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Electron flow in an iron‐rich acidic sediment—evidence for an acidity‐driven iron cycle
Author(s) -
Peine Ariane,
Tritschler Andrea,
Küsel Kirsten,
Peiffer Stefan
Publication year - 2000
Publication title -
limnology and oceanography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.7
H-Index - 197
eISSN - 1939-5590
pISSN - 0024-3590
DOI - 10.4319/lo.2000.45.5.1077
Subject(s) - schwertmannite , sulfate , chemistry , alkalinity , goethite , anoxic waters , sulfur , sulfide , environmental chemistry , pyrite , sediment , sulfate reducing bacteria , sedimentation , mineralogy , geology , adsorption , paleontology , organic chemistry
The anoxic sediment of an acidic (pH ∼3) iron‐ and sulfate‐rich lake and its pore water was studied with respect to the turnover rates of solid 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 sediment zone (pH < 4) with schwertmannite (Fe 8 O 8 (OH) x (SO 4 ) y (∼ 350 g kg −1 ). Microbial iron‐reduction rates measured by closed vessel incubation technique were highest close to the sediment‐water interface (250 nmol cm −3 d −1 ), sulfate reduction measured by the 35 S‐tracer technique was not detectable in this zone. The absence of sulfide allowed complete reoxidation of dissolved Fe(II) diffusing into oxic parts of the lake water. Thus, an iron cycle is established where acidity generation through this process (1.0–4.7 mol m −2 a −1 ) balanced the alkalinity gain through microbial iron reduction in this zone (0.65–4.0 mol m −2 a −1 ). Predominance of iron over sulfate reduction under acidic conditions is further stabilized 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‐water pH increased to values between 5 and 6, sulfate reduction occurred with a maximum rate of 14 nmol cm −3 d −1 ), sulfate reduction measured by the 35 S‐tracer technique was at 9 cm depth. Release of Fe(II) and a short turnover time of reduced sulfur relative to the sediment age implies that most of the sulfide formed seemed to be recycled to sulfate at this depth, presumably coupled to the reduction of iron. Consequently, net alkalinity is generated at low rates only (0.12 mol m −2 a −1 ).