A major advance of tropical Andean glaciers during the Antarctic cold reversal

A moraine chronology determined by surface exposure dating shows that glaciers in the northern tropical Andes expanded to a larger extent during the Antarctic cold reversal (14,500 to 12,900 years ago) than during the Younger Dryas stadial (12,800 to 11,500 years ago), contrary to previous studies; as a result, previous chronologies and climate interpretations from tropical glaciers may need to be revisited. Glaciation in the tropical Andes The Younger Dryas was a cold period immediately prior to the present warm interglacial period. There is clear evidence that it occurred throughout much of the Northern Hemisphere, but its global extent remains debatable. Prior work in the tropical Andes suggested extensive glacial advances during the Younger Dryas. Vincent Favier and colleagues present new cosmogenic dating of glacial deposits — and a reassessment of existing datasets from other glaciated Andean landscapes — to show that instead, maximal glacial advances took place in the preceding Antarctic cold reversal. Rather than a major advance, the Younger Dryas now appears to have been associated with modest glacial retreat. The Younger Dryas stadial, a cold event spanning 12,800 to 11,500 years ago, during the last deglaciation, is thought to coincide with the last major glacial re-advance in the tropical Andes 1 . This interpretation relies mainly on cosmic-ray exposure dating of glacial deposits. Recent studies, however, have established new production rates 2 , 3 , 4 for cosmogenic 10 Be and 3 He, which make it necessary to update all chronologies in this region 1 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 and revise our understanding of cryospheric responses to climate variability. Here we present a new 10 Be moraine chronology in Colombia showing that glaciers in the northern tropical Andes expanded to a larger extent during the Antarctic cold reversal (14,500 to 12,900 years ago) than during the Younger Dryas. On the basis of a homogenized chronology of all 10 Be and 3 He moraine ages across the tropical Andes, we show that this behaviour was common to the northern and southern tropical Andes. Transient simulations with a coupled global climate model suggest that the common glacier behaviour was the result of Atlantic meridional overturning circulation variability superimposed on a deglacial increase in the atmospheric carbon dioxide concentration. During the Antarctic cold reversal, glaciers advanced primarily in response to cold sea surface tempe