Abstract

The origin of carbon coming from volcanic arc outgassing is a fundamental yet unanswered question. In a newly compiled global data set for carbon and helium isotopes from volcanic arcs we demonstrated that the carbon isotope composition of mean global volcanic gas is considerably heavier than the canonical mid-ocean ridge basalt value of –6.0‰, suggesting a significant carbonate contribution to global volcanic CO2 emissions. The largest volcanic CO2 emitters, such as Mount Etna, outgas carbon with higher δ13C and are often located in mature continental arcs that have accreted carbonate platforms, indicating that reworking of crustal limestone is an important source of volcanic carbon.

The flux of carbon into and out of Earth’s surface environment has implications for Earth’s climate and habitability. Our revised value for the global average isotopic composition of volcanic CO2 calls for a re-examination of estimates of fractional organic carbon burial (and therefore the progressive oxygenation of the planet) over Earth’s history. It also challenges the widely-held assumption that this value has not changed over geological time. Supercontinent formation and break-up has modulated the make-up of volcanic arcs and thus the isotopic composition of the carbon released throughout Earth’s history.

Changes in isotopic compositions of volcanic gases have also been linked to increases in eruptive activity. With the development of devices that can make in-situ measurements, the use of isotopes as a fundamental tool in volcanic monitoring has the potential to improve our predictive capabilities.