HR: 17:00h
AN: GP24A-05 INVITED     [Abstracts]
TI: Toward using millennial-scale geomagnetic field models to constrain in situ cosmogenic nuclide production rate variations
AU: * Lifton, N A
EM: lifton@geo.arizona.edu
AF: University of Arizona, Geosciences Department and NSF-Arizona AMS Facility, Tucson, AZ 857210077 United States
AB: The geomagnetic field exerts a strong influence on the trajectories of charged primary cosmic-ray particles incident on the Earth's atmosphere (and therefore, on the geographic distribution of their progeny). The resulting cascade of secondary particles then interacts with the top few meters of the Earth's surface to produce various nuclides in situ (in situ cosmogenic nuclides, or CNs). With appropriate interpretive models, concentrations of these nuclides measured in terrestrial materials can be used to infer the duration of surface exposure (or of burial following exposure) and rates of various surficial processes. However, the potential precision with which CNs can be applied is limited by how well their production rates are known. Nuclide production rates are typically determined by measuring CN concentrations in surficial rocks with well-constrained exposure histories. Scaling these time-integrated production rates to other locations with different exposure durations, though, requires knowledge or assumptions of how instantaneous production rates have varied in space and time. Spatial scaling of CN production rates is typically described by models based on modern secondary cosmic-ray measurements, ordered by parameters linked to the geomagnetic field. In general, the most robust parameter for ordering these measurements is the effective vertical cutoff rigidity (R_C), a measure of the energy required for primary cosmic rays to penetrate the geomagnetic and magnetospheric fields and interact with the atmosphere at a given location. Temporal variations in CN production rates at a given site result primarily from changes in the orientation and intensity of the geomagnetic field. These geomagnetic variations have been modeled for CN applications using estimates of Holocene variations in dipole orientation combined with records of global dipole intensity variations. It has typically been assumed in CN research that secular variation of the geomagnetic pole averages to first order to a geocentric axial dipole (GAD) over a time scale of 10 to 100 ka, leading to the use of dipolar approximations for R_C. However, R_C values derived using this assumption may not be valid for Holocene or late Pleistocene samples since the geomagnetic field may not average to a GAD in that time frame. One of the key questions in CN research is thus how to apply scaling models based on modern, detailed R_C values over Holocene and late Pleistocene time scales - a critical period for many CN applications. Recent publications extending higher-order spherical harmonic geomagnetic field models into the Holocene may help resolve this question (e.g., Korte and Constable, 2005, Geochem., Geophys., Geosyst. v. 6, no. 2, doi:10.1029/2004GC000801). I will present a preliminary approach to using these new field models in CN scaling techniques, a comparison with current techniques, and potential implications for CN research.
DE: 1105 Quaternary geochronology
DE: 1150 Cosmogenic-nuclide exposure dating (4918)
DE: 1521 Paleointensity
DE: 1522 Paleomagnetic secular variation
DE: 1527 Paleomagnetism applied to geologic processes
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005