TOWARDS A MODEL LINKING WEATHERING ADVANCE RATES TO EROSION DURING SPHEROIDAL WEATHERING

The rate of erosion can be greater than, equal to, or less than the rate of transformation of bedrock to saprolite (the weathering advance rate). Where [lateral transport of soil or saprolite is minimal but] the local rate of erosion exceeds the weathering advance rate, bedrock is exposed at the earth surface. In contrast, in locations where the erosion rate is slower than weathering advance rates, saprolite accumulates. Since large expanses of the earth's surface are regolith-mantled, erosion rates must commonly be ≤ weathering advance rates, leading to the question: how can weathering advance rates be maintained ≤ erosion rates? One common form of bedrock weathering on many types of rock is spheroidal weathering, where intact bedrock is rounded by subparallel spheroidal fractures that create rindlets surrounding intact corestones. Observations of spheroidal weathering and chemical weathering in the Rio Blanco quartz diorite in Puerto Rico led us to develop a model for spheroidal weathering. Sets of rindlets separating Puerto Rican corestones from saprolite are 40 cm thick. The model assumes a mechanism for fracturing driven by oxidation of the Fe(II)O component in the biotite to an Fe(III)-containing component which builds up strain energy due to the increase in volume. Once strain energy accumulates to the point that it equals the surface energy of a fracture, a spheroidal fracture forms and creates the first rindlet. Rindlet formation brings reactive fluid into the bedrock and plagioclase weathering begins or accelerates. As rindlets continue to form, both biotite oxidation and plagioclase dissolution occurs. The outer rindlet transforms to saprolite at the point where plagioclase disappears. Thus, movement of the weathering boundary inward with time is driven by biotite oxidation and goethite precipitation while movement of the outer boundary (rindlet-saprolite interface) is driven by the consumption rate of plagioclase. These reactions are coupled together with the surface through the concentrations of dissolved oxygen and protons in porewaters: faster erosion results in thinner saprolite and higher diffusion-controlled concentration of dissolved oxygen at depth, driving a faster weathering advance rate.