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Exposure age, or years BP?

July 12, 2009

Q: Do the online exposure age calculators compute years before present? That is, years before 1950, like calibrated radiocarbon ages?

Short answer: The exposure age calculator computes the exposure age, that is, the length of time the sample has been exposed to the cosmic-ray flux. It does not give an absolute age in years before 1950 (or any other year).

For example, say a moraine boulder was emplaced 1000 years ago, that is, in 1009 AD. Barring any other complicating factors, if you computed the exposure age of this sample using the online calculator you would find that its exposure age was 1000 years. However, if you could date the same event by radiocarbon dating, and everything also worked perfectly, you would obtain a calibrated radiocarbon age of 942 years before 1950.

Obviously, this discrepancy is smaller than the age uncertainty in nearly all exposure dating applications, so in the vast majority of cases it is not necessary to worry about it.

Long answer: The calculator is always computing the exposure age in the sense of years the sample has been exposed before analysis, not as “years before present” where the present is some defined calendar year.

The only places that 1950 comes in are as follows:

1. Some of the production rate calibration sites have independent ages in calibrated radiocarbon years before 1950. I did not correct these to 2008, or whatever date the samples were analysed, before computing the production rates. That is, if a calibration site is, say, 15,000 calibrated radiocarbon years old and the samples were analysed in 1996, I did not correct the age to 15,046. This is a ballpark average age for the calibration sites, which means that the production rates are, technically, too high by a factor of approximately (50/15000) = 0.3 %. True, this is wrong, but as you can see it is not very important in light of the numerous other uncertainties in the production rate.

2. For the time-dependent scaling schemes, the time step is 500 years during the Holocene. The magnetic field at time zero is represented by the 1950 DGRF magnetic field. This is technically wrong too, because time zero in an exposure age calculation is not 1950, it’s the time you collected the sample. Then the cutoff rigidity is assumed to change linearly from whatever it is at time 0 to whatever it is at time 500 years ago.  No shorter-time-scale magnetic field variability is represented. No shorter-time-scale solar variability is represented either — that record gets filtered at 500 years as well. Sorry Nat.

So “years before 1950” are NOT what is being calculated. In order to do it that way, you would have to actually subtract some number of atoms/gram from the measured value to get back to whatever the Be-10 concentration was in 1950, and in order to do that the user would have to enter the year of sample collection. As users do not have to do this, there would be no way to implement this scheme. To be honest, it is not clear to me why anyone would do it this way — to me, the concept of an “exposure age” clearly indicates the number of years that a sample has been exposed.  However, recently a bunch of people have asked about this so there appears to be some confusion.

One implication of this is that the absolute time to which the exposure age is referenced (and the absolute times to which all the time steps in the paleomagnetic field reconstruction are referenced) is a moving target. That is, an exposure age of 200 years measured in 2000 gives the year 1800, whereas an exposure ages of 200 years measured in 2008 gives the year 1808. At present low-level measurements are not precise enough to care about this issue, but, sure, we should probably fix it. Certainly this will need to be fixed after, say, another hundred years have elapsed.

I didn’t think that any parts of the calculator output pages or documentation contained the phrase “years before present” instead of just “exposure age.” Unfortunately, as of July 11 I am embarassed to note that I did place “yr BP” on the axes of some of the diagnostic plots on the single-sample output page. That is now fixed. I apologize for the oversight.

The main thing that really should be fixed at some point is to include shorter-timescale magnetic field variability as you approach the present. Realistically, this would be over-precise with regard to the production rate calibration (because all the production rate calibration sites are much older) so I don’t think it would result in a real improvement in the results. In fact, in the absence of any additional calibration data from young sites, I believe it would result in an overly optimistic estimate of the precision of the dating technique for young samples.  However, it would be the proper way to do things.

Finally, the reference production rates refer to sea level, high latitude, at time zero.

2 Comments leave one →
  1. Nat Lifton permalink
    September 22, 2009 14:25

    Hmm. This looks familiar…

    Great job on the blog – I’ll be following it!


  1. Data reporting madness « The bleeding edge of cosmogenic-nuclide geochemistry.

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