  We present an analysis of CH band strengths in Keck LRIS spectra of a sample
of 81 stars in M13 within 2 magnitudes of the main-sequence turnoff. The
subgiants clearly exhibit a substantial (a factor of ~ 6) spread in [C/Fe].
Moreover, the bulk of the subgiants possess C abundances larger than those
found among their more luminous counterparts. The turnoff stars themselves are
too warm for appreciable CH formation, but the relatively small range in the
observed CH band strength for stars just below the turnoff nevertheless
translates into this same spread in [C/Fe]. Still fainter, the sample size is
small, but the same range in [C/Fe] appears to be present. On the basis of
these observations we suggest that a process external to the present stars has
resulted in a substantial star-to-star dispersion in [C/Fe] (and possibly other
light elements) among all stars in M13. In addition, the surface C abundances
among the more luminous stars have been further modified by the operation of an
internal deep-mixing mechanism during red giant branch ascent. The amplitude of
the scatter we find in [C/Fe] at all luminosities may prove difficult to
explain via accretion from intermediate mass AGB stars as the external
"polluting" mechanism.
