PATTERNS OF ELEMENTAL TRANSPORT IN THE BEDLOAD OF THE MEURTHE RIVER (NE FRANCE)

Meurthe (NE France) is a short river which drains the gneisses of the Variscan Vosges mountains, and the carbonates, clays and sandstones from their sedimentary piedmont. A systematic geochemical investigation of major and trace elements on three sand size fractions from the river and its tributaries shows the tight control of the bedrock geology on the geochemistry of the bedload. The downstream trend of decreasing Al2O3/SiO2 - commonly observed in the bedload of major rivers in the form of increasing quartz abundance - is shown to reflect erosion of lowland quartz-rich detrital rocks and not progressive feldspar attrition or dissolution. It is shown that, although short-range geochemical consistency is preserved for hydrodynamically equivalent fractions, non-conservative mass-balance relationships between the bedloads of tributaries require both some dissolution of the bedload and contribution from runoff sources of sands. The geochemistry of the least mobile elements in fine-grained fractions shows that mobility during weathering increases in the order REE(3+) < Eu < TiO2 < (Fe, Sc, V, Cr). The alkali-Ti-REE relationships show that weathering affects a phase with little Eu anomaly. Alkali feldspars resist rather well while, upon weathering of minerals such as amphibole, biotite and sphene, Fe-oxyhydroxide suspensions are released entraining some of the REE. The residual fraction shows a pronounced negative Eu anomaly whereas the suspended load is known to have a positive anomaly. The regular deepening of the Eu negative anomaly with increasing REE(3+) contents supports an incremental dissolution-precipitation process of weathering. Fifty to eighty percent of the initial fine fraction material may have been removed into the dissolved and suspended loads before incorporation of the residue into the bedload.