Reactive Iron-Bound Organic Carbon Shows Unexpected Persistence under Anoxic Burial

Abstract

Abstract The interaction between organic carbon (OC) and reactive iron (FeR) plays a critical role in suppressing OC mineralization, with the resulting FeR-bound OC (FeR-OC) constituting an important carbon sink in marine sediments. Traditional views suggest that under anoxic conditions, associations between FeR and OC not only enhance the OC accumulation and persistence but also inhibit the reductive dissolution of FeR. However, this framework leaves unresolved uncertainties regarding how FeR and OC mutually promote their costabilization under anoxic conditions. In this study, sequential chemical extraction was employed to distinguish organically complexed iron-bound OC (Fep-OC) from iron-oxide-bound OC (Feox-OC). Our results reveal that Fep represents a previously underrecognized iron-associated OC pool that contributes to the OC preservation in marine sediments. Under anoxic conditions, Fep promotes OC retention and offsets the decline in Feox-OC. Despite its lower abundance, Fep accounts for the stabilization of an average of 20.59% of the sedimentary OC. Fourier transform infrared spectroscopy (FTIR) analyses further indicate that Feox-OC is relatively enriched in aromatic and alcohol/phenol functional groups compared with bulk OC, whereas Fep-OC exhibits no distinct enrichment of specific functional groups. Collectively, this study provides field-based evidence for the redistribution of OC among different FeR-bound fractions and highlights the potentially important contribution of Fep to OC preservation under anoxic conditions.