SEASONAL CYCLING OF FE IN SALT-MARSH SEDIMENTS

This study combines an analysis of porewater chemistry with new, solid phase wet chemical extractions to examine the seasonal cycling of Fe in vegetated and unvegetated (cyanobacterial mat) saltmarsh sediments. Saltmarsh sediments are shown to contain more solid phase reactive Fe than other marine sediments studied so far. From the partitioning and speciation of solid Fe, and solid/soluble reduced S analysis in 10 se diment cores, we have observed that a majority of solid Fe in these se diments is cycled rapidly and completely between oxidized reactive Fe and reduced Fe as pyrite. Vegetated porewaters showed a lower pH and m uch higher Fe(II) concentrations on average than unvegetated porewater s in the top 10 cm, whereas sulfate, alkalinity, and sulfide concentra tions were similar in the two environments. The amorphous Fe(III) oxid e fraction showed a high negative correlation to solid and soluble red uced S (r(2) = -0.86 and -0.71, respectively) in surface vegetated sed iments whereas the crystalline Fe(III) oxide fraction showed a high ne gative correlation (r(2) = -0.96) to sulfide only at depth. Though rea ctive Fe was observed in unvegetated sediments, no seasonal trend was apparent and the speciation of solid Fe revealed that most of it was r educed. Solid phase and porewater chemistry support the dominant role of the biota (Spartina alterniflora and bacteria) in controlling the r eactivity of Fe and suggest that the current definition of solid phase , reactive Fe should be expanded to include crystalline Fe(III) minera ls which are available for pyrite formation in saltmarsh sediments. In support of previous saltmarsh studies, we present evidence that the r edox cycle of solid Fe is controlled by sulfate reduction and sediment oxidation which respond to both annual cycles (light, temperature) an d to short-term, episodic effects such as weather and tides.