Audrey Margirier

The storage and release of sediment from glacierised catchments is an important process in mountain landscape evolution, and yet sediment transport pathways and residence times within glaciers remain poorly constrained. We quantified headwall erosion rates and englacial sediment transport and storage times in the Mer de Glace catchment in the Mont Blanc massif, French Alps, during deglaciation through the Holocene (11.7 ka to present). Englacial sediment transport and storage times were constrained using luminescence rock surface burial ages of granitic clasts sampled along the central flow line of the Mer de Glace ablation area. We also used luminescence rock surface exposure dating and terrestrial cosmogenic nuclide (¹⁰Be) measurements to constrain headwall erosion rates for this catchment. These headwall erosion and sediment transport data were compared with simulated erosion, sediment trajectories and transport rates derived from the glacier model iSOSIA. Measured headwall erosion rates were ~0.1–5 mm a⁻¹ and are consistent with other estimates from the Mont Blanc massif. Luminescence rock surface burial ages ranged from ~0.6 to ~6.7 ka and clustered into distinct age populations at ~0.8 ka, ~1.5 ka, ~2.2 ka, and ~6.7 ka. The youngest age population is consistent with continuous englacial transport times predicted by the glacier modelling and observations of present-day glacier surface velocity, whereas the older age clusters indicate prolonged sediment storage within the catchment. Comparison of our results with results from Miage Glacier, Italian Alps, shows that long-term sediment storage with durations exceeding 1 ka is common in steep alpine glacierised catchments, despite high erosion rates and active ice flow. Luminescence burial ages indicate that sediment can be stored during periods of glacier minima, then released during more active phases. Glacierised catchments therefore act as millennial-scale sediment reservoirs, introducing time lags between sediment production and downstream transport, that modulate climatic signals recorded in proglacial stratigraphy during deglaciation.

Rockfalls and other gravitational mass movements are expected to become more frequent under ongoing global warming in temperate and cold mountainous regions. Tropical mountain ranges that have experienced past climatic warming, such as the Rwenzori Mountains (Uganda), provide natural laboratories for studying slope stability under warm and humid conditions. However, although rockfalls are frequently observed in these environments, their controls and evolution through Holocene climatic changes remain poorly understood. We used ¹⁰ Be surface exposure dating to establish a chronology for seven rockfall deposits in the Rwenzori Mountains and assess the climatic conditions associated with slope destabilisation. The ¹⁰ Be ages range from 13.6 ± 0.9 ka to 1.0 ± 0.1 ka and cluster into three distinct periods: 11–9 ka, 7–5 ka and ~2 ka. These periods correspond to local temperature fluctuations, suggesting that rockfalls occurred episodically in response to Holocene temperature variations. Early Holocene rockfall activity (~11–9 ka) likely reflects enhanced mechanical weathering and fracture propagation because of glacier retreat and freeze–thaw cycles. In contrast, mid‐Holocene (7–5 ka) and late Holocene (~2 ka) rockfall activities coincide with warmer conditions that promoted chemical and biological weathering, fracture propagation and slope destabilisation. These results highlight the role of temperature as a key climatic control on progressive rock failure mechanisms in the Rwenzori Mountains. Rising temperatures enhance chemical and biological weathering, promoting slope destabilisation and rockfall activity in tropical mountain environments.

The storage and release of sediment from glacierised catchments is an important process in mountain landscape evolution, and yet sediment transport pathways and residence times within glaciers remain poorly constrained. We quantified headwall erosion rates and englacial sediment transport and storage times in the Mer de Glace catchment in the Mont Blanc massif, French Alps, during deglaciation through the Holocene (11.7 ka to present). Englacial sediment transport and storage times were constrained using luminescence rock surface burial ages of granitic clasts sampled along the central flow line of the Mer de Glace ablation area. We also used luminescence rock surface exposure dating and terrestrial cosmogenic nuclide (¹⁰Be) measurements to constrain headwall erosion rates for this catchment. These headwall erosion and sediment transport data were compared with simulated erosion, sediment trajectories and transport rates derived from the glacier model iSOSIA. Measured headwall erosion rates were ~0.1–5 mm a⁻¹ and are consistent with other estimates from the Mont Blanc massif. Luminescence rock surface burial ages ranged from ~0.6 to ~6.7 ka and clustered into distinct age populations at ~0.8 ka, ~1.5 ka, ~2.2 ka, and ~6.7 ka. The youngest age population is consistent with continuous englacial transport times predicted by the glacier modelling and observations of present-day glacier surface velocity, whereas the older age clusters indicate prolonged sediment storage within the catchment. Comparison of our results with results from Miage Glacier, Italian Alps, shows that long-term sediment storage with durations exceeding 1 ka is common in steep alpine glacierised catchments, despite high erosion rates and active ice flow. Luminescence burial ages indicate that sediment can be stored during periods of glacier minima, then released during more active phases. Glacierised catchments therefore act as millennial-scale sediment reservoirs, introducing time lags between sediment production and downstream transport, that modulate climatic signals recorded in proglacial stratigraphy during deglaciation.

The storage and release of sediment from glacierised catchments is an important process in mountain landscape evolution, and yet sediment transport pathways and residence times within glaciers remain poorly constrained. We quantified headwall erosion rates and englacial sediment transport and storage times in the Mer de Glace catchment in the Mont Blanc massif, French Alps, during deglaciation through the Holocene (11.7 ka to present). Englacial sediment transport and storage times were constrained using luminescence rock surface burial ages of granitic clasts sampled along the central flow line of the Mer de Glace ablation area. We also used luminescence rock surface exposure dating and terrestrial cosmogenic nuclide (¹⁰Be) measurements to constrain headwall erosion rates for this catchment. These headwall erosion and sediment transport data were compared with simulated erosion, sediment trajectories and transport rates derived from the glacier model iSOSIA. Measured headwall erosion rates were ~0.1–5 mm a⁻¹ and are consistent with other estimates from the Mont Blanc massif. Luminescence rock surface burial ages ranged from ~0.6 to ~6.7 ka and clustered into distinct age populations at ~0.8 ka, ~1.5 ka, ~2.2 ka, and ~6.7 ka. The youngest age population is consistent with continuous englacial transport times predicted by the glacier modelling and observations of present-day glacier surface velocity, whereas the older age clusters indicate prolonged sediment storage within the catchment. Comparison of our results with results from Miage Glacier, Italian Alps, shows that long-term sediment storage with durations exceeding 1 ka is common in steep alpine glacierised catchments, despite high erosion rates and active ice flow. Luminescence burial ages indicate that sediment can be stored during periods of glacier minima, then released during more active phases. Glacierised catchments therefore act as millennial-scale sediment reservoirs, introducing time lags between sediment production and downstream transport, that modulate climatic signals recorded in proglacial stratigraphy during deglaciation.

The storage and release of sediment from glacierised catchments is an important process in mountain landscape evolution, and yet sediment transport pathways and residence times within glaciers remain poorly constrained. We quantified headwall erosion rates and englacial sediment transport and storage times in the Mer de Glace catchment in the Mont Blanc massif, French Alps, during deglaciation through the Holocene (11.7 ka to present). Englacial sediment transport and storage times were constrained using luminescence rock surface burial ages of granitic clasts sampled along the central flow line of the Mer de Glace ablation area. We also used luminescence rock surface exposure dating and terrestrial cosmogenic nuclide (¹⁰Be) measurements to constrain headwall erosion rates for this catchment. These headwall erosion and sediment transport data were compared with simulated erosion, sediment trajectories and transport rates derived from the glacier model iSOSIA. Measured headwall erosion rates were ~0.1–5 mm a⁻¹ and are consistent with other estimates from the Mont Blanc massif. Luminescence rock surface burial ages ranged from ~0.6 to ~6.7 ka and clustered into distinct age populations at ~0.8 ka, ~1.5 ka, ~2.2 ka, and ~6.7 ka. The youngest age population is consistent with continuous englacial transport times predicted by the glacier modelling and observations of present-day glacier surface velocity, whereas the older age clusters indicate prolonged sediment storage within the catchment. Comparison of our results with results from Miage Glacier, Italian Alps, shows that long-term sediment storage with durations exceeding 1 ka is common in steep alpine glacierised catchments, despite high erosion rates and active ice flow. Luminescence burial ages indicate that sediment can be stored during periods of glacier minima, then released during more active phases. Glacierised catchments therefore act as millennial-scale sediment reservoirs, introducing time lags between sediment production and downstream transport, that modulate climatic signals recorded in proglacial stratigraphy during deglaciation.

The storage and release of sediment from glacierised catchments is an important process in mountain landscape evolution, and yet sediment transport pathways and residence times within glaciers remain poorly constrained. We quantified headwall erosion rates and englacial sediment transport and storage times in the Mer de Glace catchment in the Mont Blanc massif, French Alps, during deglaciation through the Holocene (11.7 ka to present). Englacial sediment transport and storage times were constrained using luminescence rock surface burial ages of granitic clasts sampled along the central flow line of the Mer de Glace ablation area. We also used luminescence rock surface exposure dating and terrestrial cosmogenic nuclide (¹⁰Be) measurements to constrain headwall erosion rates for this catchment. These headwall erosion and sediment transport data were compared with simulated erosion, sediment trajectories and transport rates derived from the glacier model iSOSIA. Measured headwall erosion rates were ~0.1–5 mm a⁻¹ and are consistent with other estimates from the Mont Blanc massif. Luminescence rock surface burial ages ranged from ~0.6 to ~6.7 ka and clustered into distinct age populations at ~0.8 ka, ~1.5 ka, ~2.2 ka, and ~6.7 ka. The youngest age population is consistent with continuous englacial transport times predicted by the glacier modelling and observations of present-day glacier surface velocity, whereas the older age clusters indicate prolonged sediment storage within the catchment. Comparison of our results with results from Miage Glacier, Italian Alps, shows that long-term sediment storage with durations exceeding 1 ka is common in steep alpine glacierised catchments, despite high erosion rates and active ice flow. Luminescence burial ages indicate that sediment can be stored during periods of glacier minima, then released during more active phases. Glacierised catchments therefore act as millennial-scale sediment reservoirs, introducing time lags between sediment production and downstream transport, that modulate climatic signals recorded in proglacial stratigraphy during deglaciation.

Ma thèse se focalise sur l'étude des mécanismes qui ont conduit au soulèvement et à la construction du relief dans les Andes du Nord du Pérou. Dans cette région, la Cordillère Blanche forme les plus hauts sommets péruviens (> 6000 m) et constitue une anomalie à l'échelle des Andes. La morphologie de cette région des Andes est marquée par un pluton de forme atypique, allongé et à l'affleurement sur plus de 150 km. Ce pluton est bordé par une faille normale de plus de 200 km de long. La présence de cette faille normale majeure en contexte de subduction plane reste surprenante car ces zones de subduction planes semblent induire une augmentation du raccourcissement dans la plaque chevauchante. Mon travail a eu pour objectifs de caractériser les variations de l'état de contraintes régional, l'âge du soulèvement et de discuter les processus géodynamiques qui ont contribué à la formation du relief. Dans ce cadre, j'ai utilisé une approche pluridisciplinaire impliquant sur plusieurs échelles spatio-temporelles et comprenant à la fois de nouvelles données de terrain, leur analyse et leur modélisation.Mes données de microtectonique indiquent qu'il est possible de générer de l'extension au dessus d'une subduction plane à l'échelle régionale. Ces données sont en contradiction avec l'augmentation du raccourcissement classiquement attendue dans la plaque chevauchante. Mes nouvelles données de thermochronologie basse température et leur modélisation montrent une augmentation de l'exhumation induite par le soulèvement de la Cordillère Occidentale à 15 Ma. En les confrontant aux modèles précédents, je propose un soulèvement régional lié à l'aplatissement de la subduction et à la topographie dynamique associée.J'ai également étudié l'impact de l'arc Miocène sur le soulèvement à une échelle plus locale. Pour cela, j'ai compilé tous les âges de refroidissement du pluton disponibles dans la littérature. En parallèle, j'ai obtenu les premières données de profondeur de mise en place du batholite de la Cordillère Blanche. Cela m'a permis de proposer une structure du batholite en sills empilés puis basculés vers l'est. De plus, la modélisation des variations spatio-temporelle des taux d'érosion à partir des données de thermochronologie basse température indique une augmentation importante des taux d'érosion dans la Cordillère Blanche à partir de 2 Ma. L'arc Miocène ne semble donc pas contribuer significativement au soulèvement malgré sa probable contribution à l'épaississement de la lithosphère. En revanche, l'érosion glaciaire récente semble contribuer fortement à l'exhumation de la Cordillère Blanche et au basculement du batholite.Dans la dernière partie de ma thèse, pour quantifier l'importance de l'érosion dans la création du relief et le soulèvement, j'ai modélisé l'évolution du paysage de la région (FastScape). Mes modélisations numériques démontrent le rôle majeur de l'érosion et du rebond flexural associé dans la création du relief et les taux de soulèvement. Pour finir, basée sur les données de la littérature et celles apportées par mon travail de thèse, je propose un nouveau modèle pour expliquer la faille normale de la Cordillère Blanche dans son contexte régional. Ce modèle implique une faille normale d'extrado et l'érosion importante du mur de la faille.

The scarcity of terrestrial temperature proxies has been a major challenge in the reconstruction of continental climate evolution throughout the Last Glacial Maximum (LGM) and the Pleistocene-Holocene transition. Understanding such extreme climatic conditions and major system shifts in Earth’s history is paramount for constraining climate sensitivity and predicting future climate evolution in the light of rising greenhouse gas concentrations.Our research aims to develop a globally applicable method for temperature sensing during this timescale using the low-temperature (i.e., 200–280 °C) thermoluminescence (TL) signal of near-surface bedrock feldspar which has been demonstrated to be sensitive to terrestrial surface air temperature fluctuations over geological timescales (Biswas et al., 2020). As such, palaeothermometry represents one of few available proxies for terrestrial temperature and can aid in quantifying the magnitude of rapid climate changes on a more local scale.While the theoretical feasibility of TL palaeothermometry has been demonstrated (Biswas et al., 2020), it still requires accurate validation on additional samples of well-constrained temperature history. Furthermore, the method has not yet been applied to a broad set of samples for temperature reconstruction purposes.Our contribution aims to close this knowledge gap by benchmarking recent methodological improvements against samples from borehole sites located in Germany and Japan. We further present first surface air temperature reconstructions at a number of study sites, which we intend to use to constrain the evolution of altitudinal and latitudinal temperature gradients since the LGM. We show that TL palaeothermometry can be used to retrieve accurate rock and surface air temperatures and may now be more routinely applied. ReferencesBiswas, R.H., Herman, F., King, G.E., Lehmann, B., Singhvi, A.K., 2020. Surface paleothermometry using low-temperature thermoluminescence of feldspar. Clim. Past 16, 2075-2093.