Eroding mountains could release, not trap, greenhouse gases

The hills are hiding a carbon cache. For decades, scientists believed that the erosion of mountains caused carbon dioxide (CO 2 ) in the atmosphere to drop, as silicate rocks newly exposed to rainwater would “weather,” taking up carbon in carbonate minerals that would sluice down rivers and be sequestered on the sea floor.

But a new line of research, published this week in Science , is complicating that picture. A team of scientists has found that, thanks to opportunistic microbes, some mountain ranges may be sources, not sinks, of carbon. The discovery won’t mean much for climate change: The process occurs over millions of years, and the amounts involved are small compared with human-driven emissions. But it is a new type of feedback mechanism for Earth, one that could have helped the planet maintain its carbon thermostat prior to human interference. “This is part of the carbon cycle that people don’t think about—or don’t really know exists,” says Jordon Hemingway, a geochemist at Harvard University and the study’s lead author.

Hemingway’s coauthors worked in the Central Mountain Range of Taiwan, one of the fastest rising—and eroding—belts in the world. It’s a dramatic landscape formed by the collision of two tectonic plates, with sheer peaks plummeting into the Pacific Ocean. About 0.5% to 1% of the rocks in the range contain carbon, the organic remains of fossilized life buried in sedimentary rocks like shale. This locked-up “petrogenic” carbon comes at the end of a long journey. It started when the corpses of microbes and algae fell to the sea floor and got sucked into Earth’s mantle by diving tectonic plates. There, the carbon was crushed and cooked until it was eventually returned to the surface by the clashing plates.

Scientists thought the story stopped there, as this metamorphosis was thought to render the carbon inaccessible to bacteria and other bugs that could use it as food. But in recent years, researchers have discovered “radiocarbon-dead” microbes that lack the radioactive isotopes of carbon present in all life on Earth’s surface. The only plausible way that could happen would be if the microbes were subsisting on the petrogenic carbon, which would have shed its radioactive signature long ago, thanks to its subsurface journey.

Using samples collected from the Liwu and Wulu river basins in Taiwan, which run off the central range, the team compared the radiocarbon profiles of organic carbon in the rock with the soil directly above it. It became apparent that the rock, on average, lost some 67% of its organic carbon as it first crumbled into soil. The team then went a step further, putting the rock and soil samples in a controlled combustion chamber that released carbon at different temperatures, allowing the carbon molecules to be sorted by their latent energy, an indicator of their chemical structure, and the amount of radiocarbon they contain. They found a category of molecules that didn’t look like petrogenic carbon or organic molecules derived from surface life like plants. Something, likely microbes, had fed on the petrogenic carbon, it appears, altering its composition and releasing CO 2 to the atmosphere.

The idea isn’t new, but this is the first time the process has been identified and quantified, says Mark Torres, a geoscientist at Rice University in Houston, Texas, who is unaffiliated with the study. Although the combustion technique used to sort the carbon molecules by chemistry isn’t perfectly understood, he says, the team makes a convincing case. “These ancient rocks can actually fuel modern ecosystems.”

By looking at the petrogenic carbon lost from the rocks to the soil, the team estimates that the mountain belt releases roughly 6.1 to 18.6 tons of carbon per square kilometer each year through this mechanism—double or more the amount of carbon estimated to be sucked out of the atmosphere by traditional weathering. That means the range could be releasing about the same amount of CO 2 emissions per year as a small U.S. town, Hemingway says.

This may seem like a small number, Torres says, but it upends understanding of the weathering process. And the findings likely apply to mountain ranges around the world, Hemingway says, though it remains unclear how much where the mountains differ in shape and composition.