Plausible models of the early history of Titan suggest that ammonia and water were present in liquid form at the surface. We show here by thermodynamic modeling that such an ocean could have reacted with silicates to put substantial quantities of sodium and potassium into solution. Following the formation of an ice crust by cooling, mantle ammonia-water fluids enriched in potassium would have been brought to the surface through the cryogenic equivalent of volcanism. Later impacts would have released the 40Ar produced by decay of the 40K into the atmosphere. The abundance of atmospheric 40Ar, measurable by the Huygens probe gas chromatograph mass spectrometer, may be dominated by this source and hence gives a proxy indication of the volume of ammonia-water resurfacing on Titan over geologic time. ¿ American Geophysical Union 1994