HR: 1340h
AN: H43D-0398    [Abstracts]
TI: The Role of Plants in the 238U-234U Disequilibria of Stream Waters: The Example of the Strengbach Watershed (Vosges, France)
AU: Pierret, M
EM: pierret@illite.u-strasbg.fr
AF: CGS-EOST Strasbourg, 1 rue Blessig, Strasbourg, 67084 France
AU: * Chbaaux, F
EM: fchabaux@illite.u-strasbg.fr
AF: CGS-EOST Strasbourg, 1 rue Blessig, Strasbourg, 67084 France
AB: Recent TIMS or MC ICPMS analyses of U disequilibria in the dissolved load of stream and river waters have confirmed the potential of the U activity ratio in river waters as a specific tracer of chemical fluxes coming from rocks and soils (e.g., 1). These precise measurements have also outlined that occurrence of U activity ratios lower than one in dissolved load of river waters is not exceptional, especially at the scale of small watersheds. Such U values pose in turn the question of the real mechanisms controlling the supply of 234U-238U isotopes to the freshwaters. In order to address this question U activity ratios, Sr isotope ratios and the concentrations of major and some trace elements were analyzed in the different compartments of a small granitic watershed: the Strengbach environmental observatory (Vosges, France) ( http://ohge.u-strasbg.fr). In addition of the different streamwaters draining this watershed, the main horizons of weathering profiles, the associated soil solutions and the main tree species growing around were analyzed. The data confirm that the Strengbach stream water samples have generally U activity ratios lower than one and point out that soils solutions display a similar range of U and Sr variations. By contrast, tree and plant samples define different trends of variation in a plot of U activity ratios against Sr isotope ratios with, above all, U activity ratios systematically greater than one. These data show that trees and plants collected during this work cannot directly pomp their nutriments from soils and soil solutions sampled here, i.e. gravity solutions. They also suggest that, in this watershed, the plants, especially trees, play a central role in the weathering processes of rocks and minerals, and control a large part of the geochemical signature of the water samples collected on this watershed, including their U activity ratios. 1Riotte J and Chabaux F. (1999) GCA 63, 1263-1275.
DE: 1655 Water cycles (1836)
DE: 1045 Low-temperature geochemistry
DE: 1615 Biogeochemical processes (4805)
DE: 1625 Geomorphology and weathering (1824, 1886)
DE: 0330 Geochemical cycles
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting