Rock weathering could be a major source of CO2, study says

A recent University of Oxford-led study has challenged the conventional belief that natural rock weathering serves as a carbon dioxide (CO2) sink. Instead, it suggests that this process can act as a substantial CO2 source, potentially rivaling volcanic emissions. These findings, published in the journal Nature, carry significant implications for climate change modeling.

Rocks hold a vast reservoir of carbon derived from ancient organisms that lived millions of years ago, playing a role as a thermostat in regulating Earth’s temperature. Chemical weathering, for example, allows rocks to absorb CO2 when certain minerals react with rainwater’s weak acid. This counters the ongoing CO2 emissions from volcanoes and is part of Earth’s natural carbon cycle, which has sustained a habitable environment for eons.

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

However, this new study for the first time measured an additional process: CO2 release from rocks to the atmosphere, shown to be as significant as volcanic emissions. Curiously, this process isn’t typically considered in most natural carbon cycle models.

This phenomenon occurs when rocks formed on ancient seafloors, where organic matter was buried, resurface as mountains like the Himalayas or Andes form. This exposure allows the organic carbon in rocks to react with air and water, releasing CO2. In essence, weathering rocks may act as a source of CO2 rather than the previously assumed sink.

Measuring this CO2 release from weathering organic carbon in rocks has been a challenge, but the researchers used a tracer element, rhenium, released into water when rock organic carbon reacts with oxygen. By analyzing river water for rhenium levels, they could quantify CO2 release. However, obtaining a global estimate by sampling all river water would be a formidable task.

To overcome this, the researchers calculated the amount of organic carbon in surface rocks and identified locations where erosion rapidly exposed these rocks, particularly in steep mountainous areas. They then used a supercomputer to simulate the complex interplay of physical, chemical, and hydrological processes to estimate the total CO2 emitted as rocks weather.

Landslides in the high Andes of Peru, exposing rocks full of organic matter to weathering which can release CO2. Credit: Robert Hilton.

Comparing this to the CO2 absorbed by natural rock weathering of silicate minerals, they found many regions where weathering acted as a CO2 source, challenging existing views on its impact on the carbon cycle.

Notably, CO2 hotspots were concentrated in areas with high uplift rates, like the eastern Himalayas, Rocky Mountains, and Andes. The global CO2 release from rock organic carbon weathering was estimated at 68 megatons of carbon annually.

Although this figure is much smaller than human-induced CO2 emissions from fossil fuels, it’s similar to the amount released by volcanoes worldwide, making it a significant player in Earth’s natural carbon cycle.

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

Ongoing research will investigate how human-induced changes in erosion and the warming of rocks due to climate change might increase this natural carbon release. The team also questions whether this natural CO2 release will grow in the coming century.

While rock weathering’s CO2 release is minor compared to current human emissions, understanding these natural fluxes can enhance our predictions of the carbon budget and its implications for climate change.

Source: University of Oxford

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