Solving a conundrum of a steady-state hilltop with variable soil depths and production rates, Bodmin Moor, UK
Solving a conundrum of a steady-state hilltop with variable soil depths and production rates, Bodmin Moor, UK
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DOI：
10.1016/j.geomorph.2010.12.023
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发表时间：
2011-05
期刊：
影响因子：
3.9
通讯作者：
S. Riggins;R. Anderson;S. P. Anderson;A. Tye
中科院分区：
文献类型：
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作者：
S. Riggins;R. Anderson;S. P. Anderson;A. Tye
普遍的看法是，稳定状态的山顶应该呈凸形，既有均匀的土层厚度，又有均匀的土壤-基岩界面下降速率。英国西南部Bodmin Moor的一条120米高的山顶样带显示出抛物线山顶形状，这意味着它处于地貌稳定状态，但测量到的基岩深度从14到80厘米几乎是原来的6倍，土壤界面岩石中10Be浓度得出的土壤生产力是10到20米/年的2倍。为了解决这一难题，我们开发了一种理论来解释土壤-基岩边界逐块下降导致的土壤深度分散，并用一个涉及块体释放和扩散运移的简单模型来支持我们的理论。我们表明，在很长的时间尺度上，山顶确实可以处于地貌稳定状态，而在任何时间点，都会显示出从基岩到基岩的广泛深度。基岩深度的变化导致在风化岩层底部取样的岩石中的10Be浓度可变，这产生了计算出的土壤生产率，其变化大约是长期平均水平。因此，在许多景观中，土壤生产量作为土壤深度函数的分散是预期的，如果景观处于地貌稳定状态，这种分散反映了基岩界面下沉的块体释放机制的随机性质。我们发现，在我们的模型中，通常观察到的驼峰和指数下降的土壤生产函数只在向地貌稳态的过渡过程中保持不变。
The common wisdom is that steady-state hilltops should display convex shape with both uniform soil thickness and uniform rates of lowering of the soil–bedrock interface. A 120m hilltop transect on Bodmin Moor, southwest UK, displays a parabolic hilltop form, which implies it is in geomorphic steady state, yet the measured depths to bedrock range almost 6-fold from 14 to 80cm, and the soil production rates derived from10Be concentrations in rock at the soil interface range by 2-fold from 10 to 20m/My. We address this conundrum by developing a theory to explain the scatter in soil depths that would result from block-by-block lowering of the soil–bedrock boundary, and support our theory with a simple model involving block release and diffusive transport. We show that over long timescales a hilltop can indeed be in geomorphic steady state while at any point in time exhibiting a broad range of depths to bedrock. Variable depth to bedrock results in variable10Be concentrations in the rock sampled at the base of regolith, which yields calculated soil production rates that vary about the long-term average. Hence, scatter in plots of soil production as a function of soil depth are to be expected in many landscapes, and if the landscape is in geomorphic steady state, this scatter reflects the stochastic nature of the block release mechanism by which the bedrock interface is lowering. We find in our models that the commonly observed humped and exponentially declining soil production functions are only preserved during the transient approach toward geomorphic steady state.