Shale, a sedimentary rock that covers 25% of Earth’s continental surface, is one of the world’s largest carbon inventories because of inorganic and organic matters such as carbonate and decaying plants that get pressurized and transfixed into the rock. EESA scientists Jiamin Wan, Markus Bill, Tetsu Tokunaga, Kenneth Williams, Wenming Dong, investigated how water below Earth’s surface, an understudied factor that affects shale weathering, can influence carbon dioxide (CO2) release during this process. Their study, a collaboration with scientists from the Rocky Mountain Biological Laboratory and the Water Resources Institute, is detailed in a recent Nature Water publication which found that 80% of carbon released from these rocks was in the form of carbon dioxide during warmer seasons when the water table was lower.

Depending on what types of minerals are involved, the weathering or breakdown of shale has the potential to either capture or release CO2 or bicarbonate, a dissolved chemical compound released after shale weathering occurs. Studying this process can help scientists better understand this significant part of the carbon cycle that can affect atmospheric CO2 levels.

While it’s known that water plays a key role in cycling elements in the environment, scientists know less about how water flow controls the release of carbon from ancient rocks and how this could be affected by climate change. To study how subsurface water affects carbon in rocks, the researchers collected shale samples and measurements of water fluxes, pore-water, pore-gas, and temperature at various depths and seasons over five years with different precipitation averages along a hillslope in Colorado’s East River watershed. Their findings suggest that water in Earth’s subsurface has the biggest impact on rock weathering in this environment.

“Mountainous regions, despite being only 32% of the terrestrial surface area globally, are the source for about 63% of rock-carbon discharge,” explained EGD Wan, lead author of the study. “Water in the subsurface governs this export of carbon from rocks in these mountainous regions, making it important to study,” said Wan.

The scientists used chemical equations of rock weathering and carbon release to analyze the elements in collected pore water and determine how much carbon was being released, and in what form, over this five year period..This allowed them to identify processes that were occurring, quantify rock weathering rates, and understand the role of different environmental factors such as temperature and precipitation.

Their results showed that hydrology is the primary control on rock-carbon weathering and release through two processes. The first was the annual change of the water table depth that affects the amount of oxygen available to fuel the weathering process. The second is the water entering and leaving the subsurface impacted by processes like precipitation and infiltration that determines the export forms and rates of weathered rock-carbon. They also found that the depth of the water table determines how deep the weathering process is able to occur.

Eighty percent of the carbon released from rocks was emitted as CO2 during warmer and lower water table seasons, while the other 20% was released as bicarbonate, a dissolved chemical compound released after shale weathering occurs, during months of snowmelt.

“Our results highlight the interconnectedness of climate and shale weathering, and that this is an important relationship to consider when predicting changes to the global carbon cycle and atmospheric CO2 levels under climate change,” said Wan.