NATURE OF ENVIRONMENTAL-CHANGES IN SOUTH-AMERICA AT THE LAST GLACIAL MAXIMUM

A review is given of geomorphological evidence suggesting the nature of environmental changes in South America during the Last Glaciation Maximum (LGM). The data are from glacial, geocryogenic, alluvial, colluvial and aeolian systems and span 68-degrees of latitude, making the most complete land transect of major climatic systems in the Southern Hemisphere. Glaciers in the northern and central Andes may have reached their maximal extent by 27,000 yr B.P.; reduced precipitation at the LGM, caused by lower temperatures and lower atmospheric humidity, probably led to slight glacier recession in the tropical Andes. Icefields in the Southern Andes were most expansive when global temperature and sea level were lowest, because Pacific westerlies supplied abundant moisture, and western outlets advancing across the continental shelf were constrained by sea level. Low-gradient river networks became incised by at least 100 m into narrow channels as global sea level fell by 120 m during the LGM. ''Draw-down'' of water-tables possibly impacted the forest cover, enhancing the drying influence of reduced sea surface temperature and atmospheric humidity. As forest and grass covers diminished in extent, effecting greater atmospheric cooling because of reduced evapotranspiration and convective condensation, colluvial and aeolian processes became more active and widespread. Almost 25% of the continent is covered with palaeo-aeolian features, presumed to have been active when airflow was stronger due to steeper atmospheric pressure and temperature gradients at the LGM. Substantial changes in precipitation totals and incidence, and in ground-water availability, probably eliminated modern-type tropical rainforest except in areas currently receiving ca. 5000 mm of annual rainfall. Equivalent changes in vegetation cover spread across much of the Guyana and Brazilian highlands and throughout the southern temperate regions.