The Himalaya mountain range is characterized by highly glacierized, complex,
dynamic topography. The ablation area of Himalayan glaciers often features a highly
heterogeneous debris mantle comprising ...ponds, steep and shallow slopes of
various aspects, variable debris thickness, and exposed ice cliffs
associated with differing ice ablation rates. Understanding the composition
of the supraglacial debris cover is essential for a proper understanding of glacier
hydrology and glacier-related hazards. Until recently, efforts to map
debris-covered glaciers from remote sensing focused primarily on glacier
extent rather than surface characteristics and relied on traditional
whole-pixel image classification techniques. Spectral unmixing routines,
rarely used for debris-covered glaciers, allow decomposition of a pixel into
constituting materials, providing a more realistic representation of glacier
surfaces. Here we use linear spectral unmixing of Landsat 8 Operational Land Imager (OLI) images (30 m) to obtain fractional abundance maps of the various supraglacial surfaces
(debris material, clean ice, supraglacial ponds and vegetation) across the
Himalaya around the year 2015. We focus on the debris-covered glacier
extents as defined in the database of global distribution of supraglacial debris cover. The spectrally
unmixed surfaces are subsequently classified to obtain maps of composition
of debris-covered glaciers across sample regions. We test the unmixing approach in the Khumbu region of the central Himalaya,
and we evaluate its performance for supraglacial ponds by comparison with
independently mapped ponds from high-resolution Pléiades (2 m) and
PlanetScope imagery (3 m) for sample glaciers in two other regions with
differing topo-climatic conditions. Spectral unmixing applied over the
entire Himalaya mountain range (a supraglacial debris cover area of 2254 km2) indicates that at the end of the ablation season, debris-covered
glacier zones comprised 60.9 % light debris, 23.8 % dark debris, 5.6 % clean ice, 4.5 % supraglacial vegetation, 2.1 % supraglacial
ponds, and small amounts of cloud cover (2 %), with 1.2 % unclassified
areas. The spectral unmixing performed
satisfactorily for the supraglacial pond and vegetation classes (an F score
of ∼0.9 for both classes) and reasonably for the debris
classes (F score of 0.7). Supraglacial ponds were more prevalent in the monsoon-influenced
central-eastern Himalaya (up to 4 % of the debris-covered area) compared
to the monsoon-dry transition zone (only 0.3 %) and in regions with lower
glacier elevations. Climatic controls (higher average temperatures and more
abundant precipitation), coupled with higher glacier thinning rates and
lower average glacier velocities, further favour pond incidence and the
development of supraglacial vegetation. With continued advances in satellite data and
further method refinements, the approach presented here provides avenues
towards achieving large-scale, repeated mapping of supraglacial features.
We investigate the role of glacier structures in controlling ice‐front morphology and dynamics of four Himalayan lake‐terminating glaciers over a 20‐year period. At Imja, Trakarding, Lumdin and Dang ...Pu glaciers, lake area was mapped between 2000 and 2020 using Landsat 5/7/8 and Sentinel‐2 imagery. Discrete glacier flow units were identified, with glacier structures (e.g., open crevasses, transverse structures, longitudinal structures) digitised using the finest resolution panchromatic bands in each year (30, 15, or 10 m). Mapping revealed a distinct pattern of transverse structures towards the terminus of each glacier that influence ice‐front position and morphology and are then exploited via iceberg calving events. Our structural analysis also illustrates the role of subsurface conduits in calving events. During subsurface conduit collapse, glacier recession is enhanced, leading to calving along adjacent transverse structures. Furthermore, our analysis shows that ice‐front morphology influences the pattern of glacier recession. Ice fronts with distinct ice aprons undergo slower periods of recession than ice fronts with ice cliffs. We conclude that glacier structures are important in determining ice‐front morphologies at lake‐terminating Himalayan glaciers, and therefore, structural analysis is vital when assessing future ice‐front positions and behaviour, as well as rates of glacier recession.
This study investigates the role of glacier structures in controlling ice‐front morphology and dynamics of four Himalayan glaciers over a 20‐year period. Discrete glacier flow units were identified, with glacier structures digitised using the finest‐resolution panchromatic bands in each year. Glacier mapping highlights the importance of ice structures in determining ice‐front morphologies at lake‐terminating Himalayan glaciers and is a useful predictive tool for assessing likely future ice‐front positions and behaviour, and rates of glacier recession and of lake expansion.
Heterogeneity in Karakoram glacier surges Quincey, Duncan J.; Glasser, Neil F.; Cook, Simon J. ...
Journal of geophysical research. Earth surface,
July 2015, Letnik:
120, Številka:
7
Journal Article
Recenzirano
Odprti dostop
Many Karakoram glaciers periodically undergo surges during which large volumes of ice and debris are rapidly transported downglacier, usually at a rate of 1–2 orders of magnitude greater than during ...quiescence. Here we identify eight recent surges in the region and map their surface velocities using cross‐correlation feature tracking on optical satellite imagery. In total, we present 44 surface velocity data sets, which show that Karakoram surges are generally short‐lived, lasting between 3 and 5 years in most cases, and have rapid buildup and relaxation phases, often lasting less than a year. Peak velocities of up to 2 km a−1 are reached during summer months, and the surges tend to diminish during winter months. Otherwise, they do not follow a clearly identifiable pattern. In two of the surges, the peak velocity travels down‐ice through time as a wave, which we interpret as a surge front. Three other surges are characterized by high velocities that occur simultaneously across the entire glacier surface, and acceleration and deceleration are close to monotonic. There is also no consistent seasonal control on surge initiation or termination. We suggest that the differing styles of surge can be partly accounted for by individual glacier configurations and that while some characteristics of Karakoram surges are akin to thermally controlled surges elsewhere (e.g., Svalbard), the dominant surge mechanism remains unclear. We thus propose that these surges represent a spectrum of flow instabilities and the processes controlling their evolution may vary on a glacier by glacier basis.
Key Points
Karakoram glacier surges are heterogeneous in their character
Karakoram surges do not conform to classic thermal and hydrological surge models
Controls on surging may differ on an individual glacier basis
Supraglacial debris cover is an important component of glacier mass balance, especially in areas characterised by widespread glacier recession. Mapping of the spatial and temporal changes in debris ...cover on the surface of the receding outlet glaciers of the temperate North Patagonian Icefield (NPI) in southern South America between 1987 and 2015 shows that the total amount of debris cover has increased over time, from 168km2 in 1987 to 307km2 in 2015. The number of debris-covered glaciers increased from 24 in 1987, to 31 in 2001 and 32 out of 43 studied glaciers in 2015. The proportion of debris-covered area has also increased, from 4.1% in 1987 to 7.9% in 2015, with the largest proportional increases occurring east of the ice divide (where 15.2% of the glacier ice is now debris covered). Over this time, the total area of the NPI decreased from 4133 to 3887km2. The area occupied by proglacial and ice-proximal lakes also increased from 112 to 198km2. Between 1987 and 2015, the terminal environment of many of the outlet glaciers of the NPI changed from land-terminating to lake-calving, and these glaciers are now receding into terminal lakes. The change in the area of debris-covered ice is influenced by the loss of ice at debris-covered termini and by an increase in debris cover at higher elevations. The glaciers of the NPI remain highly dynamic as they recede and are therefore behaving very differently to high-elevation glaciers, such as those of the Himalaya, where debris cover leads to glacier stagnation at the termini.
Meltwater and runoff from glaciers in High Mountain Asia is a vital freshwater resource for one‐fifth of the Earth's population. Between 13% and 36% of the region's glacierized areas exhibit surface ...debris cover and associated supraglacial ponds whose hydrological buffering roles remain unconstrained. We present a high‐resolution meltwater hydrograph from the extensively debris‐covered Khumbu Glacier, Nepal, spanning a 7 month period in 2014. Supraglacial ponds and accompanying debris cover modulate proglacial discharge by acting as transient and evolving reservoirs. Diurnally, the supraglacial pond system may store >23% of observed mean daily discharge, with mean recession constants ranging from 31 to 108 h. Given projections of increased debris cover and supraglacial pond extent across High Mountain Asia, we conclude that runoff regimes may become progressively buffered by the presence of supraglacial reservoirs. Incorporation of these processes is critical to improve predictions of the region's freshwater resource availability and cascading environmental effects downstream.
Key Points
The monsoon season runoff hydrograph from Khumbu Glacier displays progressive changes in diurnal timing and recession characteristics
We propose that observed hydrological behavior results from seasonal evolution of supraglacial ponds and connections
Predicted expansion of debris‐covered areas and pond extents will influence downstream timing, availability, and quality of meltwater in the Himalaya
Abrasion and quarrying are significant processes of subglacial erosion for ice masses in direct contact with hard substrates, yet their relative efficacy and spatio-temporal variability is unclear. ...Here, we investigate the glacial impact of these processes on a 70 m by 60 m bedrock surface at Moel Ysgyfarnogod in the Rhinog Mountains, Wales, using a combination of high-resolution digital photographs, analysis of a Digital Terrain Model derived from an Unmanned Aerial Vehicle survey, and regional ice sheet modelling. We map and analyze the distribution of grooved and striated surfaces, abraded surfaces, quarried blocks and open fractures in addition to the orientation of pre-existing bedrock fractures and joints. The grooves and striations are orientated in a single, consistent direction across the bedrock surfaces related to regional ice flow during the Late Pleistocene. Abraded and smoothed bedrock dominates the proximal edges of the bedrock outcrop and quarrying prevails on the distal edges of the bedrock outcrop, which are dominated by detached and partially detached blocks. We propose these blocks were removed during the final stages of the last glacial cycle when subglacial meltwater was plentiful in this otherwise predominantly frozen subglacial setting. A minimum estimate of 2000 m3 displaced material at this site implies that subglacial quarrying would have been an important erosional process during final stages of deglaciation.
•A glaciated bedrock surface at Moel Ysgyfarnogod in Wales shows evidence of abrasion and quarrying.•Grooved and striated surfaces, abraded surfaces, quarried blocks and open fractures were mapped.•Landforms can be used to infer former subglacial regimes.
Rock debris covers ~30% of glacier ablation areas in the Central Himalaya and modifies the impact of atmospheric conditions on mass balance. The thermal properties of supraglacial debris are ...diurnally variable but remain poorly constrained for monsoon-influenced glaciers over the timescale of the ablation season. We measured vertical debris profile temperatures at 12 sites on four glaciers in the Everest region with debris thickness ranging from 0.08 to 2.8 m. Typically, the length of the ice ablation season beneath supraglacial debris was 160 days (15 May to 22 October)—a month longer than the monsoon season. Debris temperature gradients were approximately linear (r2 > 0.83), measured as −40°C m–1 where debris was up to 0.1 m thick, −20°C m–1 for debris 0.1–0.5 m thick, and −4°C m–1 for debris greater than 0.5 m thick. Our results demonstrate that the influence of supraglacial debris on the temperature of the underlying ice surface, and therefore melt, is stable at a seasonal timescale and can be estimated from near-surface temperature. These results have the potential to greatly improve the representation of ablation in calculations of debris-covered glacier mass balance and projections of their response to climate change.
Using an ensemble of close- and long-range remote sensing, lake bathymetry and regional meteorological data, we present a detailed assessment of the geometric changes of El Morado Glacier in the ...Central Andes of Chile and its adjacent proglacial lake between 1932 and 2019. Overall, the results revealed a period of marked glacier down wasting, with a mean geodetic glacier mass balance of −0.39 ± 0.15 m w.e.a−1 observed for the entire glacier between 1955 and 2015 with an area loss of 40% between 1955 and 2019. We estimate an ice elevation change of −1.00 ± 0.17 m a−1 for the glacier tongue between 1932 and 2019. The increase in the ice thinning rates and area loss during the last decade is coincident with the severe drought in this region (2010–present), which our minimal surface mass-balance model is able to reproduce. As a result of the glacier changes observed, the proglacial lake increased in area substantially between 1955 and 2019, with bathymetry data suggesting a water volume of 3.6 million m3 in 2017. This study highlights the need for further monitoring of glacierised areas in the Central Andes. Such efforts would facilitate a better understanding of the downstream impacts of glacier downwasting.
Large freshwater lakes formed in North America and Europe during deglaciation following the Last Glacial Maximum. Rapid drainage of these lakes into the Oceans resulted in abrupt perturbations in ...climate, including the Younger Dryas and 8.2 kyr cooling events. In the mid-latitudes of the Southern Hemisphere major glacial lakes also formed and drained during deglaciation but little is known about the magnitude, organization and timing of these drainage events and their effect on regional climate. We use 16 new single-grain optically stimulated luminescence (OSL) dates to define three stages of rapid glacial lake drainage in the Lago General Carrera/Lago Buenos Aires and Lago Cohrane/Pueyrredón basins of Patagonia and provide the first assessment of the effects of lake drainage on the Pacific Ocean. Lake drainage occurred between 13 and 8 kyr ago and was initially gradual eastward into the Atlantic, then subsequently reorganized westward into the Pacific as new drainage routes opened up during Patagonian Ice Sheet deglaciation. Coupled ocean-atmosphere model experiments using HadCM3 with an imposed freshwater surface "hosing" to simulate glacial lake drainage suggest that a negative salinity anomaly was advected south around Cape Horn, resulting in brief but significant impacts on coastal ocean vertical mixing and regional climate.
This paper presents the sediment, landform and dynamic context of four avalanche-fed valley glaciers (Khumbu, Imja, Lhotse and Chukhung) in the Mount Everest (Sagarmatha) region of Nepal. All four ...glaciers have a mantle of debris dominated by sandy boulder-gravel that suppresses melting to an increasing degree towards the snout, leading to a progressive reduction in the overall slope of their longitudinal profile. Prominent lateral–terminal moraine complexes, also comprising sandy boulder-gravel, enclose the glaciers. These terminal moraines originally grew by accretion of multiple sedimentary facies of basal glacial and supraglacial origin, probably by folding and thrusting when the glaciers were more dynamic during the Little Ice Age. The four glaciers are in various stages of recession, and demonstrate a range of scenarios from down-wasting of the glacier tongue, through moraine-dammed lake development, to post-moraine-dam breaching. Khumbu Glacier is at the earliest stage of supraglacial pond formation and shows no sign yet of developing a major lake, although one is likely to develop behind its >250
m high composite terminal moraine. Imja Glacier terminates in a substantial body of water behind a partially ice-cored moraine dam (as determined from geophysical surveys), but morphologically appears unlikely to be an immediate threat. Chukhung Glacier already has a breached moraine and a connected debris fan, and therefore no longer poses a threat. Lhotse Glacier has an inclined, free-draining tongue that precludes hazardous lake development. From the data assembled, a conceptual model, applicable to other Himalayan glaciers, is proposed to explain the development of large, lateral-terminal moraine complexes and associated potentially hazardous moraine dams.
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