SSP3.11

Glaciers are strong agents of erosion and play a key role in the evolution of mountain ranges. In order to improve our understanding of the influence of glacial erosion dynamics on landscape evolution and mountain building, it is essential to quantitatively constrain glacial erosion rates across multiple topographic and climatic settings.

In situ cosmogenic ¹⁰Be concentrations measured in river sediments have been widely used over the last twenty years to infer denudation rates integrated at the catchment scale. This approach was mainly applied to fluvial settings because in this case, the ¹⁰Be concentration of detrital sediments is a simple function of denudation. In regions covered by glaciers, river sediments result from a mixture of material produced in the pure fluvial domain and sediments produced by glacier erosion. The ¹⁰Be concentration measured in such settings thus results from the mixture of these two sources. Here, we use a simple mass conservation approach to estimate pure glacial erosion rates from the ¹⁰Be concentration measured in watersheds combining glacial and fluvial domains. In practice, we first established an empirical power-law linking denudation rates to the mean slope of non-glaciated catchments. For each partially glaciated catchment, this law was used to constrain the pure fluvial ¹⁰Be end-member using slopes derived from a DEM. Finally, this input was used to compute the pure glacial erosion rate required to satisfy the ¹⁰Be concentration measured in rivers. This new approach was applied to 2 different datasets:

• Present-day glacier erosion in the Alps. We apply this approach to determine the erosion of modern glaciers across the entire Alps. We used previously published ¹⁰Be concentration measured in river sediments covering partially glaciated watersheds. The fluvial denudation power law was constrained from 148 fluvial – glacier free catchments. We then selected 11 watersheds with glaciers bigger than 5 km² and a glacial cover of at least 5% of their total surface. The so-obtained glacial erosion rates from these 11 watersheds range from 0.2 to 1.5 mm.yr^-1. Finally, we compare those values to satellite-derived glaciers' sliding velocity which is thought to be the main factor controlling glacial erosion rates.

• Paleo-erosion in the Var (Southern Alps) setting over the last 75 ka. We apply the same approach to the Var catchment (Southern French Alps) to estimate past glacial erosion rates over the last 75 ka (Mariotti et al., 2021). This basin has been deglaciated since the Holocene and ¹⁰Be modern denudation rates were estimated across 9 sub-basins (Mariotti et al., 2019) providing the required dataset to estimate the local fluvial denudation power law. ¹⁰Be concentrations were measured in two 75 ka sedimentary cores drilled in the Mediterranean Sea when the Var catchment was previously glaciated (Mariotti et al., 2021). Our findings show that during the LGM, the pure glacial erosion rates were 3 times higher (1.5 +/- 1 mm.yr^-1) than during MIS 3-4 (0.4 +/- 0.5 mm.yr^-1). This suggests a nonlinear forcing of climate on glacial erosion, mainly controlled by the interplay between glacier velocity, climate, and basin topography.

How to cite: Charreau, J., Mariotti, A., Blard, P.-H., Breton, S., and Toucanne, S.: Glacial erosion rates across the Alps derived from in situ 10Be in river sediments, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8375, https://doi.org/10.5194/egusphere-egu22-8375, 2022.

The extent and distribution of glaciers on the Swiss Plateau during the Last Glacial Maximum (LGM) can be determined from the geological record. However, similar reconstructions for the glaciations that preceded the LGM are far more difficult to be made due to the inaccessibility of suitable sedimentary records. Here, we explored Quaternary sediments which were deposited during the MIS 8 glaciation at least 250 ka ago, and which were recovered in a drilling that was sunk into an overdeepening W of Bern (Switzerland). We analyzed the sediment-bulk chemical composition of the deposits to investigate the supply of the material to the area by either the Aare Glacier or the Valais Glacier. The potential confluence of these two glaciers in the Bern area makes this location ideal for such an analysis. We determined the sediment-bulk chemical signal of the various lithological units in the central Swiss Alps where the glaciers originated, which we used as endmembers for our provenance analysis. We then combined the results of this fingerprinting study with the existing information on the sedimentary succession and its deposition history. This sedimentary suite is composed of two sequences A (lower) and B (upper), both of which comprise a basal till that is overlain by lacustrine sediments. The till at the base of Sequence A was formed by the Aare Glacier. The overlying lacustrine deposits of an ice-contact lake were mainly supplied by the Aare Glacier. The basal till in Sequence B was also formed by the Aare Glacier. The provenance signal points towards a simultaneous material supply by both the Aare and the Valais Glaciers during the formation of the lacustrine sediments in Sequence B. We use these findings for a paleogeographic reconstruction. During the time when Sequence A and the basal till in Sequence B were deposited, the Aare Glacier dominated the area. This strongly contrasts with the situation during the LGM, when the Aare Glacier was deflected by the Valais Glacier towards the NE. Probably, the Valais Glacier was less extensive during MIS 8. However, part of the lacustrine sediments deposited within Sequence B could only have been supplied by the Valais Glacier, indicating that the glacier did not cover the study area, yet had been in close proximity to the study area. We thus postulate that during the deposition of Sequence B both the Aare Glacier and the Valais Glacier were connected to this lake that had formed at the foot of these glaciers. These glaciers potentially also dammed this lake. In conclusion, we could outline a detailed scenario of sediment supply to the investigated overdeepening during the MIS 8 glacial period based on the provenance and sedimentological data, and that glaciers were arranged in a different way than during the LGM.

How to cite: Schwenk, M., Schlunegger, F., Stutenbecker, L., Bandou, D., and Schläfli, P.: The provenance of the sediment in an overdeepening and its implications for the distribution of glacier ice in the Bern area (CH), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4736, https://doi.org/10.5194/egusphere-egu22-4736, 2022.

It is undoubtedly the observation of modern European Alps glaciers along with their erosive, transportational and depositional actions shaping the landscapes that first led scientists to reveal the existence of a ‘past ice age’ during which glaciers not so long ago covered northern Europe and North America. Based on similar observations, evidence for much more ancient (Permian, ca. 300 Ma) glaciers were simultaneously discovered in Wales. Since then, others glacial episodes punctuating the Earth history were successively discovered (Eyles, 2008), the oldest of which being the ‘Barberton diamictites of South Africa, dated back to 3.5 Ga (deWit & Furnes, 2016).

The modern glaciation may serve as the basis to decipher past ice ages and associated climate dynamics that remain obscure as growing evidences indicate that these ancient glacial epochs share similarities, but also discrepancies, with the Cenozoic one, in term of tempos (ice ages encompassing periods of contraction-dilatation of ice) or forcing parameters (e.g., Ghienne et al., 2014; Kochhann et al., 2021; Montanez, 2021). Past ice dynamics may therefore be unraveled by the integration in time and space of punctual glacial processes whose interpretation is made on the basis of their modern and recent equivalent. The glacio-isostatic adjustment, for example, is a process well-known for the ultimate glacial cycle, as marked by widespread evidences such as the raised beaches of Scandinavia and Canada. Given its time span of completion however (a few tens of thousands of years), this process is hardly decipherable for ancient epochs, for which temporal resolution is intrinsically too low, therefore hindering our ability to constrain ancient ice-sheet dynamics. A stratigraphic model built upon recent glacial strata has been successfully extrapolated to both the Ordovician and Carboniferous-Permian ice ages, providing clues about pattern of glacial retreat of postglacial relative sea level changes. Similarly, the geomorphic and stratigraphic imprints of fjords nowadays dissecting high-latitude continental margins were used as an analog that permitted the characterization of fossil fjords and associated glacial dynamics tied to the Carboniferous-Permian glaciation in Namibia (Dietrich et al., 2021). On the other hand, strata related to ancient ice ages may provide novel insights into the understanding of modern glacial processes that remain obscure by granting access to sectors otherwise ‘locked’ such as the ice-substrate interface or sediments nowadays on continental margins, covered in great water depths and buried under younger sediments. Finally, the window into deep and long times offered by sedimentary basins hosting deposits tied to ancient glacial epochs may provide clues on the impact of repeated or long-lasting glaciation on the earth surface evolution (Jaeger & Koppes, 2016). The presentation will briefly review how mutual benefits can be obtained from combining the investigation of ancient and recent glacial deposits (Dowdeswell et al., 2019).

How to cite: Dietrich, P.: A history of glaciations: the perks of combining recent and ancient morphostratigraphic archives, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12523, https://doi.org/10.5194/egusphere-egu22-12523, 2022.

When preserved from deep time glaciation, subglacially striated bedrock surfaces allow the interpretation of past ice characteristics that are often elusive from the study of sediments alone. Salient amongst these is the thermal regime, which has a profound influence upon ice behaviour and consequent sediment erosion, transport and deposition. Typically, striated bedrock surfaces are linked to ice at its pressure-temperature melting point, indicating a locally warm-based thermal regime. Conversely, a cold-based thermal regime is defined by ice frozen to the substrate and is linked to minimal erosion. Cold-based erosional forms have been identified in Antarctica but their recognition is next to impossible if imprinted upon a surface previously or subsequently affected by warm-based erosion (e.g. striation). In the ancient record this is especially problematic, as it is typically only through the recognition of characteristic warm-based features that a surface can be confirmed as subglacial at all. Consequently, it is likely that there is an observational bias in the rock record toward warm-based over cold-based ice. This study, through careful geomorphologic analysis of unusually well preserved striated surfaces of the North China Craton from the Ediacaran Period (c. 635 – 540 Ma), presents rare examples that record dominant cold-based and more limited warm-based erosion on the same subglacial surface. It is hoped that this approach may benefit other workers interested in identifying cold-based as well as the more obvious warm-based subglacial conditions from the record of deep time glaciation.

How to cite: Vandyk, T., Chen, X., Wang, Y., Yang, Z., Kuang, H., Liu, Y., Wu, G., Li, M., Davies, B. J., Shields, G. A., and Le Heron, D. P.: Recognising cold-based glaciation in the rock record: striated bedrock surfaces of the > 540 million year old Luoquan Formation of China, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-532, https://doi.org/10.5194/egusphere-egu22-532, 2022.

We investigated the formation mechanism of tunnel valleys, by producing 3D models of bedrock topography using gravimetry. We obtained the cross-sectional geometry of tunnel valleys in the Swiss foreland, near Bern. The combination of information about the densities of the sedimentary fill and of the bedrock together with borehole data and gravity surveys along profiles across the valleys served as input for our 3D gravity modelling software, Prisma. This finally allowed us to model the gravity effect of the Quaternary fill of the overdeepenings and to produce cross-sectional geometries of the overdeepenings. We focused on two sections situated in the Gürbe valley and in the Aare valley. We determined a density of 2’500 kg/m3 for the Upper Marine Molasse bedrock, and with Prisma we obtained a bulk density of kg/m3 for the Quaternary infill. Our gravity surveys across the valleys yielded a maximum residual anomaly of -2.9 mGal for the Gürbe valley and -4.1 mGal for the Aare valley. The application of our Prisma model showed that these anomalies can be explained by Quaternary suites with a thickness of 160 m and 235 m for the infill of the Gürbe and Aare valleys, respectively. The high-resolution information about the cross-sectional geometry of the tunnel valley flanks, from the application of Prisma, allowed us to infer a two-step formation process of the overdeepened trough.  A first glaciation, during MIS 6 or before, would have deepened the trough. And a second glaciation, during the Last Glacial Maximum  (MIS 2), would have widened the valleys. We explain this pattern by the differences between the ice thicknesses of the LGM and MIS 6 glaciers and by the relatively low erodibility of the Upper Marine Molasse bedrock. The Molasse units indeed comprise tender and porous sandstones and offer a lower erosional resistance than the Quaternary infill, which consists of cohesive and thus competent glacio-lacustrine marls. This probably offered ideal conditions for the thick and thus erosive MIS 6 glaciers to erode deeply into the Molasse bedrock. In contrast, the lacustrine fill of this trough possibly prevented the thinner and thus less erosive LGM or MIS 2 glaciers to further incise the bedrock. The consequence was that erosion of the LGM glaciers mainly occurred on the lateral sides, thereby resulting in a widening of the tunnel valleys. Finally, we apply this approach to the remaining gravity profiles, to create a 3D model of the geometry of the overdeepening network near Bern.

How to cite: Bandou, D., Schlunegger, F., Kissling, E., Marti, U., Schwenk, M., Schläfli, P., Douillet, G., and Mair, D.: Geometry of overdeepenings obtained through three-dimensional gravity modelling, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5362, https://doi.org/10.5194/egusphere-egu22-5362, 2022.

The Greenland Ice Sheet (GrIS) is a key component of the global climate system. However, our current understanding of the spatio-temporal oscillations and landscape transformation of the GrIS margins since the last glacial cycle is still incomplete. This work aims to study the deglaciation in the Zackenberg Valley, Greenland, and the origin of the derived glacial landforms. In order to reconstruct the spatial extent and geometry of past glacial phases we carried out extensive fieldwork and high-detailed geomorphological mapping, together with cosmic-ray exposure (CRE) dating to samples from erosive and depositional glacial landforms. Erratic boulders dispersed across the summits suggest that Late Quaternary glaciers filled the valleys and fjords during periods of maximum ice expansion. As glacier thickness decreased, the Zackenberg glacier was confined in the interior of the main valley, leaving several lateral moraine ridges along the slopes. The deglaciation started by ~13.7-12.5 ka and accelerated paraglacial slope processes (e.g. solifluction). By ca. 10.5 ka, the last remnants of glacial ice disappeared from the lower sections of the valley. This deglaciation chronology broadly agrees with what is observed in other sites across Greenland.

How to cite: Garcia de Oteyza de Ciria, J. N., Oliva, M., Palacios, D., Fernández-Fernández, J. M., Schimmelpfennig, I., Andrés, N., Antoniades, D., Léanni, L., Jomelli, V., Rinterknecht, V., Lane, T., and Team, A.: Major deglaciation during the Late Glacial in coastal regions of Greenland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10224, https://doi.org/10.5194/egusphere-egu22-10224, 2022.