SPEED DATING: ARTISANAL KNOWLEDGE OF COSMOGENIC-NUCLIDE GEOCHRONOLOGISTS REVEALED

Cosmogenic-nuclide geochemistry, like other fields of human endeavor, is presumably capable of documenting all its methods in well-indexed, accessible, and up-to-date literature...but, alas, has not done so. Particularly opaque aspects include (i) field sample selection (because of its site-specificity), and (ii) calculation methods for interpreting nuclide concentrations as ages or rates (because these are complicated, rapidly evolving, and generally buried in computer code).

The goal of sample selection for exposure or burial dating is for the sequence of geologic events that have befallen the sample to not be in doubt, with only times or rates of the events to be determined. If you know exactly what happened to the sample from independent geologic evidence, it is straightforward to determine when or how fast it happened from cosmogenic-nuclide measurements. However, in practice it is not always clear how to achieve this, especially in common cases where (i) there are a lot of samples that meet minimum quality-control criteria, and you need to decide which subset to collect; or (ii) there aren't any samples that meet the minimum quality threshold, and you need to decide whether to collect some anyway.

Calculation methods for interpreting nuclide concentrations as ages or rates are complicated in some cases, but for the most part they are easily available to users through simple online services. On balance, this counts as a success for the field, but it creates the problem that best-practice algorithms and parameter values used in online systems tend to evolve faster than published documentation of these systems. Thus, to what extent the current version of an online calculator corresponds to its published description is often clear only to the person responsible for the underlying code. This leads to confusion as to which online calculator, version of a particular calculator, and/or set of input parameters are or are not correct, appropriate, or au courant.

Here we try to demystify these aspects of cosmogenic-nuclide geochemistry by providing a forum to discuss (i) sample collection and experimental design, (ii) what exactly is happening inside online exposure age calculators, online databases, and various bits of computer code, or (iii) any other confusing aspects of the field.