Scientists discover how volcanoes release trapped crustal CO2 to the atmosphere

2019-06-26

En vulkan som det ryker ut. Foto. — Intense outgassing at Merapivolcano. Photo: Tom Walter

The source of carbon dioxide (CO₂) emitted from volcanoes has been the topic of a long-standing debate. Now, a team of scientists show that CO₂ is efficiently liberated from limestones by volcanic activity and released into the atmosphere. This limestone-derived CO₂ is likely a significant contributor to the global carbon cycle, but has so far been underestimated in respect to climate effects. The findings have been published in the journal Scientific Reports.

Volcanoes routinely emit CO₂ into the atmosphere via volcanic degassing. However, volcanoes which are situated on top of limestone bedrock have often an unusually high outflux of CO₂. Limestone is made of calcium carbonate, CaCO₃, and when hot magma reacts with this limestone bedrock at depth it vigorously breaks down to release large amounts of CO₂, much like classroom baking soda and vinegar volcanoes, contributing to this unusually high outflux of CO₂. Until now, the exact process during magma-limestone interaction at depth and the timescales over which this can happen have remained speculative.

The highly active and hazardous Merapi volcano in Indonesia has now been the target for a team of scientists from Keele and Swansea Universities (UK) and Uppsala University (Sweden) who used pieces of thermally altered limestone (skarn), brought to the surface during volcanic eruptions. The researchers argue that these skarn samples record the processes in the underlying magma reservoir and allows them to gain insights into the processes accompanying liberation of CO₂from limestone. Led by PhD researcher Sean Whitley from the School of Geography, Geology and the Environment at Keele University, the researchers measured different isotopes of carbon and oxygen in crystals of the mineral calcite in these skarn fragments at the Edinburgh Ion-Microprobe Facility. These data allowed them to chemically fingerprint the processes that led to formation of these calcites and the team found an unusual carbon isotopic signature that demonstrates highly efficient remobilisation of limestone-derived CO₂ into the magmatic system. They further found that this CO₂ release occurs over geologically short timescales of hundreds to thousands of years and that during non-eruptive episodes up to half of the CO₂ emissions at Merapi derive from the digestion of limestone in the magma storage region, rising to 95% during volcanic eruptions.

Although Merapi is currently considered a relatively minor emitter of CO₂ on a global scale, the results show that volcanic CO₂ liberation can potentially release large amounts of limestone-derived CO₂ during eruptive episodes. This increasingly recognised contribution of limestone-derived CO₂ to volcanic carbon budgets now requires reconsideration of global carbon cycling models throughout Earth history.

For further information please contact prof V.R. Troll or Dr. Frances Deegan

Whitley, S., Gertisser, R., Halama, R., Preece, K., Troll, V., Deegan, F. 2019: Crustal CO₂ contribution to subduction zone degassing recorded through calc-silicate xenoliths in arc lavas. Scientific Reports 9:8803, DOI: 10.1038/s41598-019-44929-2.