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An early look at the CRONUS Be-10 calibration data

July 28, 2009

One of the more suspenseful questions surrounding the periodic CRONUS-Earth and CRONUS-EU meetings is: how is the geological calibration exercise going? This is important because currently the vast majority of exposure ages are computed using a “global calibration data set”  composed of a variety of Be-10 and Al-26 production rate calibrations conducted between 1989 and 2005. Henceforth this will be abbreviated “GCDS” (however, it should not be confused with George H.W. Bush’s and this reporter’s alma mater) Also, the term ‘reference Be-10 production rate’ will henceforth refer to the Be-10 production rate due to spallation at sea level and high latitude, normalized to the Be AMS standards of Nishiizumi et al. (2007). The reference Be-10 production rate derived from the GCDS is exhaustively documented by Balco et al. (2008), and is 4.49 atoms/g/yr for the “St” scaling scheme, and 4.87 atoms/g/yr for the “Li” scaling scheme (these abbreviations are from the Balco et al. reference — St is the Stone(2000)/Lal(1991) scaling scheme, and Li is that of Lifton(various references). The Li scheme is more or less representative of several scaling schemes based on neutron monitor measurements that give similar results).

Recently a couple of studies have indicated that the reference Be-10 production rate derived from the GCDS is too high to give correct exposure ages for at least some times and places. Balco et al. (2009) compiled a handful of Be-10 production rate calibration measurements from late-glacial sites in northeastern North America; these yielded a reference Be-10 production rate about 10% lower than that derived from the GCDS. Aaron Putnam (UMaine) and colleagues recently unearthed an excellent calibration site in New Zealand — an early Holocene landslide that placed exposure-datable boulders directly on top of flattened but radiocarbon-dateable underbrush — and got similar results (these are described in Aaron’s Goldschmidt 2009 abstract).

One main point of the CRONUS-Earth project was essentially to redo the GCDS, that is, to find, exhaustively document, and comprehensively analyse a global set of high-quality calibration sites. This has now been in progress for a couple of years. So the question is, are the early CRONUS data in agreement with the GCDS, or do they also show that we have been using a production rate that is generally too high?

With regard to Be-10, there now exist new measurements from three calibration site locations: the wave-cut shorelines of Lake Bonneville, Utah (true exposure age of 17,400 years); several late-glacial moraines in Scotland (11,600 years); and a late-glacial moraine in New Hampshire (13,900 years). Most of these samples have been analysed by both the UW and Lamont prep labs, and a handful by several other labs. One important aspect of the entire project is to compare the results from multiple laboratories, but I’ll ignore this for the time being and focus on the data from UW that have been compiled by John Stone here.

First, a refresher on what the 2008 GCDS looks like. The following figure is part of Figure 5 from Balco et al. 2008, that shows the scatter of all the various calibration data around the reference production rates derived therefrom, for St and Li scaling schemes. This has not been renormalized to reflect the 2007 Nishiiumi restandardization: to bring it up to date, divide the y-axis values by 1.106. Two things are clear from this: first, a lot of the measurements have large uncertainties; second, there is a lot of scatter. Furthermore, even with some rather large measurement uncertainties, the scatter is still well in excess of that expected from measurement uncertainty.

Be_10_calibration_new.eps
Now, compare this to a similar plot for the early CRONUS results. This is a slightly different plot, requiring the following steps. First, determine the reference production rate that best fits the calibration data; second, using that production rate, calculate the exposure age of all the calibration samples from their measured Be-10 concentrations; third, compare to the actual age of the calibration sites by computing the ratio t_calculated/t_actual. If everything goes as planned, the calculated exposure ages should all be indistinguishable from the actual ages, and all data will plot on the line y = 1 within error. This plot is functionally the same as the production rate vs. elevation plot shown above, except that the best-fitting reference production rate is normalized to one. Both plots show the scatter of the calibration data set around the best-fit production rate. The choice of sample elevation as the independent variable mainly serves to spread out the data nicely.

This first plot shows the early CRONUS results by themselves on the Li and St scaling schemes:

newdata1

Two things are important here: One, the uncertainties on the individual measurements are much smaller, mostly around 3%. This is largely due to steady AMS improvements at LLNL (where these measurements were made) over the last ten years. Second, the scatter of the individual samples around the best-fit production rate is much smaller. Statistically, no excess scatter is present. This is not quite a fair conclusion yet — because the new data are not as geographically scattered as the 2008 GCDS, so scaling scheme errors are suppressed — but the scatter in the new data is in fact much less than that in any geographically equivalent subset of the 2008 data set.

Next compare these results to the 2008 GCDS. The actual values for the reference production rates derived from the new CRONUS data, for comparison to those derived from the GCDS discussed above, are 4.2 atoms/g/yr for the St scaling scheme and 4.65 atoms/g/yr for the Li scaling scheme. These are lower. This is the same figure above, with the GCDS calibration samples added by calculating their exposure ages using the best-fit production rate from the new data. Old data in red, new data in black.

newdata2The reference production rate derived from the new data systematically overestimates the exposure age of the calibration sites from the 2008 GCDS. This is another way of saying that the reference production rate implied by the new data is significantly (by almost 10%) lower than that implied by the 2008 data set.

The reference production rates implied by these new data are, however, similar to the reference production rates implied by the Northeast North America and New Zealand calibration data sets described above. The following figure makes this comparison — we are still normalizing to the reference production rate that best fits the new CRONUS data, these data are shown in black, the NE North America data are in green, and the NZ data are in blue.

newdata3The early CRONUS data (black) show good agreement – well within measurement uncertainty — with the NE North America calibration data (green), although the latter display more scatter and somewhat of a tail on the low end. Decent agreement between these two data sets is not a huge surprise because of their geographic overlap. The New Zealand calibration data, on the other hand, predict significantly lower reference production rates than the early CRONUS data for all available scaling schemes (both shown and not shown). This is unexpected — although geographically far apart, the NZ sites and the CRONUS sites are fairly similar in elevation and magnetic field characteristics. The remaining disagreement suggests that we may be missing something in the production rate scaling process, but it’s not clear what that might be.

To summarize, these early CRONUS results agree with the recently published regional calibration data sets in indicating fairly clearly that the 2008 GCDS overestimates production rates, at least at the elevations (low) and latitudes (high) of the new calibration data. Thus, exposure ages computed using the 2008 GCDS — which is incorporated into the CRONUS online exposure age calculators by default — may be systematically too young (although, it is important to note, still contained within the 10% uncertainty of the GCDS-derived production rates).  This of course, puts the CRONUS project in a situation like that of an epidemiologist who finds that patients given the experimental drug seem to be dying a lot faster than those in the control group. Should he stop the study? Should we change the default production rates in the online calculator to reflect the fact that the currently accepted production rates seem to be too high?

At present it is probably not a good idea to do this, mainly because the CRONUS geological-calibration efforts are progressing steadily but still half-baked. No one has yet compiled results from multiple labs in such a way as to determine whether inter-lab biases are important. More measurements on these calibration sites as well as from others that will increase the range of age, elevation, and magnetic field strength spanned by the data set are still in progress. New information is still coming in about the true age of many calibration sites. None of the calibration results have been comprehensively enough documented in the public literature that others can evaluate how good they are.

On the other hand, there are two things that can be done now. The first is to remember that if authors of exposure-dating papers include ALL the data necessary to recalculate the exposure ages with a different production rate calibration data set, there is no problem. If all these data are recorded — in the paper itself or in a permanent and easily accessible online data repository, not in the author’s file cabinet — then it will be easy to recalculate exposure ages in future to account for improved production rate calibrations.

The second thing, of course, is that one can calculate exposure ages using one of the new calibration data sets. In order to facilitate this, I’ve placed a handful of pages on the ‘developmental’ section of the CRONUS-Earth online exposure age calculators that allow one to calculate exposure ages using i) the Balco et al. 2009 northeast North America calibration data set, ii) Aaron Putnam and colleagues’ New Zealand calibration data set, and iii) the early CRONUS results from the UW lab described above. Will these yield more accurate ages? In the situation where one is applying a calibration data set that is close in space and time to one’s unknown-age sites — that is, using the NE North America calibration to compute exposure ages for late-glacial NE North America or the NZ calibration to compute ages in NZ — yes, the resulting ages are almost certainly more accurate. On the other hand, whether or not the new CRONUS calibration data will yield more accurate ages than the GCDS for all sites globally is still unknown. At this point, the main thing that is certain is that there will be some future revisions to the Be-10 production rate. Again, the observation that there still seems to be a lot to learn about production rates emphasizes the importance of complete data reporting — any exposure-dating papers that don’t contain enough data to recalculate the ages will rapidly and invariably become not only incorrect, but useless to future researchers.

Incomplete list of references:

Balco G., Stone J., Lifton N., Dunai T., 2008. A simple, internally consistent, and easily accessible means of calculating surface exposure ages and erosion rates from Be-10 and Al-26 measurements. Quaternary Geochronology 3, pp. 174-195.

Balco G., Briner J., Finkel R.C., Rayburn J., Ridge J.C., Schaefer J.M., 2009. Regional beryllium-10 production rate calibration for late-glacial northeastern North America. Quaternary Geochronology 4, pp. 93-107.

K. Nishiizumi, M. Imamura, M. Caffee, J. Southon, R. Finkel, and J. McAnich. Absolute calibration of Be-10 AMS standards. Nuclear Instruments and Methods in Physics Research B, 258:403–413, 2007.

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