  It is now widely accepted that globular cluster red giant branch stars owe
their strange abundance patterns to a combination of pollution from progenitor
stars and in situ extra mixing. In this hybrid theory a first generation of
stars imprint abundance patterns into the gas from which a second generation
forms. The hybrid theory suggests that extra mixing is operating in both
populations and we use the variation of [C/Fe] with luminosity to examine how
efficient this mixing is. We investigate the observed red giant branches of M3,
M13, M92, M15 and NGC 5466 as a means to test a theory of thermohaline mixing.
The second parameter pair M3 and M13 are of intermediate metallicity and our
models are able to account for the evolution of carbon along the RGB in both
clusters. Although, in order to fit the most carbon-depleted main-sequence
stars in M13 we require a model whose initial [C/Fe] abundance leads to a
carbon abundance lower than is observed. Furthermore our results suggest that
stars in M13 formed with some primary nitrogen (higher C+N+O than stars in M3).
In the metal-poor regime only NGC 5466 can be tentatively explained by
thermohaline mixing operating in multiple populations. We find thermohaline
mixing unable to model the depletion of [C/Fe] with magnitude in M92 and M15.
It appears as if extra mixing is occurring before the luminosity function bump
in these clusters. To reconcile the data with the models would require first
dredge-up to be deeper than found in extant models.
