Quantifying Changes in Erosion and Relief with Detrital Apatite (U-Th)/He Thermochronlogy and Cosmogenic Nuclides

项目总结利用碎屑磷灰石(U-Th)/He热年代学和宇宙成因核素定量侵蚀和起伏的变化技术描述：本研究利用沉积物中的碎屑磷灰石(U-Th)/He热计时和宇宙成因核素详细研究了加利福尼亚州内华达山脉南部的集水区侵蚀的分布和速率。我们的第一线研究提出，碎屑磷灰石(U-Th)/He冷却年龄可以作为沉积物示踪剂，因此可以用来测试一套陡峭的集水区是均匀侵蚀还是以点源侵蚀过程为主。我们的第二线研究比较了现代河流沉积物和古老沉积物的冷却年龄分布，特别是保存在洞穴中的河流沉积物，以研究地形和集水区高度测量的时间变化。我们使用一个三维热模型来解释等温线的地形弯曲对来自沉积物的碎屑颗粒年龄分布的影响。此外，我们建议使用河流和洞穴沉积物中宇宙成因的10Be和26Al浓度来研究流域平均侵蚀速率的空间和时间变化。碎屑磷灰石(U-Th)/氦测时和宇宙成因核素是很容易集成的工具，因为它们利用同一袋河沙中的不同矿物。通过在碎屑环境中开发和集成这些工具，并利用大量的(U-Th)/He数据和一组年代准确的洞穴，我们调查了加利福尼亚州内华达山脉南部从最早的上新世到今天的地形演化。我们集成的热计时和宇宙成因方法很容易应用到其他环境中。例如，我们开发的碎屑磷灰石(U-Th)/He方法将确定这项技术是否适用于世界各地造山带中较老的沉积物(例如沉积盆地)，以量化古岩屑和古侵蚀速率。广泛的意义：这个项目解决了地质学家和外行都非常感兴趣的山脉中地球表面演化的基本问题。我们的研究应该有助于回答内华达山脉陡峭的山谷是如何侵蚀的，它们侵蚀和产生沉积物的速度，以及过去200万年来山谷内的海拔分布是如何随着气候变化(例如反复的冰川)而演变的。此外，我们将开发的地球化学工具在量化更长时间尺度(200万年)的山区地形和侵蚀速率变化的总体努力中应该被证明是有价值的。对科学界更广泛的意义包括开发一种新的地球化学工具(磷灰石(U-Th)/氦热计时)的应用，并将这项技术与其他更传统的地球化学工具(例如，宇宙成因的同位素)相结合。与该项目相关的职业发展和培训包括培训一名博士后科学家，并为在学术界从事职业做准备。这项研究还将帮助至少一名本科生完成与该项目相关的高级论文。将通过以下形式开展公众宣传和K-12教育：(1)在红杉-国王峡谷国家公园对国家公园管理局的口译员进行教育，负责向公园游客传授内华达山脉南部的地质和地貌；(2)让一名小学教师参与培养对当前地球科学研究主题以及新的教学计划和课程材料的认识。

PROJECT SUMMARYQuantifying changes in erosion and relief with detrital apatite (U-Th)/He thermochronlogy and cosmogenic nuclidesTechnical Description: This study performs a detailed study of the distribution and rates of catchment erosion in the southern Sierra Nevada of California using integrated detrital apatite (U-Th)/He thermochronometry and cosmogenic nuclides in sediment. Our first line of research proposes that detrital apatite (U-Th)/He cooling ages can act as sediment tracers, and thus be used to test whether a suite of steep catchments, in order of increasing size and complexity, are eroding uniformly or are dominated by point source erosion processes. Our second line of research compares the distribution of cooling ages from modern river sediments with those from older deposits, specifically river sediments preserved in caves, to investigate temporal changes in relief and catchment hypsometry. We use a 3D thermal model to account for the influence of the topographic bending of isotherms on interpreted detrital grain-age distributions from sediment. In addition, we propose to use cosmogenic 10Be and 26Al concentrations in river and cave sediments to investigate spatial and temporal changes in catchment-average erosion rates. Detrital apatite (U-Th)/He thermochronometry and cosmogenic nuclides are easily integrated tools because they utilize different minerals from the same bag of river sand. By developing and integrating these tools in detrital settings, and by exploiting a wealth of previous (U-Th)/He data and a set of well-dated caves, we investigate the topographic evolution of the southern Sierra Nevada, California, from the earliest Pliocene to today. Our integrated thermochronometric and cosmogenic approach is readily applied to other settings. For example, our development of the detrital apatite (U-Th)/He approach will determine the suitability of this technique for older deposits (e.g., sedimentary basins) for quantifying paleorelief and paleoerosion rates in orogenic belts around the world.Broader Significance:This project addresses fundamental problems in evolution of the Earth's surface in a mountain range of keen interest to geologists and laypersons alike. Our research should help answer how steep mountain valleys in the Sierra Nevada erode, the rates at which they erode and produce sediment, and how the distribution of elevation within valleys has evolved in light of climate change (e.g. repeated glaciations) over the last 2 million years. Furthermore, the geochemical tools we will develop should prove valuable in the overall effort of quantifying topography and erosion rate changes in mountain belts over longer timescales (2 million years). Broader significance to the scientific community include development of a new application of a geochemical tool (apatite (U-Th)/He thermochronometry) and integration of this technique with other more conventional geochemical tools (e.g. cosmogenic isotopes). Career development and training associated with this project includes training and preparation of a postdoctoral scientist for a career in academia. The study will also facilitate at least one undergraduate student completing a senior thesis associated with this project. Public outreach and K-12 education will occur in the form of (1) education of National Park Service interpreters at Sequoia-Kings Canyon National Parks charged with teaching park visitors about the geology and landscapes of the southern Sierra Nevada, and (2) involvement of an elementary school teacher to development an awareness of current research topics in the Earth sciences as well as new lesson plans and course materials.