The global CO 2 discharge from subaerial volcanism has been estimated at ~300 Mt yr -1 . However, estimates of CO 2 emissions from volcanic lakes have not been considered. In order to improve this information, extensive research on CO 2 emissions of volcanic lakes worldwide has been performed. The observed normalized average CO 2 emission rates increase from alkaline (5.5 t km -2 d -1 ), to neutral (201.2 t km -2 d -1 ), to acid (614.2 t km -2 d -1 ) in volcanic lakes. Taking into account (1) normalized CO 2 emission rates, (2) the number of volcanic lakes in the world (~769), and (3) the fraction and average areas of the investigated alkaline, neutral, and acid volcanic lakes, the estimated global CO 2 emission from volcanic lakes is 117 ± 19 Mt yr -1 , with 94 ± 17 Mt yr -1 as deep-seated CO 2 . This study highlights the importance of a revision of the actual global CO 2 discharge from subaerial volcanism. Printed in USAFigure 4. Probability plot of normalized CO 2 emission rates from studied volcanic lakes. P-population.on June 3, 2015 geology.gsapubs.org Downloaded from

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On October 12, 2011, a submarine eruption began 2 km off the coast of La Restinga, south of El Hierro Island. CO2 and H2S 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 CO2 and H2S effluxes showed precursory signals that preceded the submarine eruption. Beginning in late August, the CO2 efflux time series started increasing at a relatively constant rate over one month, reaching a maximum of 19 gm−2d−1 one week before the onset of the submarine volcanic eruption. The H2S efflux time series at HIE07 showed a pulse in H2S 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 H2S efflux recorded during the observation period. Since CO2 and H2S 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 CO2 and H2S 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.

Volcanic eruptions are commonly subdivided into effusive and explosive activity depending on conduit dynamics that can either result in the outflow of lava when the degassed magma reaches the surface (effusive eruptions), or in the fragmentation of magma at various depths (dry explosive eruptions, from Hawaiian to Ultra-Plinian, and wet explosive eruptions from Surtseyan to phreato-Plinian in the classification of Walker ( 1980)). Nonetheless, some