Speciation and spatial distribution of solid-phase iron in surface sediments of the East China Sea continental shelf

Speciation and reactivity characterization of solid-phase Fe in marine sediments are of significance to understanding its heterogeneous mineralogy and crystallinity, the diagenetic cycling of Fe and its regulating roles on many other elements in sediments. In this study, a combination of sequential and single-step extractions was used for the determination of seven Fe pools in surface sediments of the East China Sea (ECS) continental shelf: (1) carbonate associated Fe (Fe(II)carb) plus acid volatile sulfide-Fe (Fe(II)AVS), (2) easily reducible amorphous/poorly crystalline Fe oxides (Feox1), (3) reducible crystalline Fe oxides (Feox2), (4) magnetite (Femag), (5) poorly reactive sheet silicate Fe (FePRS), (6) pyrite-Fe (Fepy), and (7) unreactive silicate Fe (FeU). Total Fe (FeT) in the sediments is largely determined by terrestrial aluminosilicate particles as indicated by a great similarity of the FeT with that of the Yangtze River and global riverine particulates. The size of FePRS is found to be the largest pool, followed by FeU, Feox2, Femag, Fe(II)AVS+carb, Feox1 and Fepy. The large FePRS may result from neoformation of Fe-rich clay minerals via reverse weathering and subsequent ageing. The small sizes of Fe(II)AVS+carb and Fepy pools is believed to be the result of low SO4 reduction due to generally low labile organic matter together with the oxic/suboxic, dynamic environments of the surface sediments. The occurrence of Feox1, Feox2 and FePRS in the sediments is closely associated with the clay fraction as indicated by a high spatial correlation between the former and the latter. Highly reactive Fe(FeHR) in the sediments is comparable to that in global marine sediments, but apparently lower than in the Yangtze River and global riverine particulates due probably to sequestration in the Yangtze Estuary. The ratios of FeHR/FeT, FePR/FeT and FeU/FeT in the ECS surface sediments consistently show more similarity to those in the Yangtze River particulates than in the global continental margin or deep-sea sediments. The surface sediments maintain a high level of buffering capacity toward sulfidation suggested by a large fraction of highly reactive Fe(III) oxides (Fe(III)HR) in FeHR.