Late Quaternary Vegetational and Climatic Changes in the Pampa Grassland of Argentina ALDO RAU ´ L PRIETO Lab. Palinologi ´a. Dpto Biología, Centro de Geología de Costas y del Cuaternario, Facultad de Ciencias Exactas y Naturales, Universidad Nacional de Mar del Plata, Funes 3250, 7600 Mar del Plata, Argentina Received December 21, 1994 The vegetation and climate of the Pampa grassland, Argentina, during the late Quaternary are reconstructed from pollen recov- ered from dated stratigraphic sections from arroyo walls and from archaelogical excavations. Prior to 10,500 yr B.P., herba- ceous psammophytic steppe existed in the central part of the Pampa grassland while xerophytic woodland associated with psammophytic and halophytic steppe occurred in the southwest- ern part of the Pampa. These types of vegetation and the conti- nental conditions that prevailed in the area of the present-day coast (38°S), indicate subhumid-dry climate and annual precipi- tation 100 mm lower than present. A subsequent change toward a vegetation characteristic of ponds, swamps, and foodplains, or toward environments with locally more effective moisture, oc- curred ca. 10,500 yr B.P. suggesting annual precipitation close to modern levels or a higher availability of water in the central part of the Pampa grassland, this type of vegetation existed until 8000 yr B.P., when it was replaced by grassland communities that lasted until 7000 yr B.P. In the southwestern part of the Pampa grassland, this vegetation developed before 7000 yr B.P. and per- sisted until ca. 5000 yr B.P. Sea level higher than the present ca. 6200 yr B.P. is consistent with sharp modification of the vegeta- tion and development of local halophytic communities dominant at 38°S. A return to subhumid-dry conditions occurred after 5000 yr B.P. The late Holocene vegetation is characterized by pollen assemblages similar to the psammophytic and halophytic commu- nities of the Southern pampa grassland, associated with commu- nities with more edaphic conditions. At the same time, at 38°S a sea level regression is suggested by the dominance of fresh-water pollen assemblages and micropaleontological remains. The trend toward humid, temperate conditions ca. 1000 yr B.P. suggested by vertebrate remains characteristic of temperate and humid condi- tions, as well as a very short but dry episode during the 18th century suggested by the geology, are not clearly evidenced in the pollen sequences. Vegetational and climatic changes are explained by the latitudinal shifts and changes in intensity of the southern atmospheric circulation and changes in sea level. © 1996 University of Washington. INTRODUCTION Several paleoclimatic models based primarily on pollen records from the Andean forest region have been proposed to explain vegetational changes during late and postglacial time southern South America (Markgraf, 1983, 1991; Markgraf et al., 1992; Villagrán, 1993). Pollen records from the Pampa grassland, however, are few and problematic. The few earlier records were either undated (e.g., Fernández and Romero, 1984; Guerstein and Quattrocchio, 1984; Quattrocchio et al., 1988) or, if dated, the pollen sum was inadequate for the cal- culation of reliable relative frequencies (Nieto and D’Antoni, 1985). Information on late Pleistocene and Holocene paleoenviron- mental changes in the Pampa grassland has come from geo- morphologic and stratigraphic studies, including study of fossil vertebrate assemblages (e.g., Tonni and Fidalgo, 1978; Tonni, 1992; Iriondo and García, 1993; Zárate and Blasi, 1993). This information, however, is in part controversial, inasmuch as some of the Holocene climatic fluctuations suggested by the geologic evidence have not been detected in the faunal analy- ses (Tonni et al., 1988; Tonni, 1992). Also, paleoenvironmen- tal inferences for the late Pleistocene are largely problematic because many are based on studies of extinct taxa, and there are few possibilities of verifying such inferences due to the lack of modern counterparts. The aim of this study is to reconstruct past plant communi- ties in the Pampa grassland for the late Quaternary, to infer regional paleoclimatic trends, and to relate these trends to glob- al changes in atmospheric circulation. Several dated pollen records were analyzed and interpreted using modern pollen spectra. The comparison of fossil pollen sequences with mod- ern pollen provided information about changes in the compo- sition and distribution of late Quaternary vegetation. GENERAL DESCRIPTION OF THE REGION Vegetation The phytogeographical province of the Pampa grassland is located in eastern Argentina between latitudes 31° and 39°S (Cabrera, 1968) (Fig. 1). The present vegetation is the result of natural factors and of agricultural and grazing activities during the last 100 years. Today, the Pampa grassland is characterized by crop farming similar to that of any temperate region of the world and by small plantations of exotic forests. However, it is QUATERNARY RESEARCH 45, 73–88 (1996) ARTICLE NO. 0007 73 0033-5894/96 $18.00 Copyright © 1996 by the University of Washington. All rights of reproduction in any form reserved.