Geochemical characterization of soil organic matter and variability of a postglacial detrital organic supply (Chaillexon Lake, France)

Geochemical analysis of sedimentary organic matter in recent lacustrine sed iments appears to be a useful tool in providing information concerning past environmental conditions. However, such analysis is often made without kno wing the geochemical characteristics of the organic matter derived from the watershed and, more explicitly, its soils. The present work deals with (i) a geochemical investigation (Rock-Eval pyrolysis) of soil organic matter s ampled in a lake watershed, and (ii) the study of the sedimentary organic m atter trapped in the lake deposits. The research was conducted on Chaillexo n Lake which was created by a rock collapse that dammed the palaeovalley of the Doubs River about 12 000 years ago. Since this event, the sediment tra p provides a continuous palaeoclimatic record for the Postglacial period. Results obtained lead to two main conclusions. First, the variability of Ro ck-Eval pyrolysis values observed in soils modifies the common interpretati on given to these parameters in the characterization of sedimentary organic matter. Indeed, variations in these parameters point not only to varying p roportions of terrestrial and lacustrine organic matter in a lacustrine inf illing but also to variations of the terrestrial supply linked with the evo lution of vegetal cover in the catchment. The second conclusion is that the story of the Chaillexon lacustrine system is marked by a rather sudden soi l and forest development at the Preboreal-Boreal transition (9000BP). (C) 1 998 John Wiley & Sons, Ltd.