Early diagenesis of fatty acids in lacustrine sediments - III. Changes in fatty acid composition in the sediments from a brackish water lake
A 3-m sediment core taken from Lake Suigetsu, in which a shift from fresh to brackish water occurred about three hundred years ago, has been examined for variation with depth of organic carbon and fatty acids. From the difference in total amounts of sulphur between sediments under fresh and brackish water environments, the surface sediments above approximately 35 cm depth were deduced to be accumulated under a brackish water environment. The total contents of organic carbon and fatty acids, and percentage composition of fatty acids gave discontinuous profiles above and below the 35-40 cm sediment layer. At a depth of 12.5 cm, the distribution in chain length of the fatty acids changed from a unimodal (the predominance of C 12-C 18 over C 20-C 34) to a bimodal pattern, which was mirrored by the composition diversity index (CDI). Although the fatty acids in the surface sediments (0-40 cm) from Lake Suigetsu seemed to suffer milder degradation through microbial activity than those in a core (0-150 cm) from Lake Suwa, a freshwater eutrophic lake, both lacustrine sediments showed similar trends in the alteration of fatty acid composition with depth.