Volcanologists have made amazing progress in measuring the carbon dioxide escaping from volcanoes worldwide. But some volcanoes – including some that may be the biggest emitters of carbon dioxide – remain inaccessible due to geographical, political, or safety reasons. Without data from these volcanoes, it is difficult to make accurate global estimates of volcanic gas emissions.
In a new paper  in Scientific Reports, DCO researchers figured out a way to fill in those gaps in the data using rocks from hard-to-access volcanoes. Reservoirs and Fluxes Community members used trace element measurements from volcanic rocks, combined with sulfur dioxide emissions detected by satellites, and carbon emission data from similar volcanoes, to estimate the carbon released by remote volcanoes. Alessandro Aiuppa (Università di Palermo, Italy), Tobias Fischer (University of New Mexico, USA), Terry Plank (Columbia University, USA), and Philipson Bani (Université Blaise Pascal - CNRS -IRD, France) estimate that, all together, these previously “unmeasured” volcanoes release about 11.4 megatons of carbon each year.
“We ended up with a much more precise estimate of the carbon dioxide flux from Earth’s most actively degassing volcanoes than previous estimates,” said Aiuppa.
The project began in spring 2018 with a meeting of the synthesis group of DCO’s Deep Earth Carbon Degassing (DECADE) program, an initiative to estimate the global flux of carbon dioxide from subaerial volcanoes through better data collection. The assembled researchers put together all the information they had on the 91 volcanoes that give off the most gases, but they had direct carbon dioxide measurements from only about 65 percent of those volcanoes.
The researchers had satellite data from all 91 volcanoes, but only for sulfur dioxide. There is already so much carbon dioxide in the atmosphere that, with current technology, scientists can’t easily discern the carbon dioxide coming from volcanoes using satellites. But previous work by Aiuppa and colleagues showed that there is a predictable relationship between the amounts of these two gases that volcanoes emit.
The relationship between the carbon dioxide and sulfur dioxide coming from a volcano varies, however, depending on where it is located. Most volcanoes on land occur in long arcs along the edges of subduction zones, where an ocean plates sinks beneath another tectonic plate. So the concentration of volatile elements in the subducting ocean crust and overlying sediments affects what comes out of the arc of volcanoes. For example, when carbon-rich sediments enter a subduction zone, volcanoes in the overlying arc will degas more carbon than when carbon-poor sediment sinks into the mantle.
Using rock samples from these volcanoes, the researcher could estimate the contributions from fluids from the subducting plate. “Getting a rock sample is much easier than getting a gas sample,” said Aiuppa, who is a DECADE board member. Then they could determine the relationship between the carbon dioxide and the sulfur dioxide.
Specifically, the researchers looked at the ratios of barium to lanthanum in the rocks. Barium tends to be enriched in fluids relative to lanthanum, so the barium to lanthanum ratio is a proxy for the contribution of seafloor sediments to the material coming out of the volcano. “Barium and lanthanum are very good tracers of processes happening in Earth, and that ratio correlates pretty well with carbon to sulfur ratio coming out of volcanoes,” said Fischer, who is chair of the DECADE initiative.
This “ratio trick” allowed the researchers to estimate carbon flux from inaccessible volcanoes and to fill in gaps in the data. “Some of these volcanoes are quite significant emitters, so it’s really important to have the contributions of those big ones filled in,” said Fischer. “That’s the really cool thing about this paper.” They could even group the volcanoes based on the relative carbon content in the sediments entering the nearby subduction zone, which allowed them to estimate carbon dioxide emissions from volcanoes, even without trace element data.
The researchers extrapolate that these 91 volcanoes release about 38.7 megatons of carbon each year, which is at the low end of previous estimates. This can be explained, in part, because volcanic gas emissions vary greatly over time, so even measurements taken over a decade may not accurately reflect the average amount of degassing for a volcano over millennial timescales, which can be captured in the rocks.
The new study also further clarifies the relationship between the carbon that enters the subduction zone and what escapes from arc volcanoes. “The fact that more carbon is subducted than recycled at arcs, which is clearly shown by our study, means that the majority of the carbon being subducted may be recycled into the deep mantle,” said Aiuppa.
DECADE will end officially in 2019, and for their final project the researchers are compiling the massive amounts of new data they have collected and their refined estimates of volcanic emissions to come up with a global estimate of carbon flux from the approximately 900 volcanoes that have erupted in the recent past. “Based on all the things we have learned using satellite data, ground-based sulfur dioxide flux data, and the rock extrapolation data method, we will hopefully come up the most up-to-date, most comprehensive approach to looking at carbon dioxide emissions from volcanoes,” said Fischer.
Main image: Bagana, viewed from offshore to the southwest, with a strong degassing plume from the summit. Credit: Brendan McCormick