Sediment magnetization: climatic and biological constraints

This thesis deals with both aspects –  paleoenvironmental and geomagnetic – contained in the magnetic signal of deep-sea sediments. For this, sedimentological, geochemical, mineralogical and magnetic investigations were conducted. This multi-proxy approach is essential in order to decipher the origins and the components of the signals. This PhD thesis has been split in two research axis.

The first one deals with the paleoenvironmental aspects and is based on the study of two oceanic cores from the Indian Ocean (cores MD92-1002 and MD92-1003) and one core from the Red Sea (core MD92-1008). This is the subject of two chapters in this manuscript. The description of magnetic signals recorded in these cores could be linked to the climate changes of the last 200,000 ka (sea-level variations, glacial-interglacial cyclicity and Heinrich Events). The part of the role played by the amount of organic matter on the preservation  of magnetic minerals and on the resultant signals of magnetic susceptibility in those two basins is widely discussed in both those chapters.

The second axis of this study deals with geomagnetic aspects presented in a long sediment core by using a multi-approach including relative paleointensity analyses and geochemical investigations: δ¹⁸O, indicative of climate changes and the evolution of the ¹⁰Be flux, whose production is modulated by the strength of the geomagnetic field. ¹⁰Be analyses constitute an independent approach, from the relative paleoïintensity investigations, for recording. the intensity o f the geomagnetic field changes. For our investigations, we selected the long-piston core MD90-0961, coming from the same studied site that core MD90-0963. That core is known for the accuracy of its oxygen isotope stratigraphy and for giving an estimated age of the Brunhes-Matuyama boundary o f about 775±10 ka [Bassinot et al., 1994]. CoreMD90-0961 allows us to investigate the high-resolution magnetic and geochemical signals related to the Brunhes-Matuyama reversal and it gives us the ability of using the ¹⁰Be tracer. Inside this core, this reversal is contemporaneous with the marine isotopic stage MIS 19 at around 773.5 ka. The weakening signal of the geomagnetic field is positively correlated with the maximum production of the ¹⁰Be. Relative paleointensity and 10Be signals show that there is no evidence of delay in the magnetization acquisition in this core.