The pollen record of a 240 cm peat profile in the Ittlemit Lake area in southwest Yukon Territory presents a vegetation development history of the last 9OOO radiocarbon years. Spruce migrated into the area at least 9OOO radiocarbon years ago. From 9OOO yr BP to approximately 5000 yr BP the area supported a sparse Piceu-Sulk-Betufu forest-tundra vegetation. By 5000 yr BP local environmental change created a different habitat primarily affecting the local taxa. Alnus invaded the general area shortly after 5000 yr BP. A local Betuludominated community replaced the previous Cyperaceae-dominated one at about 3000 yr BP. A Cyperaceae-dominated community again occupied the area at about 1900 yr BP. Although the local community changed several times, the present regional forest-tundra vegetation has had little change during the last 9000 radiocarbon years.

ABSTRACTThis paper is a summary of all known late Quaternary palynostratigraphic records from the southwest Yukon Territory. Thirty two pollen sites available by the end of 1988 are reviewed. Most pollen records in the region are of Holocene age. During the late-glacial to early Holocene, the southwest Yukon supported a herb-dominated tundra vegetation which was replaced by a birch-dominated shrub-tundra at about 10,000 yr BP. Spruce invaded the area between 9000 and 8600 yr BP at different localities, and a southward time transgression is visible in the Aishihik Basin. The current regional vegetation has been stable since 7600-8000 yr BP when dense spruce forest and/or spruce forest-tundra was established in most localities. In the Snag area, however, dense spruce forest developed only around 5700 yr BP, which is about 2000 years later than in the Aishihik Basin. The exotic pine pollen records in the region exhibit an interesting pattern, suggesting a frequent shift of the atmospheric circulation system. Anomalous records of alder pollen from the Aishihik Basin and adjacent regions suggest that alder has never been widespread in these areas due to aridity, and alder pollen is greatly overrepresented in pollen spectra. Spruce arrival dates suggest that further investigations in the Tintina Valley, Yukon River Valley, and Car-macks region might provide useful information concerning the spruce migration routes.

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Pollen analysis of three sections from the Grizzly Creek basin, Yukon Territory was undertaken in order to reconstruct the paleoecology of the region over the last two millennia.Diagrams for two sites show three well defined pollen zones, whose boundaries are based on fluctuation of pollen spectra. The oldest zone corresponds to vegetation comparable to that of the present and a climate similar to or a little colder than the present climate. In zone 2, the strong decline of Picea is interpreted as a break in pollen production caused by a decrease in precipitation and a warming of the climate. The forest survived for several centuries in a state of degeneration, which favoured the development of a stratum of shrubs and herbaceous plants. Zone 3 reflects a return to vegetation and to climatic conditions similar to those of zone 1.The positions of two layers of volcanic ash, dated at approximately 1230 and 1890 years BP and corresponding to two lobes of White River Ash, indicate the rhythm of sedimentation. The presence of loess in the sediments helps explain the great accumulation of sediment before 1250 years BP and supports an increase in aridity during this period. The pollen record also suggests that the deposition of volcanic ash had an influence on the geomorphological evolution of the area.

RÉSUMÉL'étude de travertins, situés sur la rive gauche de la Coal River (SE du Yukon), permet de mieux comprendre la genèse de ces dépôts par leur description morphologique, soutenue d'observations hydrologiques et microfaciologiques. L'analyse morphologique révèle que les barrages de travertin sont des phytohermes, à croissance discontinue attribuable aux variations de débit. Une forme nouvelle appelée « encorbellement inverse » est décrite et correspond à une croissance en eau calme. L'analyse microscopique identifie : 1 ) un faciès algaire de type Phormidium/Schizothrix qu'on trouve sur des supports ligneux et des formes en vase, 2) un faciès en glomérule d'origine probablement bactérienne, observé sur la face amont de grands barrages, et 3) un faciès bryophytique qui est le plus fréquent et qui apparaît au sommet des barrages, c'est-à-dire dans les zones de croissance verticale. Les mousses les plus fréquentes sont Brachytecium sp., Bryum sp., Cratoneuron sp., et Gymnostonum recurvirostrum (Hedw.). Les phytohermes sont situés dans le cadre stratigraphique de l'Holocène, après l'encaissement de la Coal River dans ses alluvions. Ils se construisent selon le modèle suivant : leur croissance verticale ainsi que l'épaississement de leur face aval se font surtout par l'encroûtement des mousses et l'épaississement de leur face amont se réalise davantage par l'intermédiaire des faciès cyanobactériens.