Description du sujet de thèse

PHD Title : Chroniques des conditions hydroclimatiques de la façade atlantique sud-métropolitaine (golfe de Gascogne) : dynamique et processus au cours des derniers siècles -CHRONHYCLESS, Financial support from the GDR OMER“Océan et Mers” AAP 2025 - Contrats doctoraux

1) scientific background
Aquitaine coasts are subject to chronic erosion, influenced by climate and ocean dynamics in the Northern Hemisphere, and notably related to storminess. In the current context of rapidly rising sea levels, this observation calls for a better understanding of the phenology of hydroclimatic events (extreme or not) that affect these environments, and requires a reconsideration of the calibration of the hydro-morpho-sedimentary models (conceptual and numerical) applied until now. Actually, the set of instrumental data that support this calibration is too limited to properly encompass the range of processes at work, and presents a major intrinsic bias: used descriptors have already been deeply affected by anthropogenic effects over the last century (e.g., Waters et al., 2016, 2018). These descriptors are based on recent monitoring of hydrological parameters in the surface ocean; as such, they neglect larger historical dimensions, which are, however, essential for qualifying initial and natural states, apart from rapid environmental changes linked to human activities over at least the last 150 years. A better understanding of the hydroclimatic trajectories of the Aquitaine coastal environments thus requires an integration of historical and past events, accessible by decoding both sedimentary and historical archives.
Off the main Aquitaine rivers, the continental shelf exhibits natural sedimentary traps, shaped by alternative excavation (during low sea levels) and filling (during high sea levels) of the palaeo-valleys incising the bedrock (e.g., Blum et al., 2013). These specific geomorphological contexts favour mudflats and high sedimentation rate deposits (up to 1 cm/year, Lesueur et al., 2001, 2002; Mary et al., 2015, 2017; Eynaud et al, submitted), which provide access to exceptionally well-resolved archives of the hydroclimatic and ecological evolutions of the ocean margins and adjacent continents. Combined with an inventory of historical records of extreme events and hazards (storms, cold or heat waves, etc.), these series can be used to build retrospective chronicles of multi-decadal environmental changes. Together with prospective modelling efforts, these chronicles, as the product of an intrinsically multidisciplinary approach (combining history, geography and bio-geosciences), are key to characterize the long-term trajectories envisaged by OMER. They will provide a framework for environmental changes at a very high temporal resolution, which is still lacking at the regional scale and very rare in Europe (e.g. Martin-Puertas, 2021).

2)objectives
By combining data from palaeoclimatology and historical inventories (Athimon et al., 2022), this PhD project aims to reconstruct the detailed framework of hydroclimatic events and parameters that have affected the Aquitaine margin over the last few centuries (at least following the Roman warm anomaly). This chronicle may offer the possibility to:
- define the frequency of climatic and meteorological phenomena (including extremes) and their impact on the Aquitaine territories and biomes, and link them to the dynamics of human settlement
- identify periods of hydroclimatic instability (such as existing for the Gulf of Lion, Tavenne, 2023)
- decipher the climatic modes dominating the morphosedimentary evolution of the margins
- relate the climatic processes driving the evolution of the Aquitaine environments to the dynamics of the North Atlantic Ocean (subpolar gyre in particular)
- contextualise the changes in coastal landscapes (Péret and Sauzeau, 2014) that have accompanied this history (state/fractionation of land cover, geomorphology of coastal embayments-islands, dune mobility), trying also to deconvolute the role of anthropogenic development (land/shore artificialisation) in this evolution.

3) methodology
The methodological approach is multidisciplinary and integrative in nature, encompassing a synthesis of data from palaeoceanographical archives and historical records. Within the Bay of Biscay, the aim will be to reconstruct physical and ecological descriptors of the surface ocean at a very high temporal resolution since the Roman warm anomaly. In addition, for the recent period (the last 300 years), data collected by Scientific Societies, working and research groups will be inventoried with the objective of identifying marine extreme weather events. This work will be structured in two parts: (1) the first two years will be dedicated to the analysis of sediment cores and historical data in order to obtain resolved chronologies that will allow us to combine these two types of archive sources. This chronostratigraphy will be complemented by correlations at the metropolitan scale, through the compilation of natural archives of various origins (lacustrine, dendrological, speleothems, etc.). For recent periods, the qualitative indices obtained will be compared with series of meteorological observations in order to document each event in terms of atmospheric pressure and wind direction and strength. In addition, the maree.shom.fr online application will offer a section on sea level, considered exclusively from the harmonic point of view, which can be cross-referenced with meteorological data in order to specify the atmospheric and dynamic components. This part of the project implies the collection of bibliographic data and descriptors, which will be archived in a public database (hosted by OASU, or Heurist - HumaNum); (2) the final year will be focused on the design of a conceptual model of environmental changes for the study area, targeting the integrating hydroclimatic mechanisms and modes that link the Atlantic coast of metropolitan France to the regimes of the Northern Hemisphere. Trajectories will be established for use by scientists and managers through the construction of key index chronicles.

Contexte de travail

The doctoral work is supported by funding from the GDR OMR (AAP 2025 - Doctoral Contracts) via the CNRS DGDS MITI (Mission pour les Initiatives Transverses et Interdisciplinaires )-Theses.
The student will be registered at the University of Bordeaux in the ED304 'Sciences and Environment' doctoral school (https://doctorat.u-bordeaux.fr/avant-le-doctorat/les-ecoles-doctorales/SE), whose research training is based on multidisciplinarity (earth and universe sciences, ecology, environment, archaeology, philosophy and history of science).
He/she will be attached to the Paléoclimats research team of the EPOC laboratory (Environnements et Paléoenvironnements Océaniques et Continentaux, UMR5805). Co-supervision with teams from the CRIHAM laboratory (Centre de Recherches Interdisciplinaires en Histoire, Histoire de l'Art et Musicologie - UR 15507) at the University of Poitiers will involve constant interaction throughout the PhD, both in person and at distance.
The scientific, functional and collaborative environment for the thesis is well established, as the project is integrated into the PGSAR - RNA (Région Nouvelle Aquitaine) CORALi (COnnaissances inteRdisciplinaires pour une meilleure Adaptation face aux risques Littoraux), which offers an original research consortium where the issue of reconstructing the trajectories and evolutionary history of neritic and coastal environments is central. All the partners work closely together, and two PhD in History will support the extensive historical data collection work.
This PhD project is following the dynamics of several research projects that have consolidated a network of experts at national and international levels. It benefits from the collaborations established during the initial works carried out in the framework of the COTE/LASCARBX LITAQ Interlabex project (From the Pleistocene to the Anthropocene: understanding past mechanisms in the evolution of populations - plant, animal and human - and environments in order to predict future responses. L'exemple du littoral aquitain; 2013-2015) and the EC2CO DRIL FAST-LITAQ project (Formations Argilo-Sableuses & Tourbes LITtorales AQuitaines: de leur intérêt dans l'évolution des falaises dunaires du Médoc; 2016-2017). It will complement studies conducted on Holocene climate variability as part of the ANR HAMOC (Holocene North-Atlantic Gyres and Mediterranean Overturning dynamic through Climate Changes' /2013-2018) and on the links between human settlements and climates already examined as part of the RNA- ESTRAN project (Erosion et Sociétés dans le Temps Longue sur les Rivages de l'Aquitaine Nouvelle; 2019-2024) and of the OFB- Interestuaire TRESSE project (TRajectoires - EStuaires - Sediments: Morphosedimentary trajectory of French estuaries since the Little Ice Age. Impact du climat et de l'Homme; 2021-24).
Finally, this thesis will contribute to the work of the Tempêtes et Submersions Historiques (TSH) group, which is co-chaired by BRGM, ARSN, Cerema and SHOM, and among which CRIHAM has been a member since its creation. It will also be part of the discussions supported by the GiS Humanités et sciences de la Mer (Humanities and marine sciences), of which Prof Sauzeau is a founding member and president of the CS.

Contraintes et risques

Funding for this thesis project has been granted as part of the AAP 2025 - Contrats doctoraux of the GDR 'Océan et Mers', which, via the CNRS MITI, supports research initiatives at disciplinary boundaries. As a result, the thesis will be supervised by two experts from two different laboratories (the EPOC laboratory at the University of Bordeaux and the CRIHAM laboratory at the University of Poitiers) attached to different, but complementary, sections and institutes of the CNRS and CNU (coupling of Palaeoclimatology and History/Archaeology). Interdisciplinary interaction is therefore a concrete goal for the success of the project.