Comment on gchron-2022-18

Abstract

Cosmic-ray exposure (CRE) dating of boulders on terminal moraines has become a well-established technique to reconstruct glacier chronologies. If topographical obstructions are present in the surroundings of sampling sites, CRE ages need to be corrected for topographic shielding. In recent years, geographical information system (GIS)-based approaches have been developed to compute shielding factors with elevation data, particularly two toolboxes for the ESRI ArcGIS software. So far, the output of the most recent toolbox (Li, 2018) has only been validated with a limited number of field data-based shielding factors. Additionally, it has not been systematically evaluated how the spatial resolution of the input-elevation data affects the output of the toolbox and whether a correction for vegetation leads to considerably more precise shielding factors. This paper addresses these issues by assessing the output of the toolbox with an extensive set of field data-based shielding factors. Commonly used elevation data with different spatial resolutions were tested as input. To assess the impact of the different methods on CRE ages, ages of boulders at three sites with different topography were first recalculated with GIS-based and then with field data-based shielding factors. For sampling sites in forested low mountainous areas and in high Alpine settings, the shielding factors were stable, i.e. independent of the spatial resolution of the input-elevation data. Vegetation-corrected elevation data allowed more precise shielding factors to be computed for sites in a forested low mountainous area. In most cases, recalculating CRE ages with different shielding factors for the same sampling sites led to minor changes in ages. Only in a few cases, the shifts were in the order of a few percent. It is shown that the use of elevation data with a very high resolution requires precise xy-coordinates of sampling sites. Otherwise, there is a risk that small-scale objects in the vicinity of sampling sites will be misinterpreted as topographic barriers. Overall, the toolbox provides an interesting avenue for the determination of shielding factors. Together with the guidelines presented here, it should be more widely used.