Weathering of Ancient Rocks Releases CO2 Into the Atmosphere, Contributes As Much Emission As the World’s Volcanoes

The scientific community has accepted the traditional belief that natural rock weathering acts as a CO2 sink. A new study, however, has overturned this view, indicating that it may instead act as a large CO2 source.

Role of Weathering in Carbon Cycle

Rocks store enormous amounts of carbon from the ancient remains of plants and animals that lived millions of years ago. This means that the "geological carbon cycle" serves as a thermostat that aids in regulating the temperature of the Earth.

During chemical weathering, rocks can absorb CO2 when some minerals are attacked by the weak acid in rainwater. This process helps counter counter the continuous CO2 released by volcanoes worldwide. It also forms part of the natural carbon cycle, which helps keep the planet's surface habitable for a billion years or more.

Weathering Organic Carbon in Rocks

For the first time, the new study measured the natural process of CO2 release from rocks to the atmosphere. Although this process is currently not included in most models of the natural carbon cycle, it was found that the amount of CO2 released by rocks is as significant as the CO2 released from volcanoes worldwide.

The process occurs when rocks on ancient seafloors containing buried plants and animals are pushed back up to the surface of the Earth. As the organic carbon in the rocks is exposed to air and water, it reacts with oxygen and forms CO2. This means that weathering rocks can be a significant source of CO2.

Measuring the amount of released CO2 from weathering organic carbon in rocks has been difficult. In the new study, the scientists used rhenium, a tracer element released into water when the organic carbon in rock reacts with oxygen. By sampling river water to measure rhenium levels, it becomes possible to quantify CO2 release. However, it would be challenging to sample all river waters to obtain a global estimate.

Led by Dr. Jesse Zondervan from the Department of Earth Sciences at the University of Oxford, the researchers carried out two steps to upscale the Earth's surface. First, they determined the amount of organic carbon in rocks near the surface. Second, they identified the locations where they were exposed more rapidly.

To combine the global maps with the river data, the research team fed all of their data into a supercomputer at Oxford, where the system simulated the complex interplay of physical, chemical, and hydrological processes. As the data were pieced together, the experts could finally estimate the total carbon dioxide emitted into the atmosphere during the weathering of ancient rocks.

The findings reveal that the global CO2 released from rock weathering is about 68 megatons of carbon per year. According to Professor Robert Hilton, this is about 100 times less than modern CO2 from burning fossil fuels, but it is comparable to the amount of CO2 released by volcanoes worldwide.

The result of this study may have significant implications for modeling climate change scenarios. Although the CO2 release from rock weathering is small compared to present-day human emissions, the new insights about these fluxes can help experts predict the carbon budget.