Lacustrine archives in Jura lakes since the Late Glacial Period

During the retreat of the Würm ice sheet in the Jura region, numerous glacial paleolakes took place in the French and Swiss Jura. Lake deposits are excellent continental archives for recording climatic fluctuations as well as extreme events such as volcanism through the deposition of tephra layers. Two sites were investigated: the Amburnex Valley site (Switzerland) and the Lake Val (France). During the Late Glacial period, both sites were glacial lakes characterized by significant accumulation of lacustrine sediments allowing a better and complete paleoclimatic and paleoenvironmental evolution record.

The main goal of this study is to reconstruct the paleoclimatic and the paleoenvironmental evolution recorded in lacustrine sediments deposits since the last 19’000 years, and to determine whether major volcanic events have been recorded during this period. The Amburnex core exhibit 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 consists of the same lithological succession. A multiproxy approach based on palynological, mineralogical and geochemical analyses (TOC, Nitrogen, Phosphorus, stable isotopes) have been used to characterize the hydrological and climatic fluctuations, the trophic level and the origin of organic matter in order to reconstruct the paleoenvironmental and paleoclimatic evolution of this area.

The Bølling-Allerød, the Younger Dryas and the beginning of the Preboreal period have been recognized in the Amburnex site, by palynological analyses and carbon 14 dating. During the Oldest Dryas, oligotrophic conditions took place as suggested by the very low concentrations in nitrogen and organic matter. Then, during the warmer Bølling period, an enrichment in total organic carbon (TOC) implying the development of eutrophic conditions. Later in the Allerød period, low TOC and phosphorus contents, associated with varved carbonate deposits, indicate a return to more oligotrophic conditions. New organic matter enrichments are observed in the interval corresponding to the colder Younger Dryas period. These trends are quite consistent with those observed in the Lake Val and reflect significant changes in runoff and nutrient inputs at least at regional scale.

Tephra layers are absent in the Amburnex core. The latter are indeed not always well preserved in sediments due to mineralogical transformation. But thanks to magnetic susceptibility and to phosphorus anomalies (linked to apatite present in ash deposits), two fallouts of atmospheric volcanism such as the Laacher See Tephra event (Eifel, Germany) and the Vedde Ash tephra event (Iceland) have been highlighted.