The silent degassing of volcanoes: a useful tool for volcanic surveillance and a significant contributor to the global CO2 emission from subaerial volcanism

Volcanoes emit significant amounts of gases into the atmosphere through visible and non-visual degassing manifestations regardless of whether volcanoes are active or quiescent. The latter, also known as diffuse or silent degassing, occurs across the entire volcanic building. Water vapour (H₂O), carbon dioxide (CO₂), and sulfur (S) are the three most abundant magmatic volatiles, with CO₂ being the least soluble in silicate melts. Diffuse volcanic degassing alters the chemical composition of volcanes' ground/soil gas atmosphere, resulting in enrichment of CO₂, He, and other gas species. Over the last 25 years, extensive research on diffuse CO₂ degassing has been conducted at volcanic and geothermal systems, indicating that silent CO₂ degassing is an important mechanism for dissipating energy at volcanoes and contributes significantly to global CO₂ emissions from subaerial volcanism. As a result, diffuse CO₂ degassing studies have been regarded as a powerful tool in geochemical monitoring programs for volcanic surveillance, particularly in volcanic areas lacking visible gas manifestations (plume, fumaroles, hot springs, etc.), a valuable tool for identifying productive geothermal reservoirs, and a potential source of large amounts of CO₂ to the atmosphere via gobal subaerial volcanism.

Diffuse degassing investigations on volcanoes involve primarily in-situ ground CO₂ efflux measurements and the collecting of gases at a certain depth for later chemical and isotopic analysis. CO₂ and He are the two most interesting gas species to investigate in diffuse degassing studies due to their similar low solubility in silicate melts at low pressures and suitability as geochemical tracers of magmatic activity. However, once exsolved from the silicate melts, their journey through the crust to the surface is considerably different. While CO₂, as a reactive gas, is influenced by interfering processes (gas scrubbing by groundwaters and interaction with rocks, decarbonatation processes, biogenic production, and so on), He is chemically inert, radioactively stable, non-biogenic, highly mobile, and relatively insoluble in water. These properties minimize the interaction of this noble gas with the surrounding rocks or fluids during its ascent towards the surface. Their geochemical differences yield higher relative He/CO₂ ratio in the fumarole gases than is actually present in the magma, but it decreases when the magma reservoir reaches enough pressure to generate incipient fracture systems approaching the eruption, thus releasing considerably more of the magma volatiles.

Quantifying global volcanic CO₂ emissions from subaerial volcanism is critical for gaining a better knowledge of the rates and mechanisms of carbon cycling, as well as their effects on the long-term development of Earth's climate across geological timescales. Recent studies show that diffuse degassing contributes 47 to 174 Tg·y^-1 to the atmosphere, although our understanding of the global diffuse CO₂ degassing from subaerial volcanism could be larger.

Several examples of diffuse degassing research on many different volcanic systems around the world performed by our research team and collaborators during the last 25 years will be presented during my award/medal lecture, strongly supporting that diffuse degassing is a useful tool for volcanic surveillance and a significant contributor to the global CO₂ emissions from subaerial volcanism.