Weathering of Rocks Can Release Carbon Dioxide – GWC Mag

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When the silicate rocks that make up the majority of Earth’s mantle are weathered by acids found in rain, they undergo a chemical reaction, soaking up carbon from the atmosphere and creating elements like calcium.

But as carbon-rich sedimentary rocks experience geological weathering from forces such as mountainous uplift and erosion, they actually release carbon dioxide, a study published in Nature reported. In fact, the total amount they release—about 68 megatons per year—is on par with that of Earth’s volcanoes, said Robert Hilton, a geochemist at the University of Oxford and one of the study’s authors.

“This study is exciting because it highlights the significance of a previously understudied flux in the global carbon cycle—the oxidation of ancient organic carbon stored in sedimentary rocks,” Marisa Repasch, a geochemist at the University of Colorado Boulder, wrote in an email.

Repasch, who wasn’t involved with the study, noted that although previous studies have suggested that certain types of rock may release carbon dioxide (CO2) because of weathering, this study provided the first global estimate of the phenomenon.

To obtain that estimate, the researchers had to find a way to measure it. Whereas chemical silicate weathering forms easily measurable elements like calcium, the carbon dioxide released from geological weathering is more difficult to track, Hilton said. Instead, the researchers opted to track the trace element rhenium as a proxy for carbon dioxide because it’s released into water when carbon contained in rocks reacts with oxygen.

“Mountain ranges with sedimentary rocks are not that common over the surface of Earth’s landmass, and so finding that they punch significantly above their weight on a global scale was important.”

With the help of a supercomputer—which enabled them to run simulations that would take months on a typical computer in about a half hour—they created a global spatial model based on data from 59 river basins, covering 18% of Earth’s surface.

Through this modeling, the researchers gained a greater understanding of what’s happening when rocks weather in the eastern Himalayas, the Rockies, and the Andes. Along with other mountain ranges that feature sedimentary rock exposed during the mountain uplift process, these ranges release high levels of carbon dioxide, creating CO2 “hot spots.”

“We know that steep mountain ranges have high erosion and thin soils, and these things can combine to increase rock organic carbon weathering and oxidation,” Hilton said.

In that way, the findings align with what researchers would expect. But until now, researchers didn’t know that these mountainous areas were releasing such high levels of carbon dioxide. “Mountain ranges with sedimentary rocks are not that common over the surface of Earth’s landmass, and so finding that they punch significantly above their weight on a global scale was important,” Hilton noted.

One limitation of the study is that “the authors calibrate their model of global [rock organic carbon] oxidation using a relatively small dataset of dissolved rhenium concentrations,” Repasch wrote. “It is possible that the model overestimates the CO2 flux in low relief landscape.”

A “Hopeful Message”

Though the carbon contribution of weathering rocks is significant, it’s still dwarfed by the amount produced by anthropogenic activities.

Humans emit 10,000 megatons of carbon per year, Hilton said. “It’s really important to remember that, in terms of the relative importance of these geological processes compared to the human-made emissions.”

“I take it as a very hopeful message.”

For Jesse Zondervan, a geomorphologist at University College London and lead author of the study, the research represented a step toward better understanding of how Earth has remained habitable for so long. That habitability requires a delicate balance between CO2 sources and sinks, and better understanding this balance could help researchers identify new or improved carbon dioxide removal methods. “I take it as a very hopeful message,” he said.

The results brought new information to the conversation about using enhanced rock weathering, or grinding up silicate-rich rocks and spreading them out, to increase the amount of CO2 the rocks remove from the atmosphere. Some companies already have products that are marketed for this purpose. But Zondervan noted that the process of producing and using them is energy intensive and that their effects on soil and crop yields are not yet fully understood.

The researchers’ improved understanding of global carbon fluxes could enable scientists to “think of other solutions that are maybe more aligned with things like ecological restoration,” Zondervan said.

The results could also be used to improve certain climate models, according to the researchers. These carbon fluxes are small compared to other human-induced changes, so models may not need to include them over the short term (100 years), Hilton said. “Now, whether they would need to be included over the next 500 to 1000 years—it looks like maybe they do.”

—Rachel Crowell (@writesRCrowell), Science Writer

Citation: Crowell, R. (2023), Weathering of rocks can release carbon dioxide, Eos, 104, https://doi.org/10.1029/2023EO230459. Published on 28 November 2023.
Text © 2023. The authors. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.

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