Soil Gas Controls on Chemical Weathering Fluxes

The chemical weathering of rocks by carbonic acid as rainfall percolates through the ground sequesters CO₂ in the form of alkalinity, which is transported via rivers to the ocean. Rates of chemical weathering are thought to scale with climate and act as a negative feedback to atmospheric CO₂ concentrations. This natural process is hypothesized to be the primary mechanism by which the Earth’s climate has remained habitable over the past 4 billion years despite significant changes in solar luminosity and tectonic degassing rates. My research is investigating the relationship between CO₂ concentrations in soil environments (controlled by atmospheric concentrations, soil respiration of organic carbon, and soil diffusivity) and thermodynamic limits that set the potential amount of chemical weathering that can occur.

This research has implications for a number of different problems relating to the long- and short-term carbon cycle and the links between them. For example, soil CO₂ controls on weathering may represent a major connection between environmental conditions and weathering fluxes observed in streams and rivers; however, much work remains in untangling the effects of subsurface flowpaths, hydrologic controls on soil pCO₂, and complex weathering reaction networks that all contribute to observed river solute concentrations.

Ongoing Questions:

• How do soil respiration processes control weathering potential across different rock types?
• To what extent do river solute concentrations reflect thermodynamic limits v. kinetic rates of mineral dissolution/precipitation and hydrologic mixing?
• Can relationships between subsurface pCO₂ and chemical weathering potential be used to better understand the silicate weathering feedback and the evolution of climate through Earth history?

To address these questions, I utilize a combination of reactive transport models of soil weathering profiles, environmental monitoring of soil gas conditions along with solute concentrations in soil-, ground-, and surface- waters, and analyses of large datasets of solute concentrations and discharge from river systems around the world.

Our most recent paper lays out the theoretical framework for addressing these questions:

Winnick, M.J. and Maher, K. (2018) Relationships between CO₂, thermodynamic limits on silicate weathering, and the strength of the silicate weathering feedback. Earth and Planetary Science Letters 485, 111-120. 10.1016/j.epsl.2018.01.005