PALEOENVIRONMENTAL CHANGES AND GLACIAL STADES OF THE LAST 50,000 YEARS IN THE CORDILLERA CENTRAL, COLOMBIA

Using data from glacial geomorphology, tephra-soil stratigraphy and mi neralogy, palynology, and radiocarbon dating, a sequence of glacial an d bioclimatic stades and interstades has been identified for the past ca. 50,000 yr in the Ruiz-Tolima massif, Cordillera Central. Six cold stades separated by warmer interstades occurred before 48,000, between 48,000 and 33,000, between 28,000 and 21,000, from greater than or eq ual to 16,000 to ca. 14,000, ca. 13,000-12,400, and ca. 11,000-10,000 yr B.P. Although the radiocarbon ages are minimum-limiting ages obtain ed from tephra layers on top of tills, the tills are not significantly older because most are bracketed by dated tephra sets in measured str atigraphic sections. Two minor moraine stages likely reflect glacier p auses during cold intervals ca. 7400 yr B.P. and slightly earlier. Fin ally, glaciers readvanced between the 17th and 19th centuries, In cont rast to the glacier cover (ca. 34 km(2)) on volcanoes of the massif du ring the last glacial maximum (LGM) the ice cover expanded to 1200 km( 2) and was still 800 km(2) during late-glacial time (LGT). Glacier rec onstructions based on the moraines suggest depression of the equilibri um line altitude (ELA) by ca. 1100 m during the LGM and 500-600 m duri ng LGT relative to the modern ELA which lies at ca. 5100 m in the Cord illera Central. Glaciers in this region apparently reached their great est extent when the climate was cold and moist, e.g., during stades co rresponding to marine isotope stage 3; glaciers were still expanding d uring the LGM ca. 28,000-21,000 yr B.P., but they shrank considerably after 21,000 yr B.P. because of greatly reduced precipitation. (C) 199 6 University of Washington.