Quaternary Science Reviews, Vol. 11, pp. 449--480, 1992. 0277-3791/92 $15.00 Printed in Great Britain. All rights reserved. © 1992Pergamon Press Lid GLACIAL TO INTERGLACIAL VEGETATION CHANGES IN THE NORTHERN AND SOUTHERN PYRI~NI~ES: DEGLACIATION, VEGETATION COVER AND CHRONOLOGY G. Jalut,* J. Monserrat Marti,t¶ M. Fontugne,:~ G.Delibrias,, J.M. Vilaplana§ and R. Juliall *Laboratoire de Botanique et Biogdographie, Universitd Paul Sabatier, 39, Alides J. Guesde, 31062 Toulouse Cedex, France ? Instituto Pirenaico de Ecologia, Apdo. 64, 22700 Jaca (Huesca), Spain ,Centre des Faibles Radioactivitds, Laboratoire Mixte CNRS-CEA, Domaine du CNRS, Avenue de la Terrasse, 91198, Gif sur Yvette Cedex, France §Departament de Geologia Dinamica, Geofisica i Paleontologia, Facultat de Geologia, Zona Universitaria de Pedralbes, 08071 Barcelona, Spain Illnstituto de Ciencia de la Terra Jaime Almera, 08028 Barcelona, Spain New palynological investigations have been carried out in the southern, northern and Mediterranean part of the Pyrenean chain. Pollen percentage and concentration diagrams are compared for a period including the end of the glacial period, the Late Glacial and Postglacial periods. From the new results and from comparisons with previous published data, the steppe and semi-desert vegetation of the end of the last glacial period are described, as well as the plant communities of the period 15-13 ka. A dry climate was responsible for the early glacial retreat before 30 ka. Deglaciation was continuous up to about 15-14 ka, without any readvance around 20-18 ka. Different aspects of the Late Glacial vegetation are studied and the first stages of the Postglacial are described. The Younger Dryas event is found everywhere, but particularly clearly in the Mediterranean area. The most important '4C dates are discussed, and a plateau of ~4C ages is clearly visible around 10,000 BP. INTRODUCTION During the last ten years, combined palynological and sedimentological studies have been carried out in the French and Spanish Pyrrn6es (Jalut et al., 1982, 1988; Mardonrs, 1992; Andrieu, 1987, 1989; Montser- rat Marti and Vilaplana, 1987). The goals of this research were to determine the dynamics of the development of the vegetation cover from the end of the last glacial period to the present in connection with the study of the deglaciation of these southern Euro- pean mountains. From the results of the CLIMAP Program, particu- larly those concerning the deglaciation of the northern Atlantic (Ruddiman and Mclntyre, 1981), it could be assumed that the Pyrrnres were, as in northern Europe, characterized by a glacial maximum around 20 ka. On the northern slope of this region, results have been obtained at middle elevations in the Ari~ge basin (Jalut et al., 1982), at the border between the Mediter- ranean and Atlantic regions, and at low elevations near Lourdes in the Atlantic region (Mardon~s, 1982; Mardon~s and Jalut, 1983). On the Spanish slope, recent studies (Bru et al., 1985; Vilaplana et al., 1983, 1989) have shown that the glacial maximum occurred prior to 38 ka, and that the beginning of glacial retreat also occurred before 38 ka. Furthermore, on the basis of the results obtained by Sorriaux (1982) in the Niaux- Lombrive-Sabart karstic system (Ari~ge basin), it ¶This paper is dedicated to the memory of Dr Joan Montserrat Marti. appeared that the Ari~ge glacier and its Vicdessos tributary had significantly regressed around 20 ka with respect to their positions during the phase of maximum advance (Bakalowicz et al., 1984). On the basis of palynological studies in the Mediterranean part of the Pyrrnres (Jalut, 1974), it was clear that around 14-15 ka, if the glaciers still existed, they were situated higher than 1800--1900 m, the deglaciation most likely being completed around 13 ka (Taillefer, 1983). New studies were developed in cooperation with the palaeoclimato- logical programs of the EEC in the western, central and Mediterranean parts of the chain (Andrieu, 1987, 1989; Jalut et al., 1988; Andrieu and Jalut, 1987) as well as in the central and south-western Spanish part (Montserrat and Vilaplana, 1987; Monserrat, 1989, 1991). Our goal was to extend the results on the chronology of the deglaciation and to improve our knowledge of the contemporary vegetation cover in order to have a better understanding of the climatic changes which control this development. The present synthesis comprises the sedimentological and palynological study of four sites situated on the eastern and western part of the chain, on the Spanish and French slopes. The results are compared with previously published data. LOCATION OF THE SITES The sites are glaciolacustrine basins of low and middle altitudes (Fig, 1). During the last glacial period they were pro- or juxtaglacial lakes in which sediments transported by melt waters were deposited. Because of 449