QUANTIFICATION OF EARTH'S SURFACE PROCESSES AND DATING USING THE COSMOGENIC RADIONUCLIDES 10BE, 26AL AND 36CL

Produced in the terrestrial environment during nuclear reactions between energetic particles from the cosmic ray and the constituent atoms of the Earth's environment, long-lived cosmogenic radionuclides such as Beryllium-10 (Be-10, T-1/2 similar to 1.4 Ma), Aluminium-26 (Al-26, T-1/2 similar to 0.7 Ma) and Chlorine-36 (Cl-36, T-1/2 similar to 0.3 Ma) allow using the fraction produced in the atmosphere to study the intensity variations of parameters that control production (solar activity and geomagnetic field) and dating on temporal scales covering the last ten million years marine and continental sedimentary deposits in which they accumulate before decreasing. Fraction produced in the crust (in situ production) precisely measurable since only twenty years, allows measuring in specific mineral fractions accumulated amount that depends on both duration of exposure to cosmic radiation and the stability of the surface studied to quantify surface processes modeling the Earth's surface and dating different types of surface markers on the time range 0.1-5 Ma. To achieve these quantifications, a specific method able to separate the cosmogenic radionuclide of interest from its mass interfering, at least several million times more abundant, had to be developed. Based on the unambiguous characterization of both the mass number (N) and atomic number (Z), the technique of Accelerator Mass Spectrometry (AMS) applied on geosciences including reconstruct variations in the intensity of Earth's magnetic field over the past 1.3 million years, to date the fossil layer containing the skull of Sahelanthropus tchadensis (Toumai) and to clarify the chronology of the last deglaciation in different environmental contexts.