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

Microfaunal Recording of Recent Environmental Changes in the Herschel Basin, Western Arctic Ocean

Microfaunal assemblages of benthic foraminifera, ostracods, and tintinnids from two marine sediment cores retrieved from the Herschel Basin of the Canadian Beaufort Sea shelf document relationships with environmental parameters such as salinity, sea-ice cover, and turbulence. Cores YC18-HB-GC01 and PG2303-1 were collected at 18 and 32 m water depth, respectively. At these sites, sediment accumulation rates range between 0.6 and 1.7 cm yr–1 allowing a near-annual temporal resolution over the last 50 years. Multivariate analyses indicate that benthic foraminiferal assemblages respond primarily to food supply. Dissimilarities between the microfaunal assemblages of the two cores are mainly the result of bottom water salinity levels linked to water depth. High abundance of the benthic foraminiferal species Elphidium clavatum and occurrences of Elphidium bartletti point to varying, but relatively low, salinities at the shallow core site YC18-HB-GC01, which may be affected by variations in the summer halocline depth. Higher species diversity and more abundant Cassidulina reniforme and Stainforthia feylingi characterize the deeper core PG2303-1, which might reflect more stable conditions and higher bottom-water salinities throughout the studied time interval. The most important microfaunal shift of the last 50 years, observed in the shallower longer core YC18-HB-GC01, occurred at the turn of the 21st century. Prior to ∼2000 CE, the presence of Islandiella norcrossi indicates more stable and saline conditions. Since ∼2000 CE, increased abundances of Haynesina nivea and of the ciliate Tintinnopsis fimbriata suggest decreased salinity and increased turbidity. An increased abundance of Eoeponidella pulchella after ∼2000 CE suggests a concurrent increase in productivity in the last two decades. This shift is nearly synchronous with a decrease in mean summer sea-ice concentration, which can play an important role in bottom water stability on the shelf. Easterly winds can induce a reduction in the sea-ice cover, but also foster a westward spreading of the Mackenzie River plume and the upwelling of nutrient-rich Pacific waters onto the shelf. Both factors would explain the increased freshening and productivity of the Herschel Basin. The last two decades were also marked by a decrease in ostracod abundance that may relate to higher water turbidity. This study shows that combining information from benthic foraminifera, ostracods, and tintinnids provides a comprehensive insight into recent hydrographic/climatic changes in nearshore Arctic habitats, where productivity is critical for the food security of local communities

An up-to-date Holocene catalog of sedimentary events recorded in volcanic lakes from the French Massif Central

International audienceA decade of scientific investigation, combining subsurface geophysical mapping techniques and coring operations, highlighted several sedimentary events in volcanic lakes of the French Massif Central1,2,3. Here, we present a synthesis of recent studies dedicated to lakes located around the Puy-de-Sancy stratovolcano (i.e., Pavin, Chauvet, Aydat, Montcineyre, Guéry, and Crégut) with new data collected in Lake Lacassou, nearby Lake Chambon, and in the northernmost maar lake Gour de Tazenat. A multi-proxy characterization allowed the identification of mass-movement deposits and coarse-bed turbidites related to subaqueous slope failures. The radiocarbon-based chronological framework allows the creation of a catalog of sedimentary events over the late Holocene that give insights about past geohazards in a low-medium seismogenic area. The dating of coeval deposits between sites was attributed to historical earthquakes documented during the VIth, XIIIth and XIXth centuries. Among them, an undocumented earthquake has been identified as the regional trigger for synchronous deposits in AD 1275 in four lakes around the Puy-de-Sancy stratovolcano1. New chronological constrains from Lake Lacassou suggest this event could have also triggered the `Dent du Marais` collapse and the subsequent formation of Lake Chambon. Regular occurrences of single sedimentary events suggest either a growing human pressure and/or unsettled climatic conditions have also been involved in the generation of extreme events such as for the 5500 cal BP slope failure in Lake Cregut, the AD 180 delta collapse in Lake Aydat, and the AD 600 outlet break in Lake Pavin. The absence of deposit related to the major earthquake dated AD 1490 (Mw > 6) also suggest studied lakes are too distant and/or sediment loadings were too low since the last slope failures two centuries prior. Finally, we reported unusual deposits across the subaqueous plateau (oxic waters) in Lake Pavin during late XVIIIth and early XXth centuries. Their iron-rich signature suggests a formation under an iron-rich mixolimnion consecutive of a lake rollover carrying dissolved iron species from anoxic deep waters to oxic waters2. Consequently, these layers constitute first evidences of recent roll-over phenomena in Lake Pavin and may support further work about similar hazards in volcanic settings

Anthropogenic versus natural control on lacustrine sediment yield records from the French Massif Central

International audienceA quantitative assessment of historical sediment yields (SY) was performed using sediment budgets from lacustrine records located in the Mont Dore and Cezallier volcanic provinces (French Massif Central). A source-to-sink approach combining hydro-acoustic images, organic geochemistry (Rock-Eval and quantitative organic petrography) and radiocarbon dating of sediment cores has been adopted on three lake-catchment systems, namely Pavin, Chauvet and Montcineyre. SY was estimated from the quantification of red Amorphous Particles (rAP), a terrigenous organic tracer identified in both soils and sediments. Historical SY range between 3 and 320 t km−2.yr−1, which is comparable to the magnitude reached in other European lake-based SY records in similar geographical and climatic settings. Comparison of recent SY with predicted values of soil erosion rates from the RUSLE2015 model highlights large differences linked to scale differences between the model at plot scale and lake-based SY reflecting erosion export from the catchment to the lake. In this sense, the role of peatlands as sediment traps within two studied catchments must be considered to explain the large differences between modelled soil erosion rates and reconstructed SY data. SY differences between sites can be firstly attributed to morphology, size and lithology of the catchments as well as to vegetation cover whereas fluctuations reconstructed for each record seem to be mainly related to human-induced land use management. Historical SY from Chauvet and Montcineyre synchronously recorded two events in 850 CE and 1450 CE, respectively. The first marked the rise of SY to their maxima following land-use changes in the catchments. Nearby palaeoenvironmental records from Lake Aydat, Chambedaze peatland, and Espinasse marsh suggest this rise was consecutive to intensification of agro-pastoral activities recorded at regional scale. The second event followed a land-use shift characterized by a ten to fifteen-fold decrease in SY values. The driver remains unclear but could be possibly related to historical events causing a demographic decline (i.e., the Black Plague and/or the Hundred Years War) and/or cultural adaption in response to the onset of the Little Ice Age. Overall, both records suggest erosion in the area has been historically more susceptible to human-induced land use change rather than to precipitation and temperature changes induced by climate variability of the past millennium.Keyword

Lake Pavin Sedimentary Environments

International audienceIn the recent years, Lake Pavin sedimentary basin has been intensively studied by several acoustic surveys (high resolution seismic refl ection profi ling, multibeam bathymetry) and gravity coring campaigns. This new data set combining acoustic images and multidisciplinary study of sediment cores allows characterizing contrasted subaquatic sedimentary environments along the littoral slopes, a subaquatic plateau (close to the lake outlet), steep slopes and its deep central basin. Two main types of lacustrine sediments are identifi ed (i) between the lake shore and 26 m water depth (massive organic rich sandy silts), and (ii) below 26 m water depth when the lake fl oor slopes are less than 15° steep (organic rich laminated diatomites). A large and recent slide scar is in particular clearly identifi ed at the edge of the plateau just above the deep central basin. Evidences of former gravity reworking phenomena within the crater ring draining into Lake Pavin also include a large subaquatic slump deposit accumulated on the subaquatic plateau and several small scale rock fall deposits originating from outcropping lavas within the crater ring. The identifi cation of two recent outstanding erosive sandy layers in Pavin littoral environment also suggests that some of this gravity reworking phenomena have been associated with unusually violent waves and/or abrupt lake level drop. Lake Pavin geomorphology and sedimentary environments are in addition compared to the ones of the nearby Lake Chauvet based on a similar acoustic and sedimentary data base in order to highlight the infl uence of maar age and geomorphology on the development of sedimentary environments and Natural Hazards in this volcanic region of the French Massif Central. Lake Chauvet is comparatively to Lake Pavin characterized by a shallower central basin, less steep slopes and no subaquatic plateaus. A recent and relatively large mass wasting deposit is, however, clearly identifi ed along the slopes of a small delta facing the only tributary of this maar lake. This work 366 suggest that maar lakes from the study area are concerned by subaquatic slope stabilities, especially in Lake Pavin were slope failures may in addition impact the development or the stability of its meromicticity

Holocene Vegetation Dynamics and First Land-Cover Estimates in the Auvergne Mountains (Massif Central, France): Key Tools to Landscape Management

International audienceA multi-proxy palaeoecological investigation has been undertaken at high spatio-temporal resolution in the Lower Auvergne Mountains (France). It allows us to investigate the Holocene trajectories of landscape evolution arising from the interplay between human impact and adaptability, climate oscillations and environmental evolution. The mechanistic models for the regional vegetation reconstruction applied here provide the first quantification of land cover changes in this region. The results obtained allow an improved understanding of past vegetation dynamics and a discussion of: (1) the natural variability of the vegetation to climate oscillations; (2) the development of the cultural landscape and the land uses involved; (3) the timing and the extent of the landscape openness; and (4) the richness in vegetation units within the landscape mosaic measured by the floristic diversity. These long-term changes highlight the sensitivity of these mountainous landscapes: having formed socio-ecosystems that have been shaped over millennia. It is therefore crucial to consider this ecological and cultural heritage when directing future sustainable management plans

Establishing the geometry and nature of sediments trapped in either natural or artificial dam lakes in contrasted drainage basins from Western Europe (French Massif Central and Pyrenees)

International audienceLacustrine sedimentary archives from artificial dam lakes are poorly documented both in terms of basin fill geometries and dominating sedimentary processes. In order to better understand their sensitivities to regional environmental changes, we performed a similar multidisciplinary study of French natural and artificial dam lakes in contrasted drainage basins from the volcanic Massif Central (lakes Aydat and Crégut) and two granitic sectors of the northern Pyrenees (lakes Fourcat and Orédon). Our approach combined high-resolution sub bottom profiling (14 kHz and 4 kHz chirp) and a detailed study of sediment cores based on qualitative and quantitative analysis (radiographies, sediment physical and chemical properties) together with radionuclide and radiocarbon dates. In all cases either changes in land uses within the drainage basin or the flooding of natural lakes by dams and the production of hydroelectricity induced changes in sedimentation rates and modes. Human activities affecting either the catchment or the lake itself favored enhanced clastic sediment supply in the lake basins and/or higher and fluctuating lake levels. Subaquatic slopes failures are also identified in Lake Aydat formed by a lava flow 8.5 kYrs ago and in glacial lakes Crégut (Massif Central) and Orédon (Pyrenees) now used to produce hydroelectricity and suggest that lake level changes and ground accelerations during earthquakes can remobilize distinct sectors of the basin fills and not only deltaic environments

Establishing the geometry and nature of sediments trapped in either natural or artificial dam lakes in contrasted drainage basins from Western Europe (French Massif Central and Pyrenees)

International audienceLacustrine sedimentary archives from artificial dam lakes are poorly documented both in terms of basin fill geometries and dominating sedimentary processes. In order to better understand their sensitivities to regional environmental changes, we performed a similar multidisciplinary study of French natural and artificial dam lakes in contrasted drainage basins from the volcanic Massif Central (lakes Aydat and Crégut) and two granitic sectors of the northern Pyrenees (lakes Fourcat and Orédon). Our approach combined high-resolution sub bottom profiling (14 kHz and 4 kHz chirp) and a detailed study of sediment cores based on qualitative and quantitative analysis (radiographies, sediment physical and chemical properties) together with radionuclide and radiocarbon dates. In all cases either changes in land uses within the drainage basin or the flooding of natural lakes by dams and the production of hydroelectricity induced changes in sedimentation rates and modes. Human activities affecting either the catchment or the lake itself favored enhanced clastic sediment supply in the lake basins and/or higher and fluctuating lake levels. Subaquatic slopes failures are also identified in Lake Aydat formed by a lava flow 8.5 kYrs ago and in glacial lakes Crégut (Massif Central) and Orédon (Pyrenees) now used to produce hydroelectricity and suggest that lake level changes and ground accelerations during earthquakes can remobilize distinct sectors of the basin fills and not only deltaic environments

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

https://hal-insu.archives-ouvertes.fr/insu-01290806International audienceL’occurrence de séismes en domaine continental peut être enregistrée au sein des bassins lacustres 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 des sismomètres naturels et leur compréhension s’avère essentielle pour recenser des séismes historiques afin d’évaluer le risque présent et futur. Aujourd’hui, le zonage sismique de la France continentale montre une sismicité modérée voire forte au sein des principales chaînes de montagne. Parmi elles, la région volcanique des Monts Dore 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 cet objectif, 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 et Montcineyre). Cette étude s’appuie sur une caractérisation des remplissages sédimentaires via une cartographie acoustique par sismique réflexion et des analyses multi-paramètres à haute résolution (radiographies, XRF, spectrophotométrie...) complétées par des datations au radiocarbone sur des carottes sédimentaires. La lecture des profils sismiques permet d’identifier des faciès acoustiques chaotiques assimilés à des dépôts en masse de type slumps et/ou turbidites. En complément, les analyses multiparamètres permettent de discriminer une sédimentation évènementielle d’une sédimentation de fond dont 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 à la sismicité historique de la région, comme la récurrence d’évènements sédimentaires au sein des archives de Guéry et Pavin durant le XIXème siècle, période durant laquelle l’activité sismique au Mont-Dore et au sud de la faille de la Limagne était assez intense. De manière similaire, des évènements synchrones datés aux alentours de l’an 1300 ont été répertoriés au sein des quatre lacs, soulignant un facteur de déclenchement tectonique régional jusqu’alors non recensé. Néanmoins, l’enregistrement de cette période de sismicité est contrasté en raison d’effet de sites importants et très différents d’un système à l’autre. Les principales raisons invoquées concernent la morphologie subaquatique (degré de pentes, architecture sédimentaire), la cohésion du sédiment ainsi que le degré d’anthropisation du milieu, trois paramètres qui semblent influencer la stabilité du matériel sédimentaire et donc conditionner la génération de glissements en masse durant des épisodes sismiques