To understand how the planet’s climate will evolve and has evolved over geologic time, scientists need to understand the Earth’s carbon cycle. In work recently published in Science, researchers offer new insights into that cycle, upending calculations about just how much carbon stays buried as organic carbon over long time periods.
The carbon isotope signature of volcanic gases indicates whether they came from organic carbon, limestone, the tectonic plates that compose Earth’s crust, or deep within the mantle. When scientists estimate how much carbon is buried in the form of organic carbon, they generally assume that long-term inputs of carbon into the ocean and atmosphere have remained constant over time. Now volcanologist Marie Edmonds of the University of Cambridge and colleagues find that volcanoes don’t always emit carbon dioxide that contains carbon of a constant isotopic composition.
“Instead, we show that in the modern day, subduction zone volcanoes are very heavy in their carbon isotope composition,” Edmonds says, referring to volcanoes that form where crustal plates converge and one slides below another. “And the heaviest ones are recycling carbon from the overlying crust and subduction zone.”
This analysis implies that long-term climate variations are related to subduction zones, which can oscillate between continental arc and island arc types. The former are more likely to generate more carbon dioxide, as the researchers note in the paper, due to interaction of magmas with ancient carbon stored in the continents. The analysis also relates climate variations to the buildup of carbon in the continents, which itself is intimately related to life, according to geochemist Cin-Ty Lee of Rice University, who was not involved in the study. “One begins to wonder how evolution of life, tectonics, and climate might all be linked.”
One key link appears to be the assimilation of carbon dioxide derived from crustal carbonate by magmas rising through the crust, an important source of volcanic carbon as noted in the study. Many marine organisms use carbon dioxide, along with minerals, to construct their shells and skeletons. “Over geologic time, these shells and skeletons build up on the continental shelf,” explains climate scientist Ken Caldeira of the Carnegie Institution for Science, who was not involved in the study. “The sea floor acts as a conveyor belt that drags these shells and skeletons deep into subduction zones where they are exposed to hot pressurized conditions.” There, heat and pressure free the carbon dioxide and it streams out of the volcanoes into the atmosphere.
Edmonds and colleagues focused on arc volcanoes, those that form in subduction zones. They did not have to climb fiery peaks to take new measurements. Rather, they compiled and analyzed data that had been collected and shared by many other scientists.
The team relied on measurements of not only carbon but helium, which is produced due to the radioactive decay of uranium and thorium in the overlying crust. The ratio of helium isotopes in volcanic gases indicates the proportions of magmatic and crustal components of volcanic gases. These data allowed the team to distinguish the sources of heavy carbon—though to do so they had to narrow their analysis to all volcanoes for which both carbon isotopic composition and helium isotopic composition had been recorded, or roughly 80 volcanoes worldwide.
“That’s not many, because at any one time there are 150 active volcanoes in the world and there are hundreds of volcanoes, up to thousands of volcanoes, still degassing,” Edmonds says. The contribution of these hundreds of volcanoes to carbon in the atmosphere over the past century is a small consideration, compared to the recent climate changing input from humans, as noted in this 2011 study. But these numerous volcanoes offer an opportunity to learn about the climate over geologic time.
“The next frontier in this business,” says Lee, “is to begin bridging the gap between the deep Earth and the surface processes, from sedimentation to biology, that control where and how carbon, in all its forms, is distributed.”