RELATIONSHIPS BETWEEN ORGANO-MINERAL SUPPLY AND EARLY DIAGENESIS IN THE LACUSTRINE ENVIRONMENT - A STUDY OF SURFICIAL SEDIMENTS FROM THE LAC-DU-BOUCHET (HAUTE-LOIRE, FRANCE)

The relationships between organo-mineral composition and early diagene tic processes in surface (< 50 cm) sediments from the Lac du Bouchet ( Haute Loire, France) are presented. Parallel pore-water studies reveal downcore increases in dissolved organic carbon, inorganic carbon and CH4, illustrating the dominant methanogenetic degradation, but also hi ghlighting regions of enhanced degradation due to sulphate reduction ( confirmed by solid phase sulphur and pyrite distributions). Three sedi mentary facies are observed: IA (0-25 cm) where the majority of compac tion and de,gradation of the organic matter by diagenesis occurs in th e absence of any significant change in the type of organic material or sedimentation rate; IB (25-40 cm) characterised by an increased grain size and diatom abundance; II (below 40 cm), whose base dates at 2500 +/- 350 BP (i.e. within the Sub-Atlantic) shows high organic carbon c ontents and large grain size. Autochthonous amorphous organic matter i s the dominant petrographical organic component, whereas pedogenetic ( allochthonous) amorphous organic matter is slightly enriched in facies II and IA. This is coincident with the disappearance of the spores an d pollen and suggests a greater continental runoff from the surroundin g basin at these times. Proportions of kaolinite, chlorite and illite content do not vary greatly, indicating that the detrital input has no t changed significantly over the last 2500 years. Downcore decreases i n the Si/Al curve (proxy for biogenic silica) parallel those of total organic carbon suggesting the dissolution of solid phase silica over t ime. The anthrapogenic deforestation and consequent greater nutrient r unoff into the oligotrophic lake encouraged an increased production of diatoms around 30 cm. The increased sedimentation rate at the beginni ng of the Sub-Atlantic (facies II), as a result of,greater continental runoff due to installation of present climatic conditions, enhanced t he preservation of the organic material by reducing the dia-genetic ef fects and by increasing the proportion of diagenetically inert and rel atively resistant organic matter. These findings underline the critica l role that bulk sedimentation rate acid the nature of organic matter play in the final determination of sediment composition, both in terms of input and in post-depositional degradation. (C) 1996 Elsevier Scie nce Ltd