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Sedimentary and geochronological record of Ypresian thermal events in Occitanie: stratigraphic, paleogeographic, climatic and tectonic implications M/W

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Date Limite Candidature : lundi 28 juin 2021

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General information

Reference : UMR5243-HELOUR-019
Workplace : MONTPELLIER
Date of publication : Monday, June 7, 2021
Scientific Responsible name : Flavia GIRARD - Fabrice LIHOREAU - Johan YANS
Type of Contract : PhD Student contract / Thesis offer
Contract Period : 36 months
Start date of the thesis : 1 November 2021
Proportion of work : Full time
Remuneration : 2 135,00 € gross monthly

Description of the thesis topic

The thesis project is part of the ANR EDENs project entitled "Life during past super-warm climate events: Evolutionary Dynamics of Early EoceNe mammals from Southwestern France" which focuses on the study of potential relationships between past climate warming events and macro-ecological/macro-evolutionary processes during the Lower Eocene or Ypresian (56-47.8 Ma) in Occitanie. This period is of major importance as it represents the warmest time interval of the last 66 million years (Zachos et al., 2001) and corresponds to a key phase of mammalian radiation (Rose, 2006). The Ypresian is characterized by a series of short-term (~150 kyr) global warming events (Eocene hyperthermals or thermal maxima, ETM). In terms of magnitude, the major ETM is the Paleocene-Eocene Thermal Maximum (PETM, ~56 Ma) (McInerney and Wing, 2011); this event was followed by other, but smaller, warming events (e.g. ETM2 ~54.1 Ma, ETM.I1 ~53.7 Ma, ETM3 ~53Ma, ETM.L, ETM.M, ETM.N, etc) (Westerhold et al. 2018). All ETMs are characterized by negative carbon isotopic excursions, phenomena explained by the rapid introduction of a large mass of isotopically depleted carbon into the atmosphere. During the PETM, global temperature increased by 5-8°C; this event is generally considered the best paleo-analogue of present-day climate change, although modern carbon emission rates are about 10 times higher than those of the early PETM (Gingerich, 2019). In the second part of the Ypresian, a long-term global warming event, namely the Lower Eocene Climatic Optimum (EECO, ~53-49 Ma), overlaps with a series of hyperthermals (Westerhold et al. 2018). The role of this EECO on mammals remains relatively undocumented.
At this time, in southern France, the Pyrenean orogeny plays a major role in the deposition of thick sedimentary sequences from the Bay of Biscay to Provence, recording important variations in chronology, tectonic structures and depositional environments (Ford et al., 2016; Angrand et al., 2018). This complexity is particularly pronounced in the transition zone between the Corbières region and the Provencal part of the Pyrenean belt (Arthaud and Séguret, 1981; Leleu et al., 2009), corresponding to the proposed study area for this thesis. Sedimentation is concentrated in marine to continental foreland basins recording marine, proximal detrital (alluvial fans, fluvial and lacustrine systems) or lacustrine to palustrine carbonate deposits (Freytet and Plaziat, 1982; Rasser et al., 2005).
Current depositional models of study area take little or no account of the diversity of sedimentary facies with respect to basin architecture and their temporal evolution over the period considered. Chronostratigraphic models for this period are being revised in light of new interpretations of fossiliferous material from paleontological remains under study (ISEM work); thermal events are only poorly (e.g. Noiret et al., 2016) or not yet identified in the sedimentary record.
The objective of the thesis project will be to establish a high-resolution sedimentological model as well as the precise chronology of the evolution of sedimentary paleoenvironments during the Ypresian in Occitanie. This will be possible thanks to the analysis of short-term (ETMs) and long-term (EECO) climatic changes, and tectonic events, the two main forcing's that contributed to the development of local mammalian living environments, in the different studied sedimentary basins. The thesis work will focus on the characterization of the sedimentary and geochemical signature of these thermal events while studying the interactions with tectonics in order to try to avoid them. To this end, the thesis project will focus on the Ypresian of the Occitanie region in four sub-regions: Corbières, Lauragais, Minervois and Montpellierain. A multi-scale analysis of the sedimentary record will be conducted, based on detailed sedimentological analysis of depositional systems. Particular attention will be paid to the architecture of these deposits in relation to Pyrenean tectonics by creating sedimentological sections along the basins, tectonic sections through the basins, and structural deformation analysis. The PhD student will have to carry out important field work including macro and microscopic facies studies (thin section sampling) associated with the realization of sedimentological logs, stratonomical studies (depositional architecture) and stratigraphy (correlation of marker beds, etc), as well as field mapping (dip measurements, etc). U-Pb geochronology analyses on calcite will be carried out in order to potentially determine the absolute age of the different sedimentary deposits and to compare these ages with the temporal constraints provided by paleontological data. Chemostratigraphy (carbon isotopes on various materials) will also be used to identify thermal events as this method has proven to be an excellent temporal proxy. Carbon isotopic variations (δ13C) during TMEs and OTEC show a synchronous record in marine and continental strata allowing for global correlations (McInerney and Wing, 2011; Yans et al, 2014 a and b; Noiret et al, 2016; Yans et al, 2019). Recently, new high-resolution data from ODP reference boreholes have provided a robust chemostratigraphic framework for the studied period. MTEs will be used as "geochronometers" to correlate the proximal marine-continental sections of Occitanie with marine standards and thus date the studied sedimentary sections. The outcrop conditions in the region and their sedimentological and structural analysis, the complete revision of the ages of the fossiliferous deposits and the new geochemical dating approaches available at the Montpellier Geosciences Laboratory offer us an exceptional opportunity to better understand the emplacement of the basins considered and to extract major climatic signals.
Beyond the characterization of geological processes (sedimentation-tectonic links), the study of these basins will bring to the student various skills in more applied fields such as the high-resolution characterization of reservoir heterogeneity in analogy to petroleum or phreatic reservoirs for fluvio-lacustrine to lacustrine and palustrine systems and foreland basins.

Références :
Angrand et al. 2018 Tectonics 37:430-449
Arthaud & Séguret 1981 Bull. Soc. Geol. Fr. 23:51-63
Ford et al. 2016 J. Geol. Soc. 173:419-437
Freytet & Plaziat 1982 Contrib. Sediment. Geol. 12:1-217
Gingerich 2019 Paleoceanography and Paleoclimatology 34:329-335
Leleu et al. 2009 Basin Res. 21:157-187
McInerney & Wing 2011 Annu. Rev. Earth Planet. Sci. 39:489-516
Noiret et al. 2016 Newsl. Stratigr. 49:469-480
Rasser et al. 2005 Facies 51:227-242
Rose 2006 The Beginning of the Age of Mammals 431p
Westerhold et al. 2018 Paleoceanography and Paleoclimatology 33:626-642
Yans et al. 2014a Gondwana Research 25:257-269
Yans et al. 2014b Newsl. Stratigr. 47:331-353
Yans et al. 2019, Symposium PalEurAfrica
Zachos et al. 2001 Science 292:686-693

Work Context

This thesis is one of the objectives of the ANR EDENs project funded by the CNRS. It will be carried out at Geosciences Montpellier and will be done in collaboration with ISEM and research teams from the University of Poitiers, and the University of Namur (Belgium).
The PhD student will be co-supervised by Flavia Girard (Géosciences-Montpellier, sedimentology), Fabrice Lihoreau (ISEM, paleontology) and Johan Yans (University of Namur, isotope geochemistry) and co-supervised by Grégory Ballas (Géosciences-Montpellier, structural analysis), Delphine Bosch (Geosciences-Montpellier, geochronology) and Rodolphe Tabuce (ISEM, EDENs coordinator, paleontology) in order to integrate sedimentological, structural and geochemical results in a multidisciplinary context.
The PhD student should have a Master's degree in Earth Sciences and should have a thorough knowledge of sedimentary environments (facies sedimentology), ideally continental, and of the concepts of sequence stratigraphy, tectonics, and ideally isotope geochemistry. The PhD student should have experience and be highly motivated by field geology as the project will require significant fieldwork (3-4 months).

Constraints and risks

Several field missions in the Occitanie region (3-4 months cumulated) as well as geochemical analyses in specific rooms under confined atmosphere are planned. The PhD student will be assisted by a technician for the preparation and realization of the geochemical analyses. The supervisors of the project are assured of the feasibility of the project because their research work is already in progress on the study areas and the geochemical analyses and the preliminary field missions have shown very encouraging results. The PhD student will work primarily in the Corbières and Minervois regions. The integration and correlation of existing data from the working group will be essential.

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