So far it has been considered a reducing factor in the global carbon budget. But now a study shows that the global weathering of rocks may release as much CO₂ as it binds in minerals. Accordingly, significant amounts of the greenhouse gas are apparently formed by the oxidation of bound carbon in the eroding material. The previously ignored contribution of CO₂ release from rock weathering should now be included in climate modeling, say the researchers.
Since ancient times, the Earth's carbon cycle has been characterized by natural processes that release CO₂ into the atmosphere and those that capture the greenhouse gas. However, humans then interfered with this system by releasing enormous amounts of carbon dioxide, thereby causing climate change. In order to better assess further developments and CO₂ budgets, detailed knowledge of which natural processes are at work in the carbon cycle is now required.
So far, global rock weathering has been recorded as a CO₂ sink. It has long been known that silicates released during erosion, in combination with precipitation, lead to the binding of gas from the atmosphere. The chemical processes form carbonates, which can lead to long-term deposition of carbon.
Rocks with ancient carbon in sight
However, less attention has been paid to a process that works in the opposite direction: some rocks contain oxidizable carbon, which, when weathered, can lead to the release of CO₂ through reaction with atmospheric oxygen. It is fossil material that comes from creatures that lived millions of years ago. Their carbon-rich biomass was deposited back then, integrated into rock and is now exposed to the air again through geological processes such as the folding of mountains. The research team led by Jesse Zondervan from the University of Oxford has now dedicated itself to the task of assessing the significance of this process globally.
As an indication, the scientists used test results of levels of the element rhenium in water. As they explain, it can serve as a tracer element because rhenium is typically released into water when organic carbon in the rock reacts with oxygen. The analysis results of some river waters with known geological connections enabled the team to quantify the CO₂ release during the weathering of carbon-containing rocks. To project the results to a global scale, the researchers then used data on organic carbon levels in rocks from different parts of the world. Their calculations also included information about the extent to which these formations are exposed to weathering processes.
Important natural source of CO₂
As the researchers report, their results revealed a surprisingly great importance for the oxidation of ancient carbon: The calculations showed an annual release of 68 megatons of CO₂ from rocks near the surface. In comparison with known estimates of CO₂ binding through silicate weathering, this means that the release corresponds to or even exceeds the binding on a global scale. As was shown in detail, certain mountain ranges with high uplift rates, such as the Himalayas, the Rocky Mountains and the Andes, form the major hotspots for CO₂ release.
“While it is 100 times lower than today’s CO₂ contributions from burning fossil fuels, it is similar to the amount of CO₂ released by volcanoes around the world. “This means it is an important player in Earth’s natural carbon cycle,” says co-author Robert Hilton from the University of Oxford. Zondervan adds: “Although the release appears small compared to human emissions, improved understanding of these natural inflows can now help us better predict our carbon budget.”
The scientists will now stay on the ball: In further studies, they want to investigate the question of how climate change could affect the process. It is possible that the increasing warming of rocks will increase the CO₂ contribution through the oxidation of carbon.
Source: University of Oxford, specialist article: Nature, doi: 10.1038/s41586-023-06581-9