Chemical weathering and erosion rates in Lesser Antilles: an overview in Guadeloupe, Martinique and Dominica
Résumé
Guadeloupe, Martinique and Dominica islands, alike numerous tropical environments, present extreme weathering regimes. Physical denudation is mainly controlled by landslide, which reflects the torrential dynamics of the rivers. In Guadeloupe, the mechanical weathering rates are comprised between 800 and 4000 t/km2/yr. The lithology is very porous with high infiltration rates, which induces that most of the elements fluxes are produced on subsurface with chemical erosion rates 2 to 5 times higher than the rates from surface water. We show how kinetic of chemical weathering rates depends on the age of the lava and subsurface circulation. In addition, timescale of erosion have been calculated from U-series analyses sediments from rivers. Our results show a large range: from 0 to 150 ka in Martinique and from 0 to 60 ka in Guadeloupe. From analyses of the dissolved loads, we propose to evaluate residence times in the river water. It appears that water circulation is globally 3 times longer for subsurface water than for surficial water (Rad et al. 2011). Moreover these islands are highly impacted by agriculture, however our result show that human activity brings no disturbance on Critical zone processes contrary to what one might think. Indeed, 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 rates. We could observe a combined effect between the higher erodibility and a higher climate erosivity of the younger reliefs.