In the last decades, particular emphasis has been addressed to the study of mercury (Hg) from Earth degassing, in light of its high toxicity, long-range atmospheric transport and its tendency to bio-accumulate in aquatic ecosystems through methylation processes. All of these features have motivated intensive research on Hg within the framework of a pollutant of global concern. In order to better understand the role of Earth degassing in the global Hg budget, a number of field campaigns were carried out to evaluate the Hg/SO2 and Hg/CO2 ratios in volcanic/hydrothermal gases needed in estimating Hg fluxes in the atmosphere. Real-time measurements of SO2, H2S, H2O and CO2 were carried out in parallel with Hg collection within the emissions from open-conduit vents, fumaroles and soil diffuse degassing, supporting the role of early CO2 escape from magmas as a carrier gas for Hg. Concentrations of gaseous Hg elevated above background levels were observed on most occasions. Based on our dataset, we propose that an average Hg/SO2 plume mass ratio of about 7.8×10-6 (±1.5×10-6; nvolcanoes= 13) is best representative of open-conduit quiescent degassing. Taking into account the uncertainty in global SO2 emissions, we infer a global volcanic Hg flux from persistent degassing of about 76±30 t yr-1. These data suggest that open-conduit volcanoes in a state of passive degassing represent an important contribution to the global volcanic Hg emissions into the atmosphere. It is therefore likely that volcanic contributions to the global atmospheric Hg budget will be even more important during large eruptive events. Conversely, based on our dataset and previous works, we propose that an average GEM/CO2 molar ratio of ~2×10-8 is best representative of hydrothermal degassing. Taking into account the uncertainty in global hydrothermal CO2 emissions from sub-aerial environments (~1012 Mol yr-1; Seward & Kerrick, 1996), we infer a global volcanic Hg flux from hydrothermal environments of ~ about 8.5 t yr-1. Our calculations here suggest that hydrothermal contribution to the global volcanic non-eruptive Hg is small if compared to persistently degassing open-vent volcanoes, which dominate the global volcanic Hg budget. This minor contribution mainly reflects both different CO2 degassing rates and the wall-rock-gas interactions which may support Hg deposition in the hydrothermal envelope, thus limiting its release into the atmosphere. Finally, while Hg contribution from sub-aerial volcanism is now more or less constrained, recent evidences have indicated the influence of submarine geothermal activity in controlling the dispersion of Hg as well. This makes the study on submarine hydrothermal activity among the most challenging and significant scientific advances of the 21th century in terms of Hg research as a global pollutant.
The role of Earth degassing in the global atmospheric mercury budget
Bagnato E;Sprovieri M;Barra M;
2015
Abstract
In the last decades, particular emphasis has been addressed to the study of mercury (Hg) from Earth degassing, in light of its high toxicity, long-range atmospheric transport and its tendency to bio-accumulate in aquatic ecosystems through methylation processes. All of these features have motivated intensive research on Hg within the framework of a pollutant of global concern. In order to better understand the role of Earth degassing in the global Hg budget, a number of field campaigns were carried out to evaluate the Hg/SO2 and Hg/CO2 ratios in volcanic/hydrothermal gases needed in estimating Hg fluxes in the atmosphere. Real-time measurements of SO2, H2S, H2O and CO2 were carried out in parallel with Hg collection within the emissions from open-conduit vents, fumaroles and soil diffuse degassing, supporting the role of early CO2 escape from magmas as a carrier gas for Hg. Concentrations of gaseous Hg elevated above background levels were observed on most occasions. Based on our dataset, we propose that an average Hg/SO2 plume mass ratio of about 7.8×10-6 (±1.5×10-6; nvolcanoes= 13) is best representative of open-conduit quiescent degassing. Taking into account the uncertainty in global SO2 emissions, we infer a global volcanic Hg flux from persistent degassing of about 76±30 t yr-1. These data suggest that open-conduit volcanoes in a state of passive degassing represent an important contribution to the global volcanic Hg emissions into the atmosphere. It is therefore likely that volcanic contributions to the global atmospheric Hg budget will be even more important during large eruptive events. Conversely, based on our dataset and previous works, we propose that an average GEM/CO2 molar ratio of ~2×10-8 is best representative of hydrothermal degassing. Taking into account the uncertainty in global hydrothermal CO2 emissions from sub-aerial environments (~1012 Mol yr-1; Seward & Kerrick, 1996), we infer a global volcanic Hg flux from hydrothermal environments of ~ about 8.5 t yr-1. Our calculations here suggest that hydrothermal contribution to the global volcanic non-eruptive Hg is small if compared to persistently degassing open-vent volcanoes, which dominate the global volcanic Hg budget. This minor contribution mainly reflects both different CO2 degassing rates and the wall-rock-gas interactions which may support Hg deposition in the hydrothermal envelope, thus limiting its release into the atmosphere. Finally, while Hg contribution from sub-aerial volcanism is now more or less constrained, recent evidences have indicated the influence of submarine geothermal activity in controlling the dispersion of Hg as well. This makes the study on submarine hydrothermal activity among the most challenging and significant scientific advances of the 21th century in terms of Hg research as a global pollutant.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
