Reconstructing the Evolution and Dynamics of Central Alpine Glaciers During the Last Glacial Maximum on the Basis of Their Geomorphological Footprints and Cosmogenic Nuclide Surface Exposure Dating
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Date
2022
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Doctoral Thesis
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Abstract
During the climax of the last glaciation, the Last Glacial Maximum (LGM; ~26.5-19 ka ago), large parts of the European Alps were covered by ice. Only the highest peaks protruded from the ice as nunataks. Interconnected transectional valley glaciers emerged and advanced far onto the northern, southern, and western Alpine forelands where they terminated as extensive piedmont glacier lobes. On the basis of preserved glacial and subglacial landforms, formed at the margin and at the bed of the former glaciers, respectively, extents, fluctuations, and flow patterns of long-gone glaciers can be reconstructed. Insights on the temporal evolution of glaciations, can be gained from surface exposure dating of large erratic boulders that were glacially transported and deposited, or from radiocarbon dating of organic material in glaciofluvial deposits.
Whether Alpine glaciers reached their LGM maximum in or out of phase is a long-raised question among Quaternary scientists. Clarifying LGM glacier (a)synchrony comes with implications for paleoclimate and past atmospheric circulation and addresses the need to understand the role of topography in modulating climatically driven glacier evolution. To date, timing and extent of the LGM is unequally well constrained across the Alps. With this work we aim to close a distinct gap in knowledge of LGM glacier timing in the Central Alps. Catchments of the former Ticino-Toce, Reuss, and Rhine glaciers are aligned on a longitudinal axis and were linked in their high-Alpine accumulation areas across several passes. All three were impacted to some extent by the proposed Vorderrhein ice dome. Reuss and Ticino-Toce glaciers additionally received ice from the Rhone ice dome. By reconstructing and comparing the LGM histories of the Ticino-Toce, Reuss, and Rhine glacier systems we take a step towards a better understanding of lateral and longitudinal differences in LGM glacier fluctuations across time. Underpinned by 41 10Be and 36Cl exposure dates we establish a first glacier chronology of the LGM Ticino-Toce glacier on the Alpine foreland in northern Italy.
We find Ticino-Toce glacier to have reached its greatest LGM extent at 25.0 ± 0.9 ka followed by a 5000 year long period of frequent but likely minor glacier fluctuations during which a number of closely spaced lateral and frontal moraines were deposited. Our results provide evidence that the piedmont lobe of the LGM Ticino- Toce glacier was approximately 200 km² bigger than proposed in earlier reconstructions. Thus, substantial parts of the Verbano morainic amphitheatre must have been reshaped during the LGM advance(s). A short-lived late LGM readvance at 19.7 ± 1.1 ka completed the LGM maximum cycle. Upstream exposure and radiocarbon dates indicate, that the Ticino-Toce glacier thereafter decayed rapidly. For north of the Alpine divide, we present an extensive data set of new exposure ages, radiocarbon dates, and high-resolution geomorphological mapping from the Reuss and Rhine glacier sites. This allows a reframing of previously published dates and permits close-up comparison of the timing of ice margin fluctuations in the two systems. The LGM Rhine glacier terminated as a more than 100 km wide piedmont lobe on the northeastern Swiss and southwestern German Alpine foreland. A prominent, largely continuous set of frontal moraines was thereby built up (outer Schaffhausen ice margins). Results from dating of outwash deposits of this and an earlier study chronologically frame the advance to and reaching of the Rhine glacier's LGM maximum position to 26-22 ka. The Reuss glacier system to the west, flowed out of a distinctly smaller catchment and due to topographic constraints did not develop into a broad piedmont lobe. A recent luminescence study, found that the eastern of the several narrow Reuss glacier arms, the Bünz-Reuss lobe, reached its LGM maximum by 25/24 ± 2 ka. Using surface exposure dating, the glacier's withdrawal from the corresponding frontal moraines of Untertannwald and Mellingen is dated to 22 ± 1 ka and 21 ± 1 ka, respectively. In both Rhine and Reuss glacier systems, the LGM maximum advance was followed by oscillations of the glacier fronts and interrupted by distinct glacier stillstands. The last marked foreland stabilization of Rhine and Reuss glaciers occurred largely contemporaneous during late LGM readvances to the Stein am Rhein and Bremgarten positions just after 20.6 ± 1.7 ka and 20.8 ± 1.3 ka, respectively.
New data from the Central Alps suggest in phase maxima, readvances, and decays of LGM glaciers. The consistency between northward and southward draining glaciers is striking given the distinctly different topographic preconditions (size, hypsometry, run out path) and the discussed preferential southern precipitation delivery pathways during the LGM. These exciting findings lead us to the hypothesis that the high-elevation accumulation areas in the Central Alps received sufficient precipitation throughout the LGM period. On the contrary, differences in LGM glacier behaviour of Central Alpine and Eastern Alpine glacier systems are marked when comparing chronologies on the southern side of the Alps. Apparent variations may point to a pronounced west-east gradient as a reaction to shifting precipitation centres across the LGM period. Complementing glacier chronologies, the subglacial record of past glaciations can yield fascinating insights into ice flow dynamics. Where subglacial landforms were streamlined at the ice-bed interface, the opportunity for reconstructing ice flow patterns arises. Using high-resolution elevation data, we did extensive mapping and present a first digital inventory of subglacial landforms on the foreland of the former Rhine piedmont lobe. The dataset comprises more than 2500 drumlins, glacial lineations, and subglacial ribs. Drumlins are by far the most common feature on the Rhine foreland. The typical Rhine glacier drumlin is 330 m long, 150 m wide, 14 m high and is located within one of seven drumlin fields that are arrayed in a semi-circular pattern between the Stein am Rhein ice margin and the basin of Lake Constance. Based on location in respect to mapped ice margins (Schaffhausen and Stein am Rhein ice margins), parallel conformity, and morphological similarity to neighbouring features, drumlins and glacial lineations are assigned to two flowsets. We interpret the Schaffhausen and Stein am Rhein flowsets to have formed during the advances to and while active ice stabilized at the LGM maximum and LGM readvance positions, respectively. Local ice flow directions during the LGM maximum were visually assembled on the basis of the Schaffhausen flowset. A continuous flow field of the late LGM Rhine glacier was reconstructed from the comprehensive Stein am Rhein flowset (>2400 streamlined bedforms) using a recently published kriging interpolation approach from Ng and Hughes (2019). Both qualitative and quantitative reconstructions of LGM flow patterns mimic the radial flow of Rhine glacier ice that emerged from its confined valley outlet and fanned out to the margins. Clusters of highly elongated subglacial features point to fast ice flow in major foreland troughs coherent with results of recent modelling studies. Basal ice flow during the late LGM Stein am Rhein readvance exhibited converging and diverging flow lines that underline the overall and unequivocal impact of topography on glacier flow on the foreland.
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Examiner : Ivy-Ochs, Susan
Examiner : Willett, Sean
Examiner : Synal, Hans-Arno
Examiner : Monegato, Giovanni
Examiner : Hein, Andrew S.
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ETH Zurich
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Subject
Glacial geomorphology; Quaternary glaciations; Last Glacial Maximum (LGM); European Alps; Glacial geochronology; Cosmogenic nuclide dating; 10Be exposure dating; 36Cl exposure dating
Organisational unit
08619 - Labor für Ionenstrahlphysik (LIP) / Laboratory of Ion Beam Physics (LIP)
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Funding
175794 - Deciphering the timing and dynamics of glacier advances in the Alps during the Last Glacial Maximum with cosmogenic nuclide dating: a north-south perspective (SNF)
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