Description du sujet de thèse

Over the past several millennia, the tropical forests of Central Africa have undergone significant changes in composition and structure. Two main drivers are thought to have contributed to these transformations: anthropogenic activities and/or climatic variations. Indeed, these forests have been inhabited and exploited for thousands of years, with a marked increase in human impact during the expansion of Bantu-speaking farmer-metallurgist populations around 2,500 years ago (Garcin et al., 2018; Koile et al., 2022). At the same time, however, a widespread drying trend—often associated with the end of the African Humid Period—was recorded in the Central Congo Basin (Garcin, 2022), as well as more broadly across tropical Africa (Tierney & deMenocal, 2013; Shanahan et al., 2015).
These concurrent drivers have sparked considerable debate about their respective roles (Garcin et al., 2018; Clist et al., 2018; Giresse et al., 2018), yet no comprehensive and convincing scenario has emerged. It is likely that these two forcing factors played different roles in vegetation change depending on location and time, potentially triggering abrupt ecological shifts linked to a loss of forest resilience.
However, few paleoenvironmental studies have been able to combine precise (both temporally and quantitatively) reconstructions of precipitation, anthropogenic disturbances, and vegetation responses. This PhD project aims to reconstruct the detailed dynamics of precipitation changes at two Central African sites (in Gabon and Cameroon), where previous studies have already identified major shifts in vegetation and fire activity over the past millennia. This comparative approach will help improve our understanding of the sensitivity of Central African ecosystems to these various drivers.
To achieve this, we will use hydrogen isotopes from plant leaf waxes (such as long-chain n-alkanes), molecular fossils preserved in lake and peat sediments, to reconstruct past hydroclimatic variations (e.g., rainfall amounts) at high temporal resolution.

Contexte de travail

• Primary location: CEREGE (UMR 7330, Aix-en-Provence), within the CLIMAT team.
• Secondary location: Institute of Evolutionary Sciences of Montpellier (ISEM).
• Contract duration: 36-month doctoral contract funded by the ANR via CNRS.
• Supervision: Yannick Garcin (CEREGE) and Laurent Bremond (ISEM).
• Part of the ANR project RainForStory: “Emergence and spread of agriculture in the Congo Basin forests.”

Contraintes et risques

• Fieldwork: Missions in remote tropical environments (Gabon, Cameroon).
• Laboratory work: Geochemical and isotopic analyses (solvent handling, organic extractions).
• Multi-site collaboration: Occasional travel between CEREGE (Aix-en-Provence), ISEM (Montpellier), and Central African field sites.