Inhibition of carbonate synthesis in acidic oceans on early Mars

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Several lines of evidence have recently reinforced the hypothesis that an ocean existed on early Mars1,2,3,4,5,6,7. Carbonates are accordingly expected to have formed from oceanic sedimentation of carbon dioxide from the ancient martian atmosphere7,8. 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.

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Figure 1: Stability boundaries of siderite and minnesotaite (under reducing conditions) and acidic solutions and haematite (under oxidizing conditions) on early Mars.
Figure 2: Evolution of the acidic martian environment over time.
Figure 3: Schematic representation of atmosphere–land–ocean interactions generating acidic environments in early Mars.


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Special acknowledgements to the MER team, as their compelling evidence probing the acidity of martian palaeoenvironments was unfolded while our work was in progress, and resulted in adjustments in our model following our initial submitted draft. We also thank S. Clifford, I. Fairchild and J. Kasting for comments and suggestions that refined and focused this paper.

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Correspondence to Alberto G. Fairén.

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