Rock weathering does not act as CO2 sink – study

Sedimentary rocks on the banks of the Mackenzie River, Canada, a major river basin where rock weathering is a CO2 source. (Image by Robert Hilton, University of Oxford).

A new study published in the journal Nature has overturned the view that natural rock weathering acts as a CO2 sink, indicating instead that this can also act as a large carbon dioxide source, rivalling that of volcanoes.

In the paper, University of Oxford researchers explain that rocks contain an enormous store of carbon in the ancient remains of plants and animals that lived millions of years ago. This means that the “geological carbon cycle” acts as a thermostat that helps regulate the earth’s temperature.

For instance, during chemical weathering rocks can suck up CO2 when certain minerals are attacked by the weak acid found in rainwater. This process helps to counteract the continuous CO2 released by volcanoes and forms part of the earth’s natural carbon cycle that has helped keep the surface habitable to life for a billion years or more.

For the first time, this new study measured an additional natural process of CO2 release from rocks to the atmosphere, finding that it is as significant as the CO2 released from volcanoes around the world. Currently, this process is not included in most models of the natural carbon cycle.

How it works

The process occurs when rocks that formed on ancient seafloors are pushed back up to the surface, for example, when mountains form. This exposes the organic carbon in the rocks to oxygen in the air and water, which can react and release CO2. This means that weathering rocks could be a source of CO2, rather than the commonly assumed sink.

Up to now, measuring the release of this CO2 from weathering organic carbon in rocks has proved difficult. But for the new paper, the researchers used rhenium as a tracer element because it 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. However, sampling all river water in the world to get a global estimate would be a significant challenge.

To upscale over the planet’s surface, the researchers worked out how much organic carbon is present in rocks near the surface and where these were being exposed rapidly by erosion in steep, mountain locations.

“The challenge was then how to combine these global maps with the river data while considering uncertainties,” lead researcher Jesse Zondervan said in a media statement. “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.”

This could then be compared to how much CO2 could be drawn down by natural rock weathering of silicate minerals. The results identified many large areas where weathering was a CO2 source, challenging the current view about how weathering impacts the carbon cycle.

CO2 hotspots

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.

“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,” Robert Hilton, co-author of the article, said.

Hilton noted that these fluxes could have changed during earth’s past. For instance, during periods of mountain building that bring up many rocks containing organic matter, the CO2 release may have been higher, influencing global climate in the past.

Ongoing and 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.

A question the team is now asking is if 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,” Hilton said.