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.
The purpose of this work is to quantify the relationship between magnetic field strength variation and cosmogenic nuclide production rates. In order to lay the groundwork for this, we are defining the timing of the entire reversal process in the equatorial regions which has been modeled to occur more rapidly than in higher latitudes. This is key to determining the spatial relationship between Earth’s surface and the magnetosphere during the Matuyama-Brunhes magnetic reversal. We have sampled consecutive basalt layers and are dating them using Ar -Ar geochronologyr, making the assumption that the difference between each pair of layers represents the time of exposure of the lower layer to direct bombardment of secondary cosmic rays, and hence, the cosmogenic production in the lower basalt layer. Wehave collected our data 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 during the time of formation. In addition, each layer contains enough potassium to determine the time of eruption with Ar-Ar dating. This unique combination of petrological features offers us a rare opportunity to define paleomagnetic changes through time. In particular, our high resolution ampling and Ar-Ar dating will provide one of the best dated constraints on this reversal. This research supported the PhD research of a Hispanic (Mexican-American) female. The research will benefit society through providing an enhanced understanding of regional cosmogenic nuclide production rates, which is important for helping us better define the pacing of rates of change at the Earth’s surface. Data regarding the timing of the Matuyama-Brunhes magnetic reversal will increase our overall understanding of the causes of such events and better understand possible hazards associated with them.
