Spatially Variable CO₂ Degassing in the Main Ethiopian Rift: Implications for Magma Storage, Volatile Transport, and Rift‐Related Emissions

Abstract

Deep carbon emissions from historically inactive volcanoes, hydrothermal, and tectonic structures are among the greatest unknowns in the long‐term (∼Myr) carbon cycle. Recent estimates of diffuse CO2 flux from the Eastern Rift of the East African Rift System (EARS) suggest this could equal emissions from the entire mid‐ocean ridge system. We report new CO2 surveys from the Main Ethiopian Rift (MER, northernmost EARS), and reassess the rift‐related CO2 flux. Since degassing in the MER is concentrated in discrete areas of volcanic and off‐edifice activity, characterization of such areas is important for extrapolation to a rift‐scale budget. Locations of hot springs and fumaroles along the rift show numerous geothermal areas away from volcanic edifices. With these new data, we estimate total CO2 emissions from the central and northern MER as 0.52–4.36 Mt yr−1. Our extrapolated flux from the Eastern Rift is 3.9–32.7 Mt yr−1 CO2, overlapping with lower end of the range presented in recent estimates. By scaling, we suggest that 6–18 Mt yr−1 CO2 flux can be accounted for by magmatic extension, which implies an important role for volatile‐enriched lithosphere, crustal assimilation, and/or additional magmatic intrusion to account for the upper range of flux estimates. Our results also have implications for the nature of volcanism in the MER. Many geothermal areas are found >10 km from the nearest volcanic center, suggesting ongoing hazards associated with regional volcanism.

AGU Centennial Grand Challenge: Volcanoes and Deep Carbon Global CO2 Emissions From Subaerial Volcanism—Recent Progress and Future Challenges

“…While not volcanic per se (diffuse degassing predominantly occurs along extensional faults; Tamburello et al, ), these regional degassing structures may represent a significant flux of mantle‐derived CO 2 to the atmosphere. Recent estimates of diffuse CO 2 flux from the East African Rift, for example, range from 3.9–33 Tg CO 2 /year (Hunt et al, ) to 38–104 Tg CO 2 /year (Lee et al, ). More work is needed to quantify the flux CO 2 from continental rifts, but it may be quite significant and around 30–40 Tg CO 2 /year (or 8–11 Tg C/year) and potentially on the same order as global arc fluxes.…”

“…A related aspect is that continental rift degassing remains poorly constrained between 18 ± 14 Tg CO 2 /year (Hunt et al, ) and 70 ± 33 Tg CO 2 /year (Lee et al, ) and depends on which sector of the East African Rift is measured and used to extrapolate to the entire rift length. The causes for this variability remain a still open question and may be related to the efficiency of lithospheric age and efficiency of C storage and release (Foley & Fischer, ), the efficiency of melting during rift extension and assumed C content in the upper mantle (Hunt et al, ), or localization of C release along faults and degree of extensional processes (Muirhead et al, ). Recent work shows that continental rifts potentially contribute between 40% and 60% of the total C outgassing to the atmosphere, significantly more than ARCs and MORs, and that this high contribution persisted through the past 200 Ma (Wong et al, ).…”

Volatile Degassing From Magma Chambers as a Control on Volcanic Eruptions

“…Direct measurements of soil CO 2 from a variety of volcanic environments (Burton et al, ; Inguaggiato et al, ; Viveiros et al, ) confirm this general result. In particular, many geologic regions with limited to no recent active volcanism (e.g., Apennines: Italy Chiodini et al, ; French Massif Central: Gal et al, ; sections of the East African Rift Valley Hunt et al, ; Lee et al, ) show evidence for significant passive volatile fluxes derived from deep magma sources.…”

Magmatic carbon outgassing and uptake of CO2 by alkaline waters

“…Melts may also assimilate carbon from the mantle lithosphere, or from the crust (Lee et al 2013;Carter and Dasgupta 2015;Mason et al 2017), where carbon resides as carbonate and organic carbon, and transfer this carbon to the atmosphere. Because of the low solubility of carbon in silicate melts, intrusive magmatism contributes to the outgassing flux of carbon, leading to diffuse emissions of CO 2 in volcanic regions, often mediated by large-scale faults and fractures (Allard et al 1991;Chiodini et al 1995;Farrar et al 1995;Hunt et al 2017).…”

“…In this paper, we review the current state of understanding of both reactions in detail, with application to the case of alkaline melts and fluids generated in continental rifts, which have had considerable importance for supplying CO 2 fluxes to the surface reservoir through supercontinent cycles of the past (Brune et al 2017) and in the modern day (Lee et al 2016;Hunt et al 2017;Werner et al 2019). We discuss the dependence of these reactions on other parameters in the geological system, and we consider their importance for the Earth's deep carbon cycle, tracking carbon reactions from the mantle to the surface.…”