Partitioning and speciation of solid phase iron in saltmarsh sediments

A calibrated chemical extraction scheme was developed for partitioning reactive Fe(III) minerals in the solid phase of marine sediments. The following chemical extractants were used: ascorbate (pH 8.0), oxalate (pH 2.5), dithionite (pH 4.8), and HC1 (0.5 M). The dissolution of goethite was catalyzed by Fe(II) bound to oxalate, and its dissolution is compared to that of other crystalline Fe(III) oxides (hematite), a crystalline iron mineral of mixed oxidation state (magnetite), and an Fe-containing silicate (chlorite). Ascorbate was shown to leach only amorphous iron whereas the other extradants dissolved both amorphous and crystalline iron. Dithionite attacked all crystalline Fe(III) minerals including magnetite. Amorphous Fe(III) minerals were shown to comprise a large fraction (>45%) of the total iron in surficial saltmarsh sediments during the growth season. Crystalline Fe(III) oxides were also abundant (20–33% of total Fe) to 20 cm depth in the same saltmarsh cores. Iron sulfides, pyrite and acid volatile sulfide (AVS), were measured along with oxide extractions to completely partition oxidized and reduced fractions of solid iron in saltmarsh sediment. Application of this extraction scheme to saltmarsh sediments questions the use of oxalate in defining amorphous Fe(III) oxides and suggests that crystalline Fe(III) minerals may make up a larger fraction of marine sediments than previously shown. These conclusions have bearing on all marine sediments in which solid phase iron has been studied using the common extradants listed above. Our calibrated chemical extraction scheme together with the documented reactivity of crystalline iron minerals and oxalate suggests that a broadening of the definition of reactive iron in marine sediments may be necessary.