Rocks may seem chemically inert, but carbon and oxygen constantly move between the atmosphere and the land. A new study reveals insights into tiny players in this cycle. Microorganisms in Earth’s surface release carbon dioxide from eroding rocks to the atmosphere more quickly than researchers expected, according to the study.
The work from Jordon Hemingway of Harvard University and colleagues adds much-needed details to our understanding of Earth’s carbon cycle. Scientists previously thought that eroding mountains served as carbon sinks, pulling CO2 from the atmosphere by forming carbonate minerals. The new findings suggest that the process is more complicated.
Researchers have known that microbes play a part in cycling carbon over many millions of years. But the size of the organisms’ contribution has been unclear. To get a better picture of microbes’ role in the cycle, Hemingway and his team studied carbon release in the Central Mountain Range of Taiwan. These mountains erode quickly thanks to frequent typhoons and landslides that churn up bedrock.
The team used a pyrolysis and oxidation technique to measure the amount of organic carbon in bedrock samples, in the soil above the bedrock, and in riverbeds. By analyzing the carbon isotopes in the different soil samples and the bedrock below the soil using mass spectrometry, Hemingway’s team estimated that at least 67% of the organic carbon held in the bedrock had been released—presumably by oxidation to CO2—after those rocks were exposed to air by erosion.
To determine whether microbes were responsible for this oxidation, the researchers looked at the ratio of carbon-13 to carbon-12 in fatty acids present in the soil. The team knew that only bacteria could produce these fatty acids. The carbon isotope ratio matched that of the organic carbon found in the rock, indicating that the microbes were the ones oxidizing the carbon. Hemingway’s team estimates that 6.1 to 18.6 metric tons of carbon per square kilometer get oxidized every year by microbes at these rapidly eroded sites (Science 2018, DOI: 10.1126/science.aao6463).
“We weren’t really expecting to see the organic carbon in the rocks being oxidized so quickly,” Hemingway says. “The traditional thinking was that this rock-derived organic carbon is hard for these microbes to access because it’s been chemically altered under high temperature and pressure in the Earth, so this microbial oxidation was assumed to occur pretty slowly. Our evidence suggests that they can actually access it quite readily.”
Steven Petsch, a geoscientist at the University of Massachusetts, Amherst, says that Hemingway’s work is “a well-crafted and useful study,” that will hopefully lead to more work on the fate of carbon and oxygen as the elements cycle around the planet. Petsch says that few sites around the world erode as quickly as the mountains in the Central Mountain Range of Taiwan. It makes sense that carbon oxidation in these places is relatively rapid. “We need the process to be rapid at these high-erosion-rate sites,” he says, otherwise the lower oxidation rates in areas with modest erosion rates would be insufficient to account for the amount of rock-derived CO2 measured globally.
Hemingway points out that this microbial oxidation of carbon is important on the scale of millions of years. “In terms of anthropogenic climate change, the processes we looked at in our study are unlikely to have any measurable effect,” he says.
As a next step, Hemingway is looking to study other mountain ranges across the globe, including the Southern Alps in New Zealand. He hopes to perform genomic analyses to determine which microbes are responsible for turning the carbon under our feet into CO2 in the air.
CORRECTION: This story was updated on April 19, 2018, to correct the photograph associated with it. The original image was not of mountains in Taiwan. It showed Mount Lu in China.