The Importance of Chemical Equilibrium in Controlling Chemical Weathering Rates: Insights from Reactive Transport Modeling of Soil Chronosequences

Reactive transport model simulations of soil profile development can be used to uniquely constrain the rates of soil production, the weathering rates of individual minerals and to explore the correlation between different factors of soil formation when appropriate constraints exist. A multi-component reactive transport model (CrunchFlow) was used to interpret soil profile development and mineral precipitation and dissolution rates at the 226 ka marine terrace chronosequence near Santa Cruz, CA. Aqueous compositions, fluid chemistry, transport, and mineral abundances are well characterized and were used to constrain the reaction rates for the weathering and precipitating minerals in the reactive transport modeling. Model results suggest that the argillic horizon at Santa Cruz can be explained almost entirely by weathering of primary minerals and in situ clay precipitation accompanied by undersaturation of kaolinite at the top of the profile, with no substantial illuviation or translocation of clays. In addition, primary mineral weathering rates constrained by the model are consistent with laboratory values when kinetic rate laws with a non-linear dependence on thermodynamic equilibrium are considered. Overall, our results suggest that the proximity to thermodynamic equilibrium of the reacting phases is extremely important for understanding how soils develop temporally, and in relation to other factors of soil formation.

Our model simulations, combined with data from other chronosequence studies where hydrologic data exists, suggests that the processes that control the departure from thermodynamic equilibrium, such as fluid flow or fluid residence time, secondary mineral precipitation, and microbially- supported reactions predominantly control the rate at which the weathering reactions proceed, and therefore rate of soil development. The extent to which these factors are affected by climatic and tectonic processes will determine the overall chemical weathering rate for a given system.