bluuebunny/crossref_metadata_2025_split huggingface dataset:
Electromagnetic Sounding of Bottom Crevasses on the Ross Ice Shelf, Antarctica
Authors: Jezek Kenneth C. , Bentley Charles R. , Clough John W. 
Published: September 2017

AbstractDuring 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

AbstractElectrical 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

AbstractSeasonal 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

AbstractSince 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

AbstractWe 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

AbstractA 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

AbstractThe 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

AbstractAlthough 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

AbstractSize, 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

AbstractLate-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

AbstractThe 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

AbstractTaking 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

AbstractA 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

AbstractAn 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

AbstractSince 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

AbstractThis 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.