A rock hard technique to harvest atmospheric CO2

Carbonate minerals are formed when carbon dioxide reacts with magnesium and calcium-rich rocks. But where does that CO₂ come from?

If it comes from the atmosphere, this process at sufficient scale may be able to reliably draw down atmospheric greenhouse gas levels, according to new research by Lawrence Livermore National Laboratory (LLNL) scientists. The research appears in the journal Nuclear Instruments and Methods in Physics Research.

Carbon mineralization — the formation of solid carbonate minerals from CO₂— is one of the most stable methods for sequestering carbon. This process occurs naturally at the Earth’s surface when magnesium (Mg)- and calcium (Ca) - enriched rock types, known as ultramafics, are exposed to CO₂-rich water. After CO₂ dissolves in water to form bicarbonate, it can react with the Mg or Ca ions released during rock weathering. Previous work has demonstrated that they actively draw down local carbon dioxide (CO₂) concentrations.

But there hasn’t been a method for unambiguously attributing the sequestered carbon solid product to atmospheric sources until now. LLNL researchers used radiocarbon to verify that the carbon being incorporated into carbonate minerals during carbon mineralization is atmospheric in origin.

“We were able to identify some environments and locations where all the carbon in the carbonates almost certainly came from the atmosphere,” said LLNL scientist Kari Finstad, lead author of the paper. “It proved what we suspected, that carbonate minerals passively forming in these ultramafic environments are naturally sequestering atmospheric CO₂.”

This passive reaction is well-documented at mine sites composed of processed ultramafic material. Previous work demonstrated that these mine and tailings wastes actively convert available CO₂ to more stable solid carbonate materials, but future large-scale implementation of this process to mitigate climate change will require a method to “fingerprint” both the nature and quantity of the CO₂ source in the final material (e.g., “new” atmospheric versus geologically “old” and recycled carbon).

If only atmospheric CO₂ is incorporated into a material (as opposed to CO₂ derived from other geological, biological or anthropogenic processes), then the radiocarbon (carbon 14) content of the material should match that in the atmosphere at the time of formation.

“The radiocarbon content of carbonates may provide a unique tool for verifying the sequestration of atmospheric CO₂ and determining the proportion of the carbon that is atmospheric in origin,” Finstad said.

The team, along with researchers from the University of British Columbia and Université Laval, Québec, used LLNL’s Center for Accelerator Mass Spectrometry to analyze the radiocarbon content of samples collected from recently exposed ultramafic rock surfaces. The samples were taken from an experimental mine site and an active landslide.

“This is not always the case though, we found some instances where older carbon is being incorporated, likely geologic carbon,” Finstad said. “Future work to understand why you sometimes get all atmospheric CO₂ and other times it’s a mix is ongoing.”

LLNL scientists Megan Smith and Roger Aines also contributed to this work. The research is funded by LLNL’s Laboratory Research and Development program.