Geomagnetic effects on time-integrated cosmogenic nuclide production with emphasis on in situ 14C and 10Be

Abstract

Production of cosmogenic nuclides (CNs) in geologic material is a function of the cosmic-ray flux at the Earth's surface, which in turn is a function of the intensity and orientation of the Earth's geomagnetic field. Temporal variations in the intensity of the geomagnetic field and the position of the geomagnetic dipole axis (i.e., polar wander) must be considered when calculating production rates that are integrated through time. We have developed a model, based in part on protocols set forth by Desilets and Zreda [Earth Planet. Sci. Lett. 206 (2003) 21–42], that accounts for these variations in an effort to systematically determine their impact on time-integrated production of short-lived (in situ 14C; t1/2=5.73 ka) and long-lived (in situ 10Be; t1/2=1.5 Ma) CNs. Our modeling results show that for samples exposed for the last 3 ka, integrated in situ 14C and 10Be production rates that account for temporal variations in the intensity of the Earth's geomagnetic field are up to ∼13% lower than modern rates at the same location [modern rates are referenced to the 1945.0 Definitive Geomagnetic Reference Field (DGRF)]. In contrast, intensity-corrected 10Be rates are up to ∼30% higher than modern for samples exposed for >25–30 ka. Intensity variations have little effect (<5%) on integrated CN production for samples exposed for the last 15–20 ka, regardless of site location or nuclide used.Our modeling results also show that the impact of polar wander on integrated CN production is secondary compared to intensity variations. Accounting for polar wander is critical, however, when determining modern production rates at midlatitudes (30–40°) because of the current offset between the geomagnetic and geographic poles. At sea level, integrated in situ 14C production rates that account for both intensity variations and polar wander range from 27% higher to 24% lower than modern rates at the same location, and integrated in situ 10Be rates range from 48% higher to 26% lower than modern. Differences between integrated and modern rates increase significantly at higher altitudes. Based on these results, we recommend correcting the modern production rate (referenced to the 1945.0 DGRF or another specific geomagnetic reference field) at site latitudes <60° for variations in the intensity of the Earth's geomagnetic field during exposure and for polar wander over the last 10 ka.