Department of Earth and Planetary Science, University of California, Berkeley, CA 94720.
Proc Natl Acad Sci U S A. 2014 May 6;111(18):6576-81. doi: 10.1073/pnas.1404763111. Epub 2014 Apr 23.
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, Zb. 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.
景观下未风化基岩的深度影响地下径流路径、侵蚀过程、生物群的水分可利用性以及水通量到大气。在这里，我们提出了一个定量模型来预测土壤覆盖的山坡下风化岩石的垂直范围。我们假设，一旦新鲜的基岩被近乎停滞的流体饱和，通过抬升和侵蚀被输送到近地表，通道切割就会在新鲜基岩内产生侧向水头梯度，导致水流向通道。新鲜基岩的排水导致风化，通过干燥和允许引入大气和生物控制的酸和氧化剂，风化和未风化基岩之间的边界由未排水的新鲜基岩的最高高程 Zb 确定。新鲜基岩的缓慢排水对风化前沿的推进施加了“自下而上”的控制。风化带的厚度计算为预测地形表面轮廓（由侵蚀驱动）和预测地下水轮廓（由新鲜基岩排水驱动）之间的差异。对于稳态、土壤覆盖的情况，出现了一个耦合的解析解，其中两个轮廓都由通道切割驱动。该模型预测风化带在上坡方向变厚，因此，风化带中基岩的停留时间在上坡方向逐渐增加。出现了两个对应于平均山坡坡度和平均地下水位梯度的无量纲数，它们的比值定义了未风化的山坡抬升幅度的比例。来自三个野外站点的野外数据与模型预测一致。
