Palaeosols in the loess section of Baix (Rhône Rift Valley, SE-France), compared to those of Collias: a unique Late-Pleistocene record of the transition zone between the presently temperate and the Mediterranean region of Europe

Paleosol horizons preserved in loess-palaeosol sections (LPS) provide valuable archives of Quaternary palaeoenvironmental changes over time and spatial palaeoenvironmental gradients during the same period. Here, we present the characteristics of paleosol horizons in two LPS near the western edge of the Rhône Rift Valley in southeastern France: (1) the LPS “Baix” (total thickness: 14 m), located about 17 km north of Montélimar (44°42’36”N, 4°43’21”E), thus, in the transition zone between the presently temperate and the Mediterranean region of Europe; (2) the LPS “Collias” (total thickness: 9 m), located in the Uzès Basin, about 15 km northeast of Nîmes (43°57’11.94”N, 4°27’56.71”E), thus, in presently fully Mediterranean climate. Investigation of the paleosol horizons in the main profile at Collias was complemented by those of three smaller nearby LPS, “Collias-North_D112” (43°57’12.55”N, 4°27’55.83”E), “Collias-South_D112” (43°57’12.44”N, 4°27’53.36”E), and “Collias-North” (43°57’21.67”N, 4°28’6.99”E), in order to capture the spatial variability of the characteristics of some key horizons.

To our knowledge, no LPS have been analysed yet in such a transitional position between the presently temperate and Mediterranean climate. Primarily the LPS Baix may provide a crucial link between the rigorously analysed LPS in the presently temperate regions further north (e.g., in northern France, the Alsace region and Germany) and the LPS in the Mediterranean region (e.g., in southern France, Catalonia, Italy and Croatia), including the LPS Collias. Therefore, we aimed to decipher the paleoenvironmental record of the LPS Baix and Collias, and to identify similarities and differences between them. Optically stimulated luminescence (OSL) dating provided a chronological frame for both LPS.

The basal part of the LPS Baix starts with reddish Bt(g) horizons of a Stagnic Luvisol, representing the remains of an Eemian to Early Würmian (MIS 5) pedocomplex formed under warm and - at least temporarily - relatively moist conditions. The corresponding pedocomplex in the profile Collias-North_D112 displays an intensive red (chromic) Bt horizon overlain by several Bw horizons formed in reworked soil sediment and underlain by a massive calcrete. In the main profile at Collias, this red horizon has been entirely reworked by slope processes and has regained an angular blocky structure afterwards. Thus, it appears as a dark orange-red Bw horizon. Both, the LPS Baix and Collias include a prominent brown Bw horizon of a truncated Cambisol that developed in middle Pleniglacial (MIS 3) deposits. It is associated with large, elongated, vertically oriented calcium carbonate nodules, indicating that considerable amounts of calcium carbonate must have been leached from the former middle Pleniglacial Cambisol and accumulated in the underlying loess unit. No distinct palaeosols were observed in the Late-Pleniglacial deposits of the LPS Baix and Collias; a slightly brownish colour indicates very weak weathering (several BCk horizons) in the Late-Pleniglacial sediments of both LPS.