A bottom-up control on fresh-bedrock topography under landscapes

Abstract

The depth to unweathered bedrock beneath landscapes influences subsurface runoff paths, erosional processes, moisture availability to biota, and water flux to the atmosphere. Here we propose a quantitative model to predict the vertical extent of weathered rock underlying soil-mantled hillslopes. We hypothesize that once fresh bedrock, saturated with nearly stagnant fluid, is advected into the near surface through uplift and erosion, channel incision produces a lateral head gradient within the fresh bedrock inducing drainage toward the channel. Drainage of the fresh bedrock causes weathering through drying and permits the introduction of atmospheric and biotically controlled acids and oxidants such that the boundary between weathered and unweathered bedrock is set by the uppermost elevation of undrained fresh bedrock, Z b . The slow drainage of fresh bedrock exerts a "bottom up" control on the advance of the weathering front. The thickness of the weathered zone is calculated as the difference between the predicted topographic surface profile (driven by erosion) and the predicted groundwater profile (driven by drainage of fresh bedrock). For the steady-state, soil-mantled case, a coupled analytical solution arises in which both profiles are driven by channel incision. The model predicts a thickening of the weathered zone upslope and, consequently, a progressive upslope increase in the residence time of bedrock in the weathered zone. Two nondimensional numbers corresponding to the mean hillslope gradient and mean groundwater-table gradient emerge and their ratio defines the proportion of the hillslope relief that is unweathered. Field data from three field sites are consistent with model predictions.

Reviews and syntheses: on the roles trees play in building and plumbing the critical zone

“…Another proposed example of a bottom-up control on the CZ is drainage (Rempe and Dietrich, 2014). Rempe and Dietrich (2014) argue that the unweathered rock within a hill acts as the valve that controls drainage of water and the advance rate of weathering.…”

“…Rempe and Dietrich (2014) argue that the unweathered rock within a hill acts as the valve that controls drainage of water and the advance rate of weathering. Much work is needed to understand all the valves for water within hills (shown for simplicity as one valve in Fig.…”

A simple model for regolith formation by chemical weathering

“…They also predict that, in rapidly uplifting and eroding regions, a local equilibrium can be achieved between surface erosion/uplift and weathering [Lebedeva et al, 2010], whereas in noneroding areas and under the assumption that solute transport is dominated by diffusion, regolith thickness should increase as the square root of time [Lebedeva et al, 2010]. More recently, Rempe and Dietrich [2014b] computed steady state regolith geometry on a hillslope undergoing uplift and surface erosion based on a simple groundwater flow model in the underlying bedrock. Assuming that the water table corresponds to the weathering front, they arrive at the conclusion that regolith thickness is controlled by the ratio of the slope of the water table to the surface slope.…”

The role of uncertainty in bedrock depth and hydraulic properties on the stability of a variably-saturated slope

“…Hilly landscapes are typically mantled with soil and underlain by a weathered bedrock zone that may extend tens of meters beneath the surface before reaching fresh bedrock [1]. The design, construction, and maintenance of buildings, infrastructure, and other man-made structures in such environments depends critically on the stability of the soil and underlying bedrock.…”