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

Abstract

ABSTRACTThe abundant production of in situ cosmogenic 36Cl from potassium renders 36Cl 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 10Be measurements in quartz. However, significant discrepancies in experimentally calibrated 36Cl production rates from spallation of potassium (36PK‐sp), referenced to sea‐level/high‐latitude (SLHL), limit the accuracy of 36Cl dating from K‐rich lithologies. We present a new 36Cl calibration using K‐feldspars, in which K‐spallation is the most dominant 36Cl production pathway (>92% of total 36Cl), thus minimizing uncertainties from the complex multi‐pathway 36Cl 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 36PK‐sp of 306 ± 16 atoms 36Cl (g K)−1 a−1 and an SLHL 36PK‐sp of 145.5 ± 7.7 atoms 36Cl (g K)−1 a−1, when scaled with a standard scaling protocol (‘Lm’). Applying this SLHL 36PK‐sp to determine 36Cl exposure ages of K‐feldspars from 10Be‐dated moraine boulders yields excellent agreement, confirming the validity of the new SLHL 36PK‐sp for surface exposure studies, involving 36Cl in K‐feldspars, in the Alps.