Changes in chemical and Sr-87/Sr-86 signature distribution patterns of suspended matter and bed sediments in the upper Loire river basin (France)

The mineralogy of the suspended particulate matter (SPM) transported by the upper Loire liver consists mainly of quartz and K-feldspar during periods of high river flow, with an increase in calcite concentration during period s of low flow. Concurrently, large fluctuations are observed in the levels of CaO and Fe2O3, the main oxides present in the SPM along with SiO2. The a nalysed trace elements also fluctuate significantly. The bed sediments (BS) have a similar oxide content to the SPM, but lower Zn and Pb levels and hi gher Zr levels. Fluctuations in the chemical-element concentrations with ri ver discharge are related to fluctuation in the mineral assemblages present in the SPM. Thus K, Ti and Rb levels increase with increasing in K-feldspa r abundance during high flow, and Ca and Sr levels decrease with increasing discharge due to a decrease in calcite abundance. These mineralogical and chemical variations relate to different sources of sediments under differen t flow conditions-sources that can be distinguished by Sr isotopic study. T he Sr isotopic composition fluctuates according to the rate of river discha rge i.e., the Sr-87/Sr-86 ratio increases with increasing discharge and rea ches a maximum Sr-87/Sr-86 ratio also increases with the combined increase in K-feldspar abundance and with peak flow, and vice versa. The decrease in calcite abundance. These similarities suggest the existence of at least tw o particulate-matter reservoirs, one with detrital silicates and the other with carbonates. Finally, the relationship between the Sr-87/Sr-86 ratios i n the suspended and dissolved loads suggests the coexistence of authigenic calcite in the carbonate reservoir. The SPM flow is related to a specific m echanical erosion rate over the whole Loire watershed ranging from 9 to 23 t km(-2) yr(-1). During the survey at Orleans, the SPM flow of the Loire ri ver was 37.10(4) t yr(-1), providing an estimate of the specific erosion ra te of around 9.8 t km(-2) yr(-1). From the shift in the SPM flux between th e sampling point and the mouth of the Loire river and the divergence in the mean annual discharge, a calculation was made of the input of solid matter by the tributaries and by supplementary erosion processes along the river. (C) 1999 Elsevier Science B.V, All rights reserved.