Coupled Pyrite Oxidation and Carbonate Dissolution in Shales

The weathering of shales, which comprise roughly 20% of Earth’s terrestrial surface-exposed rocks, involves the oxidation of pyrite minerals and dissolution of calcium carbonate. Together, these coupled weathering reactions have been hypothesized to act as primary regulators of atmospheric CO₂ and O₂ concentrations over geological timescales. Additionally, the oxidative weathering of shales is known to release metal contaminants into water supplies that can drastically affect freshwater resources. My research is looking to characterize the dynamics of these coupled weathering reactions and their implications for carbon cycling and stream geochemistry in shale catchments, primarily focusing on the Mancos shale in the East River watershed in Colorado.

Through analyses of concentration-discharge relationships of solutes in the East River we demonstrated that the transition from snowmelt to baseflow hydrology regulates the degree of pyrite oxidation through implied changes in water table height that control oxygen diffusion and subsurface flowpaths. The relative consistency of alkalinity concentrations across a wide range of discharge further suggests that intra-annual variations in relative pyrite oxidation control stream CO₂ evasion rates throughout the year as well. Ongoing and future work will seek to quantify stream CO₂ evasion rates across these hydrologic regimes, and explore the importance of these coupled weathering dynamics to other shale weathering systems.

Ongoing Questions:

• How does hydrology affect the balance of oxidative and acid-base weathering fluxes through its modulation of subsurface flowpaths and gas diffusivity?
• How does pyrite oxidation affect the release of rock carbon to environment in the form of alkalinity and CO₂ degassing from surface waters?
• How will these coupled dynamics respond to future changes in climate such as reduced snowpack and what are the implications for carbon cycling in shale systems?

Our recent paper on East River concentration-discharge dynamics is linked below: 

Winnick, M.J., Carroll, R., Williams, K., Maxwell, R., Dong, W., Maher, K. (2017) Snowmelt controls on concentration-discharge relationships and the balance of oxidative and acid-base weathering fluxes in an alpine catchment, East River, Colorado, USA. Water Resources Research, doi:10.1002/2016WR019724.