Scaling time-integrated in situ cosmogenic nuclide production rates using a continuous geomagnetic model

Abstract

Spatial and temporal variations in Earth's magnetic field affect the corresponding time-integrated distribution of in situ cosmogenic nuclide (CN) production rates. These effects can be quantified by the effective vertical cutoff rigidity (RC), a measure of the energy required for primary cosmic rays to penetrate the geomagnetic field and interact with the atmosphere at a given location. Recent CN production rate scaling models are based on atmospheric cosmic-ray measurements parameterized using RC estimates derived from detailed modern geomagnetic field representations which include both dipole and non-dipole field contributions. However, published methods for quantifying time-integrated geomagnetic effects on CN production rate scaling rely on various geocentric dipolar approximations to RC driven by separate records of paleomagnetic pole position and paleointensity. Therefore, applying dipolar paleomagnetic records spanning millennial time scales (which explicitly ignore past non-dipole effects) to scaling models derived using modern geomagnetic field representations may lead to systematic errors in any calculated results.A recently published continuous geomagnetic model covering the last 7 kyr (CALS7K.2) [M. Korte and C.G. Constable, Continuous geomagnetic field models for the past 7 millennia: 2. CALS7K, 2005. Geochem., Geophys., Geosyst. 6 Q02H16, doi:10.1029/2004GC000801.] may allow reduction of such errors by bridging the gap between detailed modern geomagnetic and simplified paleomagnetic models. We have developed a new model framework describing temporal and spatial variation in RC for 0–7 ka and earlier, based on CALS7K.2, which explicitly accounts for non-dipole field effects while attempting to mitigate systematic scaling biases.Scaling factors derived using the new RC framework predict significant longitudinal variability in time-integrated CN production, while predictions using dipolar geomagnetic approximations do not. One can test these predictions using in situ cosmogenic 14C (in situ 14C) in quartz. Due to its short half-life (5.73 kyr), 14C attains secular equilibrium between production and decay after approximately 25 kyr of exposure, at which point its measured concentration is only a function of its integrated average production rate. Initial in situ 14C results from samples at secular equilibrium from 38°N and 3.5 km in Tibet and eastern California are consistent with the longitudinal variability predicted by the new framework.