Speciation of iron and sulphur in seasonal layers of varved lake sediment:: an XPS study

Under specific conditions, annually-laminated (varved) sediments are formed in lakes. Such lake sediments are rare, but of great interest to studies of past environmental conditions since they provide annual or even seasonal time resolution, and can be used to follow environmental changes over hundreds or thousands of years. The chemical composition of the varves and speciation of chemical elements reveal information on past conditions. The colour of sediment varves can vary between light brown to almost black, and it was hypothesised that black layers were deposited in seasons with oxygen deficit in the hypolimnion (summer and winter) and light layers were formed at spring and autumn over-turn periods when oxygen-rich water flow occurred near the lake bottom. We have tested this hypothesis by analyses of seasonal layers of varves formed in 1968 and 2005 using X-ray photoelectron spectroscopy with fast-frozen samples. We found that the organic (C, N, O, S) and inorganic (Al, Si, Ca, Fe, S) elemental composition was similar for all samples studied, but the chemical states of iron and sulphur differed remarkably depending on the season. The black colour of the summer layer was caused by the precipitation of inorganic FeS, while in the light layers only organic thiol (-SH) and sulphate SO42- together with Fe(III) hydroxides were found. Copyright (C) 2008 John Wiley & Sons, Ltd.