Establishment of a precise chronicle of the lateglacial and holocene
climatic variations requires the use of various dating methods. Among
them, tephrochronology allows high-resolution dating and synchronization
of these events at a regional scale and sometimes at a continental
scale.
Study of the record of the climatic variations occuring in western
Europe since twenty thousands years leads to the discovery of tephras
levels and ``cryptotephras'' in sediments cored in lakes from the Jura
and Vosges mountains, and the Swiss plateau. Some of these levels are
invisible to the naked eye. Their detection was obtained using magnetic
susceptibility measurement with a 5 min step along the cores.
Our observations allow to complete the western boundary of the southern
distribution of the Laacher See Tephra (ca. 12,900 yr, Eifel, Germany).
This tephra has been already described in others lacustrine sequences
from the Jura and the western and northern Europe. In the sites we have
studied, the volcanic glass shards show geochemical compositions
comparable to those of the youngest phases of the Laacher See Tephra
eruption. Other tephra levels, never described in this region until
then, have been detected and characterized. Two very discrete tephra
levels, only observed in sites from the Jura and the Swiss plateau,
present ages, mineral assemblages and chemical compositions close to
those of tephras that were emitted by the Puy de la Nugere (Chaine des
Puys, France) during the Allerod (about 13,300 yr). Sites from the
Vosges mountains are characterized by the occurrence of the rhyolitic
Vedde Ash (ca. 12,000 yr, from the icelandic Katla volcano), deposited
during the Younger Dryas. Presently, this is the most southwestern
occurrence of the Vedde Ash. Our data make up the Eastern France
Lateglacial tephrochrology and offer an additional chronological bridge
between sedimentary sequences from northern and central Europe and those
situated more westerly. They show the importance of the Laacher See
Tephra and the Vedde Ash for palaeoenvironmental studies in western
Europe since they are flanking the Allerod-Younger Dryas transition and
allow to compensate the lack of radiochronological data.