The anatomy of a climatic oscillation: vegetation change in eastern North America during the Younger Dryas chronozone

Century-scale climate changes reshaped circulation patterns over the North Atlantic and adjacent regions during the last glacialto-interglacial transition. Here, we show that vegetation across eastern North America shifted dramatically at the beginning and end of the Younger Dryas chronozone (YDC: 12,900–11,600 cal yr B.P.), when changes in ocean circulation rapidly cooled and then warmed the North Atlantic sea-surface. On both the site-specific scale and the continental-scale, vegetation changed only gradually during the millennia before (15,000–13,000 cal yr B.P.) and after (11,000–9000 cal yr B.P.) the YDC, but climate changes ca 12,900 and 11,600 cal yr B.P. altered the vegetation on both spatial scales within centuries. Plant associations changed and some taxa rapidly migrated hundreds of kilometers (>300 km within B100 yr). In limited regions near the North Atlantic coast, abrupt cooling ca 12,900 cal yr B.P. resulted in a return to earlier vegetation types. Elsewhere, however, the vegetation patterns during the YDC were distinct from those of both earlier and later intervals. They indicate abrupt, ‘non-reversing’ seasonal temperature changes that were probably related to atmospheric circulation changes during the YDC, rather than to the direct influence of North Atlantic sea-surface temperatures. Atmospheric circulation patterns during the YDC were unique within the last 21,000 yr because of a unique combination of climate controls. Insolation, ice sheet extent, and atmospheric composition were significantly different from their full-glacial states, even when the North Atlantic returned to near full-glacial conditions. The YDC vegetation patterns demonstrate (1) rapid ecological responsiveness to abrupt climate change and (2) spatially varied patterns of YDC climate change. r 2002 Elsevier Science Ltd. All rights reserved.