Stability of Reactive Iron-Bound Organic Carbon During Sulfidization of Iron Oxides: Insights From Methane-Seep Sediments

The association of organic carbon (OC) to reactive iron oxides (Fe_R), forming OC-Fe_R complexes, represents a significant OC sink in marine sediments. However, the impact of diagenetic processes, such as sulfate reduction and iron sulfide formation, on the stability of OC-Fe_R in marine sediments remains poorly understood. Here, we compare sulfidic sediments from three cores taken at methane seeps with a non-sulfidic sediment record from a nearby site. Our results show that an overall 6.3% decrease in OC-Fe_R is associated with a 42% reduction in Fe_R during the transformation from iron oxides to iron sulfides, suggesting that OC-Fe_R is resistant to sulfidization. We observed highly ¹³C-depleted OC-Fe_R in the sulfidic sediments, likely due to the interaction between OC and Fe_R during anaerobic oxidation of methane. Our findings highlight the stability of OC-Fe_R in natural sulfidic sediments, offering new insights into the role of OC-Fe_R in continental margin sediments.