Reduction of bedrock blocks as corestones in the weathering profile: Observations and model

Blocks of metavolcaniclastic rock exposed at the surface in uplandcatchments of the Espiritu Santo and Mameyes rivers within the Luquillo ExperimentalForest (Puerto Rico) are interpreted as corestones, reduced from initial joint-boundedbedrock blocks by subsurface weathering. Maximum corestone size, expressed as thegeometric mean of the three dimensions, S= ³√ abc, shows a smooth envelope whenplotted against elevation. We postulate that, for each catchment, they represent in situcorestones within a stratified weathering profile, many tens of meters in thickness, thathas been subsequently exhumed by younger erosion. We formulate a simplifiedone- dimensional model for reduction in corestone size within a steady-state weatheringprofile that incorporates: (i) vertical fluid transport of the reactant and the solubleproducts of chemical weathering; (ii) linear kinetics of corestone reduction; and,subsequently, (iii) erosion. The rate of advance of a steady-state weathering profile is astatement of the mass balance between entering reactants and weathering components,here idealized as H⁺ and albite. The mathematical relations,1/2kvλ/fAB(γc_R) ^N = K(γc_R)^N = V (S0/6L*),tie the laboratory-determined rate constant for dissolution of albite (k) to a generalizedkinetic constant for the rate of decrease (K) in corestone diameter to the advance rateof the weathering profile (V ). The last parentheses contain an effective roughness atthe scale of the weathering profile, where S0 is the maximum size of initial bedrockblocks, inferred to be set by initial bedrock fracture spacing, and 3L* is the profilethickness. The laboratory scale roughness value, is the ratio of the surface areaaccessed by BET analysis to that of the corestone grain scale. In the model, erosion isnot coupled with weathering, although the presence of corestones of finite size, SE>0,exiting at the erosional surface may be postulated to affect the erosional flux. Thethickness of the corestone weathering profile derived for the model for the distancebetween bedrock and a corestone-free saprolite cap is approximately3L* =(βqc_R)(M_abf _abρ_ab)S₀/2[K(γc_R)N].This expression is the product of the effective pH buffering-adjusted input reactantflux per unit area times a stoichiometeric factor linking this to net albite dissolution,divided by the rate of corestone size reduction at the input concentration of protons.Further, the profile thickness scales with the input "particle" size, S0. The model fit,which yields the ratio S ₀/3L* ≈ 0.02, is consistent with a rate constant for albite dissolution that lies between laboratory measuredand field-estimated values. Sensitivity to the reaction order of albite dissolutionwith respect to H ⁺, N, is small, except near the base of the profile. This modelyields insights into the relationship between fracture spacing and the evolution of particle size and chemistry in weathering profiles.