Showing papers on "Accumulation zone published in 2000"

Effect of summer accumulation on glacier mass balance on the Tibetan Plateau revealed by mass-balance model

Abstract: The characteristics and sensitivities of a cold-based glacier on the Tibetan Plateau, where the summer monsoon provides most of the mass input to glaciers, are discussed using an energy-balance model incorporating the process of water refreezing. The model accurately represents the observational results related to the mass balance of Xiao Dongkemadi glacier on the central plateau during 1992/93. Our data revealed that the mass balance of cold glaciers cannot simply be described by the surface mass/heat balances, because about 20% of infiltrated water is refrozen and thus does not run off from the glacier. Model calculations demonstrate that glaciers in an arid environment can maintain their mass since the monsoon provides precipitation during the melting season. Snowfall in summer keeps surface albedo high and largely restrains ablation. Nevertheless, the calculations also make clear that glaciers on the plateau are more vulnerable than those of other regions because of summer accumulation. In the monsoon climate, warming would cause not only a decrease in accumulation, but also a drastic increase in ablation in combination with surface-albedo lowering. Therefore, although glaciers on and around the plateau can be sustained by summer accumulation, they are more vulnerable to warming than winter-accumulation-type glaciers.

Glacier surge propagation by thermal evolution at the bed

Abstract: Bakaninbreen, southern Svalbard, began a prolonged surge during 1985. In 1986, an internal reflecting horizon on radio echo sounding data was interpreted to show that the position of the surge front coincided with a transition between areas of warm (unfrozen) and cold (frozen) bed. Ground-penetrating radar lines run in 1996 and 1998 during early quiescence show that the basal region of the glacier is characterized by a strong reflection, interpreted as the top of a thick layer of sediment-rich basal ice. Down glacier of the present surge front, features imaged beneath the basal reflection are interpreted as the bottom of the basal ice layer, the base of a permafrost layer, and local ice lenses. This indicates that this region of the bed is cold. Up glacier of the surge front, a scattering zone above the basal reflection is interpreted as warm ice. There is no evidence for this warm zone down glacier of the surge front, nor do we see basal permafrost up glacier of it. Thus, as in early surge phase, the location of the surge front is now at the transition between warm and cold ice at the glacier bed. We suggest that the propagation of the front is associated with this basal thermal transition throughout the surge. Because propagation of the front occurs rapidly and generates only limited heat, basal motion during fast flow must have been restricted to a thin layer at the bed and occurred by sliding or deformation localized at the ice-bed interface.

Englacial water distribution in a temperate glacier from surface and borehole radar velocity analysis

Abstract: We have obtained common offset, common midpoint (CMP) and borehole vertical (VRP) ground-penetrating radar profiles close to the margin of Falljokull, a small, steep temperate valley glacier situated in southeast Iceland. Velocity analysis of CMP and VRP surveys provided a four-layered velocity model. This model was verified by comparison between the depths of englacial reflectors and water channels seen in borehole video, and from the depths of boreholes drilled to the bed. In the absence of sediment within the glacier ice, radar velocity is inversely proportional to water content. Using mixture models developed by Paren and Looyenga, the variation of water content with depth was determined from the radar velocity profile. At the glacier surface the calculated water content is 0.23−0.34% (velocity 0.166 m ns−1), which rises sharply to 3.0−4.1% (velocity 0.149 m ns−1) at 28 m depth, interpreted to be the level of the piezometric surface. Below the piezometric surface the water content drops slowly to 2.4−3.3% (velocity 0.152 m ns−1) until ∼102 m depth where it falls to 0.09−0.14% (velocity 0.167 m ns−1). The water content of the ice then remains low to the glacier bed at about 112 m. These results suggest storage of a substantial volume of water within the glacier ice, which has significant implications for glacier hydrology, ice rheology and interpretations of both radar and seismic surveys

A model of ablation-dominated medial moraines and the generation of debris-mantled glacier snouts

Abstract: Medial moraines form striking dark stripes that widen non-linearly, steepen laterally and increase in relief down-glacier from the equilibrium line. Coalescence of these low-ablation-rate features can feed back strongly on the mass balance of a glacier snout. Ablation-dominated medial moraines originate from debris delivered to glacier margins, producing a debris-rich septum between tributary streams of ice below their confluence. Emergence of this ice below the equilibrium line delivers debris to the glacier surface, which then moves down local slopes of evolving morainal topography. A quantitative description of moraine evolution requires specification of the debris concentration field within the glacier, treatment of the melt-rate dependence on debris thickness, and characterization of processes that transport debris once it emerges onto the ice surface. Debris concentration at glacier tributary junctions scales with the erosion rates and the lengths of the tributary-valley walls, and inversely with the tributary ice speeds. Melt rate is damped exponentially by debris, with a ∼10 cm decay scale. Debris flux across the glacier surface scales with the product of debris thickness and local slope. Analytical and numerical results show that medial moraines should develop cross-glacier profiles with parabolic crests and linear slopes, and should widen with age and hence distance down-glacier. Debris should be both thin and uniform over the moraine. Observed faster-than-linear growth of moraine widths with distance reflects the increasing ablation rate down-glacier. Increase in medial moraine cover reduces the local average ablation rate, allowing the glacier to extend further down-valley than meteorology alone would suggest. This feedback is especially effective when moraines merge.

Characteristics of cryoconite holes on a Himalayan glacier, Yala Glacier Central Nepal

Abstract: Cryoconite holes, water-filled cylindrical melt-holes on ice, are common features of glaciers. Morphology, distribution, stability, and organisms of cryoconite holes on a Himalayan glacier (Yala Glacier, Langtang region of Nepal) were investigated. On this glacier, cryoconite holes were distributed in the bare-ice area from 5110 m to 5240 m a.s.l. in altitude. Their size ranged 3.0 - 12.0 cm in depth (mean 5.3 cm), 1.0-36.0 cm in diameter (mean 7.0 cm). At flat bottom of the holes, dark colored mud -like material (cryoconite) was deposited. The material contained much filamentous blue-green algae and organic matter. Besides the blue-green algae, green algae, copepods, and insect larvae were found in the holes. Twenty-five days observation of 17 holes revealed that cryoconite holes on this glacier were short -lived and unstable; 13 holes were broken during the observation period. Hole-depth increased as albedo of the ice surface surrounding the holes increased by snowfall. Altitudinal increase of hole -depth and relationship between hole-depth and heat balance at the glacier surface, both of which have been reported on glaciers of other region, were not observed in this glacier. Negative correlation between hole-diameter and surface inclination suggests that life span of the holes on steep slopes is shorter than those on gentle slopes. Results suggest that cryoconite holes of this glacier were shallow and unstable because albedo of the glacier surface was reduced by much dark-colored material and it could drastically change by snow falls and/or relocation of the material.

Dynamic Features of Thinning and Retreating Glaciar Upsala, a Lacustrine Calving Glacier in Southern Patagonia

Abstract: Glaciar Upsala, which calves into a lake on the eastern side of the Hielo Patagonico Sur, South America, has significantly retreated by 5 km during the last 20 yr. The glacier near the terminus has thinned by about 33 m from 1990 to 1993. Bathymetric surveys made in 1998 at the proglacial lake revealed the existence of bedrock rises spreading out from exposed islands in the western part of the lake. Between 1978 and 1990, the western half of the glacier terminus was located at the bedrock rises, which suggests that the front fluctuations were strongly controlled by the bed topography. During 1990-93, the glacier terminus was located upstream from the bedrock rises, and it is considered to have been floating after estimation of the buoyancy. Significantly large extending strain rate of 0.22 a-1 was deduced from the continuity consideration in 1990-93. These results are discussed with those obtained at Columbia Glacier, Alaska.

Shrinkage of the Khumbu Glacier, east Nepal from 1978 to 1995

Abstract: Surface lowering of the Khumbu Glacier, a large debris-covered glacier in the Nepal Himalayas, was detected by means of ground surveying in 1978 and in 1995. Over this interval the surface of the glacier lowered about 10 m throughout the debris-covered ablation area. Lowering in the lowermost part of the glacier, where surface ablation may be negligible, might result from subglacial meltwater interaction. Indication that ice flow is slowing suggests that shrinkage may accelerate even if ablation conditions remain unchanged.

The influence of a debris cover on the midsummer discharge of Dome Glacier, Canadian Rocky Mountains

Abstract: Meltwater discharge patterns of two glacierized mountain basins in the Canadian Rocky Mountains are compared over the same 25-day period in 1994 and 1995. The glaciers under study are the Dome and Athabasca; both situated in the Columbia Icefield. The two glaciers lie adjacent to one another and are similar in size, orientation, and range in elevation. They differ however, in their surficial characteristics. While the ablation zone of the Athabasca Glacier is mostly debris free, the ablation zone of the Dome Glacier displays an extensive debris cover. It is postulated that this debris cover significantly influences the diurnal discharge patterns of the Dome Glacier's meltwater stream producing different discharge patterns to those observed in the Athabasca Glacier meltwater stream. Results indicate that the debris cover on the Dome Glacier acts as a regulator of streamflow producing annual variances of volumetric discharge of only 1.0% between 1994 and 1995 as compared with 24% for the debris-free Athabasca Glacier.

Mass balance of adjacent debris-covered and clean glacier ice in the North Cascades, Washington

Abstract: As part of the ongoing annual mass balance measurements on Lyman Glacier and Columbia Glacier, North Cascades, Washington, measurements were completed from 1986-1998 on adjacent clean and debris-covered sections of each glacier. On Columbia Glacier annual ice ablation is 3.3 m water equivalent for clean glacier ice and 2.3 m water equivalent for debris-covered areas. On Lyman Glacier from 1986 to 1999, average annual ablation on the clean glacier ice is 3.4 m and under the debris cover average annual ablation is 2.6 m water equivalent. On both glaciers annual ablation is significantly reduced (25-30%) under the debris cover. Late-summer ablation, after both the clean glacier ice and debris-covered ice have lost all snow cover, is reduced 30-40% under the debris cover. The debris cover on Columbia Glacier has reduced ablation more substantially. The debris cover on Columbia Glacier is a complete cover (100%) and is a relatively fine-grained clay-sand mixture. The Lyman Glacier debris cover is not a complete cover (85%), and is comprised of sand-boulder size particles. In the North Cascades temperate-maritime climate finer grain size debris cover provides a better insulation from ablation conditions.

Spatial and morphological change on Eliot Glacier, Mount Hood, Oregon, USA

Abstract: Eliot Glacier is a small (1.6 km 2 ) glacier on Mount Hood, Oregon, USA, and its ablation zone is largely covered with rock debris. We examine the interrelated processes of ablation rates, ice thickness and surface velocities to understand the retreat rate of this glacier. Since measurements began in 1901, the glacier has retreated 680 m, lost 19% of its area and thinned by about 50 m at the lower glacier profile before the terminus retreated past that point. The upper profile, 800 m up-glacier, has shown thinning and thickening due to a kinematic wave resulting from a cool period during the 1940s-70s, and is currently about the same thickness as in 1940. Overall, the glacier has retreated at a slower rate than other glaciers on Mount Hood. We hypothesize that the rock debris covering the ablation zone reduces Eliot Glacier's sensitivity to global warming and slows its retreat rate compared to other glaciers on Mount Hood. Spatial variations in debris thickness are the primary factor in controlling spatial variations in melt. A continuity model of debris thickness shows the rate of debris thickening down-glacier is roughly constant and is a result of the compensating effects of strain thickening and debris melt-out from the ice.

Tidal movement of Nioghalvfjerdsfjorden glacier, northeast Greenland: observations and modelling

Abstract: Nioghalvfjerdsfjorden glacier is a >60 km long and 20 km wide floating outlet glacier located at 79°30'N, 22° W, draining a large area of the northeast Greenland ice sheet. Climate, mass-balance and dynamics studies were carried out on the glacier in three field seasons in 1996, 1997 and 1998. As part of this work, tidal-movement observations were carried out by simultaneous differential global positioning system (GPS) measurements at several locations distributed on the glacier surface. The GPS observations were performed continously over several tidal cycles. At the same time, tiltmeter measurements were carried out in the grounding zones along the glaciers margins and upstream, where the glacier leaves the main ice sheet. A tide gauge installed in the sea immediately in front of the glacier front recorded the tide in the open sea during the field seasons. The observations show that the main part of the glacier tongue responds as a freely floating plate to the phase and amplitude of the local tide in the sea. However, kilometre-wide flexure zones exist along the marginal and upstream grounding lines. Attempts to model the observed tidal defectionand tilt patterns in the flexure zone by elastic-beam theory are unsuccessful, in contrast to previous findings by other investigators. The strongest disagreement between our measurements and results derived from elastic-beam theory is a significant variation of the phase of the tidal records with distance from the grounding line (most clearly displayed by the tilt records). We suggest that the viscous properties of glacier ice must be taken into account, and consequently that a viscoelastic-beam model must be used to adequately describe tidal bending of floating glaciers.

Recent areal and altimetric variations of Miage Glacier (Monte Bianco Massif, Italian Alps)

Abstract: Changes in ice thickness and area of the debris-covered tongue of Miage Glacier have been measured using 1975 and 1991 aerial photographs, supplemented by measurements of ice flow from the displacement of 24 supraglacial boulders. Results show an increase in surface elevation of the lower glacier of >40 m, but thinning upstream. Overall ice volume increased in the study period, with negligible detectable change to terminus position and glacier area. The pattern of thickness variation is interpreted as a response to positive mass balance sometime after c. 1951. The debris cover may have prevented ablation from reducing the amplitude of a kinematic wave. Thus, the downstream amplification of thickening in the zone of compressive flow has been largely preserved. Debris-covered glaciers, whose termini are often held to be unresponsive to climatic variability, may actually magnify mass balance perturbations if ice remains mobile to the terminus.

Distribution of sediment phosphorus pools and fluxes in relation to alum treatment.

Abstract: The distribution of sediment physical characteristics, sediment phosphorus (P) pools, and laboratory-based rates of P release from the sediments were used to identify regions and dosage for alum treatment in Wind Lake, Wisconsin. Using variations in sediment moisture content, we identified an erosional zone at depths 2.6 m. Mean concentrations of porewater P, loosely-bound P, iron- and aluminum-bound P, and mean rates of P release from sediments under anoxic conditions were high in the accumulation zone compared to sediment P characteristics in the erosional zone, indicating focusing of readily mobilized sediment P pools from shallow regions and accumulation to deep regions. We determined that a future alum treatment for control of internal P loading would be most effective at depths > 2.6 in the accumulation zone. The mean rate of anoxic P release from sediments encountered in the accumulation zone (8.3 mg m -2 d -1 ) was used in conjunction with a summer anoxic period of 122 d, and a treatment area of 1.6 km 2 to estimate an internal P load of 1,600 kg to be controlled. Our results suggest that an understanding of the distribution of sediment P pools and P fluxes in lakes provides a strategy for estimating alum dosage and application areas.

Resources and distribution of glaciers on the tibetan plateau

Abstract: Glaciers on the Tibetan Plateau are calculated based on the Chinese glacier inventory.Statistics shows that that there are 36 793 existing glaciers on the Tibetan Plateau,with a total area of 49 873.44 km 2 and ice volume of 4561.3857km 3,corresponding to 79.4%,84.0% and 81.6% of the total glaciers in China respectively.Tibetan Plateau is the largest existing glaciation area in low mid latitude in the world.Glaciers are distributed mainly in Kunlun Mts.,Himalayas Mts.and karakorum Mts.,accounting for over half of the total glacier number and glacier volume on the Tibetan Plateau.The total glacier number and glacier volume in the other 8 mountains account for only less than half of the total glacier number and glacier volume on the Tibetan Plateau.Due to the influence of climate and topography,the glacier number and glacier volume is larger in the northern side of the mountains than that in the southern side,except the Nyainqenlanglha Mts.and Gangdisi Mts..There are few large valley glaciers,accounting for only 4.6% of the total glacier number on the plateau.However,the glacial area and ice volume account for 33.5% and 48.6% respectively of the total glaciers on the plateau.There are 21 large valley glaciers covering an are a of over 100km 2 each.The Yinsugaiti glacier in the northern side of the Karakorum Mts.is 42km in lenghth,with an area of 379.97km 2. The total ice volume of all the glaciers on the Tibetan Plateau is 39 228×10 8 m 3 of water equivalent,which is about 10.8 times of the annual surface runoff on the plateau.The gigantic volume of fresh water reservoir on the Tibetan Plateau can provide 504×10 8 m 3 of water to river runoff.The spatial distribution of glacial melt water varies in different river basins.The runoff factor in the marine glacial area is much higher than that in the continental glacial area.

Debris entrainment and polythermal structure in the terminus of Storglaciären

Abstract: Ground penetrating radar recordings and measurements of ice motion have provided a detailed picture of movement and internal structure of the terminus of Storglaciaren, Sweden. The glacier is polythermal with a cold surface layer resulting in a frozen rim along the glacier margins and terminus. Englacial debris layers emerge at the glacier surface, forming supra-glacial ice-cored ridges. Measurements show that surface velocity decreases sharply by -50% across the ridge line. The transport mechanism through the cold ice to the surface is inferred to be through shearing of clean ice intercalated with the debris layers and not along discrete shear planes. The sediment is likely frozen on at the glacier bed. However, the way in which the sediment layers are brought up into the ice and sandwiched between clean ice layers is not answered.

Contemporary Significance of Snow and Ice Indicated by the Record in a Shallow Ice Core from a Temperate Glacier in Southwestern Monsoon Region

Abstract: The Mount Yulong is the southmost glacialized area both in China and in Eurasia, where 19 typical subtropics temperate glaciers exist. mainly formed under the climate of southwestern monsoon. In the summer of 1999. a firn core. 10. 10 m long to the glacier ice. was successfully recovered in the accumulation area of the largest glacier. Glacier Baishui No. 1. on the Mount Yulong. Periodic variations of climatic signals above the depth of 7.8 m are apparent. and five-year's boundaries of net accumulation can be identified by the annual changes of isotopic and ionic compositions. verified by some higher values of electrical conductivity and pH as well as the position of refrozen dirty ice layers in summer horizons. Average annual net accumulation from 1994/1995 to 1997 / 1998 is calculated to be about 900 mm water equivalent. Variation amplitude of δ18O decreases with increasing core depth, and isotopic homogenization occurs below the depth of 7.8 m. as a result of meltwater percolation. Above results suggest that, although high melting occurred in the temperate firn core. climatic signals had still kept for some years as a result of a protection effect of percolation-proof ice layers within the firn, especially of the thick ice layers in the summer horizons. However, because of slow melting existed between these ice layers. homogenization and a smoothed distribution of climatic records were finally caused during the formation process of glacial ice. Therefore, the high- melting temperate from core is quite limited for studying long-term climate change. Cycles of δ18O in the core roughly correlate to the variation trend of mean temperature and precipitation at Lijiang Station during the winter months of five balance years. Concentrations of Ca2+ and Mg2+ are much higher than that of Na+ and K+, reflecting that the air mass for precipitation was mainly from continental sources. as well as the materials in the core accumulated during the winter. Cl- and Na+ show a corresponding variation trend, indicating their same genesis. Concentrations of SO42- and NO3- are low. reflecting that the pollution caused by human activities is very low in this area. According to the sum of net income recovered from the core and an estimated ablation amount, the average annual precipitation above the equilibrium line is approximately assumed in the scope of 2 400- 3 100 mm. but it needed to be verified by long-term observation of mass balance.

Glaciological research of Bilchenock Glacier in Kamchatka, 1998

Abstract: Bilchenock Glacier is a surging glacier in the Kamchatka Peninsula, Russia, which most recently surged in 1982 and is currently in its quiescent phase. To get information on the quiescent phase behavior of Bilchenock Glacier, a preliminary research on the glacier dynamics and meteorological features was carried out from July 18 to August 17, 1998. The surface of the ablation area of Bilchenock Glacier exhibits repeated patterns of a ridge and a gentle slope, and the distribution of flow speeds was considerably irregular ranging from about 0. 05 m day -1 to 0.17 m day -1 within a 2-km part in the lower reach. The terminus has risen by about 250 -350 m from 1984 to 1998. Daily mean air temperature varied from 7 to 17 °C and relative humidity was often close to 100 % near the glacier terminus. Averaged daily amount of icemelt was 56 mm water equivalent near the glacier terminus. Though the next surge may be predicted to occur in the near future based on its interval of surge about 23 years, no remarkable premonition of surge was found during this research.