Systematic Analysis of Climatic and Topographic Controls on Long-Term Erosion Rates

小行星9614442 了解地形和气候如何影响基岩风化和侵蚀仍然是一个重要的问题，一个与地貌学，构造，古气候学，古水文学和环境影响评价的影响。 到目前为止，大多数侵蚀和剥蚀研究都局限于河流沉积物通量或水库沉积速率的长期测量碰巧可用的地点。 这使得它们容易受到岩性和土地使用变化的影响，而岩性和土地使用变化改变了侵蚀速率，从而掩盖了地形和气候的影响。 然而，宇宙成因核素技术现在使我们能够测量长期侵蚀率，而不需要几十年的沉积量数据。 这大大扩大了可以测量侵蚀率的地点范围。 现在可以选择地点来隔离感兴趣的特定因素（如气候和地形），并尽量减少其他变量（如岩性）的影响。 该项目结合了测量侵蚀速率的新宇宙成因核素技术和有效的实验设计，以系统地探索气候和地形如何影响未冰川花岗岩地形的风化和侵蚀速率。 为了尽量减少岩性变化的影响，本研究将集中在内华达州的闪长岩和花岗闪长岩。 选择了九个地点，这些地点的年降水量相差十倍，年平均气温相差超过10摄氏度。 在每个地点内，将选择一系列小的子集水区，包括至少五倍范围的平均山坡坡度。 选址协议是专门设计的，以涵盖降水，温度，风化率和地形的最大可能范围，几乎所有可能的组合。 通过这种实验设计，人们可以探索在几种不同的气候条件下，风化和侵蚀速率如何随地形而变化;相反，人们也可以探索气候如何在一系列地形情况下影响风化和侵蚀。 在每个集水区，将根据从每个集水区的溪流或沟壑中排出的沉积物中宇宙成因核素的浓度来测量长期侵蚀率。 最近完成的一项试点研究表明，冲积沉积物中宇宙成因核素浓度记录了贡献区域的空间平均侵蚀速率（± %或1cm/ka），平均超过2-15 ka。 这种测量侵蚀率的方法很有用，原因有几个：（a）它反映的是空间平均侵蚀率，而不是单个点（可能是异常点）的侵蚀率;（B）它反映的是长期侵蚀率，而不是最近的土地利用扰动（以往研究中的一个主要混淆因素），（c）气候和沉积物产量短期波动的平均值，但主要反映了全新世气候，以及（d）它平均侵蚀的时间尺度是相关的山坡过程，但仍然足够短，底层地形没有改变。 在几个地点，化学风化率将推断流溶质通量，并将风化层剖面取样，以确定化学蚀变的程度。 这项研究的目的是量化，第一次，风化和侵蚀速率如何取决于气候和地形在一个单一的明确的岩性。 它应该有助于我们理解构造强迫，侵蚀和气候之间的反馈，并应帮助我们更现实地模拟地貌演变。 它应有助于在大陆尺度地球化学研究中解释河流沉积物通量和化学剥蚀率，并应提供广泛的新数据集，用于风化和侵蚀的过程研究。 最后，这项研究应有助于估计长期的“背景”侵蚀率，作为衡量人类土地利用的侵蚀后果的基准。

9614442 Kirchner Understanding how topography and climate affect bedrock weathering and erosion remains an important problem, one with implications for geomorphology, tectonics, paleoclimatology, paleohydrology, and environmental impact assessment. To date, most erosion and denudation studies have been confined to sites where long-term measurements of river sediment flux or reservoir sedimentation rates happen to be available. This makes them vulnerable to variations in lithology and land use, which alter erosion rates and thus obscure the effects of topography and climate. However, cosmogenic nuclide techniques now permit us to measure long-term erosion rates, without requiring decades of sediment yield data. This dramatically broadens the range of sites where erosion rates can be measured. Sites can now be chosen to isolate specific factors of interest (such as climate and topography), and to minimize the influence of other variables (such as lithology). This project combines new cosmogenic nuclide techniques for measuring erosion rates, and an efficient experimental design, to systematically explore how climate and topography affect weathering and erosion rates in unglaciated granitic terrain. To minimize the effects of lithologic variation, this study will focus on dirites and granodiorites of the Sierra Nevada. Nine localities have been chosen, across which annual precipitation varies ten-fold and annual average temperatures vary by over 10oC. Within each locality, a series of small subcatchments will be selected, encompassing at least a five-fold range of average hillslope gradients. The site selection protocol is specifically designed to encompass the widest possible ranges of precipitation, temperature, weathering rates, and topography, in nearly all possible combinations. With this experimental design, one can explore how weathering and erosion rates vary with topography under several different climatic conditions; conversely, one can also explore how climate affects we athering and erosion in a range of topographic situations. At each subcatchment, long-term erosion rates will be measured from the concentration of cosmogenic nuclides in sediment draining from streams or gullies in each subcatchment. A recently-completed pilot study shows that cosmogenic nuclide concentrations in alluvial sediments record the spatially averaged erosion ate of the contributing area (to within+% or 1cm/ka), averaged over 2-15 ka. This method of measuring erosion rates is useful for several reasons: (a) it reflects a spatially averaged erosion rate, rather than the erosion rate of an individual--and possibly anomalous--point, (b) it reflects long-term erosion rates rather than recent land-use disturbance (a major confounding factor in previous studies), (c) it averages over short-term fluctuations in climate and sediment yield, but predominantly reflects the Holocene climate, and (d) it averages erosion over timescales that are relevant to hillslope processes, but still short enough that the underlying topography has not changed. At several sites, chemical weathering rates will be inferred from stream solute fluxes, and the regolith profile will be sampled to determine the degree of chemical alteration. This research is designed to quantify, for the first time, how weathering and erosion rates depend on climate and topography in a single well-defined lithology. It should contribute to our understanding of feedbacks between tectonic forcing, erosion, and climate, and should help us to model landform evolution more realistically. It should help in interpreting river sediment fluxes and chemical denudation rates in continental-scale geochemical studies, and should provide an extensive new data set for process-oriented studies of weathering and erosion. Finally, this study should aid in estimating long term "background" erosion rates, as a benchmark for measuring the erosional consequences of human land use.