Scientific Method —

A recipe for global cooling—put seafloor on dry land near the equator

Tectonic plate collisions could explain 40 million years of climatic history.

Oceanic crust sometimes ends up on land—like the Ingalls Peak area in Washington. This stuff pulls carbon dioxide out of the atmosphere as it weathers.

Earth’s climate has stayed within a pretty narrow range of temperatures over its history if you compare it to the inhospitable heat and cold found elsewhere in our Solar System. This relative stability has been maintained by an intricate system of interactions. On geologic timescales, the chemical commerce between the atmosphere and the rock of Earth’s crust acts as a thermostat. The weathering of common minerals includes a reaction that removes CO2 from the atmosphere. High temperatures (caused by higher CO2) mean faster weathering, which gradually brings CO2 and temperature back down. It’s a moderating influence.

But plate tectonics also fiddle with the dial on that thermostat. Arcs of volcanoes along subduction zones (where one plate dives beneath the other) provide a constant source of CO2, and subduction zones come and go over time. Research using tough zircon crystals as records of volcanic arcs has found a correlation with climate over geologic time. In fact, a new study published this week in Science extends that comparison over the last 720 million years by finding evidence that volcanic activity rises and falls with the great swings in Earth’s climate.

A second study—published in the Proceedings of the National Academy of Sciences and led by MIT’s Oliver Jagoutz—looks at the flip side of the equation: the ability of plate tectonics to strengthen the weathering feedback that eats CO2. Although climate change can increase or decrease the rate of weathering, the amount of exposed and easily weatherable rock makes a huge difference. The igneous rocks that make up oceanic crust, for example, make excellent CO2 sponges—or at least they would, if they weren’t at the bottom of the sea.

There are situations where plate tectonics shoves oceanic crust up onto continents. In the last 100 million years, Africa, Arabia, and India all docked with the Eurasian continent, closing up an ocean basin we call the Tethys. In addition to subduction zones along its continental boundaries, there were oceanic subduction zones with arcs of volcanic islands in the middle of the Tethys. As this mess smushed together, huge amounts of oceanic crust piled up on dry land. This area was located in the tropics at the time, where warmth and prodigious rainfall combine for the strongest weathering on the planet.

Between 90 and 40 million years ago, Earth’s climate went through major shifts. (Also, some creatures called “dinosaurs” you may have heard of disappeared.) Earth was a balmy “hothouse” 90 million years ago, with sea level so high that the central part of what is now the US was a seaway. Over the following 20 million years, things cooled off considerably. The trend then flipped to warming between 70 and 50 million years ago, although temperatures fell short of the previous hothouse peak. And from 50 to 40 million years ago, it was back to cooling—the Antarctic ice sheet was born shortly after.

So how does the slow death of the Tethys Ocean match up with this sequence of cooling, warming, and cooling? During the first cooling phase, the African and Arabian plates ran into the western end of the oceanic subduction zone. That shut off those volcanoes and pushed a 4,000-kilometer-long section of oceanic crust up to 500 kilometers onto the tropical continent. The same thing was happening on the eastern end of the subduction zone, which collided with Eurasia. The resulting weathering could have removed a tremendous amount of CO2 from the atmosphere as that exposed rock crumbled and eroded.

During the warming phase, India was headed north at full steam, driving a vigorous subduction zone that consumed oceanic crust and spit CO2 into the sky. That more than made up for the volcanoes shutting off elsewhere in the Tethys. The huge sections of stranded oceanic crust would have started to shrink by this time as erosion took its toll, weakening the strength of the CO2 sponge.

Around 50 million years ago—the start of the last cooling phase—the Indian plate reached the middle section of the oceanic subduction zone, again shutting off volcanoes and gathering its own slice of oceanic crust onto the surface. That rejuvenated the removal of CO2 by weathering and slowed the volcanic source.

Snapshots of the plate tectonic timeline at 90, 80, and 50 million years ago. Black lines with teeth denote subduction zone boundaries between plates.

The researchers used a model of all these processes to total the CO2 deposits and withdrawals over time. Comparing the results to records of temperatures in the deep ocean, they found that the timings match up. More work is needed to figure out exactly how much atmospheric CO2 concentrations changed and how much of the recorded temperature changes subduction could account for. But the hypothesis that plate tectonic interactions played a big role is plausible.

The researchers found that it’s only plausible because all this took place in the tropics. Simulations using a mid-latitude climate couldn’t get the job done because the rocks didn’t weather fast enough. Looking through history, the researchers note that there are other examples of similar plate collisions. Some occurred in the tropics and happen to coincide with periods of major cooling. But those that occurred at higher latitudes weren’t associated with cooling.

In the staggering expanse of geologic time, even oceans are born and die. A continent rifts apart, creating a widening ocean basin. At some point the separation sputters out and the basin shrinks until the continents are put back together again—it’s a process geologists call the Wilson cycle. The researchers write, “This tight connection between tectonism and global climate represents an extension of the Wilson cycle from the solid Earth to the oceans and atmosphere.” No part of the Earth system exists in isolation.

PNAS, 2016. DOI: 10.1073/pnas.1523667113  (About DOIs).

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