Cycling and persistence of iron-bound organic carbon in subseafloor sediments
Y. Chen, Liang Dong, Weikang Sui, Mingyang Niu, Xingqian Cui, Kai‐Uwe Hinrichs, Fengping Wang
Abstract
Reactive iron (FeR) 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 (FeR-OC). However, the fate of FeR-OC in subseafloor sediments and its availability to microorganisms, remain undetermined. Here, we reconstructed continuous FeR-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 FeR-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 FeR-OC, are observed in the sulfate-methane transition zone (SMTZ). FeR-OC is suggested to be remobilized by microbially mediated reductive dissolution of FeR and subsequently remineralized, the flux of which is 18–30% of the methane consumption in the SMTZ. The global reservoir of FeR-OC in microbially active Quaternary marine sediments could be 19-46 times the size of the atmospheric carbon pool. Thus, the FeR-OC pool may support subseafloor microorganisms and contribute to regulating Earth’s carbon cycle. This study shows that iron-bound organic carbon (FeR-OC) is generally persistent but can be remobilized during iron reduction and utilized by microbes in subseafloor sediments. This sedimentary FeR-OC pool may contribute to regulating Earth’s carbon cycle.