LATE QUATERNARY GLACIAL STADES IN THE CORDILLERA CENTRAL, COLOMBIA, BASED ON GLACIAL GEOMORPHOLOGY, TEPHRA-SOIL STRATIGRAPHY, PALYNOLOGY, AND RADIOCARBON DATING

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 last ca.50000 yr in the Ruiz-Tolima massif, Cordillera Central, Colombia. S ix Pleistocene cold stades separated by warmer interstades occurred: b efore 48 000, between 48000 and 33000, between 28000 and 21000, from g reater than or equal to 16000 to ca.14000, ca. 13 000-12 400, and ca. 11 000-10 000 yr BP. Although these radiocarbon ages are minimum-limit ing ages obtained from tephra layers on top of tills, the tills are no t significantly older because most are bracketed by dated tephra sets in measured stratigraphic sections. Two minor moraine stages likely re flect glacier standstill during cold intervals ca. 7400 yr BP and slig htly earlier. Finally, glaciers readvanced between the seventeenth and nineteenth centuries. In contrast to the ice-clad volcanoes of the ma ssif, ca. 34km(2) in area above an altitude of ca. 4800 m, the ice cov er expanded to 1200 km(2) during the Last Glacial Maximum (LGM) and wa s still 800 km(2) during Late-glacial time (LGT). Glacier reconstructi ons based on the moraines suggest depression of the equilibrium line a ltitude (ELA) by ca. 1100m during the LGM and 500-600 m during LGT rel ative to the modern ELA, which lies at ca. 5100 m in the Cordillera Ce ntral. Glaciers in this region apparently reached their greatest exten t when the climate was cold and wet, e.g. during stades corresponding to Oxygen Isotope Stage 3; glaciers were still expanding during the LG M ca. 28 000-21 000 yr BP, but they shrank Considerably after 21 000 y r BP because of greatly reduced precipitation. (C) 1997 by John Wiley & Sons, Ltd.