Soutenance
Mangalaa Kameswari Rajasekaran (LOCEAN)
Date et heure : Le 16-12-2016 à 13h30
Type : thèse
Université qui délivre le diplôme : UPMC
Lieu : UPMC - 4 place Jussieu - 75005 Paris - Grande Salle de Vidéoconférence – Atrium (RDC)
Me. Isabelle BASILE-DOELSCH Directrice de Recherche INRA, Rapporteure
M. Damien CARDINAL Professeur UPMC,Directeur de thèse
Me. Josette GARNIER Directrice de Recherche CNRS, Examinatrice
M. Alain SALIOT Professeur UPMC, Examinateur
M. Vedula V.S.S. SARMA Directeur de Recherche CSIR, Co-directeur de thèse
M. Nicolas SAVOYE Physicien-Adjoint INSU, Rapporteur
Silicon is the second most abundant element in Earth’s crust and one ofthe key nutrient in aquatic ecosystems. There are strong interactions ofSi with carbon cycle and biogeochemical processes. The present thesisfocused on variability of silicon (amorphous-ASi, lithogenic-LSi anddissolved-DSi) and Si isotopes along the land to ocean continuum.We investigated the seasonal and spatial variability of ASi, LSi & DSi andSi isotopes in ~20 Indian estuaries. We categorize the estuaries usingstatistical analysis (PCA and cluster analysis). Diatom uptake seems to bethe main process controlling ASi during dry period, especially in theSouth. Weathering and erosion control the variability of LSi in theremaining estuaries. Similarly lithogenic supply controls Si during wetperiod in all estuaries and no impact of diatoms was seen because of highsuspended load. Si isotopic compositions trace the Si sources andbiogeochemical pathways. The isotopic results exhibit clear seasonaldifference with high impact of type of weathering during both seasons.Theyshow that southwest watersheds are very special in terms ofweathering regime compared to the other watersheds because of topographyand climate. The impact of agriculture and forest cover on Si cycle isalso clearly evidenced in all the basins during wet period. We show thatgroundwater Si isotopic variability results from a combination ofdissolution and production of minerals. Overall, this study shows thepreponderant influence of weathering and type of secondary clays on Siisotopes irrespective to the seasons, rather than the biological uptake ormixing as reported elsewhere.
krmlod@locean-ipsl.upmc.fr