Evolution of palaeoenvironmental and palaeoclimatic conditions recorded in Jura lakes since the Late Glacial Period

Abstract

Since the retreat of the Würm ice sheet, numerous glacial paleolakes have formed in the French and Swiss Jura. This project investigates two sites: the Amburnex Valley site (Switzerland) and the Lake Val (France). During the Late Glacial period, both sites were glacial lakes with significant accumulation of lacustrine sediments, providing a complete record of paleoclimatic and paleoenvironmental evolution. The aim of this study is to reconstruct the paleoclimatic and the paleoenvironmental evolution recorded in lacustrine sediment deposits over the last 15’000 years. The Amburnex core displays a basal morainic deposit from the Würm period, overlain by three meters of lacustrine deposits and four meters of peatland deposits. The Lake Val core exhibits the same lithological succession. A multiproxy approach based on palynological, mineralogical and geochemical analyses was used to characterise hydrological and climatic fluctuations, trophic levels, and the origin of organic matter. This approach was employed to reconstruct the paleoenvironmental and paleoclimatic evolution of this area. The analyses included total organic carbon (TOC), nitrogen, phosphorus and mercury contents, major and trace elements, and organic carbon isotopes. The Bølling-Allerød, the Younger Dryas and the beginning of the Preboreal period have been recognised in the Amburnex site, by palynological analyses and carbon 14 dating. The Oldest Dryas was characterised by oligotrophic conditions, as evidenced by the very low concentrations of nitrogen and organic matter. During the warmer Bølling period, there was an enrichment in TOC, indicating the development of eutrophic conditions. Later, during the Allerød period, low TOC and phosphorus contents, associated with varved carbonate deposits, indicate a return to more oligotrophic conditions. In the interval corresponding to the colder Younger Dryas period, new organic matter enrichments were observed. These trends are quite consistent with those observed in the Lake Val and reflect significant changes in runoff and nutrient inputs at least at a regional scale. No evidence of a tephra layer or significant mercury anomalies were found at the Amburnex site. However, significant increases in magnetic susceptibility were observed during the Allerød and Younger Dryas periods at the Amburnex site. These increases are well correlated with those recorded in other Jura lakes containing tephra layers, corresponding to the Laacher See Tephra and the Vedde Ash, respectively. A significant peak in Ptot is observed at the same level as the magnetic susceptibility peak during the Allerød period. This peak appears to be independent of nitrogen and TOC and is related to apatite present in tephra glass in the volcanic ash deposits of the Laacher See Tephra.