LAST GLACIAL MAXIMUM EQUILIBRIUM-LINE ALTITUDE RECONSTRUCTIONS, PALEO-TEMPERATURE ESTIMATES AND DEGLACIAL CHRONOLOGY OF THE MÉRIDA ANDES, VENEZUELA

The magnitude and underlying cause of glacial variability in the Venezuelan Andes during the late Quaternary are investigated in order to better understand tropical climate and its forcing mechanisms. Paleo-glaciers were mapped using field observations, aerial photographs, satellite imagery and high-resolution digital topographic data. Paleo-glacier equilibrium-line altitudes (ELAs) were reconstructed using the accumulation-area ratio (AAR) and the accumulation-area balance ratio (AABR) methods. During the local Last Glacial Maximum (LGM) in Venezuela (~20,600 to 17,800 cal yr BP), ELAs were ~870 to 1420 m lower. Paleo-temperature estimates were calculated for the northern Sierra de Santo Domingo region which had a ÄELA of -980 m. Using a combined energy and mass-balance equation (Kuhn, 1989) it appears that temperatures were at least 8.8 ± 2°C cooler than today. This is greater than that estimated by an atmospheric lapse rate calculation, which yields a value of 6.4 ± 1°C cooler. The paleo-ELAs presented here are consistent with other northern tropical sites and the maximum estimates from southern tropical/sub-tropical Andean records. Our paleo-temperature results, based on estimates that take into account the total energy budget across a glacier's surface, are greater than Porter's (2001) overall tropical average of 5.4 ± 0.8°C cooler temperatures during the LGM using an atmospheric lapse rate calculaion. The pattern of the LGM gradient in ELA values of Venezuela is consistent with the modern precipitation and cloud cover patterns suggesting similar moisture sources and circulation patterns at both times. The regional deglacial history is characterized using lake sediments and bog deposits. Analyses include AMS radiocarbon dating, sedimentology studies and magnetic susceptibility on a series of cores from 7 lakes and 2 bogs in the Venezuelan Andes. Results show that by 15,950 cal yr BP, glaciers began to retreat. This retreat was interrupted by at least one readvance between 14,000 and 10,000 cal yr BP. Glaciers rapidly retreated after 10,000 cal yr BP in arid areas, whereas ice remained in north facing humid areas until after 6,200 cal yr BP. The deglacial history of the Venezuelan Andes cannot be attributed to insolation forcing alone and must be combined with changes in temperature and moisture availability