Clay Minerals Modulate Early Carbonate Diagenesis

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 O-2, 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 O-2 and SO42- reduction and moderate saturation during Fe3+ reduction and CH4 production if the pore-fluid pH and clay mineral pK, values are within similar to 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 O-2 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.