Q: Should I report exposure ages in ‘Be-10 years?’ It seems like some papers do this.
A: No, and here’s why.
Basically, a number of exposure-dating papers in the past couple of years have used the term “Be-10 years” (also “Cl-36 years,” etc.) as a unit for exposure ages calculated from Be-10 concentrations. Presumably, the purpose of these terms is to indicate that the exposure ages are model-dependent and may differ from true calendar years. This practice has most likely been suggested by the term “C-14 years” or “radiocarbon years” used to report radiocarbon ages. However, this analogy is misleading and has the potential to cause serious confusion. “C-14 years” is a defined term that is simply a way of restating the measured carbon-14 activity of a sample in a chronologically meaningful fashion. The definition is described in many references (Stuiver and Polach, 1977; Mook and van der Plicht, 1999; van der Plicht and Hogg, 2006) and has been used consistently throughout the Earth science literature for several decades. The carbon-14 activity of a sample can always be uniquely recovered from its age in “C-14 years” using this definition.
In contrast, there is no accepted, published, or even proposed definition of “Be-10 years.” Ages reported in “Be-10 years” in different papers have been calculated using different reference Be-10 production rates, different production rate scaling schemes, and different assumptions about the depth dependence of nuclide production. This has two important consequences. First, “Be-10 years” reported in different publications are, in general, inconsistent and cannot be directly compared. Second, in contrast to the carbon-14 example, the Be-10 concentration of a sample cannot be uniquely recovered from an age given in “Be-10 years.”
In addition, cosmogenic nuclide production rates have been measured at geological calibration sites whose true exposure age has been independently estimated in calendar years. Thus, correct dimensional analysis of an exposure age computed using these production rates implies that the exposure age is in calendar years. No additional unit is required.
Should we define a unit of “Be-10 years?” Again, no. Production rates and scaling schemes are the subject of constant and active research at present. They are continually being updated, so no lasting definition would be possible. Also, of course, the goal of this research is to ensure that cosmogenic-nuclide exposure dating actually reflects true calendar ages of surfaces, i.e., to render terms like this unnecessary.
Remember that the goal of using a term like “Be-10 years” is to indicate to readers that an exposure age is model-dependent and may need to be recalculated in future when more information is available. The best way to achieve this goal is to make sure to include all the data (nuclide concentrations, site and sample characteristics) that are needed to recalculate any exposure ages that you publish. Then the ages can be updated in future to reflect better information, and the study will remain useful to future researchers.
To summarize: report exposure ages in units of years.
W. C. Mook and J. van der Plicht. Reporting C-14 activities and concentrations. Radiocarbon, 41:227-239, 1999.
M. Stuiver and H. Polach. Reporting of C-14 data. Radiocarbon 19:355-363, 1977.
J. van der Plicht and A. Hogg. A note on reporting radiocarbon. Quaternary Geochronology, 1:237-240.