When it comes to binding CO₂ from the atmosphere, it is often about biomass, such as forests or certain algae in oceans. For decades, researchers have also been focusing on the ability of rocks to remove CO₂ from the atmosphere. In the so-called “accelerated weathering” (enhanced weathering), for example, it is proposed to spread ground basalt on fields and forests. Since the stone powder also contains micronutrients, it would even support plant growth in addition to storing CO₂. However, there are also rocks that do exactly the opposite: they emit CO₂ into the atmosphere. A group of researchers at the University of Oxford has now found that natural rock weathering can also be a major source of carbon dioxide (CO₂) emissions, even rivaling those from volcanoes.
This happens in rock formations that formed long ago on the seabed from sediments in which dead plants and animals are also buried. If these soils are pushed to the surface as mountains over millions of years, the enormous amounts of organic carbon they contain react with the oxygen in the air and with water. CO₂ is created.
However, most models of the natural carbon cycle do not yet take these CO₂ emissions from so-called petrogenic carbon into account. After all, this mechanism could mean that the earth’s crust acts as a thermostat to regulate the earth’s temperature because, in addition to the rocks that act as a CO₂ sink, it also houses rocks that are a source of CO₂.
Trick about rhenium
Until now, however, it was difficult to even measure the release of CO₂ during the weathering of organic carbon in rocks. However, the researchers from Oxford used a trick: When organic rock carbon reacts with oxygen, rhenium is always released. This is a very rare, heavy transition metal that is added in small quantities to a variety of technical alloys.
When rocks erode, rhenium ultimately ends up in rivers. So the scientists examined river water samples for the content of this metal and were thus able to draw quantitative conclusions about the amount of CO₂ being released and calculate how much organic carbon is present in rocks near the surface.
In their investigation, the researchers limited themselves to places where CO₂ is released particularly quickly through erosion in steep mountain regions. To do this, they examined, among other things, the rhenium content of the major rivers Amazon, Yangtze, Mekong, Congo and Nile.
“We fed all of our data into a supercomputer in Oxford that simulates the complex interplay of physical, chemical and hydrological processes,” says Jesse Zondervan, leader of the study. “By piecing together this giant planetary puzzle, we were finally able to estimate the total amount of carbon dioxide released as these rocks weather and release their old carbon into the air.”
As a result, they found that the hotspots of CO₂ release are concentrated in mountain ranges with sedimentary rocks and high uplift rates. These include the Eastern Himalayas, the Rocky Mountains and the Andes. The authors estimated that around 68 megatons of carbon and almost 250 megatons of CO₂ would enter the atmosphere worldwide.
That’s a hundred times less than the CO₂ emissions from burning fossil fuels, says co-author Robert Hilton, “but it’s roughly equivalent to the amount of CO₂ released by volcanoes around the world.” This makes it an important factor in the earth’s natural carbon cycle.
However, Thorben Amann from the Center for Earth System Research and Sustainability at the University of Hamburg sees a need for further research, as he said in an email. He points out that there are also studies that show that mobilized, petrogenic carbon is retained and sedimented in lakes, meaning that it does not fully enter the atmosphere as CO₂.
Amann writes that he is not a specialist in the organic carbon components in rocks, but he counters the Oxford study by comparing processes on different time scales. To almost equate organic carbon as a CO₂ source and silicate weathering as a CO₂ sink would not take the different time scales into account. The carbonation of material, as the researchers led by Zondervan and Hilton describe it, is relevant on long-term time scales of 10,000 to probably 100,000 years.
Amann is co-author of a study that estimated the possibilities and costs of CO₂ removal through enhanced weathering five years ago. She concluded “that accelerated weathering, particularly of basalt rock, could be an attractive option to promote climate protection. […] But given the costs and the mass of rock that would have to be moved, it will probably only be able to make a manageable additional contribution.”
The team from Oxford suspects that the interplay between CO₂-emitting and CO₂-binding rocks may well have changed in Earth’s past. During periods of mountain building, when a lot of rock containing organic material is stirred up, CO₂ release may have been higher and thus influenced the global climate hundreds of thousands or millions of years ago.
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