Showing papers in "Journal of Glaciology in 1986"

Combined measurements of Subglacial Water Pressure and Surface Velocity of Findelengletscher, Switzerland: Conclusions about Drainage System and Sliding Mechanism

Abstract: During the snow-melt season of 1982, basal water pressure was recorded in 11 bore holes communicating with the subglacial drainage system In most of these holes the water levels were at approximately the same depth (around 70 m below surface) The large variations of water pressure, such as diurnal variations, were usually similar at different locations and in phase In two instances of exceptionally high water pressure, however, systematic phase shifts were observed; a wave of high pressure travelled down-glacier with a velocity of approximately 100 m/h The glacier-surface velocity was measured at four lines of stakes several times daily The velocity variations correlated with variations in subglacial water pressure The functional relationship of water pressure and velocity suggests that fluctuating bed separation was responsible for the velocity variations The empirical functional relationship is compared to that of sliding over a perfectly lubricated sinusoidal bed On the basis of the measured velocity-pressure relationship, this model predicts a reasonable value of bed roughness but too high a sliding velocity and unstable sliding at too low a water pressure The main reason for this disagreement is probably the neglect of friction from debris in the sliding model The measured water pressure was considerably higher than that predicted by the theory of steady flow through straight cylindrical channels near the glacier bed Possible reasons are considered The very large disagreement between measured and predicted pressure suggests that no straight cylindrical channels may have existed

Hydraulics of Subglacial Cavities

Abstract: A theoretical model is developed to describe the steady-state behavior of interconnected, water-filled cavities at the glacier bed. Physically plausible cavities should contain constrictions along the flow path, with flow in the wider sections being relatively sluggish. Mean flow rates in cavities may be at least one order of magnitude less than in channels incised into the basal ice (R channels). Melting due to viscous dissipation - the process that allows R channels to exist - probably plays a minor or negligible role, as compared to glacier sliding, in determining the size of cavities. Furthermore, a system of subglacial cavities should not show a tendency for localization of flow in a few main conduits, as does an R-channel system. If water pressure rises to within several bars of overburden pressure, the rate of cavity closure by creep falls below the rate of cavity opening by sliding and melting, with cavities then becoming unstable. Subsequent evolution of the drainage system should depend upon the total melt-water flux. Circumstances may arise in which cavities and channels act as conduits for melt water; such a configuration would probably show unusual transient behavior.

Stress-gradient coupling in glacier flow: I. Longitudinal averaging of the influence of ice thickness and surface slope

Abstract: For a glacier flowing over a bed of longitudinally varying slope, the influence of longitudinal stress gradients on the flow is analyzed by means of a longitudinal flow-coupling equation derived from the "vertically" (cross-sectionally) integrated longitudinal stress equilibrium equation, by an extension of an approach originally developed by Budd (1968), Linearization of the flow-coupling equation, by treating the flow velocity u ("vertically" averaged), ice thickness h, and surface slope ɑ in terms of small deviations Δu, Δh, and Aɑ from overall average (datum) values u)0, h_0, and ɑ_0 results in a differential equation that can be solved by Green's function methods, giving Δu(x) as a function of Δh(x) and Δɑ(x), x being the longitudinal coordinate. The result has the form of a longitudinal averaging integral of the influence of local h(x) and ɑ(x) on the flow u(x): Δu(x)= u0/2l ∫^L_0 Δln(ɑ^nƒ^nh^(n+l)exp(-│x' -x│/l)dx' where the integration is over the length L of the glacier. The Δ operator specified deviations from the datum state, and the term on which it operates, which is a function of the integration variable x', represents the influence of local h(x' ), ɑ(x'), and channel-shape factor ƒ(x'), at longitudinal coordinate x', on the flow u at coordinate x, the influence being weighted by the "influence transfer function" exp(-│x' - x│/l) in the integral. The quantity l that appears as the scale length in the exponential weighting function is called the longitudinal coupling length. It is determined by rheological parameters via the relationship l = 2h√n ƒn/3n, where n is the flowlaw exponent, n the effective longitudinal viscosity, and n the effective shear viscosity of the ice profile. n is an average of the local effective viscosity n over the ice cross-section, and (n^(-1) is an average of n^(-1) that gives strongly increased weight to values near the base. Theoretically, the coupling length l is generally in the range one to three times the ice thickness for valley glaciers and four to ten times for ice sheets; for a glacier in surge, it is even longer, l ~ 12h. It is distinctly longer for non-linear (n = 3) than for linear rheology, so that the flow-coupling effects of longitudinal stress gradients are markedly greater for non-linear flow. The averaging integral indicates that the longitudinal variations in flow that occur under the influence of sinusoidal longitudinal variations in h or ɑ, with wavelength λ, are attenuated by the factor 1/(1 + (2πl/λ)^2) relative to what they would be without longitudinal coupling. The short, intermediate, and long scales of glacier motion (Raymond, 1980), over which the longitudinal flow variations are strongly, partially, and little attenuated, are for λ ≾ 2l, 2l ≾ λ ≾ 20l , and λ ≳ 20l. For practical glacier-flow calculations, the exponential weighting function can be approximated by a symmetrical triangular averaging window of length 4l, called the longitudinal averaging length. The traditional rectangular window is a poor approximation. Because of the exponential weighting, the local surface slope has an appreciable though muted effect on the local flow, which is clearly seen in field examples, contrary to what would result from a rectangular averaging window. Tested with field data for Variegated Glacier, Alaska, and Blue Glacier, Washington, the longitudinal averaging theory is able to account semi-quantitatively for the observed longitudinal variations in flow of these glaciers and for the representation of flow in terms of "effective surface slope" values. Exceptions occur where the flow is augmented by large contributions from basal sliding in the ice fall and terminal zone of Blue Glacier and in the reach of surge initiation in Variegated Glacier. The averaging length 4l that gives the best agreement between calculated and observed flow pattern is 2.5 km for Variegated Glacier and 1.8 km for Blue Glacier, corresponding to l/h ≈ 2 in both cases. If l varies with x, but not too rapidly, the exponential weighting function remains a fairly good approximation to the exact Green's function of the differential equation for longitudinal flow coupling; in this approximation, l in the averaging integral is l(x) but is not a function of x'. Effects of longitudinal variation of l are probably important near the glacier terminus and head, and near ice falls. The longitudinal averaging formulation can also be used to express the local basal shear stress in terms of longitudinal variations in the local "stope stress" with the mediation of longitudinal stress gradients.

Grain Growth in Polar Ice: I. Theory

Abstract: Many observations regarding grain growth in ice sheets are glaciologically interesting but imperfectly understood. Here we develop the theory of grain growth in ice that is not deforming rapidly, and in the succeeding paper we use this theory to explain observations from glacial ice. In the absence of significant strain energy, the driving force for grain growth arises from grain-boundary curvature. Grain growth is slowed by the interaction of grain boundaries with extrinsic materials (microparticles, bubbles, and dissolved impurities). If the driving force for growth is not large enough to cause boundaries to separate from an extrinsic material, then the grain-boundary velocity is determined by the velocity characteristic of the extrinsic material (low-velocity regime). If the driving force is large enough to cause separation, then boundaries migrate more rapidly than the extrinsic material (high-velocity regime) but the net driving force is reduced through transient pinning by the extrinsic material. Polar ice is typically in the low-velocity regime relative to dissolved impurities and the high-velocity regime relative to microparticles and bubbles. Cross-sectional area of grains is predicted to increase linearly with time under most but not all circumstances.

Subglacial hydrology for an ice sheet resting on a deformable aquifer

Abstract: Subglacial hydrology is investigated for an ice sheet where the substrate consists of a deformable aquifer resting on an aquitard If sliding velocities are low or absent, subglacial melt-water drainage is dominated by drainage through the aquifer to water channels Drainage along the bed is negligible Efficient melt-water drainage requires that a system of subglacial water channels exists; otherwise, pore-water pressures will exceed the overburden pressure In general, aquifer deformation near (away from) the terminus is most likely to occur during the winter (summer) The effect of short-term high channel pressures is, in general, not critical to aquifer deformation because the pressure pulse does not propagate far into the aquifer (For aquifers of high permeability, short periods of high channel pressures constitute the most critical condition) Aquifer deformation at the terminus is very likely to occur if the terminus ice slope exceeds tan ϕ, where ϕ is the Coulomb friction angle of the aquifer material Upwelling of basal melt water near the terminus will normally cause soil dilation if the aquifer has a low permeability (eg till) Maximal profiles are computed corresponding to various aquifer materials using channel spacings which provide efficient drainage (A maximal profile is the highest ice profile which the aquifer can sustain without deformation) In general, maximal profiles lie well above observed profiles (such as h(x) = 3x 1/2 (m)) except near the terminus However, if channel spacings are sufficiently large, pore-water pressures are increased and maximal profiles can lie well below h(x) = 3x 1/2

On the special rheological properties of ancient microparticle-laden Northern Hemisphere ice as derived from bore-hole and core measurements

Abstract: In the Northern Hemisphere, ice layers which have high microparticle concentrations (in particular late Wisconsin) are “softer” than modern or Holocene ice. Such ice deforms more readily in bore-hole tilt and closure measurements. This enhancement in flow, which is shown not to be related to c-axis concentration, has a maximum of three for late Wisconsin ice. The closure and tilt of a bore hole in the Agassiz Ice Cap, Ellesmere Island, drilled in 1977, has been followed every year since its drilling and the flow enhancement observed has been compared to the following quantities measured in the cores: concentration, δ(18O), crystal size, c-axis, Ca, Na, conductivity, and density. Flow enhancement of the ice age and bottom ice was found to be unrelated to c-axis concentration and density. Enhancement of flow is best related to microparticle (or Ca) concentration which in turn seems to be inversely related to crystal size. The latter relationship also seems to hold for the Devon Island Ice Cap and Greenland. In future, modellers of northern ice ages should use model ice that is three times softer than modern or Holocene ice.

Grain growth in polar ice. II: Application

Abstract: Grain growth observed in polar ice that is not deforming rapidly can be accounted for if concentrations and distributions of extrinsic materials (microparticles, bubbles, and dissolved impurities) are characterized fully. Dissolved impurities segregate to grain boundaries and slow grain growth in all cold glacial ice. The high concentration of soluble impurities in Wisconsinan ice from the Dome C (Antarctica) ice core (and perhaps other ice cores) probably causes the small grain-sizes observed in that ice. Microparticles have little effect on grain growth in ordinary ice. In ice layers that appear dirty owing to concentrations of volcanic tephra (such as in the Byrd Station (Antarctica) ice core) or of morainal material, micro particles reduce grain-growth rates significantly. The relatively high vapor pressure of ice allows rapid growth and high mobility of intergranular necks, so grain growth in firn is limited by boundary migration rather than by neck growth. Bubbles formed by pore close-off at the firn-ice transition are less mobile than grain boundaries, causing bubble-boundary separation whenever geometric constraints are satisfied; however, such separation reduces grain-growth rates by only about 10%. The observed linear increase of grain area with time is thus predicted by theory, but the growth rate depends on soluble-impurity concentrations as well as on temperature.

Detection of the depth-hoar layer in the snow-pack of the Arctic Coastal Plain of Alaska, U.S.A., using satellite data

Abstract: The snow-pack on the Arctic Coastal Plain of Alaska has a well-developed depth-hoar layer which forms each year at the base of the snow-pack due to upward vapor transfer resulting from a temperature gradient in the snow-pack. The thickness of the depth-hoar layer tends to increase inland where greater temperature extremes (in particular, lower minimum temperatures) permit larger temperature gradients to develop within the snow-pack. Brightness temperature (T B) data were analyzed from October through May for four winters using the 37 GHz horizontally polarized Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR). By mid-winter each year, a decrease in T B of approximately 20K was found between coastal and inland sites on the Arctic Coastal Plain of Alaska. Modeling has indicated that a thicker depth-hoar layer in the inland sites could be responsible for the lower T Bs. The large grain-sizes of the depth-hoar crystals scatter the upwelling radiation moreso than do smaller crystals, and greater scattering lowers the microwave T B. Using a two-layered radiative transfer model, the crystal diameter in the top layer was assumed to be 0.50 mm. The crystals in the depth-hoar layer may be 5–10 mm in diameter but the effective crystal diameter used in the radiative-transfer model is 1.40 mm. The crystal size used in the model had to be adjusted downward, relative to the actual crystal size, because the hollow, cup-shaped depth-hoar crystals are not as effective at scattering the microwave radiation as are spherical crystals that are assumed in the model. In the model, when the thickness of the depth-hoar layer was increased from 5 cm to 10 cm, a 21K decrease in T B resulted. This is comparable to the decrease in T B observed from coastal to inland sites in the study area.

Drainage-basin characteristics of Nordaustlandet ice caps, Svalbard

Abstract: Recent mapping of ice-surface and bedrock topography from airborne radio-echo sounding has shown that the ice caps of Nordaustlandet, Svalbard, are divided into a series of well-defined drainage basins. Three lines of evidence indicate that several distinctive modes of ice-flow regime characterize these basins: (1) comparison between observed and theoretical ice-surface profiles; (2) analysis of driving stresses; and (3) observations of ice-surface features on satellite imagery and air photographs. The drainage basins are inferred to behave in the following ways. First, basins with low driving stresses and surface profiles, some of them clearly stagnant, are associated with the quiescent phase between glacier surges. Secondly, the ice streams draining southern Vestfonna have low surface profiles, relatively low driving stresses, and marked shear zones at their margins. They are interpreted to be flowing continuously at a relatively faster rate than the ridges between them. Basal melting, perhaps combined with substrate deformation, is probably responsible for the regime of these glaciers. Thirdly, the remaining basins studied on Nordaustlandet have relatively high marginal driving stresses and high surface profiles. They are interpreted to be frozen to their beds, at least near their margins. Some of these basins may also surge, particularly those where a part of the basin is below sea-level, and therefore is probably underlain by considerable thicknesses of deformable sediments.

Changes in the Salinity and Porosity of Sea-Ice Samples During Shipping and Storage

Abstract: A theoreti cal examination of salinity and porosity changes introduced in sea-ice samples by brine expulsion and gas entrapment caused by thermal cycling during shipping and storage shows that in extreme cases such effects can be significant, resulting in 15 % reductions in porosity (1/). More representative scenarios give porosity changes of less than 2% which, assuming that ice-property variations scale with n 1 / 2 , result in property variations of less than 1%.

Discharges of turbid water during mini-surges of variegated glacier, alaska, u.s.a.

Abstract: Discharges of water, sediment, and dissolved impurities from Variegated Glacier, Ala ska, were monitored in the early summers of 1980 and 1981 during the occurrence of mini-surg es. Sea sonal trends, weat her-related eve nts, and diurnal variat ions similar to behavior of other temperat e gl acier streams were found. The prin cipal effect in the stream associated with mini-surge occurrence was a brief discharge of extremely turbi d water. The turbidity is assumed to be introduced into the basal hydraulic system by initiation of the fast motion of a mini-surge at a time and locat ion on the upper glacier known from oth er meas ure­ ments. The mean water velocity in the hydraulic system over the int erven ing distance is thereby determined (0.3 m S-I). The mean water velocity, together with the water discharge ("'16 m3 S-1 at the terminus), places con straints on the distribution of water velocity u and total cros s-sectional area AT of the flow paths along the gl acier base. This leads to the con clusion that within the zone of mini-surge occurrence in its unpe rturbed state: u is about 0.1 m S-1 or possibly less; AT is about 10 2 m2 or possibly more, and it must be divided into a very large number of small passag eways, be blocked by constrict ions, or both. The total water cro ss-se ction corresponds to a layer 0.1-{).2 m thick when spr ead unifo rmly over the glacier width. The water velocity is clos e to or less than the propa gation velo city of the mini-s urges. Bet ween the zone affected by mini-surges and the stream, a dyna mically less active lower section of the glacier is proba bly underlain by a small number of conduits, in which the water velocity may be very high (�2 m S-I). Water di scharge follo wing the mini­ surges puts an upper limit on water-storage changes ass ociated with the anomalous ice motion.

Role of Debris Cover in the Thermal Physics of Glaciers

Abstract: A mathematical model is presented of non-stationary melting processes of ice including particles of morainic material. The problem is treated as a Stephen-type one with the phase boundary of ice melting being located under the debris cover. The main terms of the heat-balance equation for a glacier surface are solar radiation and convective heat transfer. The quantitative relationships characterizing the effect of glacier run-off augmentation from under a thin layer of debris cover are obtained for different bulk moraine concentrations inside the ice. The concept of equivalent time is introduced. It is defined as the time elapsed until the moment the sub-moraine ice-ablation rate becomes equal to the ablation rate of clean ice. This moment signifies the beginning of the shielding stage. Thus, a glacier can be considered as a self-controlling system with respect to its summer run-off. A series of numerical tests for Djankuat glacier, Central Caucasus, has been carried out. The dynamics of moraine-cover growth and alterations of seasonal ablation rate under debris show perfect agreement between the computed data and the results of 14 years of direct observations. Some practical recommendations concerning artificial blackening of a glacier surface for augmentation of liquid run-off are presented. Conditions promoting increase of run-off are: relatively high albedo, relatively low summer air temperature, and relatively small convective heat transfer between the air and the ice surface. The method of artificially blackening a glacier surface is by means of a durable thin dark polymer film. In conclusion, some further aspects of the problem are discussed.

Modeling the influence of till rheology on the flow and profile of the lake Michigan lobe, southern Laurentide ice sheet, U.S.A.

Abstract: The late Wisconsin Shelbyville till was deposited in southern Illinois c. 20000-21 000 year B.P. and records the maximum southern advance of the Lake Michigan lobe of the Laurentide ice sheet. The yield strength calculated for a representative till debris flow found just south of the ice margin is 8 kPa (0.08 bar), and probably approximates yield strength of basal Shelbyville till. An ice-profile model assuming plastic behavior in basal till suggests the southern Lake Michigan lobe may have been unusually thin. Reconstructed Laurentide glacier profiles from the south-west and western Great Plains (South Dakota, Alberta, Minnesota, and Montana), and the MacKenzie Delta, N.W.T., are similar to those inferred for the southern Great Lakes area, and much thinner than those of most modern ice sheets. The Pleistocene Laurentide ice sheet may have been asymmetric: thicker in the east than in the west. Glaciers resting on weak sediments can move both by subglacial sediment deformation (creep) and sliding at the sediment-ice interface. Till rheology is complex; shearing of till by over-riding glaciers would increase porosity and further reduce yield strength. RESUME. Modelisation du r(jle de la rheologie des moraines basales sur l'ecoulement et le profif de la surface du lobe du lac Michigan, sud de l'indlandsis des Laurentides. U.S.A. La moraine de Shelbyville a ete deposee, dans le sud de l'Illinois, vers 20000-21 000 B.P. Elle marque I'avance maximum vers le sud du lobe du lac Michigan de l'indlandsis Nord-americain. La resistance au cisaillement d'une coulee de moraine representative situee juste du sud du bord de I'indlandsis est de 8 kPa (0,08 bar); c'est probablement l'ordre de grandeur de la resistance au cisaillement de la moraine basale de Shelbyville. Un mode le du profil de la glace, dans I'hypothese d'un comportement plastique de la moraine basale, suggere que l'extremite sud du lobe du lac Michigan a pu etre inhabituellement mince. Les profils de I'indlandsis des Laurentides dans la region des grandes plaines du sud-ouest et de I'ouest (South Dakota, Alberta, Minnesota, Montana) ainsi que dans la region du delta du MacKenzie (N.W.T.) sont corn parables a

Accumulation distribution in Terre Adélie, Antarctica: effect of meteorological parameters

Abstract: Along the 1040 km extending from Cape Prud’homme (lat. 66°41’S., long. 139°55’ E.), near Dumont d’Urville station, to Dome C (lat. 74°39’S., long. 124°10’E.), the variations in annual accumulation can be analysed by a division of the entire data set into three sub-sets depending on the types of measurements and the character of the spatial distribution. Along the first 33 km, from the coast to stake E40, annual measurements show considerable inter-annual variability, 52% of which can be explained by the spatio-temporal homogeneity of the balance distribution. However, we obtain a better result (64%) for the fluctuation homogeneity standardized using the standard deviation. This means that there is a strong space-time dis-tribution structure, characterized by an equal variation of the balance around the mean value specific to each location. This is so in spite of the existence of considerable surface roughness (sastrugi), the influence of which should be reduced by averaging values around each stake. From stake E40 to stake R60, a distance of 170 km, the almost periodic oscillations in the accumulation with a wavelength close to 40 km can be explained by the formation of a gravity-inertia wave, disturbing the geostrophic equilibrium, occurring at the break in slope 200 km from the coast. The very low values of accumulation for stakes D55 and D58S show that the oscillations were almost stationary during the study period (about 25 years). Finally, along the 840 km from stake R60 to Dome C we can observe a decrease in accumulation resulting from the decrease in mean temperature.

The Effects of Basal Melting on the Present Flow of the Ross Ice Shelf, Antarctica

Abstract: We use a hybrid finite-element/finite-difference model of ice-shelf flow and heat transfer to investigate the effects of basal melting on the present observed flow of the Ross Ice Shelf, Two hypothetical basal melting scenarios are compared: (i) zero melting everywhere and (ii) melting sufficient to balance any large-scale patterns of ice-shelf thickening that would otherwise occur. As a result of the temperature-dependent flow law (which we idealize as having a constant activation energy of 120 kJ mol−1, a scaling coefficient of 1.3 N m−2 s1/3, and an exponent of 3), simulated ice-shelf velocities for the second scenario are reduced by up to 20% below those of the first. Our results support the hypothesis that melting patterns presently maintain ice thickness in steady state and conform to patterns of oceanic circulation presently thought to ventilate the sub-ice cavity. Differences between the simulated and observed velocities are too large in the extreme south-eastern quarter of the ice shelf to permit verification of either basal melting scenario. These differences highlight the need to improve model boundary conditions at points where ice streams feed the ice shelf and where the ice shelf meets stagnant grounded ice.

Ice conditions of an Arctic polynya: North Water in winter

Abstract: The surface condition of the North Water was investigated during two winters (i.e. the three polynyas: Smith Sound polynya, Lady Ann Strait polynya, and Barrow Strait polynya). Since no detailed information was available on ice conditions and the extent of open water during winter, radiometric temperature measurements of the sea surface had to be taken along a flight line of 2650 km from an altitude of 300 m. From November to March 1978-79 and 1980-81, 14 remote-sensing flights were carried out. On the basis of the radiometric measurements, the following ice types were identified: ice-free, dark nilas, light nilas, grey ice, grey-white ice, and white ice. A comparison between the thermal and the visual ice classification (the latter being based on grey tones of the aerial images) showed a deviation of 3%. The analysis showed that in November, December, and January more than 50% of the Smith Sound polynya was covered by young ice, nilas, and ice-free, whereas in February and March white ice was dominant. Moreover, it was found that the two polynyas in Smith Sound and Lady Ann Strait were much smaller than previously believed. In Barrow Strait, a semi-permanent polynya was observed in the winter of 1980-81. The occurrence of polynyas in Barrow Strait seems to be connected with the location of the fast-ice edge. On the basis of the calculated ice-type distribution and heat-flux rates for different ice types, an energy loss of 178 W m-2 was found on the surface of the Smith Sound polynya due to open water and thin ice for the winter months November to March. Compared with other ice-covered sea surfaces in the Arctic, the heat release by the sea-water in the Smith Sound polynya is about 100 W m-2 larger.

A numerical study of plane ice-sheet flow

Abstract: The plane steady flow of a grounded ice sheet is numerically analysed using the approximate model of Morland or Hutter. In this, the ice behaves as a non-linear viscous fluid with a strongly temperature-dependent rate factor, and ice sheets are assumed to be long and shallow. The climate is assumed to be prescribed via the accumulation/ablation distribution and the surface temperature, both of which are functions of position and unknown height. The rigid base exerts external forcings via the normal heat flow, the geothermal heat, and a given basal sliding condition connecting the tangential velocity, tangential traction, and normal traction. The functional relations are those of Morland (1984) or motivated by his work. We use equations in his notation. The governing equations and boundary conditions in dimensionless form are briefly stated and dimensionless variables are related to their physical counterparts. The thermo-mechanical parabolic boundary-value problem, found to depend on physical scales, constitutive properties, and external forcing functions, has been numerically solved. For reasons of stability, the numerical integration must proceed from the ice divide towards the margin, which requires a special analysis of the ice divide. We present this analysis and then describe the versatility and limitations of the constructed computer code. Results of extensive computations are shown. In particular, we prove that the Morland–Hutter model for ice sheets is only applicable when sliding is sufficiently large (satisfying inequality (30)). In the range of the validity of this inequality, it is then demonstrated that of all physical scaling parameters only a single π-product influences the geometry and the flow within the ice sheet. We analyse the role played by advection, diffusion, and dissipation in the temperature distribution, and discuss the significance of the rheological non-linearities. Variations of the external forcings, such as accumulation/ablation conditions, free surface temperature, and geothermal heat, demonstrate the sensitivity of the ice-sheet geometry to accumulation conditions and the robustness of the flow to variations in the thermal state. We end with a summary of results and a critical review of the model.

Antarctic Iceberg Distribution and Dissolution

Abstract: Shipboard observations (in accordance with Norsk Polar-Institutt guidelines) from 6 years of Australian National Antarctic Research Expeditions (ANARE) voyages have provided data giving a detailed knowledge of iceberg sizes and concentrations in the Southern Ocean between long. 60° and 140° E. The resulting size–frequency distributions are examined in conjunction with a knowledge of water movement along known drift tracks in a selected study area (between lat. 59° and 64° S., and long. 90° and 120° E.) to determine iceberg-dissolution rates. The “median life” (before breaking) of icebergs less than 1000 m in horizontal dimension is estimated to be 0.2 a, which is significantly lower than was previously thought. The mean melt rate is estimated to be 0.12 m d–1, which agrees broadly with previous laboratory studies. The relative contributions of melt, calving, and breakage, plus the enhancement effect of roll-over, are examined in estimating the natural dissolution rate. Breakage appears to be the dominant mechanism for larger icebergs with melt and calving able to explain the disappearance of icebergs in the smallest categories only (within the mean “median-life” period). Examination of the historical records of Captain Cook indicates that iceberg concentrations, as well as the northerly extent in this region 200 years ago, were compatible with the present data.

Sub-temperate basal sliding

Abstract: Recent observations of glacier sliding at temperatures below the melting point are discussed. It is pointed out that these observations can be simply explained by including solid friction in the sliding law. Furthermore, we re-emphasize the point that such sub-temperate sliding has an important effect on the basal boundary conditions which should be applied in model studies of ice sheets and glaciers.

Experiments on freeze-bonding between ice blocks in floating ice rubble

Abstract: Le regel entre les blocs de glace a ete etudie en fonction de la pression normale sur le plan de contact, de la duree et de la surface de contact de la salinite de l'eau

Propagating strain anomalies during mini-surges of Variegated Glacier, Alaska, U.S.A

Abstract: Wire strain meters and seismometers spaced longitudinally along the upper part of Variegated Glacier, Alaska, showed quasi-periodic episodes of increased velocity (mini-surges), which lasted about 1 day and recurred at intervals of a few days to 2 weeks during the early part of the melt seasons of 1979, 1980, and 1981 The zone affected by these mini-surges corresponds to the zone of highest velocity and basal stress increase over the previous decade, and the initiation of the most recent surge in 1982 Mini-surges initiate locally; as a single melt season progresses, the later mini-surges start at higher locations and show a distinct down-glacier propagation of a characteristic strain pattern and associated zone of acoustic emissions at speeds of 01–06 km h−1 During mini-surges, extensile and compressive strain-rates exceed 10 × 10−4 d−1 and 40 × 10−4 d−1, respectively; typical strain-rates between mini-surges were less than 2 × 10−4d−1 in magnitude Seismic activity jumped by two orders of magnitude and was distinctly audible during a mini-surge Maximum strain-rate during mini-surges decreased from year to year The high time resolution of the strain allows short time-scale structure of velocity variations to be deduced As a propagating mini-surge passes, the velocity anomaly at a fixed location is characterized by a rapid initial rise over a few hours to two distinct peaks separated by a few hours, followed by a slower return to normal velocity taking up to a day The double peak in velocity may arise from a single, very sharp, transient peak in the basal slip velocity associated with the initial opening of cavities at the base in response to a sudden rise in basal water pressure (observed by Kamb and Engelhardt) This supports an important role for basal cavitation in the mini-surge mechanism

Mass-balance measurements: problems and two new methods of determining variations

Abstract: The optimum continuation of series of mass-balance measurements and their extension to unmonitored glaciers are important problems in contemporary glaciology. For this purpose, two new practical survey methods are proposed, based on the linear-balance variations model of Lliboutry (1974). The first method is a simplified application of the linear model that uses only a data set limited to selected fixed-measurement sites. It was developed to obtain the mass-balance variation in cases where data are too scarce to obtain the global mass balance or to apply the Lliboutry algorithm. This simplified linear model is used with the 8 years’ of surveys on glacier d’Argentiere. The second method uses the continuity equation to derive the mass balance of a glacier sector delimited by two cross-profiles where the surface velocities, surface altitudes, and depths are known. By using this continuity method, the entire mass-balance series is established for a sector of glacier de Gebroulaz (Vanoise area, France) from 1908 to 1950, as well as for two sectors of Unteraargletscher (Oberland, Switzerland) from 1924 to 1981.

Stress-gradient coupling in glacier flow. IV: Effects of the T term

Abstract: The "T term" in the longitudinal stress equilibrium equation for glacier mechanics, a double y-integral of ∂^2T_(xy)/∂x^2 where x is a longitudinal coordinate and y is roughly normal to the ice surface, can be evaluated within the framework of longitudinal flow-coupling theory by linking the local shear stress T_(xy) at any depth to the local shear stress T_B at the base, which is determined by the theory. This approach leads to a modified longitudinal flow-coupling equation, in which the modifications deriving from the T term are as follows: 1. The longitudinal coupling length I is increased by about 5%. 2. The asymmetry parameter σ is altered by a variable but small amount depending on longitudinal gradients in ice thickness h and surface slope ɑ. 3. There is a significant direct modification of the influence of local h and ɑ on flow, which represents a distinct "driving force" in glacier mechanics, whose origin is in pressure gradients linked to stress gradients of the type ∂T_(xy)/∂_x. For longitudinal variations in h, the "T force" varies as d^2h/dx^2 and results in an in-phase enhancement of the flow response to the variations in h, describable (for sinusoidal variations) by a wavelength-dependent enhancement factor. For longitudinal variations in ɑ, the "force" varies as dɑ/dx and gives a phase-shifted flow response. Although the "T force" is not negligible, its actual effect on T_B and on ice flow proves to be small, because it is attenuated by longitudinal stress coupling. The greatest effect is at shortest wavelengths (λ ≾2.5h), where the flow response to variations in h does not tend to zero as it would otherwise do because of longitudinal coupling, but instead, because of the effect of the "T force", tends to a response about 4% of what would occur in the absence of longitudinal coupling. If an effect of this small size can be considered negligible, then the influence of the T term can be disregarded. It is then unnecessary to distinguish in glacier mechanics between two length scales for longitudinal averaging of T_B, one over which the T term is negligible and one over which it is not. Longitudinal flow-coupling theory also provides a basis for evaluating the additional datum-state effects of the T term on the flow perturbations Δu that result from perturbations Δh and Δɑ from a datum state with longitudinal stress gradients. Although there are many small effects at the ~1% level, none of them seems to stand out significantly, and at the 10% level all can be neglected. The foregoing conclusions apply for long wavelengths λ ≳ h. For short wavelengths (λ ≾ h), effects of the T term become important in longitudinal coupling, as will be shown in a later paper in this series.

A New Instrument for Determining Strength Profiles in Snow Cover

Abstract: The Digital Thermo-Resistograph is a portable microprocessor-based data-probe system for quick and accurate field collection of snow-cover strength. This was accomplished by constructing a probe with a load cell, a small snow platform for probe-position information, and a Z-80 microprocessor-based data acquisition system. A 64 × 240 dot matrix LCD graphic display unit is used to show immediately complete strength profiles in the field. Sufficient memory for the storage of 25 profiles is provided. Temperature and temperature-gradient collection is also planned as a part of the instrument but as yet this work has not been completed. The results of winter 1984 field tests are presented. The Digital Thermo-Resistograph proved to be fast and reliable in collecting snow-strength information. Comparisons with the ram penetrometer are shown and suggestions for future developments are discussed.

On the Mechanics of Surging Glaciers

Abstract: Observations of surging glaciers indicate that the flow regime is one dominated by extensional flow. The stress state has substantial longitudinal deviatoric stress. This regime is very different from the conventional model for glacier dynamics which is dominated by shearing flow. In addition, the initiation of surging often involves a compression front which travels down the glacier. The compression front seems to divide an up-stream region of high drag at the base of the glacier from one of low drag which allows the rapid sliding. We develop a framework for the mechanics of glaciers undergoing surging. Relevant issues are the extensional and compression flows, high longitudinal deviatoric stress, and the stress state near the basal discontinuity. We find that some of the down-slope component of glacier weight is borne by longitudinal stress in the rapidly sliding region. This stress thrusts against the slowly moving parts of the glacier. We hypothesize that this effect causes the rapidly sliding part to spread and causes the compression front to travel down the glacier. A criterion for spreading of the rapidly sliding part is developed. The mechanics outlined above are used to develop a highly idealized model for glacier surging. We propose that regions of low drag are relatively common features of glaciers. The surge initiates when conditions are met which allow the surge nucleus to spread. The rapidly sliding region of low drag spreads to a large part of the glacier. Surging ends when the low-drag conditions terminate. Because of the changed state of the glacier, surge nuclei are now stable against spreading. Several years of rebuilding must occur before nuclei are once more unstable. Calculations are performed for the evolution of the shape of Medvezhy Glacier during the surge of 1963. We find a remarkable similarity between the data and our computations.

Rate of short-term ablation of exposed ground ice, Banks Island, Northwest Territories, Canada

Abstract: Energy f1uxes and resultant short-term ablation rates were measured at a ground-ice slump on south-west Banks Island. Net radiation, as a proportion of the ablation flux, is greatest on days with high incident solar radiation, whereas on overcast days, sensibleand latent-heat inputs may supply more than half the necessary energy. A multiple regression equation using net radiation and a turbulent-energy term as independent variables explains 79% of the variation in the measured ablation fluxes. Overall, at least 60% of the energy used for ablation at the slump is derived from net radiation. RESUME. Vitesse d'ablation sur de courtes periodes de temps pour de la glace reposant sur le sol, Banks Island , Terriloires du Nord Ouest , Canada. Les flux d'energie et les taux d'ablation qui en resultent ont ete mesures sur les falaises de glace de sol dans le sud ouest de Banks Island. La radiation nette, comme element du flux d'ablation, est la plus forte les jours de forte radiation solaire incidente, tandis que les jours de ciel couvert, ce sont les chaleurs sensibles et latentes qui fournissent plus de la moitie de

Explosive growth of shear-heating instabilities in the down-slope creep of ice sheets

Abstract: Modelisation de la croissance explosive d'instabilites de cisaillement thermique d'amplitude finie pour la deformation le long des pentes. On integre l'equation d'evolution temporelle du profil vertical de temperature a la suite d'un changement brusque de l'epaisseur de la glace

Recent fluctuations of Rakhiot Glacier, Nanga Parbat, Punjab Himalaya, Pakistan

Abstract: A 1985 survey at Rakhiot Glacier, Nanga Parbat, Punjab Himalaya, indicates that the glacier terminus is advanced about 200 m from its 1954 position. Between 1930 and 1954, the dates of previous surveys, the glacier terminus had thinned and back-wasted 450 m. The recession prior to the 1960s, followed by advance into the 1970s and perhaps 1980s, parallels patterns of glacier fluctuation found in the adjacent western and central Karakorum.