Calibration of the production rate of cosmogenic 36Cl from Fe

Abstract

Abstract We present a new calibration of the in-situ production rate of cosmogenic 36Cl from Fe with the aim of enabling the use of Fe-oxides and other Fe-rich minerals as 36Cl target materials. The calibration was conducted using magnetite mineral separates from rocks with well-constrained exposure histories, including late-Pleistocene glacial erratic boulders from the Sierra Nevada, California, USA and samples from a slowly eroding ridgetop in the Quadrilatero Ferrifero, Minas Gerais, Brazil. At both sites the 36Cl production rate from Fe was calibrated against the production rate of 10Be in co-occurring quartz, while in the Sierra Nevada the calibration was also conducted against 14C-based deglaciation ages. The data produce a mean 36ClFe to 10BeQtz production ratio of 0.417 ± 0.010 at the calibration sites. This ratio may depend on altitude and latitude. A reaction-specific scaling model predicts that the calibrated ratio scales to a reference value at sea level and high latitude of 0.326 ± 0.008. When considering uncertainties in reference 10Be production rate, this is equivalent to a sea level and high latitude production rate of 1.28 ± 0.11 atoms g Fe−1 yr−1. Additional data are needed to confirm the scaling behavior.