Kazuko Ogura

Organic geochemistry of a lacustrine sediment (Lake Haruna, Japan)

Abstract Total organic matter, lipids, humic compounds, pheopigments, aliphatic and aromatic hydrocarbons, aliphatic mono- and dicarboxylic acids and sterols were analyzed for a 133-cm core sediment sample taken from a mesotrophic freshwater lake, Lake Haruna, Japan. Vertical distributions of major organic compounds can be interpreted in terms of the variation of their possible precursor materials (autochthonous or allochthonous) rather than their post-depositional chemical changes. Variations in the net production of the lake in the past 2500 years was estimated, indicating that a drastic change of the eutrophic level of the lake from oligotrophic to mesotrophic occurred approximately 1000 years ago.

Early diagenesis of organic matter in the water column and sediments: Microbial degradation and resynthesis of lipids in Lake Haruna

Particulate matter, sediment trap, and surface sediment samples collected in freshwater Lake Haruna were studied to understand early diageesis of organic materials in the water column and in bottom sediments. The samples were analyzed for biomarkers, including aliphatic and aromatic hydrocarbons, fatty alcohols, saturated and unsaturated fatty acids, β- and ω-hydroxyacids, and α,ω-dicar☐ylic acids. Decreases in concentrations of autochthonous saturated C12–C19 fatty acids and polyunsaturated C18 acids relative to TOC occured with the settling of organic matter ot the lake bottom, whereas the amounts of terrestial saturated C20–C30 acids remained almost constant. Conversely, the concentrations of monosaturated fatty acids, branched chain fatty acids, and β- and ω-hydroxyacids, which are probably produced by microbial activity, increased. These results indicate that preferential degradation of algal lipids accompanies microbial resynthesis of lipids during settling, however, terrigenous lipids are relatively stable.

Diagenesis of biomarkers in Biwa Lake sediments over 1 million years

Abstract Lipid biomarker compounds ( n -alkanoic acids, n -alkanols, sterols, n -alkanes, chlorophyll derivatives) and total lipid carbon in a chloroform extractable fraction, isolated from sedimentary deposits up to ∼2 Ma old of a 1400 m core sample of Lake Biwa, were analyzed by gas chromatography, gas chromatography-mass spectrometry and a thin-layer chromatography-flame ionization detection analyzer (TLC-FID) to investigate their diagenesis and their utilities as biomarkers. The samples are divided into two parts at a depth of 911.4 m. The samples below 911.4 m are basement rock. The upper core is composed of clay from the surface to 249.5 m in depth, and alternated with slit, clay and gravels from 249.5 to 911.4 m. The organic compound carbon, obtained by hydrolysis of the lipid fraction by alkali solution, comprised from 11 to 35% of the total lipid carbon. TLC-FID analysis indicated that significant amounts (about 70–80%) of the total lipid carbon were present in the polar fraction and small proportions were simple lipids. The amounts of n -alkanols were comparable to those of n -alkanoic acids and sterols, while contemporary lacustrine sediments usually contain a larger amount of n -alkanoic acids than of n -alkanols. Normal alkanes, maximizing in abundance at C 27 or C 29 , were most abundant among the lipid compounds. Variation in the distributions of sterols and stanols seemed to preserve the information of input sources and the C 27 /C 29 ratios were correlated to the C/N of the sediments. The composition of n -alkanols, maximizing at C 24 or C 18 , appeared to indicate an aquatic paleoenvironment—a paleowater depth at least in the upper 250 m. Ratios of shorter to longer chain n -alkanoic acids, which had been believed to be an indicator of autochthonous to allochthonous origins, clearly increased with time. A marked correlation between chlorophyll derivatives (ratio of chlorophyll derivatives to total lipid carbon) and paleotemperature was found.