Abstract An open-system diagenetic (transport) model is presented which accounts for the concurrent behavior of all the dissolved carbonate species ( i.e. CO 2(aq), HCO 3 −, and CO 3 =) as well as hydrogen and hydroxyl ions in the porewaters of marine sediments during the oxic and suboxic decay of organic-matter. The model includes interconversion between the dissolved carbonate species due to association/dissociation reactions as well as production by organic decay and CaCO 3 dissolution. The existence of rapid association/dissociation reactions has important consequences. First, the transport of a dissolved carbonate species is facilitated, because it can react and diffuse as another carbonate species. This action modifies the concentration profiles which would be expected without interconversion. As a consequence, the rate of CaCO 3 dissolution is increased because it is more difficult for CO 3 = to reach and maintain the saturation concentration. Finally, CO 2(aq) and HCO 3 − produced by decay affect the concentration of CO 3 = and, therefore, the saturation state of porewaters with respect to carbonate minerals. The model is applied to the carbonate alkalinity and pH data from the Guatemala Basin and MANOP Site C. The model reproduces the sharp near-surface minimum in pH, observed at the Guatemala Basin sites; however, the carbonate alkalinity increase is underpredicted. This model result implies that there is an additional source of HCO 3 − that is not presently recognized, probably in the form of sulfate reduction at depth. The MANOP Site C data are well explained by using organic-matter oxidation rates in accord with the minimum O 2 fluxes suggested by Reimers et al. (1984).