Weathering is the process by which rocks, rain, and carbon dioxide help regulate the Earth’s climate over thousands of years, much like a thermostat. A new study guided by Penn State scientists could help better understand how this thermostat reacts to temperature changes.
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The Amazon, Earth’s largest river, transporting weathering solutes from the Andes to the Atlantic Ocean in Brazil. Image Credit: J. Gaillardet, Institut de Physique du Globe de Paris.
“Life has been on this planet for billions of years, so we know Earth’s temperature has remained consistent enough for there to be liquid water and to support life. The idea is that silicate rock weathering is this thermostat, but no one has ever really agreed on its temperature sensitivity,” states Susan Brantley, Evan Pugh University Professor and Barnes Professor of Geosciences at Penn State.
Since weathering is affected by so many factors, the researchers said it has been difficult to generate global estimates of how weathering responds to temperature changes based solely on laboratory experiments.
To better comprehend weathering of the major rock types on Earth, the team combined lab measurements and soil analysis from 45 soil sites around the world and many watersheds and then used those observations to build a global estimate of how weathering responds to temperature.
When you do experiments in the laboratory versus taking samples from soil or a river, you get different values. So what we tried to do in this research is look across those different spatial scales and figure out how we can make sense of all this data geochemists around the world been accumulating about weathering on the planet. And this study is a model for how we can do that.
Susan Brantley, Pennsylvania State University
Weathering is a part of the carbon dioxide balancing act in the Earth’s atmosphere. Throughout Earth’s history, volcanoes have emitted large amounts of carbon dioxide, but rather than turning the planet into a hot house, the greenhouse gas is systematically removed by weathering.
Rain absorbs carbon dioxide from the atmosphere, forming a weak acid that falls to Earth and erodes silicate rocks on the surface. According to scientists, the byproducts are carried by streams and rivers to the ocean, where the carbon is finally locked away in sedimentary rocks.
It has long been hypothesized that the balance between carbon dioxide entering the atmosphere from volcanoes and being pulled out by weathering over millions of years holds the temperature of the planet relatively constant. The key is when there is more carbon dioxide in the atmosphere and the planet gets hotter, weathering goes faster and pulls more carbon dioxide out. And when the planet is cooler, weathering slows down.
Susan Brantley, Pennsylvania State University
However, due to the large spatial and time scales involved, much remains unknown about how sensitive weathering is to changing temperatures.
In a soil profile, you are seeing a picture of soil where the camera shutter was open for sometimes a million years—there are integrated processes happening for a million years and you’re trying to compare that with a two-year flask experiment.
Susan Brantley, Pennsylvania State University
According to Brantley, the field of critical zone science, which investigates landscapes from the tallest vegetation to the deepest groundwater, has assisted scientists in better understanding the complex interactions that influence weathering.
For instance, rocks must fracture for water to enter and break down the materials. That requires the rock to have large, exposed surface areas, which is less likely in areas where the soil is deeper.
Brantley adds, “It’s only when you start crossing spatial and time scales that you start seeing what’s really important. Surface area is really important. You can measure all the rate constants you want for that solution in the lab, but until you can tell me how does surface area form out there in the natural system, you are never going to be able to predict the real system.”
Temperature sensitivity measurements in the lab were lower than estimates from soils and rivers in their research. The study was published in the journal Science. They upscaled their findings using observations from the lab and field sites to estimate the global temperature dependence of weathering.
Their model could help researchers understand how weathering will respond to future climate change, as well as evaluate man-made efforts to increase weathering to remove more carbon dioxide from the atmosphere, such as carbon sequestration.
Brantley notes, “One idea has been to enhance weathering by digging up a lot of rock, grinding it, transporting it and putting it out in the fields to let weathering happen. And that will work—it’s already working. The problem is, it’s a very slow process.”
Though warming may hasten weathering, researchers believe it will take thousands or hundreds of thousands of years to remove all the carbon dioxide that humans have added to the atmosphere.
Other Penn State scientists who took part in the research were Andrew Shaughnessy, a doctoral candidate in the Department of Geosciences and Marina Lebedeva and Victor Balashov, senior scientists at the Earth and Environmental Systems Institute.
The research was funded by the National Science Foundation and the Hubert L. Barnes and Mary Barnes Professorship.
Journal Reference:
Brantley, S. L., et al. (2023) How temperature-dependent silicate weathering acts as Earth’s geological thermostat. Science. doi.org/10.1126/science.add2922.