Composition and early diagenesis of fatty acids in lacustrine sediments, lake Aydat (France)

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 analyt ical procedure yields three groups of acids-free, H+-labile and OH--labile- , successively obtained by solvent extraction, acid hydrolysis and saponifi cation in toluene, in the presence of a phase transfer catalyst (PTC). The free components are relatively minor except for the presence of amounts of beta beta-hopanoic acids (C-32>>C-30) 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 an d terrestrial plants, survive in the deepest samples analysed, i.e. > 40 cm . The various fatty acids (FA) identified comprise normal saturated FA (wit h a strong C-20-dominance), monounsaturated FA (cis and trans C-16:1, C-18: 1, C-24:1 With omega 5, omega 7 and omega 9 double bond position), branched (iso, anteiso) FA, 3-hydroxy FA and 3-methoxy FA, alpha,omega- dicarboxyli c 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 cla ssically attributed to methanotrophs, which implies that markers from other micro-organisms are already destroyed in the water column. A distinctive a cid composition at 20 cm depth might be explained by intensive microbial ac tivity induced by unusual climatic conditions (i.e. an exceptional drought) . (C) 2000 Elsevier Science Ltd. All rights reserved.