04.4.2019
Volcanic collisions drive long-term climate trends
The presence or absence of collisions between volcanic arcs and continents in the tropics set long-term trends in Earth’s climate, according to new research. For the entire history of our species, humans have lived on a planet with a a chunk of ice capping each pole. But Earth has been ice-free for about 75 percent of the time since complex life first appeared. This variation in background climate, between partly glaciated and ice-free, has puzzled geologists for decades. Now, researchers report in Science that tectonic activity may be the culprit.
Sources or sinks?
“There’ve been a few hypotheses but no agreements as to why we have warmer or colder climates on these very long timescales,” says Francis Macdonald, a professor in the earth science department at the University of California, Santa Barbara. And when Macdonald says “long timescales,” he’s talking about 10 million-year periods, at a minimum. These are broad climatic trends, the backdrop against which natural and human-made fluctuations play out. Scientists have a relatively good understanding of what factors influence the climate on a thousand-year timescale, according to Macdonald. On any scale, though, the primary agent of climate change is carbon dioxide (CO2). The question is what factors influence the amount of CO2 in atmosphere. Some processes produce CO2, while others absorb it. Scientists call these sources and sinks. The debate among geologists is whether sources or sinks affect the climate more.
Collision course
Rock type also plays a key role. Stone rich in iron and magnesium has simpler chemical bonds that break down more easily. This makes these mafic rocks, like basalt, better carbon sinks than rocks such as granite, which have more complex bonds. Plate tectonics is what drives this geologic carbon cycle. When one tectonic plate slides under another—usually a dense ocean plate under a continent—the melting rock fuels a row of volcanoes on the top plate called a volcanic arc. The Cascade Range of the Pacific Northwest is one example of this. Macdonald and his colleagues reckoned that when these volcanic arcs collide with another continent, the collision uplifts mafic rocks. These rocks are readily eroded, particularly in warm, wet, tropical latitudes, and the sediment is sent out to oceans where it consumes CO2. So, he reasoned, when these collisions happen in the tropics, they drive the climate toward cooling. “The tropics are where the rocks weather best because it’s the warmest and wettest,” explains coauthor Lorraine Lisiecki, an associate professor also in the earth science department.Our planet, in flux
To test their hypothesis, the team used reconstructions of the continents and mountain-building events that scientists had built up over the past decades. This gave them an idea where and when arc-continent collisions happened. They limited themselves to the last 500 million years, since the geologic record is much less complete, and reconstructions less certain, before that time. Temperature is harder to get a read on than geography, so the team used a simple metric: Was there ice on the poles at a given time or not? They reconstructed this information from the literature by looking at data on rocks that form only in the presence of ice. What they found was that Earth had significant ice cover during only four periods in their time window.