Enhanced preservation of dark organic carbon by reactive iron in cold-seep sediments

Reactive iron (Fe_R) significantly influences sedimentary organic carbon (OC) preservation in marine systems, yet its role in cold seeps remains unclear. This study examines OC-Fe_R associations and microbial communities at the Formosa Ridge cold seep (South China Sea). Cold-seep sediments exhibit higher OC content and lower δ¹³C values than non-seep sediments, suggesting dark OC formation, which is primarily derived from methane and consists of microbial biomass and its byproducts. The dark OC is formed through the assimilation of methane-derived carbon by sulfur-oxidizing bacteria (SOB), sulfate-reducing bacteria (SRB), and anaerobic methanotrophic archaea (ANME). Above the sulfate-methane transition zone (SMTZ), the fraction of OC-Fe_R in bulk OC (f_OC-FeR) remains comparable to non-seep sediments, despite a decrease in the availability of Fe_R due to iron sulfides formation. Within the SMTZ, Fe reduction driven by anaerobic oxidation of methane leads to OC-Fe_R loss. Below the SMTZ, high f_OC-FeR and unchanged Fe_R demonstrate the role of Fe_R in long-term OC sequestration. A mixing model indicates significantly higher proportion of dark OC associated with Fe_R relative to bulk OC, especially exceeding 70 % within the SMTZ, suggesting either preferential association of dark OC with Fe_R or in-situ OC-Fe_R formation. This indicates that Fe_R effectively preserves dark OC even under strongly reducing conditions. Compared to other marine environments, cold seeps show higher OC loadings but lower f_OC-FeR due to anaerobic microbial activity. We demonstrate that Fe_R plays a critical role in preserving dark OC across redox gradients, supporting long-term OC burial in reducing environments.