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

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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.