Predominance of arid climates indicated by mammals in the pampas of Argentina during the Late Pleistocene and Holocene

South American climates during the Pleistocene and Holocene are poorly known, especially in the lowlands east of the Andean ranges. However, there is relatively good biostratigraphic and climatic information for the late Lujanian and Platan local ages (latest Pleistocene to Holocene) of the Pampas region. Most of the climatic data from the Pampean area are based on land mammal assemblages, pollen records, and geology. Contrasting with North American faunal associations, there are many non-analogue assemblages in the Holocene of the Pampean area. The climate was arid and relatively cold during most of the early Lujanian. However, several relatively wet and warm events are suggested by the presence of subtropical mammals and evidence of tchernozoid paleosoils. The late Lujanian is well represented by the fossiliferous sediments of the La Chumbiada and Guerrero members of the Lujan Formation (ca. 30,000 and 21,000-13,000 yr B.P., respectively), cropping out in stream valleys in the Buenos Aires Province. Mammal assemblages suggest a change from a temperate to a cool climate from the La Chumbiada Member to the Guerrero Member, the latter representing the last glacial maximum. The lower portion of the eolian La Postrera Formation has been dated at 11,000-8000 yr B.P. This section was deposited on basin divides under dry and cool conditions. The mammal association was similar to that of the Gueffero Member and includes the last megaherbivore mammals in South America (e.g. Megatherium americanum). Mammals occurring in Platan beds usually indicate dry but not cool conditions. Warm and probably wetter conditions were detected in continental beds deposited synchronously with a mid-Holocene marginal marine ingression (ca. 7000-6000 yr B.P.). A Holocene non-analogue assemblage of subtropical mammals and dry/cold indicators occurred at latitude 38 degrees 30'S at ca. 2000 yr B.P Subtropical mammals that require wetter conditions have been migrating southwards in the Pampean region since 1500-1000 yr B.P. Eolian sediments in the La Postrera Formation dated ca. 440 yr B.P. yielded an arid to semiarid mammal assemblage which could relate to the Little Ice Age period. Remarkably, most of the present-day manual biocenoses and climatic conditions in the Pampas seem to have been recently established. Paleogeographic maps for the last glacial maximum and the mid-Holocene are also provided. (C) 1999 Elsevier Science B.V. All rights reserved.