Description du sujet de thèse

Past dynamics of Amazon hydroclimate and vegetation during the last two glacial cycles. Insights from molecular biomarkers and their isotopic composition.

The aim of this thesis is to provide high-resolution reconstructions of past changes hydroclimate and vegetation changes that affected the Amazon Basin during the last 220 ka by using molecular biomarkers and their isotopic compositions (H and 13C) applied to marine sediment cores off the Amazon fan (cores MD23- 3652Q/53 and MD23-3655Q/56). Focus will be made on the impacts of interglacial and millennial-scale climatic variations on Amazon hydroclimate and vegetation over the last two glacial cycles.

The successful candidate will work on “master” cores of stations S6 or S7 of the AMARYLLIS-AMAGAS campaign. He/she will analyse long-chain n-alkyl lipids (n-alkanes and fatty acids) primarily derived from plant waxes to provide the type of vegetation through chain length distributions. Compound-specific 13C values will serve as indicators to differentiate between plants performing the C3 (humid forest) or the C4 (savanna) photosynthetic pathways.
More specific biomarkers such as pentacyclic triterpene methyl ethers (Poaceae) and acetates (Asteraceae) will also be identified and quantified. Their d13C values will be used to precise their plant source and, eventually, discuss climate conditions and pCO2. These results will give clues to the evolution of Amazon ecosystems through time. The results will be compared to pollen abundances and afford key information on the input of plant remains to marine sediments.
Hydroclimate changes in the Amazon Basin will be reconstructed through the determination of the hydrogen isotopic composition (d2H) of n-alkyl lipids. It will bring constraints on the hydroclimate in which plants thrived. The d13C of molecular biomarkers will be used to account for potential fractionation due to different plant metabolisms. These results will be compared to those acquired on the same cores by partners of the AMACLIM project.
In total, molecular and isotopic records compared to other tracers and proxies will allow discussing the interactions between climate and vegetation during key periods of the last 220 000 yrs at the light of more global change such as changes in oceanic and atmospheric dynamics.

We are seeking for a highly motivated candidate who recently obtained a Master 2 diploma in environmental sciences, geochemistry or palaeoclimates. The candidate must have a solid background in organic geochemistry and palaeoclimates and, potentially, a first experience on a similar topic during previous internships. The candidate must have proven abilities in labwork (with specific training in good labwork practises), interdisciplinary and international environments. He/she must be fluent in English.

Contexte de travail

This PhD project is part of the ANR-FAPESP AMACLIM project lead by Aline Govin (LSCE, France) and Cristiano Chiessi (University of Sao Paulo) funded for the 11/2025 – 10/2029. It aims at providing the first integrated understanding of the atmosphere-land-ocean interactions that control natural changes in Amazon climate and vegetation under different climatic forcings of the past two glacial cycles, shedding new light on the future response of Amazon climate and vegetation to anthropogenic climate change.
The PhD project beneficiates from the ANR AMACLIM grant for salary, analyses and travel expenditures (congress and visit to partners). Lipids will be extracted at LSCE from sediment samples collected every ~25 cm along the “master” cores of S6 or S7 AMARYLLIS stations. After separation into different fractions, molecular biomarkers will be identified and quantified by GC-FID and GC-MS, and compound-specific d2H and d13C values will be measured by GC-IRMS at LSCE.

Contraintes et risques

Labwork and missions abroad