Distributions of Ferrous Iron and Sulfide in an Anoxic Hypolimnion

In the anoxic hypolimnion of Lake 227, Experimental Lakes Area, northwestern Ontario, ΣH 2 S exhibits a mid-depth maximum, while Fe 2+ increases with depth. At the mid-depth ΣH 2 S maximum and below, saturation with respect to amorphous FeS is reached, and the concentration of ΣH 2 S is limited by the high Fe 2+ concentrations, in accord with the FeS solubility product. Values for pK sp [Formula: see text] for FeS determined from the ΣH 2 S maximum and below averaged 3.16 in 1979 and agree well with other in situ and laboratory measurements. In the top 10 cm of sediment, pore water ΣH 2 S and Fe 2+ are in equilibrium with amorphous FeS. Analyses of cores confirms the existence of an iron sulfide phase. Fe 2+ , which is produced in the pore water from the decomposition of organic matter, increases to concentrations at which siderite may form, although the presence of siderite has not been verified. Comparison of calculated pore water fluxes of Fe 2+ with the observed increase of Fe 2+ in the anoxic hypolimnion reveals that about 90% of the observed flux originates at the sediment–water interface, while the remainder is derived from pore water flux.