It is likely that early in Mars' history, abundant liquid water was available. Under a thick (several bars) carbon dioxide atmosphere, this water could have formed an ocean, located primarily in the lowlands of the northern hemisphere. An equilibrium geochemical model of this ocean and its interactions with the atmosphere and regolith of Mars was developed, and the results of this model were used to discuss the evolution of the volatile budget of Mars, including the deposition of large carbonate beds on the northern plains. Differential solutional weathering of these carbonate beds may have caused the formation of some of the enigmatic features seen on the northern plains of Mars, such as the thumbprint terrain and enclosed depressions. ¿ American Geophysical Union 1990