The weathering of rock is a crucial process to keep the earth livable. But on a global scale, things are progressing a lot slower than expected, partly because many places on earth are too dry for it. This is evident from research which was published on Thursday in Science.
The weathering of rock is also referred to as the Earth’s thermostat. Without this process, the earth would become uninhabitable in many millions of years. Because volcanoes continuously release small amounts of gas, including the greenhouse gas CO2, in the atmosphere. Without a counter-reaction, the concentration of CO2 in the atmosphere, and with it the temperature on earth, continue to rise over time. “The Earth would become a kind of Venus,” says Appy Sluijs, professor of paleoceanography at Utrecht University, who was not involved in the research.
The fact that this did not happen is because CO2 is taken off the air. CO dissolves in the atmosphere2 up in rainwater. That rainwater is therefore somewhat acidic and therefore dissolves rocks. The CO2 is converted into a reaction product that flows to the oceans. Organisms turn it into carbonates (including calcium carbonate, also known as calcite) and these precipitate on the ocean floor. Will there be more CO2 in the atmosphere and the temperature on earth rises, the weathering reaction accelerates and more CO disappears2 from the sky to the ocean floor. When it cools, the reaction slows down.
French chemist
The scientific basis of this weathering mechanism was laid in the mid-nineteenth century by the French chemist Jacques-Joseph Ébelmen. But it has always been the question of how strongly the weathering reaction responds to temperature changes. “For a long time, that was only studied in laboratories,” says Susan Brantley, first author of the now published paper, and a professor of earth sciences at Pennsylvania State University. The results of those studies varied widely. “Partly because weathering is a very slow process,” says Brantley.
In the current study, Brantley and colleagues collected a large amount of data, from laboratory studies, but also from experiments in the field, in different types of soils, on slopes, in different regions. “We’ve been researching real-world weathering ourselves for years,” says Brantley.
The results show that outdoor weathering, if it occurs, is faster than in the lab. But extended to a global scale, this translates differently, because there are many places where weathering cannot or hardly take place. “Because it’s too dry. Or because the rock is hidden under such a thick layer of soil that the water cannot penetrate,” says Brantley. On balance, the researchers conclude, weathering is slower than previously thought.
Holistic approach
Sluijs says he is “impressed” by the amount of data that has been processed and the holistic approach of the study. “Weathering has been studied at all scales, from lab to regional to global scale.”
Commenting on the study, Robert Hilton, professor of sedimentary geology at Oxford University, writes that naturally occurring weathering “unfortunately […] is too slow to account for the large excesses of CO2 released each year by mopping up human activities.” What could be done is crushing rock and spreading it over large surfaces, in order to stimulate weathering and accelerate CO2 to remove from the atmosphere. It is an approach to which the necessary research is taking place in the Netherlands, in particular with the mineral olivine. But Brantley has her doubts about it. “Because of the enormous amounts of CO2 to extract those that are now airborne, you need a lot of crushed rock and huge surfaces.” According to her, we must first of all stop emitting greenhouse gases. And secondly, we must try “everything” to reduce CO2 from the atmosphere, such as planting trees, and CO2 extracting and recording from the air with installations.
Read about a Dutch trial: Olivine lives up to its promise as a carbon dioxide capturer
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