Hidden erosion on volcanic islands
Résumé
We have studied rivers from 3 volcanic tropical Islands (Guadeloupe and Martinique in Lesser Antilles and Reunion) in order to evaluate chemical weathering rates. These islands are constituted of very porous pyroclastic deposits, andesitic or basaltic lava flows, allowing significant water infiltration. From hydrological budgets, we estimate that infiltration represents 10% in Guadeloupe and 40% in Martinique. Element fluxes transported by subsurface water to the ocean have been quantified by coupling the hydrological budget with the chemical composition of the sampled subsurface waters. This is the first estimate of chemical weathering rates from volcanic islands, which takes into account the high temperature weathering contribution to the chemical weathering rates. Subsurface waters are impacted by high-temperature water–rock interactions and present therefore higher concentrations (4 to 100 times) of major and trace elements compare to surface waters, representing thus respectively 90%, 60% and 75% of the total (surface + subsurface) weathering dissolved flux to the ocean for Martinique, Guadeloupe and Reunion. This shows the importance of these fluxes, which until now were not estimated in the basalt weathering geochemical budgets. Chemical weathering rates from subsurface water are 2 to 5 time higher than the rates from surface waters. The total average chemical rates calculated are 290 t/km2/yr in Guadeloupe, 1080 t/km2/yr in Martinique and 270 t/km2/yr in Réunion. These values are among the highest worldwide. Extrapolating this budget to the surface of all volcanic islands world-wide, we estimate that these could contribute between 23% and 31% of the global flux of dissolved load transported to the oceans by major rivers draining silicate rocks whereas their surface area represent only 9% of the silicate rock total surface area