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1.
An ice‐dammed lake at the margin of the glacier Sälkaglaciären, in the Kebnekaise Mountains in northern Sweden, drained suddenly in July 2003 producing a flood with a measured peak discharge of 9.5±0.25 m3s‐1. The total lake volume of 4.55×105 million3 drained within two days. The hydrograph of this event is characteristic of a jökulhlaup controlled by a single basal ice tunnel that enlarges due to melting. The jökulhlaup had an exponential rise to a peak discharge, and following the peak, a very steep fall in discharge as the water supply to the drainage system ceased. A similar jökulhlaup was observed in August 1990 with an estimated release of 8.05×105 m3 water. Jökulhlaups at Sälkaglaciären are recurring events and have been indirectly observed since the 1950s.  相似文献   

2.
The most well known sub‐glacial lake is probably Grímsvötn under Vatnajökul, Iceland, from where jökulhlaups regularly burst forth. It is created by thermal melting under the ice cap. The Antarctic Lake Vostok, on the other hand, is considered to be located over a region with normal geothermal heat transfer, where it can exist because the ice is so thick that its base is at the pressure melting point. This makes it a candidate for testing the captured ice shelf (CIS) hypothesis, which states that the motion of a totally confined ice shelf creates a hydrostatic seal in the form of an ice rim over the threshold. The CIS hypothesis may offer a source of water for the controversial Laurentian jökulhlaups inferred from field data, implicated in dramatic climatic changes. Here I show that Lake Vostok agrees with the hypothesis, and that it may be on the verge of a jökulhlaup, which could create an ice stream and regional downdraw. The result also implies that the lake may well be of pre‐glacial origin, and that it may have experienced jökulhlaups during previous interglacials.  相似文献   

3.
This paper presents geomorphological and sedimentological evidence for three large‐scale floods to the west of the ice‐capped volcano Katla around the time of Norse settlement or Landnám (AD 870–930). These glacial outburst floods (jökulhlaups), the most recent prehistoric events in a series of Holocene floods in the Markarfljót valley, are securely dated by tephrochronology and radiocarbon dating to between c. AD 500 and c. AD 900. The environmental impact of these events would have been extensive, affecting both the highlands and about 40–50 km2 of the coastal lowlands where about 15 of the 400 or so landnám farms in Iceland were sited. An awareness of environmental conditions and landscape stability around the time of the Norse colonisation of Iceland is important to understand the earliest settlement patterns because of the different constraints and opportunities that they represent.  相似文献   

4.
The varved sediment record from glacially-fed Lake Tuborg, Ellesmere Island, Nunavut, shows that only three large jökulhlaups have occurred there in the last millennium: 2003, 1993, and 1960. Detailed analyses of sediment microstructure and particle size, combined with in-situ hydrometeorological and limnological process studies, allowed jökulhlaup facies identification and discrimination from deposits from other processes. Deposits from large jökulhlaups are anomalously thick, typically lack internal structure, have sharp bases, and fine upwards. The ice-dammed lake above Lake Tuborg (the source of the jökulhlaups) likely changed its drainage style in 1960, from ice-dam overtopping to ice-dam flotation and glacial tunnel enlargement by melt widening, which allowed the lake to drain completely and catastrophically. Complete drainage of ice-dammed lakes by ice-dam flotation is rare in the region is due to the pervasiveness of cold-based ice. Twentieth century warming is likely responsible for some combination of dam thinning, lake expansion and deepening, and changing the thermal regime at the base of the dam. Anomalously thick individual varves were periodically deposited beginning in the nineteenth century, and their thickness increased with time. This likely reflects a combination of increased ice dam overtopping, subaqueous slope failures, sediment availability and rising air temperature. The varve record presented here significantly correlates with a previous, shorter record from Lake Tuborg. However, generally weak correlations are found between the new varve time series, regional records of air temperature, and glacial melt from ice cores on the Agassiz Ice Cap. It is hypothesized that on short timescales, sedimentation at the coring location reflects a complex and varying integration of multiple hydroclimatic, geomorphic and limnologic influences.  相似文献   

5.
Storglaciären is a 3.2 km long polythermal valley glacier in northern Sweden. Since 1994 a number of small (1–2 m high) transverse debris‐charged ridges have emerged at the ice surface in the terminal zone of the glacier. This paper presents the results of a combined structural glaciological, isotopic, sedimentological and ground‐penetrating radar (GPR) study of the terminal area of the glacier with the aim of understanding the evolution of these debris‐charged ridges, features which are typical of many polythermal glaciers. The ridges originate from steeply dipping (50–70°) curvilinear fractures on the glacier surface. Here, the fractures contain bands of sediment‐rich ice between 0.2 and 0.4 m thick composed of sandy gravel and diamicton, interpreted as glaciofluvial and basal glacial material, respectively. Structural mapping of the glacier from aerial photography demonstrates that the curvilinear fractures cannot be traced up‐glacier into pre‐existing structures visible at the glacier surface such as crevasses or crevasse traces. These curvilinear fractures are therefore interpreted as new features formed near the glacier snout. Ice adjacent to these fractures shows complex folding, partly defined by variations in ice facies, and partly by disseminated sediment. The isotopic composition (δ18O) of both coarse‐clear and coarse‐bubbly glacier ice facies is similar to the isotopic composition of the interstitial ice in debris layers that forms the debris‐charged ridges, implying that none of these facies have undergone any significant isotopic fractionation by the incomplete freezing of available water. The GPR survey shows strong internal reflections within the ice beneath the debris‐charged ridges, interpreted as debris layers within the glacier. Overall, the morphology and distribution of the fractures indicate an origin by compressional glaciotectonics near the snout, either at the thermal boundary, where active temperate glacier ice is being thrust over cold stagnant ice near the snout, or as a result of large‐scale recumbent folding in the glacier. Further work is required to elucidate the precise role of each of these mechanisms in elevating the basal glacial and glaciofluvial material to the ice surface.  相似文献   

6.
During the initial stages of the November 1996 jökulhlaup at Skeiðarárjökull, Iceland, floodwaters burst onto the glacier surface via a series of fractures. This supraglacial drainage led to the formation of a number of distinct ice surface depressions, one of which is investigated in detail. The morphology and structural characteristics of this feature are described, as well as the sedimentology of an associated assemblage of debris-filled fractures. This work suggests that debris-charged subglacial floodwaters travelled up to the glacier surface, where supraglacial flow occurred initially via an extensive network of fractures, orientated parallel to the glacier margin. Supraglacial discharge became progressively more focused into a series of discrete outlets, leading to the mechanical erosion of a number of depressions on the glacier surface. The associated transfer of subglacially derived floodwaters to high levels within the glacier resulted in the rapid entrainment of large volumes of sediment which may influence the patterns, processes and products of ice-marginal sedimentation in the future.  相似文献   

7.
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8.
A geomorphological and sedimentological case study is made of the Sprongdøla, a medium-sized river flowing in a predominantly bedrock channel in the mountains of southern Norway. Particular attention is given to bed material characteristics and potential sediment sources. Downstream patterns in clast size and shape, including sphericity and roundness, are poorly developed in the Sprongdøla. There is an apparent absence of down-stream fining and a large proportion of bedload in the lower channel remains in very angular and angular roundness categories. Covariance analysis of clast shape and roundness allows this to be attributed to the widespread input of coarse, angular rock particles derived from snow-avalanche activity on the valley sides and especially from frost-shattering of bedrock in the channel, which produces characteristically slabby clasts. The latter process is important in channel widening and in steepening the channel walls. Strictly fluvial processes, avalanche activity and frost-shattering are controlled in different ways by related annual environmental cycles. Their interaction in the context of the periglacial river channel produces a unique process-sediment-landform association that distinguishes alpine periglacio-fluvial systems from those associated with glacio-fluvial rivers and fluvial landscapes in the temperate zone.  相似文献   

9.
Suspended sediment concentration (SSC) in the Ova da Morteratsch, Switzerland, measured during July 2007 was closely associated with discharge (Q) and showed statistically significant relationships at the p < 0.001 level at the proximal and distal ends of the 600 m pro‐glacial zone. SSC predicted from 10‐minute turbidity records gives a much more detailed insight into SSC fluctuations and identified SSC peaks which do not coincide with discharge peaks. Net (proximal – distal) 10‐minute suspended sediment loads (SSL) are predominantly positive (i.e. suspended sediment is being stored in the reach) for most of the 7–19 July 2007 record. Net (proximal – distal) SSLs correlate closely with discharge for the first part of the record (7–13 July) but from 14 to 19 July suspended sediment exhaustion is in evidence and discrete phases of negative net SSL (i.e. sediment flushing) are likely for up to six hours on three separate days which coincide with phases of high discharge and exhaustion of the glacial suspended sediment sources. Analysis of Q at the Berninabach–Pontresina gauging station (5 km downstream) for the past five years revealed that maximum monthly discharges capable of generating sediment flushing events occur in an average of four months each year. The study emphasises the rapid change in suspended sediment transport and yields with distance from the glacier snout and highlights the importance of measurements as close to the glacier snout as possible if data are to be representative of the glaciated land up‐valley. A better understanding of the processes of sediment exchange and the colonisation and stabilisation of sediment stores by vegetation in such pro‐glacial zones is essential if we are to improve predictions of the impacts of climate change on river sediment dynamics and the subsequent effects on aquatic ecology.  相似文献   

10.
Depending on thickness, debris‐cover can enhance or reduce ablation, compared to bare‐ice conditions. In the geological record, hummocky moraines often represent the final product of the melt‐out of ice‐cored moraines, and the presence or absence of such moraine deposits can have paleoclimatic implications. To evaluate the effects of varying debris‐cover and climate on ice‐melt in a maritime mid‐latitude setting, an 11‐day ablation stake study was undertaken on ice‐cored moraine at Fox Glacier, on the western flank of the New Zealand Southern Alps. Ablation rates varied from 1.3 to 6.7 cm d?1, with enhancement of melt‐rate under thin debris‐covers. Highest melt‐rates (effective thickness) occurred under debris‐cover of c. 2 cm, with ～3 cm being the debris thickness at which melt‐rates are equal to adjacent bare‐ice (critical thickness). Air temperature from nearby Franz Josef Glacier allowed for a simple degree‐day approach to ablation calculations, with regression relationships indicating air temperature is the key climatic control on melt. Digital elevation models produced from topographic surveys of the ice‐cored moraine over the following 19 months indicated that ablation rates progressively decreased over time, probably due to melt‐out of englacial debris increasing debris‐cover thickness. The morphology of the sandur appears to be strongly determined by episodic high‐magnitude fluvial flows (jökulhlaups), in conjunction with surface melt. Thus, ‘hummocky’ moraine appears to be a transient landform in this climatic setting.  相似文献   

11.
This paper examines the occurrence of ephemeral hoarfrost crystals at funnel openings (funnel hoarfrost) detected between large blocks at the surface of the presumably relict Schöneben Rock Glacier. Field mapping on 25 November 2011 identified 51 individual funnel openings with notable hoarfrost crystals distributed over the entire rock glacier. Hoarfrost was no longer detectable a few days after the initial mapping campaign. At least in the period 20–25 November 2011 temperature conditions at the rock glacier surface were favourable for hoarfrost formation and preservation as indicated by different types of measurements. A period of 24–48 h of hoarfrost‐suitable weather conditions would have been sufficient to form the observed hoarfrost if crystal growth rates of 2–4 mm h?1 are assumed. The void systems with funnel hoarfrost seem to be rather localised and limited in horizontal (10s of metres) and vertical (some metres) extent. Presumably the observed funnel hoarfrost was caused by the so‐called chimney effect, although no larger reversible air circulation systems with warm air exhalation were identified. Continuous ground temperature data from several sites at the rock glacier surface (period November 2011–December 2012) showed that hoarfrost sites are cooler and thermally buffered compared with non‐hoarfrost sites at similar elevations. This seems to be related to the decoupling of the air above the rock glacier and the pore air during periods of atmospheric warming. Only the combination of specific micro‐climatic (temperature/humidity), geometric (open void systems) and sedimentological (grain size/sediment structure) conditions allow the formation of the ephemeral funnel hoarfrost at this rock glacier.  相似文献   

12.
Widespread till and moraines record excursions of middle-Pleistocene ice that flowed up-slope into several watersheds of the Valley and Ridge Province along the West Branch of the Susquehanna River. A unique landform assemblage was created by ice-damming and jökulhlaups emanating from high gradient mountain watersheds. This combination of topography formed by multiple eastward-plunging anticlinal ridges, and the upvalley advance of glaciers resulted in an ideal geomorphic condition for the formation of temporary ice-dammed lakes. Extensive low gradient (1°–2° slope) gravel surfaces dominate the mountain front geomorphology in this region and defy simple explanation. The geomorphic circumstances that occurred in tributaries to the West Branch Susquehanna River during middle Pleistocene glaciation are extremely rare and may be unique in the world. Failure of ice dams released sediment-rich water from lakes, entraining cobbles and boulders, and depositing them in elongated debris fans extending up to 9 km downstream from their mountain-front breakout points. Poorly developed imbrication is rare, but occasionally present in matrix-supported sediments resembling debris flow deposits. Clast weathering and soils are consistent with a middle Pleistocene age for the most recent flows, circa the 880-ka paleomagnetic date for glacial lake sediments north of the region on the West Branch Susquehanna River. Post-glacial stream incision has focused along the margins of fan surfaces, resulting in topographic inversion, leaving bouldery jökulhlaup surfaces up to 15 m above Holocene channels. Because of their coarse nature and high water tables, jökulhlaup surfaces are generally forested in contrast to agricultural land use in the valleys and, thus, are readily apparent from orbital imagery.  相似文献   

13.
Advance of part of the margin of the Greenland ice sheet across a proglacial moraine ridge between 1968 and 2002 caused progressive changes in moraine morphology, basal ice formation, debris release, ice‐marginal sediment storage, and sediment transfer to the distal proglacial zone. When the ice margin is behind the moraine, most of the sediment released from the glacier is stored close to the ice margin. As the margin advances across the moraine the potential for ice‐proximal sediment storage decreases and distal sediment flux is augmented by reactivation of moraine sediment. For six stages of advance associated with distinctive glacial and sedimentary processes we describe the ice margin, the debris‐rich basal ice, debris release from the glacier, sediment routing into the proglacial zone, and geomorphic processes on the moraine. The overtopping of a moraine ridge is a significant glaciological, geomorphological and sedimentological threshold in glacier advance, likely to cause a distinctive pulse in distal sediment accumulation rates that should be taken into account when glacial sediments are interpreted to reconstruct glacier fluctuations.  相似文献   

14.
The presented paper analyses the variability of grain size distribution parameters of bedload transported by the gravel‐bed Scott River (Svalbard) draining a glacier catchment with an area of 10 km2. The grain size distribution analysis is one of the basic elements of identification of the fluvial transport mechanisms in gravel‐bed rivers. It is used for the determination of threshold values for bedload movement. It is also treated as an important indicator of the origin, routes of distribution, and conditions of transport and deposition of fluvial bedload. The field study in a natural proglacial gravel‐bed channel was carried out at two reaches in the mouth section of Scott River. The study revealed relatively high temporal variability and similar mean parameters of grain size distribution in conditions of low discharges. Bedload transport rates reached a mean of 71.9–76.0 kg d?1 in channel cross‐section. Bedload texture was dominated by gravels with a proportional contribution of the fine‐grained fraction along with very fine‐grained gravels (8‐2 mm) of 38.8%. The medium‐grained fraction (16‐8 mm) constituted 33.7%, with a lower contribution from the coarse‐grained fraction (32‐16 mm) of 23.2%, and the very coarse‐grained fraction (64‐32 mm) of 4.4%. Two periods in the course of bedload transport and distribution of grain size distribution parameters were distinguished based on variation of hydro‐meteorological conditions. The first half of the measurement period was distinguished by significantly higher values of daily loads and increased contribution of the coarse‐grained and very coarse‐grained fraction (28–31% and 6.2–6.6%, respectively). During this time, the river discharged up to 94% of bedload. This resulted in a clear tendency for riverbed scouring. The second half was distinguished by generally low daily bedload transport rates (<10 kg d?1), an increase in contribution of fine‐ and very fine‐grained gravels (42–55.6%), and a change in the tendency to aggradation. Grain size indices were more varied, and grains were usually finer and better sorted. Selective transport processes, often related to redeposition, were dominant in the channel. Along with an increase in flow velocity, conditions for material deposition became more variable. This was manifested in weaker sorting and an increase in grain diameter.  相似文献   

15.
Artificial neural networks were applied to simulate runoff from the glacierized part of the Waldemar River catchment (Svalbard) based on hydrometeorological data collected in the summer seasons of 2010, 2011 and 2012. Continuous discharge monitoring was performed at about 1 km from the glacier snout, in the place where the river leaves the marginal zone. Averaged daily values of discharge and selected meteorological variables in a number of combinations were used to create several models based on the feed‐forward multilayer perceptron architecture. Due to specific conditions of melt water storing and releasing, two groups of models were established: the first is based on meteorological inputs only, while second includes the preceding day's mean discharge. Analysis of the multilayer perceptron simulation performance was done in comparison to the other black‐box model type, a multivariate regression method based on the following efficiency criteria: coefficient of determination (R2) and its adjusted form (adj. R2), weighted coefficient of determination (wR2), Nash–Sutcliffe coefficient of efficiency, mean absolute error, and error analysis. Moreover, the predictors' importance analysis for both multilayer perceptron and multivariate regression models was done. The performed study showed that the nonlinear estimation realized by the multilayer perceptron gives more accurate results than the multivariate regression approach in both groups of models.  相似文献   

16.
This paper describes the structural glaciology of the lower Fox Glacier, a 12.7 km‐long valley glacier draining the western side of the Southern Alps, New Zealand. Field data are combined with analysis of aerial photographs to present a structural interpretation of a 5 km‐long segment covering the lower trunk of the glacier, from the upper icefall down‐glacier to the terminus. The glacier typifies the structural patterns observed in many other alpine glaciers, including: primary stratification visible within crevasse walls in the lower icefall; foliation visible in crevasses below the lower icefall; a complex set of intersecting crevasse traces; splaying and chevron crevasses at the glacier margins; transverse crevasses forming due to longitudinal extension; longitudinal crevasses due to lateral extension near the snout; and, arcuate up‐glacier dipping structures between the foot of the lower icefall and the terminus. The latter are interpreted as crevasse traces that have been reactivated as thrust faults, accommodating longitudinal compression at the glacier snout. Weak band‐ogives are visible below the upper icefall, and these could be formed by multiple shearing zones uplifting basal ice to the glacier surface to produce the darker bands, rather than by discrete fault planes. Many structures such as crevasses traces do not show a clear relationship with measured surface strain‐rates, in which case they may be ‘close to crevassing’, or are undergoing passive transport down‐glacier.  相似文献   

17.
Little Ice Age (LIA) moraines along the margins of Skálafellsjökull and Heinabergsjökull, two neighbouring outlet glaciers flowing from the Vatnajökull ice‐cap, have been re‐dated to test the reliability of different lichenometric approaches. During 2003, 12 000 lichens were measured on 40 moraine fragments at Skálafellsjökull and Heinabergsjökull to provide surface age proxies. The results are revealing. Depending on the chosen method of analysis, Skálafellsjökull either reached its LIA maximum in the early 19th century (population gradient) or the late 19th century (average of five largest lichens), whereas the LIA maximum of Heinabergsjökull occurred by the mid‐19th century (population gradient) or late‐19th century (average of 5 largest lichens). Discrepancies (c. 80 years for Skálafellsjökull and c. 40 years for Heinabergsjökull) suggest that the previously cited AD 1887 LIA maxima for both glaciers should be reassessed. Dates predicted by the lichen population gradient method appear to be the most appropriate, as mounting evidence from other geochronological reconstructions and sea‐ice records throughout Iceland tends to support an earlier LIA glacier maximum (late 18th to mid‐19th century) and probably reflects changes in the North Atlantic Oscillation. These revised chronologies shed further light on the precise timing of the Icelandic LIA glacier maximum, whilst improving our understanding of glacier‐climate interactions in the North Atlantic.  相似文献   

18.
Small mountain glaciers have short mass balance response times to climate change and are consequently very important for short‐term contributions to sea level. However, a distinct research and knowledge gap exists between (1) wider regional studies that produce overview patterns and trends in glacier changes, and (2) in situ local scale studies that emphasise spatial heterogeneity and complexity in glacier responses to climate. This study of a small glacier in central Austria presents a spatiotemporally detailed analysis of changes in glacier geometry and changes in glaciological behaviour. It integrates geomorphological surveys, historical maps, aerial photographs, airborne LiDAR data, ground‐based differential global positioning surveys and Ground Penetrating Radar surveys to produce three‐dimensional glacier geometry at 13 time increments spanning from 1850 to 2013. Glacier length, area and volume parameters all generally showed reductions with time. The glacier equilibrium line altitude increased by 90 m between 1850 and 2008. Calculations of the mean bed shear stress rapidly approaching less than 100 kPA, of the volume–area ratio fast approaching 1.458, and comparison of the geometric reconstructions with a 1D theoretical model could together be interpreted to suggest evolution of the glacier geometry towards steady state. If the present linear trend in declining ice volume continues, then the Ödenwinkelkees will disappear by the year 2040, but we conceptualise that non‐linear effects of bed overdeepenings on ice dynamics, of supraglacial debris cover on the surface energy balance, and of local topographically driven controls, namely wind‐redistributed snow deposition, avalanching and solar shading, will become proportionally more important factors in the glacier net balance.  相似文献   

19.
Enhanced delivery of water‐saturated, ice‐marginal sediments to the glacier surface is a response to glacier thinning that has the potential to increase both levels of sediment transfer through the glacier hydrological system and total basin sediment yields. Preliminary observations made during summer 2007 at Austre Brøggerbreen, Svalbard, confirm that ice‐marginal debris flows in the upper reaches of the glacier are actively delivering sediments to the glacier surface, which may then be flushed into the glacier's hydrological system. During a four‐day observation period, several stochastic pulses in water turbidity were observed at a single portal where solely supra‐ and englacial drainage emerge at the glacier margin. The erratic suspended sediment fluxes were hypothesized to originate from ice‐marginal sources. Quantitative analysis of continuous turbidity and discharge data confirm that discharge is not driving these turbidity pulses and, combined with observational data, that the most likely origin is the delivery of water‐saturated sediments to the glacier surface from ice‐marginal, debris flows with subsequent transfer to the portal via the glacial drainage system. These observations illustrate the potential importance of the paraglacial component to the overall sediment cascade of deglaciating basins and highlight the need for careful interpretation of turbidity records, where stochastic pulses in turbidity may be attributed to sources and processes other than ice‐marginal sediment inputs.  相似文献   

20.
Glacier advance and recession are considered key indicators of climate change. Understanding the relationship between climatic variations and glacial responses is crucial. Here, we apply archival digital photogrammetry to reconstruct the decadal scale glacial history of an unmonitored Alpine valley glacier, the Haut Glacier d'Arolla, Switzerland, and we use the data generated to explore the linkages between glacier recession and climate forcing. High precision digital elevation models were derived. They show continual recession of the glacier since 1967, associated with long‐term climatic amelioration but only a weak reaction to shorter‐term climatic deterioration. Glacier surface velocity estimates obtained using surface particle tracking showed that, unlike for most Swiss glaciers during the late 1970s and early 1980s, ice mass flux from the accumulation zone was too low to compensate for the effects of glacier thinning and subsequent snout recession, especially during the rapid warming that occurred through the 1980s. The results emphasise the dangers of inferring glacier response to climate forcing from measurements of the terminus position only and the importance of using remote sensing methods as an alternative, especially where historical imagery is available.  相似文献   

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