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