David L. Dettman escribis:

>         One of our students is working on the stable isotope ratios of carbonate
> dust blowing around Tucson. One of the issues involved is alteration of the
> original composition through interaction of this very fine carbonate powder
> with atmospheric water vapor and CO2. It is well known in the 14C community
> that fine grain carbonate powder will pick up modern CO2 and give anomalous
> ages. This got me to thinking about the old standard NBS-20 and whether
> interaction with atmosphere can result in measurable shifts in either d13C
> or d18O. Can anyone describe the problems that led to the abandonment of
> NBS-20? Did the increased variability (or trend in composition) show up in
> oxygen or in carbon or both?
>       Throughout the 1980s we used NBS-20 at the Univ. of Michigan. The
> original supply was split into small amounts and each was sealed and stored
> in a desiccator. We never saw any drift in its measured composition.
> Many thanks,
> David Dettman
>
>                David Dettman          [log in to unmask]
>                Dept. of Geosciences   ph. (520) 621-4618
>                Univ. of Arizona       FAX (520) 621-2672
>                Gould-Simpson Bldg.
>                Tucson, AZ 85721