Palaeoenvironmental changes inferred from malacofaunas in the Lateglacial and early Holocene uvial sequence at Conty, northern France NICOLE LIMONDIN-LOZOUET AND PIERRE ANTOINE Limondin-Lozouet, N. & Antoine, P. 2001 (June): Palaeoenvironmental changes inferred from malacofaunas in the Lateglacial and early Holocene uvial sequence at Conty, northern France. Boreas, Vol. 30, pp. 148– 164. Oslo. ISSN 0300-9483 Environmental changes are reconstructed from a Lateglacial and early Holocene sequence at Conty, northern France. The molluscan succession is put into a chronostratigraphic framework supported by numerous radio- carbon dates. Malacofaunas from the Bølling chronozone are reported for the rst time in northern France and show progressive expansion of marshy communities within organic deposits. This biozone ended in a calcar- eous silt with the appearance of several species of arctic–alpine afnities. These sedimentological and malaco- logical data point to colder climatic conditions after 12 220 § 90 BP, but before 11 640 § 80 BP, allowing al- location to the Older Dryas event. The rst part of the Allerød appears to have been drier and relatively stable. After 11 400 BP, a decline in species richness and diversity in the malacofaunas suggests increasing dryness. During the Younger Dryas, two molluscan biozones are identied in a homogeneous calcareous silt, reecting an early wet phase followed by a drier episode. At the onset of the Holocene malacofaunas show a higher diversity, suggesting climatic improvement. Nicole Limondin-Lozouet (e-mail: limondin@cnrs-bellevue.fr), UMR 8591 CNRS, Laboratoire de Ge ´ographie Physique, 1 Place A. Briand, 92195 Meudon cedex, France. Pierre Antoine (e-mail: Pierre.Antoine@univ-lille1.fr), ESA 8018 CNRS, Unite ´ Stratigraphie et Pale ´oenvironnements quaternaires, Bat. de Ge ´ographie, Av. P. Langevin, Universite ´ des Sciences et Technologies de Lille, 59655 Villeneuve d’Ascq cedex, France; received 10th April 2000, accepted 26th October 2000 The Weichselian Lateglacial in northern France has been studied intensively during the past few years. Valley bottoms have been the focus of special attention as these provide the best-preserved Lateglacial sequences, and several papers have been published recently about the evolution of river systems in response to climatic variation (Emontspohl & Veermersch 1991; Antoine 1997a–c, 2000; Leroyer et al. 1997; Pastre et al. 1997, 2000). These results reveal that higher sedimentation rates are found mostly in small valleys, which produce the best sequences and allow high resolution environmental and climatic reconstructions to be made. Conty is located on the right bank of the river Selle near the conuence with Les Evoissons stream, 20 km south of Amiens (Fig. 1). Archaeological survey of the site, excavated for industrial gravel extraction, led to the discovery of three nal Upper Palaeolithic levels within the Lateglacial sequence (Fagnart & Coudret 1995) and a Mesolithic settlement in the Holocene peat (Ducrocq 1999). Numerous boreholes and trenches allowed a very accurate stratigraphic reconstruction, which revealed the presence of organic deposits (Antoine 1997b) that were sampled for various fossil groups. Several of these studies are still in progress and will be published later. In this paper we present the malacological results in their stratigraphical context along with some new 14 C results. Palaeoenvironments and climatic impact regis- tered by mollusc communities are described and discussed on a regional scale, and compared with other molluscan communities reported from NW Europe. The primary aim is to highlight major environmental and climatic changes in an area where until recently little was known on the last glacial–Holocene transition. In valley bottoms from the Somme area, cold phases are represented by calcareous minerogenic deposits and climatic improvements by organic levels (Antoine 1997a–c). It is therefore possible to dene a succession of climatic events where maximum peaks are broadly comparable to an ice-core oxygen isotope signal. Nevertheless, resolution of the sedimentary record is not high enough to allow the precise correlation of climate variations, and consequently reference to the new classication for the Last Termination based on isotopic variations in the GRIP ice-core record (Bjo ¨rck et al. 1998; Walker et al. 1999) appears unjustied. Thus, as the Conty chronostratigraphic scheme is supported by a range of 14 C dates yr BP and rst palynological results, use of the Nordic terminology (Mangerud et al. 1974) seems more appropriate in order to provide the basis of comparisons with other continental NW European sequences. Stratigraphical context and chronology Geomorphological and stratigraphical studies through the Selle valley (Antoine 1997b) have identied four