Rock weathering may be a major source of carbon dioxide, rivaling volcanic emissions, a University of Oxford study suggests. This insight is critical for future carbon budget forecasts. New research overturns conventional wisdom that natural rock weathering acts as a carbon dioxide sink that removes carbon dioxide from the atmosphere. Conversely, natural rock weathering is also a huge source of carbon dioxide, comparable to that of volcanoes.
Sedimentary rocks along the banks of Canada's Mackenzie River, a major river basin where rock weathering is a source of carbon dioxide. Photo credit: Robert Hilton
These results have important implications for modeling climate change scenarios, but climate modeling currently does not capture the release of carbon dioxide from rock weathering. Future work will focus on whether human activity increases the release of carbon dioxide from rock weathering and how this can be controlled.
A paradigm shift in understanding the carbon cycle
A new study led by the University of Oxford overturns the idea that natural rock weathering acts as a carbon dioxide sink, showing that natural rock weathering can also act as a huge source of carbon dioxide, with a role comparable to that of volcanoes. The research results were published in the journal Nature on October 4 and are of great significance for simulating climate change scenarios.
Shale high in Canada's remote Mackenzie Mountains contains large amounts of rock organic carbon and is a hotspot for carbon dioxide releases. Source: Robert Hilton
Rocks and the Carbon Cycle
Rocks contain vast amounts of carbon and are the ancient remains of plants and animals from millions of years ago. This means that the "geological carbon cycle" acts like a thermostat, helping to regulate the Earth's temperature. For example, during chemical weathering, when certain minerals are eroded by weak acids in rainwater, rocks absorb carbon dioxide. This process helps offset the constant release of carbon dioxide from volcanoes around the world and forms part of Earth's natural carbon cycle, keeping the planet's surface habitable for life for a billion years or more.
New carbon dioxide release mechanism discovered
However, the new study is the first to measure another natural process by which rocks release carbon dioxide into the atmosphere, finding that it is as important as that released by volcanoes around the world. Currently, most natural carbon cycle models do not include this process.
Landslides in the highlands of the Peruvian Andes cause rocks full of organic matter to weather, releasing carbon dioxide. Source: Robert Hilton
This process occurs when rocks formed on ancient sea floors (in which plants and animals were buried in sediments) are pushed back to the Earth's surface, for example when mountain ranges such as the Himalayas or the Andes were formed. This allows the organic carbon in the rock to come into contact with oxygen in the air and water, causing it to react and release carbon dioxide. This means weathered rocks may be a source of carbon dioxide, rather than a sink as is commonly thought.
Methods and findings
Until now, measuring the carbon dioxide released by weathering organic carbon in rocks has been difficult. In the new study, the researchers used a tracer element (rhenium) that is released into water when organic carbon in rocks reacts with oxygen. The release of carbon dioxide can be quantified by measuring rhenium levels in river water. However, sampling all river waters around the world to obtain a global estimate will be a major challenge.
To expand the extent of Earth's surface, researchers did two things. First, they calculated the amount of organic carbon in rocks near the surface. Second, they calculated where this organic carbon would be exposed most quickly, namely in steep mountainous areas where it is subject to erosion.
Severe erosion in southern France has exposed these sedimentary rocks to weathering processes, which release carbon dioxide as ancient organic carbon breaks down. Source: Robert Hilton
Dr. Jesse Zondervan, a researcher at the University of Oxford's Department of Earth Sciences who led the study, said: "The challenge was to combine these global maps with river data, taking into account uncertainties. We fed all the data into a supercomputer at the University of Oxford, simulating the complex interplay of physical, chemical and hydrological processes. By piecing together this giant planetary puzzle, we can ultimately estimate the total amount of carbon dioxide released as these rocks weather and emit their ancient carbon into the air.
We can then compare this figure to the amount of carbon dioxide that can be absorbed by natural rock weathering of silicate minerals. The findings find that weathering is a source of carbon dioxide over many large areas, challenging current ideas about how weathering affects the carbon cycle. Hotspots of carbon dioxide release are concentrated in mountain ranges with high uplift rates and exposed sedimentary rocks, such as the eastern Himalayas, Rocky Mountains and Andes. The study found that the global release of carbon dioxide from the weathering of rock organic carbon is 68 megatons of carbon per year. "
Professor Robert Hilton (Department of Earth Sciences, University of Oxford) is director of the ROC-CO2 research project, which funded the research: "This is around 100 times less CO2 than humans emit today from burning fossil fuels, but is similar to the amount released by volcanoes around the world, meaning it is a key player in Earth's natural carbon cycle."
Impact and future directions
These fluxes may have changed in Earth's past. For example, during periods of orogeny, when many rocks containing organic matter were uplifted, the release of carbon dioxide may have been higher, affecting past global climate.
Current and future work is examining how changes in erosion caused by human activities, as well as warming rocks due to anthropogenic climate change, may increase this natural carbon leakage. One question the team is now asking is whether this natural release of carbon dioxide will increase over the next century. "We don't know yet -- our approach allows us to provide a reliable global estimate, but not yet to assess how it will change," Hilton said.
"While the carbon dioxide released by rock weathering is insignificant compared to today's human emissions, a greater understanding of these natural fluxes will help us better predict the carbon budget," concludes Dr. Zondervan.