Fletcher RC, Brantley SL. 2010. Reduction of bedrock blocks as corestones in the weathering profile: observations

and model. Am. J. Sci. 310:131–64

Abstract:

the Espiritu Santo and Mameyes rivers within the Luquillo Experimental
Forest (Puerto Rico) are interpreted as corestones, reduced from initial joint-bounded
bedrock blocks by subsurface weathering. Maximum corestone size, expressed as the
geometric mean of the three dimensions, S 3 abc, shows a smooth envelope when
plotted against elevation. We postulate that, for each catchment, they represent in situ
corestones within a stratified weathering profile, many tens of meters in thickness, that
has been subsequently exhumed by younger erosion. We formulate a simplified
one-dimensional model for reduction in corestone size within a steady-state weathering
profile that incorporates: (i) vertical fluid transport of the reactant and the soluble
products of chemical weathering; (ii) linear kinetics of corestone reduction; and,
subsequently, (iii) erosion. The rate of advance of a steady-state weathering profile is a
statement of the mass balance between entering reactants and weathering components,
here idealized as H and albite. The mathematical relations, tie the laboratory-determined rate constant for dissolution of albite (k) to a generalized
kinetic constant for the rate of decrease (K) in corestone diameter to the advance rate
of the weathering profile (V ). The last parentheses contain an effective roughness at
the scale of the weathering profile, where S0 is the maximum size of initial bedrock
blocks, inferred to be set by initial bedrock fracture spacing, and 3L* is the profile
thickness. The laboratory scale roughness value, , is the ratio of the surface area
accessed by BET analysis to that of the corestone grain scale. In the model, erosion is
not 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. The
thickness of the corestone weathering profile derived for the model for the distance
between bedrock and a corestone-free saprolite cap is approximately This expression is the product of the effective pH buffering-adjusted input reactant
flux 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 is consistent with a rate constant for albite dissolution that lies between laboratorymeasured
and field-estimated values. Sensitivity to the reaction order of albite dissolution
with respect to H, N, is small, except near the base of the profile. This model
yields insights into the relationship between fracture spacing and the evolution of
particle size and chemistry in weathering profiles.