Understanding the environmental fate of organic carbon associated with iron (OC-Fe) is critically important for investigating OC preservation in aquatic systems. Here, we first investigate ¹³C and ¹⁴C isotopes of OC-Fe within grain size-fractionated sediments retrieved from the East China Sea and estimate their sources and reactivities of OC-Fe through isotope-mixing models and thermal pyrolysis approaches in order to reveal the fate of OC-Fe on continental shelves influenced by hydrodynamic processes. Our results show that the OC-Fe proportion in total OC ( f _OC‑Fe)in the sortable silt fractions (20−63 μm) is the highest among three grain size fractions, likely suggesting that hydrodynamics may enhance the iron protection on OC. In addition, Δ¹⁴C_OC‑Fe values fall within the range of from −358.73 to −64.03 ‰, and both Δ¹⁴C_OC‑Fe values and ancient OC-Fe% exhibit strong positive linear relationships with f _OC‑Fe. This emphasized that the hydrodynamic processes may cause the ancient OC to be tightly associated with Fe, accompanying OC-Fe aging. Our findings shed new light on the preservation of OC-Fe in marginal seas to advance the recognition of carbon “rusty sinks” in seafloor sediments.

organic carbon, marine sediments, iron, carbon cycle, hydrodynamic processes