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

项目概述利用碎屑磷灰石(U-Th)/He热年代学和宇宙成因核素量化侵蚀和地形变化技术描述：本研究利用碎屑磷灰石(U-Th)/He热年代学和沉积物中的宇宙成因核素，对加州内华达山脉南部流域侵蚀的分布和速率进行了详细研究。我们的第一步研究提出，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，按照大小和复杂程度的增加顺序，是均匀侵蚀还是由点源侵蚀过程主导。我们的第二项研究比较了现代河流沉积物与更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，以调查地形和集水区低纬度的时间变化。我们使用三维热模型来解释等温线的地形弯曲对沉积物中解释的碎屑颗粒年龄分布的影响。此外，我们建议利用宇宙成因的河流和洞穴沉积物中的10Be和26Al浓度来研究流域平均侵蚀速率的时空变化。碎屑磷灰石(U-Th)/He热时计和宇宙成因核素是很容易整合的工具，因为它们利用来自同一袋河砂的不同矿物。通过在碎屑环境中开发和整合这些工具，并利用大量以前的(U-Th)/He数据和一组年代确定的洞穴，我们研究了加利福尼亚内华达山脉南部从最早上新世到今天的地形演变。我们的综合热时学和宇宙学方法很容易应用于其他环境。例如，我们的碎屑磷灰石(U-Th)/He方法的发展将决定该技术在世界各地造山带中用于量化古地貌和古侵蚀速率的更古老矿床（如沉积盆地）的适用性。更广泛的意义：该项目解决了地质学家和外行人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被侵蚀的，它们侵蚀和产生沉积物的速度是多少，以及在过去200万年里，山谷内的海拔分布是如何随着气候变化（例如反复的冰川作用）而演变的。此外，我们将开发的地球化学工具应该在更长的时间尺度（200万年）上量化山带地形和侵蚀速率变化的总体努力中证明是有价值的。对科学界更广泛的意义包括开发了地球化学工具（磷灰石(U-Th)/He热时测定法）的新应用，并将该技术与其他更传统的地球化学工具（例如宇宙成因同位素）相结合。与本项目相关的职业发展和培训包括为博士后科学家在学术界的职业生涯提供培训和准备。该研究还将促进至少一名本科生完成与该项目相关的高级论文。公共宣传和K-12教育将以以下形式进行：(1)对红杉国王峡谷国家公园的国家公园服务口译员进行教育，负责向公园游客介绍南内华达山脉的地质和景观；(2)让一名小学教师参与，以提高对当前地球科学研究课题的认识，以及新的课程计划和课程材料。

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.