Degassing of CO2 and CH4 to the atmosphere and oceans is the process whereby carbon is restored to the surficial system after being buried in rocks. Carbon dioxide is released by a variety of processes. This includes volcanic emissions from the mantle and metamorphic and diagenetic decarbonation of limestones and organic matter. Volcanic degassing can occur over subduction zones, at mid-ocean rises, on the continents, and in the interior of oceanic plates. Degassing can be sudden and violent, as during volcanic eruptions, or slow and semi-continuous in the form of fumaroles, springs, gas vents, and continually degassing volcanic vents. An outstanding example of the latter is Mt. Etna, which contributes about 10% to total global degassing (Caldeira and Rampino, 1992). Metamorphic degassing is concentrated in zones of seafloor subduction (Barnes et al., 1978), crustal convergence (Kerrick and Caldeira, 1998), and crustal extension (Kerrick et al., 1995). Most methane degassing on a geologic time scale occurs from organic matter diagenesis slowly from coal, oil, and kerogen maturation and suddenly from methane hydrate breakdown. A smaller amount of CH4 emanates from mid-ocean hydrothermal vents. Estimates of present-day global volcanic degassing rates are under constant revision (e.g., see Gerlach, 1991; Brantley and Koepenick, 1995; Sano and Williams, 1996; Marty and Tolstikhin, 1998; Kerrick, 2001). A compilation of recent estimated rates of most degassing processes is shown in table 4.1. A constraint on estimates is that none can exceed total global degassing. The latter can be determined from the steady-state assumption that CO2 release by global degassing must be balanced by global uptake by Ca and Mg silicate weathering (Berner, 1990; Berner and Caldeira, 1997). (This assumes essential balance of the organic C subcycle.) Global Ca and Mg silicate weathering, based on river fluxes of these elements to the sea, has been estimated to be about 6 ± 3 × 1018 mol/my (Berner, 1990). Gaillardet et al. (1999) estimate a minimum value for Ca and Mg silicate weathering of 3.6 × 1018 mol/my.
Quantifying tropospheric volcanic emissions with AIRS: The 2002 eruption of Mt. Etna (Italy)