The role of trace minerals in chemical weathering in a high-elevation granitic watershed (Estibere, France):: Chemical and mineralogical evidence

We investigated chemical weathering in a high elevation granitic environment in three selected watersheds located in the Pyrenees (France). The sites were located on glacial deposits derived from similar Hercynian (similar to300 Ma) granites characterized by the occurrence of zoned plagioclases and trace calcic phases (epidote, prehnite, spheric, apatite). The surface waters at those sites show high Ca/Na molar ratios (> 1) which could not be explained by the dissolution of the major plagioclase (oligoclase) present in the rocks. The coupled approach of investigating stream water chemistry and the mineralogy and chemistry of rocks and soils allowed us to explore the role of the weathering of trace calcic minerals in calcium export at the watershed scale. The weathering of the trace calcic minerals which represent similar to 1% of the total rock volume are responsible for more than 90% of the calcium export at the sites. Annual cationic fluxes(similar to 23.10(4) eq/km(2)/yr) calculated for the Estibere watershed are among the highest reported for high elevation systems draining granitic rocks and similar to 80% of this annual cationic flux can be attributed to the weathering of trace calcic phases. Calculations based on isotopic values (Sr-87/Sr-86) go in the same direction. Except apatite, the trace calcic phases appear to be mainly silicates, thus the type of chemical weathering observed in the Estibere watershed may have an influence on atmospheric CO2 consumption by granite weathering. However, comparison with other watersheds draining granitic environments worldwide, and with the two other sites in the Pyrenees, indicate that the role of trace calcic phases is important in most young environments exposed to chemical weathering (e.g., high elevation catchments on glacial deposits). Other factors Such as the date of glacial retreat, the physical denudation rate, the hydrological functioning of the watershed and the nature and structure of the soil cover are also important. Copyright (C) 2004 Elsevier Ltd.