The convenient fiction of steady-state soil thickness

Abstract

Steady-state regolith or soil thickness (SSST), whereby surface removals are approximately balanced by production of new soil by bedrock weathering, is a common assumption in most models of hillslope and landscape evolution. SSST is also a fundamental assumption in the use of cosmogenic radionuclides (CRN) to estimate erosion and weathering rates. The steady-state concept is based on feedbacks between soil thickness and weathering at the base of the regolith, such that (sometimes after an optimal or threshold thickness is achieved), thicker soils lead to lower weathering rates (and vice versa). SSST is thus only applicable to soils formed chiefly from weathering of the underlying bedrock, where sufficient time has elapsed for regolith accumulation, and where effects of processes other than weathering and surface removals on thickness are negligible. Even within this domain, the widespread occurrence of deep weathering profiles, regolith stripping, and inherited regolith features makes SSST problematic as a conceptual model for pedogenesis or weathering profile development. The ratio of soil thickness to total weathering profile thickness is proposed as a simple index of steady state. Steady-state profiles formed on weathered bedrock should exhibit ratios close to unity. Data from the Cumberland Plateau region of eastern Kentucky show that soil/weathering profile ratios in shallow (< 1.5 m) profiles formed on sandstone may reach or approach unity, but are generally < 1. Geotechnical core data show depths to bedrock of 2 to > 20 m, and generally significantly greater than soil thicknesses in the region, suggesting ratios <<1. However, while evidence shows that SSST is likely rare and not a viable conceptual framework for assessing soil and weathering profile development, deviations from SSST may have limited influence on results of CRN-based estimates of erosion and weathering and simulation model results. This is because in the landscape settings where SSST is typically assumed, and over the customary time scales involved, rates of denudation and weathering are very small compared to regolith thickness, such that imbalances do not materially affect results of calculations. Steady-state in development of soil, regolith, or weathering profile development thus represents a convenient fiction facilitating the use of some models and tools. The potential pitfalls arise from the possibility that the utility of SSST as a convenient fiction in some contexts may be mistaken for a realistic representation of the dynamics of pedogenesis and weathering profile evolution.