7200 years of Rhône river flooding activity in Lake Le Bourget, France: a high-resolution sediment record of NW Alps hydrology.

Magnetic susceptibility (MS) was measured with high resolution (5 mm) on a 9 m long, 14C dated core from Lake Le Bourget (Savoie, France), spanning the last 7200 years. The strong correlation (R = 0.85) of the MS with the silicate-borne suite of elements (Si, Al, Fe,Mg, K) and anti-correlation with the carbonate content (R = -0.87) allows it to be used as a proxy for the fluctuations of the abundance of riverborne clastic fraction versus authigenic carbonates in sediment. As the Rhône is the only river bringing a significant amountof silicate minerals to the coring site, the MS downstream is interpreted as a proxy of the Rhône suspended load discharge in Lake Le Bourget. This is confirmed over the last 3000 years by the good match with the evolution of hydrological activity of the Rhône as it is known throughgeomorphological studies of well-dated archaeological sites. Over the last 7200 years, the record is consistent with the regional record of lake water-level fluctuations. While the intensity of the MS signal might be widely affected by the human impact on soil stability, the timing of theperiod of enhanced hydrological activity appears to be mostly climate-related, and should thus constitute a first step toward a high-resolution (<8 yr) continuous history of hydrological conditions in the NW Alps

Past Holocene detritism quantification and modeling from lacustrine archives in order to deconvoluate human-climate interactions on natural ecosystem over long time-scale

International audienceWater budget is one of the main challenges to paleoclimate researchers in relation to present-day global warming and its consequences for human societies

Origin of the tsunami of may 1960 in the Lake Nahuel Huapi, Patagonia: application of the high-resolution bathymetric and seism ic techniques

El tsunami ocurrido en el lago Nahuel Haupi, Patagonia Argentina, el 22 de Mayo de 1960, constituye el primer ejemplo de tsunami en lagos continentales en Argentina. La batimetría de detalle obtenida con sonar batimétrico por medición de fase (SBMF) y dos perfiles sísmicos de alta resolución junto con tres testigos sedimentarios cortos del lago permitieron vincular al tsunami con el gran movimiento sísmico conocido como el terremoto de Valdivia, el sismo más fuerte registrado instrumen- talmente a escala global (Mw 9,5). El impacto de las ondas sísmicas provocó la movilización de grandes volúmenes de sedi- mentos lacustres del sustrato, especialmente por debajo de los 70-80 m de profundidad de agua. El fallo fue probablemente inducido por la presencia de una superficie de baja coherencia (tefra?) que funcionó como plano de deslizamiento posibilitan- do la remoción de sedimentos y conformando una megaturbidita en la parte más profunda. La movilización de estos mate- riales provocó el desplazamiento de un gran volumen de agua, originando el tsunami que golpeó las costas de Bariloche y des- truyó el muelle de la ciudad.The tsunami recorded in the Lake Nahuel Huapi, Patagonia Argentina, on May 22, 1960 is the first example of a tsu- nami in a continental lake in Argentina. High-resolution bathymetry (SBMF), 2 high-resolution seismic profiles together with three short sedimentary cores allowed linking this tsunami to the 1960 earthquake of Valdivia, the strongest (Mw 9.5) ever ins- trumentally recorded earthquake. The impact of the seismic waves produced huge mass-failure below 70-80 m water depth. The failure was probably induced by the presence of a non-cohesive surface (tephra layer?) that acted as a sliding surface, and the mo- bilized material evolved into a mega turbidite in the deep basin. A large volume of water was displaced by the mobilization of these sediments producing a tsunami that hit the coasts of Bariloche and destroyed the harbor of the city.Fil: Villarosa, Gustavo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigaciones en Biodiversidad y Medioambiente. Universidad Nacional del Comahue. Centro Regional Universidad Bariloche. Instituto de Investigaciones en Biodiversidad y Medioambiente; ArgentinaFil: Outes, Ana Valeria. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; ArgentinaFil: Gomez, Eduardo Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; Argentina. Universidad Tecnológica Nacional; ArgentinaFil: Chapron, Emmanuel. Universidad de Orleans; FranciaFil: Ariztegui, Daniel. Universidad de Ginebra; Suiz

Impacts of new agricultural practices on soil erosion during the Bronze Age in the French Prealps

International audienceIn order to better understand the evolution of past climate-human-environment interactions in the North-western Alps during the Holocene, we have analysed the lipid content of two cores taken from the sediments of Lake le Bourget (French Alps). By using a specific molecular biomarker of Panicum miliaceum (broomcorn millet) previously defined and a new molecular marker of soil erosion, we demonstrate that the onset of millet cultivation coincides with the onset of major soils erosion in the catchment during the Middle Bronze Age. Although archaeological and archaeobotanical investigations indicate a discrete human occupation of the lakeshores at this period, they also point to a regional change in agricultural practices that deeply affected soils. The evolution of millet cultivation appears in strong connection with climatic variations, estimated in the same cores from the variations in titanium, a proxy of hydrological changes in the region. Social and cultural triggers cannot be discarded at this stage. Such an approach applied to more sedimentary archives shows high potential to unravel the temporal and spatial dynamics of human land-use

Paléosismicité en Auvergne à travers l'étude régionale comparée des enregistrements sédimentaires lacustres au Mont-Dore.

National audienceL’occurrence de séismes en domaine continental peut être enregistrée au sein des bassinslacustres à travers une grande variété de dépôts sédimentaires reliés à différents processus gravi- taires.A ce titre, les archives lacustres confrontées à un fort aléa sismique peuvent être assimilées à dessismomètres naturels et leur compréhension s’avère essentielle pour recenser des séismes historiques afind’évaluer le risque présent et futur.Aujourd’hui, le zonage sismique de la France continentale montre une sismicité modéréevoire forte au sein des principales chaînes de montagne. Parmi elles, la région volcanique des MontsDore permet de s’intéresser à des systèmes lacustres très contrastés, tant sur le plan limnologique,géomorphologique, que dans leur degré d’anthropisation au cours du dernier millénaire. Dans cetobjectif, une approche régionale comparée a été menée sur quatre sites entourant le massif du Sancy,incluant au total deux lacs de maar (Pavin et Chauvet) et deux lacs de barrage volcanique (Guéry etMontcineyre). Cette étude s’appuie sur une caractérisation des remplissages sédimentaires via unecartographie acoustique par sismique réflexion et des analyses multi-paramètres à haute résolution(radiographies, XRF, spectrophotométrie...) complétées par des datations au radiocarbone sur descarottes sédimentaires.La lecture des profils sismiques permet d’identifier des faciès acoustiques chaotiques assimilésà des dépôts en masse de type slumps et/ou turbidites. En complément, les analyses multiparamètrespermettent de discriminer une sédimentation évènementielle d’une sédimentation de fonddont les flux semblent reliés à l’anthropisation du milieu (Guéry, Montcineyre et Chauvet). Enfin,les modèles d’âges réalisés à partir des datations radiocarbones relient une part de ces évènements àla sismicité historique de la région, comme la récurrence d’évènements sédimentaires au sein des archivesde Guéry et Pavin durant le XIXème siècle, période durant laquelle l’activité sismique au Mont-Doreet au sud de la faille de la Limagne était assez intense. De manière similaire, des évènements synchronesdatés aux alentours de l’an 1300 ont été répertoriés au sein des quatre lacs, soulignant un facteur dedéclenchement tectonique régional jusqu’alors non recensé. Néanmoins, l’enregistrement de cettepériode de sismicité est contrasté en raison d’effet de sites importants et très différents d’un systèmeà l’autre. Les principales raisons invoquées concernent la morphologie subaquatique (degré depentes, architecture sédimentaire), la cohésion du sédiment ainsi que le degré d’anthropisation dumilieu, trois paramètres qui semblent influencer la stabilité du matériel sédimentaire et doncconditionner la génération de glissements en masse durant des épisodes sismiques

Millet cultivation history in the French Alps as evidenced by a sedimentary molecule

International audienceWe report on the detection, in a sediment core drilled in Lake Le Bourget (French Alps), of a fossil molecule (miliacin) that was synthesized by broomcorn millet cultivated in the watershed, and then exported to the sediment. The variation in abundance of this molecule allows us reconstructing the history of millet cultivation around Lake Le Bourget. Our results support the introduction of millet around -1700 BC in the region. After an intensive cultivation during the Late Bronze Age, the failure of millet cropping during the Hallstatt period coincides with a phase of climatic deterioration. Millet cultivation recovers during the Roman and Mediaeval periods before falling most probably due to the introduction of more productive cereals. These pioneering results constitute the first continuous record of an agrarian activity for the last 6000 yrs and emphasize the close relationships between local hydrology, land use and agro-pastoral activities around Lake Le Bourget

Millet cultivation history in the alps during the last 6000 yrs as revealed by a sedimentary biomarker.

International audienceLacustrine sedimentary archives provide clue information on past Human-climate-environment interactions in order to predict the future responses of societies and ecosystems to global climate change. Within a multidisciplinary project aiming at documenting these interactions during the Holocene in the French Alps, we have examined by gas chromatography-mass spectrometry the lipid content of a sedimentary series covering the last 6 ka, drilled in Lake le Bourget (core LDB04, Fig. 1A)

Mass-movement and flood-induced deposits in Lake Ledro, southern Alps, Italy: implications for Holocene palaeohydrology and natural hazards

International audienceHigh-resolution seismic profiles and sediment cores from Lake Ledro combined with soil and riverbed samples from the lake's catchment area are used to assess the recurrence of natural hazards (earthquakes and flood events) in the southern Italian Alps during the Holocene. Two welldeveloped deltas and a flat central basin are identified on seismic profiles in Lake Ledro. Lake sediments have been finely laminated in the basin since 9000 cal. yr BP and frequently interrupted by two types of sedimentary events (SEs): lightcoloured massive layers and dark-coloured graded beds. Optical analysis (quantitative organic petrography) of the organic matter present in soil, riverbed and lacustrine samples together with lake sediment bulk density and grainsize analysis illustrate that light-coloured layers consist of a mixture of lacustrine sediments and mainly contain algal particles similar to the ones observed in background sediments. Light-coloured layers thicker than 1.5 cm in the main basin of Lake Ledro are synchronous to numerous coeval mass-wasting deposits remoulding the slopes of the basin. They are interpreted as subaquatic mass-movements triggered by historical and pre-historical regional earthquakes dated to AD2005, AD1891, AD1045 and 1260, 2545, 2595, 3350, 3815, 4740, 7190, 9185 and 11 495 cal. yr BP. Darkcoloured SEs develop high-amplitude reflections in front of the deltas and in the deep central basin. These beds are mainly made of terrestrial organic matter (soils and lignocellulosic debris) and are interpreted as resulting from intense hyperpycnal flood event. Mapping and quantifying the amount of soil material accumulated in the Holocene hyperpycnal flood deposits of the sequence allow estimating that the equivalent soil thickness eroded over the catchment area reached up to 5mm during the largest Holocene flood events. Such significant soil erosion is interpreted as resulting from the combination of heavy rainfall and snowmelt. The recurrence of flash flood events during the Holocene was, however, not high enough to affect pedogenesis processes and highlight several wet regional periods during the Holocene. The Holocene period is divided into four phases of environmental evolution. Over the first half of the Holocene, a progressive stabilization of the soils present through the catchment of Lake Ledro was associated with a progressive reforestation of the area and only interrupted during the wet 8.2 event when the soil destabilization was particularly important. Lower soil erosion was recorded during the mid-Holocene climatic optimum (8000-4200 cal. yr BP) and associated with higher algal production. Between 4200 and 3100 cal. yr BP, both wetter climate and human activities within the drainage basin drastically increased soil erosion rates. Finally, from 3100 cal. yr BP to the present-day, data suggest increasing and changing human land use

Archives lacustres de l'évolution du climat et des activités humaines récentes dans les Pyrénées ariégeoises au cours de l'Holocène (étang majeur, vallée du Haut-Vicdessos, Pyrénées, France)

International audienceDans le cadre de l'Observatoire Hommes Milieux Haut-Vicdessos, des archives lacustres sont utilisées pour distin-guer les impacts de l'Homme ou du climat dans les Pyrénées (Ariège, 42°N). Associée à la cartographie acoustique de l'Étang Majeur, les analyses des sols et des sédi-ments lacustres mettent en évidence une sédimentation Tardiglaciaire riche en ti-tane, qui contraste avec une sédimentation Holocène de type dy résultant de l'érosion diffuse des tourbes présentes en amont du lac. L'enregistrement indique des périodes plus humides, datées en 1200, 1950, 3400 et 4550 cal BP et associées aux apports d'un canyon drainant les zones d'altitude. Depuis 1907, le niveau d'eau du lac est ré-gulé par deux barrages hydroélectriques. Il en résulte un marnage de 10 m affec-tant jusqu'à 37 % du bassin. Ceci a pour conséquences de remobiliser le matériel issu des berges, et d'augmenter la produc-tivité algaire et les taux d'accumulation

North western Alps Holocene paleohydrology recorded by flooding activity in Lake Le Bourget, France and possible relations with Mont-Blanc glaciers fluctuations

International audienceA 14-m long piston core was retrieved from Lake Le Bourget, NWAlps (France), in order to provide a continuous record of flooding events of the Rhone River during the Holocene. The selection of the coring site was based on high resolution seismic profiling, in an area with limited mass wasting deposits and accumulated proximal Rhone River inter-and underflow deposits. The age-depth model of this core is based on (i) 14 AMS radiocarbon dates, (ii)radionuclide dating(137Cs) and (iii) the identification of historical data (flood events, eutrophication of the lake).The sedimentary record dates back to 9400 cal BP, and includes a thin mass wasting event deposited around 4500 cal BP. A multi-proxy approach was used to track the evolution and origin of clastic sedimentation during the Holocene, in order to identify periods of higher hydrologic al activity in the catchment area. Spectrophotometry was used to detect fluctuations in clastic supply and the study of clay minerals (especially the Illite crystallinity index) allowed locating the main source area of fine grained clastic particles settling at the lake after flood events. This dataset highlights up to 12 periods of more intense flooding events over the last 9400 years in Lake Le Bourget and shows that the main source area of clastic particles during this period is the upper part of the Arve River drainage basin. This part of the catchment area drains several large glaciers from the Mont-Blanc Massif, and fluctuations in Rhone River flood supply in Lake Le Bourget is interpreted as resulting essentially from Mont-Blanc Glacier activity during the Holocene.The comparison of clastic sedimentationin Lake Le Bourget with periods of increasing land use and periods of Alpine glacier and mid-European lake level fluctuations, suggest that the core LDB04 clastic record in Lake Le Bourget is a continuous proxy of the Holocene hydrologic al history of the NW Alps