Precursory diffuse CO₂ and H₂S emission signatures of the 2011–2012 El Hierro submarine eruption, Canary Islands

Abstract

On October 12, 2011, a submarine eruption began 2 km off the coast of La Restinga, south of El Hierro Island. CO 2 and H 2 S soil efflux were continuously measured during the period of volcanic unrest by using the accumulation chamber method at two different geochemical stations, HIE01 and HIE07. Recorded CO 2 and H 2 S effluxes showed precursory signals that preceded the submarine eruption. Beginning in late August, the CO 2 efflux time series started increasing at a relatively constant rate over one month, reaching a maximum of 19 gm −2 d −1 one week before the onset of the submarine volcanic eruption. The H 2 S efflux time series at HIE07 showed a pulse in H 2 S emission just one day before the initiation of the submarine eruption, reaching peak values of 42 mg m −2 d −1 , 10 times the average H 2 S efflux recorded during the observation period. Since CO 2 and H 2 S effluxes are strongly influenced by external factors, we applied a multiple regression analysis to remove their contribution. A statistical analysis showed that the long‐term trend of the filtered data is well correlated with the seismic energy. We find that these geochemical stations are important monitoring sites for evaluating the volcanic activity of El Hierro and that they demonstrate the potential of applying continuous monitoring of soil CO 2 and H 2 S efflux to improve and optimize the detection of early warning signals of future volcanic unrest episodes at El Hierro. Continuous diffuse degassing studies would likely prove useful for monitoring other volcanoes during unrest episodes.