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

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 (R-C), 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 R-C estimates derived from detailed modem 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 R-C 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:10.1029/2004GCO00801.] may allow reduction of such errors by bridging the gap between detailed modem geomagnetic and simplified paleomagnetic models. We have developed a new model framework describing temporal and spatial variation in R-C 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 R-C 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 C-14 (in Situ C-14) in quartz. Due to its short half-life (5.73 kyr), C-14 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 C-14 results from samples at secular equilibrium from 38 degrees N and 3.5 km in Tibet and eastern California are consistent with the longitudinal variability predicted by the new framework. (C) 2008 Elsevier B.V. All rights reserved.