New research says that the Earth’s past ice ages may have been caused by tectonic pile-ups in the tropics.
Our planet has braved three major ice ages in the past 540 million years, seeing global temperatures plummet and ice sheets stretching far beyond the poles. Needless to say, these were quite dramatic events for the planet, so researchers are keen to understand what set them off. A new study reports that plate tectonics might be the culprit.
Cold hard plates
“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 and a co-author of the new study.
“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.”
“Arc-continent collisions” is a term that describes the slow, grinding head-butting that takes place when a piece of oceanic crust hits a continent (i.e. continental crust). Generally speaking, oceanic crust (OC) will slip beneath the continental crust (CC) during such collisions, as the former is denser than the latter. Arc-continent collisions are a mainstay of orogen (mountain range) formation, as they cause the edges of CC plates ‘wrinkle up’. But in geology, as is often the case in life, things don’t always go according to plan.
The study reports that the last three major ice ages were preceded by arc-continent collisions in the tropics which exposed tens of thousands of kilometers of oceanic, rather than continental, crust to the atmosphere. The heat and humidity of the tropics then likely triggered a chemical reaction between calcium and magnesium minerals in these rocks and carbon dioxide in the air. This would have scrubbed huge quantities of atmospheric CO2 to form carbonate rocks (such as limestone).
Over time, this led to a global cooling of the climate, setting off the ice ages, they add.
The team tracked the movements of two suture zones (the areas where plates collide) in today’s Himalayan mountains. Both sutures were formed during the same tectonic migrations, they report: one collision 80 million years ago, when the supercontinent Gondwana moved north creating part of Eurasia, and another 50 million years ago. Both collisions occurred near the equator and proceeded global atmospheric cooling events by several million years.
In geological terms, ‘several million years’ is basically the blink of an eye. So, curious to see whether one event caused the other, the team analyzed the rate at which oceanic rocks known as ophiolites can react to CO2 in the tropics. They conclude that, given the location and magnitude of the events that created them, both of the sutures they investigated could have absorbed enough CO2 to cool the atmosphere enough to trigger the subsequent ice ages.
Another interesting find is that the same processes likely led to the end of these ice ages. The fresh oceanic crust progressively lost its ability to scrub CO2 from the air (as the calcium and magnesium minerals transformed into carbonate rocks), allowing the atmosphere to stabilize.
“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.”
The team then expanded their analysis to older ice ages to see whether they were also associated with tropical arc-continent collisions. After compiling the location of major suture zones on Earth from pre-existing literature, they reconstruct their movement and that of the plates which generated them over time using computer simulations.
All in all, the team found three periods over the last 540 million years in which major suture zones (those about 10,000 kilometers in length) were formed in the tropics. Their formation coincided with three major ice ages, they add: one the Late Ordovician (455 to 440 million years ago), one in the Permo-Carboniferous (335 to 280 million years ago), and one in the Cenozoic (35 million years ago to present day). This wasn’t a happy coincidence, either. The team explains that no ice ages or glaciation events occurred 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.”
Jagoutz notes that there is a major suture zone active today in Indonesia. It includes some of the largest bodies of ophiolite rocks in the world today, and Jagoutz says it may prove to be an important resource for absorbing carbon dioxide. The team says that the findings lend some weight to current proposals to grind up these ophiolites in massive quantities and spread them along the equatorial belt in an effort to counteract our CO2 emissions. However, they also point to how such efforts may, in fact, produce additional carbon emissions — and also suggest that such measures may simply take too long to produce results 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.”
The paper “Arc-continent collisions in the tropics set Earth’s climate state” has been published in the journal Science.