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Knowledge of past natural flood variability and controlling climate factors is of high value since it can be useful to refine projections of the future flood behavior under climate warming. In this context we present a seasonally resolved 2000 year long flood frequency and intensity reconstruction from the southern Alpine slope (North Italy) using annually laminated (varved) lake sediments. Floods occurred predominantly during summer and autumn whereas winter and spring events were rare. The all season flood frequency and particularly the occurrence of summer events increased during solar minima suggesting solar induced circulation changes resembling negative conditions of the North Atlantic Oscillation as controlling atmospheric mechanism. Furthermore the most extreme autumn events occurred during a period of warm Mediterranean sea surface temperature. Interpreting these results in regard to present climate change our data set proposes for a warming scenario a decrease in summer floods but an increase in the intensity of autumn floods at the South Alpine slope.

Prior to the collection of a series of sediment cores, a high- and very-high-resolution reflection seismic survey was carried out on Lago Puyehue, Lake District, South-Central Chile. The data reveal a complex bathymetry and basin structure, with three sub-basins separated by bathymetric ridges, bedrock islands and interconnected channels. The sedimentary infill reaches a thickness of >200 m. It can be sub-divided into five seismic-stratigraphic units, which are interpreted as: moraine, ice-contact or outwash deposits (Unit I), glacio-lacustrine sediments rapidly deposited in a proglacial or subglacial lake at the onset of deglaciation (Unit II), lacustrine fan deposits fed by sediment-laden meltwater streams in a proglacial lake (Unit III), distal deposits of fluvially derived sediment in an open, post-glacial lake (Unit IV) and authigenic lacustrine sediments, predominantly of biogenic origin, that accumulated in an open, post-glacial lake (Unit V). This facies succession is very similar to that observed in other glacial lakes, and minor differences are attributed to an overall higher depositional energy and higher terrigenous input caused by the strong seismic and volcanic activity in the region combined with heavy precipitation. A long sediment core (PU-II core) penetrates part of Unit V and its base is dated as 17,915 cal. yr. BP. Extrapolation of average sedimentation rates yields an age of ca. 24,750 cal. yr. BP for the base of Unit V, and of ca. 28,000 cal. yr. BP for the base of Unit IV or for the onset of open-water conditions. This is in contrast with previous glacial-history reconstructions based on terrestrial records, which date the complete deglaciation of the basin as ca. 14,600 cal. yr. BP. This discrepancy cannot be easily explained and highlights the need for more lacustrine records from this region.

An 11-m-long sediment core was collected in Lago Puyehue (40ºS, Lake District, Chile). The coring site was selected on basis of a thorough seismic-stratigraphic analysis that highlighted it as an area of relatively condensed, continuous and undisturbed sedimentation in this otherwise highly dynamic post-glacial lake. The 11-m core extends back to 17,915 cal. yr BP and a substantial part of it is annually laminated. An age-depth model was established by 9 AMS 14C dates, constrained by 210Pb, 237Cs, 241Am measurements, by the identification of event deposits related to earthquakes and/or volcanic eruptions, and by varve counting for the past 600 yrs. The core was submitted to a multi-proxy analysis, including sedimentology, mineralogy, grain-size, major geochemistry and organic geochemistry (C/N ratio, d13C), loss-on-ignition, magnetic susceptibility, diatom analysis, palynology and varve thickness analysis. Interpretation of the seismic data and extrapolation of the age-depth model puts the age for the onset of open-water conditions in Lago Puyehue at ca. 28,000 cal. yr BP. This contrasts with previous glacial-history reconstructions from the area based on terrestrial records, which date the complete deglaciation of the basin as ca. 14,600 cal. yr. BP. Along-core variations in sediment composition reveal that the area of Lago Puyehue was characterised since the Last Glacial Maximum (LGM) by a series of rapid climate fluctuations superimposed on a long-term warming trend. Among these climate fluctuations are: i) a first warming pulse sometime between 19,500 and 17,150 cal. yr BP, ii) a well-marked cold and humid interval at 13,100-12,300 cal. yr BP, roughly coeval with the Huelmo/Mascardi Cold Reversal (and out-of-phase with the Northern-Hemisphere Younger Dryas and the Antarctic Cold Reversal), iii) a second pulse towards warmer and dryer conditions between 12,300 and 11,800 cal. yr BP, marking the onset of the Holocene, iv) warm and dry climate conditions between 11,800 and 7,800 cal. yr BP, reflecting an early Holocene climatic optimum, v) relatively stable climate conditions since 7,800 cal. yr BP, except for vi) an interval of cold and/or humid conditions at 3,400-2,900 cal. yr BP, and vii) a wet period during the interval 1490-1700. Regional present-day climate around Lago Puyehue is dominated by the southern Westerlies and associated strong precipitation. Precipitation peaks during the transition from austral autumn to winter and during winter. Detailed re-analysis of available instrumental data shows El-Niño years to be characterized by lower-thannormal precipitation, particularly during the autumn/winter transition. Since precipitation during autumn/winter is the dominant factor controlling the thickness of the varves, varve-thickness and color-intensity records can be used as proxy for precipitation and as a tracer of El Niño activity over time. Spectral analysis of varvethickness records over the last 600 yrs reveals periodicities of 3, 3.2 and 4-4.4 yrs. The dominant period over the last 100 yrs is that of 3 yrs, which is consistent with the most common frequency of the present-day El Niño phenomenon. The record also contains multi-decadal periodicities of 15 and 41 yrs, which are consistent with other South Pacific and even North Pacific indexes (QBO, PDO, PNI or NOI). The dominant period over the last 600 yrs is 4-4.4 yrs and appears in the record during the period 1490-1700 (i.e., the Little Ice Age). The climate during the Little Ice Age thus appears to have been characterized by successive pronounced El Niño events. Varve-thickness analysis of the laminated interval at the bottom of the long core allows sub-dividing the Huelmo/Mascardi Cold Reversal into two separate cold/humid intervals, interrupted by a dryer and warmer phase. The sedimentary record obtained from Lago Puyehue provides the very first longterm, continuous, high-resolution record from the Pacific side of southern South America, and will undoubtedly emerge as a reference paleoclimate record.

Despite South-Central Chile’s high seismicity and the occurrence of earth’s largest instrumentally recorded earthquake (AD 1960; Mw: 9.5), paleoseismic data is still scarce for this region. In this study, very high-resolution reflection seismic profiles (3.5 kHz) in Lake Puyehue (41°S) were utilized to trace giant seismic events back into time. The seismic profiles show repeated occurrences of multiple mass-wasting deposits (slumps, debris flows, homogenites) occurring at a same seismic-stratigraphic horizon, indicating that they are coeval and caused by a single mass-wasting event of basin-wide importance. An age-depth model, based on 9 AMS radiocarbon datings and varve-counting on an 11 m-long sediment core, has been used to develop a “seismic chronostratigraphy”. It allows dating of the mass-wasting events by interpolation between dated seismic horizons to the distal parts of the mass-wasting deposits. The mass-wasting events are assumed to be earthquake-triggered because:The recentmost mass-wasting events correlate with the devastating historical earthquakes of AD 1575 and AD 1960.Synchronicity of multiple slope failures (mass-wasting events) requires a strong regional trigger, such as an earthquake. Consequently, local slope oversteepening at delta fronts or local fluid expulsion could not initiate such widespread events.South-Central Chile has been historically subjected to several strong (M > 8) subduction earthquakes and subduction processes have been constantly active since Mesozoic times.Multiple slope failures occur at water depths > 70 m, which rules out shallow instability triggers, such as storm wave action and lake-level fluctuations.This study reveals nine paleoseismic events during the Holocene with a mean recurrence rate of about 1000 yr, but with an overall relatively aperiodic occurrence (ranging between 400-2000 yrs.). The most prominent event took place around 1660 cal. yr. BP, evidenced by at least 29 simultaneous mass-movements and a homogenite deposit. Quantitative comparison of mass-wasting events related to the historical earthquakes of AD 1960 and AD 1575 showed significant differences (respectively 17 and 4 observed mass-wasting deposits) although these earthquakes are assumed to have had a comparable strength. This can be attributed to a lowered sedimentation rate on the potentially unstable slopes in the period 3000 cal. yr. BP – 500 cal. yr. BP, which would have made lacustrine earthquake recording less likely in AD 1575. The absence of mass-wasting deposits associated with other historical earthquakes (e.g.: AD 1737 (Ms: 7.5) and AD 1837 (Ms: 8)) indicates that only mega-earthquakes (Mw >8.5) within a range of about 300 km are recorded in the sedimentary sequence of Lake Puyehue.Reflection seismic profiles also show vertical fluidisation structures with large-scale sediment injections, which disturb the upper sedimentary sequences. The top of these fluidisation structures and diverse deformation levels could be spatially linked to seismically induced mass-wasting deposits and consequently indicate an additional method for lacustrine paleo-earthquake tracing.Several reconaissance seismic surveys on other glacigenic lakes in the Chilean Lake District also show promising paleoseismic records, which will offer the opportunity to correlate lacustrine records to reveal South-Central Chile’s paleoseismic history in detail and the earthquake registration capacities of its glacigenic lakes.

The recent evolution of clastic sedimentary processes in Lago Puyehue, as characterized by high-resolution seismic profiling and multidisciplinary analysis of sediment cores from two contrasted coring sites (PU-I and PU-II), is presented and compared to the catastrophic impact of the 1960 Chilean earthquake (Mw 9.5). Lake Puyehue’s catchment area was strongly influenced by this earthquake: (i) multiple earthquake-induced land slides and debris avalanches temporally dammed the course of the main tributary (Rio Golgol) and (ii) ca. 7.106 m3 of white fine pumiceous sands and black medium-sand-sized scoria were deposited in the catchment area during the Puyehue-Cordon-Caulle volcanic eruption that followed two days after the main seismic shock.Based on the correlation of seismic data and sediment cores dated by 137Cs and the identification of historical events, we argue that “normal” clastic sedimentation is essentially resulting from the development of homopycnal flows at the end of the winter season in these oligothrophic monomictic lakes from the Chilean Lake District. While distal clastic environments (PU-II coring site) are dominated by a biogenic production and appear to have been little affected by the 1960 earthquake and Puyehue volcanic eruptions in 1960 and 1921-22, this might not be the case for more proximal clastic environments (PU-I coring site) as well submitted to sporadic hyperpycnal flows during major flood events. In 1960, for example, as several landslide dams broke in the Golgol valley after the earthquake and the volcanic eruption, a mega-hyperpycnal flow reworked a mixture of volcanoclastic and soils sediments from the catchment as well as lacustrine sediments surrounding the Golgol delta during the rising limb of the flood and accumulated ca. 3.106 m3 of material in the deep basin.

The north-western corner of Lake Le Bourget is situated along an active fault zone and accommodated a large sediment supply from the Rhone River until the end of the Late Glacial period. On the delta slope, the Holocene sheet drape that covers the largest buried mass wasting deposit (the HDU) shows undulations, small fractures and discontinuities that are attributed to downslope creep. Evidence for episodes of vigorous fluid expulsion is found in association with these discontinuities. All these features are rooted at the top of the HDU and occur along two specific isobaths. These observations indicate a close link between fractures and focused fluid flow. We suggest that focused fluid flow triggered by earthquakes facilitates the formation of small-scale faults that accommodate part of the downslope movement and eventually link up to form a head-scarp of a large slide (c. 107 m3).

The Chilean Lake District, located in Southern Chile, comprises 17 lakes at the foothill of the Cordillera de los Andes. These lakes, dammed by frontal moraines, were formed during the last deglaciation (12500-12000 BP). Their sedimentary infilling has the potential to contain a complete and continuous Holocene sedimentary record of environmental and climatic changes having affected the area.High-resolution reflection seismic data (sparker and pinger) collected during the 2001-2002 expedition in the framework of the Belgian ENSO-CHILE project have allowed us to select two lakes for the collection of long and short sediment cores:Lago Icalma (38°50’S, alt. 1150 m) is located in the Cordillera de los Andes, in the upper part of the Bio-Bio River. Its watershed (148 km2) is dominated by a soft post-glacial sediment cover, interrupted by two important pumice layers. According to the high-resolution seismic survey, the 70m-thick sedimentary infilling consists of morainic deposits, under- and interflows and laminated lacustrine deposits. The western part of the main basin represents an elevated platform, free of the influence of bottom-currents and turbidites and possibly consisting of interflow deposits. Core descriptions and physical property analyses of sediments (gamma-density, low and high-resolution magnetic susceptibility) suggest that the deposits consist of an alternation of volcanic deposits and terrigenous sediments correlated on pinger profiles, showing the presence of several low-amplitude layers.Lago Puyehue (40°40’S, alt. 185 m) is located at the foothill of the Cordillera de los Andes and presents a glacial morphology much more complicated than Lago Icalma. Its watershed is larger (1267 km2) and dominated by Quaternary and Tertiary volcanic rocks. The lake is composed, in its western part, by a large basin, filled by 250 m of sediments, as can be deduced from sparker profiles. The eastern part of the lake presents a complex substratum morphology. However, it was possible to find a suitable location in underflow and interflow deposit areas for the collection of two long cores. Core description and physical property analyses of sediments of the interflow area suggest a good and continuous sedimentary record.With this contribution, we wish to illustrate the potential of high resolution geophysical site-survey data for interpreting core descriptions and physical property analyses.

The Belgian ENSO-CHILE project aims to provide a record of paleo-precipitation in the Chilean Lake District over the last 12000 years, in order to define the regional impact of ENSO-events. ENSO-events in the area induce precipitation anomalies, and should be recorded in the lakes as changes in terrigenous sediment supply. Additional attention will be paid to identifying millennium-timescale abrupt climate changes during the Holocene and their effect on the ENSO-record, but also to pluri-decadal cyclicity overprints on ENSO-intensities.Four long sediment cores were collected from two lakes in the area. The two lakes were selected after high-resolution reflection seismic reconnaissance surveys in 6 lakes. In each of the two selected lakes, two coring sites have been identified on basis of the seismic data, one in a proximal and one in a distal depositional setting. Field work around the lakes has allowed to define the sediment source characteristics and mineralogies, the present-day vegetation and the geomorphology of the drainage basins. Surface sediment sampling in the lakes provides information on the spatial variability in present-day terrigeneous sediment supply and dispersion.Multi-disciplinary analysis of the cores is currently underway and includes: physical properties, sedimentology, mineralogy, dating, tephrochronology and pollen studies. The chronology will be established by Pb, Cs and C dating, as well as by correlation of sedimentary events with historical records. The most suitable and promising proxies will be selected after the first analyses.The project result will be a well-dated, multi-proxy record of Holocene variations in terrigeneous sediment supply. It will be integrated with a pollen record of vegetation changes, the tephrostratigraphy and the regional seismic-stratigraphy, in order to reconstruct the paleoenvironmental changes that affected the area, in terms of precipitation and temperature variations.