Quantifying and understanding the natural emissions of CO2 from Earth is fundamental to our knowledge of how our planet regulates climate over long (geological) timescales, and how it has remained habitable. Despite its importance, this flux is poorly defined .
Cataloguing how much CO2 gas is coming out of the world’s volcanoes is an important part of this puzzle. But in order to understand the contribution of volcanoes to the geological carbon cycle, we need to know not only the amount of CO2 being emitted but also where all this carbon is coming from. CO2 in volcanic gases can originate from a number of sources, including Earth’s mantle, sedimentary rocks “recycled” from the crust into the mantle at subduction zones (where one tectonic plate sinks beneath another), or from rocks such as limestone in Earth’s crust when hot magma pushes its way through it to the surface .
CO2 is released from magmas deeper in Earth than most other volcanic gases. This difference means that significant quantities of CO2 gas can seep out into the atmosphere through rocks and soils on and around volcanoes, as well as being released with other gases through volcanic vents and fumaroles. This diversity of sources makes measuring CO2 emissions far more challenging. It also makes working out the sources of this carbon more difficult as traditional techniques, such as using noble gases as a proxy, are sensitive to contamination by air - an experimental challenge harder to avoid or correct for when measuring diffuse gases seeping from soils and rock.
In a recent paper in Earth and Planetary Science Letters DCO scientists, representing the UK, Italy and Greece, developed a new approach for identifying and quantifying the different sources of carbon in volcanic soil gas emissions by analyzing a combination of carbon isotopes and the radioactive gas radon . Applying this approach during a period of increased seismicity and inflation on the Kameni islands, an archipelago that form the newest part of the picturesque Santorini volcano in Greece, during a period of increased seismicity and inflation, they were able to determine that about two thirds of the CO2 emitted was from magma sitting below the volcano. Variations in carbon isotopes during the period of unrest  suggest that release from shallow crustal limestone contributes significantly to the other third of the carbon emissions.
CO2 was measured using an accumulation chamber that sits on top of the soil and collects the gas seeping out for analysis (pictured). The radon detectors are small cylindrical plastic containers encasing a piece of radiation sensitive plastic, which are deployed by burying them in the soil.
"The combination of carbon soil gas flux and isotope measurements with these radon detectors allowed us to get great high spatial resolution data. This allowed us to distinguish the deep source arising from the presence of magma in the volcano from very shallow biological sources and atmospheric contamination and to explore its make up. The exciting thing about this project was that it gave us insights not only into the carbon sources but also into the behavior and state of the volcano, so important from both a planetary and human hazard perspective." says DCO scientist and co-author Tamsin Mather.
This work complements exciting recent and ongoing developments in the measurement of noble gases such as helium in volcanic soil gases .
Photo credit: M. Parks.