Lag of the Fraser glacial maximum in the Pacific Northwest: pollen and macrofossil evidence from western Fraser Lowland, British Columbia

Abstract

Pollen and macrofossil evidence from two sites in northwestern Fraser Lowland reveals that Abies lasiocarpa – Picea cf. engelmannii forest and parkland grew there about 18 000 years ago under cold humid continental conditions. Taxus brevifolia was also a significant constituent of this forest. This plant assemblage resembles the ESSF Biogeoclimatic Zone of subalpine elevations in the northern interior of British Columbia (900–2250 m). Climate was probably cold with low to moderate rainfall and characterized by long, cold, wet winters and very short, probably dry, frost-free summers. Mean annual temperature was depressed about 8 °C and the tree line was probably 1200–1500 m lower than today.Fraser Lowland was probably removed from Pacific oceanic influence because the land–sea interface was located on the continental shelf to the west of Vancouver Island and Washington about the time of the last global glacial maximum, global depression of sea level, and Quadra Sand aggradation in the Pacific Northwest. Lowland glaciation was probably delayed because of insufficient precipitation in the drier macroclimate and the precipitation shadow created behind mountains on Vancouver Island and Olympic Peninsula. We speculate that, as Laurentide ice decayed, there was a northward shift of zonal weather patterns over the eastern Pacific, bringing very wet winters to the Fraser Lowland and providing moisture for rapid, extensive, Vashon glaciation, which culminated about 14 500 BP, lagging at least 3000 years behind the Laurentide glacial maximum.