Carbon dioxide in the atmosphere is a major driver of shifts in the Earth's climate, as we know all too well in our currently warming world. But it also works the other way – in the past too little CO2 has been associated with triggering ice ages. Now a team of scientists has found a surprising new mechanism that could lead to a drop in CO2 and bring on ice ages: Tropical tectonic activity.
Although ice ages are clearly marked in the geological record, it's still a bit murky as to what could cause such drastic shifts in climate. Some cooling events may be the result of disrupted ocean currents or even comet collisions, but the general consensus is that reduced CO2 in the atmosphere is the most reliable way to put Earth on ice.
But what exactly causes the dip in CO2 levels in the first place? A team of scientists from MIT, and the University of California at Santa Barbara and Berkeley have now pinned the blame on tectonic activity in the tropics.
When an oceanic plate and a continental plate smash into each other, they can buckle upwards and eventually create mountain ranges like the Himalayas. The fault zones that result from these collisions are known as "sutures," and the team found that large sutures seemed to appear near the equator just before each of the last three major ice ages. The researchers suggest that that's no coincidence.
"We think that arc-continent collisions at low latitudes are the trigger for global cooling," says Oliver Jagoutz, an author of the study. "This could occur over 1- 5 million square kilometers (0.4 to 1.9 million sq mi), 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."
So what do tropical sutures have to do with global cooling? The team says that when continents collide, oceanic rocks known as ophiolites are thrust out into the open. These newly-exposed rocks can react with the carbon dioxide in the air and effectively trap it. Given a large enough suture and the right environmental conditions, this new carbon sink could pull enough CO2 out of the atmosphere to bring on a global cooling event.
By the same token, this mechanism could also be responsible for ending ice ages too. After millions of years of absorbing CO2, the ophiolites would wear away in the weather, reducing the uptake of the gas. That lets it build back up in the atmosphere and gradually warm the planet back up.
"We showed that this process can start and end glaciation," says Jagoutz. "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 first noticed the connection between tropical sutures and ice ages in an earlier study, linking the mechanism to glaciations that occurred 80 million and 50 million years ago. For the new study, they looked further back, to 540 million years ago.
First, the researchers identified all the major suture zones on the planet today, then ran computer simulations of plate tectonics to figure out roughly when and where they formed.
During that half-billion year period to the present day, the team found three times when large sutures, some 10,000 km (6,210 mi) long, formed in the tropics. And importantly, in each case an ice age followed just a few million years later – a blip on the geological time scale.
These ice ages started around 455 million years ago in the Late Ordovician, 335 million years ago in the Permo-Carboniferous, and 35 million years ago in the Cenozoic. The team also noticed that when sutures formed outside of tropical regions, no glaciation events followed.
Today, there is one major suture zone still sitting in the tropics: the islands of Indonesia. The team says this is one of the most active carbon sinks in the world at the moment, but unfortunately, the process is likely too slow to help offset our rising CO2 emissions.
The research was published in the journal Science.
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