  We present CN and CH indices and CaII triplet metallicities for 34 giant
stars and chemical abundances for 33 elements in 14 giants in the globular
cluster M2. Assuming the program stars are cluster members, our analysis
reveals (i) an extreme variation in CN and CH line strengths, (ii) a
metallicity dispersion with a dominant peak at [Fe/H] = -1.7 and smaller peaks
at -1.5 and -1.0, (iii) star-to-star abundance variations and correlations for
the light elements O, Na, Al and Si and (iv) a large (and possibly bimodal)
distribution in the abundances of all elements produced mainly via the
s-process in solar system material. Following Roederer et al. (2011), we define
two groups of stars, "r+s" and "r-only", and subtract the average abundances of
the latter from the former group to obtain a "s-process residual". This
s-process residual is remarkably similar to that found in M22 and in M4 despite
the range in metallicity covered by these three systems. With recent studies
identifying a double subgiant branch in M2 and a dispersion in Sr and Ba
abundances, our spectroscopic analysis confirms that this globular cluster has
experienced a complex formation history with similarities to M22, NGC 1851 and
omega Centauri.
