Highly variable light attenuation across a gneiss rock wall in eastern Greenland

Abstract

Exposure ages and erosion rates determined using rock surface luminescence have the potential to revolutionalise Quaternary Science. However, accurately quantifying μ (the light attenuation coefficient) is a significant challenge. Our study shows how variable light attenuation properties (i.e. μ) can be for five samples taken from a small (423 m) elevation range of a near-vertical gneiss rock wall, which we would have expected to have had a shared μ value. The light attenuation properties varied on multiple scales: (i) between samples; (ii) between replicate cores; and (iii) between slices within a core. To avoid the between sample variability, we need to either measure μ directly from the rock cores used to derive the exposure age or erosion rate, or prove that these samples and the sample used to calibrate μ have identical light attenuation properties. To avoid variability between replicate cores observed in samples, it may be more accurate to derive exposure ages or erosion rates using a single core with identical light attenuation properties to the sample used to calibrate μ. Finally, where possible, we should avoid rocks that have hetergeneous mixes of minerals with different opacities (lighter and darker). However, if it is not possible, we should routinely measure red-green-blue (RGB) values to link the presence of any low datapoints in the luminescence depth profiles to overlying lighter-coloured minerals that could have caused light ‘piping’. Our findings further emphasise the detailed consideration required for deriving μ to determine accurate exposure ages and erosion rates