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The composition of the gases released by El Chichon (Chiapas, Mexico) and Viti (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 H-2/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 Chichon) of the H2S(g) contributed by the subaquatic fumaroles ultimately reaches the atmosphere. At El Chichon, 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.

Plain Language Summary Volcanic lakes are the site of some of the most unpredictable, and therefore dangerous, volcanic eruptions in nature. Their activity is driven by a feeding volcanic gas phase supplied by the underlying hydrothermal/magmatic system. These volatile species, entering the lake bottom, are absorbed into lake water at different rates/degrees depending on their water solubilities and the lake physical and chemical characteristics. Hyperacidic crater lakes (pH < 1) are degassing SO2, a gas that was earlier believed to be totally dissolved into the water. In this study, we investigate for the first time the presence of reactive S gases (SO2 and H2S) in the plumes of less acidic (pH 2-3) lakes El Chichon (Mexico) and Viti (Iceland). Our results demonstrate that H2S, coming from the sublimnic hydrothermal systems is only partially dissolved and oxidized by the lake water. In addition, we discover trace amount of SO2 coming off both lakes. We propose that SO2 is produced into the lake by H2S oxidation, with dissolved sulfite as an intermediate product. Our results thus open new piece of knowledge to our understanding and monitoring the activity of restless volcanic lakes.