Showing papers in "Journal of Glaciology in 1997"

The thermal conductivity of seasonal snow

Abstract: Twenty-seven studies on the thermal conductivity of snow (k eff ) have been published since 1886. Combined, they comprise 354 values of k eff , and have been used to derive over 13 regression equations predicting k eff vs density. Due to large (and largely undocumented) differences in measurement methods and accuracy, sample temperature and snow type, it is not possible to know what part of the variability in this data set is the result of snow microstructure. We present a new data set containing 488 measurements for which the temperature, type and measurement accuracy are known. A quadratic equation, k eff = 0.138 - 1.01ρ + 3.233ρ 2 {0.156 ≤ ρ ≤ 0.6} k eff = 0.023 + 0.234ρ {ρ < 0.156}, where ρ is in g cm -3 and k eff is in W m -1 K -1 , can be fit to the new data (R 2 =0.79). A logarithmic expression, k eff = 10 (2.650ρ-1.652) {ρ ≤ 0.6}, can also be used. The first regression is better when estimating values beyond the limits of the data; the second when estimating values for low-density snow. Within the data set, snow types resulting from kinetic growth show density-independent behavior. Rounded-grain and wind-blown snow show strong density dependence. The new data set has a higher mean value of density but a lower mean value of thermal conductivity than the old set. This shift is attributed to differences in snow types and sample temperatures in the sets. Using both data sets, we show that there are well-defined limits to the geometric configurations that natural seasonal snow can take.

Fennoscandian palaeoglaciology reconstructed using a glacial geological inversion model

Abstract: The evolution of ice-sheet configuration and flow pattern in Fennoscandia through the last glacial cycle was reconstructed using a glacial geological inversion model, i.e. a theoretical model that formalises the procedure of using the landform record to reconstruct ice sheets. The model uses mapped flow traces and deglacial melt-water landforms, as well as relative chronologies derived from cross-cutting striae and till lineations, as input data. Flow-trace systems were classified into four types: (i) time-transgressive wet-bed deglacial fans, (ii) time-transgressive frozen-bed deglacial fans, (iii) surge fans, and (iv) synchronous non-deglacial (event) fans. Using relative chronologies and aggregation of fans into glaciologically plausible patterns, a series of ice-sheet Configurations at different time slices was erected. A chronology was constructed through correlation with dated stratigraphical records and proxy data reflecting global ice volume. Geological evidence exists for several discrete ice-sheet configurations centred over the Scandinavian mountain range during the early Weichselian. The build-up of the main Weichselian Fennoscandian ice sheet started at approximately 70 Ka, and our results indicate that it was characterised by an ice sheet with a centre of mass located over southern Norway. This configuration had a flow pattern which is poorly reproduced by current numerical models of the Fennoscandian ice sheet. At the Last Glacial Maximum the main ice divide was located overthe Gulf of Bothnia. A major bend in the ice divide was caused by outflow of ice to the northwest over the lowest part of the Scandinavian mountain chain. Widespread areas of preserved pre-late-Weichselian landscapes indicate that the ice sheet had a frozen-bed core area, which was only partly diminished in size by inward-transgressive wet-bed zones during the decay phase.

Basal hydraulic system of a West Antarctic ice stream: constraints from borehole observations

Abstract: Pressure and tracer measurements in boreholes drilled to the bottom of Ice Stream B, West Antarctica, are used to obtain information about the basal water conduit system in which high water pressures are developed. These high pressures presumably make possible the rapid movement of the ice stream. Pressure in the system is indicated by the borehole water level once connection to the conduit system is made. On initial connection, here also called "breakthrough" to the basal water system, the water level drops in a few minutes to an initial depth in the range 96- 117 m below the surface. These water levels are near but mostly somewhat deeper than the flotation level of about 100m depth (water level at which basal water pressure and ice overburden pressure are equal), which is calculated from depth-density profiles and is measured in one borehole. The conduit system can be modelled as a continuous or somewhat discontinuous gap between ice and bed; the thickness of the gap δ has to be about 2 mm to account for the water-level drop on breakthrough, and about 4 mm to fit the results of a salt-tracer experiment indicating downstream transport at a speed of 7.5 mm s^-1. The above gap-conduit model is, however, ruled out by the way a pressure pulse injected into the basal water system at breakthrough propagates outward from the injection hole, and also by the large hole-to-hole variation in measured basal pressure, which if present in a gap-conduit system with δ = 2 or 4 mm would result in unacceptably large local water fluxes. An alternative model that avoids the e objections, called the "gap opening" model, involves opening a gap as injection proceeds: starting with a thin film, the injection of water under pressure lifts the ice mass a round the borehole, creating a gap 3 or 4 mm wide at the ice/bed interface. Evaluated quantitatively, the gap-opening model accounts for the volume of water that the basal watery system accepts on breakthrough, which obviates the gap-conduit model. In order to transport basal meltwater from upstream it is then necessary for the complete hydraulic model to contain also a network of relatively large conduits, of which the most promising type is the "canal" conduit proposed theoretically by Walder and Fowler (1994): flat, low conduits incised into the till, ~0.1 m deep and perhaps ~1 m wide, with a flat ice roof The basal water-pressure data suggest that the canals are spaced ~50-300 m apart, much closer than R-tunnel would be. The deepest observed water level, 117 m, i the most likely to reflect the actual water pressure in the canals, corresponding to a basal effective pressure of 1.6 bar. In this interpretation, the shallower water levels are affected by loss of hydraulic head in the narrow passageway(s; that connect along the bed from borehole to canal (s). Once a borehole has frozen up and any passageways connecting with canals have become closed, a pressure sensor in contact with the unfrozen till that underlies the ice will measure the pore pressure in the till, given enough time for pressure equilibration. This pressure varies considerably with time, over the equivalent water-level range from 100 to 113 m. Basal pressure sensors 500 m apart report uncorrelated variations, whereas sensors in boreholes 25 m apart report mostly (but not entirely) well-correlated variations, of unknown origin. In part of the record, remarkable anticorrelated variations are interspersed with positively correlated one, and there are rare, abrupt excursions to extreme water levels as deep as 125m and as shallow as 74 m. A diurnal pressure fluctuation, intermittently observed, may possibly be caused by the ocean tide in the Ross Sea. The lack of any observed variation in ice-stream motion, when large percentagewise variations in basal effective pressure were occurring according to our data, suggests that the observed pressure variations are sufficiently local, and so randomly variable from place to place, that they are averaged out in the process by which the basal motion of the ice stream is determined by an integration over a large area of the bed.

Vapor transport, grain growth and depth-hoar development in the subarctic snow

Abstract: Measurements from the subarctic snowpack are used to explore the relationship between grain growth and vapor flow, the fundamental processes of dry-snow metamorphism. Due to extreme temperature gradients, the subarctic pack undergoes extensive depth-hoar metamorphism. By the end of the winter a five-layered structure with a pronounced weak layer near the base of the snow evolves. Grain-size increases by a factor of 2-3, while the number of grains per unit mass decreases by a factor of 10. Observed growth rates require significant net inter-particle vapor fluxes. Stable-isotope ratios show that there are also significant net layer-to-layer vapor fluxes. Soil moisture enters the base of the pack and mixes with the bottom 10 cm of snow, while isotopically light water vapor fractionates from the basal layer and is deposited up to 50 cm higher in the pack. End-of-winter density profiles for snow on the ground, compared with snow on tables, indicate the net layer-to-layer vapor flux averages 6 x 10 -7 kg m 2 s -1, , though detailed measurements show the net flux is episodic and varies with time and height in the pack, with peak net fluxes ten times higher than average. A model, driven by observed temperature profiles, reproduces the layer-to-layer flux pattern and predicts the observed weak layer at the base of the snow. Calculated layer-to-layer vapor fluxes are ten times higher than inter-particle fluxes, which implies that depth-hoar grain growth is limited by factors other than the vapor supply. This finding suggests that gain and loss of water molecules due to sublimation from grains takes place at a rate many times higher than the rate at which grains grow, and it explains why grains can metamorphose into different forms so readily.

The relationship between subglacial water pressure and velocity of Findelengletscher, Switzerland, during its advance and retreat

Abstract: Findelengletscher, Switzerland, advanced about 250 m between 1979 and 1985, and retreated therearfter. Subglacial water pressure, surface velocity and surface strain rate were determined at several sites. The measurements were made early in the melt seasons of 1980, 1982, 1985 and 1994 and in the autumn of 1983 and the winter of 1984. Changes of surface geometry were assessed form aerial photographs. The estimated basal shear stress changed little between 1982 adn 1994. Nevertheless, large changes in the relationship of subglacial water pressure and surface veolcity were observed, which cannot be reconciled with the most commonly used sliding law unless it is modified substantially. Considereation of possible reasons indicates that a change in the subglacial drainage sustem occurred, probably involving a change in the degree of cavity interconnection. Isolated cavities damp the variations in sliding velocity that normally result form changes in water pressure, because the pressure in isolated cavities decreases as the sliding speed increases. In contrast, by transmitting water-pressure fluctuations to a larger area of the bed, interconnected cavities amplify the effect of water-pressure fluctuations on sliding speed. Thus we suggest that an observed decrease in velocity (for a given water pressure) between 1982 and 1994 was a consequence of a decrease in the interconnectedness of the subglacial cavity system.

Site information and initial results from deep ice drilling on Law Dome, Antarctica

Abstract: The aim of deep ice drilling on Law Dome, Antarctica, has been to exploit the special characteristics of Law Dome summit, ie low temperature and high accumulation near an ice divide, to obtain a high-resolution ice core for climatic/ environmental studies of the Holocene and the Last Glacial Maximum (LGM) Drilling was completed in February 1993, when basal ice containing small fragments of rock was reached at a depth of 1196 m Accurate ice dating, obtained by counting annual layers revealed by fine-detail δ 18 O, peroxide and electrical-conductivity measurements, is continuous down to 399 m, corresponding to a date of AD 1304 Sulphate concentration measurements, made around depths where conductivity tracing indicates volcanic fallout, allow confirmation of the dating (for Agung in 1963 and Tambora in 1815) or estimates of the eruption date from the ice dating (for the Kuwae,Vanuatu, eruption ∼1457) The lower part of the core is dated by extrapolating the layer-counting using a simple model of the ice flow At the LGM, ice-fabric measurements show a large decrease (250 to 14 mm 2 ) in crystal size and a narrow maximum in c-axis verticality The main zone of strong single-pole fabrics however, is located higher up in a broad zone around 900 m Oxygen-isotope (δ 18 O) measurements show Holocene ice down to 1113 m, the LGM at 1133 m and warm (δ 18 O about the same as Holocene) ice near the base of the ice sheet The LGM/Holocene δ 18 O shift of 70‰, only ∼1‰ larger than for Vostok, indicates that Law Dome remained an independent ice cap and was not overridden by the inland ice sheet in the Glacial

Rheology of till beneath Storglaciären, Sweden

Abstract: In order to study, in situ, the rheology of a deforming subglacial till, various instruments were emplaced in till beneath Storglaciaren, Sweden. Boreholes were used to gain access to the till beneath about 100 m of ice. Tiltmeters provided an estimate of the shear strain rate in the till. Two other instruments yielded measures of till strength. In addition, water pressures were recorded in boreholes and in the till, a computer-controlled distance meter provided an effectively continuous record of the surface velocity and data from frequent surveys of a stake network were used to estimate the mean basal drag, based on a force-balance calculation. Tilt rates varied directly with effective pressure, so decreases in water pressure apparently increased the coupling between the glacier and the bed. Surface speed was either out of phase with tilt or varied independently of tilt. Thus, increases in speed were apparently a consequence either of longitudinal coupling or of reduced coupling between the glacier and the bed; they were not a result of till deformation! Till strength varied directly with effective pressure, which is consistent with it being a Mohr-Coulomb, or frictional material. The devices measuring till strength are presumed to have been pulled through the till at a speed that varied in phase with the surface speed but till strength did not vary systematically with surface speed. This implies that the residual strength of the till is insensitive to strain rate. Thus, the appropriate constitutive equation for till rheology may be of the form: e˙∞ e kτ where k is a constant. This is consistent with experimental data reported in the geotechnical literature.

Response of sediment to ice-sheet loading in northwestern Germany: effective stresses and glacier-bed stability

Abstract: Laboratory tests on sediment over-ridden by the last ice sheet in north-western Germany reveal very low ice-induced pre-consolidation and high palaeo-pore-water pressures. Sediment consolidation at the base of the glacier was largely controlled by hydraulic properties of the substratum. Generally low permeabilities of the bed caused sustained high pore-water pressure in over-ridden sediments close to the flotation point. This implies a serious possibility of hydraulic lifting of the ice sheet. It is believed that the reduced basal coupling limited the transformation of glacier shear stress on to the bed sediments, which is indicated by a lack of sedimentological evidence for widespread pervasive bed deformation. Ice motion was probably focused at the glacier sole by some combination of sliding and ploughing. However, isolated spots with deformation occur, so that the subglacial system in the study area can be characterized as a stable/deforming mosaic.

Altimetric observations of surface characteristics of the Antarctic ice sheet

Abstract: The aim of this paper is to investigate the geophysical characteristics of the Antarctic ice sheet using radar altimetric observations. To do this, we use an altimetric waveform simulator, in situ observations, ERS-1 (European remote-sensing satellite) data and SPOT (Satellite pour l’observation de la terre) images. The small-scale study takes place at Dome C, Terre Adelie, which is a relatively flat region with gentle undulations and low wind speed. Despite this, the altimetric waveform parameters (height, energy, leading edge and trailing edge) are highly noisy. The effect of undulations on the waveform parameters is found to be dominant. The combination of a subsurface signal and a rough surface produces a linear effect on the altimetric backscattering or on the trailing edge of the waveform, but a strongly non-linear effect on the leading edge of the waveform or height estimation. As a consequence, the height measurement is very sensitive to the altimeter technical or orbital characteristics and is not reproducible from one mission to another. Observations show sastrugi fields that enhance the leading edge and affect the whole waveform. Observed local backscattering changes, probably due to local variations in surface microroughness, enhance the backscattered energy and may artificially create a topographic signal. The continental-scale study shows coherent patterns. Even if both surface and subsurface components affect the altimetric observation, the large-scale signal is mostly controlled by surface backscattering variations. The surface or near-subsurface characteristics of the snowpack may then be reached by altimetric observations.

Pore-water controlled grain fracturing as an indicator for subglacial shearing in tills

Abstract: Genetic classification of glacial deposits still proves to be highly controversial. In many cases, it remains unclear whether a particular sediment has been sub-glacially deformed. In the present paper it is suggested that micromorphological research is helpful where current techniques fail to lead to unambiguous interpretations. It is argued that the occurrence of fractured grains in glacial sediments is indicative of subglacial shearing and that such grains may be used as diagnostic evidence of glaciotectonism. Deformational mechanisms associated with the fracturing process are outlined and explained, using a series of thin sections from Wijnjewoude, The Netherlands.

The marginal shear stress of Ice Stream B, West Antarctica

Abstract: To ascertain whether the velocity of Ice Stream B, West Antarctica, may be controlled by the stress in its marginal shear zones (the "Snake" and the "Dragon"), we undertook a determination of the marginal shear stress in the Dragon near Camp Up B by using ice itself as a stress meter. The observed marginal shear strain rate of 0.14 a^(-1) is used to calculate the marginal shear stress from the flow law of ice determined by creep tests on ice cores from a depth of 300 m in the Dragon, obtained by using a hot-water ice-coring drill. The test-specimen orientation relative to the stress axes in the tests is chosen on the basis of c-axis fabrics so that the test applies horizontal shear across vertical planes parallel to the margin. The resulting marginal shear stress is (2.2 ± 0.3) × 10^5 Pa. This implies that 63-100% of the ice stream's support against gravitational loading comes from the margins and only 37-0% from the base, so that the margins play an important role in controlling the ice-stream motion. The marginal shear-stress value is twice that given by the ice-stream model of Echelmeyer and others (1994} and the corresponding strain-rate enhancement factors differ greatly (E ≈ 1-2 vs 10-12.5). This large discrepancy could be explained by recrystallization of the ice during or shortly after coring. Estimates of the expected recrystallization time-scale bracket the ~1 h time-scale of coring and leave the likelihood of recrystallization uncertain. However, the observed two-maximum fabric type is not what is expected for annealing recrystallization from the sharp single-maximum fabric that would be expected in situ at the high shear strains involved (γ~20). Experimental data from Wilson (1982) suggest that, if the core did recrystallize, the prior fabric was a two-maximum fabric not substantially different from the observed one, which implies that the measured flow law and derived marginal shear stress are applicable to the in situ situation. These issues need to be resolved by further work to obtain a more definitive observational assessment of the marginal shear stress.

The role of lateral drag in the dynamics of Ice Stream B, Antarctica

Abstract: The partitioning of resistive force between the bed and sides of Ice Stream B, Antarctica, is obtained for three large areas that have been measured using repeat aerial photogrammetry. Problems associated with data errors and local variations in ice strength and velocity are reduced by considering the areally averaged budget of forces for each photo block. Results indicate that the bed under Ice Stream B must be very weak and unable to provide muche resistance. Mechanical control on this ice stream emanates almost entirely from the lateral margins.

Seasonal variations in the properties and structural composition of sea ice and snow cover in the Bellingshausen and Amundsen Seas, Antarctica

Abstract: Sixty-three ice cores were collected in the Bellingshausen and Amundsen Seas in August and September 1993 during a cruise of the R.V. Nathaniel B. Palmer. The structure and stable-isotopic composition (18O/16O) of the cores were investigated in order to understand the growth conditions and to identify the key growth processes, particularly the contribution of snow to sea-ice formation. The structure and isotopic composition of a set of 12 cores that was collected for the same purpose in the Bellingshausen Sea in March 1992 are reassessed. Frazil ice and congelation ice contribute 44% and 26%, respectively, to the composition of both the winter and summer ice-core sets, evidence that the relatively calm conditions that favour congelation-ice formation are neither as common nor as prolonged as the more turbulent conditions that favour frazil-ice growth and pancake-ice formation. Both frazil- and congelation-ice layers have an av erage thickness of 0.12 m in winter, evidence that congelation ice and pancake ice thicken primarily by dynamic processes. The thermodynamic development of the ice cover relies heavily on the formation of snow ice at the surface of floes after sea water has flooded the snow cover. Snow-ice layers have a mean thickness of 0.20 and 0.28 m in the winter and summer cores, respectively, and the contribution of snow ice to the winter (24%) and summer (16%) core sets exceeds most quantities that have been reported previously in other Antarctic pack-ice zones. The thickness and quantity of snow ice may be due to a combination of high snow-accumulation rates and snow loads, environmental conditions that favour a warm ice cover in which brine convection between the bottom and top of the ice introduces sea water to the snow/ice interface, and bottom melting losses being compensated by snow-ice formation. Layers of superimposed ice at the top of each of the summer cores make up 4.6% of the ice that was examined and they increase by a factor of 3 the quantity of snow entrained in the ice. The accumulation of superimposed ice is evidence that melting in the snow cover on Antarctic sea-ice floes ran reach an advanced stage and contribute a significant amount of snow to the total ice mass.

Some comments on climatic reconstructions from ice cores drilled in areas of high melt

Abstract: Poor consideration has been given in many Arctic circum-polar ice-core studies to the effect of summer snow melt on chemistry, stable-isotope concentrations and time-scales. Many of these corps are drilled close to the firn line where melt is intense. Some come from below the firn line where accumulation is solely in the form of super-imposed ice. In all cases, seasonal signals are reduced or removed and, in some, time gaps develop during periods of excessive melting which situate the drill site in the ablation zone. Consequently, cross correlations of assumed synchronous events among the cores are invalid, so that time-scales along the same cores differ between authors by factors of over 2. Many so-called climatic signals are imaginary rather than real. By reference to published analyses of cores from the superimposed ice zone on Devon Ice Cap (Koerner, 1970) and Meighen Ice Cap (Koerner and Paterson, 1974), it is shown how melt affects all the normally well-established ice-core proxies and leads to their misinterpretation. Despite these limitations, the cores can give valuable low-resolution records for all or part of the Holocene. They show that the thermal maximum in the circum-polar Arctic occurred in the early Holocene. This maximum, effected negative balances on all the ice caps and removed the smaller ones. Cooler conditions in the second half of the Holocene have caused the regrowth of these same ice caps.

Glacier recession on Kilimanjaro, East Africa, 1912-89

Abstract: The varying ice extent on the Kibo cone of Kilimanjaro, East Africa, this century was reconstructed from terrestrial-photogrammetric mappings in 1912 and 1953 and satellite images in 1976 and 1989. These were all conformed to the official 1:50 000 topographic sheet. The ice distribution on Kibo is characterized by lower ice margins on the southern than on the northern flanks, and deepest-reaching glaciers to the west contrasting with least ice cover on the eastern crater rim. The meridional contrast appears to be related to the more abundant precipitation on the southern side. The even more striking zonal asymmetry in ice cover is favored by the vigorous diurnal circulations, with enhanced afternoon cloudiness blocking insolation on the western slopes. The total ice cover diminished from 12 058 to 6675 to 4171 to 3305 x 10 3 m 2 between the years 1912, 1953, 1976 and 1989, and the three large coherent ice bodies disintegrated into numerous smaller ice fragments.

Changes in glaciers in Hidden Valley, Mukut Himal, Nepal Himalayas, from 1974 to 1994

Abstract: Glaciological investigations were carried out in 1994 on the glaciers in Hidden Valley, Mukut Himal, Nepal Himalayas, in order to make a comparison with observations made in 1974. Most of the glaciers were found to have retreated by 30-60 m in terminus elevation over the 20 years between the two studies. Rikha Samba Glacier, the longest glacier in the valleys, has retreated by about 200 m. The areal average of the amount of surface lowering and the volume loss of the glacier was estimated to be 12.6 m ice equivalent and 13% of the total mass, respectively. The annual mass balance of -0.55 m a -1 water equivalent was obtained as an average for 20 years, which is one of the largest negative values amongst small glaciers of the world.

Glaciological and oceanographic evidence of high melt rates beneath Pine Island Glacier, West Antarctica

Abstract: Satellite imagery indicates that the floating terminus of Pine Island Glacier has changed little in extent over the past two decades. Data on the velocity and thickness of the glacier reveal that calving of 28 ± 4 Gt a-1 accounts for only half of the ice input near the grounding line. The apparently steady configuration implies that the remainder of the input is lost by basal melting at a mean rate of 12 ± 3 m a-1. Ocean circulation in Pine Island Bay transports +1°C waters beneath the glacier and temperatures recorded in melt-laden outflows show that heat loss from the ocean is consistent with the requirements of the calculated melt rate. The combination of iceberg calving and basal melting lies at the lower end of estimates for the total accumulation over the catchment basin, drawing into question previous estimates of a significantly positive mass budget for this part of the ice sheet.

Bubbly-ice densification in ice sheets : II. Applications

Abstract: A mathematical model for simulating the densification of bubbly glacier ice is used to interpret the following experimental data from the Vostok (central Antarctica) ice core: two ice-porosity profiles obtained by independent methods and a bubble-pressure profile obtained by direct measurements of air pressure within individual bubbles. The rheological properties of pure polycrystalline ice are deduced from the solution of the inverse problem. The model and the inferred ice-flow law are then validated, using porosity profiles from seven other ice cores drilled in Antarctica and Greenland, in the temperature range from -55° to -20°C. The following expression is adopted for the constitutive law: 2e˙ = (τ/μ 1 + τ α /μ 2 ) exp[Q(1/T s - 1/T)/R s ] where e˙ and τ are the effective strain rate and stress, respectively, α is the creep exponent taken as 3.5, R s is the gas constant and T(T s ) is the temperature (standard temperature). The numerical values obtained for the linear and non-linear viscosities are: μ 1 = 2.9 ± 1.3 MPayear and μ 2 = 0.051 ± 0.019 MPa α year, and the apparent activation energy Q is confirmed to be 60 kJ mole -1 . The corresponding flow law is in good agreement with results of both mechanical tests and independent estimations based on the analysis of different natural phenomena associated with glacier-ice deformation. When the model is constrained by the porosity and bubble-pressure profiles from Vostok, the mean air content in Holocene ice is inferred to be about 0.088 cm 3 g -1 . The corresponding mean air pressure in bubbles at the end of pore closure is about 0.083 MPa, whereas the atmospheric pressure at this depth level would be 0.063 MPa. The influence of the climatic change on the ice-porosity profile is discussed. It resulted in an increased air content in ice at Vostok during the Last Glacial Maximum: 0.096 cm 3 g -1 .

The response of two Icelandic glaciers to climatic warming computed with a degree-day glacier mass-balance model coupled to a dynamic glacier model

Abstract: A degree-day glacier mass-balance model is coupled to a dynamic glacier model for temperate glaciers The model is calibrated for two outlet glaciers from the Hofsjokull ice cap in central Iceland It is forced with a climate scenario that has recently been defined for the Nordic countries for the purpose of outlining the hydrological consequences of future greenhouse warming The scenario for Iceland specifies a warming rate of 025°C per decade in mid-summer and 035°C per decade in mid-winter with a sinusoidal variation through the year The volume of the glaciers is predicted to decrease by approximately 40% over the next century, and the glaciers essentially disappear during the next 200 years Runoff from the area that is presently covered by the glaciers is predicted to increase by approximately 05 m a -1 30 years from now due to the reduction in the volume of the glaciers The runoff increase reaches a flat maximum of 05-20 m a -1 100-150 years from now and levels off after that The predicted runoff increase leads to a significant increase in the discharge of rivers fed by meltwater from the outlet glaciers of the ice cap and may have important consequences for the operation and planning of hydroelectric power plants in Iceland

A search in north Greenland for a new ice-core drill site

Abstract: A new deep ice-core drilling site has been identified in north Greenland at 75.12°N, 42.30°W, 316 km north-northwest (NNW) of the GRIP drill site on the summit of the ice sheet. The ice thickness here is 3085 m; the surface elevation is 2919 m. The North GRIP (NGRIP) site is identified so that ice of Eemian age (115-130 ka BP, calendar years before present) is located as far above bedrock as possible and so the thickness of the Eemian layer is as great as possible. An ice-flow model, similar to the one used to date the GRIP ice core, is used to simulate the flow along the NNW-trending ice ridge. Surface and bedrock elevations, surface accumulation-rate distribution and ratio-echo sounding along the ridge have been used as model input. The surface accumulation rate drops from 0.23 m ice equivalent year -1 at GRIP to 0.19 m ice equivalent year -1 50 km from GRIP. Over the following 300 km the accumulation is relatively constant, before is starts decreasing again further north. Ice thickness up to 3250 m bring the temperature of the basal ice up to the pressure-melting point 100-250 km from GRIP. The NGRIP site is located 316 km from GRIP in a region where the bedrock is smooth and the accumulation rate is 0.19 m ice equivalent year -1 . The modeled basal ice here has always been a few degrees below the pressure-melting point. Internal radio-echo sounding horizons can be traced between the GRIP and NGRIP sites, allowing us to date the ice down to 2300 m depth (52 ka BP). An ice-flow model predicts that the Eemian-age ice will be located in the depth range 2710-2800 m, which is 285 m above the bedrock. This is 120 m further above the bedrock, and the thickness of the Eemian layer of ice is 20 m thicker, than at the GRIP ice-core site.

Recession of the southern part of Barnes Ice Cap, Baffin Island, Canada, between 1961 and 1993, determined from digital mapping of Landsat TM

Abstract: Changes along the margin of the southern half of the 5900 km 2 Barnes Ice Cap have been assessed using 1993 Landsat TM imagery in comparison with digitized 1: 50 000 NTS maps based on 1961 photogrammetry. The average recession over the 183 km long southern perimeter was found to be at least 4 m a -1 , with no significant difference between the southeast and southwest sectors. Viewed in conjunction with the sustained retreat previously reported for the northwest margin, these results indicate that a general reduction in the size of Barnes Ice Cap is occurring. The present retreat phase began under a regional climate warming in the late 19th to early 20th century period and continues, while the record of the ablation-season temperature since the midcentury has not shown any significant trend.

Basal-flow characteristics of a non-linear flow sliding frictionless over strongly undulating bedrock

Abstract: The flow field of a medium sliding without friction over a strongly undulating surface is calculated numerically. The results are used to elucidate the basal-flow characteristics of glacier flow and they are discussed with reference to known analytical solutions. Extrusion flow is found to become increasingly pronounced as the value of n, where n is a parameter in Glen’s flow law, becomes larger. For sinusoidal bedrock undulations, a flow separation occurs if the amplitude-to-wavelength ratio exceeds a critical value of about 0.28. The main flow then sets up a secondary flow circulation within the trough, and the ice participating in this circular motion theoretically never leaves it. The sliding velocity is calculated numerically as a function of the mean basal shear stress, the amplitude-to-wavelength ratio and the flow parameter n. For moderate and high slope fluctuations, the sliding velocity is significantly different from what would be expected from results based on the small-slope approximation.

Width and length scaling of glaciers

Abstract: An analysis of hundreds of mountain and valley glaciers in the former Soviet Union and the Alps shows that characteristic glacier widths scale as characteristic glacier lengths raised to an exponent of 0.6. This is in contrast to most previous analyses which implicitly of explicitly assumed scaling exponents of either 0 or 1. The exponent 0.6 implies that average glacier widths are proportional to average glacier thicknesses. Although this seems to suggest V-shaped glacier valleys, the linear width-thickness relationship is not inconsistent with parabolic valley cross-sections, because the characteristic (or average) width of a glacier depends on many other aspects of channel and glacier morphology, including variations in the channel width with distance up- and downstream.

Snow accumulation and ice flow at Dôme du Goûter (4300 m), Mont Blanc, French Alps

Abstract: Glaciological experiments have been carried out at Dome du Gouter (4300 m a.s.l.), Mont Blanc, in order to understand the flow of firn/ice in this high-altitude Alpine glacierized area. Accumulation measurements from stakes show a very strong spatial variability and an unusual feature of mass-balance fluctuations for the Alps, i.e. the snow accumulation does not show any seasonal patterns. Measured vertical velocities which should match with long-term mean mass balance are consistent with observed accumulations. Therefore, the measurement of vertical velocities seems a good way of quickly obtaining reliable mean accumulation values for several decades in such a region. A simple flow model can be used to determine the main flowlines of the glacier and to propose snow/ice age of core samples from the two boreholes drilled down to the bedrock in June 1994. These results coincide with radioactivity measurements made to identify the well-known radioactive snow layers of 1963 and 1986. We can hope to obtain ice samples 55-60 years old about 20 or 30 m above the bedrock (110 m deep). Below, the deformation of the ice layers is too great to be dated accurately.

Temperature measurements and heat transfer in near-surface snow at the South Pole

Abstract: To study near-surface heat flow on the Antarctic ice sheet, snow temperatures were measured at South Pole Station to a depth of 3 m at 15 min intervals during most of 1992. Solar heating and water-vapor transport were negligible during the 6 month Winter, as was inter-grain net thermal radiation, leaving conduction as the dominant heat-transport mechanism. The rate of temperature change at depth over 15 min intervals was smaller than that at the surface, by one order of magnitude at 20 cm depth and two orders of magnitude at 1 m depth. A finite-difference model, with conduction as the only heat-transfer mechanism and measured temperatures as the upper and lower boundary conditions, was applied to foursets of three thermistors each. The thermal conductivity was estimated as that which minimized the difference between modeled and measured 15 min changes in temperatures at the center thermistor. The thermal conductivity obtained at shallow depths (above 40 cm) was lower than that given by existing parameterizations based on density, probably because the snow grains were freshly deposited, cold and poorly bonded. A model using only vertical conduction explains on average 87% ofthe observed 15 min temperature changes at less than 60 cm depth and 92% below 60 cm. The difference between modeled andmeasured temperature changes decreased with depth. The discrepancies between model and observation correlated more strongly with the air-snow temperature difference than with the product of that difference with the square of the wind speed,suggesting that the residual errors are due more to non-vertical conduction and to sub-grid-scale variabilis of the conductivity than to windpumping. The residual heating rate not explained by the model of vertical conduction exceeds 0.2 W m−3 only in the top 60 cm of the near-surface snow.

Meteorological controls on glacier mass balance : empirical relations suggested by measurements on glacier de Sarennes, France

Abstract: Glacial mass-balance reconstruction for a long-term time-scale requires knowledge of the relation between climate change and mass-balance fluctuations. A large number of mass-balance reconstructions since the beginning of the century are based on statistical relations between monthly meteorological data and mass balance. The question examined in this paper is: are these relationships reliable enough for long-term time-scale extrapolation? From the glacier de Sarennes long mass-balance observations series, we were surprised to discover large discrepancies between relations resulting from different time periods. The importance of the albedo in relation to ablation and mass balance is highlighted, and it is shown that it is impossible to ignore glacier-surface conditions in establishing the empirical relation between mass-balance fluctuations and climatic variation; to omit this parameter leads to incorrect results for mass-balance reconstruction in the past based on meteorological data.

Variations in basal conditions on Rutford Ice Stream, West Antarctica

Abstract: Seismic reflection data from two lines on Rutford Ice Stream are presented and are compared with data already published from a third line on the ice stream The amplitude and phase of the reflections have been used to investigate the properties of the sub-ice material Multiple reflections on long record-length data allowed calibration of the reflection coefficient at the ice-bed interface and determination of the acoustic impedance of the bed material The characteristics of the bed material vary both along and across the ice stream The average acoustic impedance of the bed material across the glacier at the upstream line is 388 x 106 kg m-2s-1 This decreases to 319 x 106kg m-2s-1 52 km further downstream These values are within the range which is typical of soft sediments Using acoustic impedance as an indicator of subglacial porosity, some areas of the ice-stream bed are interpreted as dilatant water-saturated sediments undergoing pervasive deformation In other areas, the bed is not deforming and basal sliding may be a more important process The proportion of the ice-stream width over which bed deformation occurs increases downstream

Simulation of the historical variations in length of Unterer Grindelwaldgletscher, Switzerland

Abstract: The historical length variations in Unterer Grindelwaldgletscher have been simulated by coupling a numerical mass-balance model to a dynamic ice-flow model. As forcing functions, we used (partly reconstructed) local climatic records, which were transformed by the mass-balance model into a mass-balance history. The ice-flow model then computes the length variations that have occurred over the course of time. In a model run from AD 1530 to the present, with both seasonal temperature and precipitation variations as forcing functions, the observed maximum length of the glacier around AD 1860 and the subsequent retreat are simulated. The observed AD 1600 maximum, however, does not show up in the simulation. This is probably due to an incorrect reconstruction of the mass balance for this period, as detailed climatic data are available only since 1865. The root-mean-square difference between the simulated and the observed front positions is 0.28 km. The simulated glacier geometry for 1987 fits the observed geometry for that year reasonably well. Because of the success of the historical simulation, an attempt is made to predict future glacier retreat on the basis of two different greenhouse-gas scenarios. For a Business-asUsual scenario, only 29% of the 1990 volume would remain in AD 2100.

Model of wind pumping for layered snow

Abstract: Layering affects the air flow through snow caused by surface pressure variations. The horizontal and total fluxes are high in hoar layers but the pressure perturbations and vertical components of the flow do not penetrate as deeply as in homogeneous snow. That is because the layers “pipe” the flow horizontally toward the area of low pressure. An ice layer at the surface reduces the total flow everywhere. The flow decreases as ice-layer thickness increases and, in general, flow changes with permeability. However, the magnitude of the effect is proportionately weaker when the ice layers are further from the surface. The residence time is reduced when hoar layers are present due to shorter flow paths, reduced penetration into the deeper snow and higher speeds.

Surge-type glaciers in the Russian High Arctic identified from digital satellite imagery

Abstract: Landsat digital imagery was used to search the island archipelagos of Franz Josef Land, Severnaya Zemlya and Novaya Zemlya, Russian High Arctic, for the presence of looped moraines characteristic of past glacier surges. The imagery provides almost complete summer-time coverage of the 60000 km 2 of ice in these islands. Very few surge-type glaciers are identified: none in Franz Josef Land, three in Novaya Zemlya and two on Severnaya Zemlya. This contrasts greatly with Svalbard (ice-covered area 36 600 km 2 ), to the west, where 36% of glaciers and ice-cap drainage basins are inferred to surge. The strong climatic gradient across the Eurasian High Arctic, with decreasing temperature and moisture eastward, may provide a gross control on this pattern through colder glacier thermal structure, limiting basal drainage on the thinner ice masses in particular.