Reconstitution paléoclimatique et paléoenvironnementale au Valanginien ( 135 Ma, Crétacé inférieur) : vers une meilleure compréhension des processus locaux versus globaux

• Directeurs : S. Duchamp-Alphonse et C. Colin
• Allocation ministérielle
• Début de thèse : octobre 2010

Devant le jury composé de :

 Christophe Colin (Professeur Université Paris Sud XI) : directeur de thèse
 Stéphanie Duchamp-Alphonse (Maître de conférence Université Sud Paris XI) : co-directeur de thèse
 Annachiara Bartolini (Professeur Muséum histoire naturelle Paris) : rapporteur
 Jean François Deconinck (Professeur Université de Bourgogne) : rapporteur
 Guillaume Dera (Maître de conférence Université de Toulouse) : examinateur
 Giuseppe Siani (Professeur Université Paris Sud XI) : examinateur
 Karl Föllmi (Professeur Université de Lausanne) : examinateur
 Bruno Galbrun (Directeur de recherche CNRS Université Paris VI) : examinateur

Abstract : The Valanginian stage witnesses a positive carbon isotope excursion (amplitude 1.5-2‰) in the sedimentary record, the so-called « Weissert Episode » (WE). This event coincides with major palaeoenvironmental and palaeoclimatic changes. The formation of the Paraná-Etendeka large igneous province has widely been proposed as the major driving force behind these changes. However recent investigations demonstrate that the Weissert Episode precede the volcanic activity. Thus the driving force(s) behind these changes are still under debate. In the herein study, biostratigraphical, cyclostratigraphical, mineralogical and geochemical analyses have been performed on mid- and high latitudes sites (the Orpierre and Arnayon/La Charce sections in the Vocontian Basin, and the ODP hole 765C in the Argo abyssal plain) in order (i) to explore the dynamic of the local versus global processes in the palaeoenvironmental changes and (ii) to discuss the possible forcing factors behind the Weissert Episode. Local diagenetic and sedimentological processes that might bias the palaeoclimatic signals have been highlighted and discussed before any palaeoenvironmental interpretations. The astronomical calibration of the Weissert Episode performed at Orpierre, allowed for the first time, the calculation of detrital and nutrient influxes into the Vocontian basin. They highlighted rapid climate changes at mid latitudes, characterized by pronounced humid conditions during the positive δ13C shift (base of the WE : Early–Late Valanginian transition) and drier conditions in the Late Valanginian (top of the WE). In parallel, new geochemical and mineralogical analyses performed on the ODP hole 765C have been integrated to a compilation of published geochemical and mineralogical data from 23 sections, located in five depositional environments. Three latitudinal belts have been recognized during the Valanginian : (i) an arid palaeolatitudinal belt in the Proto North Atlantic Ocean ( 15–17°N), (ii) a subtropical belt in the Northwestern Tethyan margin ( 25–30°N) ; and (iii) a warm temperate belt in high latitude ( 53°S and 40 to 70°N). It appears that during this period, both tectonic and orbital factors might have controlled the distribution of the palaeoclimatic belts and the major palaeoenvironmental changes that are recorded during the Weissert Episode.

Keywords : Weissert Episode, terrigenous/nutrient influxes, local vs global processes, palaeoclimatic belt, tectonic and orbital forcings.