Natural processes such as photosynthesis, decomposition of organic matter, and rock weathering taking place in the Critical Zone – the Earth’s outer skin that extends from bedrock to the canopy – regulate Earth’s carbon cycling and influence the concentration of carbon dioxide (CO2) in the atmosphere. Rock weathering, the breakdown of rocks, is particularly important for terrestrial carbon fluxes because the chemical reactions between water and minerals can either consume or release CO2.

EESA scientists are studying processes below ground to understand how the delicate balance of carbon in the Critical Zone is shaped by local climate conditions. A team of EESA scientists including first author Postdoc Lucien Stolze, EESA Research Scientist Dipankar Dwivedi, Staff Scientist Bhavna Arora, and Staff Scientist Peter Nico developed a model that describes the interactions between life, water, air, and minerals in shale, a major sedimentary rock covering 25% of Earth’s continental land and that represents a large carbon reservoir. Their model shows how climate conditions shape the net carbon balance associated with shale weathering and how this balance might be altered by future climate factors. Their findings are summarized in a recent publication in PNAS. 

“Understanding and predicting the interconnectedness of weathering, atmospheric CO2 levels, and climate represents a formidable scientific challenge,” explained Stolze. “Models like ours, that capture different processes at play, can help predict the cascading effect of global warming on the exchange of carbon between the atmosphere and the subsurface, and are becoming increasingly necessary to understand the future of our climate.”

The team used a dataset collected at the East River Watershed of the Upper Colorado River Basin. As part of the Watershed Function Science Focus Area Project led by Lawrence Berkeley National Laboratory, this field site primarily serves as a testbed to understand the evolution of mountainous watersheds hydro-biogeochemistry. The dataset captures information about temperature, precipitation, and the transformation and movement of carbon in shale three to four meters below the surface where rock weathering prevails. By developing a model that simulates soil respiration–a key process driving weathering–and mineral reactions under different environmental conditions, the team found that the breakdown of shale is enhanced by large snowmelt or precipitation events.

This is largely because of the connection between precipitation, microbial respiration, and carbonate weathering. When water infiltration occurs, the added moisture in soil makes organic matter more available, stimulating activity from microbes that generate CO2. Exposure of rocks to such carbonic acid produced in soil in addition to water enhances the chemical weathering of carbonate minerals, resulting in increased atmospheric CO2 consumption by rocks.

“Our findings emphasize that shale weathering’s potential to act as a carbon sink is largely determined by climate conditions,” Stolze said. 

Considering these results in the development of larger Earth model frameworks can increase the accuracy of future climate trajectories. This study helps shed light on the complex relationship between shale weathering, climate conditions and the carbon cycle–an important relationship to understand and consider in climate models as Earth’s climate and the carbon cycle are rapidly evolving.