IS TERRESTRIAL COSMOGENIC NUCLIDE DATING THE NEW LICHENOMETRY?

Terrestrial cosmogenic nuclide dating (TCN) has arguably been the most important advance in reconstructing Holocene glacier fluctuations since the advent of radiocarbon dating. An advantage of TCN dating, like the radiocarbon method, is the reproducibility of a sample’s isotopic concentration for a given sample. Improvements in analytical techniques have further refined TCN dating to yield ages with reported errors (±1 σ) that are often below 10% of a sample’s age. In contrast to TCN, some have suggested that lichenometry is a pseudo-science largely because it is purported to be based on: lack of consensus on methodology; invalid assumptions about lichen identification; treatment of errors; and relation between lichen growth and factors unrelated to time since deposit stabilization. While TCN concentrations are reproducible for a given sample with high analytical precision, the calculation of a TCN age is subject to many of the same warts and wrinkles that beset lichenometry. One unsightly wart includes the lack of an objective-based approach to outlier identification that considers both statistical and non-statistical arguments. Deeply incised wrinkles include a lack of systematic treatment (or omission) of environmental factors that could influence TCN concentrations including time-varying changes in surface erosion and snow cover, and the uncertain geomorphic history of a given deposit. Further advances in TCN dating will probably not arise from improved production rates, reduced counting errors or use of tandem measurements, but, rather, a critical assessment of environmental factors that influence the final age of a given sample. Carefully designed inter-comparison studies would help to quantify the importance of these factors and how they influence the final reported age of a given TCN sample.