Climatic inferences from glacial and palaeoecological evidence at the last glacial termination, southern South America

Abstract

There is uncertainty about the interhemispheric timing of climatic changes during the last glacial–interglacial transition. Different hypotheses, relying on different lines of evidence, point variously to the Northern Hemisphere leading the Southern Hemisphere and vice versa, or to synchrony between the hemispheres. Southern South America is well placed to test the various alternatives using both glacial and palaeoecological evidence. We argue here from a synthesis of key proxy records that there was a sudden rise in temperature that initiated deglaciation sychronously over 16° of latitude at 14 600–14 300 14C yr BP (17 500–17 150 cal. yr). There was a second step of warming in the Chilean Lake District at 13 000–12 700 14C yr BP (15 650–15 350 cal. yr), which saw temperatures rise to close to modern values. A third warming step, particularly clear in the south, occurred at ca. 10 000 14C yr BP (11 400 cal. yr), the latter achieving Holocene levels of warmth. Following the initial warming, there was a lagged response in precipitation as the westerlies, after a delay of ca. 1600 yr, migrated from their northern glacial location to their present latitude, which was attained by 12 300 14C yr BP (14 300 cal. yr). The latitudinal contrasts in the timing of maximum precipitation are reflected in regional contrasts in vegetation change and in glacier behaviour. The large scale of a 80-km glacier advance in the Strait of Magellan at 12 700–10 300 14C yr BP (15 350–12 250 cal. yr), which spans the Antarctic Cold Reversal and the Younger Dryas, was influenced by the return of the westerlies to southern latitudes. The delay in the migration of the westerlies coincides with the Heinrich 1 iceberg event in the North Atlantic. The suppressed global thermohaline circulation at the time may have affected sea-surface temperatures in the South Pacific, and the return of the westerlies to their present southerly latitude only followed ocean reorganisation to its present interglacial mode.