PARTITIONING AND SPECIATION OF SOLID-PHASE IRON IN SALT-MARSH 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 HCl (0.5 M). The dissolution of go ethite was catalyzed by Fe(II) bound to oxalate, and its dissolution i s compared to that of other crystalline Fe(III) oxides (hematite), a c rystalline iron mineral of mixed oxidation state (magnetite), and an F e-containing silicate (chlorite). Ascorbate was shown to leach only am orphous iron whereas the other extractants dissolved both amorphous an d crystalline iron. Dithionite attacked all crystalline Fe(III) minera ls including magnetite. Amorphous Fe(III) minerals were shown to compr ise a large fraction (> 45%) of the total iron in surficial saltmarsh sediments during the growth season. Crystalline Fe(III) oxides were al so abundant (20-33% of total Fe) to 20 cm depth in the same saltmarsh cores. Iron sulfides, pyrite and acid volatile sulfide (AVS), were mea sured along with oxide extractions to completely partition oxidized an d reduced fractions of solid iron in saltmarsh sediment. Application o f this extraction scheme to saltmarsh sediments questions the use of o xalate in defining amorphous Fe(III) oxides and suggests that crystall ine Fe(III) minerals may make up a larger fraction of marine sediments than previously shown. These conclusions have bearing on all marine s ediments in which solid phase iron has been studied using the common e xtractants listed above. Our calibrated chemical extraction scheme tog ether 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.