Carbon Burial Capacity of a Terraformed Martian Hydrosphere: Sulfate Buffering and the Cost of the Redox Transition

Terraforming Mars to a breathable state requires reducing atmospheric CO2 below roughly 10 mbar, which requires burying the carbon as carbonate mineral. Two questions bear on whether that is achievable: whether dissolution of the Martian sulfate inventory acidifies a hydrosphere enough to prevent carbonate precipitation, and how much carbonate-forming cation the crust can supply. This analysis addresses both with an aqueous geochemical model built from published thermodynamic data with no fitted parameters, and places the results against recent work that has substantially answered the underlying geochemistry. On the first question, the acid-neutralising capacity of Martian basalt (8.02 eq/kg for a standard average-crust composition) exceeds the maximum stored acidity of the observed sulfate inventory (at most 1.50 eq/kg at 8 wt% SO3, taking all sulfur as Fe(III)-sulfate) by at least a factor of 5.3. Blocking carbonate precipitation would require 42.8 wt% SO3 held entirely as Fe(III)-sulfate, about seven times the observed abundance; even a regolith composed entirely of jarosite supplies only 6.00 eq/kg and titrates net alkaline. Calcite is supersaturated (SI +2.9 to +5.6) throughout the dilute regime. This confirms, by bulk mass balance, a result already supported experimentally by Tosca et al. (2004), Horgan et al. (2017) and Gil-Lozano et al. (2024), and consistent with the low water-to-rock-ratio interpretation of Hurowitz et al. (2006). On the second question, carbonate formation on Mars is cation-limited (Kite et al. 2025) and Martian carbonate is dominantly siderite (Tutolo et al. 2025). The coupling between iron redox state and the carbon cycle is established (Tutolo et al. 2025; Mitra et al. 2025), and the redox capacity of the Martian crust has been identified as a terraforming constraint (Mars Terraforming Workshop 2025; Turyshev 2026). The contribution of this analysis is to account for that same iron from the carbon-burial side and express it as a cost. Iron supplies 42% of the divalent cations available for carbonate formation in Martian basalt. Sequestering 1 bar of CO2 requires chemically weathering approximately 46 m of crust under anoxic conditions, 89 m once oxygenated, and 503 m if magnesite is additionally inhibited at ocean temperatures. Because carbon burial and oxygenation compete for the same iron, burial should precede oxygenation rather than run alongside it. Model code, validation benchmarks and figures are included. Preprint, not peer reviewed.