Palaeoecology of pleniglacial sediments from the Venezuelan Andes. Palynological record of El Caballo stadial, sedimentation rates and glacier retreat

Abstract

Palynological analyses of fluvio-glacial sediments from Mesa del Caballo (3500 m a.s.l.), in the Venezuelan Andes, allowed reconstruction of palaeoenvironments of high altitudes close to the Last Glacial Maximum. A stadial, called El Caballo, is reported at 16.5 ka B.P. The estimated average temperature was around 7°C lower than today, and coincides with the Colombian Fúquene stadial and the global oxygen-isotopic Stage 2. The rate of sedimentation of the whole Mesa del Caballo sequence has been up to 50 times higher than others in the Venezuelan Andes. This rate declines exponentially with altitude for a range of sites, and is probably related to the amount of erosion and the size of catchment area. Also, a direct relationship exists between sedimentation rates and the age of sediments. To explain this it is necessary to take into account the decreasing age of sediments with increasing altitude, because of the removing of older sediments by glaciers during the LGM, combined with the sequential filling of rock cavities upwards during the deglaciation. The average velocity of altitudinal glacier retreat since El Caballo stadial until the present day has been 7.3 cm/yr. It has not been constant through time. The maximum occurred in the last centuries, after the Little Ice Age (Piedras Blancas cold phase), and the minimum after the Miranda warm phase, around 2.5 ka B.P. Negative retreat values or readvance coincide with the La Culata cold phase. Taxonomic and autoecological studies of unidentified palynomorphs, as well as palaeolimnological analyses (diatoms and sediment chemistry) will be reported elsewhere.