The effects of slope degradation on lichenometric dating of Little Ice Age moraines

Over the last four decades, lichenometry has been used globally to determine late Holocene moraine ages that are necessary for developing glacier-length chronologies. However, the influence of a variety of environmental factors that can affect the demographics of a lichen population in glacial landscapes led to the development of a variety of different methods of data collection and analysis. Although the benefits of many methods have been discussed in the context of different controlling factors, none has provided information regarding the effects of landform evolution on lichenometric dating techniques. Because matrix-supported moraines widen and flatten over time, boulders at the crest are continually exhumed while those at the toe are gradually buried. Using a commonly applied sediment-diffusion model and an exhaustive field analysis of the distribution of the largest lichen on each boulder in a 18-m-wide swath on two moraine slopes, the effects of moraine degradation are related to lichen demographics. Results indicate that diffusion describes the evolution of a moraine profile, and further show that slope degradation processes affect the number, frequency, and distribution of boulders available for lichen colonization over time. This model of a constantly changing landscape typically is not considered when applying lichenometry in recently deglaciated terrain. However, the amount of degradation indicated herein is substantial on decadal to centennial time-scales and indicates that degradation processes must be considered when choosing the statistic of a lichen population for constructing growth curves and dating landforms. This study suggests that for many moraines a technique that identifies the largest lichen, or the average of the largest lichens on five different boulders, is likely to result in more accurate growth curve construction and subsequent landform ages than statistically rigorous techniques.