Air-sea exchange represents an important controlling factor for the distribution of carbon isotopes in the modern ocean, even below the surface mixed layer. Here we show that this thermodynamic influence varies between water masses with a magnitude which is large relative to the amplitude of typical deep sea sediment Δ13C records. Thus, changes in air-sea exchange processes must be regarded as a significant source of variability for any marine Δ13C signal. ¿ American Geophysical Union 1993