Université Lyon 1

Marine carbonate ecosystems are highly sensitive to environmental conditions (trophic conditions, temperature, pH, redox conditions, etc.). As a result, they are particularly fragile environments in the face of current climate warming, just as they were during past climate changes. The fossil sedimentary record shows that paleoclimatic disturbances are correlated with either crises in carbonate production (i.e., a drastic reduction or even cessation of carbonate sedimentation) or a change in the biomineralizing organisms that produce carbonates1. Such an upheaval of sedimentary systems occurred during the Lower Toarcian (Lower Jurassic, ~184-183 Ma), marked by two major climatic disturbances occurring (1) at the boundary between the Pliensbachian and Toarcian stages (Pl-To event) and (2) in the Lower Toarcian (Toarcian anoxic event, T-OAE)2. These events are marked by a succession of warming and cooling stages that are among the most intense in the Mesozoic, but also by disruptions to the carbon cycle, increased nutrient input to the oceans, and ocean acidification and oxygenation deficits2, 3. The impact of these events on carbonate systems is particularly visible in the so-called West-Tethyan domain, from present-day Morocco to northern France, where vast epicontinental platforms developed during the Upper Pliensbachian4,5. The sedimentary record is systematically characterized by a discontinuity indicating a cessation of carbonate production, followed by either (1) the replacement of carbonate sedimentation by predominantly silicoclastic sedimentation2 or (2) a change in carbonate producers6.
However, while these upheavals in carbonate systems have been highlighted on a West-Tethyan scale, for pelagic and benthic organisms, they have essentially been studied on the basis of vertical facies successions, without taking into account their lateral disparity on a depositional profile. Yet it is likely that the response of carbonate ecosystems to these climatic changes differs according to local environmental conditions. Moreover, the low sedimentation rates often associated with Toarcian climatic disturbances make it difficult to reconstruct in detail the stages of this crisis in carbonate ecosystems. Precise characterization of the spatio-temporal dynamics of this crisis is essential to understand its forcing factors, the respective roles of local and global parameters, or to identify precursor events. The aim of this work is to document the stages of collapse of neritic carbonate ecosystems in the Toarcian and their spatial variability along a proximal-distal profile. To this end, this thesis will focus on the exceptional outcrops of the central Moroccan High Atlas, which make it possible to follow the Toarcian carbonate platform over tens of kilometers from the coastal domain to the offshore. This platform is characterized by very high sedimentation rates, enabling an exceptionally detailed record of events. In addition to sedimentary rocks, particular interest will be given to discontinuity surfaces and their early diagenetic transformations, which could provide information on environmental conditions during shutdowns of carbonate production. Complementary δ18O isotopic data will be acquired to clarify the relationship between temperature and carbonate production. Finally, the sedimentary systems studied will be modeled in DionisosFlow to quantify the controlling parameters on their evolution.

With a Master’s degree or an engineering diploma in Geosciences/Earth Sciences, the candidate we are looking for is a field sedimentologist and petrographer, with experience in carbonate systems if possible. Experience or interest in isotope geochemistry and paleoclimate studies would be welcome. An interest in modeling (DionisosFlow) would be appreciated. In terms of interpersonal skills, we expect the candidate to demonstrate an aptitude for field and laboratory work, teamwork and communication. Scientific curiosity and motivation for the subject are essential.

Apply online : https://adum.fr/as/ed/voirproposition.pl?site=adumR&matricule_prop=57408#version

If needed, contact simon.andrieu@univ-lyon1.fr for further informations.