Abstract

Spheroidal weathering, a common mechanism that initiates the transformation of bedrock to saprolite, creates concentric
fractures demarcating relatively unaltered corestones and progressively more altered rindlets. In the spheroidally weathering Rio
Blanco quartz diorite (Puerto Rico), diffusion of oxygen into corestones initiates oxidation of ferrous minerals and precipitation of
ferric oxides. A positive ΔV of reaction results in the build-up of elastic strain energy in the rock. Formation of each fracture is
postulated to occur when the strain energy in a layer equals the fracture surface energy. The rate of spheroidal weathering is thus a
function of the concentration of reactants, the reaction rate, the rate of transport, and the mechanical properties of the rock.
Substitution of reasonable values for the parameters involved in the model produces results consistent with the observed thickness
of rindlets in the Rio Icacos bedrock (≈2–3cm) and a time interval between fractures (≈200–300 a) based on an assumption of
steady-state denudation at the measured rate of 0.01cm/a. Averaged over times longer than this interval, the rate of advance of the
bedrock–saprolite interface during spheroidal weathering (the weathering advance rate) is constant with time. Assuming that the
oxygen concentration at the bedrock–saprolite interface varies with the thickness of soil/saprolite yields predictive equations for
how weathering advance rate and steady-state saprolite/soil thickness depend upon atmospheric oxygen levels and upon
denudation rate. The denudation and weathering advance rates at steady state are therefore related through a condition on the
concentration of porewater oxygen at the base of the saprolite. In our model for spheroidal weathering of the Rio Blanco quartz
diorite, fractures occur every ∼250yr, ferric oxide is fully depleted over a four rindlet set in ∼1000yr, and saprolitization is
completed in ∼5000yr in the zone containing ∼20 rindlets. Spheroidal weathering thus allows weathering to keep up with the high
rate of denudation by enhancing access of bedrock to reactants by fracturing. Coupling of denudation and weathering advance rates
can also occur for the case that weathering occurs without spheroidal fractures, but for the same kinetics and transport parameters,
the maximum rate of saprolitization achieved would be far smaller than the rate of denudation for the Rio Blanco system. The
spheroidal weathering model provides a quantitative picture of how physical and chemical processes can be coupled explicitly
during bedrock alteration to soil to explain weathering advance rates that are constant in time.

Citation

Fletcher, R.C., Buss, H.L., Brantley S.L. (2006): A spheroidal weathering model coupling porewater chemistry to soil thicknesses during steady-state denudation. Earth and Planetary Science Letters. DOI: 10.1016/j.epsl.2006.01.055