There are no crocodiles near the North Pole today, but that wasn’t always the case. Fossil bones from crocodile-like reptiles called champsosaurs have been unearthed from islands hundreds of miles above the Arctic Circle, surrounded by the remnants of towering trees and prehistoric turtles. They date from 90 million years ago, a time when the now-frozen landscape resembled today’s tropics.
Those out-of-place remains tell us that Earth’s climate was once much warmer than it is now. In fact, Earth’s history has been marked by several such periods of warmth, when levels of heat-trapping carbon dioxide were several times higher than today. Earth has also experienced several cooler periods, when glaciers covered much of the planet and carbon dioxide levels were generally lower. For most of Earth’s history, its climate has fluctuated between these two states, greenhouse and icehouse. In fact, we’re living in an icehouse period today, although it might be hard to believe as Texas summer approaches.
Researchers from UT’s Jackson School of Geosciences have found that during the past 720 million years when volcanoes along continental arcs were more active, Earth was more like a greenhouse. When the planet saw less continental arc volcanic activity, it was more like an icehouse. This finding suggests that carbon dioxide from volcanoes is the primary driver of Earth’s long-term climate cycles.
“We’ve generally been able to tie these long-term shifts to levels of atmospheric carbon dioxide, but the bigger question is what controls the concentration of that atmospheric carbon dioxide,” says lead author Ryan McKenzie, a research fellow in the Department of Geological Sciences.
While much climate research focuses on recent decades or centuries, McKenzie takes the (very) long view. Large-scale greenhouse and icehouse cycles have been perhaps the biggest influences on life’s ongoing evolution, and McKenzie says that studying them helps him appreciate Earth as an integrated system rather than a collection of individual moving parts. Yet his research suggests that moving parts are precisely what’s to blame for Earth’s seesawing CO2 levels.
Volcanic continental arcs can be found today in places like the Andes and Cascade mountains, where continental and ocean tectonic plates collide. As the dense ocean floor is thrust into the scorching magma of Earth’s mantle, sinking beneath the lighter continental crust, carbon trapped in the rock bubbles up in the form of volcanic eruptions.
These same mountain-building processes can also draw carbon out of the atmosphere as new rock is exposed and weathered, and many scientists pointed to this CO2 withdrawal as the key climate driver. But according to McKenzie, “what we’ve shown is that it’s really changes in the mechanism that releases CO2 that have driven these cycles.”
Most of the rocks laid down by such ancient volcanic activity are long gone, but McKenzie and his co-researchers were able to use tiny grains of zircon, a durable mineral created mainly by continental arc volcanoes, as fingerprints of past eruptions. Using radioactive dating, they found that rock layers enriched in young zircon from recent volcanic activity were laid down during greenhouse climate periods. Conversely, they found less young zircon and reduced volcanic activity in rocks dating to icehouse periods. What’s more, McKenzie and his fellow researchers found that during periods when the ancient supercontinents like Pangea broke apart and volcanic activity increased, Earth tended to be hotter, much like when champsosaurs roamed the Arctic.
McKenzie points out that these findings don’t conflict with modern evidence of man-made climate change—since every year humans emit about 100 times as much CO2 as volcanoes—but his work provides the strongest evidence yet that the violent dance of Earth’s continents has not only shaped Earth’s geography, but its climate as well.
Licancabur, a volcano in Chile’s Atacama Desert. Photo by krheesy on Flickr.
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