Ancient carbon in rocks releases as much Carbon Dioxide as the world’s volcanoes

A new study led by the University of Oxford has overturned the view that natural rock weathering acts as a CO₂ sink, indicating instead that this can act as a large CO₂ source, rivalling that of volcanoes. The results, published today in the journal Nature, have important implications for modelling climate change scenarios.

Rocks contain an enormous store of carbon in the ancient remains of plants and animals that lived millions of years ago. Human activities are rapidly adding this carbon to the atmosphere by burning fossil fuels for energy, releasing carbon dioxide (CO₂) into the air faster than it can be removed, warming Earth’s surface.

A slower cycle, the so called “geological carbon cycle”, has been in place over millions of years acting as a thermostat that regulates the Earth’s temperature. This new study, led by scientists at the University of Oxford however has measured an additional natural process of CO₂ release to the atmosphere for the first time, finding it releases 100 times less than human activities, but is as significant as the CO₂ released from volcanoes around the world.

When air and water interact with rocks, a set of chemical reactions happen which move carbon around. Over the last 30 years or more, scientists have focused on how rocks can suck up CO₂ when certain minerals (made with silicon-oxygen bonds) are attacked by the weak acid found in rainwater during a process called chemical weathering. This process helps to counter-act the continuous CO₂ released by volcanoes around the world, and forms part of Earth’s natural carbon cycle that has helped keep the surface habitable to life for a billion years or more.

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

However, some rocks can have carbon in them, particularly those formed on ancient seafloors where plants and animals were buried in sediments. When these rocks are pushed back up to Earth’s surface, for example when mountains like the Himalaya or Andes are built, the organic carbon in the rocks is exposed to oxygen in the air and water. This can react and release CO₂, meaning that weathering rocks could be a source of CO₂, rather than the commonly assumed sink.

Measuring the release of this CO₂ from weathering organic carbon in rocks has proved difficult. Prior to this study, the estimates were very crude. To move forward, Zondervan and colleagues have instead used a tracer of the process that is contained in river waters around the world. The element rhenium is released into water when rock organic carbon reacts with oxygen so by measuring this tracer we can quantify CO₂ release. However, although the approach has been shown to be successful in specific locations, sampling all river water in the world to get a global estimate would be a significant challenge.

To upscale over Earth’s surface, the scientists did two things. First, they worked out how much organic carbon is present in rocks near the surface and they found as much carbon as is present in surface soils. Second, they worked out where these were being exposed most rapidly, by erosion in steep, mountain locations. “The challenge was then how to combine these global maps with the river data, while considering uncertainties” said Jesse Zondervan who led the study, ‘’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.”

High erosion in southern France exposes these sedimentary rocks to weathering, releasing CO2 as the ancient organic carbon breaks down. Photo by Robert Hilton

This could then be compared to how much CO₂ could be drawdown by natural rock weathering of silicate minerals. “It was remarkable – we found many large areas where weathering was a CO₂ source, challenging our current view about how weathering impacts the carbon cycle” added Zondervan. The global CO₂ release from rock organic carbon weathering was found to be 62 to 86 megatons of carbon per year. “This is much less than present day human CO₂ emissions by burning fossil fuels” said Robert Hilton who led the ROC-CO2 research project that completed the research, “but its similar to how much CO₂ volcanoes release around the world”. These fluxes could have changed during Earth’s past as Zondervan explains “if periods of mountain building bring up rocks containing organic matter, the CO₂ release may be higher”.

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 are 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.’’ says 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” concluded Dr. Zondervan.

Shale rocks high up in the remote Mackenzie mountains Canada, which contain lots of rock organic carbon and are hotspots of CO2 release. Photo by Robert Hilton

The authors would like to acknowledge the use of the University of Oxford Advanced Research Computing (ARC) facility in carrying out this work. In addition, this work is based on digital elevation products and high-performance topographic analysis services provided by the OpenTopography Facility with support from the National Science Foundation. This study was supported by the European Research Council Starting Grant ROC-CO2.