Spatial and temporal variations in rockwall erosion rates derived from cosmogenic 10Be in medial moraines at five valley glaciers around Pigne d'Arolla, Switzerland

Rockwall erosion in high-alpine glacial environments varies both temporally and spatially. Where rockwalls flank glaciers, changes in debris supply and supraglacial cover will modify ice ablation. Yet, quantifying spatiotemporal patterns in erosion across deglaciating rockwalls is not trivial. At five nearby valley glaciers around Pigne d'Arolla in Switzerland, we derived apparent rockwall erosion rates using 10Be cosmogenic nuclide concentrations ([10Be]) in medial moraine debris. Systematic downglacier sampling of six medial moraines that receive debris from rockwalls with differing orientation, slope, and deglaciation histories enabled us to assess rockwall erosion through time and to investigate how distinct spatial source rockwall morphology may express itself in medial moraine [10Be] records. Our dataset combines 24 new samples from medial moraines of Glacier du Brenay, Glacier de Cheilon, Glacier de Pièce, and Glacier de Tsijiore Nouve with 15 published samples from Glacier d'Otemma. For each sample, we simulated the glacial debris transport using a simple debris particle trajectory model to approximate the time of debris erosion and to correct the measured [10Be] for post-depositional 10Be accumulation. Our derived apparent rockwall erosion rates range between ∼ 0.6 and 10.0 mm yr−1. Whereas the longest downglacier [10Be] record presumably reaches back to the end of the Little Ice Age and suggests a systematic increase in rockwall erosion rates over the last ∼ 200 years, the shorter records only cover the last ∼ 100 years from the recent deglaciation period and indicate temporally more stable erosion rates. For the estimated time of debris erosion, ice cover changes across most source rockwalls were small, suggesting that our records are largely unaffected by the contribution of recently deglaciated bedrock of possibly different [10Be], but admixture of subglacially derived debris cannot be excluded at every site. Comparing our sites suggests that apparent rockwall erosion rates are higher where rockwalls are steep and north-facing, indicating a potential slope and temperature control on rockwall erosion around Pigne d'Arolla

Temporal evolution of headwall erosion rates derived from cosmogenic nuclide concentrations in the medial moraines of Glacier d'Otemma, Switzerland

Climate change affects the stability and erosion of high‐alpine rock walls above glaciers (headwalls) that deliver debris to glacier surfaces. Since supraglacial debris in the ablation zone alters the melt behaviour of the underlying ice, the responses of debris‐covered glaciers and of headwalls to climate change may be coupled. In this study, we analyse the beryllium‐10 (10Be)‐cosmogenic nuclide concentration history of glacial headwalls delivering debris to the Glacier d'Otemma in Switzerland. By systematic downglacier‐profile‐sampling of two parallel medial moraines, we assess changes in headwall erosion through time for small, well‐defined debris source areas. We compute apparent headwall erosion rates from 10Be concentrations ([10Be]), measured in 15 amalgamated medial moraine debris samples. To estimate both the additional 10Be production during glacial debris transport and the age of our samples we combine our field‐based data with a simple model that simulates downglacier debris trajectories. Furthermore, we evaluate additional grain size fractions for eight samples to test for stochastic mass wasting effects on [10Be]. Our results indicate that [10Be] along the medial moraines vary systematically with time and consistently for different grain sizes. [10Be] are higher for older debris, closer to the glacier terminus, and lower for younger debris, closer to the glacier head. Computed apparent headwall erosion rates vary between ~0.6 and 10.8 mm yr−1, increasing over a maximum time span of ~200 years towards the present. As ice cover retreats, newly exposed headwall surfaces may become susceptible to enhanced weathering and erosion, expand to lower elevations, and contribute formerly shielded bedrock of likely different [10Be]. Hence, we suggest that recently lower [10Be] reflect the deglaciation of the debris source areas since the end of the Little Ice Age

Raumzeitliche Entwicklung von Erosionsraten an Gletscher-Stirnwänden in glazialen Landschaften in den Schweizer Alpen

High-alpine landscapes are exposed to pronounced temperature rise due to climate change. In particular, glacial and periglacial dynamics such as glacier retreat and permafrost thaw are accelerating, affecting the stability of rockwalls and posing hazards to the alpine environment. With recent deglaciation, rockfall activity and the production of debris are observed to increase. As the erosion of so-called headwalls above glaciers and associated debris deposition onto the ice surface increase, a thick supraglacial debris cover can form, modifying glacial mass balance and potentially delaying glacier retreat. Yet, debris supply rates and their changes vary spatially. To predict the evolution of glacial landscapes with climate change therefore requires to temporally and spatially examine rockwall erosion and potential temperature-related patterns. However, in situ measurements of rockwall erosion rates, specifically at headwalls, are rare as they are potentially dangerous in the difficult-to-access terrain, and the few existing records are often based on short-term monitoring from the last few decades. This thesis examines spatiotemporal records of headwall erosion in the Swiss Alps on longer terms to study glacial landscape dynamics with climate change. Headwall erosion rates were quantified by concentrations of the in situ-produced cosmogenic nuclide 10Be in medial moraine debris, obtained by interval-sampling along longitudinal moraine profiles. In a first small-scale case study, the archive function of medial moraines and temporal evolution of headwall erosion are studied in detail. Two medial moraine records were combined with a simple glacier transport model to account for the additional accumulation of 10Be during post-depositional downglacier debris transport and to assess the time of headwall erosion. Systematically varying 10Be concentrations over approximately the last 200 years indicate an increase in headwall erosion from the end of the Little Ice Age towards the following deglaciation period. This trend is reflected by different debris grain size fractions, implying that the records are unaffected by episodic large-scale erosion. In a follow-up study on a small mountain massif, temporal and spatial patterns of rockwall erosion are examined for five nearby glaciers and in relation to spatially distinct rockwall morphology. As an extension of the first study, ice cover changes across each debris source area were quantified to assess the potential contribution of recently deglaciated bedrock of possibly low 10Be concentrations, which seems to be low at most sites. Compared to the data from the first study, temporally constant 10Be concentrations over the last 100 years imply more stable rockwall erosion throughout the deglaciation period. At the same time, rockwall erosion rates are higher at steeper north faces in the massif. In a final, still ongoing study on a large-scale glacier catchment, two high-resolution medial moraine records are combined for the first time with paired in situ 14C/10Be analysis to resolve erosional landscape transience in more detail. Preliminary analyses suggest that conditions that caused an apparent pulse in erosion in one of two major debris source areas may be minor in the other due to differing headwall deglaciation histories. Yet, such rapid changes in transient landscapes, debris supply from long-time exposed or recently deglaciated surfaces, and post-depositional 10Be accumulation pose methodological challenges to derive rockwall erosion rates directly from measured medial moraine 10Be concentrations and require debris particle tracing in glacial landscapes and future research. Eventually, the studies of this thesis demonstrate that 10Be concentrations along medial moraines provide systematic results in landscapes that typically erode stochastically. Headwall erosion seems to be controlled by slope and temperature, and to accelerate at the transition of the Little Ice Age to deglaciation before stabilizing again - both observations that ultimately highlight the climate sensitivity of glacial landscapes.Hochalpine Landschaften sind aufgrund des Klimawandels einem starken Temperaturanstieg ausgesetzt, wobei sich insbesondere glaziale und periglaziale Dynamiken beschleunigen, wie der Rückzug der Gletscher und das Auftauen des Permafrosts. Diese Veränderungen beein-trächtigen unter anderem auch die Stabilität von Felswänden und stellen eine Gefahr für die alpine Umwelt dar. Beobachtungen deuten darauf hin, dass Steinschläge und Felsstürze sowie die Produktion von Schutt mit dem rezenten Eisrückgang im Hochgebirge zunehmen. Nimmt die Erosion sogenannter Stirnwände am Kopf von Gletschern zu und steigt die damit verbundene Schuttablagerung auf der Eisoberfläche an, kann sich eine dicke supraglaziale Schuttdecke bilden, die die Massenbilanz der Gletscher verändert und den Gletscherrückzug möglicherweise verzögert. Allerdings unterscheiden sich die Raten des Schutteintrags und deren Veränderungen räumlich. Prognosen über die Entwicklung glazialer Landschaften im Zuge des Klimawandels erfordern daher sowohl zeitliche als auch räumliche Untersuchungen der Erosion alpiner Felswände sowie potenzieller temperaturbedingter Muster. Bisher gibt es jedoch nur wenige in situ Messungen von Erosionsraten an Felswänden und insbesondere an Gletscher-Stirnwänden, da diese in dem schwer zugänglichen Gelände potenziell gefährlich sind. Außerdem beruhen die wenigen vorhandenen Datensätze häufig nur auf Kurzzeitbeob-achtungen der letzten Jahrzehnte. In der vorliegenden Doktorarbeit werden raumzeitliche Daten zur Erosion von Gletscher-Stirnwänden in den Schweizer Alpen über längere Zeiträume analysiert, um die Dynamiken glazialer Landschaften in Zeiten des Klimawandels näher zu untersuchen. Erosionsraten der Stirnwände wurden dabei anhand von Konzentrationen des in situ-produzierten kosmogenen Nuklids 10Be in Mittelmoränenschutt quantifiziert, wofür Intervall-Proben entlang von Längs-profilen der Moränen genommen wurden. In einer ersten kleinräumigen Fallstudie werden zunächst die Archivfunktion von Mittelmoränen sowie die zeitliche Entwicklung der Stirnwanderosion im Detail untersucht. Hierfür wurden zwei Mittelmoränen-Datensätze mit einem einfachen Gletschertransportmodell kombiniert, um die zusätzliche 10Be-Akkumulation in Schutt nach seiner Ablagerung und während seines Transports gletscherabwärts sowie den Zeitraum der Stirnwanderosion abzuschätzen. Systematisch variierende 10Be-Konzentrationen über die letzten 200 Jahre deuten hierbei auf eine Zunahme der Stirnwanderosion vom Ende der Kleinen Eiszeit in die sich anschließende Phase des Gletscherschwundes hin. Dieser Trend spiegelt sich in verschiedenen Korngrößenfraktionen des Schutts wider, was darauf hindeutet, dass die Datensätze nicht durch episodische, erosive Großereignisse verzerrt werden. In einer Folgestudie an einem kleinen Gebirgsmassiv werden anschließend sowohl zeitliche als auch räumliche Muster der Felswanderosion an fünf nahe gelegenen Gletschern untersucht und mit räumlich unterschiedlichen Fels-Morphologien in Beziehung gesetzt. Als Erweiterung der ersten Studie wurden zudem Veränderungen in der Eisbedeckung jedes Schuttquellengebiets quantifiziert, um den eventuellen Eintrag von Festgestein abzuschätzen, das erst kürzlich eisfrei wurde und niedrigere 10Be-Konzentrationen aufweisen könnte. An den meisten Standorten wurde dieser Eintrag als gering eingestuft. Im Vergleich zu den Datensätzen der ersten Studie verweisen zeitlich stabilere 10Be-Konzentrationen über die letzten 100 Jahre auf konstantere Felswanderosion während Zeiten des Gletscherschwundes hin. Gleichzeitig erscheinen die Erosionsraten für steilere Nordwände des Massivs höher. Abschließend werden in einer noch laufenden Studie an einem großflächigeren Gletscher Einzugsgebiet zwei hochauflösende Mittelmoränen-Datensätze erstmals mit in situ 14C/10Be Analysen kombiniert, um die Erosion in Landschaften, die sich in einem Übergangzustand befinden, noch detaillierter zu erfassen. Vorläufige Analysen deuten darauf hin, dass Bedingungen, die einen offensichtlichen Erosionsschub in einem der beiden großen Schuttquellengebiete verursachten, in dem anderen Gebiet kaum oder nur in geringem Maße vorhanden sind. Dies könnte auf einen unterschiedlich stark ausgeprägten Eisrückgang an den jeweiligen Stirnwänden zurückzuführen sein. Solche rapiden Veränderungen in transienten Landschaften, der Schutteintrag von längerfristig exponierten oder erst kürzlich eisfrei gewordenen Oberflächen und die zusätzliche 10Be-Akkumulation in Schutt nach seiner Ablagerung stellen jedoch die direkte Ableitung von Felswand-Erosionsraten aus 10Be-Konzentrationen in Mittelmoränenschutt vor methodische Herausforderungen, die die Verfolgung von Schuttpartikeln in glazialen Landschaften erfordern und weiterer Erforschung bedürfen. Schließlich zeigen die Untersuchungen dieser Doktorarbeit, dass 10Be-Konzentrationen entlang von Mittelmoränen systematische Ergebnisse für Landschaften liefern, die für gewöhnlich stochastisch erodieren. Die Stirnwanderosion scheint von der Hangneigung und Temperatur abzuhängen und sich zunächst während der Übergangsphase von der Kleinen Eiszeit in die anschließende Gletscherschwund-Phase zu beschleunigen, bevor sie sich wieder stabilisiert - beides Beobachtungen, die letztlich die Klimasensitivität von Gletscherlandschaften verdeutlichen

Temporal evolution of headwall erosion rates derived from cosmogenic nuclide concentrations in the medial moraines of Glacier d'Otemma, Switzerland

Climate change affects the stability and erosion of high‐alpine rock walls above glaciers (headwalls) that deliver debris to glacier surfaces. Since supraglacial debris in the ablation zone alters the melt behaviour of the underlying ice, the responses of debris‐covered glaciers and of headwalls to climate change may be coupled. In this study, we analyse the beryllium‐10 (10Be)‐cosmogenic nuclide concentration history of glacial headwalls delivering debris to the Glacier d'Otemma in Switzerland. By systematic downglacier‐profile‐sampling of two parallel medial moraines, we assess changes in headwall erosion through time for small, well‐defined debris source areas. We compute apparent headwall erosion rates from 10Be concentrations ([10Be]), measured in 15 amalgamated medial moraine debris samples. To estimate both the additional 10Be production during glacial debris transport and the age of our samples we combine our field‐based data with a simple model that simulates downglacier debris trajectories. Furthermore, we evaluate additional grain size fractions for eight samples to test for stochastic mass wasting effects on [10Be]. Our results indicate that [10Be] along the medial moraines vary systematically with time and consistently for different grain sizes. [10Be] are higher for older debris, closer to the glacier terminus, and lower for younger debris, closer to the glacier head. Computed apparent headwall erosion rates vary between ~0.6 and 10.8 mm yr−1, increasing over a maximum time span of ~200 years towards the present. As ice cover retreats, newly exposed headwall surfaces may become susceptible to enhanced weathering and erosion, expand to lower elevations, and contribute formerly shielded bedrock of likely different [10Be]. Hence, we suggest that recently lower [10Be] reflect the deglaciation of the debris source areas since the end of the Little Ice Age.In glacial landscapes, systematic downglacier‐sampling of medial moraine debris holds the potential to assess changes in headwall erosion through time. Cosmogenic beryllium‐10 (10Be) concentrations within the medial moraines of Glacier d'Otemma, Switzerland, broadly increase downglacier and translate into increasing headwall erosion rates towards the present. These trends may reflect processes associated with the exposure of new bedrock surfaces across recently deglaciating source headwalls.European Research Council (ERC) H2020‐EU.1.1.https://doi.org/10.5880/GFZ.3.3.2021.00