The Role of Iron Minerals in the Preservation of Organic Carbon in Mangrove Sediments

Although mangrove forests occupy only 0.5% of the global coastal area, they account for 10-15% of coastal organic carbon (OC) storage, and 49-98% of OC is stored in sediments. The biogeochemistry of iron minerals and OC in marine sediments is closely related. To better reveal the role of iron minerals in OC preservation in mangrove sediments, an established dithionite-citrate-bicarbonate (DCB) extraction method was used to extract iron-bound OC (Fe-OC), and then the parameters of OC, Fe-OC, iron content, carbon isotopes, infrared spectroscopy, and XRD diffractions of sediments at a 1 m depth in four typical mangrove communities in the Gaoqiao Mangrove Reserve, Guangdong, China, were systematically measured. XRD diffractograms showed that the iron minerals in mangrove sediments may mainly exist in the form of goethite, which is consistent with the predominant types of iron minerals in marine sediments. About 10% of OC is directly bound to iron, and it is further estimated that about 2.4 x 1012-3.8 x 1012 g OC is preserved in global mangrove forests each year based on the high burial rate of OC in mangrove sediments. Lower Fe-OC/OC molar ratios indicated that iron mainly binds to OC via adsorption mechanisms. More depleted delta 13CFe-OC relative to delta 13Cbulk indicated that iron minerals are mainly associated with terrigenous OM, and the infrared spectra also revealed that iron minerals preferentially bind to terrigenous aromatic carbon. This work supports the "giant rusty sponge" view, elucidating that iron plays an important role in the preservation of OC in mangrove sediments.