In eastern North America, understanding the sensitivity and timescales of vegetation responses to rapid climate changes during the last deglaciation has been hampered by a scarcity of independent empirical datasets of past climate and vegetation. A warming climate from the Last Glacial Maximum to present is invoked as the cause of range expansion for conifers, followed later by more mesic taxa. However, the timescales of response and degree of disequilibrium remain of active interest and close analysis of vegetation responses to abrupt climate changes (decadal- to centennial-scale) requires multiproxy same-site records to avoid radiocarbon dating uncertainties. Branched glycerol dialkyl glycerol tetraethers (brGDGT) are bacterial membrane lipids well preserved in lake sediments that show promise as a paleotemperature proxy. brGDGT records have been widely used in Africa, but are just beginning to be applied to terrestrial records in eastern North America and choice of calibration function remains uncertain. Here we present a new paired deglacial brGDGT and pollen record from Bonnet Lake, Ohio and compare various temperature calibration reconstructions to inferred vegetation change, test calibrations, and assess the timescale and sensitivity of vegetation responses to abrupt temperature changes.
Results/Conclusions
Two basal radiocarbon AMS dates indicate lake initiation at 18,000 calendar years before present (cal yr BP). Eight additional radiocarbon dates anchor the age-depth model with five more dates planned. The pollen record contains two large abrupt transitions in vegetation composition. Spruce abundances decline rapidly at the onset of deglaciation, as mesic hardwoods abundances gradually increase. During this transition, white pine quickly becomes dominant, comprising 80% of the fossil pollen counted, and roughly 900 years later, abruptly declines to near absence. Among four brGDGT calibration functions, temperature reconstructions are similar in timing of events and deglacial history, but vary in magnitude of reconstructed temperature changes. Correspondence analysis (CA) indicated that the best temperature reconstruction accounted for 33% of the variance in the pollen data. Furthermore, two large and rapid transitions in temperature coincide with rapid changes in vegetation assemblages, indicating rapid and sensitive vegetation responses to abrupt hemispheric temperature changes during the last deglaciation. Our findings suggest that brGDGT has promise as a paleotemperature proxy in eastern North America and for further studies of vegetation responses to abrupt temperature changes.