Over the past 540 million years, the Earth has endured three periods of glaciation during which global temperatures collapsed, producing large ice caps and glaciers that stretched beyond the polar ice caps.
Now the scientists at MIT, the University of California at Santa Barbara and the University of California at Berkeley have identified the probable trigger for these ice ages.
In a study published in Science the team reports that each of the last three main ice ages was preceded by tropical collisions "arc-continent" ̵
Scientists say that the heat and humidity of the tropics have probably triggered a chemical reaction between the rocks and the atmosphere. Specifically, the calcium and magnesium of the rocks reacted with atmospheric carbon dioxide, extracting the gas from the atmosphere and sequestering it permanently in the form of carbonates such as limestone.
Over time, researchers say that this aging process takes millions of square kilometers, could extract enough carbon dioxide from the atmosphere to cool temperatures globally and ultimately trigger an ice age.
"We think that arc-continent collisions at low latitudes are the trigger for global cooling," says Oliver Jagoutz, associate professor in the Department of Earth, Atmospheric and Planetary Sciences at MIT. "This could happen for 1-5 million square kilometers, which seems a lot, but in reality it is a very thin strip of land, which is in the right position, which can change the global climate."
Jagoutz co-Authors are Francis Macdonald and Lorraine Lisiecki of UC Santa Barbara, Nicholas Swanson-Hysell and Yuem Park of UC Berkeley.
A tropical trigger
When an oceanic plate pushes against a continental plate, the collision generally creates a mountain range of newly exposed rock. The fault zone along which the oceanic and continental plates collide is called "suture". Today some mountain ranges like the Himalaya contain sutures that have migrated from their original collision points, as the continents have moved beyond the millennia
In 2016, Jagoutz and his colleagues retraced the movements of two stitches that today make up the Himalaya. They found that both sutures derived from the same tectonic migration. Eighty million years ago, when the supercontinent known as Gondwana moved north, part of the landmass was crushed against Eurasia, exposing a long line of oceanic rocks and creating the first suture; 50 million years ago, another collision between supercontinents created a second suture.
The team found that both collisions occurred in tropical areas near the equator and both preceded the global atmospheric cooling events of several million years, which is almost instantaneous on a geological time scale. After examining the rates at which the exposed oceanic rocks, also known as ophiolites, could react with carbon dioxide in the tropics, the researchers concluded that, given their position and magnitude, both sutures could actually sequester enough carbon dioxide to cool the atmosphere and trigger both ice ages.
It is interesting to note that this process was probably responsible for the end of both ice ages. Over millions of years, the oceanic rock that was willing to react with the atmosphere eventually eroded, replaced by a new rock that absorbed far less carbon dioxide.
"We have shown that this process can begin and end the ice age," says Jagoutz. "So we wondered, how often does it work? If our hypothesis is correct, we should find out that whenever there is a cooling event, there are many sutures in the tropics."
Exposing the sutures of the Earth
The researchers tried to see if the ice ages further back in the history of the Earth were associated with collisions similar to arc-continent in the tropics. They carried out extensive bibliographic research to compile the positions 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 continental and oceanic plates of the Earth, in the time. In this way, they were able to identify approximately where and when each originally formed suture, and for how long each suture was lengthened.
They identified three periods in the last 540 million years in which major sutures, around 10,000 kilometers long, were formed in the tropics. Each of these periods coincided with each of the three great and glacious glacial epochs, in the late Ordovician (from 455 to 440 million years ago), the Permo-Carboniferous (from 335 to 280 million years ago) and the Cenozoic (35 million of years ago to today). Above all, they discovered that there were no glaciations or glacial events during the periods when the main suture areas were formed outside the tropics.
"We found that whenever there was a peak in the suture area in the tropics, it was an ice age event," says Jagoutz. "So every time you get, say, 10,000 miles of stitches in the tropics, you get an ice age."
He notes that an important suture zone, about 10,000 kilometers long, is still active today in Indonesia, and is perhaps responsible for the current glacial period of the Earth and the appearance of large expanses of ice at the poles.
This tropical zone includes some of the largest ophiolitic bodies in the world and is currently one of the most efficient regions of the Earth to absorb and sequester carbon dioxide. Since global temperatures are rising due to carbon dioxide derived from man, some scientists have proposed to grind a large quantity of ophiolites and to diffuse minerals throughout the equatorial belt, in an attempt to accelerate this natural cooling process .
But Jagoutz says that the act of grinding and transporting these materials could produce additional unintended carbon emissions. And it is not clear whether these measures can have a significant impact on our lives.
"It is a challenge to make this process work in human times," says Jagoutz. "The Earth does this in a slow geological process that has nothing to do with what we do today on Earth. It will not harm or save us."
The ancient tectonic activity was triggered for the ice age, the study says
"The collisions between the arch and the continent in the tropics establish the climatic state of the Earth" Science (2019). science.sciencemag.org/lookup/ … 1126 / science.aav5300