Tropical tectonic collisions trigger Earth’s ice ages
Earth goes through periodic ice ages — years or decades when global temperatures become frigid, resulting in ice sheets and glaciers covering continents far beyond the planet’s polar caps. In fact, Earth has been through three major ice ages in the last 540 million years. Scientists have long wondered what triggers these chilling times, and a joint study from the Massachusetts Institute of Technology, University of California at Santa Barbara, and the University of California at Berkeley may provide the answer.
The study, published in the journal Science, finds that the last three major ice ages were prefaced by tropical "arc-continent collisions." In these "tectonic pileups" near the equator, oceanic plates drive over continent plates, burying the regular tropical crust and subjecting large swaths of oceanic rock to a tropical environment. According to the paper, this drastic and sudden change triggers a chemical reaction between the oceanic crust and the atmosphere, a weathering process where calcium and magnesium in the oceanic plates react with atmospheric carbon dioxide, producing carbonates such as limestone and reducing atmospheric carbon dioxide content. Over thousands of square kilometers, the phenomenon cuts enough carbon dioxide from the air to decrease global temperatures and trigger an ice age.
"We think that arc-continent collisions at low latitudes are the trigger for global cooling," said Oliver Jagoutz, an associate professor in MIT's Department of Earth, Atmospheric, and Planetary Sciences, in an interview with MIT News. "This could occur over one to five 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.”
A byproduct of arc-continent collisions is usually mountain ranges created at the point of the tectonic pileup. Mountain ranges such as the Himalayas contain sutures, the fault zones where the oceanic and continental plates collide, that have slowly shifted over thousands of years.
In order to study arc-continent collisions more closely, the research team tracked the movements of two sutures that comprise the Himalayas, both originating from the collision of the same tectonic plates. A supercontinent called Gondwana migrated north eighty million years ago, crushing against Eurasia and exposing a tremendously long line of oceanic rock to the atmosphere and forming part of the Himalayas we know today. Thirty million years later, the two supercontinents collided again, creating the second suture in the region.
Jagoutz and his colleagues found that both collisions occurred near Earth’s equator, in tropical zones where the weathering process that removes carbon dioxide from the air can have a serious effect on global temperatures. Indeed, both Himalayan collisions were followed by major atmospheric cooling events within several million years — a blink of an eye on a geological timescale. A subsequent investigation into the chemical reactions that most likely occurred in the Himalayan sutures found that they certainly collected enough carbon dioxide to trigger the ice ages that followed.
Of course, the next logical question is: how do ice ages end? The study posits a theory for this as well, explaining how the crust of the oceanic plate in an arc-continent collision is eventually eroded by the same weathering process that draws carbon dioxide from the atmosphere. The newly produced rock does not sequester nearly as much carbon dioxide, allowing Earth’s temperatures to gradually rise to a normal level.
"We showed that this process can start and end glaciation," Jagoutz said. But his team was not ready to stop there. "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 researchers wanted to confirm their theories by studying even older ice ages, looking for signs of preceding arc-continent collisions. They compiled the locations of all major suture zones on present-day Earth, then simulated the plate tectonics at the time and their movement patterns to trace the migration of sutures at continental and oceanic plates through several millennia. They identified three ice ages and the corresponding periods in which sutures of approximately ten thousand kilometers in length emerged in the tropics. Over the last 540 million years, the trigger of all three ice ages fit the researchers’ theory.
"We found that every time there was a peak in the suture zone in the tropics, there was a glaciation event," Jagoutz explained. "So every time you get, say, 10,000 kilometers of sutures in the tropics, you get an ice age.”
Looking ahead, Jagoutz observed that a major suture zone is still active in Indonesia today, which is likely responsible for the formation of extensive ice sheets near the polar ice caps and Earth’s upcoming glacial period. Given this finding about how oceanic rock at the suture is consuming carbon dioxide from the atmosphere to cool global temperatures and the alarming rate of human carbon dioxide emissions, some scientists have proposed churning through the oceanic rock at the Indonesian suture and spreading it throughout the equatorial belt to accelerate the atmospheric cooling process.
Jagoutz, however, has his doubts. He thinks the machinery required to grind oceanic rock would produce excessive carbon dioxide emissions in itself, and redistributing the minerals obtained might only have a negligible effect on man-made climate change. "It's a challenge to make this process work on human timescales. The Earth does this in a slow, geological process that has nothing to do with what we do to the Earth today," he said, "And it will neither harm us, nor save us."