Electron flow in an iron-rich acidic sediment - evidence for an acidity-driven iron cycle

The anoxic sediment of an acidic (pH similar to 3) iron- and sulfate-rich l ake and its pore water was studied with respect to the turnover rates of so lid and dissolved iron and sulfur species. High sedimentation rates of iron (570 g m(-2) a(-1)) lead to an enrichment of the upper (0-5 cm) acidic sed iment zone (pH < 4) with schwertmannite (Fe8O8)(OH)(x)(SO4)(y) (approximate to 350 g kg(-1)). Microbial iron-reduction rates measured by closed vessel incubation technique were highest close to the sediment-water interface (2 50 nmol cm(-3) d(-1)), sulfate reduction measured by the S-35- tracer techn ique was not detectable in this zone. The absence of sulfide allowed comple te reoxidation of dissolved Fe(II) diffusing into oxic parts of the lake wa ter. Thus, an iron cycle is established where acidity generation through th is process (1.0-4.7 mol m(-2) a(-1) ) balanced the alkalinity gain through microbial iron reduction in this zone (0.654-.0 mol m(-2) a(-1)). Predomina nce of iron over sulfate reduction under acidic conditions is further stabi lized by the transformation of schwertmannite to goethite at a depth of 3-5 cm, which releases acidity at a rate of 3.5 mol m(-2) a(-1). Below, pore-w ater pH increased to values between 5 and 6, sulfate reduction occurred wit h a maximum rate of 14 nmol cm(-3) d(-1) at 9 cm depth. Release of Fe(II) a nd a short turnover time of reduced sulfur relative to the sediment age imp lies that most of the sulfide formed seemed to be recycled to sulfate at th is depth, presumably coupled to the reduction of iron. Consequently, net al kalinity is generated at low rates only (0.12 mol m(-2) a(-1)).