Early diagenetic precipitation of authigenic carbonate has been a globally significant carbon sink throughout Earth history. In particular, SO42− and Fe3+ reduction and CH4 production create conditions in pore fluids that promote carbonate mineral precipitation; however, these conditions may be modified by the presence of acid-base buffers such as clay minerals. We integrated the acid-base properties of clay minerals into a biogeochemical model that predicts the evolution of pore-water pH and carbonate mineral saturation during O2, Fe3+, and SO42− reduction and CH4 production. Key model inputs were obtained using two natural clay mineral–rich sediments from the Integrated Ocean Drilling Program as well as from literature. We found that clay minerals can enhance carbonate mineral saturation during O2 and SO42− reduction and moderate saturation during Fe3+ reduction and CH4 production if the pore-fluid pH and clay mineral pKa values are within ~2 log units of one another. We therefore suggest that clay minerals could significantly modify the environmental conditions and settings in which early diagenetic carbonate precipitation occurs. In Phanerozoic marine sediments—where O2 and SO42− have been the main oxidants of marine sedimentary organic carbon—clay minerals have likely inhibited carbonate dissolution and promoted precipitation of authigenic carbonate.