<b>Cycling and persistence of iron-bound organic carbon in subseafloor sediments</b>

Reactive iron (Fe_R) serves as an important sink of organic carbon (OC) in marine surface sediments, which preserves approximately 20% of total OC (TOC) as reactive iron-bound OC (Fe_R-OC). However, the fate of Fe_R-OC in subseafloor sediments and its availability to microorganisms, remain undetermined. Here, we reconstructed continuous Fe_R-OC records in two sediment cores of the northern South China Sea encompassing the suboxic to methanic biogeochemical zones and reaching a maximum age of ~100 kyr. The downcore Fe_R-OC contributes a relatively stable proportion of 13.3±3.2% to TOC. However, distinctly lower values of less than 5% of TOC, accompanied by notable ¹³C depletion of Fe_R-OC, are observed in the sulfate-methane transition zone (SMTZ). Fe_R-OC is suggested to be remobilized by microbially mediated reductive dissolution of Fe_R and subsequently remineralized, the flux of which is 18-30% of the methane consumption in the SMTZ. The global reservoir of Fe_R-OC in microbially active Quaternary marine sediments could be 19-46 times the size of the atmospheric carbon pool. Thus, the Fe_R-OC pool may support subseafloor microorganisms and contribute to regulating Earth’s carbon cycle.