Sulfur Degassing From Steam-Heated Crater Lakes: El Chichón (Chiapas, Mexico) and Víti (Iceland)

The composition of the gases released by El Chichón (Chiapas, Mexico) and Víti (Askja volcano,
Iceland) volcanic lakes is examined by Multi-GAS for the first time. Our results demonstrate that H2S and
SO2 are degassed by these pH 2–3 lakes. We find higher CO2/H2S and H2/H2S ratios in the lakes’ emissions
(31–5,685 and 0.6–35, respectively) than in the fumarolic gases feeding the lakes (13–33 and 0.08–0.5,
respectively), evidencing that only a fraction (0.2–5.4% at El Chichón) of the H2S(g) contributed by the
subaquatic fumaroles ultimately reaches the atmosphere. At El Chichón, we estimate a H2S output from the
crater lake of 0.02–0.06 t/day. Curiously, SO2 is also detected at trace levels in the gases released from
both lakes (0.003–0.3 ppmv).We propose that H2S supplied into the lakes initiates a series of complex oxidation reactions, having sulfite as an intermediate product, and ultimately leading to SO2 production and degassing.