Inhibition of carbonate synthesis in acidic oceans on early Mars

Several lines of evidence have recently reinforced the hypothesis that an ocean existed on early Mars^1-7. Carbonates are accordingly expected to have formed from oceanic sedimentation of carbon dioxide from the ancient martian atmosphere^7,8. But spectral imaging of the martian surface has revealed the presence of only a small amount of carbonate, widely distributed in the martian dust⁹. Here we examine the feasibility of carbonate synthesis in ancient martian oceans using aqueous equilibrium calculations. We show that partial pressures of atmospheric carbon dioxide in the range 0.8-4 bar, in the presence of up to 13.5 mM sulphate and 0.8 mM iron in sea water⁸, result in an acidic oceanic environment with a pH of less than 6.2. This precludes the formation of siderite, usually expected to be the first major carbonate mineral to precipitate⁸. We conclude that extensive interaction between an atmosphere dominated by carbon dioxide and a lasting sulphate- and iron-enriched acidic ocean on early Mars is a plausible explanation for the observed absence of carbonates.