Doctoral Dissertation Research: Understanding the Relationship Between Cosmic Ray Intensity and the Magnetic Field: A Case Study During the Most Recent Magnetic Reversal

这个博士论文项目是关于量化地磁场的屏蔽作用，以及它如何保护地球表面免受宇宙射线的轰击。宇宙成因核素（CN）被用来划分地貌过程的速率和评估古气候事件的时空变异性，因此是古气候重建的重要工具。然而，它们的用处是有限的，因为我们对CN的生产速度了解得不多。目前，用于确定地表暴露日期的“标量”的空间变异性主要受纬度和海拔的控制。然而，人们普遍认为，这两个变量并不能准确地解释地球磁场的动态特性及其对CN生成速率的调制。本研究将制定一种策略来量化与Matuyama-Brunhes反转相关的CN产量与磁场强度之间的关系。这将提供一种独特的评价环境变化率对磁场强度几乎消失的时刻的敏感性。数据将从玄武岩中发现的矿物质中提取。熔岩流序列中的每一层玄武岩都记录了磁场的方向以及形成时磁场的绝对古强度。此外，每一层含有足够的钾，可以用Ar-Ar测年法确定喷发时间。将使用的策略包括通过测量橄榄石中宇宙形成核素3He的浓度，作为跨越反转的层状玄武岩地层中40Ar-39Ar日期确定的年龄的函数，来构建CN古生成速率的时间序列。对这些浓度的分析将有助于确定是否可以通过CN浓度的变化来确定与Matuyama-Brunhes磁极反转相关的宇宙射线通量的变化。了解全球环境变化的努力受到对古气候事件及其速度的了解的限制。CN地表暴露测年被广泛用于确定地貌过程的规模和速度，是气候变化的关键指标。加强对区域CN生产率的了解将有助于更好地确定变化的速度。此外，科学界尚未确定地磁倒转是如何通过增加宇宙辐射的方式影响地球表面的。磁层是地球抵御宇宙辐射的主要屏障，这个项目的结果之一可能是更好地定义与逆转有关的宇宙辐射是否有害。此外，确定磁极反转期间磁场的地理特征以及对生物或人类活动的任何相关影响，从多种物种迁移模式的中断到人类航空旅行的中断，对社会具有重要价值。在此研究项目中收集的经验和数据将用于俄勒冈州的公共推广项目，并纳入俄勒冈西部地区男孩和女孩俱乐部的项目。该计划的目标是让中学生接触从事研究项目的科学家，并激励他们朝着科学家的职业发展。作为博士论文研究改进奖，该项目将提供支持，使一个有前途的研究生从一个代表性不足的群体建立一个独立的研究生涯。

This doctoral dissertation project is about quantifying the shielding affects of the geomagnetic field and how it protects earth's surface from the bombardment of cosmic rays. Cosmogenic nuclides (CN) are used to delimit rates of geomorphic processes and to assess spatial and temporal variability of paleoclimatic events, and are therefore a crucial tool in paleoclimatic reconstruction. Their usefulness is limited, however, by how well CN production rates are known. Currently, the spatial variability of 'scalers' used to determine surface exposure dates are controlled largely by latitude and altitude. However, it is widely accepted that these two variables do not accurately account for the dynamic character of earth's magnetic field and its resulting modulation of CN production rates. This study will develop a strategy to quantify the relationship between CN production rates and magnetic field strength associated with the Matuyama-Brunhes reversal. This will provide a unique evaluation of the sensitivity of environmental change rates to a time when the magnetic field strength virtually disappeared. Data will be extracted from the minerals found in basalt. Each layer of basalt within a sequence of lava flows records both the orientation of the magnetic field as well as the absolute paleo-intensity of the field during the time of formation. In addition, each layer contains enough potassium to determine the time of eruption with Ar-Ar dating. The strategy that will be used involves the construction of a time series of CN paleo-production rates by measuring concentrations of the cosmogenic nuclide 3He in olivine as a function of age determined by 40Ar-39Ar dates in layered basalt formations that span the reversal. Analysis of these concentrations will help determine if changes in the cosmic ray flux related to the Matuyama-Brunhes magnetic reversal can be defined from variations in CN concentrations. Efforts to understand global environmental change are limited by the understanding of paleoclimatic events and their pacing. CN surface exposure dating is widely used to define the scale and pacing of such geomorphic processes, which are a key indicator of climate change. An enhanced understanding of regional CN production rates will help better define the pacing of change. In addition, the scientific community has yet to define how geomagnetic reversals impact the surface of the planet by way of increased cosmic radiation. The magnetosphere is earth's primary shield against cosmic radiation and one of the results of this project may be a better definition of whether cosmic radiation associated with reversals is hazardous. In addition, determining the geographic character of the magnetic field during times of magnetic reversal and any associated impacts on biological or human activity ranging from disruption in migration patterns of multiple species to disruption in human air travel is of great value to society. Experiences and data collected during this research project will be used for public outreach programs in Oregon and incorporated into a program for the Boys and Girls Club in the western Oregon area. The program goal is to expose middle and secondary school children to scientists working on research projects and inspire them to work towards careers as scientists. As a Doctoral Dissertation Research Improvement award, this project will provide support to enable a promising graduate student from an under-represented group to establish an independent research career.