Dissolved inorganic carbon (DIC), gaseous CO2 and alkalinity fluxes from intertidal sediments were investigated during periods of exposure and inundation, using laboratory core incubations, previously published field data and reactive transport model simulations. In the incubations and previous field data, it was found that during periods of alkalinity production (attributed to the accumulation of reduced sulfur species within the sediment), the flux of DIC out of the sediment was greater during inundation than the gaseous CO2 flux during exposure by a factor of up to 1.8. This finding was supported by computational simulations which indicated that large amounts of sulfate reduction and reduced sulfur burial (FeS) induce an alkalinity flux from the sediment during high tide conditions. Model simulations also found that the amount of reactive Fe in the sediment was a major driver of net alkalinity production. Our finding that CO2 fluxes can be significantly lower than total metabolism during exposure has implications for how total metabolism is quantified on tidal flats.