ABSTRACT: Late-Quaternary environmental change in northern British Columbia and northern Montana

CGRG Bibliography of Canadian Geomorphology
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Abstract

Research on the Stikine Plateau, northwestern British Columbia (Spooner, Osborn, Hills) and in the Mission Mountains, Montana (Gerloff, Osborn, Hills) focuses on resolving patterns of Holocene climate change in alpine areas. In each study a series of alpine lakes, situated near tree-line, has been cored and the palynomorph assemblages preserved have been interpreted as proxy indicators of Holocene climate change. Changes in the relative abundance of specific palynomorphs and macrofossils can indicate past elevational fluctuations of the timberline ecotone, a sensitive proxy indicator of climate change. A pollen record from the central section of the Mission Mountains reveals a four-zone sequence of Holocene vegetation change. Deposition of Glacier Peak tephra above the basal till indicates the initiation of deglaciation prior to 11,200 B.P. Initial colonizing vegetation after deglaciation was dominated by Artemisia (sage) and Alnus (alder),reflecting a relatively cool macroclimate due to persisting periglacial conditions to the north. Two vegetation units appear to contribute to Pollen Zone II: arboreal communities with Pinus (pine), Picea (spruce) and Abies (fir), and treeless vegetation dominated by Artemisia. Pollen Zone II represents a warmer macroclimate than occurred upon ice withdrawal. Alnus becomes more pervasive in Pollen Zone III at the expense of Pinus. The presence of Alnus reflects a return to cooler and probably moist conditions. The re-emergence of warmer climate conditions in Pollen Zone IV is characterized by an increase in Pinus. At the Stikine Plateau site the post-glacial colonizing vegetation appeared before 7990 ą80 B.P. and consisted of a shrub herb community dominated by Alnus (alder), Betula (birch), Artemisia (sage) and Gramineae (Grasses). Macrofossil and palynomorph data suggest that between about 7700 B.P. and 2000 B.P. timberline remained above presentlevels in response to warmer climate conditions and was dominated by Picea (spruce) and Abies (fir). A return to wetter, perhaps colder conditions after 2000 B.P. is indicated by an increase in Tsuga (hemlock) and an absence of tree macrofossils. The frequency of occurrence of turbidite deposits within the core coincides with reduced timberlines and is probably related to a decrease in stability of the alluvial fan from which the turbidites originated, possibly because of devegetation. These studies are part of an attempt to characterize Holocene climate change in western North America and when combined with the work of Reasoner et al. (1994) and Luckman and Osborn (1979) and others will produce a more comprehensive understanding of the timing and magnitude of global climate change.

Bibliography of Canadian Geomorphology