Early diagenesis of organic matter and related sulphur incorporation in surface sediments of meromictic Lake Cadagno in the Swiss Alps

Samples of Recent anoxic sediment (< 100 yr BP) from the meromictic Lake Cadagno (Switzerland) were examined to study sulphur incorporation into organic matter at a very early stage of diagenesis. Trace amounts of two organosulphur compounds were identified in the extractable organic matter. Desulphurization of the heterocompound and asphaltene fractions with nickel boride yielded hydrocarbons with a strong dominance of phytane and minor amounts of n-alkanes, steranes and squalane, but may also have converted ester-bound moieties into hydrocarbons as shown by a control experiment with a chloropyll a standard. The amount of sulphur-bound compounds is always higher than the amount of the free precursors, even in the shallowest sediments. This indicates that the formation of organosulphur compounds starts already in the water column or is very effective in the uppermost millimeters of the sediment layer. Stable carbon isotope data of the sedimentary compounds, and of biological material from the water column and from land biota growing around the lake, show that the sulphur-bound phytane in the youngest samples is dominated by phytane of bacterial origin. With increasing depth the amount of phytane of terrestrial origin slightly increases.