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

Secular and orbital-scale variability of equatorial Indian Ocean summer monsoon winds during the late Miocene

In the modern northern Indian Ocean, biological productivity is intimately linked to near-surface oceanographic dynamics forced by the South Asian, or Indian, monsoon. In the late Pleistocene, this strong seasonal signal is transferred to the sedimentary record in the form of strong variance in the precession band (19–23 kyr), because precession dominates low-latitude insolation variations and drives seasonal contrast in oceanographic conditions. In addition, internal climate system feedbacks (e.g. ice-sheet albedo, carbon cycle, topography) play a key role in monsoon variability. Little is known about orbital-scale monsoon variability in the pre-Pleistocene, when atmospheric CO2 levels and global temperatures were higher. In addition, many questions remain open regarding the timing of the initiation and intensification of the South Asian monsoon during the Miocene, an interval of significant global climate change that culminated in bipolar glaciation. Here, we present new high-resolution (<1 kyr) records of export productivity and sediment accumulation from International Ocean Discovery Program Site U1443 in the southernmost part of the Bay of Bengal spanning the late Miocene (9 to 5 million years ago). Underpinned by a new orbitally tuned benthic isotope stratigraphy, we use X-ray fluorescence-derived biogenic barium variations to discern productivity trends and rhythms. Results show strong eccentricity-modulated precession-band productivity variations throughout the late Miocene, interpreted to reflect insolation forcing of summer monsoon wind strength in the equatorial Indian Ocean. On long timescales, our data support the interpretation that South Asian monsoon winds were already established by 9 Ma in the equatorial sector of the Indian Ocean, with no apparent intensification over the latest Miocene

Recent climatic and anthropogenic imprints on lacustrine systems in the Pyrenean Mountains inferred from minerogenic and organic clastic supply (Vicdessos valley, Pyrenees, France)

High-resolution seismic profiling has been combined with geochemical analyses of both watershed samples and five lacustrine cores retrieved from two natural lacustrine basins of glacial origin: Lake Majeur and Lake Sigriou (1630 m a.s.l. and 1995 m a.s.l., respectively, Eastern French Pyrenees). Identifying specific minerogenic and organic markers of autochthonous and allochthonous supply, data allow documenting past climatic and anthropogenic pressures. Over the past century, the lacustrine sediment of Lake Majeur has been essentially composed of algae, drastically contrasting with the natural sedimentary infill of the basin, mainly resulting from soil erosion from the mid–late Holocene. Since ad 1907, the Lake Majeur has been used for hydroelectricity production. Human-induced lake-level regulations, affecting up to 37% of the lacustrine surface, have increased by fourfold the accumulation rate of the lake and favoured water enrichment. Rubidium abundance within the lacustrine sediments of the two lakes reflects the mid–late Holocene palaeohydrology. After dam construction in ad 1907, greater quantities of rubidium found in Lake Majeur sedimentary infills indicate drier climatic periods, such as from ad 1975 to ad 1982, during which water reservoirs were particularly in demand. Inversely, before the dam was built, rubidium fluctuations were correlated with wetter conditions and hydrological events were recorded as sandy layers deposited by canyon reactivation, synchronous with European climatic deterioration phases. We notably document that the Mediaeval Climate Anomaly was interrupted by some humid periods dated c. ad 940, ad 1080, ad 1100 and ad 1250. We also date the onset of the ‘Little Ice Age’ c. ad 1360 and identify that this period was wetter after c. ad 1500

The large-scale evolution of neodymium isotopic composition in the global modern and Holocene ocean revealed from seawater and archive data

Neodymium isotopic compositions (143Nd/144Nd or εNd) have been used as a tracer of water masses and lithogenic inputs to the ocean. To further evaluate the faithfulness of this tracer, we have updated a global seawater εNd database and combined it with hydrography parameters (temperature, salinity, nutrients and oxygen concentrations), carbon isotopic ratio and radiocarbon of dissolved inorganic carbon. Archive εNd data are also compiled for leachates, foraminiferal tests, deep-sea corals and fish teeth/debris from the Holocene period (< 10,000 years). At water depths ≥ 1500 m, property-property plots show clear correlations between seawater εNd and the other variables, suggesting that large-scale water mass mixing is a primary control of deepwater εNd distribution. At ≥ 200 m, basin-scale seawater T-S-εNd diagrams demonstrate the isotopic evolution of different water masses. Seawater and archive εNd values are compared using property-property plots and T-S-εNd diagrams. Archive values generally agree with corresponding seawater values although they tend to be at the upper limit in the Pacific. Both positive and negative offsets exist in the northern North Atlantic. Applying multiple regression analysis to deep (≥ 1500 m) seawater data, we established empirical equations that predict the main, large-scale, deepwater εNd trends from hydrography parameters. Large offsets from the predicted values are interpreted as a sign of significant local/regional influence. Dominant continental influence on seawater and archive εNd is observed mainly within 1000 km from the continents. Generally, seawater and archive εNd values form gradual latitudinal trend in the Atlantic and Pacific at depths ≥ 600 m, consistent with the idea that Nd isotopes help distinguish between northern/southern sourced water contributions at intermediate and deep water depths

Neodymium isotopic composition as a proxy of water mass provenance in the Atlantic Ocean: the modern ocean and the past 1200 kyr

International audienceNeodymium isotopic composition (143 Nd/ 144 Nd or e Nd) is one of the most widely used tracers to reconstruct ocean circulation and detrital inputs to the ocean. In this presentation, I will address the following topics by taking the Atlantic Ocean as an example: the distinction between conservative water mixing and local/regional Nd effects on seawater e Nd ; an assessment of sites where local/regional Nd inputs dominate; the combination of e Nd with foraminiferal d 13 C to investigate changes in oceanic circulation and the carbon cycle across the Mid-Pleistocene transition (MPT). Based on the present-day seawater database, the relationships between e Nd and other water mass tracers (d 13 C-DIC, nutrients, dissolved oxygen, 14 C, temperature and salinity) were determined for the Atlantic deep waters (≥ 1500m). Using multiple regression analysis, empirical equations were established to predict the large-scale deep-water e Nd trends from hydrographic parameters. The spatial distribution of significant local influence can be estimated as large offsets from the predicted values. The results indicate that deep waters in the northern subpolar Atlantic, close to Greenland and Iceland, are highly affected by local inputs. The authigenic e Nd values (foraminiferal oxides, fish teeth/debris, dispersive oxides) more frequently present an isotopic composition that deviates from expected values, often with a positive bias. This suggests a contribution of exchangeable radiogenic Nd from the detrital fraction to pore water and to authigenic fractions. To examine whether sites are suitable for using e Nd to evaluate water mass provenance, systematic comparison of core-top values with present-day seawater is recommended. Finally, two eastern Atlantic e Nd records were generated and combined with benthic foraminiferal d 13 C values to evaluate the role of deep-water circulation across the MPT. By comparing with existing data, the 900-ka event is suggested to be related to a reduced contribution of northern source water with slower circulation that allowed accumulation of respired carbon in deep waters. Thus, ocean circulation and carbon cycle changes seem to have contributed to the climate shift associated with the MPT

Neodymium isotopic composition as a proxy of water mass provenance in the Atlantic Ocean: the modern ocean and the past 1200 kyr

International audienceNeodymium isotopic composition (143 Nd/ 144 Nd or e Nd) is one of the most widely used tracers to reconstruct ocean circulation and detrital inputs to the ocean. In this presentation, I will address the following topics by taking the Atlantic Ocean as an example: the distinction between conservative water mixing and local/regional Nd effects on seawater e Nd ; an assessment of sites where local/regional Nd inputs dominate; the combination of e Nd with foraminiferal d 13 C to investigate changes in oceanic circulation and the carbon cycle across the Mid-Pleistocene transition (MPT). Based on the present-day seawater database, the relationships between e Nd and other water mass tracers (d 13 C-DIC, nutrients, dissolved oxygen, 14 C, temperature and salinity) were determined for the Atlantic deep waters (≥ 1500m). Using multiple regression analysis, empirical equations were established to predict the large-scale deep-water e Nd trends from hydrographic parameters. The spatial distribution of significant local influence can be estimated as large offsets from the predicted values. The results indicate that deep waters in the northern subpolar Atlantic, close to Greenland and Iceland, are highly affected by local inputs. The authigenic e Nd values (foraminiferal oxides, fish teeth/debris, dispersive oxides) more frequently present an isotopic composition that deviates from expected values, often with a positive bias. This suggests a contribution of exchangeable radiogenic Nd from the detrital fraction to pore water and to authigenic fractions. To examine whether sites are suitable for using e Nd to evaluate water mass provenance, systematic comparison of core-top values with present-day seawater is recommended. Finally, two eastern Atlantic e Nd records were generated and combined with benthic foraminiferal d 13 C values to evaluate the role of deep-water circulation across the MPT. By comparing with existing data, the 900-ka event is suggested to be related to a reduced contribution of northern source water with slower circulation that allowed accumulation of respired carbon in deep waters. Thus, ocean circulation and carbon cycle changes seem to have contributed to the climate shift associated with the MPT