Samstag, 12. September 2009

Lac du Bouchet

How Florian correctly answered, the WoGE was the maar lake of Bouchet and it´s very long stratigraphic sequence.

Located in the south eastern part of the French Massif Central, the volcanic region of Velay contains numerous maar craters. Pollen analysis has been carried out on lake sediment sequences obtained from three of these craters - Lac du Bouchet, Ribains and Praclaux. The presence of thick trachytic tephra layer has enabled correlations between the sequences. This has led to the reconstruction of a long continental sequence from 450ka ago to the present.

The attribution of the sequence to the last five climatic cycles is based on an apparently continuous succession of warm and cold phases, which correlates with the marine oxygen isotopic record. Tephra layers in the organic deposits of Lac du Bouchet provided Ar40/Ar39 dates with an average of 275ka - after this considerations the Lac du Bouchet temperate phase is correlated to the Marine Isotopic Stadium 7.

Lac du Bouchet (44°55´N, 3°47`E, 1.200m altitude) is a 28m deep lake, and has been the subject of numerous geological and biological studies. It yielded a sediment sequence extending from about 325ka to the present.

The Bouchet sequence can be subdivided by the amount of pollen taxa in three interstadials, which shows a classical succession of tree species, from cold tolerant at the beginning, to warm climate species, to again cold tolerant trees.

The proposed chronostratigraphy for the Velay maar sites (after REILLE et al. 2000, modified).

The Bouchet I interstadial is characterised by the presence of great quantities of Carpinus pollen, other thermophile taxa like Abies and Fagus are rare, or like Taxus complete missing. The Carpinus forest was probably the dominant vegetation everywhere in Velay at that time.
The Carpinus forest was then replaced directly by a Pinus forest, the latter marking the end of the interstadial and maybe a sudden cooling.
The Bouchet II interstadial shows similarities with the fist: again a Carpinus forest develops, but this time other trees like Ulmus, Corylus, Abies, Fagus and Picea must be also been present with significant numbers. As usual, the intestadial ends with a Pinus forest.
The third interstadial - Bouchet III, differs in significant taxa appearance and significance from the earlier two interstadials. Alnus viridis, today absent in the Massif Central region, but found as pioneer species in the timberline of the Alps, plays a mayor role in the first phase of the interstadial. This tree is then replaced by Ulmus, Quercus and Corylus, an Oak forest occasionally coexisting with a Picea forest develops. Carpinus this time doesn't form a forest of its own.

The establishment of tree taxa depends not only by the climatic conditions, but also from the distance of an investigated site from the glacial refugia of the species, and the capability of this species to spread. This maybe can explain the differences between the forest developments in the three interstadials.

The site of the ancient (with sediments filled) maar of Praclaux, in the vicinity of the Lac du Bouchet, today a quiet pasture.


REILLE, M.; BEAULIEU DE J.L.; SVOBODA, H.; ANDRIEU-PONEL, V. & GOEURY, C. (2000): Pollen analytical biostratigraphy of the last five climatic cycles from a long continental sequence from the Velay region (Massif Central, France). Journal of Quaternary Science (7): 665-685

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