Rocks contain an enormous store of carbon in the ancient remains of plants and animals that lived millions of years ago. Research has shown that the geological carbon cycle acts as a vital regulation of Earth’s temperature: for example, natural chemical weathering of rocks absorbs CO2, when certain minerals are attacked by the weak acid found in rainwater. This process has helped to counteract the continuous CO2 released by volcanoes around the world.
However, a new study, released on October 4, 2023, has shown that in taking into account an additional natural process, weathering rocks may actually release CO2 into the atmosphere in similar volumes to volcanoes. Currently, this process is not included in most models of the natural carbon cycle.
The process occurs when rocks that formed on ancient seafloors, where plants and animals were buried in sediments, are pushed back up to Earth’s surface when mountains like the Himalayas or Andes form. This exposes the carbon from the remains of dead organisms in the rocks to oxygen in the air and water, which can react and release CO2.
In the new study, the researchers used a tracer element, rhenium, which is released into water when rock organic carbon reacts with oxygen. Sampling river water to measure rhenium levels makes it possible to quantify CO2 release.
To upscale over Earth’s surface, the researchers worked out how much organic carbon is present in rocks near the surface, and then worked out where these were being exposed most rapidly, by erosion in steep, mountainous locations.
Dr Jesse Zondervan, lead researcher at the department of earth sciences, said: “The challenge was then how to combine these global maps with the river data, while considering uncertainties. We fed all of our data into a supercomputer at Oxford, simulating the complex interplay of physical, chemical, and hydrological processes. By piecing together this vast planetary jigsaw, we could finally estimate the total carbon dioxide emitted as these rocks weather and exhale their ancient carbon into the air."
Hotspots of CO2 release were concentrated in mountain ranges with high uplift rates that cause sedimentary rocks to be exposed, such as the eastern Himalayas, the Rocky Mountains, and the Andes. The global CO2 release from rock organic carbon weathering was found to be 68 megatons of carbon per year.
Future work is looking into how changes in erosion due to human activities, alongside the increased warming of rocks due to anthropogenic climate changes, could increase this natural carbon leak.
When asked if this natural CO2 release will increase over the coming century, Professor Robert Hilton, head of the ROC-CO2 research project that funded the study, said: “Currently we don’t know – our methods allow us to provide a robust global estimate, but not yet assess how it could change.
“While the carbon dioxide release from rock weathering is small compared to present-day human emissions, the improved understanding of these natural fluxes will help us better predict our carbon budget.”
The full paper, ‘Rock organic carbon oxidation CO2 release offsets silicate weathering sink’, can be read here.