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品暴露在地球表面的时间来解决。例如，终末冰碛是由冰川融化时沉积在冰川前部的巨石组成的；因此，巨砾的地表暴露年龄限制了冰川退缩的时间。在过去的15年里，测量地球年龄的技术已经取得了很大的进展，这种技术是通过在地球表面最上面1米处高能宇宙射线和岩石中的原子之间的相互作用所产生的稀有同位素的积累来测量的。准确的“宇宙起源定年”要求精确地知道同位素产生的速率和系统。这个项目将研究这些系统学中一个特别不为人所知的部分——发生在空气-岩石界面的过程，在那里宇宙射线从一种介质（大气）过渡到另一种非常不同的介质（岩石）。该界面的核过程将通过对宇宙射线产生的几种不同同位素（包括氦-3和氖-21）的岩石最上面几厘米的高空间分辨率分析来研究。此外，合成矿物目标将暴露在安第斯山脉高海拔的宇宙射线照射下，那里的宇宙射线通量非常高。这些目标将被检索并以高空间分辨率分析同一组同位素。合成目标补充了对天然岩石的研究，因为它们允许在化学成分和暴露历史方面更受控制的环境。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。该研究已广泛应用于许多相关过程的研究，如侵蚀速率、断层运动时间和第四纪气候变化的记录。

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.