We present the results of an observational study of the efficiency of deep mixing in globular cluster red giants as a function of stellar metallicity .
We determine [ C/Fe ] abundances based on low-resolution spectra taken with the Kast spectrograph on the 3m Shane telescope at Lick Observatory .
Spectra centered on the 4300 \mbox { \AA } CH absorption band were taken for 42 bright red giants in 11 Galactic globular clusters ranging in metallicity from M92 ( [ Fe/H ] = -2.29 ) to NGC 6712 ( [ Fe/H ] = -1.01 ) .
Carbon abundances were derived by comparing values of the CH bandstrength index S _ { 2 } ( CH ) measured from the data with values measured from a large grid of SSG synthetic spectra .
Present-day abundances are combined with theoretical calculations of the time since the onset of mixing , which is also a function of stellar metallicity , to calculate the carbon depletion rate across our metallicity range .
We find that the carbon depletion rate is twice as high at a metallicity of [ Fe/H ] = -2.3 than at [ Fe/H ] = -1.3 , which is a result qualitatively predicted by some theoretical explanations of the deep mixing process .