Evolution of the debris-covered Miage Glacier

<p>Glaciers in high-mountain regions typically exhibit a debris cover that moderates their response to climatic change. Here we present an integrated study that integrates long-term observations of debris-covered glacier mass balance, velocity, surface debris evolution and geomorphological changes (such as ponds and ice cliffs) of Miage Glacier, Italian Alps over the period 1952 – 2018. Analysis of the evolution of Miage Glacier highlighted a reduction in glacier activity associated with a period of sustained negative mass balance (-0.86 ± 0.27 metres per year water equivalent [m w.e. a<sup>-1</sup>]) and a substantial reduction in surface velocity (-46%). Ice mass loss of Miage Glacier was quantified using satellite imagery and derived digital elevation models (DEMs) applying the geodetic approach over a 28-year time period, 1990 – 2018. Temporal analysis highlighted an increase in surface lowering rates from 2012 – 2018. Further, the increase in debris-cover extent, supraglacial ponds and ice cliffs was evident since the 1990s. Supraglacial ponds and ice cliffs accounted for up to 8 times the magnitude of the average glacier surface lowering, whilst only covering 1.2 – 1.5% of the glacier area.</p><p>Ground-based photogrammetry and bathymetry surveys undertaken in 2017 and 2018 indicated the total volume of water storage at Miage Glacier increased by 46%, however, intermittent drainage events suggest this is highly variable over both seasonal and annual timescales. All ice cliffs underwent substantial vertical retreat up<sup></sup>to a maximum rate of -8.15 ma<sup>-1 </sup>resulting in ice loss of 39,569 m<sup>3</sup>. Thus, ice loss from supraglacial ponds and ice cliffs are important to account for and have the potential to substantially impact future glacier evolution.</p>

Quantifying and characterising contemporary rockfall supply to a debris-covered glacier catchment

<p>Supraglacial debris extent and thickness is an important control on the ablation rate of a debris-covered glacier. Debris is supplied to the surface of a debris-covered glacier through several pathways with the primary source of this debris originating from rockfall in both the accumulation area, where debris is transported englacially downglacier, and the upper ablation area, where debris remains in the supraglacial environment while transported downglacier. Current quantification of debris supply to debris-covered glaciers is limited to headwall erosion rates determined through the dating of headwall derived supraglacial debris using <sup>10</sup>Be concentrations, or estimations of these rates using a ratio of supraglacial debris flux to the headwall catchment area. To increase the knowledge of the contemporary short-term estimations of these processes, repeat LiDAR scans of debris-contributing slopes were acquired during a single ablation season in both July and September at Miage Glacier, Italy. An area of ~7.7 km<sup>2</sup> comprising > 1.8 billion 3D points was scanned per survey epoch, covering ~33% of the glacierised area of Miage Glacier. Sequential scans were co-registered using an iterative closest point adjustment algorithm within CloudCompare. Manual filtering was used to remove snow, artefacts, and the glacier surface from the raw point clouds. To ease processing, the rock walls were segmented both horizontally and vertically within the catchment. Change detection was carried out using the M3C2 algorithm at a projection scale of 0.3 m and point clouds representing areas of significant change within the segment were obtained using a distributed 95<sup>th</sup> percentile confidence interval. The DBSCAN clustering algorithm was used to identify individual rockfall clusters, and the volume of each rockfall was calculated using both an iterative alpha-shapes approach. Finally, a bounding box approach was used to estimate the a, b and c axes and therefore shape of the individual rockfalls. Increasing the projection scale used within the M3C2 algorithm decreases the frequency of significant rockfalls found exponentially, and an iterative alpha shapes approach is the most computationally efficient volume estimation method. Our results show that the Miage Glacier catchment is dominated by small scale rockfall events, although at least one large-scale rockfall event is evident in the upper ablation area (validated by time-lapse imagery). This failure on a recently deglaciated area of rock wall highlights that slope response to glacial erosion can be rapid following periods of deglaciation.</p>

Using climate reanalysis data in conjunction with multi-temporal satellite thermal imagery to derive supraglacial debris thickness changes from energy-balance modelling

Surface energy-balance models are commonly used in conjunction with satellite thermal imagery to estimate supraglacial debris thickness. Removing the need for local meteorological data in the debris thickness estimation workflow could improve the versatility and spatiotemporal application of debris thickness estimation. We evaluate the use of regional reanalysis data to derive debris thickness for two mountain glaciers using a surface energy-balance model. Results forced using ERA-5 agree with AWS-derived estimates to within 0.01 ± 0.05 m for Miage Glacier, Italy, and 0.01 ± 0.02 m for Khumbu Glacier, Nepal. ERA-5 data were then used to estimate spatiotemporal changes in debris thickness over a ~20-year period for Miage Glacier, Khumbu Glacier and Haut Glacier d'Arolla, Switzerland. We observe significant increases in debris thickness at the terminus for Haut Glacier d'Arolla and at the margins of the expanding debris cover at all glaciers. While simulated debris thickness was underestimated compared to point measurements in areas of thick debris, our approach can reconstruct glacier-scale debris thickness distribution and its temporal evolution over multiple decades. We find significant changes in debris thickness over areas of thin debris, areas susceptible to high ablation rates, where current knowledge of debris evolution is limited.

Carbon gas cycling in supraglacial debris covers

<p>Debris-covered glaciers extend over 4000 km2 in the high Asian Mountains and are significant and expanding features of most of the World’s glacierized mountain ranges. Within supraglacial debris covers, a combination of fresh mechanically-weathered rock and an abundance of water and energy during melt seasons provides an ideal environment for chemical rock weathering and microbial activity. These processes involve exchange of carbon dioxide CO2 and methane CH4 with the atmosphere, while daytime heating of debris leads to evaporation of meltwater from the debris matrix. Debris-covered glaciers may therefore play an important role in regional and global cycling of major greenhous gases. This new project aims to address 2 key questions: (i) What are the important chemical and microbiological processes affecting carbon gas exchange within supraglacial debris covers? (ii) What are the rates and controls on gas exchange and how do these rates vary in time and space? Initial direct measurements of CO2 flux have been made using an eddy covariance (EC) and gas analyser system installed over debris cover at Miage glacier in the Italian Alps, during the melt season. Under fine weather conditions, there is a strong daily cycle in downwardly-directed CO2 flux, closely linked to variation in energy input to the debris, driven by the flux of shortwave radiation. In contrast, rainfall is associated with short pulses of upwardly-directed CO2 flux to the atmosphere. In common with previously published findings, these data indicate that supraglacial debris covers are a strong summer sink of CO2. At Miage glacier the mean summer (June-August) flux is almost 0.5 g carbon per day per square metre of debris, more than 2 orders of magnitude higher than reported fluxes over cryoconite. Current gas flux data are limited to a few points and this project will extend measurements to varying lithologies, elevations and glaciers in different climatic environments using portable greenhouse gas analysers in conjunction with the EC system. Direct flux measurements will be supported by in-field analysis of debris strucure and composition and subsequent laboratory analysis to determine the minerals, carbon content and microbial communities present in debris covers to uncover controlling processes and determine the relative roles of chemical weathering and microbial activity in carbon gas cycling.</p>

Analysis of the Miage Glacier Lake GLOFs (Aosta Valley - Italy).

<p>Miage Glacier Lake is a glacial marginal lake that forms on the right snout of Miage Glacier, located in the Val Veny Valley (Aosta Valley – Italy). The lake has been experiencing seasonal drainages at least since the 1930’s and 15 events have been documented from 1930 to 1990. The lake position has been almost unvaried since the first existing maps of late 1700, but lake morphology experienced major changes after the drainage event of September 2004, after which the water level could not reach again a sufficient height to fill the 3 depressions that used to form a bigger lake until 2003 (36.000 m<sup>2</sup>). The lake having decreased its volume and surface, it did not seem by that time that GLOF from Miage Lake could cause any risk downstream (Deline et Al. 2004), but recent observation of Sentinel 2B satellite images  led to the individuation of unusual lake expansion towards its north shore. Thus, an UAV survey was performed to assess the actual lake area in July 2019, and the integration of satellite images and UAV surveys demonstrated a consistent lake area expansion since 2015. Moreover an emptying occurred in late August 2019 so that another UAV survey could be performed, and water volume estimation could be performed by means of DEM differencing. An important water volume was individuated, reaching 196.000 m<sup>3</sup> and an estimation of maximum subglacial GLOF debit has been performed. Global evolution trend of the glacier mass has been evaluated by analyzing different airborne Lidar surveys (1991-2008). A cumulated geodetic mass balance could be thus inferred and found good matching with remote sensed analysis (2003-2012) performed by means of stereo satellite imagery by Berthier et Al. in 2014. Average surface lowering of the glacier surface could be analyzed and average values of -1.12 m/yr could be observed around lake Miage. The strong elevation loss of Miage Glacier lower snout is probably the cause of the lowering of the piezometric level in intra-glacial water limiting maximum altitude that water level can reach in the lake, so that the bigger basin of 2004 cannot be filled anymore. Moreover, an analysis of recent GLOFs of Miage Lake gave an insight about the possible dynamics of lake subglacial drainage, suggesting the existence of 2 different mechanisms of emptying as some events occur with lower water debits, earlier in the season, and other events occur later in the summer season with major water debits. Similar GLOF behavior has been described at Plaine Morte Glacier Lake in the Canton of Bern-Switzerland (Fahrni 2018). Field surveys of 2018 showed very likely evidence of hydrostatic uplift of the ice dam, so multi temporal UAV surveys and GNSS field surveys are planned for 2020 to possibly highlight evidences of hydrostatic uplift of the glacier prior to GLOFs.</p>

Ice aprons on steep North faces: oldest surface ice in the Alps?

<p>Ice aprons are small but ubiquitous ice masses in high alpine ranges such as the Mont Blanc massif. Mainly present on its north faces above 3200 m a.s.l., they are a condition for practice of the so-called "traditional" mountaineering (now on the Intangible Cultural Heritage UNESCO list) and an indicator of the presence of permafrost in the bedrock. Most often thin (<10 m), these ice aprons are very sensitive to increasing air temperatures while their evolution during the recent decades suggests many coming disappearances in the short term and, consequently, a change in the permafrost thermal regime and a related increase in the rockfall occurrence.</p><p>Very few studied, ice aprons however represent an important glacial inheritance. We suggest that ice aprons are made up of very old ice, likely the oldest surface one in the Alps. In the north face of the Mont Blanc du Tacul (4248 m a.s.l.) for example, following the disappearance of the upper layers due to the increased occurrence of summer heatwaves, the ice on the present surface formed <em>c</em>. 2700 ago years (cold phase of Göschener I), against probably 200-300 years for the ice at the front of the Mer de Glace, the largest glacier in the French Alps. We present the ice ages acquired from five ice aprons on rock walls of the Mont Blanc massif together with ice ages from two glacier tongues of the massif (Mer de Glace and Miage Glacier).</p>

An investigation of the influence of supraglacial debris on glacier-hydrology

The influence of supraglacial debris on the rate and spatial distribution of glacier surface melt is well established, but its potential impact on the structure and evolution of the drainage system of extensively debris-covered glaciers has not been previously investigated. Forty-eight dye injections were conducted on Miage Glacier, Italian Alps, throughout the 2010 and 2011 ablation seasons. An efficient conduit system emanates from moulins in the mid-part of the glacier, which are downstream of a high melt area of dirty ice and patchy debris. High melt rates and runoff concentration by intermoraine troughs encourages the early-season development of a channelized system downstream of this area. Conversely, the drainage system beneath the continuously debris-covered lower ablation area is generally inefficient, with multi-peaked traces suggesting a distributed network, which likely feeds into the conduit system fed by the upglacier moulins. Drainage efficiency from the debris-covered area increased over the season but trace flow velocity remained lower than from the upper glacier moulins. Low and less-peaked melt inputs combined with the hummocky topography of the debris-covered area inhibits the formation of an efficient drainage network. These findings are relevant to regions with extensive glacial debris cover and where debris cover is expanding.

Assessing ice-cliff backwasting and its contribution to total ablation of debris-covered Miage glacier, Mont Blanc massif, Italy

Continuous surface debris cover strongly reduces the ablation of glaciers, but high melt rates may occur at ice cliffs that are too steep to hold debris. This study assesses the contribution of ice-cliff backwasting to total ablation of Miage glacier, Mont Blanc massif, Italy, in 2010 and 2011, based on field measurements, physical melt models and mapping of ice cliffs using a high-resolution (1 m) digital elevation model (DEM). Short-term model calculations closely match the measured melt rates. A model sensitivity analysis indicates that the effects of cliff slope and albedo are more important for ablation than enhanced longwave incidence from sun-warmed debris or reduced turbulent fluxes at sheltered cliff bases. Analysis of the DEM indicates that ice cliffs account for at most 1.3% of the 1 m pixels in the glacier’s debris-covered zone, but application of a distributed model indicates that ice cliffs account for ~7.4% of total ablation. We conclude that ice cliffs make an important contribution to the ablation of debris-covered glaciers, even when their spatial extent is very small.