). Other significant bluffs and point bar fossil locations found along the Old Crow River include, in the Old Crow Flats Special Management Area, CRH-11, 12, 15, 94 and HH68-10; and, in Vuntut National Park, CHR-44 (Figure 4.4 and see also Paleontology). The bottom layers in these bluffs originated millions of years ago, probably in the Pliocene, when the structure of Old Crow Basin was established (see Geology).

The layers of deposits seen in these bluffs have been divided by researchers into units based on certain characteristics of the deposits. (Note that the units are numbered differently at different bluffs, but the smallest number is always given to the unit at the bottom of the bluff). The cross sections of these bluffs are the glacial geologist's "library" . A composite section of bluffs in the Old Crow River Basin is shown in Figure 4.6a and a section of the Ch’ijee Bluff in Figure 4.6b. These will be used to trace the change in climate and environmental conditions in the discussions that follow. But first it is necessary to discuss the methods used to date the layers in the bluff sections. As in other forms of historical and paleohistorical research, it is the researcher's interpretation which turns the laboratory results into a picture of past events and conditions.

ii) Dating Techniques

Geomorphologists can roughly determine the age of the materials of the bluffs by relative dating, which is determined by the spacial relationships between layers at various sites, but also rely on several different techniques of absolute dating. The dating method used depends on the type of material available for dating and the age of the layer being dated. For most of the Pleistocene Ice Age only relative dating schemes, not absolute chronologies are available. All these techniques were used in dating the bluffs represented in the composite bluff sections, Figure 4.6b, a (see also Figure 4.8, from Matthews et al., 1990 which correlates the Ch’ijee Bluff site with other sites in eastern Beringia in Supplement MSJ6).

The chief methods for absolute dating are based on radioactive decay. The methods most widely used and accepted are based on the natural radioactivity of certain minerals found in rocks. Since the rate of radioactive decay of any particular isotope (the form of an element differing from other forms of the same element in terms of relative atomic mass) is known, the age of a specimen can be computed from the relative proportions of the remaining radioactive material and its decay products. Some of the radioactive elements used in dating and their decay products (their stable daughter isotopes) are uranium-238 to lead-206 and potassium-40 to argon-40. Each radioactive member of these series has a known, constant decay rate that is unaffected by any physical or chemical changes. Each of these element is only effective for a particular age range. (The following details are largely from McClelland & Stewart 1998, The 1999 Canadian Encyclopedia)

a) Radio carbon dating uses radioactive carbon-14 to determine the age of organic matter from several hundred years to approximately 50,000 years old. Carbon dating is possible because all organic matter, including bones and other hard parts, contains carbon and thus contains a scalable proportion of carbon-14 to its decay product, nitrogen-14. The carbon-14, along with non-radioactive carbon-13 and carbon-12, is converted to carbon dioxide and assimilated by plants and organisms; when the plant or animal dies, it no longer acquires carbon, and the carbon-14 begins to decay.

b) Fission track dating is based on the fact that when U-238 atoms undergo fission within a solid medium such as a mineral or a glass, they expel charged particles that leave a trail of damage (known as fission tracks) preserved in the medium. The number of tracks per unit area is a function of time and the uranium concentration. Thus it is possible to measure the time that has elapsed since the material solidified. U-238 is effective for dating materials from 100 million to 4.5 billion years old.

c) Volcanic ash layers or tephra

One of the important means of assessing the age of various strata and deposits in the Old Crow Basin area is the presence of layers of volcanic ash known as tephra. A tephra is a visible layer of volcanic ash deposited on the ground surface at the time of an eruption (tephra means ashes in Greek). The best known ash layer, called the "Old Crow Tephra" (Westgate et al.1985) occurs over a broad area, roughly triangular in shape, from Alaska's Seward Peninsula in the west, to the Wrangell Mountains in the south, and east to the Old Crow Basin (Schweger and Matthews, 1985 Figure 4.1a; in Supplement S&M1). Studies of the chemistry of the ash indicate that it originated from a volcano in the Aleutian arc of Alaska (Westgate et al.1985), likely in the Wrangell Mountains (Westgate 1982). Other ash layers also present and available for dating deposits in the northern Yukon include the "Little Timber Tephra", the "Surprise Creek Tephra" and the "Sheep Creek Tephra" (Schweger and Matthews 1985) which probably also originated in the Wrangell Mountains (Westgate et al. 1985).

Identification of ash layers is an important cross-dating tool and could theoretically provide absolute dating for the larger layer within which the ash is found, providing that the ash layer itself can be dated. The Old Crow Tephra is an important marker bed which can be identified in many cores from eastern Beringia. The changing estimates and ongoing controversy concerning the dating of this tephra (Matthews et al. 1990) are good examples of the problems faced by researchers trying to produce a chronology from bluff profiles and sediment cores. Early estimates for the age of the Old Crow Tephra, which was first collected by Hughes in 1968, have ranged from 80 to 230 thousand years before present. Dating of loess (wind blown) deposits above and below the Old Crow Tephra in Alaska suggested a limit to the age of the ash layer to about 86,000 years before present (Wintle and Westgate 1986). Studies of pollen in the layer immediately below the ash indicated that it was deposited over a tundra dominated by birch in a climate cooler than the present climate (Schweger and Matthews 1985). With the use of new isothermal fission-track dating techniques (Westgate 1985), Matthews et al. (1990) now favour a date of 140-150 thousand years before present for the Old Crow Tephra. The Little Timber Tephra (exposure CRH-94, Figure 4.6a) is fission dated at 1.1 to 1.2 million years ago, the Surprise Creek Tephra (exposure CHR-47) is still undated. The Sheep Creek Tephra was previously dated at 80 thousand years ago but Westgate’s recent fission-track date is of 190,000 years before present is now the accepted date. These tephra have not been studied in the same detail as the Old Crow Tephra in the northern Yukon.

d) Paleomagnitism is a technique used to cross date terrestrial cores with ocean cores and to establish major intervals over wide areas. By measuring the polarity of the magnetism in samples from the cliff it is possible to determine the layer in which the earth’s polarity reverses. Paleomagnetism is one of the techniques used in Old Crow Basin to indicate a Pliocene or at least early Pleistocene age at the base of the cliff where a "reversal polarity" is recorded.

e) Exposure dating using Cl ³⁶ is described in detail by Duk-Rodkin et al. (1996) and has been has been used to determine the age of shield erratics (displaced pieces of the Laurentian Shield rock) left by the Laurentide Ice Sheet along the eastern flanks of the Mackenzie Mountains.

iii) Glaciation (Glacial History)

The Quaternary is often referred to as the glacial age, although it is not the only time period during which glaciation occurred (Fulton 1989). Studies of marine and terrestrial sediments indicate that about 1.65 million years ago or earlier as indicated in some sources, at the beginning of the Quaternary, the earth's climate began a series of warm-cold cycles that apparently caused the growth and decay of continent-scale ice sheets as many as eight times.  These cycles are identified in ocean bottom sediments by the oxygen isotope ratios of the remains of plant and animal material (which indicate the relative warmth of the ocean in which the sediments were deposited). These cycles are called Oxygen Isotope Stages and have been numbered as shown in Figure 4.2b (see also supplement S&Mfig6corr for a plot of ocean core results). Cooling started in Northwestern Canada 2.9 million years ago (Duk-Rodkin and Barendregt 1997 and White et al. 1997 and 1999).

The Periods, Epochs, stages and sub-stages of the Quaternary, as identified from terrestrial features in Canada (from Fulton 1989), are shown in the first three columns of Figure 4.2b. Although these are used Canada-wide, in each area of Canada geomorphological studies have identified glaciations (periods of ice sheet advance) and interglacials (periods where ice sheets almost disappeared, such as at present). In each area distinctive names have been given to these glaciations and interglacials. In the Northern Yukon, this is further complicated by the presence of two ice sheets, the Laurentide and the Cordilleran (Figure 4.1a, Figure 4.1b). Glacial and interglacial events recognized in various areas of the Northern Yukon have been added to Figure 4.2b. It should be noted that not all events are seen in all areas. This is sometimes because they were absent from the area and sometimes because evidence of their presence has been erased or has not yet been found. The dating of these events is in many cases still controversial.

Many indicators are taken into account in determining the extent of a particular glacial or interglacial event seen in the vertical stratigraphies. These include: terminal moraines (debris deposited at the front edge of a glacier), till (debris deposited directly by a glacier without reworking by meltwater), esker (debris deposited in a sub-glacial stream), strias (gouges in the bedrock showing movement of ice), meltwater channel (channel carved by meltwater from glacier), and drumlins (smoothly rounded, oval hills which are parallel to the ice movement). Some of these are illustrated on surficial geology maps (Hughes 1972 and Rampton 1982).

a) Notes on glaciation in the Northern Yukon

Although the study area has not been glaciated (Figure 4.1a, Figure 4.1b and Figure 4.7a), the Quaternary glaciations have profoundly influenced the climate and in some cases the landscape of Vuntut National Park and Old Crow Flats Special Management Area.

Discussions of the extent of the Quaternary glaciations in the northern Yukon are found in Rampton (1982), Clague (1989), Fulton (1989), Welch et al.(1993), Duk-Rodkin and Hughes (1991, 1992, 1995), Lemmen et al. 1994, and Duk-Rodkin et al. (1996). Schweger (Whitehorse Symposium 1999) points out that over the past 2.5 million years, global climate has oscillated between glacial and interglacial conditions numerous times. During 90% of this time the climate was colder than present. Interglacials, occupying approximately 10% of the time, make ideal laboratories for a range of interests, including serving as comparisons (analogues) for global warming. Schweger (1999), Matthews (1999), and Tarnocai (1990), among others, have discussed evidence of environmental conditions in the study area during these warm periods.

The age of the all time glacial limit in the northern Yukon is controversial. In discussing the glacial history of Ivvavik National Park, Welch (1993), based on Rampton (1982), points out that although there may have been more than one glaciation in the area, the visible glacial landforms and sediments resulted from only one, the so-called Buckland Glaciation (Figure 4.2b). He places this glaciation as probably early Wisconsinan in age (pre 65,000 years before present) but recent evidence suggests that the early Wisconsinan glaciation was not experienced in Canada (Duk-Rodkin personal communication, and Duk-Rodkin and Hughes 1991) and places the Buckland glaciation as Late Wisconsinan. In the northern Yukon there is agreement on the extent of glaciation but not on the dating or sequence of events. Discussions of the conflicting interpretations are beyond the scope of this synthesis and change with ongoing discoveries (see also Dyke and Prest 1989, Vincent 1989, Lemmem et al. 1994). The most recent depiction of the extent of the glacial maximum is the Glacial Limits Map of the Yukon Territories (Duk-Rodkin 1999, copy provided with the Supplement, and Duk-Rodkin 1999 on the Yukon Geology CDROM, Gordey and Makepeace, 1999). Several recent interpretation of the extent of Laurentide and Cordillerian ice cover during the late Wisconsinian are presented in Figure 4.1b, Figure 4.7c, Figure 4.8a, and an early depiction of extent of this glaciation north and east of the study area is shown on the Surficial Geology map accompanying Hughes (1972, provided in the Supplement Hmapsnall). Discussion of the glacial history within each of the of the northern Yukon YEWG Ecoregions is also given in the Supplement. In the British and Richardson Mountains, evidence of local glacier development of undetermined age has been found in two restricted areas ( Duk-Rodkin and Hughes 1992). Neither are within Vuntut National Park.

The controversies regarding details of the glacial extent and chronology in the northern Yukon may not seem critical for the unglaciated Vuntut National Park area. However, they become particularly important in the following two contexts.

b) "Ice free corridor"

A recent discussion of the glacial controls over the "ice free corridor" between the Laurentide and Cordilleran ice sheets (Jackson and Duk-Rodkin 1996) is of interest. It is this corridor that, it has been hypothesized, was the link between Beringia and southern North America. Jackson and Duk-Rodkin map the ice free areas at three time intervals during the Late Wisconsinan based on recent studies. They conclude the following (Jackson and Duk-Rodkin 1996, page 223):

Although glaciation did not encroach on Vuntut National Park, the extent and timing of the Laurentide Ice Sheet to the west and the northern portion of the Cordillerian Ice Sheet to the south did influence the inhabitants of the refugium during the ice age.

c) Proglacial Lake Old Crow

The most important feature of the Wisconsinan Glaciation for the study area is undoubtedly the inundation of the Old Crow Basin by a proglacial lake (a lake formed by blocking of melt waters at the edge of a glacier or ice sheet). In pre-glacial times the Old Crow Flats were drained by streams that flowed east through the Richardson Mountains at McDougall Pass as seen in Figure 4.7b. These streams deposited a variety of lake, river, and river delta sediments in the area (Hughes et al.1989). It is these sediments which are found at the bottom of the cliff profiles (Figure 4.6a). Near the top of these same profiles are found the glacial-lacustrine deposits from proglacial Lake Old Crow (Figure 4.6a, Figure 4.6b) which was first mapped and described by Hughes (1972). As with other aspects of glacial history there have been a number of conflicting suggestions regarding the timing and number of inundations of the area (Hughes et al. 1989, Rocheleau et al. 1992a,b, Rocheleau and Lauriol 1994, Matthews et al. 1997, Duk-Rodkin and Hughes, 1991, 1995, and Lemmen et al., 1994). The most recent description of the extent, timing and formation of Lake Old Crow is reproduced here from the YEWG report (Duk-Rodkin and Jackson in Smith and Roots, in press) and is illustrated by Figure 4.7c and Figure 4.8a (see also Duk-Rodkin 1999a in Supplement and 1999b, and Duk-Rodkin et al. in preparation):

The maximum area of glacial Lake Old Crow was approximately 13,000 sq. km (this includes the Bell basin portion of the lake also). The lake lay entirely within unglaciated terrain, which is unusual, and was in contact with glacier ice for approximately 3 km of its shoreline at the entrance to MacDougall Pass (Lemmen et al. 1994). The lake was about 40 metres deep (Rochleau et al. 1992a). By 22,000 years ago the Laurentide ice had retreated far enough along the eastern flanks of the Richardson Mountains that drainage to the east was possible and thus further inundation of the Old Crow Basin would have occurred. Lemmen et al. (1994) show the extent of the glacial Lake Old Crow as being much reduced by 21, 000 years ago (Figure 4.8a ii) and suggest that the drainage was complete by 12,500 years ago (Figure 4.8a iii).

The ice rich silt and clay deposited by the glacial Lake Old Crow is seen in Unit 3 of the Old Crow composite profile (Figure 4.6a, Matthews 1987). This layer liquifies and flows when exposed in summer, and is responsible for the distinctive retrogressive thaw/flow slides (see geomorphic processes, section C, for definition). It is also forms Unit 5 of Matthews et al.’s (1990) portrayal of the Ch’ijee Bluffs Figure 4.6b. Joplin et al. 1981 discuss two glaciolacustrine units found in the cliff sediments. The most recent (Units 3 of Old Crow and Unit 5 of Ch’ijee ) they gave the informal name of ‘Lake Kutchin’ (now called Lake Old Crow) while the lower unit which is probably Unit 1 of the Old Crow Bluffs and Unit 3 of the Ch’ijee Bluffs they call ‘Lake Old Crow’. They present a very detailed description of the stratigraphy of this lower glacial lake unit which they suggest is of Illinoian age but which may actually be Pre-Illinoian (see Figure 4.2b and discussion below).

iv) Chronology: Paleoclimatic Reconstruction

The Quaternary is the geologic time period in which we live. It has been subdivided into the Pleistocene epoch (1,600,000 -10,000 years ago and the Holocene epoch (10,000 to present), the period since the retreat of the last glaciation (Figure 4.2b). To set the stage for the discussion of paleoclimate, we will take a brief look at the end of the last period of geologic time before the Quaternary, the Tertiary Period.

a) Pre-Quaternary (late Tertiary)

Geological evidence suggests that until the beginning of the Oligocene Epoch (36 million years ago, Figure 4.2a), the shape of the Arctic Ocean was markedly different from what it is today (McNeil, 1990). It is thought to have been an enclosed, relatively warm, possibly ice-free ocean associated with an arctic-wide boreal-type ecosystem. Temperatures dropped steadily into the Oligocene by as much as 12^o C in Alaska but leveled off in the late Oligocene. By the Late Miocene Epoch (beginning 13 million years ago, McNeil 1990), coniferous forests began to occupy increasingly large areas of the uplands of Alaska. The advance of continental glaciation in North America began some 2.4 million years ago. Reconstructions of the sequence of events in the late Tertiary Period, based on the analysis of marine organisms found in layers of ocean bottom sediments (from drill cores) and Beaufort Sea-Mackenzie Basin marine outcrops, are shown in Figure 4.2a (from McNeil 1990).

The late Tertiary macrofossil records of three exposures in the Old Crow area have been discussed by Matthews and Ovenden (1990). They suggested that the plant macrofossils yielded by the Upper Ramparts site in Alaska and the Ch’ijee Bluff site indicated boreal forests which were much richer than today in terms of the number of plant species present and suggested that these sites samples were probably late Tertiary (Pliocene) age. In fact, Matthews (Whitehorse conference 1999) suggests that the forests of today are markedly impoverished compared to those of the pre-Quaternary boreal realm. (It should be noted, however, that Storer [pers. comm.1999] recently suggested that these Upper Ramparts site plant macrofossils were more likely to be from the middle Miocene (about 16 million years ago, Figure 4.2a) rather than the Pliocene (2.5 million years ago, Figure 4.2a, Figure 4.2b). Late Tertiary fossil soils (paleosols) have been found at a number of locations in the Old Crow area (Tarnocai, 1990). The occurrence of these paleosols (luvisolic and podzolic paleosols, see Soils, Table 1), when combined with the record of fossil plants from the area, suggests a mean annual temperature of at least 4^o C (Old Crow annual mean today is -10^o C) and precipitation twice that experienced today. This is comparable to the present climate of central British Columbia.

b) Pleistocene Epoch (1.65 million years ago to 10,000 years ago)

1) Early and Middle Pleistocene

Cooling began in the late Tertiary as defined by the National Earth Sciences Series Geological Time chart (Okulitch, 1999) Canadian classification (Figure 4.2b), and the early Pleistocene was dominated by alternating glacial and interglacial periods as shown by the study of fossil soils or paleosols (Tarnocai and Schweger 1991). The study of paleosols suggests the occurrence of four interglacial periods during the Early Pleistocene (Pre-Illinoian Figure 4.2b). These interglacials produced the most highly developed paleosols in the Quaternary period (Tarnocai, 1990). Paleosols from the central Yukon suggest temperatures about 8 EC higher than today and precipitation slightly higher than now during these early Pleistocene interglacials.

The climate was very cold at the time of the deposition of the Old Crow Tephra and for some time thereafter (Matthews et al. 1990). The new date for the Old Crow Tephra of 140 to 150 thousand years ago puts it at the time of the glaciation in northwestern Canada known as the Illinoian Stage (see Figure 4.2b) which occurred after the deposition of the Sheep Creek Tephra around 190 thousand years ago. This stage is thought to be correlative (occurring at the same time as) with the Reid glaciation in the central Yukon, the Thompson glaciation on Banks Island, the Intermediate glaciation for the Ogilvie Mountains, the Mirror Creek glaciation in for SW Yukon, and the Mountain River glaciation for the Mackenzie Mountains (Figure 4.2b). The new date, shown in the Ch’ijee Bluff stratigraphy and pollen record in Figure 4.6b, is much more consistent with the marine isotope record and corresponds to Isotope Stage 6 (Figure 4.2b, see also Supplement S&M6corr). However, the new dating of the Old Crow Tephra requires re-examination of some previous chronologies. For example, Unit 4 of Ch’ijee Bluff must now be at least 150 thousand years old (Figure 4.6b). Some of the inconsistencies between previous datings and this new chronology are thought to be a result of materials in the bluff being moved around and redeposited by natural processes (Matthews et al. 1990, Table 1). Other inconsistences were probably due to the fact that the dates were at the limit of radiocarbon dating techniques.

The new date for the Old Crow Tephra also requires some adjustment to the interpretations of Joplin et al. (1981 see Supplement Jopetal81fig3 and 11), basically that the whole profile below the upper lake beds is older than thought at the time. Based on the dates for the Little Timber and Surprise Creek Tephra (Matthews et al. 1987) the lower lake beds (Unit 1 in the Old Crow Bluffs Figure 4.6a) may be Pre-Reid, (Pre-Illinoin) rather than Ilinoian. The details of their sediment analysis are however very interesting in that they suggest the return of permafrost after the draining of the lake accompanied by the local formation of pingos (Jopetal81fig11).

2) Late Pleistocene (Sangamonian Interglacial,Koy-Yukon Interglacial)

The new date for the Old Crow Tephra allows the subsequent warm period, informally named the Koy-Yukon interglacial by Matthews et al. (1990), to fall into a warmer-than-present interglacial period in the marine record (Isotope Stage 5 Figure 4.2b). Evidence of the climate of this interglacial period is seen in stratigraphies from various sites in Beringia (Schweger and Matthews 1985). The fossil evidence suggests a forest more closed than at present, with the tree line located further north. Trees may have extended onto the coastal plain of the northern Yukon, as they are thought to have done in northern Alaska (Carter and Ager 1989). In the Old Crow Basin area, evidence from the Koy-Yukon interglacial for warmer temperatures and deeper thaw depths than today or any time during the Holocene comes from the pollen, large fossils, and ice wedge pseudomorphs (the shape of ice-wedges preserved by replacement of ice by other materials after thawing has taken place) from the Bluffs (Figure 4.6a and MWet4all in Supplement). Matthews et al. (1990) suggest that this interglacial is represented in the Old Crow River bluffs by the discontinuity or "unconformity A" (evidence of a loss of materials at that layer, i.e., not a continuous record) seen between Units 2a and 2b (Figure 4.6a) and describe the climate of the time period as represented by the base of Unit 4c of Ch’ijee Bluff, as "much warmer than present."

The existence of habitat suitable for several species of ducks suggests that the Sangamon /Koy-Yukon interglacial conditions were as warm or warmer than today (Fitzgerald 1991). The central Yukon paleosols suggest this area was probably averaged 1^o C warmer than today (Tarnocai 1990).

3) Late Pleistocene (Early Wisconsinan)

On top of the interglacial record is evidence of cooling, probably starting in the unique zone of cryoturbation (mixing by freezing) structures seen in Unit 4c of Ch’ijee Bluff (Figure 4.6b and Mwet4all in the Supplement) and 2b in the Old Crow River composite (Figure 4.6a). Tarnocai (1990) suggests that when the Old Crow Cryosolic paleosols developed 42,000 years ago, the climate was similar to or slightly cooler than today (Figure 4.2b). Little climate information is seen in the rest of the Ch’ijee Bluff Unit 4 except that there was another smaller thawing shown by the layer of ice-wedge pseudomorphs between 37,000 and 30,000 years ago. Controversy still exists as to whether by this time there were people living in the area of the Old Crow Basin (see Archaeology).

4) Late Pleistocene (Middle and late Wisconsinan)

Unit 4 of the Ch’ijee Bluffs (Figure 4.6b) and Unit 2 of the Old Crow River Bluffs (Figure 4.6b), and the equivalent units in sections from other locations in the Old Crow and Bluefish Basins, contain the incredibly rich faunal and archeological records which have made the Old Crow Basin an internationally-known site for the study of ice age mammals and early human history. As this rich layer is overlain by the sediments of at least the last glacial lake flooding, it must be older than 20,000 years (Morlan 1986). Duk-Rodkin, Hughes and Lemmen place the beginning of the inundation at 30, 000 years ago. Thus, by at least 25,000 (or as early as 30,000 years ago), the Late Wisconsinan, mammoths, bison, giant beavers, camels, and horses roamed the Old Crow area. Scientific controversy surrounds the date of arrival of humans in the Old Crow Basin, as discussed in Archeology, but artifacts have been found which suggest that people were also present by 25,000 years ago. The climate in the unglaciated Old Crow Basin (southeastern Beringia) during this important period was strongly influenced by the presence and behavior of the Laurentide ice sheet to the east and the Cordilleran to the south.

As seen above in the discussion of glacial history, about 30 ka BP the Laurentide Ice Sheet reached it maximum extent and blocked drainage of the Peel and Porcupine Rivers as it moved along the eastern slopes of the northern Cordillera. It filled Bonnet Plume Depression and McDougall Pass (Figure 4.7c and Figure 4.8ai) diverting Peel River northward and the Porcupine River westward. This caused the inundation of Old Crow and Bluefish basins creating glacial Lake Old Crow in the Old Crow Basin and glacial Lake Hughes on the Peel River Basin (Lemmen et al., 1994, Duk-Rodkin and Jackson 2000.) By 21,000 years ago Lemmen et al. (1994) shows the lake as having drained considerably (Figure 4.8bii) and they give a minimum age for sedimentation by the lake of 12,500 years ago (Figure 4.8a iii). Drainage of the Lake Old Crow via Porcupine River was westward through the Ramparts as is the case today (see above for details). The existence and draining of Lake Old Crow and the proximity of massive ice sheets to the east and south obviously dominated life in the area and can be seen in the paleoclimate records from this Late Wisconsinan period.

By 30,000 years ago the stratigraphic records are suggesting a climate colder than today. The pollen record (which indicates the number and kinds of plants present) from Hanging Lake in the northern Richardson Mountains, which begins 30,000 years ago, shows very low pollen influx, suggesting relatively few plants present, until after 25,000 years ago (Cwynar and Ritchie 1980 see particularly Supplement C&R80fig2).

The pollen record from Unit 4 at Ch’ijee Bluffs shows evidence of a return to tundra conditions (Lichti-Federovich 1974) and a layer of fossils from the top of sub-unit 4c is dominated by tundra insects (Matthews 1975). A dated middle Wisconsinan record from Hungry Creek (southeast of Old Crow) also shows tundra conditions (Hughes et al. 1981). Fossil evidence also points to the lack of boreal forest in eastern and northern Yukon during this period. Cwynar and Ritchie (1980) state that the herbacious plants of the time period, from between 30 and 14 thousand years ago, at Hanging Lake are clearly of the arctic-alpine type. This evidence suggests the climate of the Old Crow Basin area was definitely colder and drier than today, during the period from 30,000 to 16,000 years ago. This is in agreement with the paleoenvironmental reconstructions for the Bluefish Basin (Morlan 1980, Hopkins 1982, and Ritchie 1982), suggesting a Late Pleistocene environment characterized by cold winters, cool summers, low annual precipitation (190-375 mm), dry summers and continuous permafrost. These cold conditions are reflected in the description of autumn on the edge of the Old Crow Flats 25,000 years ago by McClellan et al. (1987) in her history of the Yukon Indians. "The day is windy and raw. The low sun rays that touch only the tops of the bare brown hills on either side of the valley give little warmth. Six men crouch in the tall grass watching a herd of mammoth .."

c) Holocene Epoch (10,000 years ago to present)

Recent glacial geology results suggest that by 12,000 Lake Old Crow had completely drained (Figure 4.8a iii). At this time also they suggest several "ice free corridors" had opened up through which people could move south from Beringia (Lemmen et al. 1994, Jackson and Duk-Rodkin 1996). These factors and climatic conditions which accompanied the retreat of the massive ice sheets had a profound effect on life in the Old Crow Basin.

By 12 thousand years ago the climate began to warm, as evidenced in the pollen and macrofossil records from Hanging Lake, Polybog and Bluefish Caves (Ritchie et al. 1982). Although controversy exists regarding the vegetation existing at this time (Cwynar and Ritchie 1980, see also discussion in Paleontology) it is hypothesized that around this time saiga antelope and other animals began to die out as the former steppe-like or arctic-alpine tundra terrain was replaced by spruce forest and tundra (Harington and Cinq-Mars 1995) due to increased warmth and precipitation (Schweger 1997). McClellan et al. (1987) reflects this in her description of the ancestors of the Vuntut Gwitchin in late winter on a branch of the Porcupine River 11,000 years ago. "Six families are camped in the shelter of some popular and spruce that have managed to grow in a sunny, south-facing spot along a branch of a river which later will be called the Porcupine. Not far from here, men hunted mammoth 15,000 years earlier. Nowadays hunters hardly ever see mammoths; they seem, in fact, to be disappearing."

Rapid warming began around 10,000 years ago, and the climate was warmer than today, until about 5000 years ago, when it began to cool slowly (Ovenden 1992, Anderson et al. 1989, Ritchie 1984, Clague and MacDonald 1989). Climate reconstructions for the Old Crow Basin have generally focused on earlier time periods directly related to the question of the peopling of North America (Lichti-Federovich 1972, Hughes et al. 1981, Matthews 1975, Cumbaa et al. 1981, Morlan and Matthews 1983), however, in other areas of the Yukon and NWT the warmer than present period of the mid-Holocene (10,000 to 4,000 years ago) and subsequent slow decline have been studied in some detail (Anderson 1982, Cwynar 1982, Ritchie 1984, Clague and MacDonald 1989, Holdsworth 1992). In many parts of the world this slow decline was interrupted about 1000 years ago by the Medieval Warm Period during which temperatures were similar to today (Environment Canada, 1999). The climate and landscape at the time of this warm period are reflected in McClellan’s description of the Vuntut Gwitchin’s ancestral land in the spring some 1000 years ago. "The Yukon country now looks quite different from the way it did 24,000 or 10,000 years earlier. Much of what was then glacial ice, arctic tundra, grass or shrubs is now covered with spruce, birch, cottonwood and willow. There is still tundra in the mountains, especially in the Brooks Range, and on the coastal plain beyond, and huge herds of caribou that winter in the forests further south move north each spring to the arctic slope for calving and summer browsing" (McClellan et al.1987).

From this warm period temperatures declined into the "Little Ice Age". This decline to the climate of the 1600s to the 1800s which was cooler then present conditions, and the subsequent increase in temperature to present conditions is shown in the tree ring data presented in Figure 7.22a of Climate. No detailed analysis is available for the Late Holocene from the Old Crow Basin and it was beyond the scope of the present study to analyse the literature on the subject of climate change during the Holocene in Northwestern North America (see for instance Supplement C&M89Fig1.33), however, this is a gap that should be addressed in future studies.