Article Bottom-current and wind-pattern changes as indicated by Late Glacial and Holocene sediments from western Lake Geneva (Switzerland) GIRARDCLOS, Stéphanie, et al. Abstract The Late-Glacial and Holocene sedimentary history of the Hauts-Monts area(western Lake Geneva, Switzerland) is reconstructed combining high resolution seismic stratigraphy and well-dated sedimentary cores. Six reflections and seismic units are defined and represented by individual isopach maps, which are further combined to obtain a three-dimensional age-depth model. Slumps, blank areas and various geometries are identified using these seismic data. The sediment depositional areas have substantially changed throughout the lake during the end of the Late-Glacial and the Holocene. These changes are interpreted as the result of variations in the intensity of deep lake currents and the frequency of strong winds determining the distribution of sediment input from the Versoix River and from reworking of previously deposited sediments within the lacustrine basin. The identified changes in sediment distribution allowed us to reconstruct the lake's deep-current history and the evolution of dominant strong wind regimes from the Preboreal to present times. Reference GIRARDCLOS, Stéphanie, et al. Bottom-current and wind-pattern changes as indicated by Late Glacial and Holocene sediments from western Lake Geneva (Switzerland). Eclogae Geologicae Helvetiae, 2003, vol. 96, Supplement 1, p. 39-48 DOI : 10.1007/978-3-0348-7992-7_6 Available at: http://archive-ouverte.unige.ch/unige:18868 Disclaimer: layout of this document may differ from the published version. 1 / 1 0012-9402/03/01S039-10 Eclogae geol. Helv. 96 (2003) Supplement 1, S39–S48 Birkhäuser Verlag, Basel, 2003 Bottom-current and wind-pattern changes as indicated by Late Glacial and Holocene sediments from western Lake Geneva (Switzerland) STÉPHANIE GIRARDCLOS1, 2, IRA BASTER1,3, WALTER WILDI 1, ANDRÉ PUGIN1, 4 & ANNE-MARIE RACHOUD-SCHNEIDER 5 Key words: limnogeology, seismic stratigraphy, Lake Geneva, Late-Glacial, Holocene, isopachs, bottom currents Mots-clés: limnogéologie, stratigraphie sismique, Lac Léman, Tardiglaciaire, Holocène, isopaques, courants profonds. ABSTRACT RESUME L’histoire sédimentaire tardiglaciaire et holocène de la zone des Hauts-Monts The Late-Glacial and Holocene sedimentary history of the Hauts-Monts area (partie occidentale du Lac Léman, Suisse) est reconstruite grâce à la combi- (western Lake Geneva, Switzerland) is reconstructed combining high resolu- naison d’une stratigraphie sismique à haute résolution et de datations de tion seismic stratigraphy and well-dated sedimentary cores. Six reflections and carottes de sédiment. Six réflecteurs et unités sismiques sont définis et seismic units are defined and represented by individual isopach maps, which représentés sous forme de cartes isopaques individuelles, qui réunies, établis- are further combined to obtain a three-dimensional age-depth model. Slumps, sent un modèle âge-profondeur tridimensionnel. Des slumps, des zones ‘sour- blank areas and various geometries are identified using these seismic data. des’ et la géométrie des réflecteurs sont identifiés à l’aide des données sis- The sediment depositional areas have substantially changed miques. throughout the lake during the end of the Late-Glacial and the Holocene. Les principales zones de dépôt sédimentaire ont considérablement These changes are interpreted as the result of variations in the intensity of changé durant la fin du Tardiglaciaire et l’Holocène. Ces changements, inter- deep lake currents and the frequency of strong winds determining the distribu- prétés en terme d’intensité des courants lacustres profonds et de fréquence des tion of sediment input from the Versoix River and from reworking of previ- forts vents, déterminent la distribution des apports sédimentaires de la Versoix ously deposited sediments within the lacustrine basin. et du remaniement des sédiments précédemment déposés au sein du bassin la- The identified changes in sediment distribution allowed us to re- custre. construct the lake’s deep-current history and the evolution of dominant strong Ces modifications dans la distribution des sédiments ont permis de wind regimes from the Preboreal to present times. reconstruire l’histoire des courants lacustres profonds et l’évolution des régimes dominants des fort vents du Préboréal à nos jours. 1.- Introduction Lake sediments are considered to be among the most sensitive mate changes allowing inter-site comparisons over distances of archives of environmental and climate changes on the conti- hundreds of kilometres (Eicher & Siegenthaler 1976, Kelts nents. The size and morphology of lakes and their associated 1978, Lotter et al. 1992, Niessen et al. 1992), and eventually to features greatly determine the sediment record (Håkanson the marine and ice core records. Depending on the part of the 1977, Pourriot & Meybeck 1995). In particular, large lakes lacustrine basin –lake-bottom or delta/coastal areas- the sedi- register major events averaging long-term environmental mentary record is dominated by internal or external processes, changes. Their sediment infills, therefore, record regional cli- respectively. 1 Institut F.A. Forel, route de Suisse 10, CH-1290 Versoix (Switzerland) 2 Present address: Geologisches Institut – ETH Zentrum, CH-8092 Zürich (Switzerland) 3 Present address: Departement of surface water - EAWAG, CH-8600 Dübendorf (Switzerland) 4 Illinois State Geological Survey, 615 E. Peabody, Champaign, IL 61820 (USA) 5 Tattes d’oies 19, CH-1260 Nyon (Switzerland) Corresponding author: [email protected] Bottom currents and wind patterns Lake Geneva S39 Fig. 1. Location map of Lake Geneva and 3D representation of the Hauts-Monts study area. Bathymetry is indicated in meters. The sedimentary infill of numerous perialpine lakes has been 20°C. The lake basin is traditionally and geographically divid- studied during the last several years in Switzerland (Gaillard & ed into two components: the large, 300-m-deep “Grand-Lac” Moulin 1989, Lister 1988, Lotter 1999, Niessen & Kelts 1989, and the elongated, 50-70-m-deep “Petit-Lac”. Schwalb 1992, Sturm & Matter 1978, Wohlfarth & Schneider The studied “Hauts-Monts” zone is located in the 1991), France (Chapron 1999, Magny 1992, Van Rensbergen well-oxygenated and mixed waters of the Petit-Lac. The sedi- et al. 1998) and Austria (Schmidt et al. 1998, Wessels 1998). mentation in this area is influenced by the Versoix River However, the use of both high resolution seismic and sedimen- mouth, which has an estimated median flow rate of 3 m3/s tary studies is still scarce. Lake Geneva, the largest freshwater (Département de l’Intérieur 1996), and by a large underwater basin in western Europe, is part of the Rhône River system and promontory 9-14 m below the present lake level (alt. 372.05 cuts across the Alpine foreland basin from the Alps to the Jura m, Fig. 1). This subaquatic platform is a topographycally high Mountains (Fig. 1, Wildi & Pugin 1998). Its bedrock morphol- piece of Molasse bedrock that resisted glacial erosion during ogy and Pleistocene glacial sediment infill have been previ- the last ice ages. ously described by Vernet & Horn (1971), Vernet et al. (1972), This paper presents the Late-Glacial and Holocene Wildi et al. (1997) and Moscariello et al. (1998). Detailed paleoenvironmental evolution of a bottom-current dominated studies of the Holocene sediment record have also been pub- area in western Lake Geneva. Based on both seismic and sedi- lished by Loizeau (1998), Moscariello (1996), Girardclos mentological approaches we have reconstructed the changes in (2001), Baster (2002) and Baster et al. (this volume). the geometry, lateral extent and thickness distribution of indi- Lake Geneva is monomictic and mesotrophic, and its vidual seismo-stratigraphic units throughout time and the asso- surface-water temperature varies yearly between 5°C and ciated paleoenvironmental history of the lacustrine basin. S40 S. Girardclos et al. 2.- Methodology 20299). The other reflections’ ages (reflections n° 9/10 and 14) were estimated compiling and correlating previous palynolog- 2.1. Seismic data acquisition and processing ical analyses (Burrus 1980, Châteauneuf & Fauconnier 1977, Lüdi 1939, Moscariello 1996, Reynaud 1981). The time scale In 1997, 200 km of seismic profiles within a 4 x 3.5 km area displayed in Fig. 2 is based on chrono- and biozones defined have been acquired using a high resolution BATHY-1000 for the Swiss Plateau (Ammann et al. 1996, Lotter 1999). echosounder (Girardclos 2001), connected to a Differential Unit c1* represents the end of the Late-Glacial, from Global Positioning System (DGPS) with an accuracy of ±2-10 the end of the Bølling to the beginning of the Preboreal (Fig. m. The dominant wave frequency of this seismic system is 1.5- 2). Unit c2 spans the first half of the Holocene, from the Pre- 2 kHz. Following the Rayleigh’s criterion, the calculated verti- boreal to the end of the Younger Atlantic. Unit c3 encompass- cal resolution of the echosounder data is 0.25 m (Girardclos es the entire Subboreal, the Older Subatlantic and the very be- 2001). After standard seismic processing, the lines were digi- ginning of the Younger Subatlantic. All the remaining units tally analysed with the PC software SEISVISION 4.0 of GEO- d1, d2 and d3 represent the Younger Subatlantic. The high res- GRAPHIX LTD. The seismo-statigraphic analyses was exported olution of the Younger Subatlantic