Introduction to Volcanic Carbon Cycling

The flux of carbon into and out of Earth's surface environment significantly impacts climate and habitability. This study compiled a global dataset of carbon and helium isotopes from volcanic arcs, revealing that the mean global volcanic gas carbon isotope composition (δ¹³C) is considerably heavier (-3.8 to -4.6‰) than the canonical mid-ocean ridge basalt value (-6.0‰).

Sources and Tracers of Volcanic Carbon

Earth's carbon is primarily stored in the core, mantle, and crust, with a smaller fraction in surface reservoirs. Volcanic outgassing is a major carbon input, but the origin of carbon in arc volcanoes has been a fundamental question. Carbon and helium isotopes serve as geochemical tracers to determine carbon sources:

Carbon Isotopes (δ¹³C): Mantle carbon typically ranges around -6.0‰, sedimentary organic carbon is < -20 to -40‰, and carbonate carbon is ~0‰.
Helium Isotopes (³He/⁴He or R/RA): This ratio indicates contributions from magmatic versus crustal components. High δ¹³C combined with low R/RA strongly suggests carbon derived from the overlying crust.

Key Findings on Arc Emissions

The study found that the largest volcanic emitters, particularly in mature continental arcs with accreted carbonate platforms, outgas carbon with higher δ¹³C. This indicates that reworking of crustal limestone is a significant source of volcanic carbon. Many arcs, including those in Italy, Central America, Indonesia, Papua New Guinea, and parts of the Andes, emit carbon with δ¹³C values higher than the typical mantle range.

Isotopic Signatures: Global arc mean δ¹³C is -4.3 ± 2.6‰, with a weighted average of -3.8 to -4.6‰, significantly heavier than the mantle.
CO₂ Flux Correlation: Volcanic arcs with the highest CO₂ fluxes also exhibit higher δ¹³C values, suggesting that assimilation and outgassing of crustal carbon may dominate global volcanic CO₂ fluxes.
Schematic Diagram (Fig. 1): Illustrates carbon sources in a subduction zone, including organic and inorganic carbon in sediments, and remobilization from the slab or assimilation of crustal carbonate platforms into ascending magmas.
Isotope Plots (Fig. 2 & 3): These figures visually demonstrate that many volcanic arcs plot outside the canonical mantle ranges for both carbon and helium isotopes, indicating significant crustal influence.
CO₂ Flux vs. δ¹³C (Fig. 4): Shows a positive correlation, reinforcing that arcs with higher CO₂ output tend to have heavier carbon isotopic signatures.

Implications for Earth's Carbon Cycle

These findings challenge the traditional assumption of a constant δ¹³C for volcanic input over geological time. If a large fraction of outgassed carbon originates from the overlying crust in some arcs, a greater proportion of subducted carbon may return to the deep mantle. The reinterpretation of the global carbon isotope mass balance suggests that the fractional burial of organic carbon (f_org) may be lower today (~15%) than previously assumed (~20%). This revised understanding has cascading effects on the calculated fraction of carbon buried as organic carbon throughout Earth's history, modulated by supercontinent formation and breakup events.

Remobilization of Crustal Carbon May Dominate Volcanic Arc Emissions

Remobilization of Crustal Carbon May Dominate Volcanic Arc Emissions