HOLOCENE WATER-LEVEL FLUCTUATIONS OF A SUB-ARCTIC LAKE AT THE TREE LINE IN NORTHERN QUEBEC

Water-level changes of a small subarctic lake, located near the tree line in northern Quebec, were inferred from stratigraphic analysis of buried peat and minerogenic sediments deposited over the last 6000 C-14 years, i.e. the time lapse since the final withdrawal of the postglacial Tyrrell Sea waters. Two major periods of water-level fluctuations were recorded: a generally low level of the lake from 5400-5300 BP to 3600-3500 BP and a predominantly high water level from 3500 BP to present. The most important lowering occurred between 4600 and 4100 BP, when the water level was at least 60-100 cm lower than present. Three brief lowerings also occurred around 2600-2400, 2100-2000 and 1300 BP. An important lowering at 300-250 BP coincided with the Little Ice Age period. At that time the lake level was at least 45-50 cm lower than present, and this facilitated tree growth in the shore zone. The generally low lake level before 3500 BP was probably caused by dry and warm conditions (with high evaporation), whereas the 300-250 BP lowering was most likely due to a decrease in the annual snow fall. The formation of permafrost mounds in the shore zone after 2100-2000 BP was associated with a lower lake level. The absence of any pedogenic development in sandy deposits at the top of the mounds suggests a rather recent origin, possibly during the Little Ice Age. The overall chronology of predominantly high and low water levels is partly similar to that of other lakes from temperate North America and northern Europe.