ESR as a method for the characterization of alluvial sediments
Résumé
The Electron Spin Resonance is now commonly used as a dating method for quartz grains extracted from alluvial sediments of ages ranging from one hundred thousand years to several million years. In the Loire valley area, the ESR method has been successfully applied on alluvial deposits in order to establish the chronological framework for the Loire fluvial system evolution during the Quaternary period. Several parameters are involved in the ESR dating of optically bleached quartz grains. They are mainly the radionuclides content of the surrounding sediment estimated for the dose rate determination; the absolute intensity of the paramagnetic centers Al and Ti (most commonly used); and, regarding Al paramagnetic center, the ratio between the optically bleachable aluminum traps (OBAT) and the "Deep" aluminum traps, or non-optically bleachable aluminum traps (DAT). These parameters have been also considered in this study in order to characterize the alluvial sediments. As a conclusion we observed that the U,Th and K contents, as well as the OBAT/DAT ratio plotted as a function of the age of the sample, gave information about the heterogeneity of the sediment in terms of quartz grain sources. It was then possible to distinguish alluvial units containing quartz grains originating from a local parent rock and deposited by a small river from heterogeneous alluvia deposited by a large river, as the Loire River, after cutting various bedrocks. Moreover we observed that the heterogeneity in the quartz source has a visible incidence on the correlation coefficient of the dose response curves. In order to increase the accuracy of the ESR ages, the number of samples analyzed and the number of measurements of each sample should be adapted to the heterogeneity of the alluvial unit from which they are extracted.