Chemical weathering and erosion rates in Lesser Antilles: an overview in Guadeloupe, Martinique and Dominique
Résumé
Lesser Antilles are located in a tropical climate with high temperatures, high precipitations, very dense vegetation, sharp relief and very thick soils. It is characterized by a uniform andesitic lithology and presents an East-West variation in precipitations and hence in runoff. Moreover, the implantation of agriculture induces important land use changes which replace existing native forest cover with banana and sugar cane plantations. Guadeloupe, Martinique and Dominique Islands alike numerous tropical environment present therefore extreme weathering regimes. Physical denudation is mainly controlled by landside, especially where pyroclastic formations are thick. It is discontinuous and proceeds by pulses during tropical storms. This reflects the torrential dynamics of the rivers. For Guadeloupe, the mechanical weathering rates are 800-4000 t/km2/yr. As for many volcanic islands erodible lithology such as pyroclastic flows with ashes or even massif lava flows involve important material transported during the erosion processes. The lithology is also very porous with high infiltration rates, which induces that most of the elements fluxes are produced on subsurface as the chemical erosion rates are 2 to 5 time higher than the rates from surface water (Rad et al., 2007). We show how kinetic of chemical weathering rates depends on the age of the lava and subsurface circulation with local hydrothermal springs produce by the acidity of volcanoes, which highly increases chemical weathering rates. These islands present some of the highest surface chemical denudation rates that can reach 1290 t/km2/yr. We show that among the combined impact of all parameters (climate, runoff, slopes, vegetation...), the basins age seems to be the control parameter on chemical weathering and land use: the younger the basin, the higher the weathering rate. We could observe a combined effect between the higher erodibility and a higher climate erosivity of the younger reliefs.