EXPLORING TIME-DEPENDENT GEOMAGNETIC AND ATMOSPHERIC EFFECTS ON COSMOGENIC NUCLIDE PRODUCTION-RATE SCALING

The scaling implications of two recent time-dependent spherical harmonic geomagnetic models spanning the Holocene are explored using a recently published cosmogenic nuclide (CN) production-rate scaling model (Lifton et al., 2014 EPSL 386, 149–160, termed the LSD model). Korte and Constable (2011, Phys. Earth. Planet. Int. 188, 247–259) and Korte et al. (2011, EPSL 312, 497–505) updated earlier global paleomagnetic models used by Lifton et al. (2014) and extended coverage to 0-10 ka, based on both archeomagnetic/volcanic and sedimentary records. Another recent time-dependent spherical harmonic geomagnetic model from 0-14 ka by Pavón-Carrasco et al. (2014, EPSL 388, 98–109) is based solely on archeomagnetic/volcanic records. With the new models as input, trajectory-traced estimates of effective vertical cutoff rigidity (R_C - the standard method for ordering cosmic ray data) yield significantly different time-integrated LSD scaling predictions when compared to each other and to results using the earlier models. These predictions using R_C are also compared to those using a dipolar approximation to R_C that estimates the cutoff effects over all arrival directions (termed apparent cutoff rigidity, or R_CA).

Scaling predictions are tested empirically using recently published production rate calibration data for both ¹⁰Be and ³He. Results for the few calibration sites from geomagnetically sensitive regions suggest that the Pavón-Carrasco et al. (2014) model with an R_CAparameterization tends to predict sea level, high latitude production rates more in line with those from calibration sites not affected by geomagnetic variations, suggesting a global resolution of regional production rate differences is possible with additional high-quality calibrations and improved geomagnetic models.

In addition, the effects of a time-dependent atmospheric model (SynTraCE-21, e.g., Liu et al., 2009, Science 325, 310–314) on LSD scaling predictions are evaluated from 0-21 ka. Given the dominance of altitudinal over latitudinal (geomagnetic) scaling effects on CN production, incorporating such transient global simulations of atmospheric structure into scaling frameworks may contribute to improved understanding of long-term production rate variations and their implications for surficial process studies.