University of Massachusetts Amherst researchers have added a new source of carbon dioxide to the complex climate change puzzle by showing that ancient rocks can release substantial amounts of organic matter into Earth’s rivers and oceans, and that this organic matter is easily converted by bacteria to carbon dioxide, which enters the atmosphere and contributes to climate change.

“Sedimentary rocks contain the largest mass of organic carbon on Earth, but these reservoirs are not well-integrated into modern carbon budgets” says Steven Petsch, a professor of geosciences. “Since we need to know the budget of the natural carbon cycle in order to determine human climate impacts, this information will lead to more accurate climate modeling.” The research was conducted by Petsch and UMass Amherst graduate student Sarah Schillawski.

In a study published in the July issue of Global Biogeochemical Cycles, Petsch and Schillawski focused on black shales from Kentucky. Black shales are rich in a type of organic matter called kerogen that contains carbon. Kerogen can turn into oil and natural gas when the rocks are heated. The first step was to determine how much organic carbon could be released from the rocks by simulating the weathering process in the laboratory.

Samples of the shale were placed in glass columns, and the effects of weathering were duplicated by running water through the samples for one year. Kerogen is thought to be difficult to dissolve, but the results of the column studies showed a slow, sustained release of organic matter from the rock. Over the course of one year, the rock samples had lost approximately 0.3 percent of their total organic carbon.

The next step was to determine whether this hard-to-digest organic matter could be broken down by bacteria into carbon dioxide. Using common bacteria found in natural waters, including the Quabbin Reservoir, Petsch found that essentially all of the dissolved organic matter in water from the column studies was rapidly degraded by bacteria over a period of nine days.

“This was the most surprising finding in the study, since these bacteria are adapted to digest organic matter from things like leaves and acorns, which is similar to carbohydrates consumed by humans,” says Petsch. “The presence of microorganisms capable of using kerogen may have significant implications for the global-scale cycling of carbon and oxygen.”

Petsch has also studied the release of carbon from sedimentary rocks by soil bacteria, which is another way that ancient carbon can be converted into carbon dioxide. “We have found outcrops of the New Albany Shale, which is usually black, that have turned a light brown color as bacteria consume carbon where the overlying soil meets the weathered rock,” says Petsch.

According to Petsch, the bottom line is that the release of organic material from sedimentary rocks contributes approximately 2 percent of the carbon dioxide that enters the atmosphere each year. While this may seem like a small amount, it is another piece of the puzzle that can be used when determining how to reduce greenhouse gas emissions in the coming decades.