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 Cott¨¦s (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 Cott¨¦s 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 Cott¨¦s 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.