Chronostratigraphy of two Late Pleistocene loess-palaeosol sequences in the Rhône Valley (southeast France)

Abstract

A sedimentological and chronostratigraphical investigation was carried out on two loess sections located in the Mediterranean area in southeast France along the Rhône River (Lautagne) and the lower reach of a tributary of the Rhône River (Collias). High-resolution sampling (5-20 cm) for magnetic susceptibility, grain size distribution (including nonparametric end-member modelling), colour reflectance and geochemistry was performed. The chronology was based on luminescence dating of quartz grains and radiocarbon dating of small gastropod shells, coupled with hierarchical Bayesian modelling. The Collias section (~8 m thick) records the whole last climatic cycle. It comprises a thick red basal pedocomplex S1 developed during the Last Interglacial and the Early Glacial, similar to that observed elsewhere in southern and southeastern Europe. Loess deposition occurred during the Lower (L1L2) and the Upper Pleniglacial (L1L1). It was interrupted by soil formation during the Middle Pleniglacial, of which a brown Bwk horizon has been preserved (L1S1). By contrast, the ~5 m thick Lautagne section provides a detailed record of the Upper Pleniglacial. Weakly developed hydromorphic soils are correlated with the Greenland Interstadials GI-4 to GI-2, while the main period of coarse loess sedimentation corresponds to the Greenland Stadials GS-5 to GS-2. At a regional scale, the time of loess deposition ranges between 38.5 ka and 12 ka, with a peak at ~28-24 ka, overlapping with the maximal advance of the Alpine Ice Sheet (AIS). This strongly suggests that regional glacier dynamics was the main driver of loess sedimentation.

The relative contributions of weathering and aeolian inputs to postglacial formation of Mediterranean alpine loess

“…The upwards fining of M1 and M2 along with the decrease of their concentrations, may result from either a decrease in the transport capacity from the source, or from a decrease of their in-situ production. A fine mode with a respective grain-size of ~3 μm, has been reported in almost all European loess representing either regional, or supracontinental sources (Bosq et al, 2020). This may be the case for either M1 and/or M2, with their origin derived either from local sources such the glacial debris and/or the adjacent alluvial plane of Katerini.…”

“…Rantitsch et al, 2003), which may be the case for Mount Olympus, as interbedded shale bands are visually observed within the cirques' headwall Cretaceous limestones. Rb shows similar behavior with K, but is less mobile (Bosq et al, 2020) and is generally hosted K-feldspars, micas and clays (Nesbitt et al, 1980), which constitute integral compartments of Mount Olympus bedrock. Thus, it is safe to consider that the Mn and Rb enrichment of PM loess (Fig.…”

“…8B) are explained by the slow mobility of Sn during weathering (Kabata-Pendias, 2001), whereas low Ca counts likely result from decalcification of the loess profile, a process common in other loess deposits (e.g. Bosq et al, 2020). Mn and Rb are also present in MK and TZ samples (Fig.…”

“…Therefore, synergistic to the weathering of Mount Olympus carbonates and deposition of detrital components with subsequent post depositional production of fine particles and aggregates rich in Fe-Ti oxides, is the deposition of fine dust incorporated into M1 and M2. Background dust with grain size similar to M1 and M2 (~3μm), is found in many European loess sequences and represents local, regional, or supracontinental sources (Bosq et al, 2020).…”

“…to syn or post-depositional Ca removal and is described by the Ca/Sr ratio (Bosq et al, 2020). Ca/Sr decreases with increasing intensity of Ca weathering (Bokhorst et al, 2009) and within PM loess it ranges from 8 to 2 from the base towards the surface of the PM profile.…”