Quantitative organic petrography and reconstruction of paleoenvironments in Alps / A. Simonneau
Contribution of quantitative organic petrography to the reconstruction of high altitude mountains paleoenvironments during the Holocene : a comparative study of Bramant & Blanc Huez proglacial lakes sedimentary infills (Grandes Rousses Massif, Western Alps, France). SIMONNEAU A. In "La construction des territoires montagnards : exploitation des ressources et mobilité des pratiques, organisé par les Laboratoires TRACES et GEODE (Toulouse) et le Centre d'Archéologie Préhistorique du Rhône aux Alpes (Valence, France). Université Toulouse II-Le Mirail, 8-11 octobre 2009. [Première journée.]
This report introduces results of lithological, optical and geochemical investigations made on lacustrine sediments from two high altitude proglacial lakes of Grandes Rousses massif (western Alps, France) : the lakes Blanc Huez and Bramant. The aim of this work is to reconstitute mountain paleoenvironments of the massif during the last 10 000 years (Holocene) and to put it in relation with high altitude human activities identified by archaeology. Lithological analyses show succession of organic and clastic sequences. Geochemical analyses of organic matter show that total organic carbon could be correlated with physical parameters like magnetic susceptibility, gamma densimetry or spectrocolorymetry. The entirety allows characterizing transitions between our facies. Optical analyses on organic sequences allow us to clarify composition, sources and geochemical conditions in basins. Thus, we underline fossil organic matter from Blanc Huez cathment in the Blanc Huez lacustrine sediments. It leads us to modify bulk dating. We created an age"depth model which allowed us to describe alpines mountain paleoenvironments since Younger Dryas (~12765 cal BP). Two glacial overhangs have been identified during Younger Dryas and since Upper Atlantic (~6800 cal BP). Organical facies of gytja were deposed at the beginning of Upper Dryas, and from Preboreal (~10200 cal BP) to Upper Atlantic. However, climatic variations have been identified during Subboreal (~5550 à 2590 cal BP) and can be correlated with human activities in the catchments in particular during Bronze Age. More studies in the massif would allow us to validate our results.