The role of reaction affinity and secondary minerals in regulating chemical weathering rates at the Santa Cruz Soil Chronosequence, California

Abstract

ABSTRACT:In order to explore the reasons for the apparent discrepancy between laboratory and field weathering rates and to determine the extent to which weathering rates are controlled by the approach to thermodynamic equilibrium, secondary mineral precipitation and flow rates, a multicomponent 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 (White et al., 2008, GCA) and were used to constrain the reaction rates for the weathering and precipitating minerals in the reactive transport modeling. When primary mineral weathering rates are calculated with either of two experimentally determined rate constants, the nonlinear, parallel rate law formulation of Hellmann and Tisserand [2006] or the aluminum inhibition model proposed by Oelkers et al. [1994], modeling results are consistent with fieldscale observations when independently constrained clay precipitation rates are accounted for. Experimental and field rates, therefore, can be reconciled at the Santa Cruz site.Observed maximum clay abundances in the argillic horizons occur at the depth and time where the reaction fronts of the primary minerals overlap. The modeling indicates 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. The rate constant for kaolinite precipitation was also determined based on model simulations of mineral abundances and dissolved Al, SiO 2 (aq) and pH in pore waters. Changes in the rate of kaolinite precipitation or the flow rate do not affect the gradient of the primary mineral weathering profiles, but instead control the rate of propagation of the primary mineral weathering fronts and thus total mass removed from the weathering profile. Our analysis suggests that secondary clay precipitation is as important as aqueous transport in governing the amount of dissolution that occurs within a profile because clay minerals exert a strong control over the reaction affinity of the dissolving primary minerals. The modeling also indicates that the weathering advance rate and the total mass of mineral dissolved is controlled by the thermodynamic saturation of the primary dissolving phases plagioclase and K-feldspar, as is evident from the difference in propagation rates of the reaction fronts for the two minerals despite their very similar kinetic rate laws.

Upscaling calcium carbonate precipitation rates from pore to continuum scale

“…The code CrunchFlow, or its predecessors, has been used for over 20 years to simulate mineral precipitation (and dissolution) in porous media (Steefel and Van Cappellen [1990], Steefel and Lasaga [1990], Steefel and Lasaga [1994], Steefel and Lichtner [1994], Maher et al [2009]). The software solves a set of partial differential equations for the conservation of aqueous and solid mass, with treatment of fluid flow, molecular diffusion, and multicomponent reaction at the continuum scale:…”

Reviews and syntheses: on the roles trees play in building and plumbing the critical zone

“…Some holes in our understanding are obvious. For example, many numerical models are available to simulate chemical weathering and erosion (Lichtner, 1988;Lebedeva et al, 2007;Minasny et al, 2008;Maher et al, 2009) but most only model trees indirectly by incorporating the assumption that trees can reduce the water flow through the soil through evapotranspiration. Where the impact of trees or biota has been incorporated into models of weathering or landscape development, the models typically focus on one aspect of trees' impact (Gabet and Mudd, 2010;Roering et al, 2010;Corenblit et al, 2011;Reinhardt et al, 2011;Godderis and Brantley, 2014).…”

Mineralogical transformations set slow weathering rates in low-porosity metamorphic bedrock on mountain slopes in a tropical climate

“…The successful application of these models relies on an accurate knowledge of the weathering system's transport properties (e.g., soil and saprolite permeability) and chemical parameters (e.g., kinetics of dissolution and precipitation, or equilibrium constants). The nature and properties of the secondary precipitates forming during weathering (i.e., their mineralogy, crystallinity, or ability to form aggregates) exert a strong control on the outputs of such models (Maher et al, 2009). In particular, many weathering reactions have been reported to produce amorphous intermediate phases rather than forming crystalline clays directly from primary minerals (Chadwick and Chorover, 2001;Dahlgren et al, 1997;Hellmann et al, 2012;Steefel and van Cappellen, 1990).…”