MULTI-PROXY EVIDENCE FOR RAPID CLIMATE CHANGE DURING THE LATE-GLACIAL PERIOD IN CENTRAL VERMONT

Late-glacial (9.0-13.5 cal ka BP) pollen and ostracode records from Twin Ponds, Vermont are compared with a previously published lake-sediment δ¹⁸O record from the same core. The records indicate that both aquatic and terrestrial ecosystems were influenced by a rapidly changing climate and by ecological succession following deglaciation. Low pollen accumulation rates, the dominance of Picea (likely P. glauca) and limited ostracode diversity at the base of the core (13.5-13.3 cal ka BP) are in contrast to relatively high values of δ¹⁸O. This mismatch suggests that terrestrial and aquatic ecosystems were limited by ecological constraints, such as soil development and lake turbidity, despite favorable climatic conditions. A peak in seasonal stream-adapted ostracodes at 13.3 cal ka BP corresponds to a decrease in δ¹⁸O values and suggests a short-lived increase in winter precipitation. The onset of the Younger Dryas period (12.7 cal ka B.P.) is marked by small increases in δ¹⁸O values, cold- groundwater-adapted ostracodes and pollen from herbaceous taxa, which together indicate cold and dry conditions. Starting at 12.5 cal ka BP the data show the lowest δ¹⁸O values and increases in minerogenic sediment, seasonal stream-adapted ostracodes and Picea pollen accumulation rates. This shift indicates the continuation of cold conditions and an increase in meltwater from winter precipitation. Following the Younger Dryas period (after 11.5 cal ka BP), warming is shown in the pollen data by a more diverse and closed boreal forest, in the ostracode record by an increase in diversity and summer lake productivity and the isotope record by an increase in δ¹⁸O values. At about 11.0 cal ka BP the pollen record shows a shift from coniferous to mixed forest that is marked by a peak in Betula and a decline in Abies and Picea percentages. This terrestrial event is correlated with an increase in groundwater-adapted ostracodes and a plateau in the δ¹⁸O record suggesting drier than previous conditions. After 10.9 cal ka BP, the pollen data indicates a mixed forest dominated by Pinus and Quercus and warmer conditions. The ostracode record shows the continued influence of groundwater, decreasing turbidity, and increasing lake temperatures, while the climatic significance of variability in the δ¹⁸O record is more difficult to interpret.