10Be and 26Al production rates deduced from an instantaneous event within the dendro-calibration curve, the landslide of Köfels, Ötz Valley, Austria

Abstract

Surface exposure dating requires the knowledge of cosmogenic nuclide production rates. When determining time-integrated production rates the exposure ages of the calibration samples need to be accurately known. The landslide of Köfels (Austria) is very well suited for this purpose. It is the largest landslide in the crystalline Alps of Austria dating back to 7800±100 years BC (AMS 14C dating of buried wood), which is well within the 14C dendro calibration curve. Exposed quartz veins were sampled from the tops of large boulders from the toe of the landslide for analysis of 10Be and 26Al. To calculate sea level, high geomagnetic latitude (≥60°), open sky radionuclide production rates, corrections were applied for altitude and latitude, for shielding by surrounding mountains, for sample geometry, vegetation and snow cover, and for sample thickness. The production rates for an exposure age of 10,000 years are 5.75±0.24 10Be atoms/yr g SiO 2 and 37.4±1.9 26Al atoms/yr g SiO 2. A 26Al/ 10Be ratio of 6.52±0.43 can be calculated. The influence of the geomagnetic field on these production rates has been estimated using two different geomagnetic field records. Our production rates should be a good approximation for the use of surface exposure dating between about 5000 and 13,000 years BP.