Natural rock weathering is a CO2 source comparable to volcanoes

A new study conducted published in the journal Nature challenges the conventional belief that natural rock weathering functions exclusively as a carbon dioxide (CO2) sink. 

Contrary to the traditional view, the research reveals that rock weathering can also serve as a significant source of CO2 emissions, comparable to the emissions generated by the world’s volcanoes.

Ancient carbon 

The ancient carbon embedded in rocks constitutes a crucial component of the Earth’s “geological carbon cycle,” acting essentially as a thermostat moderating the Earth’s temperature. 

Rocks store vast amounts of carbon from the remains of plants and animals that inhabited the Earth millions of years ago. 

Chemical weathering 

During chemical weathering, certain minerals within rocks interact with the weak acid found in rainwater, absorbing CO2 from the atmosphere. 

This interaction counterbalances the continuous emission of CO2 from volcanic activities, thereby sustaining a natural carbon cycle that maintains Earth’s surface conditions conducive to life.

Natural CO2 release process 

The new study, however, reveals a new natural CO2 release process from rocks that is as significant as volcanic emissions. 

This overlooked process comes into play when rocks that originated from ancient seafloors (containing buried plants and animals) are elevated to Earth’s surface, such as during the formation of mountain ranges like the Himalayas or Andes. 

Upon exposure, the organic carbon in these rocks interacts with oxygen from the air and water, leading to CO2 release.

How the research was conducted 

The research team used a tracer element called rhenium and extensive sampling of river water to quantify the CO2 emissions from this rock weathering process. 

By analyzing the amount and location of organic carbon in surface rocks, especially in erosion-prone mountainous regions, the researchers were able to map areas with high CO2 emissions.

Planetary jigsaw puzzle

The study was led by Dr. Jesse Zondervan at the University of Oxford’s Department of Earth Sciences.

“We fed all of our data into a supercomputer at Oxford, simulating the complex interplay of physical, chemical, and hydrological processes,” said Dr. Zondervan.

“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.”

What the study revealed

The results were startling. Hotspots of CO2 release were identified in mountain ranges like the eastern Himalayas, the Rocky Mountains, and the Andes, with the global CO2 release from rock organic carbon weathering estimated at 68 megatons of carbon annually.

To put this in perspective, Professor Robert Hilton from the University of Oxford, explained: “This is about 100 times less than present day human CO2 emissions by burning fossil fuels, but it is similar to how much CO2 is released by volcanoes around the world, meaning it is a key player in Earth’s natural carbon cycle.”

Study implications

With the climate’s history shaped by these emissions, especially during mountain-forming periods, the study now paves the way for exploring the potential impact of human activities and global warming on this natural carbon release. 

The team wonders whether this natural CO2 release will increase over the coming century. “Currently we don’t know – our methods allow us to provide a robust global estimate, but not yet assess how it could change,’’ said Hilton.

“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,” said Dr. Zondervan.

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