Characterizing iron-bound organic carbon in cold seep sediment cores: impacts of anaerobic oxidation of methane

Abstract Reactive iron oxides (Fe R ), often termed the “rusty sink” of organic carbon (OC), has been well studied in estuarine settings but remains poorly constrained in deep-sea sediments. Here, we investigate reactive iron-bound OC (Fe R -OC) dynamics in cold seep sediments from the Qiongdongnan Basin, northern South China Sea, with emphasis on the role of anaerobic oxidation of methane (AOM). The relative contribution of Fe R -OC to total OC ( f FeR-OC ) ranged from 5.2 to 40.0% (mean: 15.7% ± 8.1%) and decreased from reference sites to methane-rich sediments, primarily due to iron oxide sulfidization and subsequent pyrite formation. Methane-derived OC accounted for 0–11.1% of TOC (2.2 ± 2.6%) and 0–12.7% of Fe R -OC (3.8 ± 4.1%), with higher values in sulfate-methane transition zones (SMTZs) than in non-SMTZ layers. These results indicate that sulfate-dependent AOM is a major source of methane-derived OC, which associates with Fe R through adsorption and co-precipitation, partially offsetting the decline in f FeR-OC . The greater contribution of methane-derived OC to Fe R -OC than to bulk TOC, together with more depleted δ 13 C FeR-OC relative to δ 13 C TOC , supports the preferential binding of methane-derived OC by Fe R . Importantly, Fe R and Fe R -OC in methane-rich sediments were not primarily controlled by bulk Fe content but rather by TOC source and the Fe R -OC/Fe R values. Collectively, our findings reveal dual processes of Fe R -OC preservation—loss through sulfidization and retention via methane-derived OC—and underscore the significance of AOM-driven iron–carbon coupling for carbon sequestration in deep-sea environments.