Geopolymerization threatens the persistence of organic carbon associated with iron in anoxic environments.

The sequestration of organic carbon (OC) through mineral association in soils and sediments is a crucial process that regulates carbon sink dynamics and the global carbon cycle. However, minerals can participate in both abiotic and biotic OC transformations, altering the persistence of mineral-associated OC under anoxic conditions. In this work, we report that synergistic interactions among metal (oxyhydr)oxides, such as iron (Fe), manganese (Mn), and aluminum (Al) drive the polymerization of simple organic molecules into macromolecular geopolymers, increasing their electron transfer capacity by 52-115%. These geopolymers function as electron shuttles, enhancing OC decomposition through microbial dissimilatory iron reduction. This reduces the mean retention time (MRT) of OC bound to active and inert Fe minerals by 51.4 ± 15.6% and 74.1 ± 13.7%, respectively. Future carbon turnover models should explicitly account for the mineral composition, redox fluctuations, and microbial metabolic pathways to advance the understanding of the Earth's carbon sink.