³⁶Cl production rate from K‐spallation in the European Alps (Chironico landslide, Switzerland)

Abstract

ABSTRACT The abundant production of in situ cosmogenic 36 Cl from potassium renders 36 Cl measurements in K‐rich rocks or minerals, such as K‐feldspars, potentially useful for precisely dating rock surfaces, either in single‐nuclide or in multi‐nuclide studies, for example combined with 10 Be measurements in quartz. However, significant discrepancies in experimentally calibrated 36 Cl production rates from spallation of potassium ( 36 P K‐sp ), referenced to sea‐level/high‐latitude (SLHL), limit the accuracy of 36 Cl dating from K‐rich lithologies. We present a new 36 Cl calibration using K‐feldspars, in which K‐spallation is the most dominant 36 Cl production pathway (>92% of total 36 Cl), thus minimizing uncertainties from the complex multi‐pathway 36 Cl production systematics. The samples are derived from boulders of an ∼13.4 ka‐old landslide in the Swiss Alps (∼820 m, 46.43°N, 8.85°E). We obtain a local 36 P K‐sp of 306 ± 16 atoms 36 Cl (g K) −1 a −1 and an SLHL 36 P K‐sp of 145.5 ± 7.7 atoms 36 Cl (g K) −1 a −1 , when scaled with a standard scaling protocol (‘Lm’). Applying this SLHL 36 P K‐sp to determine 36 Cl exposure ages of K‐feldspars from 10 Be‐dated moraine boulders yields excellent agreement, confirming the validity of the new SLHL 36 P K‐sp for surface exposure studies, involving 36 Cl in K‐feldspars, in the Alps.