RAPID HOLOCENE HYDROLOGIC CHANGE ALONG BOREAL TREELINE REVEALED BY DELTA-C-13 AND DELTA-O-18 IN ORGANIC LAKE-SEDIMENTS, NORTHWEST-TERRITORIES, CANADA

Analysis of delta(18)O(cellulose), delta(13)C(organic matter), and del ta(13)C(cellulose) at about 100 year intervals from organic matter dep osited in Toronto Lake, Northwest Territories, Canada, revealed an 800 0-year history of rapid, post-glacial hydrologic change at the treelin e zone. Several mid-Holocene phases of enriched delta(13)C(org) and de lta(13)C(cell), caused by elevated lake productivity, declining [CO2(a q)], and closed basin conditions, were abruptly terminated by interval s of open hydrology recorded by sharply depleted delta(18)O(cell). Two of these events, at 5000 and 4500 BP, are correlated with increased t otal organic content and Picea mariana pollen concentration, which ind icate that high levels of productivity were also accompanied by northe rn treeline advances. A third treeline advance at about 2500 BP is als o marked by an apparent outflow event from Toronto Lake, but this was not associated with delta(13)C(org/cell) enrichment in the sediment re cord because rapid and substantial lake water renewal probably prevent ed productivity-driven enrichment of the dissolved inorganic carbon an d replenished the CO2(aq) supply to thriving phytoplankton. However, h igh sediment organic content during this period suggests increased pro ductivity. Increases in the inflow:evaporation ratio at about 6500 and 3500 BP were also sufficient to cause Toronto Lake to overflow but th e prevailing climate during these periods apparently did not favour ap preciable northward treeline migration or changes in lake productivity .