Cosmogenic nuclide burial dating
So to compare the precision of burial dates with various nuclide pairs over different age ranges, a few ingredients are needed.One is the precision of the half-life determinations.So, again, exposure goes to the right and burial goes down. Although I have not made a systematic historiographic study of this phenomenon, I believe that the European style is largely just due to the fact that the “Cosmo Calc” software put together by Pieter Vermeesch does it this way. Nearly all the two-nuclide diagrams in the existing literature involve the normal implementation of the Al-26/Be-10 diagram, so anyone familiar with this literature expects exposure to go to the right on a tw0-nuclide diagram, and burial to go down.On the other hand, here is a Ne-21/Be-10 diagram from a very cool paper by Florian Kober and Vasily Alfimov: This figure has a lot of data in it that are beside the point from the perspective of this post, but the point is that it has the opposite axes: Be-10 concentration on the x-axis and Ne-21/Be-10 ratio on the y-axis. I think inverting the diagram so that burial goes up just confuses readers. Thus, I advocate always plotting the longer-lived nuclide of the pair on the x-axis, and the ratio of the shorter-lived to longer-lived nuclide on the y-axis. Of course, I am in the US, but I am not just cheering for my own team here.So, basically, in the Al-26/Be-10 two-nuclide diagram (let’s not use “banana diagram” any more…historically, it probably should be called a “Lal-Klein-Nishiizumi diagram,” although that is a bit cumbersome), exposure goes to the right and burial goes down. The problem arises when other nuclides are involved.
Of these, Ne-21 is stable, so there is no uncertainty in its half-life. The following plot shows the concentration-measurement uncertainty relationship for all the Al-26 and Be-10 concentrations I could assemble from readily available data.
Then if you bury the sample deeply enough to stop new nuclide production, inventories of both nuclides (or at least one of the nuclides, if the other is stable) decrease due to radioactive decay.
Because they decay at different rates, the actual ratio of the two nuclides gradually diverges from the production ratio.
So samples that are “below the banana” have experienced both a period of exposure and a period of burial.
Here’s an example: The lines are contours of burial time in Myr. This is the foundation of the method of cosmogenic-nuclide burial dating.
The uncertainty of a cosmogenic-nuclide burial age is set by a number of factors: measurement precision for the nuclides in question; the actual values of the production ratios and decay constants; how precisely the decay constants of the nuclides in question are known; how precisely the production ratios are known; and geological factors, mainly to do with the burial history of the sample.