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Seventh Annual V. M. Goldschmidt Conference 2288.pdf Coupling modern chemical and mechanical weathering rates of silicates using rivers. J. Gaillardet , P. Louvat, B. Dupré§ and C. J. Allègre Laboratoire de Géochimie et Cosmochimie, Institut de Physique du Globe de Paris - URA CNRS D1758 - UER des Sciences de la Terre Université Paris VII Denis Diderot, 4 Place Jussieu, 75252 Paris cedex 05, France §Laboratoire de Géochimie, CNRS OMP, Université Paul Sabatier, 38 rue des 36 ponts, Toulouse France. On a global scale, the control of modern weathering rates and hence of CO2 consumption by rock weathering, is a subject of great debate. Among the factors that may exert a role in determining the way and the rate whereby continental material is transferred from land to sea, temperature and precipitation (hence climate) and relief (hence tectonics) have been suspected as being key parameters. Whether climate or tectonic play the major role is still debated (Berner, 1996; Edmond, 1996). However, the role of physical erosion, by creating surfaces and hence preparing chemical weathering has attracted little attention. In addition to experimental results, field data are clearly needed to address this issue. Rivers, and especially large rivers, as they integrate large portions of the continental crust are particulary well suited to identify the parameters controlling the modern weathering rates. Their dissolved and suspended loads give insights into the chemical and mechanical weathering processes respectively. Although the number of studies focusing on river chemistry is increasing, these studies are still focused on the chemical load rather than on an integrated study of suspended and dissolved load and, thus, the relationships between chemical and mechanical denudation. We have made an effort to fill in this gap, by systematically associating the sampling of dissolved and suspended loads in large basins such as the Congo, Niger, Amazon, Mackenzie, S. Lawrence, Huanghe, Yangtze, Red river, Mekong river systems and in volcanic islands under variable climatic conditions, such as the Reunion, Azores, Iceland and Java. The chemical composition of river sediments in these settings together with the scarce data from the litterature, show that modern products of physical erosion are characterized by a huge depletion in all the most soluble elements compared to the bed rocks from which they are derived. The less depleted sediments are those from volcanic islands. On a world scale, the contrast between lowlands rivers (with sediments strongly depleted in solutes) and mountaineous or volcanic island rivers (thar are much less depleted) is evident. The influence of climate on the intensity of solute solubilisation is therefore not obvious. Extending a formalism previously described, the dissolved load of each river can be used to derive chemical weathering rates for silicates. Again, on a global scale, the influence of climatic factors is secondary and the lithology appears to be the predominant factor. Finally, the complementarity between dissolved loads (once corrected from non-silicate weathering inputs) and suspended loads enables to calculate a mechanical weathering rate for each river. These rates are calculated according to a steady-state hypothesis and are independant from field estimations of mechanical denudation. Such estimations suffer from numerous problems such as for example the deposition of suspended sediments within the basins. A correlation between the so-calculated mechanical erosion rates and the chemical rates of silicate weathering is observed in a log-log space, tending to show the physical degradation of continental rock has a major role on a global scale on chemical weathering and hence CO2 consumption.