The pollen stratigraphy of a dated section of Late Pleistocene lake sediment from central Alberta

A 5.5 m section of limnic sediment from Lofty Lake in the Mixedwood Section of the Boreal Forest in central Alberta has yielded the first complete Late Pleistocene pollen stratigraphy for the province. The basal organic sediment was radiocarbon dated at 11 400 ± 190 y (GSC-1049) and a layer of Mount Mazama type ash was recognized at the 398 cm level. This represents the furthest extension into Canada of Mazama ash records. Five pollen assemblage zones have been identified—at the base, a Populus – Salix – Shepherdia – Artemisia assemblage, which is unique in the Late Pleistocene of N. America, and is interpreted as a pioneer forest and shrub community which occupied the area immediately following deglaciation. This is succeeded by a spruce dominated assemblage which conforms in general to many early Late Pleistocene Picea assemblages from western Canada and adjacent United States, interpreted as a pioneering version of the boreal forest. There follows a tree birch-dominated assemblage with poplar and hazel, suggesting a slight amelioration of climate, and this trend appears to have continued to about 6000 B.P. when a birch – alder – herb assemblage reaches its maximum; this is followed by a spruce – birch – alder assemblage, which continued to the present and is interpreted as an expression of a deterioration in climate about 3500 B.P. The apparent absence at the site of grassland, although the birch – alder – herb assemblage suggests that the grassland might have advanced closer to the site than at present (240 km (150 miles)), supports the hypothesis that there was never a Late Pleistocene connection between the Peace River and the main southern grasslands.