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 mineralogy, palynology, and radiocarbon dating, a sequence of glacial and bioclimatic stades and interstades has been identified for the past ca. 50,000 yr in the Ruíz-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 ≥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 obtained from tephra layers on top of tills, the tills are not significantly older because most are bracketed by dated tephra sets in measured stratigraphic sections. Two minor moraine stages likely reflect glacier pauses during cold intervals ca. 7400 yr B.P. and slightly earlier. Finally, glaciers readvanced between the 17th and 19th centuries. In contrast to the glacier cover (ca. 34 km 2) on volcanoes of the massif during the last glacial maximum (LGM) the ice cover expanded to 1200 km 2and was still 800 km 2during late-glacial time (LGT). Glacier reconstructions based on the moraines suggest depression of the equilibrium line altitude (ELA) by ca. 1100 m during the LGM and 500-600 m during LGT relative to the modern ELA which lies at ca. 5100 m in the Cordillera Central. Glaciers in this region apparently reached their greatest extent when the climate was cold and moist, e.g., during stades corresponding to marine isotope stage 3; glaciers were still expanding during 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.