Iron oxides are important terminal electron acceptors for organic carbon (OC) remineralization in subterranean estuaries, particularly where oxygen and nitrate concentrations are low. In Indian River Lagoon, Florida, USA, terrestrial Fe-oxides dissolve at the seaward edge of the seepage face and flow upward into overlying marine sediments where they precipitate as Fe-sulfides. The dissolved Fe concentrations vary by over three orders of magnitude, but Fe-oxide dissolution rates are similar across the 25-m wide seepage face, averaging around 0.21 mg/cm²/yr. The constant dissolution rate, but differing concentrations, indicate Fe dissolution is controlled by a combination of increasing lability of dissolved organic carbon (DOC) and slower porewater flow velocities with distance offshore. In contrast, the average rate constants of Fe-sulfide precipitation decrease from 21.9 × 10^− 8 s^− 1 to 0.64 × 10^− 8 s^− 1 from the shoreline to the seaward edge of the seepage face as more oxygenated surface water circulates through the sediment. The amount of OC remineralized by Fe-oxides varies little across the seepage face, averaging 5.34 × 10^− 2 mg/cm²/yr. These rates suggest about 3.4 kg of marine DOC was remineralized in a 1-m wide, shore-perpendicular strip of the seepage face as the terrestrial sediments were transgressed over the past 280 years. During this time, about 10 times more marine solid organic carbon (SOC) accumulated in marine sediments than were removed from the underlying terrestrial sediments. Indian River Lagoon thus appears to be a net sink for marine OC.