Rocks don’t appear to do much. But over millions of years, chemical reactions involving rocks can have a tremendous influence on Earth’s climate. Now researchers have amassed evidence that tectonic activity could have exposed rocks that soaked up large amounts of carbon dioxide in the tropics, helping to trigger every ice age over the past 540 million years (Science 2019, DOI: 10.1126/science.aav5300).
For the most part, researchers have explained transitions between ice ages and relatively warmer periods in Earth’s history by assuming there were fluctuations in the main source of CO2—prior to the industrial revolution, volcanic activity. Scientists thought that when volcanic activity increased, Earth warmed as more CO2 went into the atmosphere. When the activity dropped, carbon sinks pulled CO2 from the atmosphere, cooling the planet and allowing glaciers to form. The biggest sink is rock weathering. Over time, weathering exposes calcium and magnesium minerals that react with CO2, sequestering it in carbonates such as limestone. This weathering, scientists thought, was a background process, while CO2 production by volcanoes drove the big changes in climate.
The emphasis has always been on volcanoes as a CO2 source, and scientists have overlooked the other side of the equation, says Oliver Jagoutz, a geologist at the Massachusetts Institute of Technology. Jagoutz is part of a camp of researchers who believe volcanic output hasn’t varied enough to have been the main climate-control switch. “We think there is not a lot of good evidence of variation in volcanoes’ output of CO2,” he says.
Jagoutz and other geochemists have been studying cooling triggered by rock weathering, particularly in the tropics, where warm, wet conditions accelerate carbonate-forming chemistry. They think that changes in geochemical activity drove climate more than volcanic activity did. For example, previous research by Dennis Kent of Rutgers University and by Jagoutz’s group has tied rock weathering to the most recent ice age, which started about 35 million years ago. About 50 million years ago, what’s now India collided with the continent that became Asia, exposing magnesium- and calcium-rich rocks. These “sutures” can be seen in the Himalayas today. This tectonic collision was followed by glaciation.
Jagoutz wanted to know whether this sort of tectonic activity followed by weathering and carbonate production triggered other ice ages as well. “Was this just a coincidence? Or is this something Earth regularly does?” he wondered.
Working with researchers at the University of California, Santa Barbara, he created a database of all such rock formations from the past 540 million years, and used computer models to track how they would have been formed by the movement and collision of ancient continents, islands, and other land masses, as well as where these formations were located at the time. The researchers were interested in collisions that took place in the tropics. Then, they checked the timing of these ancient collisions against the timing of ice ages. The team found a significant correlation in the timing between the two events. Throughout the geological record, when tectonic activity shoved the right sort of minerals to the surface in the tropics, an ice age followed.
Rutgers University’s Kent says the MIT and UC Santa Barbara group make a strong argument for the wide-ranging importance of tectonic activity in controlling the ancient climate. It’s not that volcanic activity is not important, he says. But scientists need to account for both CO2 sources and sinks.
Today, human activity is the main driver of climate change. The geochemical CO2 sinks analyzed in this study work on timescales of tens of millions of years, so they’re “not relevant for human problems,” Jagoutz says. Natural rock weathering simply cannot absorb enough CO2 quickly enough to make a dent in anthropogenic emissions. Some scientists have tried to find ways to speed it up. In 2015, a group of researchers proposed accelerated rock weathering as a way to soak up CO2 while ameliorating ocean acidification (Nat. Climate Change 2015, DOI: 10.1038/nclimate2882). Transporting pulverized silicate rocks to the tropics could indeed create a significant CO2 sink, but it’s likely to be a big engineering challenge, Jagoutz says.
Kent notes that there are already carbon sequestration projects that take advantage of similar chemistry. In Iceland, geologists have injected CO2 emitted by a geothermal plant into a nearby magnesium-rich basalt formation. Cores drilled by the CarbFix project, less than two years later, revealed that the pressurized gas had reacted with the rock to form stable carbonates (Science 2016, DOI: 10.1126/science.aad8132). The kinds of natural rock weathering that caused past ice ages only occur in the warm, wet tropics, Kent says. But projects like CarbFix show how humans might put similar chemistry to work elsewhere.
Given the urgency of addressing climate change, Jagoutz wouldn’t rule out a role for paleogeochemistry-inspired CO2 sinks. “People should think about everything,” he says.