Cosmogenic Production Near the Air-Rock Interface

许多重要的地球科学问题可以通过确定特定岩石样本暴露在地球表面的时间来解决。例如，冰川末端由融化时沉积在冰川前部的巨石组成；因此，巨石的表面暴露年龄限制了冰川消退的时间。在过去的15年里，通过在地球表面最高约1米处高能宇宙射线与岩石中的原子相互作用产生的稀有同位素的积累，在测量这种年龄的技术方面取得了巨大的进步。准确的“宇宙成因测年”要求准确地知道同位素生产的速率和系统。这个项目将研究这些分类学中一个特别鲜为人知的部分--在空气-岩石界面上发生的过程，在那里宇宙射线从一种介质(大气)转换到另一种非常不同的介质(岩石)。这一界面上的核过程将通过高空间分辨率分析在最上面几厘米的岩石中由宇宙射线产生的几种不同的同位素来研究，包括氦-3和霓21。此外，在宇宙线流量非常高的安第斯山脉的高海拔地区，合成矿物目标将受到宇宙线照射。对于同一组同位素，将以高空间分辨率检索和分析这些目标。合成靶标是对天然岩石研究工作的补充，因为它们允许在化学成分和暴露历史方面有一个更受控制的环境。这项研究的最终目标是提高使用宇宙成因同位素作为确定地球表面年龄的工具的准确性和可靠性。这项研究广泛应用于许多相关过程的调查，如侵蚀速率、断层运动的时间安排，以及记录第四纪气候变化。

Many important Earth science questions can be addressed by determining the time at which specific rock samples became exposed at the Earth's surface. For example, a terminal moraine consists of boulders deposited at the front of a glacier as it melts; hence the surface exposure age of the boulders constrains the timing of glacial retreat. Over the last fifteen years great progress has been made in the development of techniques to measure such ages via the accumulation of rare isotopes produced by interactions in the uppermost ~ 1 meter of Earth's surface between high energy cosmic rays and the atoms in rocks. Accurate "cosmogenic dating" requires that the rate and systematics of isotope production be accurately known. This project will investigate a particularly poorly understood part of these systematics - the processes occurring at the air-rock interface, where cosmic rays transition from one medium (atmosphere) in to a very different one (rock). Nuclear processes at this interface will be investigated through high spatial resolution analyses in the uppermost few cm of rock of several different isotopes produced by cosmic rays, including helium-3 and neon-21. In addition, synthetic mineral targets will be exposed to cosmic ray irradiation at high elevation in the Andes, where the cosmic ray flux is very high. These targets will be retrieved and analyzed at high spatial resolution for the same set of isotopes. The synthetic targets complement the work on natural rocks because they permit a far more controlled environment in terms of chemical composition and exposure history. The ultimate goal of this research is to increase the accuracy and reliability of using cosmogenic isotopes as tools for determining the age of the Earth's surface. This research has widespread application to investigations of many relevant processes such as rates of erosion, timing of fault movements, and in documenting Quaternary climate change.