Description of the thesis topic

The Amazon basin is a massive source of heat and humidity, playing a central role in the hydrological balance of equatorial regions. Changes in climate and land use in Amazonia may force the rainforest (whose extent and biodiversity depend on rainfall) into a self-perpetuating dieback process. Climate models simulate a drop in rainfall in the Amazon by 2100, which could be amplified by the probable slowdown in the deep Atlantic circulation.
Paleoclimatic reconstructions allow us to assess the mechanisms controlling natural hydroclimatic changes in Amazonia under different forcings. Existing records reveal past variations in Amazon hydroclimate on orbital and millennial time scales over the last 50,000 years. But many questions remain. While rainfall in the Andean regions of the Amazon seems to intensify during periods of high austral summer insolation, what is the response of the Amazonian plains on an orbital scale? The slowdown of the deep Atlantic circulation during millennial-scale events of the last glacial period also led to an overall intensification of precipitation in Amazonia, but many uncertainties remain regarding the spatial heterogeneity of this response, the differences observed between events, and the climatic mechanisms at play. Finally, how has the Amazon hydroclimate responded to an overall warmer climate than today, such as that observed during past interglacial periods and that could be reached in the coming decades? These questions can now be answered using older, high temporal resolution palaeoclimate reconstructions from sediment cores collected in 2023 during the AMARYLLIS-AMAGAS II oceanographic cruise.
This PhD thesis aims to determine the climatic mechanisms that control the natural variability of the Amazon hydroclimate over the last two climate cycles (the last 220,000 years). It is based on a multi-tracer experimental approach applied to several sediment cores collected about 700 km downstream from the mouth of the Amazon River. Thanks to a unique temporal resolution over this period (~100 years), this study will, for the first time, (1) deepen our understanding of the mechanisms controlling glacial-interglacial, orbital and millennial variations in the Amazon hydroclimate over the last two climate cycles, and (2) clarify the response of the Amazon hydroclimate to the generally warmer conditions of the last two interglacial periods.
Specifically, the student will reconstruct past variations in the Amazon hydroclimate using geophysical (magnetic properties of the sediment) and geochemical (concentrations of major and minor elements, radiogenic isotopes of neodymium and strontium) measurements on sediment cores stored at the LSCE. He/she will also have access to the collection of modern Amazon samples available at the University of São Paulo (USP, Brazil), in order to refine the modern magnetic and geochemical signatures of the Amazon detrital material. Finally, the student will be able to interact with USP paleoceanographers and LSCE climate modellers in order to determine the climatic mechanisms involved. The experimental part of the PhD thesis will provide the student with analytical skills in palaeomagnetism and geochemistry. He/she will have access to the samples and instrumentation available at the LSCE and the USP.

Work Context

The PhD thesis will take place at the Laboratoire des Sciences du Climat et de l'Environnement (LSCE), a joint research unit (UMR 8212) of the Commissariat à l'Énergie Atomique et aux énergies alternatives (CEA), the Centre National de la Recherche Scientifique (CNRS) and the Université de Versailles Saint-Quentin en Yvelines (UVSQ), located on the CEA's Orme des Merisiers site at the heart of the Paris-Saclay University campus. It is part of the Institut Pierre-Simon Laplace (IPSL). The LSCE, which brings together over 350 researchers, PhD students, engineers, technicians and administrative staff, studies past, present and future mechanisms in the evolution of the Earth's climate and external environment, using observations and modeling exercises.
The PhD thesis is part of the Climate-Magnetism team (CliMag) of LSCE's “Archives and Tracers” research theme. The team is interested in the reconstruction of modern and past environments using the magnetic, sedimentological and elemental properties of sediments. It also studies high-resolution variations in the Earth's magnetic field, a geochronological tool. The team boasts a wealth of cutting-edge environmental magnetism instruments, including three cryogenic magnetometers suitable for discrete samples or continuous sedimentary series and placed in a wide shielded room. These instruments are complemented by several furnaces and high-sensitivity equipment for the study of magnetic mineralogy. The team also manages the X-ray fluorescence and grain-size laboratories, which are shared with several other LSCE teams.
The PhD thesis is part of the AMACLIM-PhD "joint PhD program" funded by CNRS and the University of São Paulo. Close links are expected between this PhD thesis funded by the CNRS in France and the other PhD thesis (paleoceanographic reconstructions in the western equatorial Atlantic from the same sediment cores) funded by the University of São Paulo in Brazil. These two PhD theses are also part of a close Franco-Brazilian collaboration in paleoclimatology (joint exploitation of material collected during the AMARYLLIS-AMAGAS II cruise, International Research Project SARAVA, CAPES-COFECUB project), from which the students will benefit directly.
Research stays at the University of São Paulo (USP) in Brazil are expected for a few months during every year of the PhD thesis.