Rocks, rain and carbon dioxide have helped control Earth’s climate for thousands of years – like a thermostat – through a process called weathering. A new study led by Penn State scientists may improve our understanding of how this thermostat responds to changes in temperature.
“Life has existed on this planet for billions of years, so we know that Earth’s temperature has remained stable enough for liquid water to exist and support life,” said Susan Brantley, a professor at Evan Pugh University and the Barnes Professor of Geosciences at Pennsylvania State University. . “The idea is that the weathering of silicate rocks is a thermostat, but no one has ever agreed on its temperature sensitivity.”
Because many factors affect weathering, it has been difficult to use only the results of laboratory experiments to create global estimates of how weathering responds to changes in temperature, the scientists said.
The team combined laboratory measurements and soil analysis from 45 soil sites around the world and many watersheds to better understand the weathering of Earth’s major rock types, and used the findings to create a global assessment of how weathering responds to temperature.
“When you do experiments in the lab versus taking samples from soil or a river, you get different values,” Brantley said. “So what we tried to do in this study is look at these different spatial scales and figure out how we can make sense of all this data that geochemists around the world are accumulating about weathering on the planet. And this study is a model of how we can do that.”
Weathering is part of the balance of carbon dioxide in the Earth’s atmosphere. Volcanoes have emitted large amounts of carbon dioxide throughout Earth’s history, but instead of turning the planet into a hot house, the greenhouse gas is slowly removed through weathering.
Rain takes carbon dioxide from the atmosphere and creates a weak acid that falls to Earth and wears away silicate rocks on the surface. The byproducts are carried by streams and rivers into the ocean, where the carbon is eventually locked up in sedimentary rocks, scientists say.
“It has long been hypothesized that the balance between carbon dioxide entering the atmosphere from volcanoes and being removed by weathering over millions of years keeps the planet’s temperature relatively constant,” Brantley said. “The key is that when there’s more carbon dioxide in the atmosphere and the planet gets hotter, weathering happens faster and pulls out more carbon dioxide. And when the planet is colder, weathering slows down.”
But much remains unknown about how sensitive weathering is to temperature change, in part because of the long spatial and temporal scales.
“In a soil profile, you see a picture of the soil where the shutter of the chamber has been open for sometimes a million years – there are complex processes going on over a million years, and you’re trying to compare that to a two-year flask experiment. ” Brantley said.
Brantley said critical zone science, which examines landscapes from the highest vegetation to the deepest groundwater, has helped scientists better understand the complex interactions that influence weather.
For example, rocks must break down for water to enter the cracks and begin to break down the materials. For this to happen, the rock must have a large exposed surface, which is less likely in regions where the soil is deeper.
“It’s only when you start to cross spatial and temporal scales that you start to see what’s really important,” Brantley said. “Surface area is very important. You can measure all the rate constants you want for this solution in the lab, but until you tell me how the surface area is formed in a natural system, you will never be able to predict the real system.’
Scientists reported this in the journal Science that laboratory temperature sensitivity measurements were lower than soil and river estimates in their study. Using laboratory and field observations, they refined their results to estimate the global temperature dependence of weathering.
Their model could be useful for understanding how weather will respond to climate change in the future, and for assessing human attempts to increase the amount of weather to get more carbon dioxide out of the atmosphere — such as carbon sequestration.
“One of the ideas was to increase the weathering by excavating large amounts of rock, crushing it, transporting it and placing it in fields to allow the weathering to occur,” Brantley said. “And it will work – it already works. The problem is that it’s a very slow process.”
While warming may speed up weathering, it could take thousands or hundreds of thousands of years to pull all the carbon dioxide that humans have added out of the atmosphere, scientists say.
Other Penn State researchers involved in the study include Andrew Shaughnessy, a doctoral student in the Department of Earth Sciences, and Marina Lebedeva and Viktor Balashov, senior scientists in the Institute for Earth and Environmental Systems.
The National Science Foundation and the Hubert L. Barnes and Mary Barnes Professorship supported this work.
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