Organic fluxes and early diagenesis in the lacustrine environment: the superficial sediments of the Lac du Bouchet (Haute Loire, France).

Superficial sediments from the Lac du Bouchet were analysed to study the degradation processes which affect sedimentary organic matter in an oligotrophic lacustrine environment. This was undertaken by the study of the evolution of the organic matter composition, in terms of both early diagenesis and any possible variations in organic inputs. The petrographical study of the resistant organic matter shows that some marked variations in the type, nature and abundance of organic inputs have occurred through time over the last 2500 years. The geochemical study of the bulk organic matter and the distribution of alkalisoluble components indicate that the effects of early diagenesis are only visible when the organic inputs are unchanged (<1400 years). Due to the very low sedimentation rates, methanogenesis seems only to affect the autochthonous (algal, phytoplanktonic) organic matter which consists of a degraded and amorphous organic matter, whereas the organic matter deriving from the surrounding basin has been partially or totally degraded before its deposition. [ABSTRACT FROM AUTHOR]