Composition and early diagenesis of fatty acids in lacustrine sediments, lake Aydat (France)
Résumé
The organic acids from recent sediments from lake Aydat, a small eutrophic lake located in the French Massif Central, have been analysed to determine their possible source(s) and their fate during early diagenesis. The analytical procedure yields three groups of acids — free, H+-labile and OH−-labile — , successively obtained by solvent extraction, acid hydrolysis and saponification in toluene, in the presence of a phase transfer catalyst (PTC). The free components are relatively minor except for the presence of amounts of ββ-hopanoic acids (C32>>C30). The H+-labile components, assumed to be mostly of bacterial origin, disappear very rapidly at depth. In contrast, notable proportions of OH−-labile compounds, mostly derived from algae and terrestrial plants, survive in the deepest samples analysed, i.e. >40 cm. The various fatty acids (FA) identified comprise normal saturated FA (with a strong C20- dominance), monounsaturated FA (cis and trans C16:1, C18:1, C24:1 with ω5, ω7 and ω9 double bond position), branched (iso, anteiso) FA, 3-hydroxy FA and 3-methoxy FA, α,ω- dicarboxylic acids, as well as hopanoic acids and a few lignin-derived aromatic acids. A notable part of the most unusual of these various compound types are classically attributed to methanotrophs, which implies that markers from other micro-organisms are already destroyed in the water column. A distinctive acid composition at 20 cm depth might be explained by intensive microbial activity induced by unusual climatic conditions (i.e. an exceptional drought).