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Palaeoecological studies based on the analysis of pollen in lake sediments offer the potential for high resolution and well-dated independent records of past vegetation and climate. Sulphur Lake, located in the southwest Yukon (60.95$\sp\circ$N, 137.95$\sp\circ$W; 847 m), was chosen for a paleoecological study to explore postglacial vegetation dynamics in this region of the boreal forest. A 5 m sediment core was raised from the deepest section of Sulphur Lake using a modified Livingstone piston corer. The sequence spans the full postglacial and reveals significant late glacial and Holocene vegetation changes that provide new information on the regional paleoecological history of the southwest Yukon. The pollen spectra indicate that between approximately 12,000 and 11,250 yr BP, the vegetation was an open alpine tundra marked by the presence of Artemisia. The vegetation then progressed from an open birch shrub tundra to a poplar woodland at 10,250 yr BP. Juniperus populations expanded at 9500 yr BP and by 8400 yr BP, spruce invaded the region. The relatively closed white spruce forest that occupies the region today was established by approximately 8000 yr BP. Alnus crispa increased at 6000 yr BP, however the increase in Picea mariana found at this time at most sites in the Yukon was not present at Sulphur Lake. Black spruce was not a dominant component of the vegetation in the Shakwak Trench as it was to the immediate southeast. The basal radiocarbon date demonstrates that the chronology of regional deglaciation needs to be more firmly established.
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  • Document
Wollaston Peninsula and most of southern Victoria Island comprise Palaeozoic carbonate lowlands, scarps, and tableland situated between rises and arches of underlying Precambrian sedimentary and igneous rocks. Quaternary sediments are hummocky, thick and ice-cored near escarpments where ice flow resistance, thrusting and meltwater concentrated glacial debris; thinner, streamlined drift occurs in lowlands. Quaternary sediments are mainly Late Wisconsinan in age. Glacial sediments predominate but surficial raised marine and periglacial sediments are noted. Fluvial modification of the landscape is minor. Many of the spectacular glacial landforms on Wollaston Peninsula are streamlined and indicate formation under thick, warm-based (i.e. free subglacial water) glaciers. A set of distinctive landforms, including ground moraine (with small moraines and marginal channels), hummocky moraine, lateral moraines, and streamlined forms, relates to varying flow conditions within one major glacial advance. Stratified drift within moraines indicates the importance of glaciofluvial processes in addition to ice action. Collectively, these landforms record ice-marginal retreat, marginal stagnation following compressional flow, surging, flooding and regional stagnation during deglaciation. Freeze-on and ice stagnation trapped extensive bodies of drift-rich ice in zones of hummocky moraine. Arborescent networks of narrow eskers record subglacial meltwater drainage beneath major ice lobes and long broad eskers record subaerial deposition by meltwater of mainly supraglacial derivation. Late glacial events are dated relative to the incursion of the sea during deglaciation of northwest areas of Wollaston Peninsula by about 12 000 BP. Active ice-marginal conditions existed just before 10 000 BP, during formation of the large Colville moraines. Ice downwasted in the area causing glacier thinning. Prominent ground-ice features include pingos, thermokarst scars, and debris-flow lobes. Ground ice occurs as massive icy bodies, ice-wedge ice, and buried pingo ice. Based on its setting in hummocky moraine, its stratigraphy, debris content, and isotopic composition, the massive ice is likely buried glacial ice. Landscape modification by thermokarst erosion has produced ubiquitous diamictons similar to till, and landforms similar to glacial forms. Thick ground ice bodies exist only above marine limit and the limits of streamlined landforms because permafrost degradation occurred below these limits. Widespread thaw slumps and large-scale thermal contraction cracks also indicate ice-cored terrain.
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  • Document