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Be-10 measurements really have improved in the last ten years

July 22, 2010

This plot shows relative uncertainty (one standard error, expressed in percent) on measurements of Be-10 concentrations in quartz. It’s hard to say for sure what is included in these uncertainties because I didn’t compute all of them myself, but to the best of my knowledge they include i) AMS measurement uncertainty, ii) uncertainty in the amount of Be-9 added as carrier, and iii) uncertainty in the blank correction.

The red circles show measurements made by Paul Bierman and his co-workers in the late 1990’s (various papers by Bierman and Marsella selected to include a good range of Be-1o concentrations; citations below). These measurements represent the approximate state of the art at the time, and share two important features. First, they used a commercially-available Be-9 carrier that had a Be-10/Be-9 ratio somewhere around 1 x 10^-14. Second, the isotope ratio measurements were made at LLNL-CAMS on accelerator targets prepared by mixing BeO with silver powder as the conductive binder.

The black circles are a compilation of measurements on samples analysed in the University of Washington lab by me, Greg Balco, between approximately 2002 and 2009. The black triangles are measurements from Schaefer et al. (2009) that extend to lower Be-10 concentrations. These measurements represent the approximate state of the art at the present time and share two important features. First, they employed a low-blank Be-9 carrier prepared from deep-mined beryl according to a recipe by John Stone. This carrier has a Be-10/Be-9 ratio somewhere around 2-6 x 10^-16. Second, the isotope ratio measurements were made at LLNL-CAMS on accelerator targets prepared with Nb rather than Ag as the binder. Sometime around 2002, the LLNL-CAMS staff discovered that this modification significantly increased Be beam currents and thus Be-10 count rates. There have also been a handful of other incremental improvements over the years in increasing beam currents at CAMS.

This plot makes no attempt to control for sample size, which of course affects the Be-10/Be-9 ratio that is actually being measured, in either of these data sets. As the available range of adjustment of sample size is about one order of magnitude, that presumably accounts for most of the half-an-order-of-magnitude scatter in relative uncertainty.

These two major improvements — the low-blank Be-9 carrier and the Nb-BeO targets — make a difference. In the 1990’s data it is evident that precision is blank-limited at lower concentrations…uncertainties diverge from the overall log-linear relationship and become large as sample ratios approach blank levels somewhere around a few tens of thousands of atoms per gram. The low-blank carrier removes this limitation and permits maintenance of counting statistics at much lower concentrations. In addition, higher beam currents push this relationship down across the entire range of Be-10 concentrations.

Homework: plot your own measurements on this figure.

Some references:

Bierman, P., and Turner, J., 1995, 10Be and  26Al evidence for exceptionally low rates of Australian bedrock erosion and the likely existence of pre-Pleistocene landscapes: Quaternary Research, v. 44, p. 378-382.

Bierman, P.R., Gillespie, A., and Caffee, M., 1995, Cosmogenic age-estimates for earthquake recurrence intervals and debris-flow fan deposition, Owens Valley, California: Science, v. 270, p. 447-450.

Marsella, K.A., 1998. Timing and extent of glaciation in the Pangnirtung Fjord region, Baffin Island: determined using in situ produced cosmogenic 10Be and 26Al. Ms thesis, University of Vermont.

Marsella, K.A., Bierman, P.R., Davis, P.T., Caffee, M.W., 2000. Cosmogenic 10Be and 26Al ages for the last glacial maximum, eastern Baffin Island, arctic Canada. Geol. Soc. Am. Bull. 112, 1296-1312.

J. Schaefer, G. Denton, M. Kaplan, A. Putnam, R. Finkel, D. Barrell, B. Andersen, R. Schwartz, A. Mack- intosh, T. Chinn, and C. Schlu ̈chter. High frequency glacier fluctuations in New Zealand differ from the northern signature. Science, 324:622–625, 2009.

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