Determination limits for cosmogenic ¹⁰Be and their importance for geomorphic applications

Abstract. When using cosmogenic nuclides to determine exposure ages or denudation rates in rapidly evolving landscapes, challenges arise related to the small number of nuclides that have accumulated in surface materials. Improvements in accelerator mass spectrometry have enabled analysis of samples with low ¹⁰Be content (< 10⁵ atoms), such that it is timely to discuss how technical limits of nuclide determination, effects of laboratory cleanliness, and overall sample preparation quality affect lower blank limits. Here we describe an approach that defines a lower threshold above which samples with low ¹⁰Be content can be statistically distinguished from laboratory blanks. In general, this threshold depends on the chosen confidence interval. In detail, however, we show that depending on which ensemble of blank values and which approach is chosen for the calculation of this threshold, significant differences can arise with respect to when a sample can be distinguished from a blank. This in turn dictates whether the sample can be used to determine an exposure age or a denudation rate, or when it only constrains a maximum age or a minimum denudation rate. Based on a dataset of 57 samples and 61 blank measurements obtained in one laboratory, we demonstrate how these different approaches may influence the interpretation of the data.