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4 Research products, page 1 of 1

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  • Open Access English
    Authors: 
    Waelbroeck, Claire; Pichat, Sylvain; Böhm, Evelyn; Missiaen, Lise;
    Publisher: PANGAEA - Data Publisher for Earth & Environmental Science
    Project: EC | ACCLIMATE (339108), ANR | RETRO (ANR-09-BLAN-0347)

    Thanks to its optimal location on the northern Brazilian margin, core MD09-3257 records both ocean circulation and atmospheric changes. The latter occur locally in the form of increased rainfall on the adjacent continent during the cold intervals recorded in Greenland ice and northern North Atlantic sediment cores (i.e. Greenland stadials). These rainfall events are recorded in MD09-3257 as peaks in ln(Ti/Ca). New sedimentary Pa/Th data indicate that mid-depth western equatorial water mass transport decreased during all of the Greenland stadials of the last 40 kyr. Using cross-wavelet transforms and spectrogram analysis, we assess the relative phase between the MD09-3257 sedimentary Pa/Th and ln(Ti/Ca) signals. We show that decreased water mass transport between a dept of ~1300 and 2300 m in the western equatorial Atlantic preceded increased rainfall over the adjacent continent by 120 to 400 y at Dansgaard-Oeschger (D-O) frequencies, and by 280 to 980 y at Heinrich-like frequencies. We suggest that the large lead of ocean circulation changes with respect to changes in tropical South American precipitation at Heinrich-like frequencies is related to the effect of a positive feedback involving iceberg discharges in the North Atlantic. In contrast, the absence of widespread ice rafted detrital layers in North Atlantic cores during D-O stadials supports the hypothesis that a feedback such as this was not triggered in the case of D-O stadials, with circulation slowdowns and subsequent changes remaining more limited during D-O stadials than Heinrich stadials.

  • Other research product . Lecture . 2016
    French
    Authors: 
    Idmhand, Fatiha;
    Publisher: HAL CCSD
    Country: France
    Project: ANR | CHispa (ANR-13-JSH3-0006)

    Master; Intervention réalisée à Lorient, le 22-09-2016, pour le Master mention Métiers du Livre et Humanités Numériques

  • Open Access English
    Authors: 
    Fouinat, Laurent; Sabatier, Pierre; David, Fernand; Montet, Xavier; Schoeneich, Philippe; Chaumillon, Eric; Poulenard, Jérôme; Arnaud, Fabien;
    Publisher: PANGAEA - Data Publisher for Earth & Environmental Science
    Project: ANR | CLIMCOR (ANR-11-EQPX-0009)

    Understanding wet avalanche intensity and the role of past environmental changes on wet avalanche occurrence is a main concern especially in the context of a warming climate and accelerated environmental mutations. Avalanches are closely related to fast cryosphere changes and may cause major threats to human society. Here, we used the sedimentary archive of the alpine Lake Lauvitel (western French Alps) to establish the first long-term avalanche record in this Alpine region. For this purpose, we used a novel CT scan methodology that allows the precise identification of coarse material - from sand to pebble - transported to the lake and embedded within the finer continuous sedimentation. We identified a total of 166 deposits over the last 3300 yrs cal. BP. In parallel, a detailed pollen analysis gave an independent record of environmental changes. Based on modern observation, lake monitoring, seismic investigations and sedimentological evidences, coarse material deposits were attributed to wet avalanche events. Our results highlight the effect of vegetation cover on the avalanche hazard while a period of strong frequency increase occurred after 780 yrs cal. BP. In Lake Lauvitel, this period corresponds to a major forest clearance induced by the rise of human land-use. Climate forcing on the avalanche hazard was investigated before and after the vegetation shift. On a multi-centennial scale, wet avalanches preferably occur during periods of larger glacier extent, in which higher winter precipitations probably generate a sufficiently thick snow cover. On a sub-centennial scale, avalanches are more frequent during periods of relative warming, resulting in a destabilisation of the same snow cover in spring season. Our results highlight as well the role of forest cover to mitigate wet snow avalanches occurrence. In the context of predicted warmer temperatures, this study raises the question of whether a wet avalanche hazard increase may be expected in the near future especially at higher altitudes.

  • Open Access English
    Authors: 
    Burckel, Pierre; Waelbroeck, Claire; Luo, Yiming; Roche, Didier M; Pichat, Sylvain; Jaccard, Samuel L; Gherardi, Jeanne-Marie; Govin, Aline; Lippold, Jörg; Thil, François;
    Publisher: PANGAEA - Data Publisher for Earth & Environmental Science
    Project: SNSF | Quantifying changes in th... (111588), EC | ACCLIMATE (339108), SNSF | SeaO2 - Past changes in S... (144811), ANR | RETRO (ANR-09-BLAN-0347)

    We reconstruct the geometry and strength of the Atlantic Meridional Overturning Circulation during Heinrich Stadial 2 and three Greenland interstadials of the 20-50 ka period based on the comparison of new and published sedimentary 231Pa/230Th data with simulated sedimentary 231Pa/230Th. We show that the deep Atlantic circulation during these interstadials was very different from that of the Holocene. Northern-sourced waters likely circulated above 2500 m depth, with a flow rate lower than that of the present day North Atlantic Deep Water (NADW). Southern-sourced deep waters most probably flowed northwards below 4000 m depth into the North Atlantic basin, and then southwards as a return flow between 2500 and 4000 m depth. The flow rate of this southern-sourced deep water was likely larger than that of the modern Antarctic Bottom Water (AABW). Our results further show that during Heinrich Stadial 2, the deep Atlantic was probably directly affected by a southern-sourced water mass below 2500 m depth, while a slow southward flowing water mass originating from the North Atlantic likely influenced depths between 1500 and 2500 m down to the equator.