Cosmogenic nuclides, topography, and the spatial variation of soil depth
Cosmogenic nuclides, topography, and the spatial variation of soil depth
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DOI：
10.1016/s0169-555x(98)00095-6
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发表时间：
1999-02-01
期刊：
影响因子：
3.9
通讯作者：
Finkel, RC
中科院分区：
文献类型：
--
作者：
Heimsath, AM;Dietrich, WE;Finkel, RC
如果基岩转化为活动土层的速度取决于当地土层的厚度，那么在接近动态平衡的景观中，均匀基岩上的山坡应该被均匀的土层覆盖。相反，如果土壤深度在侵蚀活跃的地貌上有所不同，那么土壤生产率也会不同，因此，地貌将不会处于形态平衡状态。相对于气候变化和构造调整的节奏，山坡的缓慢演变表明，许多景观可能会出现局部地貌不平衡。在这里，我们通过一个先前开发的模型来探索这个平衡景观的问题，该模型预测了由于土壤生产和侵蚀之间的局部平衡而导致的土壤厚度的空间变化。首先，我们用两种独立的方法证实了模型中土壤生产量与土层厚度成反比的假设。一种方法采用理论预测，即在局部稳定状态(产土量等于去除量)下，土层深度应与坡度成反比。第二种方法依赖于直接测量土柱底部基岩中宇宙成因Be-10和Al-26的现场生成浓度。对于我们在加利福尼亚州北部的研究地点，这两种方法一致并得出了土壤生产量随土层厚度的指数下降的表达式，从无土层时的0.077 mm/年下降到1米土层下的0.0077 mm/年。然后，我们在泥沙运动的生产扩散耦合模型中使用这个土壤生产函数，来探索土壤生产函数的数据在四个独立的支脊(鼻子)上对土壤深度空间变化的控制。模型预测对边界条件、网格比例和运行时间非常敏感。尽管如此，只要我们使用土壤生产的观测函数，我们发现预测的土壤深度与观测的土壤深度之间有很好的一致性。四个鼻子在空间上都有不同的曲率，因此，土壤的深度也不同，这意味着形态不平衡。我们认为，由于构造和气候振荡的频率比地貌的响应时间短，我们的研究地点遭受了一波切割和不同强度的侵蚀。(C)1999 Elsevier Science B.V.保留所有权利。
If the rate of bedrock conversion to a mobile layer of soil depends on the local thickness of soil, then hillslopes on uniform bedrock in a landscape approaching dynamic equilibrium should be mantled by a uniform thickness of soil. Conversely, if the depth of soil varies across an actively eroding landscape, then rates of soil production will also vary and, consequently the landscape will not be in morphologic equilibrium. The slow evolution of hillslopes relative to the tempo of climatic variations and tectonic adjustments would suggest that local morphologic disequilibrium may be expected in many landscapes. Here, we explore this issue of equilibrium landscapes through a previously developed model that predicts the spatial variation in thickness of soil as a consequence of the local balance between soil production and erosion. First, we confirm the assumption in the model that soil production varies inversely with the thickness of soil using two independent methods. One method uses the theoretical prediction that at local steady state (soil production equals removal), the depth of soil should vary inversely with hillslope curvature. The second method relies on direct measurements of in situ produced concentrations of cosmogenic Be-10 and Al-26 in bedrock at the base of the soil column. For our study site in Northern California, the two methods agree and yield the expression that the rate of soil production declines exponentially with the thickness of soil from 0.077 mm/year with no soil mantle to 0.0077 mm/year under 1 m of soil. We then use this function of soil production in a coupled production and diffusive model of sediment transport to explore the controls on the spatial variation of the depth of soil on four separate spur ridges (noses) where we measured the data for the function of soil production. Model predictions are sensitive to boundary conditions, grid scale, and run time. Nonetheless, we found good agreement between predicted and observed depths of soil as long as we used the observed function of soil production. The four noses each have spatially varying curvature and, consequently, have varying depths of soil, implying morphologic disequilibrium. We suggest that our study site has been subjected to a wave of incision and varying intensities of erosion because of tectonic and climatic oscillations that have a frequency shorter than the morphologic response time of the landscape. (C) 1999 Elsevier Science B.V. All rights reserved.