Runoff control of soil degassing at an active volcano. The case of Piton de la Fournaise, Réunion Island

Airborne atmospheric and in-situ soil gas analyses demonstrate that distal gas leaks are non-existent at Piton de la Fournaise (PF) despite frequent eruptions. Airborne traverses made at a constant altitude indicated that no detectable large-scale CO2 degassing occurs from the volcano. In-situ soil gas traverses (CO2, 4He, 222Rn) across the active rift-zones, the summit crater and the caldera rims, show both a lack of positive anomalies and evidence of strong depleted concentrations for He and CO2 with respect to air. Depletions in soil He is explained by intense rain water infiltration. A more unexpected result lies in the very low CO2 concentrations (down to 92 ppm) in bare soils in the summit region. Such a depletion was not previously reported and is attributed to considerable basalt weathering operating as the result of huge runoff. A model has enabled both the runoff and the CO2 consumption occurring within the basaltic layers to be calculated. Data obtained are consistent with values expected from large-scale erosion models and with the local calculated diffusive flux of CO2 in soils. The huge runoff, together with the basaltic lithology of the volcano, are found to be the main factor controlling the soil gas signatures at PF.