Remobilization of crustal carbon may dominate volcanic arc emissions

The flux of carbon into and out of Earth's surface environment has implications for Earth's climate and habitability. We compiled a global data set for carbon and helium isotopes from volcanic arcs and demonstrated that the carbon isotope composition of mean global volcanic gas is considerably heavier, at -3.8 to -4.6 per mil (‰), than the canonical mid-ocean ridge basalt value of -6.0‰. The largest volcanic emitters outgas carbon with higher δ¹³C and are located in mature continental arcs that have accreted carbonate platforms, indicating that reworking of crustal limestone is an important source of volcanic carbon. The fractional burial of organic carbon is lower than traditionally determined from a global carbon isotope mass balance and may have varied over geological time, modulated by supercontinent formation and breakup.

Carbon dioxide emissions from fumarolic ice towers, Mount Erebus volcano, Antarctica

Degassing at Mount Erebus occurs as a plume from a persistent convecting anorthoclase phonolite lava lake, and by flank degassing through warm ground and fumarolic ice towers within the summit caldera. The fumarolic ice towers offer a unique and simple approach to quantifying the flank CO2 emissions. Carbon dioxide effluxes were determined at openings in the ice towers by measuring the CO2 concentration, air-flow velocity, and size of the exit orifice. Fluxes ranged from <0.0001 to 0.034 kg s−1 at 43 actively degassing ice towers. Small patches of steaming warm ground contributed 0.010 kg s−1. The δ13C isotopic compositions of the CO2 samples ranged from −2.1 to −4.7‰, suggesting a magmatic origin for the CO2. Fumarolic ice towers allow diffuse degassing to be visually identified, providing a strong advantage in determining the total flux rate of these passive emissions. The estimated output of flank CO2 degassing is 0.46 kg s−1 (40 Mg d−1). Compared with direct airborne measurements of the volcanic plume, passive flank emissions constitute less than 2% of the total volcanic CO2 budget emitted from Mount Erebus.