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

气候和地形对长期侵蚀率控制的系统分析

• 批准号: 9614442
• 负责人: James Kirchner
• 金额: $ 26.09万
• 依托单位: University of California-Berkeley
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 1997
• 资助国家: 美国
• 起止时间: 1997-02-01 至 2002-01-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=9614442&HistoricalAwards=false
• 关键词: Systematic; Analysis; Climatic; Topographic; Controls

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.

9614442基什内尔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问题，涉及地貌学、构造学、古气候学、古水文学和环境影响评估。到目前为止，大多数侵蚀和剥蚀研究仅限于对河流泥沙通量或水库沉积速率进行长期测量的地点。这使得它们很容易受到岩性和土地利用变化的影响，这些变化改变了侵蚀速度，从而掩盖了地形和气候的影响。然而，宇宙源核素技术现在允许我们测量长期的侵蚀速率，而不需要几十年的泥沙产量数据。这极大地扩大了可以测量侵蚀速率的地点范围。现在可以选择地点来隔离特定的感兴趣的因素(如气候和地形)，并将其他变量(如岩性)的影响降至最低。该项目结合了用于测量侵蚀速率的新的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山脉的闪长岩和花岗闪长岩上。选择了9个地区，这些地区的年降水量变化了十倍，年平均气温变化超过了10摄氏度。在每个地区内，将选择一系列小流域，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、风化速率和地形的最大可能范围，几乎所有可能的组合。通过这个实验设计，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；反过来，人们也可以探索气候如何影响我们在一系列地形情况下的风化和侵蚀。在每个分集水区，将根据从每个分集水区的溪流或沟渠排出的沉积物中宇宙成因核素的浓度来测量长期侵蚀率。最近完成的一项初步研究表明，冲积沉积物中的宇宙成因核素浓度记录了贡献区(在+%或1厘米/ka以内)的空间平均侵蚀率，平均超过2-15ka。这种测量侵蚀速率的方法很有用，原因有几个：(A)它反映的是空间平均侵蚀速率，而不是单个点的侵蚀速率--可能还有异常点的侵蚀速率；(B)它反映的是长期侵蚀速率，而不是最近的土地利用干扰(以前研究中的一个主要混淆因素)；(C)它是气候和产沙量短期波动的平均值，但主要反映全新世气候；(D)它是与山坡过程有关的时间尺度上的平均侵蚀，但仍然足够短，以至于基本地形没有变化。在几个地点，将从水蒸气溶质通量推断化学风化速率，并将采样风化层剖面以确定化学变化的程度。这项研究的目的是首次量化风化和侵蚀速度如何在单一明确定义的岩性中取决于气候和地形。这将有助于我们理解构造强迫、侵蚀和气候之间的反馈关系，并有助于我们更真实地模拟地貌演变。它应有助于解释大陆尺度地球化学研究中的河流沉积物通量和化学剥蚀率，并应为以过程为导向的风化和侵蚀研究提供广泛的新数据集。最后，这项研究应该有助于估计长期的“背景”侵蚀率，作为衡量人类土地利用的侵蚀后果的基准。