scholarly journals Electrical Resistivity Of George VI Ice Shelf, Antarctic Peninsula

1982 ◽  
Vol 3 ◽  
pp. 279-283 ◽  
Author(s):  
John M. Reynolds
Keyword(s):  
Apparent Resistivity ◽  
Sea Water ◽  
Free Water ◽  
Melting Rate ◽  
Small Range ◽  
Ice Shelf ◽  
Ross Ice Shelf ◽  
Melt Water ◽  
Electrical Sounding ◽  
Basal Melting

A georesistivity survey was made on part of George VI Ice Shelf (71°55'S, 67°20'W). The principal objectives were to determine the electrical structure of the 1ce shelf, in particular how refrozen melt water differs in electrical behaviour from dry firn, and to Investigate the environment beneath the ice shelf.Apparent resistivity profiles using a Schlumberger electrode configuration have been interpreted using Ghosh's convolution method for vertical electrical sounding (VES), adapted for use where extreme resistivity contrasts are present.Warm, wet surface conditions tend to reduce the gross resistivity of shallow permeable layers. The electrical results indicate that the refrozen free water has affected the resistivity only indirectly; the mean density of firn is raised to about 0.915 Mg m−3 within the uppermost 10 m of the ice shelf at which point the resistivity is comparable to that of Ice of the same density but formed by compaction of firn. The apparent resistivities in the top 100 m reflect the variation of density with depth; a small range of resistivities implies that the range of density 1s narrow and that densification is affected by the percolation and refreezing of melt water.The bulk of the ice behaves as if resistivity either Is independent of temperature or has only a slight dependence (activation energy ~0.15 eV) with a basal melting rate in excess of 1 to 2 m a−1. The principal resistivities determined for two sites on George VI Ice Shelf were within 10% of those at station BC on the Ross Ice Shelf, allowing for differences in temperature. This Indicates that polar ice, I.e. non-temperate ice, has a very narrow range of resistivity. The apparent resistivity profiles are consistent with there being sea-water of oceanic salinity under the Ice shelf.

scholarly journals Electrical Resistivity Of George VI Ice Shelf, Antarctic Peninsula

1982 ◽  
Vol 3 ◽  
pp. 279-283 ◽  
Author(s):  
John M. Reynolds
Keyword(s):  
Apparent Resistivity ◽  
Sea Water ◽  
Free Water ◽  
Melting Rate ◽  
Small Range ◽  
Ice Shelf ◽  
Ross Ice Shelf ◽  
Melt Water ◽  
Electrical Sounding ◽  
Basal Melting

A georesistivity survey was made on part of George VI Ice Shelf (71°55'S, 67°20'W). The principal objectives were to determine the electrical structure of the 1ce shelf, in particular how refrozen melt water differs in electrical behaviour from dry firn, and to Investigate the environment beneath the ice shelf.Apparent resistivity profiles using a Schlumberger electrode configuration have been interpreted using Ghosh's convolution method for vertical electrical sounding (VES), adapted for use where extreme resistivity contrasts are present.Warm, wet surface conditions tend to reduce the gross resistivity of shallow permeable layers. The electrical results indicate that the refrozen free water has affected the resistivity only indirectly; the mean density of firn is raised to about 0.915 Mg m−3within the uppermost 10 m of the ice shelf at which point the resistivity is comparable to that of Ice of the same density but formed by compaction of firn. The apparent resistivities in the top 100 m reflect the variation of density with depth; a small range of resistivities implies that the range of density 1s narrow and that densification is affected by the percolation and refreezing of melt water.The bulk of the ice behaves as if resistivity either Is independent of temperature or has only a slight dependence (activation energy ~0.15 eV) with a basal melting rate in excess of 1 to 2 m a−1. The principal resistivities determined for two sites on George VI Ice Shelf were within 10% of those at station BC on the Ross Ice Shelf, allowing for differences in temperature. This Indicates that polar ice, I.e. non-temperate ice, has a very narrow range of resistivity. The apparent resistivity profiles are consistent with there being sea-water of oceanic salinity under the Ice shelf.

scholarly journals In Situ Measurements of the Activation Energy for D.C. Conduction in Polar Ice

1978 ◽  
Vol 21 (85) ◽  
pp. 698-699
Author(s):  
Charles R. Bentley
Keyword(s):  
Activation Energy ◽  
Apparent Resistivity ◽  
Personal Communication ◽  
Depth Range ◽  
Polar Ice ◽  
Ice Shelf ◽  
Ross Ice Shelf ◽  
True Value ◽  
Basal Melting ◽  
Ice Water

Abstract During the 1976-77 Antarctic field season, electrical resistivity profiling was carried out in the south-eastern quadrant of the Ross Ice Shelf. Drilling to a depth slightly greater than 300 m at the same site, where the total ice thickness is 425 m, permitted tem-perature determinations (personal communication from B. L. Hanson and J. H. Rand) that can be extrapolated to the ice-water boundary. Numerical modelling of the apparent resistivity, allowing for a continuous variation of temperature and density, and hence con-ductivity, with depth, was done in the same manner as has been described previously (Bentley, 1977). Temperatures calculated by assuming no basal melting or freezing show excellent agreement with those measured. Two models of apparent resistivity, taking the activation energy in the solid ice to be 0.15 eV and 0.25 eV, respectively, bracket the observed data, with the points tending to favor the lower value. This is in satisfactory agreement with (although perhaps slightly lower than) other measurements on polar ice. Assuming that the same temperature model applies at the site of the earlier measurements (Bentley, 1977), only 30 km away and approximately "up-stream", leads to apparent resistivity models, with activation energies of 0.15 eV and 0.25 eV, that again bracket the observations. The effect of other possible causes for the change of conductivity with depth besides temperature, such as varying grain size, crystal orientation, CO2 content, etc., is unknown but believed to be small because of the similar history of all the ice in the relevant depth range, about 100-350 In, over which the conductivity increases by a factor of 2. The conductivity in the ice at 100 m depth (temperature —23°C) at both sites is within ± 10% of 1.4 × 10-5 Ω-1. We conclude that an activation energy of 0.20 ± 0.05 eV not only can be used for modelling, but also closely represents the true value for ice-shelf ice.

scholarly journals The Recent Advance of the Ross Ice Shelf Antarctica

1986 ◽  
Vol 32 (112) ◽  
pp. 464-474 ◽  
Author(s):  
S. S. Jacobs ◽  
D. R. Macayeal ◽  
J. L. Ardai
Keyword(s):  
Large Scale ◽  
Melting Rate ◽  
Spreading Rate ◽  
Ice Shelf ◽  
Ice Shelves ◽  
Ross Ice Shelf ◽  
Front Position ◽  
Iceberg Calving ◽  
Basal Melting ◽  
Available Information

AbstractThe seaward edge of the Ross Ice Shelf advanced northward at a minimum average velocity of 0.8 km a–1 between 1962 and 1985. That advance approximated velocities that have been obtained from glaciological data, indicating little recent wastage by iceberg calving. West of long. 178° E., the ice shelf has attained its most northerly position in the past 145 years, and has not experienced a major calving episode for at least 75 years. Since 1841 the ice-front position has advanced and retreated within a zone from about lat. 77° 10’S. (near long. 171° E.) to lat. 78° 40’ S. (near long. 164° W.). The central ice front is now farthest south but has the highest advance rate. Calving may occur at more frequent intervals in that sector, which also overlies the warmest ocean currents that flow into the sub-ice-shelf cavity. Available information on ice-shelf advance, thickness, spreading rate, and surface accumulation indicates a basal melting rate around 3 m a–1 near the ice front. These data and independent estimates imply that basal melting is nearly as large a factor as iceberg calving in maintaining the ice-shelf mass balance. In recent years, the Ross, Ronne, and Filchner Ice Shelves have contributed few icebergs to the Southern Ocean, while projections from a contemporaneous iceberg census are that circumpolar calving alone may exceed accumulation on the ice sheet. Large-scale ice-shelf calving may have preceded historical sightings of increased numbers of icebergs at sea.

scholarly journals Crary Ice Rise, Antarctica: Formed in Response to a Surging Ice Stream? (Abstract)

1988 ◽  
Vol 11 ◽  
pp. 202
Author(s):  
D. R. MacAyeal ◽  
R. A. Bindschadler
Keyword(s):  
Stress Resistance ◽  
Field Data ◽  
Back Stress ◽  
Melting Rate ◽  
Current Development ◽  
Ice Shelf ◽  
Ross Ice Shelf ◽  
Ice Stream ◽  
Ice Stream B ◽  
Basal Melting

Field data is presented to support the hypothesis that Crary Ice Rise (on Ross Ice Shelf, Fig. 1) has substantially increased in area over the last 500 years, in response to ice advection through the mouth of Ice Stream B. The up-stream end of the ice rise is now surrounded by ice shelf that is currently thickening at 0.44 0.06 m/year (under an assumed zero basal melting rate). This rate of thickening suggests that the ice rise's contribution to back-stress resistance of Ice Stream B's flow, presently calculated to be 50% of the total back stress, is growing in the course of time. We speculate that this current development of the ice rise is the precursor to the possible future stagnation of Ice Stream B. It is convenient to conceptualize a possible see-saw oscillation between ice-stream surging and ice-rise build-up.

scholarly journals In Situ Measurements of the Activation Energy for D.C. Conduction in Polar Ice

1978 ◽  
Vol 21 (85) ◽  
pp. 698-699
Author(s):  
Charles R. Bentley
Keyword(s):  
Activation Energy ◽  
Apparent Resistivity ◽  
Personal Communication ◽  
Depth Range ◽  
Polar Ice ◽  
Ice Shelf ◽  
Ross Ice Shelf ◽  
True Value ◽  
Basal Melting ◽  
Ice Water

AbstractDuring the 1976-77 Antarctic field season, electrical resistivity profiling was carried out in the south-eastern quadrant of the Ross Ice Shelf. Drilling to a depth slightly greater than 300 m at the same site, where the total ice thickness is 425 m, permitted tem-perature determinations (personal communication from B. L. Hanson and J. H. Rand) that can be extrapolated to the ice-water boundary. Numerical modelling of the apparent resistivity, allowing for a continuous variation of temperature and density, and hence con-ductivity, with depth, was done in the same manner as has been described previously (Bentley, 1977). Temperatures calculated by assuming no basal melting or freezing show excellent agreement with those measured. Two models of apparent resistivity, taking the activation energy in the solid ice to be 0.15 eV and 0.25 eV, respectively, bracket the observed data, with the points tending to favor the lower value. This is in satisfactory agreement with (although perhaps slightly lower than) other measurements on polar ice. Assuming that the same temperature model applies at the site of the earlier measurements (Bentley, 1977), only 30 km away and approximately "up-stream", leads to apparent resistivity models, with activation energies of 0.15 eV and 0.25 eV, that again bracket the observations. The effect of other possible causes for the change of conductivity with depth besides temperature, such as varying grain size, crystal orientation, CO2 content, etc., is unknown but believed to be small because of the similar history of all the ice in the relevant depth range, about 100-350 In, over which the conductivity increases by a factor of 2. The conductivity in the ice at 100 m depth (temperature —23°C) at both sites is within ± 10% of 1.4 × 10-5 Ω-1. We conclude that an activation energy of 0.20 ± 0.05 eV not only can be used for modelling, but also closely represents the true value for ice-shelf ice.

scholarly journals The Recent Advance of the Ross Ice Shelf Antarctica

1986 ◽  
Vol 32 (112) ◽  
pp. 464-474 ◽  
Author(s):  
S. S. Jacobs ◽  
D. R. Macayeal ◽  
J. L. Ardai
Keyword(s):  
Large Scale ◽  
Melting Rate ◽  
Spreading Rate ◽  
Ice Shelf ◽  
Ice Shelves ◽  
Ross Ice Shelf ◽  
Front Position ◽  
Iceberg Calving ◽  
Basal Melting ◽  
Available Information

AbstractThe seaward edge of the Ross Ice Shelf advanced northward at a minimum average velocity of 0.8 km a–1between 1962 and 1985. That advance approximated velocities that have been obtained from glaciological data, indicating little recent wastage by iceberg calving. West of long. 178° E., the ice shelf has attained its most northerly position in the past 145 years, and has not experienced a major calving episode for at least 75 years. Since 1841 the ice-front position has advanced and retreated within a zone from about lat. 77° 10’S. (near long. 171° E.) to lat. 78° 40’ S. (near long. 164° W.). The central ice front is now farthest south but has the highest advance rate. Calving may occur at more frequent intervals in that sector, which also overlies the warmest ocean currents that flow into the sub-ice-shelf cavity. Available information on ice-shelf advance, thickness, spreading rate, and surface accumulation indicates a basal melting rate around 3 m a–1near the ice front. These data and independent estimates imply that basal melting is nearly as large a factor as iceberg calving in maintaining the ice-shelf mass balance. In recent years, the Ross, Ronne, and Filchner Ice Shelves have contributed few icebergs to the Southern Ocean, while projections from a contemporaneous iceberg census are that circumpolar calving alone may exceed accumulation on the ice sheet. Large-scale ice-shelf calving may have preceded historical sightings of increased numbers of icebergs at sea.

scholarly journals Crary Ice Rise, Antarctica: Formed in Response to a Surging Ice Stream? (Abstract)

1988 ◽  
Vol 11 ◽  
pp. 202-202
Author(s):  
D. R. MacAyeal ◽  
R. A. Bindschadler
Keyword(s):  
Stress Resistance ◽  
Field Data ◽  
Back Stress ◽  
Melting Rate ◽  
Current Development ◽  
Ice Shelf ◽  
Ross Ice Shelf ◽  
Ice Stream ◽  
Ice Stream B ◽  
Basal Melting

Field data is presented to support the hypothesis that Crary Ice Rise (on Ross Ice Shelf, Fig. 1) has substantially increased in area over the last 500 years, in response to ice advection through the mouth of Ice Stream B. The up-stream end of the ice rise is now surrounded by ice shelf that is currently thickening at 0.44 0.06 m/year (under an assumed zero basal melting rate). This rate of thickening suggests that the ice rise's contribution to back-stress resistance of Ice Stream B's flow, presently calculated to be 50% of the total back stress, is growing in the course of time. We speculate that this current development of the ice rise is the precursor to the possible future stagnation of Ice Stream B. It is convenient to conceptualize a possible see-saw oscillation between ice-stream surging and ice-rise build-up.

scholarly journals High basal melting forming a channel at the grounding line of Ross Ice Shelf, Antarctica

2016 ◽  
Vol 43 (1) ◽  
pp. 250-255 ◽  
Author(s):  
Oliver J. Marsh ◽  
Helen A. Fricker ◽  
Matthew R. Siegfried ◽  
Knut Christianson ◽  
Keith W. Nicholls ◽  
...  
Keyword(s):  
Ice Shelf ◽  
Ross Ice Shelf ◽  
Grounding Line ◽  
Basal Melting

scholarly journals Tabular Icebergs: Implication from Geophysical Studies of Ice Shelves

1980 ◽  
Vol 1 ◽  
pp. 55-55
Author(s):  
Sion Shabtaie ◽  
Charles R. Bentley
Keyword(s):  
Mass Balance ◽  
Sea Water ◽  
Ice Thickness ◽  
Ice Shelf ◽  
Frictional Drag ◽  
Ice Shelves ◽  
Ross Ice Shelf ◽  
Rift Zones ◽  
Different Characteristics ◽  
Ice Water

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 the elevation above sea-level and total ice thickness for three ice shelves (Ross, Brunt, and McMurdo) are given; from them, expressions for the 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 the thickness of the saline ice at the bottom of icebergs calving from the Ross Ice Shelf is no more than 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 the 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 crevasses could 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 the 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 temperature-depth function, and from that the basal mass-balance rate. With some modifications, the technique may also be applied to estimating the basal mass-balance rates of tabular icebergs.

scholarly journals Tabular Icebergs: Implications from Geophysical Studies of Ice Shelves

1982 ◽  
Vol 28 (100) ◽  
pp. 413-430 ◽  
Author(s):  
Sion Shabtaie ◽  
Charles R. Bentley
Keyword(s):  
Mass Balance ◽  
Sea Water ◽  
Ice Thickness ◽  
Ice Shelf ◽  
Frictional Drag ◽  
Ice Shelves ◽  
Ross Ice Shelf ◽  
Rift Zones ◽  
Different Characteristics ◽  
Ice Water

AbstractRecent 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, ridge/troughs, and other features could substantially modify the hydraulics of iceberg towing and lead to disintegration in the course of transport.The relationships between the elevation above sea-level and total ice thickness for three ice shelves (Ross, Brunt, and McMurdo) are given; from them, expressions for the 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 the thickness of the saline ice at the bottom of icebergs calving from the Ross Ice Shelf is no more than 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 the 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 crevasses could 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 front at four sites in flow bands of very different characteristics are shown. In some places rifting up-stream from the front shows regular spacings, suggesting a periodic calving. Differential bottom melting near the front 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 temperature-depth function, and from that the basal mass-balance rate. With some modifications, the technique may also be applied to estimating the basal mass balance rates of tabular icebergs.

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