Rocks, rain and carbon dioxide help control Earth's climate over thousands of years -- like a thermostat -- through a process called weathering. A new study may improve our understanding of how this thermostat responds as temperatures change.
From: Penn State
February 5, 2023 – "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," said Susan Brantley, Evan Pugh University Professor and Barnes
Professor of Geosciences at Penn State. "The idea is that silicate rock
weathering is this thermostat, but no one has ever really agreed on its
temperature sensitivity."
Because many factors go
into weathering, it has been challenging to use results of laboratory
experiments alone to create global estimates of how weathering responds to
temperature changes, the scientists said.
The team combined
laboratory measurements and soil analysis from 45 soil sites around the world
and many watersheds to better understand weathering of the major rock types on
Earth and used those findings to create a global estimate for how weathering
responds to temperature.
"When you do
experiments in the laboratory versus taking samples from soil or a river, you
get different values," Brantley said. "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."
Weathering represents
part of a balancing act of carbon dioxide in Earth's atmosphere. Volcanoes have
emitted large amounts of carbon dioxide through Earth's history, but instead of
turning the planet into a hot house, the greenhouse gas is slowly removed via
weathering.
Rain takes the carbon
dioxide from the atmosphere and creates a weak acid that falls to Earth and
wears away silicate rocks the surface. The byproducts are carried by streams
and rivers to the ocean where the carbon is eventually locked away in
sedimentary rocks, the scientists said.
"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," Brantley said.
"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."
But much remains
unknown about how sensitive weathering is to changing temperatures, partly because
of the long spatial and time scales involved.
"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," Brantley said.
Brantley said the field
of critical zone science -- which examines landscapes from the tallest
vegetation to the deepest groundwater -- has helped scientists better
understand the complex interactions that influence weathering.
For example, rocks must
fracture for water to get in cracks and start breaking down the materials. For
that to happen, the rock must have large, exposed surface areas, and that is
less likely to happen in regions where soil is deeper.
"It's only when
you start crossing spatial and time scales that you start seeing what's really
important," Brantley said. "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."
The scientists reported
in the journal Science that temperature sensitivity
measurements in the laboratory were lower than estimates from soils and rivers
in their study. Using observations from the lab and field sites, they upscaled
their findings to estimate the global temperature dependance of weathering.
Their model may be
helpful for understanding how weathering will respond to future climate change,
and in evaluating human-made attempts to increase weathering to draw more
carbon dioxide from the atmosphere -- like carbon sequestration.
"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," Brantley said.
"And that will work -- it's already working. The problem is, it's a very
slow process."
Though warming may
speed up weathering, pulling all the carbon dioxide out of the atmosphere that
humans have added could take thousands or hundreds of thousands of years, the
scientists said.
Other Penn State
researchers who participated on the study were Andrew Shaughnessy, doctoral
candidate in the Department of Geosciences and Marina Lebedeva and Victor
Balashov, senior scientists in the Earth and Environmental Systems Institute.
The National Science
Foundation and the Hubert L. Barnes and Mary Barnes Professorship supported
this work.
https://www.sciencedaily.com/releases/2023/02/230205081323.htm
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