• Acronyme :
  • Référence :
  • Domaine de recherche :
    Sciences de la Terre et de l'Univers
  • Région :
  • Site :
    Laboratoire d'Océanologie et de Géosciences
  • Responsable du projet :
    Armynot du Châtelet Eric

Investigation of present and past climate change thanks to a multiproxy approach (testate amoebae, clay mineralogy and organic matter)

Due to their specific location, under the influence of Southern Hemisphere Westerly winds and within the main core of the Antarctic Circumpolar Current, the sub-Antarctic Kerguelen Islands are perfect natural experimentation site for studying both atmospheric and oceanic coupling under changing climate. Recent studies highlight that the observed present-day glacier wastage is mainly controlled by reduced precipitations. In that frame, temperature and precipitation reconstructions are necessary for improving model simulations: we propose a coupled biological-mineralogical-organic geochemical approach in order to better constrain the past and ongoing climatic change on the Kerguelen Islands. Biological approach – The Distribution of testate amoebae (or thecamoebians) have been studied quite extensively in Antarctica and sub-Antarctica islands. These studies reveal the presence of cosmopolitan species with a few of them with more restricted distribution. While diversity decreases with increasing latitude, the proportions of the different species depend on the hygrometric degree, pH, temperature and biogeographic factors. Connecting the testate amoebae diversity with the nature of the substrate on which they develop (physical and biogeochemical context: redox conditions, chemical characteristics of sediment and organic matter), themselves related to temperatures and moisture conditions, will enable us to propose a conceptual model for biotope in the sub-Antarctic zone under the constraints of climate change. Mineralogical approach- Because clay minerals derive from parent-rock through erosion and weathering processes, they are commonly used as tracers of hydrolysis intensity, and their crystallographic parameters provides valuable information on influence of authigenic processes. The Kerguelen Islands are composed of basaltic rocks and volcanic formations, which mainly altered into various type of clay among which mica, celadonite and smectites with specific cristallography and chemical signatures depending on the respective effects of hydrolysis, pedogenesis as well as on the presence of organic compounds. Investigating the mineralogical, crystallographic and geochemical signatures of clay minerals from various substrates as well as from short cores will allow characterizing hydrolysis patterns and their past variations, under climatic control. Organic geochemistry approach – Lipid biomarkers are widely used to reconstruct palaeoenvironment and palaeoclimate in the geological history. These organic compounds permit to link the fossil organic matter to a specific metabolism and in many cases to specific organisms. Biomarkers are built upon the biochemical principle that the organisms adjust their metabolic pathways upon environmental constrain, food and water supply. In addition, biomarkers are subjected to a different set of diagenetic alteration than fossil shells, cysts, or even plant tissues, registering changes in redox conditions and thermal evolution. Lipid biomarkers may be preserved in environments where skeletal fossils are poorly preserved, which makes of biomarkers an excellent tool to screen the organic matter producers and reworkers in a given environment. To constitute a theoretical biotope of Kerguelen island, the study of lipid biomarkers will permit to characterize: i. the molecular scaffold in which the life is edified, ii. to characterize the organisms contributing to the CO2 cycling iii. to monitor the changes organic matter producers (microorganisms and vegetation), pH and the hydrography of Kerguelen plateau. After the definition of a biodiversity model that adjusts the different species in the different biotopes present on the Kerguelen islands, the mineralogical model that adjust the sedimentological response to weathering and the organic model that adjust the molecular biodiversity and the organic response to climate, thanks to core analyses, the project will tend to propose a model for climatic reconstruction.