The cycling of dissolved inorganic carbon (DIC) and the role of tidal marshes in estuarine DIC dynamics were studied in a Virginia tidal freshwater marsh and adjacent estuary. DIC was measured over diurnal cycles in different seasons in a marsh tidal creek and at the junction of the creek with the adjacent Pamunkey River. In the creek, DIC concentrations around high tide were controlled by the same processes affecting whole‐estuary DIC gradients. Near low tide, DIC concentrations were 1.5–5‐fold enriched relative to high tide concentrations, indicating an input of DIC from the marsh. Similar patterns (although dampened in magnitude) were observed at the creek mouth and indicated that DIC was exported from the marsh. Marsh pore‐water DIC concentrations were up to 5 mmol L⁻¹ greater than those in the creek and suggested a significant input of sediment pore water to the creek. A model of tidal marsh DIC export showed that, on a seasonal basis, DIC export rates were influenced by water temperature. The composition of exported DIC averaged 19% dissolved CO₂ and 81% HCO₃⁻ and CO₃²⁻. Although CO₂ can be lost to the atmosphere during transit through the estuary, DIC in the form of carbonate alkalinity is subject to export from the estuary to the coastal ocean. When extrapolated to an estuarywide scale, the export of marsh‐derived DIC to the York River estuary explained a significant portion (47 ± 23%) of excess DIC production (i.e., DIC in excess of that expected from conservative mixing between seawater and freshwater and equilibrium with the atmosphere) in this system. Therefore, CO₂ supersaturation, by itself, does not indicate that an estuary is net heterotrophic.