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PurposeDespite occupying only 0.5% of the global coastal ocean, mangroves play a disproportionately large role in the capture and retention of atmospheric carbon dioxide as organic carbon (OC) in its sediments. However, the capacity of mangrove sediments to store high amounts of OC has never been explained mechanistically. This study elucidates the role of reactive iron (Fe-R) in long-term carbon sequestration in mangrove sediments.Materials and methodsSediment samples of up to 1m in depth were collected from six selected mangrove areas across the Philippines. The samples were characterized using X-ray diffraction (XRD) and X-ray fluorescence (XRF). A citrate-bicarbonate-dithionite reduction procedure was employed to extract Fe-R from the sediments. The OC concentration and C-13 signatures before and after Fe-R extraction were determined using a dichromate oxidation technique and isotope ratio mass spectrometry (IRMS), respectively.Results and discussionXRD diffractograms showed that the mangrove sediment samples varied in terms of mineralogical characteristics, which reflected their different parent materials. It was found that the OC concentration increased exponentially (OC=8.38e(0.37FeR); R-2=0.88; p<0.0001) with increasing Fe-R concentration. C-13 signatures of Fe-R-associated OC revealed that Fe-R preferentially preserved terrestrial over marine-derived organic matter. Finally, Fe-R was estimated to coprecipitate with up to 5.44x10(12)g OC in mangrove sediments per year.ConclusionsThe findings of this study suggest that Fe-R is responsible for the preservation of OC, thus making mangrove sediments a giant rusty sponge for carbon. This mechanistic understanding of thelong-term carbon storage in mangrove sediments could help draft better strategies for blue carbon initiatives that include the mangrove ecosystems.