The Science
Shale bedrocks hold Earth’s largest carbon (C) inventory. Although water is recognized for cycling elements through terrestrial environments, understanding how hydrology controls ancient rock carbon (Crock) release from shale bedrock is limited. We measured depth- and seasonally-dependent subsurface fluxes of water, pore-water, and pore-gas geochemical composition (including radiocarbon) over five distinct water years along a mountain hillslope. The data revealed how hydrology, through water table excursions and concomitant subsurface water fluxes, controls shale bedrock weathering and Crock release. The maximum depth to the water table determines the maximum extent of weathering, and subsurface water fluxes determine the types and rates of weathered Crock that are released from bedrock. 80% of released Crock is emitted as CO2 to the atmosphere, primarily during warmer and deeper water table seasons, with 20% of released Crock exported to streams as bicarbonate mostly during snowmelt. Thus, the rates and forms of Crock weathering and release are governed by climate-driven hydrological processes that regulate oxygen availability and subsurface flow dynamics.
The Impact
▪ We revealed that hydrology is the primary control of subsurface rock-carbon weathering and release.
▪ Mountainous regions contribute about 63% of rock-carbon discharge globally, despite being only 32% of the terrestrial surface area. The average shale carbon release rates from mountainous regions are 4.6 to 9.2 Mg km2 yr-1, supporting the broad importance of subsurface flow within weathering zones for releasing C in mountainous regions.
▪ The approaches developed here can be applied to other environments.
Summary
Shale bedrocks contain the largest carbon reserves on Earth. While it’s known that water plays a key role in cycling elements in the environment, we have a limited understanding of how water flow controls the release of carbon from ancient rocks. To explore this, we combined five years of field sampling, lab analyses, and modeling at a mountain hillslope in the East River watershed, part of the Colorado River Basin.
Our study found that hydrology—the movement and distribution of water—is the main factor controlling the weathering and release of carbon from ancient rocks, through two key processes. First, the annual changes in water table depth influence how deep oxygen can penetrate, setting the depth of the weathering front. Second, the movement of water below the surface controls the form and rate at which weathered carbon is released. We discovered that 80% of this carbon is released as CO2 gas into the atmosphere, mainly during warmer seasons when the water table is lower. The remaining 20% is released as bicarbonate, primarily during snowmelt, and flows into rivers and streams.
Overall, the rates and forms of carbon release from weathered rocks are strongly controlled by climate, which affects water flow, oxygen availability, and subsurface dynamics.
Contact
Eoin L. Brodie, Watershed Function SFA LRM
Lawrence Berkeley National Laboratory
Funding
Work was supported by the Watershed Function Science Focus Area funded by the US Department of Energy, Office of Science, Office of Biological and Environmental Research under Contract No. DE-AC02-05CH11231.
Publications
Wan, J.; Tokunaga, T. K.; Beutler, C. A.; Newman, A. W.; Dong, W.; Bill, M.; Brown, W. S.; Henderson, A. N.; Tran, A. P.; Williams, K. H. Hydrological control of rock carbon fluxes from shale weathering, Nature Water, Aug. 2024, https://www.nature.com/articles/s44221-024-00293-8
Data availability https://doi.org/10.15485/2322567.
Nature, News and Views paper review: https://doi.org/10.1038/s44221-024-00292-9