Sion Shabtaie

Force, mass, and energy budgets of the Crary Ice Rise complex, Antarctica

The stress, mass, and energy-dissipation budgets of Crary Ice Rise are analyzed using field data collected during the 1983-1985 austral summers and in previous field programs. In addition, the net back pressure and ice-discharge rate along the grounding lines of ice streams are calculated to assess the effect of the ice rise on the surrounding flow. Comparison of the ice-rise budgets with the analysis of grounding-line data confirms the influence of the ice rise on ice-sheet stability, and suggests that Crary Ice Rise may have formed recently in response to an acceleration of one of the ice streams. It is concluded that feedback between ice-stream acceleration and ice-rise formation may control the future evolution of the above ice stream and promote long-term grounding-line stability in the face of strong natural fluctuations.

Electrical resistivity sounding of the East Antarctic Ice Sheet

Electrical resistivity soundings using a Schlumberger array have been carried out at Dome C, East Antarctica (74°39′S, 124°10′E, elevation 3400 m), to sound the entire 3500-m depth of the ice sheet. Changes in density and temperature are largely separated in the ice at Dome C, so activation energies for both firn and ice could be determined: we find an activation energy of 0.25 eV in both solid ice and firn between −15°C and −58°C. A common value of the activation energy points to a single transport regime in which the charge carriers and conduction paths are the same in firn and ice. To evaluate the variation of resistivity with density, we have considered five dielectric mixture models that fit the available data on the high-frequency dielectric constant of firn. Only Looyengas equation fits the field data for dc resistivity. In the upper 900 m of the ice sheet, where impurity concentrations are known from core samples, we find no correlation between resistivities and the concentrations of salts or acids. Instead, we find it likely that resistivities are correlated with the crystal size, hence with the Holocene-Wisconsin boundary in the ice column. A pronounced increase in resistivity, to a value comparable with that in temperate glacier ice, occurs deep within the ice sheet. We attribute this to a large increase in the size and irregularity of the ice crystals, which destroys the continuity of the impurity shells surrounding the ice crystals that we believe supply the conduction paths (Shabtaie and Bentley, 1994a). High resistivity does not imply removal of the impurities from the system; moderate concentrations of impurities can be accommodated by locating them in disconnected domains.

Ice-shelf flow at the boundary of Crary Ice Rise, Antarctica

Surface velocity and deformation, radar sounding , and aerial photography data are used to describe the flow of Ross Ice Shelf around Crary Ice Rise. A continuous band of crevasses around the ice rise now allows the complete boundary to be mapped for the first time. The dynamics of three distinctly different areas of ice flow are studied . Just up-stream of the ice rise, there is a region of ice rumples dominated by intense longitudinal compression (0.01 a -I) and lateral tension. On the south-west side of the ice rise, intense shear (0.03 a-I) dominates, with the boundary layer of affected ice-shelf motion extending over 20 km from the ice-rise edge into the ice shelf. North-west of the ice rise, a crevasse-free block of ice, 40 km x 7 km , appears to have separated from the main ice rise and is now moving with the ice shelf. We refer to such moving blocks of ice as rafts. The separation of this raft is calculated to have occurred 20 ± 10 years ago. Other possible rafts are identified, including one on the south-west side of the ice rise which appears to be in the process of separating . Mechanisms for the formation of rafts are discussed.

Measurement of Radio Wave Velocities in the Firn Zones of Polar Ice Sheets (Abstract only)

A common-reflection point profiling experiment to obtain electromagnetic wave velocities in the ice at Dome C at 35 MHz was carried out to a maximum antenna separation of 2 km. Four different recording systems were used for this experiment. The echoes from numerous internal reflecting horizons within the ice and the bedrock were recorded in four different ways: in A-display form on film using an oscilloscope, in intensity-modulated form using the Honeywell Visicorder and thermal intensifier on heat-sensitive silver paper, and in both raw and signal-averaged form on magnetic tape. Travel times of oblique reflections from nearly 160 internal layers down to a depth of 2 600 m and reflections from the ice bottom were measured at each station along the profile. The average wave velocities from the surface of each internal layer were measured to obtain a continuous mean velocity vs depth profile. Velocity-density models derived from the dielectric mixture equations of Looyenga, Bottcher, Lichtenrecker, Hanai-Bruggeman and Wiener were compared against the measurements. In addition Robins empirical relationship was also used in this study. The preliminary results show that the observed velocitydepth profile for the ice column is compatible with Wieners equation (with Formzahl = 0) and a newly derived empirical relationship (from this study) of

Grounding-Line Retreat of Ice Stream B, Antarctica

depth can be obtained. The measurements and model studies at several locations in Antarctica (Ice Stream B, Ross Ice Shelf, and Dome C) have resulted in the discovery of a rather sharp change in resistivity at a depth identified with the Holocene/Wisconsin transition at two locations (19 on the Ross Ice Shelf and Dome C in East Antarctica), where core studies have been made. The change in resistivity in polar ice sheets can be attributed primarily to changes in crystal size. In general, crystal size increases with depth down to the transition zone, decreases rather sharply over a short depth, and then gradually increases again . Resistivities show a similar behavior, and indeed there is a good correlation between the two at Dome C and J9. At a location on Ice Stream B where the ice is about 1060 m thick, our best model, using that correlation, shows the Holocene-Wisconsin boundary at 750 m depth. Furthermore, the shape of the resistivity change with depth (after correction for temperature) shows a good correlation with the crystal-size profile across that boundary at Byrd Station. Another pronounced increase in resistivity occurs deeper in the ice, especially at locations on the Ross Ice Shelf where the ice has come from West Antarctic ice streams or the major outlet glaciers of East Antarctica. We attribute this increase also to a large increase in crystal size, such as occurs in the basal ice at Byrd Station, up-stream from Ice Stream D, and at Little America Station V on the Ross Ice Shelf.

A Few Preliminary Results from the Glaciogeophysical Survey of the Interior Ross Embayment (GSIRE)

We report here on some of the results of our first two seasons work along the Siple Coast. These results are all preliminary in nature and could be modified substantially with further analysis. Furthermore, we have selected from a much larger body of data only a few points that we believe will be of interest to this workshop.

Electrical Resistivity Sounding Related to Ice-Crystal Size: A Technique for Probing the Holocene-Wisconsin Boundary

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Tabular Icebergs: Implication from Geophysical Studies of Ice Shelves

Recent geophysical and glaciological investigations of the Ross Ice Shelf have revealed many complexities in the ice shelf that can be important factors in iceberg structure . The presence of rift zones, surface and bottom crevasses, corrugations, ridges and troughs, and other features could substantially modify the hydraulics of iceberg towing and lead to disintegration of the berg in the course of transport. The relationships between t he elevation above sea-level and total ice t hickness for three ice shelves (Ross, Brunt, and McMurdo) are given; from them, expressions for t he thickness/ freeboard ratios of tabular icebergs calved from these ice shelves are obtained. The relationships obtained from the measured values of surface elevation and ice thickness are in agreement with models derived assuming hydrostatic equilibrium . Areas of brine infiltration into the Ross Ice Shelf have been mapped. Examples of radar profiles in these zones are shown . Absorption from the brine layers results in a poor or absent bottom echo. It is probable that little saline ice exists at the bottom of the Ross Ice Shelf front due to a rapid bottom melting near the ice front, and that t he thickness of the saline ice at the bottom of icebergs cal ving from the Ross Ice Shelf is no more t han a few meters, if there is any at all. We have observed many rift zones on the ice shelf by airborne radar techniques, and at one site t he bottom and surface topographies of (buried) rift zones have been delineated. These rift zones play an obvious role in iceberg formation and may also affect the dynamics of iceberg transport . Bottom crevasses with different shapes, sizes, and spacings are abundant in ice shelves; probably some are filled with saline ice and others with unfrozen sea-water . Existence of these bottom cr~vasses cou ld lead to a rapid disintegration of icebergs in the course of transport , as well as increasing the frictional drag at the ice-water boundary. Radar profiles of the ice-shelf barrier at four sites in flow bands of very different characteristics are shown . In some places rifting upstream from t he barrier shows regular spacings, suggesting a periodic calving. Differential bottom melting near the barrier causes the icebergs to have an uneven surface and bottom (i . e . dome-shaped) . Electrical resistivity soundings on the ice shelf can be applied to estimate the temperaturedepth function, and from t hat the basal mass balance rate. With some modifications, the technique may also be applied to estimating the basal mass-balance rates of tabular icebergs. *Geophysical and Polar Research Cent er , University of Wisconsin-Madison , Contribution Number 384