The preservation of organic carbon in marine sediments is fundamentally important for Earth’s carbon and oxygen cycles, but the controls on carbon preservation remain unclear. Preservation can be enhanced by limiting exposure of carbon to oxygen, but on continental margins, where the majority of carbon preservation occurs, the relationship between oxygen exposure and burial efficiency is weak. In these environments in particular, additional preservation mechanisms are proposed, including the protection and preservation of carbon with sediment minerals. We show that the sorption of carboxyl-rich carbon [1] and its chemical transformation [2] with reactive forms of iron and manganese provides a hitherto unrecognised mechanism for the stabilisation and preservation of carbon in sediments, such that the flux of iron and manganese into the oceans may provide a new control on planetary climate and oxygenation [2,3].