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 (increasing atmospheric depth), so production rate scaling will differ for production reactions that have different energy sensitivities. Here, we assess the possibility of the 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 a.s.l.) in western North America. The data are broadly consistent with model predictions. The null hypothesis that 36ClFe/10Beqtz and 36ClFe/36ClK production ratios are invariant with altitude can be rejected at the 90 % confidence level. Thus, reaction-specific scaling factors will likely yield more accurate results than non-reaction-specific scaling factors when scaling 36Cl production in Fe-rich rocks and minerals.