Study of the recurrence of African Humid Periods based on the characterization of Saharan dusts deposited in the western Mediterranean and in the Tropical North-East Atlantic Ocean over the course of the last two climatic cycles.

Over the course of the Quaternary, variations in received insolation over the African continent have governed the monsoon dynamic giving rise to a recurrence of periods of intense precipitation known as “African Humid Periods” (AHPs). The latter have caused major transformations of the Saharan hydrological cycle, including the development of rivers, lakes and even lush vegetation in what is today a desert region. Over the last two climatic cycles (240 ka), eight AHPs have occurred in recurrent manner. Numerous studies have focused in particular on the onset and the termination of the last AHP, dated to the beginning of the Holocene, about 9000 to 5000 years ago. The latter corresponds to a period when obliquity and local insolation reached their maxima almost synchronously. The identification of the orbital forcings associated with this AHP is still a matter of debate. In this thesis, since the oldest AHPs correspond to different orbital configurations, they are studied with the aim of providing new insights concerning the intensities, beginnings and terminations of these AHPs. In order to do this, I am carrying out high-resolution sedimentological, mineralogical and geochemical characterization of the Saharan dust deposits recorded in two sediment cores, one taken from a lacustrine context (Lake Bastani in Corsica) and the other from a marine context (off the coast of West Africa), which cover these periods. In fact, carried by air and associated with the environmental conditions prevailing in the source zones (level of vegetation cover, aridity) these Saharan dusts deposited in the sedimentary archives, provide indirect but invaluable evidence for variability in the Saharan hydrological cycle over time. Moreover, digital simulations, which will allow us to test the responses to the forcings identified through the analytical study of the dust deposits over the course of the various AHPs, will also be undertaken using an IPSL ocean-atmosphere coupled model.

Thesis director : Christophe Colin (GEOPS)

Thesis co-director : Charlotte Skonieczny (GEOPS)

Doctoral contract (Université Paris-Sud)