Fe(III) and Fe(II) organic complexes were determined by spectroscopic
methods after sephadex gel fractionation of salt marsh porewaters duri
ng June and July 1993. Fe(III), which is a significant anaerobic oxida
nt for sulfide mineral oxidation, was typically found in the 100-5000
molecular weight (MW) fraction indicative of humic and other organic c
omplexing agents. Fe(II) was found in both the < 100 and the 100-5000
MW fractions with most of the Fe(II) found in the smaller MW class. Bo
th forms of Fe precipitated with humic material when the pH of the por
ewaters became less than 3. There was a twofold decrease in the < 5000
MW fractions, a ninefold increase in the > 5000 MW class, and a tenfo
ld decrease in dissolved Fe concentration in sephadex gel fractions as
the pH decreased from 5.0 to < 3 for porewater samples. Low pH values
are attributed to sulfide mineral oxidation from severe drought condi
tions that caused significant dessication of the vegetated marsh. Micr
oelectrode measurements demonstrated that O-2 was not detected below 2
mm and that Fe(III) organic complexes should be significant oxidants
in anoxic waters.