Charlet Francois

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  • Preprint
    Reconstruction of the Holocene seismotectonic activity of the Southern Andes from seismites recorded in Lago Icalma, Chile, 39°S
    ( 2006-09-14) Bertrand, Sebastien ; Charlet, Francois ; Chapron, Emmanuel ; Fagel, Nathalie ; De Batist, Marc
    South-central Chile is one of the most geodynamically active areas in the world, characterised by frequent volcanic eruptions and numerous earthquakes, which are both recorded in lake sediments. In Lago Icalma (39°S), long piston and short gravity coring, as well as 3.5 kHz high-resolution seismic profiling, has been carried out in order to study the Holocene sedimentary infill of the lake, with a special focus on earthquake-triggered deposits. Macroscopic description of sediment cores and detailed grain-size analyses allow us to identify four types of seismically-induced deposits, or “seismites”: slump deposits, chaotic deposits, turbidites s.s. and homogenites. Homogenites are characterized by the occurrence of three distinct units on grain-size profiles (coarse base, thick homogenous unit topped by a thin layer of very fine sediment) and by the typical distribution of the grain-size parameters in a skewness-sorting diagram, while turbidites s.s. are characterized by a continuous fining upward trend. Radiocarbon, 210Pb dating, and tephrochronology allow us to demonstrate that the regional seismotectonic activity was probably very high between 2200 and 3000 cal. yr. BP as well as between 7000 and 8000 cal. yr. BP and that none of the historically documented earthquakes have triggered any seismite in Lago Icalma. The most recent seismite recognized in the sediments of Lago Icalma is a slump deposit dated at 1100 ± 100 AD, i.e. older than the period covered by historical records. The remarkable record of seismites between 2200 and 3000 cal. yr. BP is probably influenced by a major eruption of Sollipulli volcano at 3000 cal. yr. BP, which has rejuvenated the stock of terrigenous particles available for erosion, by depositing a thick layer of pumices all over the watershed of Lago Icalma and by clearing the vegetation covering the volcanic ash soils. This paper demonstrates that the record of seismically-triggered deposits in lake sediments is not only controlled by the intensity of the triggering earthquake and the occurrence of unstable sediment along the lake slopes but also by the presence of particles available for erosion/remobilisation in the watershed.
  • Preprint
    Climate variability of southern Chile since the Last Glacial Maximum : a continuous sedimentological record from Lago Puyehue (40°S)
    ( 2007-04-29) Bertrand, Sebastien ; Charlet, Francois ; Charlier, Bernard ; Renson, Virginie ; Fagel, Nathalie
    This paper presents a multi-proxy climate record of an 11 m long core collected in Lago Puyehue (southern Chile, 40°S) and extending back to 18,000 cal yr BP. The multi-proxy analyses include sedimentology, mineralogy, grain size, geochemistry, loss-on-ignition, magnetic susceptibility and radiocarbon datings. Results demonstrate that sediment grain size is positively correlated with the biogenic sediment content and can be used as a proxy for lake paleoproductivity. On the other hand, the magnetic susceptibility signal is correlated with the aluminium and titanium concentrations and can be used as a proxy for the terrigenous supply. Temporal variations of sediment composition evidence that, since the last glacial maximum, the Chilean Lake District was characterized by 3 abrupt climate changes superimposed on a long-term climate evolution. These rapid climate changes are: (1) an abrupt warming at the end of the last glacial maximum at 17,300 cal yr BP; (2) a 13,100-12,300 cal yr BP cold event, ending rapidly and interpreted as the local counter part of the Younger Dryas cold period, and (3) a 3400-2900 cal yr BP climatic instability synchronous with a period of low solar activity. The timing of the 13,100-12,300 cold event is compared with similar records in both hemispheres and demonstrates that this southern hemisphere climate change lags behind the northern hemisphere Younger Dryas cold period by 500 to 1000 years.