bluuebunny/crossref_metadata_2025 huggingface dataset:
Stability of Temperate Ice Caps and Ice Sheets Resting on Beds of Deformable Sediment
Authors: Boulton G. S , Jones A. S
Published: September 2017

Although theories of glacier movement generally assume that glaciers flow over rigid rock beds, there are many places where glaciers rest on beds of deformable sediment, and the great Pleistocene ice sheets which extended from time to time over much of Northern Europe and North America were largely underlain by such beds. Observations show that a large proportion of the forward movement of a glacier lying on such a bed may be contributed by deformation of the bed rather than the glacier. A theory is developed in which the glacier surface profile is related to the hydraulic and strength properties of potentially deformable bed materials. If these have a high hydraulic transmissibility, melt water is readily discharged sub-glacially, the bed is stable, and the profile is a normal parabolic one, governed by the rhcological properties of ice. If bed transmissibility is low, water pressures build up, the bed begins to deform, and a lower equilibrium profile will develop, so that in an extreme case the glacier approximates to a thin flat sheet, similar to an ice shelf. It is suggested that such behaviour may have occurred at the margins of large Pleistocene ice sheets over North America and Europe, and evidence in support of this is drawn from the reconstructed shapes of these ice margins, anomalously small amounts of isostatic rebound, anomalously high retreat-rates, and the presence of glaciotcctonic structures. Reasons are suggested to explain why this behaviour should have been important for Pleistocene glaciers which penetrated into currently temperate latitudes but does not appear to be important in large modern glaciers.
Ice-Sheet Flow Properties Derived from Bore-Hole Shear Measurements Combined With Ice-Core Studies
Authors: Russell-head D. S. , Budd W. F.
Published: September 2017

A 345 m deep bore hole in ice about 385 m thick, near the edge of Law Dome, Antarctica, was drilled in 1974 about 3 km up-stream from the site of a previous bore hole, nearly reaching the bed, obtained in 1969. The core from this new bore hole has been studied comprehensively, particularly with regard to the ice-crystal orientation fabrics. Samples of the ice core were subjected to simple shear at temperatures and deviatoric stresses which match the in situ conditions of the ice sheet.

Similar studies of randomly-oriented laboratory-made polycrystalline ice were undertaken. Long-term tests, lasting for up to two years, were required to determine minimum strain-rates. The flow law for the anisotropic ice was thus determined as a function of that for the isotropic ice together with a measure of c-axis fabric strength perpendicular to the shear plane.

Core studies indicate that the upper part of the ice sheet has a polycrystalline structure appropriate to the surface longitudinal stress. Deeper in the core a strong concentration of near-vertical c-axes develops. Ice having very large crystals with multiple maxima fabrics was found in the lower quarter of the ice thickness.

Shear measurements in the bore hole indicate the existence of high strain-rates in the zone of vertical c-axes, and of lower shear-rates below that level. The low values of shear-rates in the basal region cannot be explained in terms of crystallographic changes alone, and therefore it is inferred that the shear stress decreases in this layer–a result which also provides a possible explanation for the development of the observed basal crystal structure.
Antarctic Ice-Sheet Volume at 18000 Years B.P. and Holocene Sea-Level Changes at the West Antarctic Margin
Authors: Lingle Craig S. , Clark James A.
Published: September 2017

The Antarctic ice sheet has been reconstructed at 18000 years b.p. by Hughes and others (in press) using an ice-flow model. The volume of the portion of this reconstruction which contributed to a rise of post-glacial eustatic sea-level has been calculated and found to be (9.8±1.5) × 106 km3. This volume is equivalent to 25±4 m of eustatic sea-level rise, defined as the volume of water added to the ocean divided by ocean area. The total volume of the reconstructed Antarctic ice sheet was found to be (37±6) × 106 km3. If the results of Hughes and others are correct, Antarctica was the second largest contributor to post-glacial eustatic sea-level rise after the Laurentide ice sheet. The Farrell and Clark (1976) model for computation of the relative sea-level changes caused by changes in ice and water loading on a visco-elastic Earth has been applied to the ice-sheet reconstruction, and the results have been combined with the changes in relative sea-level caused by Northern Hemisphere deglaciation as previously calculated by Clark and others (1978). Three families of curves have been compiled, showing calculated relative sea-level change at different times near the margin of the possibly unstable West Antarctic ice sheet in the Ross Sea, Pine Island Bay, and the Weddell Sea. The curves suggest that the West Antarctic ice sheet remained grounded to the edge of the continental shelf until c. 13000 years b.p., when the rate of sea-level rise due to northern ice disintegration became sufficient to dominate emergence near the margin predicted otherwise to have been caused by shrinkage of the Antarctic ice mass. In addition, the curves suggest that falling relative sea-levels played a significant role in slowing and, perhaps, reversing retreat when grounding lines approached their present positions in the Ross and Weddell Seas. A predicted fall of relative sea-level beneath the central Ross Ice Shelf of as much as 23 m during the past 2000 years is found to be compatible with recent field evidence that the ice shelf is thickening in the south-east quadrant.
Late Wisconsin Reconstruction for the Ross Sea Region, Antarctica
Authors: Drewry David J.
Published: September 2017

If sea-level depression in the Ross Sea embayment were: (1) < 120–130 m, or (2) > 120–130 m but sustained for < 104 a, it is unlikely that the Ross Ice Shelf would have become fully grounded over the whole continental shelf during Wisconsin times. Sediments flooring the Ross Sea and recent estimates of sea-level lowering and post-glacial emergence yield little support for grounded ice but do provide some evidence for expanded ice-shelf conditions. A reconstruction is presented based on this premise. The model is compatible with glacial geologic results, especially those in the McMurdo Sound area.
A Catastrophe Model of the Paleoglimate
Authors: MacAyeal Douglas Reed
Published: September 2017

A simple qualitative model of global ice cover is presented to account for the apparent catastrophic transitions between glacial and interglacial climates. Unlike the Budyko (1972) and Weertman (1976) models, the catastrophe model employs a second control parameter besides the oribtally induced insolation fluctuations to differentiate between fast and slow glaciological response mechanisms. It is conjectured that this second control parameter is linked to isostatic actions occurring beneath the continental crust which shift the dynamic emphasis from terrestrial to marine ice margins.
Ice-thickness Patterns and the Dynamics of the Ross Ice Shelf, Antarctica
Authors: Bentley Charles R. , Glough John W. , Jezek Kenneth C. , Shabtaie Sion
Published: September 2017

As part of the Ross Ice Shelf Geophysical and Glaciological Survey, a detailed map of ice thickness has been produced from airborne radar measurements closely tied to the network of survey stations on the ice-shelf surface. The map, drawn with a 20 m contour interval, reveals a highly complex pattern of thickness variations reflecting presumably, at least in part, complex ice-shelf dynamics. Many features of the thickness variation pattern appear to be associated with zones of grounded ice, but not all. Features of interest include many ice thickness minima, with closures up to 120m; a narrow, greatly elongated ridge-trough system 450 km or more in length; a few ice thickness maxima; steep regional gradients of 10 m/km in freely floating ice; highly contorted contours suggesting a large-scale “turbulence”; and at least two remarkable step-like changes in ice thickness. The irregularity of many of these features suggests dynamic non-equilibrium, i.e. the existence of transients in the dynamic system, so that the ice shelf as a whole suggests a state of rather rapid change. Flow-bands constructed on the basis of the strengths of the echo from the ice-water interface clearly delineate the outflow from the main East Antarctic outlet glaciers in the grid eastern part of the shelf. A discontinuous flow band originating in a small mountain glacier (Robb Glacier) suggests a variable mesoclimate in the vicinity of the glacier within the last thousand years. Strong reflections near the ice front suggest bottom melting of saline ice previously frozen on to the underside of the ice. Several rifts or incipient rifts in the ice shelf characteristically show two lateral bands of strong reflections with a non-reflecting zone in between.
Electromagnetic Sounding of Bottom Crevasses on the Ross Ice Shelf, Antarctica
Authors: Jezek Kenneth C. , Bentley Charles R. , Clough John W.
Published: September 2017

During the 1976—77 season of the Ross Ice Shelf Geophysical and Glaciological Survey, a series of vertical electromagnetic sounding profiles of subsurface features was completed at station J-9. The survey comprised three five-kilometer north-west-south-east profiles separated by one kilometer and six two-kilometer north-east-south-west profiles, and was carried out on the surface using 35 MHz and 50 MHz radar systems. Folded-dipole antennae were used and oriented to detect reflectors both along and perpendicular to the profile path. This was done to facilitate the interpretation of the data, which indicated a complex system of bottom crevasses. Measurements of the positions, heights, and shapes of these crevasses showed at least two sets of crevasses varying in both strike and size. The larger crevasses, about 120 m high and oriented more or less normal to the flow direction, are probably associated with the movement of ice stream B across the grounding line between the West Antarctic ice sheet and the Ross Ice Shelf. A satisfactory explanation for the secondary set of crevasses, about 60 m high and forming an angle of 60° ±10° with the first set, has not yet been found.
Investigation of Bottom Mass-Balance Rates by Electrical Resistivity Soundings on the Ross Ice Shelf, Antarctica
Authors: Shabtaie Sion , Bentley Charles R.
Published: September 2017

Electrical resistivity sounding, using the four-electrode Schlumberger array, has been carried out at 11 locations on the Ross Ice Shelf. The apparent resistivity curves generally show four characteristic zones. The first, at distances from 1 to 10 m, reflects the near-surface zone of seasonal temperature changes and inhomogeneities. The second zone, from 10 m to 100 m, reflects primarily the increasing density with depth in the upper 50 m of the ice shelf, modified, in some locations, by temperature variations. The third zone, from 100 m to a distance roughly equal to the ice thickness, is affected principally by the temperature gradient in the solid ice. In the fourth zone, at distances greater than approximately twice the ice thickness, the apparent resistivity usually decreases rapidly with distance, owing to the highly conductive sea-water beneath the ice shelf. At some stations associated with ice streams and outlet glaciers, however, an increase at large spacings indicates much more resistive basal ice.

Using data from seven locations on the grid eastern half of the shelf that do not show obvious evidence of a basal resistive zone, including temperatures to 100 m at two of the sites, the mass-balance rate at the bottom of the ice is estimated to be within a few tenths of a meter per year of zero at distances between 90 and 530 km from the ice front, assuming steady-state condition over most of the ice shelf. However, the assumption of steady-state is questionable at locations close to outlet glaciers, and must be treated with great caution. The temperature measurements at the two sites, along with previously observed temperatures at the RISP drill site, make it possible to estimate the activation energy in the solid ice. The models fitted to the observed values suggest an “apparent” activation energy in the solid ice closer to 0.15 eV (14 kJ mol−1) than to 0.25 eV (24 kJ mol−1). This difference is believed to be due to a decrease in the ionic impurity content with increasing depth in the ice by a factor of two or more.
Dating of Ross Ice Shelf Cores by Chemical Analysis
Authors: Herron Michael M. , Langway Chester C.
Published: September 2017

Seasonal variations in sodium concentrations have been measured on surface-pit snow samples and on firn and ice core samples from the Ross Ice Shelf, Antarctica. Site locations include J-9 (82° 22’ S., 168° 40’ W.), Roosevelt Island dome (79° 22’ S, 161° 40’ W), C-7-1 (78° 30’ S., 177° 00’ W.), and C-7-3 (78° 20’S., 179° 51’ E.). The predominant source for the Na is sea salt, indicated by greater concentration levels at seaward sites. Al concentrations of the order of only 10–9g/g show that the input of continental dust is comparable to that at inland Antarctic locations, and that dust contributes only a negligibly small fraction of the Na on the shelf. Maximum Na concentrations occur in the winter or early spring, as is the case for Greenland ice. The annual accumulation-rate at J-9, determined by counting Na concentration peaks with depth, is 90 kg m–2year–1, in agreement with rates determined radiometrically. Annual cycles in Na concentration are also detectable at depth in the J-9 ice core. It is suggested that Na concentration is a useful diagnostic criterion for distinguishing between East Antarctic ice (10–8g Na/g), West Antarctic ice (30 × 10–9g Na/g), and ice that fell as snow on the shelf itself (> 30 × 10–9g Na/g). The transition between snow that is chemically characteristic of the ice-shelf regime to snow of an inland regime is expected to occur near the 500 m elevation contour. This position is up to 200 km inland of the grounding line. A model is presented for the large-scale decrease in Na concentration with distance inland within the ice-shelf regime. Since deeper ice in Ross Ice Shelf cores originated generally further from the ocean, the up-stream origin of shelf ice may be estimated from the chemical profile. The Little America V ice-core chemistry profile shows no discontinuity as would be expected if a recent surge of West Antarctic ice had occurred.
Satellite Studies Of Fresh-Water Ice Movement On Lake Erie
Authors: Wiesnet Donald R.
Published: September 2017

Since the winter of 1972–73, NOAA visible and thermal images of North America have been available for the study of ice on the Great Lakes. The shallowest of the Great Lakes is Lake Erie, on which ice occurs from December usually through April. The NOAA series of polar-orbiting satellites collect two thermal and one visible image per day; the Geostationary Observational Environmental Satellites (GOES) collect visible and infrared images every 30 min. NASA’s Landsat satellite has an 18 d revisit cycle (a 9 d cycle when two are operating), with superb ground resolution (79 m). Satellite images from all three satellites were used in this study.

Ice formation, movement, and break-up in Lake Erie are different in each sub-basin, Western, Central, and Eastern, owing to the variation in depth of water, geomorphology, and meteorologic factors.

Ice movement after initial break-up in 1974–77 resulted in the accumulation of ice at the entrance of the Niagara River at the eastern end of the lake. In 1973, satellite images show the unusual development of an ice dam across the entire width of Lake Erie and the “ice plug” at the mouth of the Niagara did not form.

The effect of wind direction on break-up patterns is seen to be critical to any attempt at forecasting date of complete ice melt as well as day-to-day distribution of ice. Satellite observation of ice in Lake Erie—and in all large lakes—provides information regularly, routinely, and synoptically from a point in space. These data are useful for the study of ice formation, movement, and break-up, and they provide an unrivaled and unprecedented data set of historical records.
The Use of a Rational Model in the Mathematical Analysis of a Polythermal Glacier
Authors: Fowler A.C.
Published: September 2017

We here describe the process by which a complex model set of equations and boundary conditions may be rationally reduced to a simpler and more manageable set by the. processes of non-dimensionalization and asymptotic approximation. Such a reduced model (derived elsewhere) is then presented for an incompressible, two-dimensional ice flow. It consists of two coupled equations for the stream function and enthalpy variable, together with a complex set of boundary conditions.

The important dimensionless parameters which arise are given, and various limiting values of these are commented on. Nye’s (1960) equation for kinematic waves may be reproduced, and a non-linear analysis of this reveals that disturbances reach the glacier snout in finite time, and are uniformly bounded there: in the particular case considered here, one can also show that the temperature field is stable.

It is shown that the effect of introducing a (realistic) sliding law which is continuously dependent on the temperature has a major effect on the bedrock temperature profile.

Lastly we consider seasonal waves using a kinematic wave equation based on a plausible form of the sliding law when cavitation is present. The main observed features are qualitatively reproduced.
Ice-Shelf Underwater Morphology
Authors: Orheim Olav
Published: September 2017

A side-scan sonar survey was conducted near the ice front of Riiser-Larsenisen during the 1976–77 Norwegian Antarctic Research Expedition to investigate the underwater ice-shelf morphology. The survey covered several shorter, widely-spaced sections, for a total of about 10 km along the ice front. Only a smaller part of the sections showed a smooth, straight ice front. Most of the observed ice fronts were very rough at the scale of decametres. Projections at around l00 m water depth commonly extended 50–200 m seaward of the above-water ice shelf, and had widths of similar dimensions.
Inland Ice Sheet Thinning due to Holocene Warmth
Authors: Whillans I. M.
Published: September 2017

The Holocene warmth is now affecting the flow of the central West Antarctic ice sheet. It is supposed that the ice sheet reached approximate steady-state during the Wisconsinan. A perturbation analysis of ice-sheet temperatures indicates that deep ice has warmed by one or two degrees since the Wisconsinan. Warmer ice deforms more rapidly and the ice sheet should now be flowing 10 to 30% faster than during the Wisconsinan.
Evidence Against and Factors Preventing Major Surges of the Antarctic Ice Sheet
Authors: Robin G. de Q.
Published: September 2017

Although computer modelling using realistic flow parameters can simulate surging of the Antarctic ice sheet, the present model does not take into account certain factors that make surging less probable. Before discussing these factors, knowledge of the Antarctic ice sheet that might indicate the occurrence of former surging is reviewed. The following studies appear relevant

(a)

Observed temperature–depth profiles approximate to steady-state solutions, whereas a major surge within the last 10 000 to 20000 years would have produced markedly different temperature–depth profiles at Byrd and Vostok Stations.

(b)

Isotopic profiles are estimated for steady-state and for surging behaviour of the Antarctic ice sheet. When these are compared with observed profiles no convincing evidence of surging over the past 10 000 to 50 000 years is seen.

(c)

Studies of flow lines in and around the Ross Ice Shelf do not reveal any surging of discharge glaciers in the past I 000 to 2 000 years.

(d)

Although mass-balance calculations and balance-velocity calculations on the Antarctic ice sheet are not accurate, ice discharge is generally estimated to be within a factor of two of the total mass accumulation.

Three stabilizing factors that have not been included in computer models and need consideration are

(1)

It appears unlikely that a surge will be propagated up-stream of any substantial bedrock slope that opposes the ice motion.

(2)

The very high effective viscosity of great thicknesses of ice at very low temperatures adds considerable rigidity to the ice sheet at the lateral boundaries of any incipient surge.

(3)

Strong convergence of flow lines towards ice streams and major trunk glaciers apparently provides a stabilizing factor.
Reconstruction and Disintegration of Ice Sheets for the Climap 18000 and 125000 Years B.P. Experiments: Theory
Authors: Hughes T. J.
Published: September 2017

Size, shape, and surface albedo of former ice sheets are needed in order to model atmospheric circulation for the CLIMAP 18000 years B.P. experiment. Both the size and shape of an ice sheet depend on the hardness of ice and its coupling to bedrock. Ice hardness is controlled by ice temperature and fabric, which are not adequately described by any ice flow law. Ice–bed coupling is controlled by bed roughness and basal melt water, which are not adequately described by any ice sliding law. With these inadequacies in mind, we assumed equilibrium ice-sheet conditions 18000 years ago and combined the standard steady-state flow and sliding laws of ice with the equation of mass balance to obtain separate basal shear-stress variations along ice-sheet flow lines for a frozen bed when the flow law dominates and for a melted bed when the sliding law dominates. Theoretical basal shear-stress variations were then derived for freezing and melting beds on the assumption that separate melted areas of the bed had water films of constant thickness which expanded and merged for a melting bed but contracted and separated for a freezing bed. Theoretical basal shear-stress variations were also derived for ice streams along marine ice-sheet margins and ice lobes along terrestrial ice-sheet margins on the assumption that the entire area of their bed was wet so that further melting increased the water-layer thickness, which would then be decreased by freezing. Melting was assumed to continue to the grounding line of an ice stream and the minimum-slope surface inflection line of an ice lobe, where freezing began and continued to the ice-lobe terminus. Ice–bed uncoupling is complete at an ice-stream grounding line and maximized at an ice-lobe minimum-slope inflection line, so ice velocity and consequent generation of frictional heat were assumed to reach maxima across these lines. Theoretical basal shear-stress variations were derived for the zone of converging flow at the heads of ice streams and ice lobes, and from domes to saddles along the ice divide for both frozen and melted beds.
Reconstruction and Disintegration of Ice Sheets for the Climap 18000 And 125000 Years b.p. Experiments: Results
Authors: Denton G. H. , Hughes T. J. , Fastook J. L. , Schilling D. H. , Lingle C. S.
Published: September 2017

Late-Wisconsin ice sheets were reconstructed for the CLIMAP 18000 years b.p. experiment. This experiment modeled the ice-age steady-state climate using boundary conditions that differed from present ones mainly in Earth-surface albedos, sea-surface areas, and land-surface topography. These required determinations of the area, volume, and elevation of Late Wisconsin ice sheets. An initial-value finite-difference numerical model for ice-sheet reconstruction was developed from a recursive formula which gave ice thickness for known variations of bed topography and theoretical variations of basal shear stress. Ice thicknesses were calculated in 50 km to 100 km steps along flow lines from margins to domes of late-Wisconsin ice sheets. We assumed that terrestrial margins were along the furthermost moraines, marine margins were along the present 500 m bathymetric contour, domes were centers of maximum post-glacial isostatic rebound, and flow lines were along glacial lineations (eskers, striations, drumlins, etc.) connecting margins to domes. At various locations ice-sheet margins were verified by dated moraines for terrestrial margins and Egga-type moraines for marine margins. Ice-sheet elevations and thicknesses were contoured from profiles reconstructed for 40 Antarctic flow lines and 137 Northern Hemisphere flow lines for a maximum ice-sheet extent, and 86 Northern Hemisphere flow lines for a minimum ice-sheet extent.
A Closer look at the Proposed Late-Wisconsin-Weichselian Arctic Ice Sheet
Authors: Fastook J. L. , Sweet R. , Hughes T. J.
Published: September 2017

The CLIMAP 18000 years b.p. experiment required reconstructing late-Wisconsin-Weichselian ice sheets. In the Northern Hemisphere, the greatest uncertainty in these reconstructions is the area covered by ice sheets. Two schools of thought exist (Hughes and others, in press). The minimum-ice-sheet school holds that ice sheets originated from present ice caps in the High Arctic islands, but the northern seaward margins of these ice sheets retreated as the southern landward margins advanced. This occurred because northern margins became isolated from sources of precipitation as Arctic seas became permanently ice-covered and the advancing southern margin changed atmospheric circulation patterns. In this view, these ice sheets stay about the same size and migrate southward during an ice age. Northern margins rarely reach sea-level during the later stage of the ice age so no marine portions form and ablation is by melting or sublimation. Marine portions formed only when the ice sheets migrated across shallow seas between the High Arctic islands and the mainland. At the end of the ice age, huge amounts of heat had to be transferred from the tropics to the ice sheets in order to account for late-Wisconsin-Weichselian and Holocene retreat-rates by melting along ice-sheet margins.
Isostatic Equilibrium Grounding Line Between The West Antarctic Ice Sheet And The Ross Ice Shelf
Authors: Bentley C. R. , Greischar L.
Published: September 2017

Taking various retreat-rates for the presumed grounded ice sheet in the Ross embayment during Wisconsin time, as calculated by Thomas (Thomas and Bentley, 1978), and assuming a time constant of 4400 years for isostatic rebound, a sea-floor uplift of 100±50 m still to be expected in the grid western part of the Ross Ice Shelf can be calculated. The expected uplift diminishes from grid west to grid east, and is probably negligible in the eastern half of the shelf area. There are extensive areas near the present grounding line where the water depth beneath the shelf is less than 100 m, so that uplift would lead to grounding. As grounding occurred, the neighboring ice shelf would thicken, causing grounding to advance farther. This process would probably extend the grounding line to a position running grid north-eastward across the shelf from the seaward end of Roosevelt Island, deeply indented by the extensions of the present ice streams. Floating ice would remain in the grid south-eastern half of the shelf.
Surging Glaciers—The Dilemma Continues
Authors: Meier M. F.
Published: September 2017

A glacier surge, according to most definitions, is a short-lived phase of unusually rapid glacier flow, after which the glacier returns to more normal behavior, with the surge–non-surge phases recurring on a regular or periodic basis. Recent interest is largely directed toward analyzing the effect of water at the bed on the periodic change in flow regime and on the rapid flow during a surge phase. For instance, study of a local depression of basal shear stress that dependson a “friction lubrication factor” which becomes important as the ice velocity increases, is one promising phenomenological approach. An important physical approach focuses on a water “collection zone” that occurs where and when the longitudinal pressure gradient in the subglacial wtaer film approaches zero. The data necessary for properly verifying these and other similar theories do not yet exist. Computer modeling of rapidly-surging glaciers based on a “friction lubrication factor” has been quite successful in duplicating their major features. Once rapid movement (102–103 m a–1) has begun, sufficient water is generated at the bed, from ice melted by heat dissipated in sliding, to produce some decoupling of the glacier from its bed and to maintain the surge, but only if this water is not lost by rapid drainage. Some glaciers exhibit periodic pulses in which the basal sliding velocity during the fastest part of the pulses appears to be in the range for “normal” glaciers (<102 m a–1). Some evidence suggests a continuum of behavior from steady (normal) glaciers through these “mini-surges” to classic surges. This continuum and the “mini-surges” seem to be difficult to explain quantitatively by existing theories. A few glaciers flow continuously at surging speeds (>103 m a–1) in certain reaches. The up-glacier transition reaches show speeds decreasing to “nonrmal” with no indication of intermediate surging regime, but the down-glacier transition reaches may be areas where surges are triggered.
Instability of a Calving Glacier Terminus
Authors: Hodge Steven M.
Published: September 2017

An unstable situation can develop when a calving glacier (one that ends in water but is still grounded throughout) terminates on a shoal. This instability is largely a consequence of continuity and the extending flow which occurs up-glacier of the shoal. Whenever the calving-rate exceeds the ice velocity the ice front will start to retreat; the rate of retreat is accentuated by the extending flow and the front continues to retreat at an increasing rate.
Dynamics Of Icelandic Ice Caps And Outlet Glaciers
Authors: Williams Richard S. , Thorarinsson Sigurdur , Björnsson Helgi , Gudmundsson Bragi
Published: September 2017

Since late 1972, when the first Landsat images of Iceland became available, the ice caps of Iceland and their outlet glaciers have been under intensive study with Landsat images. The original emphasis of the Landsat research of Icelandic ice caps was to record and quantitatively measure dynamic change of glaciers and phenomena related to glaciers. A comparison of Landsat images with the best available topographic maps revealed a number of discrepancies in the planimetric shape and size of Icelandic ice caps and in the mapping of prominent surface features. Some of the surface features recorded on Landsat images turned out to be the subdued morphological expression of subglacial topography and geologic structure or the surficial response to subglacial volcanic and geothermal activity. Of particular interest was a winter image of Vatnajökull, acquired at a solar elevation angle of 7°, which revealed a number of surficial features not recorded before, including a large volcanic caldera in the Kverkfjöll area. A computer-enhanced early fall image of Vatnajökull revealed similar phenomena, and computer-enhancement techniques have also been applied to summer images of Hofsjökull, Langjökull, Mýrdalsjökull, and Eyjafjallajökull. In the case of Hofsjökull a subglacial volcanic edifice with the approximate dimensions of the Dyngjufjöll massif in north-central Iceland, and possibly with a caldera, may exist. Radio echo-sounding surveys have been carried out over some parts of Vatnajökull and Mýrdalsjökull to determine the precise nature of the subglacial topography, and it is of great interest to compare the results of these surveys of glacier bed topography with the surface features recorded on Landsat and other imagery. Landsat images are also being used to plan the optimum location of field traverses and as a navigational tool (map substitute) during field work.
Wave Ogives as Second-Order Effects in the Creep of Large Ice Masses
Authors: Williams F. M.
Published: September 2017

This paper presents a mathematical analysis of the steady-state motion of a large ice mass. The purpose of the study is to determine the parameters which govern the motion of the ice and to establish the relationships between these parameters and the observable features of the motion. The study considers both land-based glaciers and floating ice shelves as different cases of the same problem. The particular features of the motion which are considered in this study are the surface waves or ogives which appear on both glaciers and ice shelves.
Water pressure and basal sliding on Storglaciären, northern Sweden
Authors: Jansson Peter
Published: September 2017

The subglacial hydrology of the ablation area of Storglaciären, a small valley glacier in northern Sweden, is dramatically affected by a subglacial ridge, or riegel. Water pressures above this riegel are relatively constant, while down-glacier from it they vary significantly. The lower part of the glacier accelerates in response to peaks in basal water pressure. The upper part may be weakly coupled to the lower part during these peaks.

A power-law fit of observed basal water pressures and measured surface velocities yields

where us is the surface velocity and PE is the effective water pressure (ice overburden pressure minus subglacial water pressure). Data from Findelengletscher, reported by Iken and Bindschadler (1986), yield an identical exponent and a coefficient one order of magnitude larger. The similar exponent implies that the process producing the velocity variations on both glaciers is similar. The variations in velocity are inferred to be due to hydraulic jacking on both glaciers.
Comments on the use of chemically based mixing models in glacier hydrology
Authors: Sharp Martin , Brown Giles H. , Tranter Martyn , Willis Ian C. , Hubbard Bryn
Published: September 2017

The assumptions involved in the use of chemically based mixing models for analysis of flow routing of meltwaters in glacierized basins are critically evaluated. The assumption that glacial drainage systems consist of only two primary flow components is arbitrary and must be supported by independent evidence. Recent studies of the processes by which meltwaters acquire solute indicate that the assumption that flow components have unique and constant chemical compositions is unlikely to be correct. Source-water composition and weathering potential will vary over the course of a melt season, and the extent of subglacial weathering is strongly dependent upon such factors as meltwater residence time and the availability of reactive sediment, both of which are known to vary on diurnal to seasonal time-scales. Mixing of flow components does not appear to be confined to the terminal regions of glaciers and is therefore unlikely to be conservative as assumed. A multi-parameter mixing model is applied to the analysis of data on the chemistry of waters sampled from boreholes drilled through Haut Glacier d’Arolla, Switzerland, to demonstrate the range of dissolved species for which the assumption of conservative mixing is violated. The consequences of this violation for quantitative hydrograph separation are shown to be highly significant. The utility of mixing models as a tool for the investigation of glacier hydrological systems is questionable and the results of previous studies are unreliable.
Changes in jökulhlaup sizes in Grímsvötn, Vatnajökull, Iceland, 1934-91, deduced from in-situ measurements of subglacial lake volume
Authors: Gudmundsson Magnús T. , Björnsson Helgi , Pálsson Finnur
Published: September 2017

A record of volumes of jökulhlaups from the subglacial Grímsvötn lake, Vatnajökull, Iceland, has been derived for the period 1934–91. The change in lake volume during jökulhlaups is estimated from the lake area, ice-cover thickness and the drop in lake level. The jökulhlaup volumes have decreased gradually during this period of low volcanic activity and declining geothermal power. The two jökulhlaups in the 1930s each discharged about 4.5 km3(peak discharge 25-30 × 103m3s−1). In the 1980s, jökulhlaup volumes were 0.6.-1.2 km3(peak discharge 2 × 103m3s−1). The lake level required to trigger a jökulhlaup has risen as an ice dam east of the lake has thickened. Water flow in a jökulhlaup ceases when the base of a floating ice shelf covering Grímsvötn settles to about 1160 m a.s.l. Apparently, the jökulhlaups are cut off when the base of the ice shelf collapses on to a subglacial ridge bordering the lake on its eastern side. The decline in melting rates has resulted in a positive mass balance of the 160-170 km2Grímsvötn ice-drainage basin. Comparison of maps shows that the average positive mass-balance rate was 0.12 km3a−1(25% of the total accumulation) in the period 1946-87. A gradually increasing positive mass balance has prevailed since 1954, reaching 0.23 km3a−1in 1976-86 (48% of total accumulation).
Degree-day glacier mass-balance modelling with applications to glaciers in Iceland, Norway and Greenland
Authors: Jóhannesson Tómas , Sigurdsson Oddur , Laumann Tron , Kennett Michael
Published: September 2017

A degree-day glacier mass-balance model is applied to three glaciers in Iceland, Norway and Greenland for which detailed mass-balance measurements are available over a period of several years. Model results are in good agreement with measured variations in the mass balance with elevation over the time periods considered for each glacier. In addition, the model explains 60-80% of the year-to-year variance in the elevation-averaged summer season mass-balance measurements on the glaciers, using a single parameter set for each glacier.

The increase in ablation on the glaciers due to a warming of 2° C is predicted to range from about 1 m w.e. year−1at the highest elevations to about 2.5 m w.e. year−1at the lowest elevations. Predicted changes in the winter balance (measured between fixed date) are relatively small, except at the lowest elevations on the Icelandic and Norwegian glaciers where the winter balance is significantly reduced. Equilibrium-line altitudes are raised by 200-300 m on the Icelandic and Norwegian glaciers. Except at the highest elevations, the winter balance of the Icelandic and Norwegian glaciers is predicted to decrease even if the warming is accompanied by a 10% increase in the precipitation.

No firm evidence of a climate-related variation in the degree-day factors or in the temperature lapse rate on the same glacier could be found. The model, furthermore, reproduces large variations in the mass balance with elevation and from year to year on each glacier using the same parameter set. We assume, therefore, that these parameters will not change significantly for the climate scenarios considered here.
An energy-balance model of lake-ice evolution
Authors: Liston Glen E. , Hall Dorothy K.
Published: September 2017

A physically based mathematical model of the coupled lake, lake ice, snow and atmosphere system is developed for studying terrestrial-atmospheric interactions in high-elevation and high-latitude regions. The ability to model lake-ice freeze-up, break-up, total ice thickness and ice type offers the potential to describe the effects of climate change in these regions. Model output is validated against lake-ice observations made during the winter of 1992–93 in Glacier National Park, Montana. U.S.A. The model is driven with observed daily atmospheric forcing of precipitation, wind speed and air temperature. In addition to simulating complete energy-balance components over the annual cycle, model output includes ice freeze-up and break-up dates, and the end-of-season clear ice, snow-ice and total ice depths for two nearby lakes in Glacier National Park, each in a different topographic setting. Modeled ice features are found to agree closely with the lake-ice observations.

Model simulations illustrate the key role that the wind component of the local climatic regime plays on the growth and decay of lake ice. The wind speed affects both the surface temperature and the accumulation of snow on the lake-ice surface. Higher snow accumulations on the lake ice depress the ice surface below the water line, causing the snow to become saturated and leading to the formation of snow-ice deposits. In regions having higher wind speeds, significantly less snow accumulates on the lake-ice surface, thus limiting snow-ice formation. The ice produced by these two different mechanisms has distinctly different optical and radiative properties, and affects the monitoring of frozen lakes using remote-sensing techniques.
Fracture toughness of ice and firn determined from the modified ring test
Authors: Fischer Mark P. , Alley Richard B. , Engelder Terry
Published: September 2017

The modified ring test is used to determine the fracture toughness of synthetic, granular, fresh-water ice average density 0.891 Mg m−3 and firn (average density 0.605 Mg m 3) from depths between 26 and 27.2 m in the E core of the Greenland Ice Sheet Project II. Average fracture toughness is 145.7kPa m2 for the manufactured ice and 108.6kPam½ for the firn. Comparison between the ice and firn suggests that ice-fracture toughness decreases with decreasing density (i.e. increasing porosity), suggesting lateral and vertical variations in the near-surface fracture resistance of glaciers and ice sheets may be related to firn densification. The modified ring test has many advantages over conventional, notch-based specimens in that complications which arise in notched specimens due to crack-length, loading-rate, notch-acuity and specimen-size effects are irrelevant for a modified ring-specimen geometry.
Glaciological studies on Nelson Island, South Shetland Islands, Antarctica
Authors: Jiawen Ren , Dahe Qin , Petit J. R. , Jouzel J. , Wenti Wang , Chen Liu , Xiaojun Wang , Songlin Qian , Xiaoxiang Wang
Published: September 2017

The ice cap on Nelson Island in the South Shetland Islands, West Antarctica, was studied between 1985 and 1989. The ice cap has an average thickness of 120 m. it is temperate, exists under the sub-Antarctic maritime climate and almost completely covers the island. Owing to intense percolation of meltwater (and, to some extent, liquid precipitation), the snow-firn layer is in the soaked facies, with a firn-ice transition at a depth of 25-26 m at the summit. A force-balance model suggests that the ice is almost linearly viscous but has a high viscosity. The model further suggests that basal sliding makes a larger contribution to the ice movement than does ice deformation. From 1970 to 1988. the average accumulation rate was 120 kg m−2a−1 at the centre, and between 1985 and 1989 the equilibrium-line elevation averaged 110m a.s.l. Analysis of chemical impurities in the surface snow suggests that the precipitation source is mainly local marine air masses and that human activity has already exerted a detectable influence on the local environment.
The limiting resolution of ice-sheet elevations derived from pulse-limited satellite altimetry
Authors: Wingham D.J.
Published: September 2017

The problem of improving the spatial resolution of ice-sheet elevations derived from satellite-altimeter observations is investigated theoretically in two dimensions. An equation for the echo is described that allows the elevation to be determined In linear operations on the echo. An approximate solution of the equation is explored to illustrate the behaviour of the solution. The modifications necessary to consider a regularly sampled solution are described and the effects of echo fluctuations on the reconstructed surface are considered. Numerical examples are described. It is shown that, in general terms, the resolution may be limited by sampling or by echo fluctuations but that in the case of current practical measurements it is the sampling that determines the resolution. This resolution is smaller than has been considered possible hitherto. Limitations of the method are discussed together with extensions to a wider class of problems including three-dimensional reconstructions.
Physical environment of drumlin formation
Authors: Patterson Carrie J. , Hooke Roger LeB.
Published: September 2017

Review of published descriptions of drumlin fields suggests that the following conditions are important to drumlin growth: (1) compressive longitudinal and possibly extending transverse strain rates in the ice, (2) thin ice such as occurs near the glacier margin, and (3) high pore-water pressure in the subglacial sediments. Most drumlin fields display all of these, and no fields of well-developed drumlins were found that did not. On the oilier hand, the lithology of drumlin-forming sediment appears not to be important in promoting drumlin growth, since it varied widely, nor are the lithology and large-scale topography of the bed.
The “faint young Sun paradox”: further exploration of the role of dynamical heat-flux feed backs in maintaining global climate stability
Authors: Molnar Gyula I. , Gutowski William J.
Published: September 2017

The climate-modeling problems associated with global change underline the importance of understanding paleoclimates. The available evidence, which suggests that the Earth has never been fully glaciated, poses an especially serious problem for the early Earth when the Sun was about 20–30% fainter than today. In conventional explanations of this “faint young Sun paradox”, presumed very high levels of atmospheric greenhouse gases are required to prevent runaway glaciation of the Earth. Here we explore other possible explanations of this paradox. As an extension of our previous work on this subject, we illustrate how-dynamical beat-flux feed backs may have prevented the early Earth from freezing. Our simulations are carried out using a two-dimensional, seasonal-climate model with physically based parameterizations for atmospheric meridional-heat transport and sea ice. It ís found that dynamical heat-flux feed backs alone may have protected the Archean Earth against a runaway glaciation to a considerable degree.
Derived bedrock elevations, strain rates and stresses from measured surface elevations and velocities: Jakobshavns Isbræ, Greenland
Authors: Fastook James L. , Brecher Henry H. , Hughes Terence J.
Published: September 2017

Jakobshavns Isbræ (69 °10′ N, 49 °59′ W) drains about 6.5% of the Greenland ice sheet and is the fastest ice stream known. The Jakobshavns Isbræ basin of about 10 000 km2 was mapped photogrammetrically from four sets of aerial photography, two taken in July 1985 and two in July 1986. Positions and elevations of several hundred natural features on the ice surface were determined for each epoch by photogrammetric block aerial triangulation, and surface velocity vectors were computed from the positions. The two flights in 1985 yielded the best results and provided most common points (716) for velocity determinations and are therefore used in the modeling studies. The data from these irregularly spaced points were used to calculate ice elevations and velocity vectors at uniformly spaced grid points 3 km apart by interpolation. The field of surface strain rates was then calculated from these gridded data and used to compute the field of surface deviatoric stresses, using the flow law of ice, for rectilinear coordinates, X, Y pointing eastward and northward, and curvilinear coordinates. L, Τ pointing longitudinally and transversely to the changing ice-flow direction, Ice-surface elevations and slopes were then used to calculate ice thicknesses and the fraction of the ice velocity due to basal sliding. Our calculated ice thicknesses are in fair agreement with an ice-thickness map based on seismic sounding and supplied to us by K. Echelmeyer. Ice thicknesses were subtracted from measured ice-surface elevations to map bed topography. Our calculation shows that basal sliding is significant only in the 10–15 km before Jakobshavns Isbræ becomes afloat in Jakobshavns Isfjord.