Controls on chemical weathering on a mountainous volcanic tropical island: Guadeloupe (French West Indies),

International audienceGuadeloupe Island is a natural laboratory, ideally suited to the study of biogeochemical processes in tropical and mountainous volcanic environments. The island’s east–west rainfall gradient (1200–8000 mm/yr) is superimposed on a north–south age gradient (2.7 Ma to present), providing a unique opportunity to investigate the influence of rainfall and rock age on the chemical weathering of volcanic terrains. Taking advantage of this configuration, we present the first temporal survey (2007–2013) of the geochemical composition of the dissolved load of rain and river waters in Guadeloupe. Our data demonstrate that the chemical composition of river water is influenced by rainfall abundance, hydrothermal alteration (from active or fossilized volcanic systems) and interactions between water and minerals during chemical weathering processes. The contribution of rain to the overall chemical balance is especially significant in the older northern part of the island, where the ferralitic soils are base-cation-depleted. Between 15% and 65% of the Ca or Mg riverine budgets comes from atmospheric deposits, highlighting the major role of rainfall in the geochemical budgets of small tropical and mountainous watersheds. The river water dataset indicates that different chemical weathering processes dominate the budget depending on the age of the local bedrock. In the younger, southern part of the island, a pool of easily-weatherable andesitic minerals from the bedrock dominates. The contribution from this pool decreases significantly (to 5–15 wt.% of the bulk soil) towards the older terrains in the north. The northern rivers are characterized by low Ca/Mg ratios (0.5–1.0), intermediate between those of fresh rocks (1.7–3.3) and soil (0.1). Weathering in the northern part of the island is therefore dominated by the dissolution of depleted secondary minerals into soils. The Ca/Mg ratio of the river water increases from north to south, eventually reaching values similar to those of the bedrocks, arguing for congruent dissolution of the youngest volcanic rocks. The magnesium isotopic composition of river water (δ26Mg) reflects inputs from both rain and weathering processes. In southern and central rivers, the Mg isotopic value of waters after correction for rain inputs (δ26Mgwea) is systematically depleted in heavy isotopes (mean value of −0.34‰) relative to that of the bedrock (−0.24‰ to −0.15‰). In the north, the δ26Mgwea of the river water (−0.09‰) is heavier than that of the andesitic bedrock, possibly reflecting the dissolution of 26Mg-rich secondary minerals (ferralitic soil measured around +0.13‰). The robustness of δ26Mg and Ca/Mg as proxies of the degree of soil weathering should be investigated further via more detailed sampling campaigns in the future. By combining high-frequency monitoring of river discharge with measurements of concentration–discharge relationships for a wide range of chemical elements, we estimate the mean annual chemical weathering fluxes for three rivers that belong to the OBSERA critical zone observatory. Fluxes vary from 51.7 to 91.8 t/km2/yr north to south along the bedrock age gradient, and are among the highest recorded in volcanic tropical regions. Flash floods can explain 21–31% of the annual chemical weathering fluxes. The results highlight the importance of monitoring rivers over periods of several years in order to obtain accurate estimates of chemical exports in tropical and mountainous environments