Preservation of organic carbon in marine sediments sustained by sorption and transformation processes

Controls on organic carbon preservation in marine sediments remain controversial but crucial for understanding past and future climate dynamics. Here we develop a conceptual-mathematical model to determine the key processes for the preservation of organic carbon. The model considers the major processes involved in the breakdown of organic carbon, including dissolved organic carbon hydrolysis, mixing, remineralization, mineral sorption and molecular transformation. This allows redefining of burial efficiency as preservation efficiency, which considers both particulate organic carbon and mineral-phase organic carbon. We show that preservation efficiency is almost three times higher than the conventionally defined burial efficiency and reconciles predictions with global field data. Kinetic sorption and transformation are the dominant controls on organic carbon preservation. We conclude that a synergistic effect between kinetic sorption and molecular transformation (geopolymerization) creates a mineral shuttle in which mineral-phase organic carbon is protected from remineralization in the surface sediment and released at depth. The results explain why transformed organic carbon persists over long timescales and increases with depth. Kinetic sorption and transformation are primary controls on organic carbon preservation in marine sediments, according to reactive transport model simulations of the cycling and breakdown of particulate and mineral-phase organic carbon.