Exceptionally steep north–south gradients in lake temperatures during the last deglaciation

During the transition from the last glacial to the present interglacial climate (late-glacial period), high summer insolation1 combined with the presence of the Laurentide ice sheet is thought to have promoted the development of strong summer thermal gradients in North America south of the ice margin2. Here we use palaeoecological methods to obtain quantitative evidence for the existence of these gradients. Our palaeoclimate 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 southeastern Maine, USA. We show that the temperature gradient shifted during the Killarney Oscillation (KO) and Younger Dryas (YD) cooling events, and that water temperature differences of 9 and 11°C existed over distances of 55 and 240 km, respectively, during parts of the late-glacial period. These gradients are much stronger than the current north–south trends of lake summer surface water temperatures for these five lakes and for lakes across the northern tree line. The rapid motion of such steep temperature gradients may have affected the progressive development of aquatic and terrestrial ecosystems in the region.

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