Technical Note: Altitude Scaling of 36cl Production from Fe

Abstract. Cosmogenic nuclide production rates depend on the excitation functions of the underlying nuclear reactions and the intensity and energy spectrum of the cosmic ray flux. The cosmic ray energy spectrum shifts towards lower average energies with decreasing altitude such that production from high-energy reactions may attenuate more rapidly in the atmosphere than other reactions and require unique scaling factors. Here, we assess the possibility of unique scaling of 36Cl production from Fe by modeling changes in the 36ClFe/36ClK and 36ClFe/10Beqtz production ratios with altitude. We evaluate model predictions against measured 36Cl concentrations in magnetite and K-feldspar and 10Be concentrations in quartz from granitic rocks exposed across an elevation transect (ca. 1700–4300 m asl) in western North America. The data are broadly consistent with model predictions. The null hypothesis, that production ratios are invariant with altitude, can be rejected at the 90 % confidence level. We recommend using reaction-specific scaling factors for scaling 36Cl production in magnetite and other Fe-rich minerals.