Abstract :
The chemical composition of bed sediments in the Loire estuary, and their evolution between landward and seaward have been investigated. The concentrations of five major oxides (SiO2, K2O, CaO, TiO2and Fe2O3) and nine trace elements (Rb, Sr, Zr, Zn, Pb, Sn, La, Ce and Ba) have been measured. Factors that govern element contents in bed sediments were examined by a combined descriptive (using indicator elements) and statistical (principal component analysis) approach.
Using descriptive tools, the results show that Fe2O3, Zn, Pb and Sn are significantly enriched down estuary referred to a fluviatile normalization, whereas K2O, Rb, Zr and Ba are depleted down estuary. SiO2, TiO2, CaO, Sr, La and Ce behave conservatively over the estuary profile.
Principal component analysis was performed for the 13 X/Fe ratios (X being Si, K, Ca, Ti, Rb, Sr, Zr, Zn, Pb, Sn, La, Ce and Ba). The variability of the data is explained by at least four components. Two components explain nine chemical elements; the first end-member is represented by the fluviatile particles, and the second one is represented by the particles producedin situin the estuary area. Two other components are required to explain the variability of Zn and Sn on one hand, and Pb on the other hand. These components can be related to removal (adsorption, co-precipitation, flocculation) onto clay minerals, Fe oxyhydroxides and humic acid complexes.
Finally, a binary mixing model using rare earth elements (REE) and iron between the fluviatile particles and the particles producedin situin the estuary area is developed. The results of the mixing model computation show that half of the particles exported by the Loire estuary system are of fluviatile origin.
