QUATERNARY RESEARCH 47, 13–28 (1997) ARTICLE NO. QR961861 A Younger Dryas Icecap in the Equatorial Andes CHALMERS M. CLAPPERTON Department of Geography, University of Aberdeen, Aberdeen AB24 3UF, Scotland, United Kingdom MINARD HALL AND PATRICIA MOTHES Instituto Geofisico, Escuela Politecnica Nacional, Apartado 2759, Quito, Ecuador MALCOLM J. HOLE AND JOHN W. STILL Department of Geology & Petroleum Geology, University of Aberdeen, Aberdeen AB24 3UF, Scotland, United Kingdom KARIN F. HELMENS AND PETER KUHRY Arctic Centre, University of Lapland, P.O. Box 122, 96101 Rovaniemi, Finland AND ALASTAIR M. D. GEMMELL Department of Geography, University of Aberdeen, Aberdeen AB24 3UF, Scotland, United Kingdom Received June 20, 1996 such periodic changes possibly pervaded most of the North- Morphologic and stratigraphic evidence shows that a late-glacial ern Hemisphere (Kotilainen and Shackleton, 1995; Kennett ice cap existed on part of the Eastern Cordillera of Ecuador(Lat. and Ingram, 1995; Keigwin, 1995) and even the tropical 0° 20 S) on ground with a mean elevation of 4200 m where none Sulu Sea (Linsley, 1996). Younger Dryas (YD) cooling was exists now. An outlet glacier from an ca. 800 km 2 ice cap termi- the last significant change prior to the global warming that nated at 3850 m altitude in the Papallacta valley on the eastern ushered in the present interglaciation. However, the global side of the plateau. Radiocarbon dates show that moraines formed distribution of YD cooling has remained somewhat of an by this advance were ice-free by 13,200 14 C yr B.P. Tephras and enigma. Whereas clear evidence of it abounds in the North- the age of organic deposits at the plateau edge indicate ice-free ern Hemisphere (Alley et al., 1993a), data confirming that conditions before 11,800 14 C yr B.P. This interval was followed by it also affected the Southern Hemisphere have been elusive the expansion of an ca. 140 km 2 ice cap that discharged glaciers and controversial (Heusser, 1984; Ashworth and Markgraf, into adjacent valleys where terminal moraines were built at 3950 m altitude. AMS and conventional radiocarbon datesfrom macro- 1989; Clapperton, 1993a, 1993b). For palaeoclimatologists fossils, peat, and gyttja above and below till of the readvance seeking the cause of climate variability during the last glacial indicate that the ice cap formed between ca. 11,000 and 10,000 cycle, and at the Pleistocene–Holocene transition, it is cru- 14 C yr B.P. and was thus coeval with the European Younger Dryas cial to know if significant cooling events like the YD affected event. The ice cap developed in response to a surface temperature the Earth’s atmosphere globally and if their effects were felt cooling of at least 3°C in the tropical Andes, a finding that is synchronously worldwide. consistent with a coupled equatorial/high latitude North Atlantic In a review of palaeoecological data for the late-glacial climate system operating at the late-glacial/Holocene transition. interval, Peteet (1995) produced a global map showing the These results are further evidence that Younger Dryas cooling distribution of sites where YD cooling has been inferred. The may have been a global event.  1997 University of Washington. map indicates that there are no sites in the entire Southern Hemisphere where pollen stratigraphy ‘‘probably’’ confirms late-glacial cooling, thereby supporting the views of those INTRODUCTION who have inferred continuous warmth for southern latitudes Spectacular millennial-scale variability of glacial-age cli- since about 13,000 yr B.P. (Markgraf, 1991, 1993; McGlone, mate has been revealed by records from Greenland ice cores 1995). Peteet concluded that in South America, the only (Johnsen et al., 1992; Dansgaard et al., 1993) and North ‘‘probable’’ evidence for YD age cooling comes from sites Atlantic sediments (Bond et al., 1993; Broecker, 1994; Bond in Colombia (north of the Equator) and that areas where existing data suggest that no palynological oscillation took and Lotti, 1995). Analyses of Pacific sediments suggest that 13 0033-5894/97 $25.00 Copyright  1997 by the University of Washington. All rights of reproduction in any form reserved.