Clay minerals of late Pleistocene sites (Jonzac and Les Cottés, SW France): Applications of X-ray diffraction analyses to local paleoclimatic and paleoenvironmental reconstructions

Abstract

The crystal-chemical properties of clay minerals are closely dependent on the physicochemical conditions prevailing at the time when they formed in soils and weathered zones. The degree of alteration of primary minerals and the composition of the secondary products are clearly linked to climate and plant cover. In particular, changes are rapid under temperate conditions which favor the development of thick soils, but slow in cold contexts favoring thin and fragile soils. In this paper, the mineral composition of two Pleistocene clay-rich deposits is investigated in order to contribute to reconstructing paleoenvironmental changes. Jonzac (Charentes, SW France) and Les Cottés (Vienne, SW France) are two rock-shelters with long archaeological sequences which include faunal remains and were recently radiometrically dated. The decomposition of XRD patterns at each site allows identification of the principal mineral phases and semi-quantitative estimation of their relative proportions. The variation of these proportions is analyzed through indexes based on the relative intensities of the decomposed bands. The clay fraction of both sites mostly results from the transformation of the minerals forming the sediments in the surrounding area of the rock-shelter terrace, including contributions from the hillside. Mineralogical clay data are compared with the faunal spectrum identified at both sites in order to discuss paleoenvironmental implications.At Jonzac, during the Quina Mousterian period dated to MIS4, the environment was cold and dry. The dominant mineralogical phases are inherited from the rocks (weak pedogenesis). This is consistent with the faunal analysis data typical of the MIS4. During the Mousterian of Acheulean Tradition period, dated to MIS3, the mineralogical composition of the clay deposits changed significantly, suggesting a less wet environment. The Aurignacian deposits dated to the end of MIS3/early MIS2 are rich in mixed layer minerals (MLMs) which are produced by the hydrolysis processes in soils. The climatic conditions probably became milder. However, because no faunal remains were found, this evolution is uncertain. The Les Cottés deposits, dated between MIS3 and early MIS2, are mineralogically typical of a cold environment where soil reactions are not very active. However, some weak variations of the indexes suggest temporary temperate conditions during the Les Cottés interstadial and in the Aurignacian. The parallelism between mineral signature and paleontological evidence in recording relatively rapid variations between contrasted climates suggests that clay mineral assemblages from rock-shelter deposits can be used to assess paleoclimatic and paleoenvironmental dynamics at a local scale.