March 14, 2019, Massachusetts
Institute of Technology
Credit: CC0 Public Domain
Over the last 540 million years, the Earth has weathered three
major ice ages—periods during which global temperatures plummeted, producing
extensive ice sheets and glaciers that have stretched beyond the polar caps.
Now scientists at MIT, the University of California at Santa
Barbara, and the University of California at Berkeley have identified the
likely trigger for these ice ages.
In a study published in Science, the team reports that
each of the last three major ice ages were preceded by tropical
"arc-continent collisions"—tectonic pileups that occurred near the
Earth's equator, in which oceanic plates rode up over continental plates,
exposing tens of thousands of kilometers of oceanic rock to a tropical
environment.
The scientists say that the heat and humidity of the tropics
likely triggered a chemical reaction between the rocks and the atmosphere.
Specifically, the rocks' calcium and magnesium reacted with atmospheric carbon dioxide, pulling the gas
out of the atmosphere and permanently sequestering it in the form of carbonates
such as limestone.
Over time, the researchers say, this weathering process, occurring
over millions of square kilometers, could pull enough carbon dioxide out of the
atmosphere to cool temperatures globally and ultimately set off an ice age.
"We think that arc-continent collisions at low latitudes are
the trigger for global cooling," says Oliver Jagoutz, an associate
professor in MIT's Department of Earth, Atmospheric, and Planetary Sciences.
"This could occur over 1-5 million square kilometers, which sounds like a
lot. But in reality, it's a very thin strip of Earth, sitting in the right
location, that can change the global climate."
Jagoutz' co-authors are Francis Macdonald and Lorraine Lisiecki of
UC Santa Barbara, and Nicholas Swanson-Hysell and Yuem Park of UC Berkeley.
A tropical trigger
When an oceanic plate pushes up against a continental plate, the
collision typically creates a mountain range of newly exposed rock. The fault zone along
which the oceanic and continental plates collide is called a
"suture." Today, certain mountain ranges such as the Himalayas
contain sutures that have migrated from their original collision points, as
continents have shifted over millenia.
In 2016, Jagoutz and his colleagues retraced the movements of two
sutures that today make up the Himalayas. They found that both sutures stemmed
from the same tectonic migration. Eighty million years ago, as the
supercontinent known as Gondwana moved north, part of the landmass was crushed
against Eurasia, exposing a long line of oceanic rock and creating the first
suture; 50 million years ago, another collision between the supercontinents
created a second suture.
The team found that both collisions occurred in tropical zones
near the equator, and both preceded global atmospheric cooling events by
several million years—which is nearly instantaneous on a geologic timescale.
After looking into the rates at which exposed oceanic rock, also known as
ophiolites, could react with carbon dioxide in the tropics, the researchers
concluded that, given their location and magnitude, both sutures could have
indeed sequestered enough carbon dioxide to cool the atmosphere and trigger
both ice ages.
Interestingly, they found that this process was likely responsible
for ending both ice ages as well. Over millions of years, the oceanic rock that
was available to react with the atmosphere eventually eroded away, replaced
with new rock that took up far less carbon dioxide.
"We showed that this process can start and end
glaciation," Jagoutz says. "Then we wondered, how often does that
work? If our hypothesis is correct, we should find that for every time there's
a cooling event, there are a lot of sutures in the tropics."
Exposing Earth's sutures
The researchers looked to see whether ice ages even further back
in Earth's history were associated with similar arc-continent collisions in the
tropics. They performed an extensive literature search to compile the locations
of all the major suture zones on Earth today, and then used a computer
simulation of plate tectonics to reconstruct the movement of these suture
zones, and the Earth's continental and oceanic
plates, back through time. In this way, they were able to pinpoint
approximately where and when each suture originally formed, and how long each
suture stretched.
They identified three periods over the last 540 million years in
which major sutures, of about 10,000 kilometers in length, were formed in the
tropics. Each of these periods coincided with each of three major, well-known
ice ages, in the Late Ordovician (455 to 440 million years ago), the
Permo-Carboniferous (335 to 280 million years ago), and the Cenozoic (35
million years ago to present day). Importantly, they found there were no ice
ages or glaciation events during periods when major suture zones formed outside
of the tropics.
"We found that every time there was a peak in the suture zone
in the tropics, there was a glaciation event," Jagoutz says. "So
every time you get, say, 10,000 kilometers of sutures in the tropics, you get
an ice age."
He notes that a major suture zone, spanning about 10,000
kilometers, is still active today in Indonesia, and is possibly responsible for
the Earth's current glacial period and the appearance of extensive ice sheets
at the poles.
This tropical zone includes some of the largest ophiolite bodies
in the world and is currently one of the most efficient regions on Earth for
absorbing and sequestering carbon dioxide. As global
temperatures are climbing as a result of human-derived carbon
dioxide, some scientists have proposed grinding up vast quantities of
ophiolites and spreading the minerals throughout the equatorial belt, in an
effort to speed up this natural cooling process.
But Jagoutz says the act of grinding up and transporting these
materials could produce additional, unintended carbon emissions. And it's
unclear whether such measures could make any significant impact within our
lifetimes.
"It's a challenge to make this process work on human
timescales," Jagoutz says. "The Earth does this in a slow, geological
process that has nothing to do with what we do to the Earth today. And it will
neither harm us, nor save us."
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