Abstracts of Symposium 55 and some papers from other related Symposia.

Upper Quaternary Paleoclimatic Evolution of Northwestern Argentina

Jose M. SAYAGO & Mirian M. COLLANTES, Instituto de Geociencias y Medio Ambiente (INGEMA), Universidad Nacional de Tucuman, Miguel Lillo 205 - (4000) Tucuman, Argentina

The study region is situated in the transition between westerly (polar front) and South Pacific and Atlantic anticyclon influences (from 22 to 28 degree, south lat.). In this paper particularly attention is devoted to the highly climatic variability of the Late Pleistocene-Holocene, the wet intervals of neo and upper Holocene and the extreme dry medieval period. Evidences from the last glacial period, based on landform and glacigenic sediments, are fragmentary and incomplete. On the contrary, a well preserved complex of loess layers interbedded with paleosols and fossil vertebrates point out a more accurate environmental record of the Pleistocene-Holocene transition. The transition shows up a highly climatic ciclicity with alternation of periods of loess sedimentation (dry) and pedogenesis (wet) parallel to faunal extinction ending at the early Holocene. During the neo-Holocene the dry Andes were afected by wetter conditions that generated shallow glacigenic sediment in the summit areas and clastic levels in the piedmonts. At the beginning of the Upper Holocene humid and warmer conditions are reflected by a well developed paleoedaphic sequence found in the dry Calchaquies valley of the west, the intermontaneous structural valley in the main pre-andean chains, and also in the western border of the Chaco plain. The expansion of the main agrarian cultures of the region coincides with this period of mild climatic conditions. However, approximately at the end of the first millennium (A.D.) the abrupt disappearance of those cultures, particularly in the southern valley, is attributed to an extreme drought extended during the next four centuries. Intensive erosion, field dune expansion and loess sedimentation characterized the geodynamic during an interval approximately contemporaneous with the medieval European warm.

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