A bottom-up control on fresh-bedrock topography under landscapes
A bottom-up control on fresh-bedrock topography under landscapes
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DOI：
10.1073/pnas.1404763111
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发表时间：
2014-04
期刊：
影响因子：
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通讯作者：
D. Rempe;W. Dietrich
中科院分区：
文献类型：
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作者：
D. Rempe;W. Dietrich
丘陵地貌通常覆盖着土壤，下面是风化的基岩区，在到达新鲜基岩之前，基岩区可能延伸到地表以下数十米。风化的基岩区影响水径流到渠道，水的化学，侵蚀的速率和过程，以及由于植物吸收水分和返回大气的大气过程。然而，下伏新鲜基岩表面的空间格局基本上是未知的。我们提出了一个可测试的模型，预测山坡的形式和深度新鲜基岩。深度增加上坡和强烈依赖于基岩的孔隙度和渗透性和在山坡的基础上的通道切口的速度。景观下未风化基岩的深度影响地下径流路径、侵蚀过程、生物区系的水分供应和大气水通量。在这里，我们提出了一个定量模型来预测风化岩石的垂直范围内的土壤覆盖的山坡。我们假设，一旦新鲜的基岩，饱和与几乎停滞的流体，平流到近表面通过隆起和侵蚀，通道切口产生的横向头梯度内的新鲜基岩诱导排水朝向通道。新鲜基岩的排水通过干燥引起风化，并允许引入大气和生物控制的酸和氧化剂，使得风化和未风化基岩之间的边界由未排水新鲜基岩的最高高程Zb设定。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的地形表面剖面（由侵蚀驱动）和预测的地下水剖面（由新鲜基岩排水驱动）之间的差异。对于稳定状态，土壤覆盖的情况下，耦合的分析解决方案，其中两个配置文件被驱动的通道切口。该模型预测的风化带上坡增厚，因此，在风化带中的基岩的停留时间逐步上坡增加。两个无量纲数对应的平均山坡坡度和平均地下水位梯度出现，它们的比例定义的山坡起伏，是未风化的比例。来自三个现场的现场数据与模型预测一致。
Significance 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. The weathered bedrock zone influences water runoff to channels, the chemistry of that water, the rates and processes of erosion, and atmospheric processes due to plant uptake of moisture and return to the atmosphere. However, the spatial pattern of the underlying fresh-bedrock surface is essentially unknown. We present a testable model that predicts hillslope form and the depth to fresh bedrock. The depth increases upslope and depends strongly on the porosity and permeability of the bedrock and the rate of channel incision at the base of the hillslope. 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.