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In simulation experiments the stable isotope, 57Fe, was used as a novel tracer of iron transport to investigate the role of microbial activity on the distribution/redistribution of metals in lake sediments. A series of microcosm sediment columns was set up, containing a layer of 57Fe-labelled ferrihydrite to track iron mobility temporally. Three series of columns were prepared, one set with unamended homogenized lake sediments, one set amended with 57Fe-labelled ferrihydrite in the bottom layer of the column and one abiotic control set amended with 57Fe-labelled ferrihydrite which was sterilized. During the experiments the iron concentrations increased in the upper sediment layers, pore water and overlying lake water and decreased in the lower amended sediment layer. The distribution of 57Fe in the abiotic cores did not change during the experiment, implying that physical disturbance during sampling and chemical iron reduction and remobilization can be excluded as causes for the changes in iron distribution in the biotic cores. Thus, the remobilization and deposition of iron in the columns was initiated by microbial redox reactions. The implications of this work are significant for the interpretation of metal profiles in fresh water sediments as historical records of metal loadings, particularly because redox-sensitive amorphous Fe and Mn oxides and oxyhydroxides can serve as sites for metal adsorption or co-precipitation. [ABSTRACT FROM AUTHOR]

Abstract: Contents and δ 34S values of several S compounds, enumerations of S-reducing bacteria (SRB) and Fe-reducing bacteria (IRB), and Fe, Pb and In concentrations were determined for 210Pb-dated sediment cores from two lakes in Quebec, Canada. Both lakes are located approximately 70km downwind of the Horne smelter and refinery in Rouyn-Noranda. Increases in Fe, Pb and In concentrations and a decrease in the δ 34S values of total S in both lake sediment cores coincide with the start-up of the smelter in 1927. The shift towards more negative δ 34S values was primarily caused by an increase in the extent of S isotope fractionation during bacterial (dissimilatory) SO4 reduction due to SO4 loading of the lakes after smelting began. Consequently, an enhanced accumulation of 32S-enriched reduced inorganic S compounds is evident in the sediments. δ 34S values of organic S in the sediments decreased only slightly due to the smelter emissions between 1930 and 1980. Hence, due to the sulfide depositing mechanisms, S isotope ratios constitute a useful tracer recording the onset of S pollution in sediments of the two previously SO4-limited lakes investigated. In contrast, total S concentrations alone are not reliable indicators for anthropogenic S loading in lake sediment records. [Copyright &y& Elsevier]