Geologists, students look for clues to Earth’s ‘carbon cycle’

Student researcher and professor at microscope
Geology senior Desiree Baker-Wright examines samples from a Costa Rican volcano collected by Daniel Hummer, right, assistant professor of geology at Southern Illinois University Carbondale. Hummer, along with assistance from undergraduate researchers, is part of an international team of scientists studying how carbon gets into a subduction zone – the dynamic place where one tectonic plate dives beneath another – as well as where it comes from, where it goes and what forms it takes when it is released.

There’s a lot of talk about carbon these years: How it gets into the atmosphere and its potential impacts on climate.

But some estimates say just 10 percent of all the Earth’s carbon is located on the surface, with the vast majority lying deep underground as part of the “carbon cycle.” But scientists know very little about where all this carbon comes from and how it behaves beneath the surface.

A geologist at Southern Illinois University, along with assistance from undergraduate researchers, is helping put together a picture of how carbon gets into a subduction zone – the dynamic place where one tectonic plate dives beneath another – as well as where it comes from, where it goes and what forms it takes when it is released.

To do this, he traveled to Costa Rica, where a chain of volcanoes is offering a window into this important mystery that may open our understanding of the planet.

“It could fundamentally change our conception of the carbon cycle, including a portion that is largely unexplored,” said Daniel Hummer, assistant professor in geology. “On the Earth’s surface the carbon you have is in plants, animals and air and such, but we’re looking at the cycle underneath the Earth and how that might work.”

Hummer began working on the project in early 2017, collaborating with researchers from six nations as they collected samples of volcanic rock, fluids, gases and even microbes living near the volcanic vents, where they feed on carbon emissions. The project is funded by the Alfred P. Sloan Foundation and carried out by the Deep Carbon Observatory organization.

We’re all familiar with pictures of volcanoes spewing magma, especially in light of the recent eruptions in Hawaii. A volcanic area, however, emits several other materials, including water-based fluids and gases. The team of scientists collected volcanic fluids from hot springs, volcanic gases and microbial samples from underwater sediments and volcanic fumaroles.

“The goal of all this sampling is to put together a picture of all the various forms of carbon that are emitted from volcanoes, how microbes living there use that carbon and where all the carbon originally came from,” Hummer said.

Hummer and his students are examining some of the volcanic rock samples in laboratories at SIU, looking closely at olivine crystals that contain inclusions of fluids such as water and gases such as carbon dioxide. Inclusions are tiny bubbles that contain clues as to the composition of the original magma and where the carbon came from.

“The amount of carbon dioxide tells us about how much carbon was in the original magma,” Hummer said.

Hummer’s team of students includes Desiree Baker-Wright and Nathan Shelton, both seniors earning degrees in geology, and Corey Swiger, a sophomore in geology. The team especially is interested in the ratio between the stable isotopes carbon-12 and carbon-13. Most carbon is carbon-12, with a variant being carbon-13. But the ratio between the two can reveal if the carbon came from the atmosphere, from deep within the Earth’s mantle or from limestone rock that is being subducted, or pushed beneath a colliding tectonic plate as they collide.

The researchers hope identifying the mix of sources will tell them more about the deep carbon cycle, which will deepen our understanding of the planet.

“Carbon is probably the most important element on Earth’s surface, as all life is based on it,” Hummer said. “With up to 90 percent of it possibly beneath the surface, what we learn could impact our understanding of the Earth’s structure and what we know about where, when and how much magma is generated by the planet.”

It will take another two years of analysis to complete the study.

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