Organic carbon was budgeted for an experimental marine ecosystem which received 21.2 mol C m–2 of allochthonous sewage sludge plus 12.4 mol C m–2 in situ net daytime production over a 99 day experiment. The fate of carbon, in order of importance, was remineralization, storage in the sediments and export. Sediment carbon metabolism was dominated by sulfate reduction which resulted in the dissociation of organic carbon remineralization from oxygen consumption and energy cycling. The sediments were inefficient in processing sedimented carbon and its associated chemical energy. About 70% of the energy reaching the sediments as organic carbon remained as accumulated carbon and sulfide minerals at the end of the experiment (71% of remaining energy was in the form of unrespired C and with the remaining 29% of energy stored as precipitated sulfides). Sediment oxygen consumption was a poor estimator of benthic metabolism. Total CO2 flux from the sediments was, however, balanced by the sum of sediment oxygen consumption plus oxygen equivalents stored as sedimentary sulfides. Sludge additions drove the experimental ecosystem to a eutrophic state with periods of severe oxygen depletion, death of macrofauna, hydrogen sulfide concentrations in excess of 1 mmolar in surface sediments, and the presence of a white filamentous bacterial mat over the sediment surface.