Distortion of the (paleo) environmental message recorded by molecular organic markers: example of an eutrophic lake (Aydat, Puy de Dôme, France).
Résumé
The reconstitution of paleoenvironnements constitutes one of the main goals of organic geochemistry. The approach that relies mostly on the identification of specific molecular markers likely to provide paleoecological indications, generally omits the losses (partial or even total) that affect the biomarkers from their production site to the sediment archives, thus distorting the primary signal. In order to appreciate the nature and the importance of such distortions, we analyzed various biochemicals from the lake Aydat recent sediments. The study was carried out on a 44 cm long core covering approximately one century of sedimentation. The following compounds were analysed: cellulosic and hemicellulosic sugars, by gas chromatography; lignin monomers by capillary electrophoresis; free and bound fatty acids (released by hydrolysis and saponification) and saturated hydrocarbons and aromatics, by gas chromatography-mass spectrometry. With a composition apparently inherited from soft tissues of Angiosperms and a content which fluctuates rather little around 1.3%TOC, lignin denotes a relatively constant input to the sediments. In contrast, higher plant monosaccharides (xylose, cellulosic glucose) which only occur in some rare levels, testify to accidental inputs, at the favour of exceptional events. In the water column, sugars of the autochthonous primary producers (mostly diatoms) are partly replaced by microbial compounds. Then, all polysaccharides appear surprisingly stable in the sediment record. Fatty acids undergo differential degradation depending on various factors (compounds unsaturated or not, free or bound...). The free and bound acids originate from different stocks and thus bring complementary information. However, only those released by saponification appear relatively stable in the sediment and thus can bring information on the autochthonous (nC20-) and allochthonous (nC20+) contributions. Other fatty acids testify to bacterial activities. If the preservation of some compounds appears more conditioned by their engagement in peculiar chemical combinations (e.g. lignin polymer, lipidic esters) than by their membership to a particular compound family (e.g. fatty acids), the cause for the preservation of others remain rather conjectural (e.g. polysaccharides of aquatic organisms). In summary, a choice of suitable markers can a priori make it possible to overcome the important distortions undergone by the original biological signal previous to contributing to the sediment record. The nature and extant of these distortions might also help deciphering the impact of certain environmental parameters on the original biological contributions. The confrontation of the geochemical data from various lakes with contrasted physiography might allow refining the relations between biomarkers and (paleo) environment of deposition.