Master of Science / Department of Geology / Kirsten Nicolaysen / The Kerguelen Archipelago and the submarine Northern Kerguelen Plateau consist primarily of basalt erupted by the intraplate Kerguelen hotspot. The exposed volcanic rocks are tholeiitic basalts (29 Ma) followed by lesser alkalic basalts, highly alkaline phonolites, and syenites (24 Ma). This study presents the mineralogy, isotopic and geochemical compositions of lavas (n=59) from a 650 m section exposed in the wall of a glacial valley at Trois Menestrels in the Kerguelen Archipelago. This basalt section, exposed south of the Cook Ice Cap and closest to the Raillier du Baty alkalic plutonic complex, is further south and west of previously studied basalts exposed on the Plateau Central. Results of 40Ar/39Ar dating show the Trois Menestrels lavas erupted approximately 25 Ma, approximately contemporaneously with other Plateau Central lavas and thus provide an opportunity to examine spatial compositional variations within the Kerguelen hotspot. Though the Trois Menestrels lavas are tholeiitic, The samples plot on the tholeiitic field at the stratigraphic section of Trois Menestrels, 80% of lavas from Mt. Tourmente are transitional to alkalic in the younger rocks (25.6 Ma). This sequence of tholeiitic to transitional to alkalic compositions is mirrored at Mt. Capitole and Mt. Marion Dufresne. Sr, Ba, Pb, Nb, Zr versus Nb plots show similar patterns across the Plateau Central and abundances overlap within error. Variations in isotopic composition reveal that Trois Menestrels is rather homogeneous compared to the other sections. A fundamental issue that derives from these results is the close relationship between these different rocks of the various region of the Plateau Central.

Les écosystèmes sub-Antarctiques sont particulièrement vulnérables aux perturbations environnementales et notamment aux invasions biologiques. L'introduction du Chat (Felis silvestris catus) sur la Grande Terre de l'archipel des Kerguelen a eu des effets désastreux sur les communautés d'oiseaux marins. Ce prédateur doit sa survie dans cet environnement à la présence d'une proie introduite, le Lapin (Oryctolagus cuniculus), qui constitue sa ressource principale pendant l'hiver lorsque les oiseaux marins sont repartis en mer. L'éradication du chat n'apparaît pas être une solution réaliste compte tenu de la taille de l'île (6675 km2) et son inaccessibilité. De plus elle serait susceptible de conduire à des résultats non souhaités comme par exemple une explosion démographique des populations de lapins. La mise en place d'une politique de gestion adaptée nécessite d'améliorer notre connaissance des facteurs qui conditionnent la dynamique spatio-temporelle des populations de chats. Au cours de cette thèse, j'ai cherché à identi_er les mécanismes des interactions entre le chat, le lapin et la végétation, car elles sont attendues conditionner celles existantes entre le chat et les oiseaux marins. J'ai analysé la dynamique spatio-temporelle de quatre populations de chats sur la Grande Terre et j'ai en particulier recherché si des conditions environnementales spatialement corrélées (effet Moran) pouvaient, en se propageant dans la chaine alimentaire, synchroniser les variations temporelles d'abondance des populations de chats. Enfin, j'ai étudié l'impact des variations spatio-temporelles d'abondance des populations de chats sur les variations spatio-temporelles de la structure génétique des populations de chats. / Sub-Antarctic ecosystems are particularly vulnerable to environmental perturbations including biological invasions. The introduction of the feral cat (Felis silvestris catus) on the Grande TerreIsland of the Kerguelen archipelago has lead to detrimental effects on seabird's communities. Cat survival in this environment is possible because of the presence of the introduced rabbit (Oryctolagus cuniculus), which constitute the main prey of the cat during winter when most seabirds are offshore. Cat eradication is not a realistic solution given the size and inaccessibility of the island (6675 km2). Moreover it can lead to unwanted results such as the demographic explosion of rabbit populations. The design of appropriate management policy needs to improve our understanding of the factors that influence the spatio-temporal dynamics of cat populations. In this thesis, I explored the mechanisms of cat-rabbit-vegetation interactions, as they are expected to determine interactions existing between cats and seabirds. I analyzed the spatio-temporal dynamics of four cat populations on Grande Terre Island and I especially sought whether environmental conditions spatially correlated (Moran effect) could synchronize cat through their effect on the trophic web pathways. Finally, I studied the impact of spatio-temporal