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 diagenetic 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 highlighting regions of enhanced degradation due to sulphate reduction (confirmed by solid phase sulphur and pyrite distributions). Three sedimentary facies are observed: IA (0–25 cm) where the majority of compaction and degradation of the organic matter by diagenesis occurs in the 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 contents and large grain size. Autochthonous amorphous organic matter is 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 and pollen and suggests a greater continental runoff from the surrounding basin at these times. Proportions of kaolinite, chlorite and illite content do not vary greatly, indicating that the detrital input has not changed significantly over the last 2500 years. Downcore decreases in the SiAl curve (proxy for biogenic silica) parallel those of total organic carbon suggesting the dissolution of solid phase silica over time. The anthropogenic deforestation and consequent greater nutrient runoff into the oligotrophic lake encouraged an increased production of diatoms around 30 cm. The increased sedimentation rate at the beginning of the Sub-Atlantic (facies II), as a result of greater continental runoff due to installation of present climatic conditions, enhanced the preservation of the organic material by reducing the dia-genetic effects and by increasing the proportion of diagenetically inert and relatively resistant organic matter. These findings underline the critical role that bulk sedimentation rate and the nature of organic matter play in the final determination of sediment composition, both in terms of input and in post-depositional degradation.