『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 A.F., Schulz M.S., Vivit D.V., Blum A., Stonestrom D.A. and Anderson S.P. (2008) Chemical weathering of a Marine Terrace Chronosequence, Santa Cruz, California. I: interpreting the long-term controls on chemical weathering based on spatial and temporal element and mineral distributions. Geochim. Cosmochim. Acta 72(1), 36-68] 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 [Hellmann R. and Tisserand D. (2006) Dissolution kinetics as a function of the Gibbs free energy of reaction: An experimental study based on albite feldspar. Geochim. Cosmochim. Acta 70(2), 364-383] or the aluminum inhibition model proposed by Oelkers et al. [Oelkers E.H., Schott J. and Devidal J.L. (1994) The effect of aluminum, pH, and chemical affinity on the rates of aluminosilicate dissolution reactions. Geochim. Cosmochim. Acta 58(9), 2011-2024], modeling results are consistent with field-scale observations when independently constrained clay precipitation rates are accounted for. Experimental and field rates, therefore, can be reconciled at the Santa Cruz site.
　Additionally, 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, SiO2(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.』

1. Introduction
2. Site description
3. Model formulation and model constraints
　3.1. Primary and secondary minerals
　3.2. Mineral surface areas
　3.3. Aqueous transport
　3.4. Aqueous chemistry
　3.5. Cation exchange
　3.6. Alternative rate law formulations
　3.7. Model conditions and parameters
4. Results
　4.1. Base case model
　　4.1.1. Profile evolution for end-member compositions
　　4.1.2. Primary mineral stoichiometry and solubility
　　4.1.3. Solute profiles and mineral saturation states
　　4.1.4. Cation exchange and the evolution of clay abundances
　4.2. Results using alternative rate law formulations
　4.3. Comparison to previous weathering rate estimates at the Santa Cruz, CA chronosequences
5. Discussion
　5.1. Solid state profile development and reaction front geometry
　　5.1.1. Primary mineral dissolution
　　5.1.2. Secondary mineral precipitation and the development of an argillic horizon
　5.2. Effect of flow rates on weathering rates and weathering profiles
　5.3. Secondary mineral precipitation as a control on primary mineral weathering
　5.4. Exporting experimental rate constants to natural systems
　5.5. Positive feedback behavior in weathering profiles
6. Conclusion
Acknowledgments
Appendix A. model sensitivities and limitations
　A.1. Effect of mass loss or mass gain via eolian deposition or erosion on observed profiles and reaction rates
　A.2. Effect of variations in soil pCO2
　A.3. Effect of kaolinite solubility and initial protolith abundance on kaolinite rates
　A.4. Parameter sensitivity and covariance
Appendix B. Model limitations and assumptions
References