Mineral carbon pump in the Earth system

The balance between the degradation and preservation of organic carbon (OC) is vital for the modulation of atmospheric CO2 and O2 in the Earth system, which regulates short-term climate as well as oxygenation of the early Earth. The mineral carbon pump (MnCP) was recently proposed to describe how soil minerals enhance the persistence and accumulation of OC, where interactions with minerals stabilize labile OC against microbial degradation (including via sorption, occlusion, aggregation, geopolymerization, and redox reactions).1 Given the widespread occurrence of metal (oxyhydr)oxides and clay minerals in terrestrial and marine environments and building on recent progress in mineral-OC interactions, we suggest that the MnCP occurs across the Earth system, where it plays a key role in OC preservation and hence the global carbon and oxygen cycles.

Contact between minerals and OC is inevitable in many terrestrial and marine environments, and interfacial reactions can happen spontaneously and commonly between minerals with charged hydroxyl groups, and/or permanent charge, and functional groups of OC (notably carboxyl, phenol, and amine). Metal (oxyhydr)oxides, particularly iron (oxyhydr)oxides, are typically an order of magnitude less abundant than clay minerals but are often disproportionately important for OC preservation because they have higher surface area and extremely reactive surfaces that facilitate dynamic interfacial reactions. Up to 80% of all OC across global terrestrial and marine depositional environments is preserved in association with reactive iron. In marine sediments, recent work finds that reactive iron and manganese minerals can geopolymerize labile OC molecules into more complex and recalcitrant macromolecular OC forms under ambient marine sediment conditions to bury around 4.1 Mt C year−1.2 Recent work also shows that in the ocean interior, the key role of the coupling between colloidal iron minerals and OC molecules is controlling iron distributions in seawater.3 This colloidal shunt mechanism highlights an important link between iron minerals and OC, like their coupling in marine sediments above. Thus, although oceanic primary production (PP) is often limited by iron, the MnCP can decouple PP from OC burial by enhancing preservation without requiring a concomitant increase in PP.