U N I V E R S I D A D D E C O N C E P C I Ó N DEPARTAMENTO DE CIENCIAS DE LA TIERRA CONGRESO GEOLÓGICO CHILENO 10° 2003 HOLOCENE SEDIMENTATION IN ICALMA AND PUYEHUE LAKES (SOUTHERN CHILE): INSTANTANEOUS VS CONTINUOUS RECORDS BERTRAND, S.1,*, CHARLET, F.2, RENSON, V.1, . BOES, X.1, VARGAS-RAMIREZ, L.3, ARNAUD, F.4, LIGNIER, V.5, CHAPRON, E.6, BECK, S.7, MAGAND, O. 8, FAGEL,N.1, & DE BATIST, M.2 1 U.R. Argiles et Paléoclimats, Department of Geology, University of Liège, Liège, Belgium 2 RCMG, University of Ghent, Belgium 3University of Liege, Paleobotany, Paleopalynology, Micropleontology 4Université des Sciences et Techniques de Lille, Lille, France 5 ENS, Lyon, France 6 Geological Institute, ETH, Zürich, Switzerland 7 LGCA, Université de Savoie, Le-Bourget-du-Lac, France 8 LGGE, CNRS Grenoble, France * Corresponding author: [email protected] INTRODUCTION El Niño is one of the most important present-day climatic phenomena affecting the Earth’s climate. But a better understanding of ENSO variability through time requires high-resolution studies of past records in suitable areas. In order to document Holocene climate variability at middle latitudes in Southern Chile and to test the possible influence of El Nino phenomena, two lakes directly submitted to the influence of the Westerlies in the Lake District area, have been selected: Icalma and Puyehue lakes. Based on high-resolution seismic reflection investigations (Charlet et al., this volume), two long coring sites where selected in each lake and within the main distal clastic environments (interflow and underflow deposits) in order to provide high- resolution reconstructions and to document past flooding activities. Although the active geodynamic setting of this part of Chile might complicate the normal layering of lacustrine sedimentary infills, a detailed understanding of sedimentary features and sedimentary processes has the potential to provide reconstructions of climate changes, paleoseismicity and former volcanic activity. GENERAL SETTING OF SELECTED LAKES Icalma is a small (11.65 km2) but deep (135 m) oligotrophic lake from the IX region (Parra et al, 1993), located in the Cordillera de Los Andes at an elevation of 1140 m near the source of the Bio-Bio River (Fig. 1). This narrow and over deepened basin is mainly of glacial origin. The glacial valley contain several rock bars delimitating sub-basins in the lake, the main one being closely associated to the development of a fault affecting the basement (Mardones et al, 1993). The catchment area covers 147 km² and is surrounded by several active volcanoes (i.e., Lonquimay and Sollipulli) and especially the Llaima, one of the most active volcano of America. Todas las contribuciones fueron proporcionados directamente por los autores y su contenido es de su exclusiva responsabilidad. Puyehue is a typical oligotrophic and moraine-dammed lake from the Lake District (X region). This large lake (164 km2) is located at the foothill of the Cordillera de Los Andes at an elevation of 185 m (Fig. 1) and is characterized by a rather limited water depth with respect to the other lakes of the area (maximum depth of 123 m). Several islands resulting from glacial erosion but also the development of inter-stadial moraines in the middle of the lake resulted in the subdivision of several sub-basins. The main tributary of the lake (Rio Golgol) forms a large delta in the deepest sub-basin of the lake along is eastern side. The watershed of Puyehue Lake (1267 km²) is delimited by several active volcanoes (i.e., Puyehue–Cordon de Caulle, Antillanca and Osorno), but the vegetation cover is here more dense than around Lake Icalma. The watersheds of these two lakes are covered by a several meters thick soft sedimentary cover composed of post-glacial volcanic ashes deposited by several volcanic eruptions during the Holocene (Bertrand & Fagel, submitted). Actual andosoils (i.e., Trumaos) are developed on these ashes. In such a setting, depending on the type and density of the vegetation cover, heavy rains will erode and rework Trumaos from the catchment area to the lake basin during major tributary floods. CORING OPERATIONS Coring operations involved an Uwitec platform (Fig. 2) and a 3 m-long piston coring system, as well as a classical short gravity coring device. With this system, composite long cores are resulting from a succession of several 3 m-long sections cored at different depths and at two nearby locations, as show in Figure 3. On each coring site, five short gravity cores where also taken, in order to provide enough material for 210Pb and 137Cs dating and to test different climate proxies. To reconstitute the composite lithology of a long coring site, sediment cores back to Europe where measured using a GEOTEK multi sensor track providing the bulk density, the P- wave velocity and magnetic susceptibility of the sediments every two centimeters. These sediment cores where then splitted into halves, described and photographed. Composite lithology reconstructions were then based on the macroscopic reconnaissance of key horizons and on the petrophysical properties measured with the GEOTEK. Fig. 1 - Location map of Icalma and Puyehue lakes in the North and the Centre of the Chilean Lake District. These two basins are submitted to heavy precipitations directly related to the Westerlies. Fig. 2 - UWITEC platform: coring on Puyehue Lake, February 2002. Fig. 3 - Composite logs of the four cores obtained on Icalma and Puyehue lakes. Overlapping 3 meters sections have been correlated with the help of macroscopic descriptions and high resolution magnetic suceptibility measurements. Grey shaded areas are sections which have been selected for sampling. In each lake, two long coring sites (Fig. 4) where selected within the distal clastic depocenters associated to the main tributary of the lake (interflow and underflow flood deposits) in order to provide high resolution reconstructions and to document large floods periodicities (e.g., Chapron et al., 2002). Coring locations presented in Figure 4 where selected on the base of the morphology of the basin and of high-resolution seismic reflection profiling (Charlet et al., this volume). The two cores collected in Icalma Lake are ICA-I (interflow deposits, 77 m water depth) and ICA-II (underflow deposits, 135 m water depth) and their composite logs reach a length of ~8 m (Fig. 3 and 5). Due to bad climatic conditions, the location of ICA-I had to be adapted, and resulted next to sediment slides visible on seismic profiles. These gravity reworking phenomena seems to be mainly related to the development of sub-aerial canyons along the steep northern flank of the catchment and to the generation of debris flows during heavy rains storms (Don Callucello-Carillo, Oral. Com.). Cores collected in Puyehue Lake are PU-I (underflow deposits, 122 m water depth) and PU-II (interflow deposits, 48 m water depth). The compaction of decimeter-thick sandy layers with high water content during coring operations in site PU-I and the presence of gas in the sediments, produced important coring disturbances (fine-grained sediments liquefaction) and this coring site could not be properly recovered. A more than 11 m long composite core has been obtained on the interflow site of Puyehue Lake (PU-II, Fig. 3 and 5) while the 3 m long sections of the core collected in the underflow site of this lake did not show good overlaps (PU-I). Fig. 4 - Long cores locations on Icalma and Puyehue lakes. Bathymetric map of Puyehue Lake according to Campos et al. (1989). ICAI = interflow location, ICAII = underflow location. PUI = underflow location, PUII = interflow location. DATING The preliminary chronologies of these cores are based on 210Pb and 137Cs profiles for the last millennium on short gravity cores and some AMS 14C dates on the long cores. 210Pb and 137Cs profiles where measured at the Laboratoire de Glaciologie et de Géophysique de l’Environnement (LGGE, France), and revealed rather low sedimentation rates for the last century: 17 mg/cm2.yr at ICA-II site (corresponding to approximately 0.7 mm/yr); 10 mg/cm2.yr at ICA-I site (~0.5 mm/yr) and 29-25 mg/cm2.yr at PU-II site (~1.08 mm/yr). Radiocarbon datings on wood samples (frequently observed in event deposits) or on bulk sediment are in progress at the Poznan Radiocarbon Laboratory (Poland). HOLOCENE SEDIMENTATION ICALMA LAKE Sediment cores from Icalma Lake consist in sandy volcanic layers intercalated in silty-clay laminated sediments (Fig. 5). Laminations are composed of diatoms, volcanic minerals and allophane, a typical secondary product coming from the weathering of volcanic ashes very widespread in the watershed (Bertrand & Fagel, submitted). Moreover, centimetric pumices have been described in both cores. Based on these new cores it appears that the volcanic activity of the area has been more intense in the recent past than it is today. The thicknesses of the laminations in the sediments are also changing down cores, indicating that sedimentation rates and the sedimentary processes might have been quite different during the Holocene. The total thickness of the volcanic sandy layers in ICA-II core (underflow location) reaches 190 cm while the half of the core coming from interflow location (ICA-I) contains volcanic sands (Fig. 5). These sandy sediments have several possible origins: - in situ air-fall tephras, - volcanic ashes eroded from the catchment during heavy rains and transported to the deep basin (ICA-II) through underflows during major floods of Rio Icalma, - volcanic ashes eroded from the catchment during heavy rains and directly transported to the coring sites by hill slopes (especially at ICA-I through steep canyons draining the northern slopes of the catchment nearby the depocenter of interflow deposits), - sub-aqueous gravity reworking of sediments affecting the steep slopes of the sub- basins (i.e.