Collaborative Research: Glacial Erosion in the Patagonian Andes: Testing the Buzzsaw

在世界上许多活跃的山脉中，冰川平衡线高度(ELA)与顶峰海拔高度之间的显著匹配导致了冰川作为侵蚀锯的假设。由于大多数观测到的冰川侵蚀速率非常高，它们被认为能够有效地消除通过ELA在构造上隆起的大多数地形。然而，冰川时代的观点主要是基于间接证据。在理论上模拟这一过程的定量预测方面只做了初步的尝试。这项拟议的研究旨在使用现代技术和计算方法来验证冰川Buzzsaw假说。由于隆起的地形往往是板块汇聚的结果，理解冰川侵蚀的重要性是复杂的，因此需要认识到造山楔形力学和地表过程之间的反馈。为了检验这些反馈，提出了一个包括冰川侵蚀的地球动力学和地表过程耦合模型。初步的模型预测表明，活动造山带中弱冰川和强冰川冰锥的抬升和侵蚀的时空模式与以被动均衡作用响应冰锥的造山带的剥蚀模式形成明显对比。通过应用组合磷灰石(U-Th)/He和裂变径迹低温热年代学来评估巴塔哥尼亚安第斯山脉晚新生代冰川开始后侵蚀速率和空间侵蚀模式的变化，将实现判断冰川侵蚀是强还是弱的模型预测。巴塔哥尼亚安第斯山脉是造山带，ELA的高度与顶峰海拔之间的匹配首次被发现。巴塔哥尼亚的安第斯山脉是一个特殊的天然实验室。除了拥有合适和有充分记录的构造和气候历史外，它们的南北范围也为研究空间变化的冰川历史提供了机会。将以两个不同的横断面为目标：一个在39至41度(瓦尔迪亚、PUCN、巴里洛什)，另一个在46至51度(运河贝克、拉戈将军卡雷拉、南巴塔哥尼亚冰原)。这将包括近垂直和跨造山带样品剖面的收集。拟议的工作将解决以下研究问题：1)冰川Buzzsaw假说是否解释了巴塔哥尼亚安第斯山脉的地表侵蚀行为？2)Buzzsaw沿巴塔哥尼亚安第斯山脉的长度是历时的还是同步的？3)造山带是接近稳定状态，还是仍然对ELA的下降做出反应？4)当前的冰川侵蚀模型在区域尺度和长时间段内的准确性如何？更广泛的影响地球系统的长期行为取决于气候、反照率、构造、造山地形、风化和温室气体(如二氧化碳)之间的动态相互作用。特别是，鉴于硅酸盐风化在调节大气中二氧化碳浓度方面的重要作用，人们一直在猜测造山带景观的侵蚀通量在减缓温室效应方面可能发挥的作用。造山地形影响降水的分布，如果地形变得足够高，也可以改变全球大气环流，就像目前喜马拉雅和安第斯山脉中央的情况一样。因此，人们对可能影响造山带地形演化或限制地形最大尺寸的因素仍有广泛的兴趣。这项研究将对理解高山冰川在控制山脉最大高度方面所起的作用做出重大贡献。这项建议的多学科方法将促进若干国家和国际教育和研究联系。来自路易斯安那州立大学、耶鲁大学、波茨坦大学、智利大学的学生和研究人员将参与这个项目。来自美国两所院校的学生将有机会将实地研究和理论结合起来。这项拟议的研究有可能为NSF资助的地球科学联盟的一名校友提供研究支持，通过与Pi Jonathan Tomkin共同参与路易斯安那州立大学的该计划，加强少数群体参与地球科学(GAEMP)项目。这项研究的结果将被整合到GAEMP暑期学校项目中，该项目强调正在进行的地球科学研究对少数族裔本科生的价值。

ABSTRACTThe remarkable match between the glacial equilibrium line altitude (ELA) and summit elevations in many active mountain ranges of the world has led to the supposition that glaciers act as an erosional buzzsaw. Since most observed rates of glacial erosion are very high they are thought to be able to effectively remove most topography tectonically uplifted through the ELA. However, the glacial buzzsaw idea is based largely on circumstantial evidence. Only preliminary attempts have been made to theoretically simulate quantitative predictions of this process. This proposed research aims to test the glacial buzzsaw hypothesis using modern technical and computational methods. Understanding the importance of glacial erosion is complicated by the fact that elevated topographies are often the result of plate convergence, and so an appreciation of the feedbacks involved between orogenic wedge mechanics and surface processes is required. Development of a coupled geodynamic and surface process model that incorporates glacial erosion in an active critical orogenic wedge is proposed to examine these feedbacks. Preliminary model predictions indicate that the temporal and spatial patterns of uplift and erosion for both a weak and strong glacial buzzsaw in an active orogen contrast significantly with erosion patterns of an orogen that responds to the buzzsaw by passive isostasy. Testing the model predictions to judge whether glacial erosion acts as a 'strong' or 'weak' buzzsaw will be achieved by applying combined apatite (U-Th)/He and fission-track low temperature thermochronology to evaluate changes in erosion rates and spatial erosion patterns following the onset of late Cenozoic glaciation in the Patagonian Andes - the orogen where the match between the height of the ELA and summit elevation was first recognized. The Patagonian Andes are an exceptional natural laboratory. As well as having a suitable and well-documented tectonic and climatic history, their north-south range provides an opportunity to study a spatially varying glacial history. Two different transects will be targeted: one at 39S to 41S (Valdivia, Pucn, Bariloche), and another between 46S and 51S (Canal Baker, Lago General Carrera, South Patagonian Icefield). This will include the collection of both near-vertical and trans-orogen sample profiles. The proposed work will address the following research questions: 1) Does the glacial buzzsaw hypothesis explain the behavior of surface erosion in the Patagonian Andes? 2) Was the buzzsaw diachronous or synchronous along the length of the Patagonian Andes? 3) Is the orogen close to steady state, or still responding to the drop in ELA? and 4) How accurate are current models of glacial erosion over regional scales and long time periods?Broader ImpactThe long-term behavior of the earth system is dependent on dynamic interactions between climate, albedo, tectonics, orogenic topography, weathering, and greenhouse gases, such as CO2. In particular, there has been much speculation about the role that erosional fluxes from orogenic landscapes might play in moderating the greenhouse effect, given the important role of silicate weathering in moderating CO2 concentrations in the atmosphere. Orogenic topography as influences the distribution of precipitation, and can also change global atmospheric circulation, if the topography gets high enough, as is presently the case in the Himalaya and central Andes. Thus, there remains broad interest in the factors that might influence the evolution of orogenic topography, or limit the maximum size of the topography. This research will make a significant contribution towards understanding the role that alpine glaciation plays in controlling the maximum height of mountain ranges. The multidisciplinary approach of this proposal will foster a number of national and international educational and research links. Students and researchers from Louisiana State, Yale, GFZ Potsdam, the Universidad de Chile will be involved in this project. Students from two US institutions will have an opportunity to combine field research and theory. The proposed study has the potential to provide research support for an alumnus of the NSF funded Geoscience Alliance to Enhance Minority Participation in the Earth Sciences (GAEMP) project, through co-PI JonathanTomkin's participation in the program at LSU. The results of the research will be integrated into a GAEMP summer school program, which emphasizes the value of ongoing earth science research to minority undergraduates.