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PhD Defense

Laurine Drugat (LSCE)

Title : Geochemistry of speleothems from the southwestern Europe (U-Th and 14C chronologies, trace elements, 87Sr/86Sr, d18O, d13C), application to climatic and environmental reconstructions during the Holocene.

Date and time : The 17-12-2018 at 14h00

Type : thèse

Université qui délivre le diplôme : UVSQ

Location : Salle 17C du bâtiment 701 du LSCE (Orme des Merisiers, Gif-sur-Yvette)
Members of jury :

Sophie VERHEYDEN, CR, Institut Royal des Sciences Naturelles de Belgique, Rapporteur

Giovanni ZANCHETTA, DR, Université de Pise-Italie, Rapporteur

Jean-Luc MICHELOT, DR, Laboratoire GEOPS, Université Paris-Sud, Examinateur

Russell DRYSDALE, DR, Université de Melbourne, Australie, Examinateur

Paolo MONTAGNA,  CR, Institut des sciences marines (ISMAR), U.O.S, CNR- Bologne, Italie, Examinateur

Edwige PONS-BRANCHU, MC, LSCE, CNRS/CEA, IPSL, UVSQ, Paris-Saclay, Directrice de thèse

Hélène VALLADAS, DR, LSCE, CNRS/CEA, IPSL, UVSQ, Paris-Saclay, Co-Directrice de thèse

Eric DOUVILLE, CR, LSCE, CNRS/CEA, IPSL, UVSQ, Paris-Saclay, Invité

Summary :

The purpose of this thesis is to use speleothem geochemistry through a multi-proxy study to reconstruct environmental and climatic variations in southwestern Europe during the Holocene. This work, based on three sites distributed between Spain and France, covers the last 14,000 years. Due to its location at the confluence of the high and low latitudes, this region of south-western Europe is especially sensitive to both the influence of the North Atlantic climate but also the Mediterranean and subtropical African and Asian climate. This particular context allows to study ideally the impact of the different climatic mechanisms driving the modifications of its environment. The choice of the study sites’ distribution aims to compare sites with main Atlantic influence (Candamo, N Spain), Mediterranean (Salamandre cave, SE France) and mixt one (Nerjà, S Spain). The understanding of geochemical behavior was the first part of this study. After the establishment of a chronological frame (14C, U/Th), elemental geochemistry (trace elements and rare earth elements) and isotopic (87Sr/86Sr, d18O, d13C) was coupled. The 87Sr/86Sr isotope ratio, traditionally used as a source tracer for Sr, has been coupled to the study of elementary variations of alkali elements (Li, Rb, Cs) in order to determine changes in the contribution of bedrock and soil in speleothems. The behavior of these alkalis has also been shown to be related to different modes of transport (particulate dissolved or bound to organic complexes) and the use of d13C and rare earths (light rare earth and heavy rare earth fractionation and cerium anomaly) made it possible to confirm or not these hypotheses. The second part of this work was to apply these geochemical mechanisms to the determination of the hydrological and environmental processes and to place them in the specific climatic context of each study site. Thus, the alkali elements (Li, Rb, Cs) have proved to be good indicators of weathering phenomena that may be related to climate change (such as the transition between Bölling-Alleröd and Younger-Dryas in the Salamandre site), and/or environmental changes (period of deforestation or establishment of vegetation in the Middle Holocene in the case of the sites of Nerjà or Candamo). These events can thus take place in very different contexts (dry climate, deforestation, detrital inputs during wet climate by leaching) the coupling to other proxis is then necessary to specify the conditions of enrichment of these alkali elements.

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