Riverine erosion rates on Sao Miguel volcanic island, Azores archipelago

Abstract

We present here a coupled study of the dissolved and suspended loads of the rivers draining the basaltic island of Sao Miguel, in order to determine both chemical and mechanical erosion rates. Dissolved elemental concentrations of the river waters are corrected for rain inputs (mainly oceanic salts) and for geothermal water inflows. They provide specific chemical denudation rates of 26–50 t/km 2/year and associated atmospheric CO 2 consumption rates of 0.26–0.87×10 6 mol/km 2/year. Suspended loads have chemical compositions similar to those of their parent rocks with slight depletions in alkaline and alkaline earths elements, which reflect their partial dissolution during rock weathering. Using the steady-state model of erosion, which assumes a relationship between chemical and mechanical erosion rates, we can establish mass budgets between the initially unaltered basaltic rocks and the dissolved and suspended erosion products in the river waters. Thereby, we calculate mechanical erosion rates of 170–500 t/km 2/year. These chemical and mechanical erosion rates are very high compared to the global average, and emphasize the ease of erosion of basaltic terrains. A comparison with the erosion rates that have been previously determined for the rivers of Réunion island highlights the importance of relief, runoff and tectonics in the erosion processes.