Climatic change during the Younger Dryas chron in northern South America: a test of the evidence

New AMS and palynological data are presented from the Colombian Andes to assess vegetational and climatic change during the Lateglacial-Holocene transition, with special emphasis on the Younger Dryas (YD) chronozone. The new evidence is compared and discussed with other Colombian cores and with data from other countries in the region. The Lateglacial climatic reversal in Colombia, known as the Fl Abra stadial, has an estimated lower boundary of 11,200 (conventional) and 10,900 (interpolated) C-14 yr BP, respectively. Although the El Abra stadial is assumed to be the equivalent of the European YD, the present data suggest that the Fl Abra stadial is a biostratigraphical signal representing both the YD chronozone and the earliest Holocene (11,000-9000 C-14 Yr BP). On the basis of new AMS dates and a re-evaluation of the pollen zones, we divided the Colombian El Abra biozone into two phases. From ca. 11,000 to ca. 10,500 C-14 yr BP there is a sharp increase of subparamo and paramo pollen, reflecting a relatively cool phase during the YD chronozone (zone Y1). After ca. 10,500 14C yr BP, a slight increase of arboreal pollen and the presence of Cactaceae (zone Z1) point toward a relatively milder but drier phase extending to ca. 9000 C-14 yr BP in the earliest Holocene. Our conclusions add detail to the concept of astronomical forcing of contrasting rainfall changes in northern South America between 12,400 and 8800 C-14 yr BP. We propose an environmental drought during the El Abra biozone, with subsequent erosion of the sediments deposited during that period, as a major factor explaining the poor presence of YD evidence in northern South America. We conclude that at hydrologically sensitive sites without a clear lithological change around 11,000-9000 C-14 yr BP, a hiatus during the YD chronozone can only be detected if other cores are available with bracketed time control. We state that the start of the temperature decline in Central America and northern South America is related to the global YD cooling event. However, much work is still required to understand the duration and amplitude of the climatic reversal during the YD chronozone in this part of the globe. (C) 2000 Elsevier Science Ltd. All rights reserved.