Influence of secular variation of the geomagnetic field on production rates of in situ produced cosmogenic nuclides

Scaling factors are presented that allow a full description of the cosmic ray flux modulation during secular variation of the geomagnetic field. These scaling factors are analogous to those determined in a similar work [Dunai, T.J., Earth Planet. Sci. Lett. (2000) 176, 157¯169] that were accurate for field strengths equivalent to present-day field strength, but cannot accommodate field strength variations. Strategies to derive the relevant geomagnetic field parameters are discussed, and are tested with existing production rate calibrations. For 3He a production rate of 103±4 (±2sigma ) atoms/g olivine/yr (sea level and for zero cutoff rigidity), was derived from calibration sites from Hawaii, Western USA and Canary Islands, covering the time range of 2¯281 kyr.