EXCEPTIONALLY STEEP NORTH SOUTH GRADIENTS IN LAKE TEMPERATURES DURINGTHE LAST DEGLACIATION

During the transition from the last glacial to the present interglacia l climate (late-glacial period), high summer insolation(1) combined wi th the presence of the Laurentide ice sheet is thought to have promote d the development of strong summer thermal gradients in North America south of the ice margin(2). Here we use palaeoecological methods to ob tain quantitative evidence for the existence of these gradients. Our p alaeoclimate reconstructions are based on water temperatures inferred from fossil assemblages of aquatic midge larvae in five lakes along a 240-km transect, extending from central New Brunswick, Canada, to sout heastern Maine, USA. We show that the temperature gradient shifted dur ing the Killarney Oscillation (KO) and Younger Dryas (YD) cooling even ts, and that water temperature differences of 9 and 11 degrees C exist ed over distances of 55 and 240 km, respectively, during parts of the late-glacial period. These gradients are much stronger than the curren t north-south trends of lake summer surface water temperatures for the se five lakes and for lakes across the northern tree line. The rapid m otion of such steep temperature gradients may have affected the progre ssive development of aquatic and terrestrial ecosystems in the region.