Local environmental changes recorded by clay minerals in a karst deposit during MIS 3 (La Chauverie, SW France)

Abstract

The four stratigraphic units of the Late Pleistocene (MIS 3) clay-rich deposit of La Chauverie (Charente, SW France) have been characterized for mineral composition. The lower part of this 75 cm-thick karst deposit preserves a mammal fauna typical of temperate climate, followed by an assemblage pointing to a colder phase. The clay fraction of both horizons mostly results from mineral transformations of the clays contained in the sediments of the Paleogene formation surrounding the cave. Because clay mineral properties and changes intimately depend on the physico-chemical conditions and reaction kinetics prevailing at a given time in the soils, they are highly temperature-related. Notably, changes are rapid under temperate conditions, but slow in tundra-like cold contexts. As shown by the altered inherited mica-illite, as well as by the increase of crystallinity of kaolinite particles, at La Chauverie these transformations were marked in the stratigraphically lower part of the deposit formed under temperate conditions. Conversely, as reflected by the reduction of the kaolinite/smectite ratio characterizing the sediments of the upper horizon, the pedological evolution was limited during the subsequent cold phase. Here, both illite/smectite and kaolinite/smectite mixed layers become smectite-richer than their equivalent in the Paleogene. The parallelism between paleontological evidence and mineral signature in recording a relatively rapid (millennial-scale) shift towards colder conditions suggests that clay mineral assemblages from cave deposits can be used to assess paleoclimatic and paleoenvironmental dynamics at local scale. Nonetheless, future research should test this potential tool in more appropriate, thicker deposits investigated using other independent paleobiological and geochemical indicators.