LIGNIN-DERIVED PHENOLS IN SEDIMENTS IN LONG CORES FROM LAC DU BOUCHET(MASSIF-CENTRAL, FRANCE) - PALEOENVIRONMENTAL SIGNIFICANCE

Lignitic remains present in the organic fraction of lake sediments con tain monomeric phenolic structures which can be regarded as specific b iogeotracers of the organic matter produced by higher plants growing o n the watershed. A study of the lignin-derived phenolic monomers (keto nic, aldehydic and acidic forms of 4-hydroxybenzyl, guaiacyl - or vani llyl -, syringyl and cinnamyl acids) has been made on samples of a 50 metres long core from the maar-lake, Lac du Bouchet. Their qualitative and quantitative determinations have been achieved by high-performanc e liquid chromatography (HPLC). The data permit analysis of the taxono mic origin of allochthonous inputs and, by comparison with palynologic al data of climate variations from at least 350.10(3) years (350 ky). At the same lime, comparison of lignin-derived phenol data and bulk or ganic matter enable the mode and the extent of organic matter alterati on to be defined. In this oligotrophic lake, sediments corresponding t o cold periods with a steppe pollen assemblage (stadial or glacial per iods) are generally poor in organic matter (OM). This OM is poor or la cking in lignin-derived phenols. It contains some ubiquitous phenols w hich map originate from algal tissues : consequently the organic compo nent of these sediments is either autochthonous - i.e. of phytoplancto nic and bacterial origin (at the time, the ancient lac du Bouchet was similar to certain present Arctic lakes) - or mixed autochthonous-allo chthonous but altered (that is particularly shown by the presence of a bundant oxidized ligno-cellulosic debris). In contrast, the OM corresp onding to temperate periods (interstadial and interglacial) contains h igh proportions of lignin indicating allochthonous organic input to th e lake from a diverse vascular plants. During these relatively warm pe riods, the allochthonous organic phase was mixed with local autochthon ous organic input.