Hi Alison, again! - and welcome to the SMOW scale for carbonates. I am almost
sure that may be the problem, otherwise it must be a weird influence of the
chicxulub impact, that somehow depleted Mexican calcites to minus 15 permil. You
have not even made any real mistake, since you did not state your scale. Just
the fact that you called the value unusual made one (some of us) jump to the
conclusion that it was SMOW. Also one normally does use the SMOW scale for
carbonate minerals, so that they become compatible with silicate data.
You also did set off some welcome communication.
Peter