HR: 0800h
AN: U21B-0425    [Abstracts]
TI: Geochemical Potamology
AU: * GAILLARDET, J
EM: gaillardet@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4 Place Jussieu, PARIS, 75252, France
AB: The surface of our planet has been reworked by erosion and weathering processes throughout its geological history. Erosion processes are so fast that only 10's of millions years are necessary to remove major reliefs formed by plate tectonics. The material of the surface of the Earth has been recycled many times and the only visible remains of the ancient continental crust are in the sediments, in the atmosphere, or dissolved in the ocean. The chemical action of water on rocks and the associated physical gravity-driven processes are the key features of the « weathering engine ». Rivers are messengers from the surface of the Earth and integrate weathering processes at various scales of time and space. They provide a unique opportunity to have average information about the chemical interactions between surface waters and rocks, about their kinetics and about the major geochemical fractionations created by this major geological process. The study of rivers has been focused on small- and large-scale approaches, both giving different information. Although the sampling of small river basin allows us to simplify nature and separate the potential controlling parameters by fixing them (climate, vegetation, type of rock, slopes…), the study of large rivers give global information about the composition of sediments, oceanic budgets, the significance of isotopic proxies. A geochemical potamology has developed over the last decades and we now more fully understand the critical questions that have to be solved. Numerous isotopic and elemental proxies have been developed to shed light on weathering processes, deciphering their control and calculate rates and timescales of erosion based on river products. A coupling between physical and chemical processes of erosion is now clear at different scales but probably the most important factor in controlling weathering style and rates are the protons. Low pH solutions are much more aggressive than neutral waters, as confirmed by experimental studies. On Earth there are a few proton donors (acids) that can contribute to feed the weathering engine : carbonic acid (the form of dissolution of atmospheric CO2), sulfuric acid , nitric and chlorhydric acids. Only carbonic and sulfuric acids have climatic implications by changing the long-term composition of the atmosphere and of the ocean. Because plants essentially control the production of CO2, the influence of ecosystem dynamics on chemical weathering is probably very important. In parallel, an international network of drainage basin is developing to select a panel of "node" river sites on which isotopic and geochemical tools will be applied. In a changing world, rivers are acting as sensors of environmental changes and their long-term monitoring appears to be essential.
DE: 3617 Alteration and weathering processes (1039)
SC: Union [U]
MN: 2007 Fall Meeting