Durham professor discovers missing link in carbon cycle

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The carbon cycle is a biogeochemical event where carbon is exchanged across the biosphere, pedosphere, geosphere, and atmosphere of earth. In this process, carbon is ferried in different forms, from CO2 to plant biomass in photosynthesis, to carbohydrates in animals after consumption, and released back into gaseous CO2 by microbial decomposition of these animals after death. This is a description well-known to all of us; studied by children in Geography classes at school.However, a new link in this cycle has been defined by Professor Robert Hilton, from Durham University’s Geography department, and Guillaume Soulet, Durham University postdoctoral researcher.

The carbon cycle is a biogeochemical event where carbon is exchanged across the biosphere.

This was done through the study of sedimentary rocks, which contain vast stores of organic carbon, 130,000 times that of the pre-industrial atmosphere. Oxidative weathering processes of these rocks release CO2 into the air. This gas can also enter rivers in an aqueous form, as a bicarbonate, and enter the ocean. This is a carbon sink and delays the release to the atmosphere by 104 years. Professor Hilton and his team aimed to research the fluctuations of the gaseous release of CO2 through different climate conditions, which has never been documented before. Global fluxes of CO2 emissions are natural and important, but the sensitivity of CO2 release from sedimentary rocks under our new changing climate is unknown and unconstrained.

The research group monitored the CO2 emissions from these rocks over a period of two years, in the Laval catchment of the INRAE Draix-Bléone observatory, France. The team worked closely with onsite experts to ensure the wildlife and environment would be preserved while sampling five areas: this consisted of drilling small holes into the rock samples and inserting a gas cylinder and tube, allowing extraction. Prof. Hilton said “It was quite a complex process, and it would take hours to make just one of the measurements. Once the gas sample was obtained, we would take it back to the labs to analyse the different isotopes and split them into carbon 12, which makes up 99% of the carbon on earth, and carbon 14, which is also known as radiocarbon. Where the relative amount of radiocarbon is low, this tells us the carbon in the sample collected derives from the rock.”

He described the process as “as if the rocks were breathing”, as the CO2 derived from organic matter in the rock formation, such as ancient vegetation and animals, was released. The study revealed that during the summer months the rocks gave off five times more the amount of CO2 released in the winter months. This rings alarm bells when we think about the results in the context of climate change: the warming acts as a feedback loop. Prof. Hilton explained: “The hotter weather will mean more CO2 being released into the atmosphere, which will in turn increase the temperature even more.”

[Prof. Hilton] described the process as “if the rocks were breathing”.

He continued: “This study has increased our knowledge of carbon dioxide being released from rocks and when further research in different climates is conducted, we would expect the findings of higher temperature causing more CO2 to still be the case.”

Previous research has shown that around 100 megaton of carbon dioxide is released into the atmosphere by sedimentary rocks every year – which is 100 times less than the carbon being released by the burning of fossil fuels. This comes as another warning to the wide-reaching effects of climate change: it influences this fundamental biogeochemical cycle, essential to life.

Image: Robert Hilton

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