We live in an age where changing levels of atmospheric carbon dioxide are driven by the actions of humans.
However, this phenomenon is recent, covering only the last few hundred years of Earth’s long history. Prior to our influence on the atmosphere, changes in carbon dioxide were governed by two main forces: life and magma.
The actions of organisms can release, emit, and capture carbon dioxide, and magmatism in Earth’s crust releases carbon dioxide and other volatiles at both mid-ocean ridges and arcs. Carbon dioxide is a major player in global climate, so unraveling how its concentration has changed in the atmosphere over the planet’s history is a question that requires an answer if we want to understand why climate has varied.
In a recent paper in Geochemistry, Geophysics, Geosystems, Barbara Ratschbacher and others tackled the potential importance of continental arcs in carbon dioxide emissions over the past 750 million years. They dissected representative examples of continental arcs from the Andes, Cascades, and Sierra Nevada, quantifying the volumes of plutonic and volcanic rocks in each to estimate the total volume of magma needed for these sections. These estimates of magma volumes in the sections allowed for a time slice of arc productivity during periods of heightened activity called flare-ups.
To get from these slices to a global estimate, Ratschbacher assumed a percentage of carbon dioxide in arc magma, a length of continental arc over time across the planet, and a ratio of flare-up and lull activity along the arcs to get the total carbon dioxide released by continental arcs across the past 750 million years.
Results revealed the warm climate of the Mesozoic correlated with the highest estimates of carbon dioxide emissions from continental arcs, about 15–18 metric tons per year. Modern estimates of continental arc carbon dioxide emissions are closer to 7 metric tons per year. The values in the new research represent carbon dioxide emitted from only continental arcs and not mid-ocean ridges or flood basalt provinces, so this is only a piece of the global carbon dioxide balance.
These calculations were built off some of the best examples of exposed continental arc crust, where both plutonic and volcanic components could be assessed.
“I have been trained to be hesitant to draw conclusions if data is missing or inaccessible. However, I do believe that the assumptions we made are well founded and the areas we choose for the calculations are the best-studied continental arc sections worldwide, making these calculations as accurate as was possible. It will be interesting to see whether our results change as more data is collected in the future, ” Ratschbacher wrote to Eos in an email.
Ratschbacher said that the idea to use magma addition rates in continental arcs to assess carbon dioxide emissions over time arose from conversations at the 2017 Cooperative Institute for Dynamic Earth Research summer program.
“Instead of trying to get at the carbon dioxide output via direct measurement at volcanic centers,” she said, “we used the input of magma to the crustal column to determine the output of carbon dioxide.”
Gary Michelfelder, an assistant professor at Missouri State University in Springfield who was not involved in the research, said in an email that the study was a great first step in understanding changing rates of magmatic addition over time and carbon dioxide emissions from continental arcs.
Anita Grunder, an emeritus professor at Oregon State University in Corvallis who also wasn’t involved in the study, agreed that the study was “a creative attempt to quantify carbon dioxide contributions” and wondered what other lines of evidence might exist for high carbon dioxide emissions during flare-ups: “More weathering? More limestone? Hotter climate?”
Ratschbacher thinks that the next avenue to pursue is examining arcs that are less productive and magma rich, such as the Appalachians. This comparison between more and less productive arcs could help answer a key question of why arcs can change in character from lulls to flare-ups. Putting all these pieces together will paint a clearer picture of the relationship between changing carbon dioxide in the atmosphere and global climate.
—Erik Klemetti (@eruptionsblog), Denison University, Granville, Ohio