When Continents Tear Apart, Stored Carbon Escapes

Earth’s crust and mantle hold a giant stockpile of carbon. This carbon can be released to the atmosphere as carbon dioxide gas from the mid-ocean ridges as tectonic plates stretch apart on the ocean floor, from arc volcanoes at the edges of colliding plates, and through volcanic islands like Hawaii. But volcanoes and ridges are not the only way that Earth lets out a little gas.

Scientists are beginning to recognize that continental rifts, where the crust rips apart in the middle of continents, are important relief valves for releasing deep carbon trapped in the crust.

DCO Reservoirs and Fluxes Community members Stephen Foley (Macquarie University, Australia) and Tobias Fischer (University of New Mexico, USA), published a new Perspectives paper in Nature Geoscience [1] to propose that continental rifts represent a major source of deep carbon released at the surface, with a single rift emitting up to 34 megatons (MT) of carbon each year. They suggest that as carbon in the mantle melts, rises, and solidifies in the crust, it accumulates and eventually funnels out through rifts. When considered over geological timescales, this carbon release likely had a huge impact on the evolution of Earth’s climate.

“We’ve increased the role of the continents in the modern deep carbon cycle by a factor of 100,” said Foley. “We can’t cross it out of the equation anymore.”

The collaboration between the two researchers began during a coffee break at a meeting sponsored by the Deep Carbon Observatory and the Cooperative Institute for Dynamic Earth Research (CIDER) at the University of California, Berkeley in 2015. Participants at the meeting were discussing a new carbon budget, which lacked input from the continents. Foley and Fischer decided to investigate if this deficit was actually true. 

To calculate the carbon output of continental rifts, the researchers combined Foley’s work on how carbon minerals melt under conditions in the mantle and Fischer’s measurements of carbon dioxide release from the East African Rift. They had to estimate how often a hot mantle plume would pass by an area, which would melt the rock and transport its carbon to the crust, placing the value at about every 700 million years. Using this information, they could calculate how much mantle carbon the continental plates have soaked up since their formation, about 2 to 3 billion years ago.

The researchers estimate that the solid part of the mantle and the crust, which makes up the lithosphere, had very little carbon at first, estimated at only about 0.25 Mt of carbon per cubic kilometer. But as cycles of melting and cooling occurred beneath the lithosphere, deep carbon concentrated beneath continental rifts, reaching concentrations of 150 to 240 Mt of carbon per cubic kilometer.

These high levels of carbon likely explain the observation that volcanic eruptions at rift zones tend to produce carbonate-rich lavas.  “People have suspected that there are reservoirs of carbon beneath rifts because experimental data has suggested that you need a lot of carbon to make some of the rocks that erupt in rifts, but it was never really quantified,” said Fischer.

Their analysis estimated that a rift the size of the East African Rift can release about 28 to 34 Mt of carbon each year; a value that is close to the estimate of 19 Mt per year, based on previous measurements of degassing carbon.

The total length of continental rifts has varied greatly over Earth’s history. When the supercontinent Pangaea broke apart, the number of rifts would have increased fivefold, spewing out huge quantities of carbon dioxide that likely affected levels in the atmosphere. A companion paper [2] published on the same day by Sascha Brune (GFZ German Research Centre for Geosciences, Germany), with DCO member Dietmar Müller (University of Sydney, Australia), expands on this idea to correlate rift emissions with past changes in global climate.

The new study illuminates an important but long overlooked component of the global carbon budget that has implications for ancient climate change. “We’re filling in the gaps to understand the connections between the carbon dioxide emission at the surface and these larger carbon dioxide reservoirs that might be at depth,” said Fischer.

 

Carbon-rich carbonatite lava erupts from the Ol Doinyo Lengai volcano, which is part of volcanic system of the East African Rift. Credit: Tobias Fischer

 
Researchers take measurements of carbon dioxide degassing near the Ol Doinyo Lengai volcano. Credit: Tobias Fischer

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