Loess-palaeosol sections along the Rhône Rift Valley (SE France) as Late Quaternary palaeoenvironmental archives
von Nora Pfaffner
Datum der mündl. Prüfung:2023-03-10
Erschienen:2024-03-05
Betreuer:Prof. Dr. Daniela Sauer
Gutachter:Prof. Dr. Daniela Sauer
Gutachter:Prof. Dr. Heinrich Thiemeyer
Dateien
Name:Dissertation_Nora Pfaffner_March 2024.pdf
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Zusammenfassung
Englisch
Loess-palaeosol sections (LPSs) represent ideal archives for gaining knowledge of past climatic conditions and changes, as their intermediate loess deposits and buried palaeosols reflect climate conditions of their specific locations. Detailed knowledge of past climatic changes is also beneficial for accurate modelling and forecasting since such past changes must have already shown similar trends. Although climate reconstructions from LPS from the last decades have improved our understanding concerning the cross-regional Pleistocene climate changes, there are wide knowledge gaps regarding reconstructions linking the central temperate and Mediterranean Europe. According to climate projections, the region between the Mediterranean and the temperate climate zone of central Europe is most vulnerable in Europe, as a northern expansion of Mediterranean conditions (e.g. higher temperatures, more arid months) is expected. Within this context, the climate reconstruction of the expansion of the Last Interglacial (Eemian) conditions is especially relevant, as this area is assumed to be several degrees warmer than current climate, a condition that is also expected for the future climate within the European vulnerable climatic transition zone. The LPSs of the Rhône Rift Valley in southeast (SE) France were chosen for this investigate as the recent climate transition zone is located in the centre of this valley (Valence, ~45° N). Thus, the LPS present important tie-points between the central European and the Mediterranean LPS region. Previous investigations on the LPS in the Rhône Rift Valley were mainly based on observations, and provided only limited insights into the lithological properties of the loess deposits, the intercalated palaeosols and the resulting pedostratigraphy. This limitation also applied to the understanding of the changing degree of weathering and interpretations of the palaeosols with respect to their relative palaeoenvironments. In particular, numerical ages of the loess deposition and soil formation phases were scarce. We aimed in this thesis to fill these gaps by reconstructing the Last Interglacial-Glacial landscape evolution changes along the climate-sensitive transition zone from LPS of the Rhône Rift Valley. The results were linked with known European LPS in order to (i) verify our findings with LPS of similar conditions (e.g. climate conditions, valley position) and (ii) to identify where possible climatic boundaries were located during the Late Pleistocene, and how these boundaries may have fluctuated over time. We focused thereby on identifying possible stratigraphic marker horizons/complexes in the LPS, and establishing a reliable chronostratigraphy. In several field campaigns, two main LPS, i.e. Baix LPS (in the climate transition zone) and Collias LPS (in the Mediterranean zone) and several aside profiles, have been investigated with multiple state-of-the-art-methods to establish reliable chronostratigraphies. The two LPS displayed several loess and weakly to strongly developed palaeosol horizons. Based on the field observations and the laboratory analyses, both main LPS were subdivided in up to four pedo-stratigraphic units, interlaced by two main loess units, respectively and formation and deposition times of soils and sediments were correlated with various OSL ages. Both LPSs begin from bottom to top with an Last Interglacial/Early Glacial soil-complex (MIS 5) represent by a mainly truncated Luvisol formation (Baix LPS: Stagnic Luvisol; Collias LPS: Chromic Luvisol + several Bw horizons), also well-know from other European LPSs (e.g. Rocourt complex). However, in contrast to European LPSs no humic-rich layer for late MIS 5 were observed in Rhône Rift Valley LPS. The following loess unit of the Lower Pleniglacial (MIS 4) comprises of least weathered silty and sandy loess (partly olive-ish at Collias LPS) and loess-like sediment horizons. At Baix LPS also a brown MIS 4 /MIS 3 Bw horizon was identified (lower interstadial soil), which may correlate with known palaeosol relicts of the Cambisol or Regosol-Cambisol types (e.g. Gräselberger Soil, Jackerath Soil, Reisberg Soil) from central Europe LPSs. The overlain pedo-unit, at both sites, is represented by a truncated Cambisol- complex (upper interstadial soil) with prominent in situ carbonate nodules and a striking former biopores system (mainly at Collias LPS). The soil remain is assumed to represents a pedo- complex spanning from (GI 15.1 or) GI 14 to GI 9 and correlates well with brown soil horizons (e.g. Remagen Soils, Gräselberg Soils) of European LPSs. Thus, the striking Cambisol soil- complex may be regarded as a stratigraphic marker horizon within the Rhône Rift Valley and serve as a Middle Pleniglacial-unit for supra-regional correlation of central European and Mediterranean LPS. In contrast to central European LPSs with further Cambisol formation (e.g. Lohne Soil complex, Sinzig Soils) for the milder climate periods GI 8 to GI 4 (ca 38–29 ka) during upper MIS 3, at Baix and Collias LPSs no significant soil formation could be detected or may not exist. Thus, times favourable of soil formation (and soil conservation) appear to alternate between the Mediterranean (early MIS 3 prior to Heinrich 4 event) and the central European loess landscapes (later MIS 3, after the Heinrich 4 event). The deposits of the following Upper Pleniglacial (MIS 2) are represented at both LPSs mainly by silt and sand sized loess with varying intensity of weathering. Also no further (weak) soil formation, e.g. tundra gley soils, especially redoximorphic features related to freeze-thaw dynamics, was observed, thus agree with assumed less severe climate conditions during loess accumulations at both LPS sites compared to central Europe. The Holocene soil formation displays similar to European LPSs a Cambisol formation, although the conditions for a Luvisol formation along the entire climatic transect is given, but the difference in terms of disturbance and duration of time period have to be respected. Based on our findings we could show that the main (pedo) -stratigraphies of Collias and Baix LPSs generally agree with the central and western European LPSs (e.g. MIS 5, MIS 3 pedo- stratigraphic marker horizons). However, the syn-sedimentary, intensively reworked and bioturbated character of the Rhône Rift Valley loess deposits as well as their partly polygenetic soils and soil-complexes reflect clearly a gradient of decreasing magnitude of Late Pleistocene climate changes from central Europe to southern Europe. Furthermore, no clear northern expansion of Mediterranean conditions based solely on soil characteristics during the Last Interglacial could be unambiguous verified. However, other archives do support a northern expansion of Mediterranean conditions in the northern Rhône Rift Valley and we can concluded that the climate conditions in the central and southern Rhône Rift Valley seemed to be significantly more humid than today. The comparison of the Last Interglacial and the recent soil formation to determine similarities and differences of the climate conditions within the Rhône Rift Valley is challenging due to various factors (e.g. different insolation, geomorphological position, intense anthropogenic use). As our results are inconclusive concerning the Last Interglacial climate conditions within the recent climatic transition zone, statements how these boundaries within the Rhône Rift Valley respond to future climate changes have to be carefully made and thus, more research to conclusively verify the intensity of rubefication under similar conditions, regionally and within the climate transect is needed.
Keywords: Loess, Loess-palaeosol section, Late Pleistocene, Palaeoenvironment, Rhône Valley