New data on the carbon isotopic composition of dissolved inorganic carbon (DIC) in deep sea pore fluids suggests that a community of chemoautotrophic bacteria exists in carbonate sediments hundreds of meters below the sea floor. Pore fluid samples, poisoned with mercuric chloride, were collected from five sites in water depths ranging from 2500 to 4300 meters on the Ceara Rise during Ocean Drilling Program Leg 154. Carbon isotope data from all sites show the same basic features. Values decrease below the sea floor over the first 10 to 30 meters, reaching 4 to 9 per mil, and then begin to rise over the next several hundred meters, reaching values as high as +5 to +10 at depths from 250 to 550 meters. This pattern is similar to data measured at sites with high organic carbon content, anoxic pore fluids, and ongoing methanogenesis. However, the Ceara Rise sediments are carbonate rich, have little organic matter, and have substantial sulfate (>10 mM) even at the base of the hole. The low carbon isotope values at the top of the hole are consistent with oxidation of organic matter by sulfate-reducing bacteria, but the high values at depth can only be explained by a kinetic isotope effect associated with carbon fixation. Because sulfate concentrations continue to decrease to the base of the hole, bacteria could be using sulfate to oxidize dissolved H2 released from the basaltic crust. Independent estimates of the rate of carbonate recrystallization can be combined with the carbon isotope data to calculate net carbon uptake rates. Future efforts should attempt to measure H2 concentrations in pore fluids, and identify genetic evidence for chemoautotrophy. Carbon isotope data from additional carbonate-rich sites would determine whether such chemoautotrophy is a general feature of the deep sea sedimentary environment.