Pleistocene glaciations of the northern Alpine Foreland

Abstract. It is now well known that climate and environment are not stable over geological time and Alpine Quaternary stratigraphy has widely evolved since the first discussions on Pleistocene glaciations nearly two centuries ago. Originally, in the late 19th century, three and later four glacial periods were postulated but with more research it appears that several more cold phases and separate ice advances occurred. To solve the history of climate and environmental change, a reliable time frame for the available geomorphological evidence is necessary. While some very interesting results have been presented in recent years, the amount of numerical dating is still very small and needs to be improved in order to unravel the full story of the Alpine palaeoclimate.


Introduction
The Alps are an area on the globe where it was first recognised that climate and environment are not stable over geological time. The « ice age » theory and later a widely accepted Quaternary stratigraphy were developed in this region. Hence, palaeoclimate andenvironmental research are long established in the Alpine region. The Alps are also important with regard to atmospheric circulation, representing the major weather divide in Europe. The area lies down¬ wind of the North Atlantic Oscillation that strongly influenced climate change in the past.
The article begins with a brief overview of the historic aspects of the recognition of Pleistocene glaciations of the northern Alpine Foreland. The principles of the still commonly used stratigraphic system of Qua¬ ternary glaciations by Penck & Brückner 1901/ 09) are then presented and critically discussed in the light of basic assumptions and new evidence. Finally, some new ideas for a revised stratigraphic model for the Alps are presented. 2 A brief history of the ice age theory The theory that Alpine glaciers are not stationary but highly dynamic bodies of ice was first recorded during the 17th century, with the massive advance of glaciers such as that at Grindelwald. Wagner 1680) wrote: « These snow masses show such growth over the last few centuries that they force back the areas close with all the trees, the charming meadows and the neighbouring huts, and force the residents to relocate their houses else¬ where » unpaged) expressing the drama of changing environment at that time. The theory that glaciers once almost completely covered the Alps was first developed by Venetz 1833) and later promoted by Agassiz 1837) in particular. However, this approach appears to have two major flaws: 1) it very much depends on which particular oxygen isotope record is being used as not all show maximum peaks of glaciations at times and 2) this approach a priori assumes that the dynamics of Alpine glaciation is synchronous with global ice volume. The latter appears to be a crucial point as it suggests that the Alpine chronology is not independent. Further¬ more, there is evidence for pronounced regional dif¬ The classical Alpine Quaternary stratigraphy is mainly based on the assumption that the gravel deposits found in the Alpine Foreland represent melt-water sediments of outwash plains that were formed through glacial erosion, transport and termination of glaciers in the lowlands cf. Habbe et al. 2007). As a consequence, almost any gravel deposit in the lowlands has been interpreted to represent a major glaciation of the Alps.
The complex interplay between tectonic fragmenta¬ tion, uplift and erosion of the Alpine body as well as climate change during the Pleistocene was, however,  & Scholz 1981), redrawn by H. Pfalz-Schwingenschlögl investigated rather poorly, even though a clear fluvi¬ oglacial origin of gravel deposits is evidenced at a very few sites only. Beside this general concern about the origin of certain sediments the chronology of Alpine glaciations is also questioned. This is mainly due to the lack of independent age control which led several researchers to carry out tentative correlations with marine records of global ice volume. More recently, the application of new methods of numerical dating to sediments has resulted in debate over previous find¬ ings.
The oldest gravel units Biber, Donau) are generally attributed to earliest Pleistocene times setting its lower boundary at 2 600 000 years) Habbe et al. 2007) and similar age constraints are, based on mammal remains, available for the « Deckenschotter » of northern Swit¬ zerland Bolliger et al. 1996). Preliminary results of burial dating using cosmogenic nuclides imply a simi¬ lar age 2 300 000+ 1 000 000/-800 000 years) for flu¬ vial sediments attributed to the « Günz gravel unit » although it should be noted that this dating approach is in a very early stage of development and is associ¬ ated with large uncertainties. The subsequent younger unit, « Mindel » was found by burial dating using cos¬ mogenic nuclides to be only 600 000+/-200 000 years Häuselmann et al. 2007). This Middle Pleistocene age is in agreement with the amount of local tectonic displacement but it is still controversial from the meth¬ odological point of view. The Meikirch drilling site in the Aare Valley, Switzerland, is a locality where Middle Pleistocene deposits have been dated by luminescence techniques and the Meikirch complex has recently been correlated with Marine Isotope Stage MIS) 7 Preusser et al. 2005b). An important issue raised is that the Meikirch complex consists of three well-devel¬ oped warm phases of interglacial character), and this casts doubt on the often applied assumption that inter¬ glacials occurred at 100 000 years-cyclicity in the past. Loess deposits of similar age MIS 7) and recording a similar pattern seem to cover « Günz gravels » in Upper Austria Wels site) Preusser & Fiebig 2008). How¬ ever, MIS 7 deposits have only recently been identified in the Alpine area and there may be many more sites with deposits of similar age.
The Penultimate Glacial Maximum classically attributed to the « Riss gravel unit » represents in most regions the most extensive glaciation of the Quaternary. Surface exposure dating of erratic boul¬ ders attributed to this glaciation in the Jura Moun¬ tains suggests an age of ca. 150 000 years MIS 6) Graf et al. 2007). However, luminescence dating of gravel deposits attributed to the Rissian identi¬ fied two phases of aggradation between 60 000 to 90 000 years and 140 000 to 170 000 years Fiebig . The first cluster of ages implies deposition during the early part of the Last Glacial Cycle and not prior to the Last Interglacial.
Finally, while it is generally accepted that the last glaci¬ ation of the Alps took place between 30 000 and 20 000 years ago, discussion is ongoing regarding further pos¬ sible glaciations during the Last Glacial Cycle. Some preliminary evidence suggests that earlier glaciations followed by an almost complete meltdown of ice, occurred during MIS 5d ca. 105 000 years) and MIS 4 ca. 65 000 years) Link & Preusser 2006;Preusser 2004;Preusser et al. 2003Preusser et al. , 2007.
Abstract: Pleistocene glaciations of the northern Alpine Foreland It is now well known that climate and environment are not stable over geological time and Alpine Quaternary stratigraphy has widely evolved since the first discus¬ sions on Pleistocene glaciations nearly two centuries ago. Originally, in the late 19th century, three and later four glacial periods were postulated but with more research it appears that several more cold phases and separate ice advances occurred. To solve the history of climate and environmental change, a reliable time frame for the available geomorphological evidence is necessary. While some very interesting results have been presented in recent years, the amount of numeri¬ cal dating is still very small and needs to be improved in order to unravel the full story of the Alpine palaeo¬ climate.