Histoire holocène du climat et de la végétation à Lanoraie (Québec)

Abstract

Four different cores were recovered from the same peat complex at Lanoraie (Québec), and have been used to evaluate, by pollen analysis, possible differences in the representation of the vegetation history. The isotopic ratio of oxygen has been used to indicate climatic variations involved in these processes.This method was first tested and calibrated with modern moss samples. A transect of 15 samples, from the temperate forest to the tundra, indicates that annual mean temperatures and evapotranspiration rates have a predominant influence on oxygen isotopic ratios. A sequence of fossil sediments, interpreted in terms of these results, shows a climatic maximum at 3500 BP and a reduction of temperature since 1500 BP in the Lanoraie region.The history of the regional vegetation shows the following succession of stages: (1) establishment of pioneer tree vegetation of pine, oak, elm, and walnut; (2) buildup of a sugar maple forest, contemporaneous with the migration of beech and correlated with a maximum pollen influx and a climatic optimum at about 3500 BP; (3) increase of the representation of spruce and fir after 1500 BP, related to a climatic cooling. Paleobotanical data–the recurrence of ruderal spectra and the presence of Iva xanthifolia– suggest the occurrence of two prehistoric anthropic periods, one before 3500 BP and the other at ca. 1500 BP.