EXPLORING INTER-RELATIONSHIPS AMONG CHEMICAL, PHYSICAL, AND BIOLOGICAL WEATHERING

The factors that control the rates of chemical weathering and physical erosion are difficult to decipher: these relationships are further confounded by the influence of biota. Many geochemists and geomorphologists categorize erosion of hillslopes or catchments within endmember regimes: in theory, chemical weathering rate of a weathering-limited system is affected by the rate constants of mineral dissolution (because chemical weathering is slow relative to the capacity for erosion to remove weathered material); in contrast, chemical weathering rate of a transport-limited system is not affected by mineral dissolution kinetics (because the capacity for erosion to remove weathered products is so slow in comparison to mineral weathering). Biota affect both these regimes: soil profiles often document the importance of biocycling.

A simple model of feldspar + inert quartz weathering to kaolinite (+ quartz) can be used to explore how changes in biota might impact denudation. In model calculations of an initially weathering-limited system, significant growth of plant biomass is expected to increase chemical weathering and decrease erosion. This perturbation will generally produce deeper regolith and will move the system from weathering-limited toward mixed or transport-control. In contrast, for an initially transport-controlled system, significant growth of biomass will push the system further into the regime of transport control: such a system should also result in deeper regolith. Either way, biota will have the net effect of driving systems toward deeper regolith and away from weathering limitation. Both model simulations and data from profiles and catchments will be discussed.