Strontium isotopic record of signatures of Holocene fluvial sediments in the Loire valley, France

Abstract. The distribution of Sr contents and isotopes of strontium Sr in the labile fraction of sediments from a channel fillings infill in the Middle Loire alluvial plain is used to highlight aspects of the evolution of the basin over the period from 10 000 yr. BP to the present. The acid extractable matter (AEM) in the sediment samples is variable in amount and in trace element contents. Carbonates and Fe–Mn oxyhydroxides are the principal carrying phases in the labile fraction. The historical record of ⁸⁷Sr/⁸⁶Sr ratio variations in AEM shows two components. In the first, the AEM is consistently low (around 10%) and the ⁸⁷Sr/⁸⁶Sr ratios range from around 0.7086 to 0.710. This corresponds to the hydrous Fe-Mn oxides endmember. Fe–Mn oxyhydroxides originate from the Massif Central and their ⁸⁷Sr/⁸⁶Sr ratios display a larger range than present day Fe–Mn oxides. Furthermore, the mixing proportions between Fe–Mn oxides generated on granitoids and on basalts fluctuated only weakly in the past. The second component has greater AEM (20–50%) and the ⁸⁷Sr/⁸⁶Sr ratio is almost constant at 0.709–0.7095. This corresponds to a carbonate endmember, clearly different from present day carbonates and may reflect the influence of both authigenic and detrital origins. The low ⁸⁷Sr/⁸⁶Sr ratio suggests a dominantly authigenic source for the carbonate endmember; this may be linked to groundwater inputs, although a minor influence from a secondary detrital source cannot be discounted. This reflects a different functioning of the river between deposition in the channel and the present day.

Keywords: River Loire, Holocene, fluvial dynamics, geochemistry, strontium isotopes