Ice Rumples Research Articles

The use of tiltmeters to study the dynamics of Antarctic ice-shelf grounding lines

AbstractNew tiltmeter data are presented from Doake Ice Rumples on Ronne Ice Shelf, Antarctica. Five sites which showed a tidal ice-shelf flexure have been analysed using an elastic beam model to investigate the variation of flexure amplitude with distance from the grounding line. An earlier study on Rutford Ice Stream which also used an elastic model required an ice thickness much less than that observed. Reworking the Rutford Ice Stream data suggests that this greatly reduced ice thickness is not required, given the current sparse data coverage. The elastic model is used to improve the estimated grounding-line position on Rutford Ice Stream. Some of the difficulties in modelling ice-shelf flexure and locating grounding lines are discussed.

The use of tiltmeters to study the dynamics of Antarctic ice-shelf grounding lines

Abstract New tiltmeter data are presented from Doake Ice Rumples on Ronne Ice Shelf, Antarctica. Five sites which showed a tidal ice-shelf flexure have been analysed using an elastic beam model to investigate the variation of flexure amplitude with distance from the grounding line. An earlier study on Rutford Ice Stream which also used an elastic model required an ice thickness much less than that observed. Reworking the Rutford Ice Stream data suggests that this greatly reduced ice thickness is not required, given the current sparse data coverage. The elastic model is used to improve the estimated grounding-line position on Rutford Ice Stream. Some of the difficulties in modelling ice-shelf flexure and locating grounding lines are discussed.

Icequakes On Ekström Ice Shelf Near Atka Bay, Antarctica

AbstractTwo seismic arrays recorded in an 11 month field experiment in 1985 the seismicity of Ekström Ice Shelf in the area of an ice rumple and an inlet, situated respectively about 10 km north-west and 7 km north of the German Antarctic station Georg von Neumayer (lat. 70°37′S., long. 08°22′W). Most of the focal depths of the icequakes considered until now are in the range 5–9 m; the ice-rumple area shows extremely high seismic activity. Tensile fracture is the most frequent fault mechanism, although there are a few shear-fracture events. The ice rumple’s seismicity provides information on the dynamics of the ice shelf in this area. A comparison of this time-dependent seismicity with tides suggests that most of this seismicity is induced by tides. The most active period of this seismicity starts at the beginning of low tide and ends at low tide. The location of the epicentres of icequakes recorded at that time and the digital recording on tapes of the seismicity without interruption for 396 h shows a jerky vertical movement of the ice shelf in response to tides; this can be interpreted as a kind of “grater effect”, especially at the southern ice-rock boundary of the ice rumple.The seismicity in the inlet is much less and tensile fracture seems to be the only fault mechanism.

Icequakes On Ekström Ice Shelf Near Atka Bay, Antarctica

Abstract Two seismic arrays recorded in an 11 month field experiment in 1985 the seismicity of Ekström Ice Shelf in the area of an ice rumple and an inlet, situated respectively about 10 km north-west and 7 km north of the German Antarctic station Georg von Neumayer (lat. 70°37′S., long. 08°22′W). Most of the focal depths of the icequakes considered until now are in the range 5–9 m; the ice-rumple area shows extremely high seismic activity. Tensile fracture is the most frequent fault mechanism, although there are a few shear-fracture events. The ice rumple’s seismicity provides information on the dynamics of the ice shelf in this area. A comparison of this time-dependent seismicity with tides suggests that most of this seismicity is induced by tides. The most active period of this seismicity starts at the beginning of low tide and ends at low tide. The location of the epicentres of icequakes recorded at that time and the digital recording on tapes of the seismicity without interruption for 396 h shows a jerky vertical movement of the ice shelf in response to tides; this can be interpreted as a kind of “grater effect”, especially at the southern ice-rock boundary of the ice rumple. The seismicity in the inlet is much less and tensile fracture seems to be the only fault mechanism.

Ice-Shelf Topography and Structure Determined Using Satellite-Radar Altimetry and Landsat Imagery

The glaciological structure and dynamics of West and Shackleton Ice Shelves, East Antarctica, are qualitatively determined using a combination of satellite remote-sensing techniques. Sketch maps traced from unenhanced imagery show the ice edge, grounding lines, flow lines, and rifts. Surface-elevation profiles and contour maps from radar altimetry provide free-board elevations of the floating ice and show flow-line undulations and rumples characteristic of grounded ice. West and Shackleton Ice Shelves consist of a combination of fast-moving ice tongues from outlet glaciers and slow-moving parts constrained by islands, ice rises, and ice rumples.

Ice-Shelf Topography and Structure Determined Using Satellite-Radar Altimetry and Landsat Imagery

The glaciological structure and dynamics of West and Shackleton Ice Shelves, East Antarctica, are qualitatively determined using a combination of satellite remote-sensing techniques. Sketch maps traced from unenhanced imagery show the ice edge, grounding lines, flow lines, and rifts. Surface-elevation profiles and contour maps from radar altimetry provide free-board elevations of the floating ice and show flow-line undulations and rumples characteristic of grounded ice. West and Shackleton Ice Shelves consist of a combination of fast-moving ice tongues from outlet glaciers and slow-moving parts constrained by islands, ice rises, and ice rumples.

Tiltmeter Observations From Doake Ice Rumples, Ronne Ice Shelf, Antarctica (Abstract)

New tiltmeter data from Doake Ice Rumples on Ronne Ice Shelf are presented. The tiltmeters detected flexing of the ice shelf close to the grounding line, due to tidal forces. In earlier studies on Rutford Ice Stream, flow was from grounded ice into the floating ice shelf. In contrast, the area studied on Ronne Ice Shelf exhibits flow from the ice shelf on to grounded ice rumples. The data span 5 km of the up-stream ice shelf and much of the grounded ice. Approximately 30 d of continuous tilt data are available for one site on the ice shelf. Analysis of this record can determine the dominant tidal frequencies present. A number of shorter records cover periods normally of a few days. These were obtained from sites both up-stream and down-stream of the long-term site. They have been used to investigate the variation in tilt amplitude with distance from the grounding line. The approximate position of the grounding line was located from the position of strand cracks and by using surface-elevation and ice-thickness data. The tiltmeters helped to confirm this position. Over grounded ice, tidal flexing is clearly present up to about 2 km from the grounding line. Beyond this it appears to be absent. The signal recorded by tiltmeters on grounded ice farther than 2 km from the grounding line is attributed, at present, to a temperature dependency of the tiltmeters. The flexing of ice shelves at tidal frequencies has previously been treated as an elastic problem. Available models which use beam theory require the use of a time-dependent function and a reduced “effective” ice thickness in the elastic modulus. On preliminary analysis, data from the ice shelf and grounding line of Doake Ice Rumples appear to be consistent with these theories. More detailed analysis and interpretation is required in order to confirm the level of agreement and to determine possible variations due to the “reversed” nature of the flow direction.

Numerical Models of Steady-State Thickness and Basal Ice Configurations of the Central Ronne Ice Shelf, Antarctica

Radar ice-thickness surveys and bore-hole measurements suggest that the central part of Ronne Ice Shelf possesses a lobe-shaped basal layer of undetermined nature (probably saline ice). This layer is characterized by high radio-wave absorbtivity and by thicknesses up to approximately 300 m. We reconstruct this basal layer and the associated ice-shelf thickness and flow distributions, using a time-dependent ice-shelf model forced with prescribed basal freezing rates. Characteristics of the basal layer are controlled by two factors: (i) long ice-column residence times in the unventilated pocket between Henry and Korff ice rises and Doake Ice Rumples, and (ii) basal freezing rates in this pocket that exceed the snow-accumulation rate (currently averaging 0.35 m/a ice equivalent across the ice shelf).

A Glaciological Map of Filchner-Ronne Ice Shelf, Antarctica

A set of Landsat-5 multispectral scanner (MSS) images of Filchner-Ronne Ice Shelf was commissioned by the Institut für Angewandte Geodäsie and recorded over a short period in early 1986. A mosaic of digitally enhanced images was constructed, using all available ground control. In due course it is planned to use the data for the production of a series of digital-image maps at a scale of 1:1 000 000. Meanwhile a line map has been prepared at a scale of 1 :2 000 000 to show the principal glaciological features. The new mosaic shows much more detail than earlier Landsat images and extends coverage to higher latitudes. More of the poorly known southern boundary of the ice shelf can be identified but floating ice still extends beyond the limit of coverage. Extensive ice rumples are revealed, many of them undetected in earlier imagery. Flow bands can be tracked without a break from their ice-stream source to the ice front, over distances of up to 800 km. Even minor tributaries, and sheet flow from the inland ice sheet between the ice streams, yield narrow but identifiable flow bands. Ice thickness, velocity, and mass-flux data are reviewed, in order to underline their dependence on good mapping.

Numerical Models of Steady-State Thickness and Basal Ice Configurations of the Central Ronne Ice Shelf, Antarctica

Radar ice-thickness surveys and bore-hole measurements suggest that the central part of Ronne Ice Shelf possesses a lobe-shaped basal layer of undetermined nature (probably saline ice). This layer is characterized by high radio-wave absorbtivity and by thicknesses up to approximately 300 m. We reconstruct this basal layer and the associated ice-shelf thickness and flow distributions, using a time-dependent ice-shelf model forced with prescribed basal freezing rates. Characteristics of the basal layer are controlled by two factors: (i) long ice-column residence times in the unventilated pocket between Henry and Korff ice rises and Doake Ice Rumples, and (ii) basal freezing rates in this pocket that exceed the snow-accumulation rate (currently averaging 0.35 m/a ice equivalent across the ice shelf).

A Glaciological Map of Filchner-Ronne Ice Shelf, Antarctica

A set of Landsat-5 multispectral scanner (MSS) images of Filchner-Ronne Ice Shelf was commissioned by the Institut für Angewandte Geodäsie and recorded over a short period in early 1986. A mosaic of digitally enhanced images was constructed, using all available ground control. In due course it is planned to use the data for the production of a series of digital-image maps at a scale of 1:1 000 000. Meanwhile a line map has been prepared at a scale of 1 :2 000 000 to show the principal glaciological features. The new mosaic shows much more detail than earlier Landsat images and extends coverage to higher latitudes. More of the poorly known southern boundary of the ice shelf can be identified but floating ice still extends beyond the limit of coverage. Extensive ice rumples are revealed, many of them undetected in earlier imagery. Flow bands can be tracked without a break from their ice-stream source to the ice front, over distances of up to 800 km. Even minor tributaries, and sheet flow from the inland ice sheet between the ice streams, yield narrow but identifiable flow bands. Ice thickness, velocity, and mass-flux data are reviewed, in order to underline their dependence on good mapping.

Tiltmeter Observations From Doake Ice Rumples, Ronne Ice Shelf, Antarctica (Abstract)

New tiltmeter data from Doake Ice Rumples on Ronne Ice Shelf are presented. The tiltmeters detected flexing of the ice shelf close to the grounding line, due to tidal forces. In earlier studies on Rutford Ice Stream, flow was from grounded ice into the floating ice shelf. In contrast, the area studied on Ronne Ice Shelf exhibits flow from the ice shelf on to grounded ice rumples.The data span 5 km of the up-stream ice shelf and much of the grounded ice. Approximately 30 d of continuous tilt data are available for one site on the ice shelf. Analysis of this record can determine the dominant tidal frequencies present.A number of shorter records cover periods normally of a few days. These were obtained from sites both up-stream and down-stream of the long-term site. They have been used to investigate the variation in tilt amplitude with distance from the grounding line.The approximate position of the grounding line was located from the position of strand cracks and by using surface-elevation and ice-thickness data. The tiltmeters helped to confirm this position. Over grounded ice, tidal flexing is clearly present up to about 2 km from the grounding line. Beyond this it appears to be absent. The signal recorded by tiltmeters on grounded ice farther than 2 km from the grounding line is attributed, at present, to a temperature dependency of the tiltmeters.The flexing of ice shelves at tidal frequencies has previously been treated as an elastic problem. Available models which use beam theory require the use of a time-dependent function and a reduced “effective” ice thickness in the elastic modulus. On preliminary analysis, data from the ice shelf and grounding line of Doake Ice Rumples appear to be consistent with these theories. More detailed analysis and interpretation is required in order to confirm the level of agreement and to determine possible variations due to the “reversed” nature of the flow direction.

Multi-Spectral Landsat Image Maps of Antarctica (Abstract)

The US Geological Survey is conducting a program to prepare digitally enhanced Multispectral Scanner (MSS) Landsat images of Antarctica. The goal is to furnish accurate planimetric, false-colour composite image maps in Lambert conformal conic projection for the following purposes: (1) to locate and delineate blue-ice areas for the collection of meteorites; (2) to produce special purpose maps showing selected features; (3) to provide synoptic views that aid in the detection and interpretation of glaciological features associated with the inland ice sheet, outlet glaciers, ice streams, and ice shelves; (4) to monitor changes in coastline and glacial features; (5) to enable the superposition and correlation of different types of digital cartographic data; and (6) to furnish spectral and (or) structural information in areas of limited bedrock outcrop to aid in regional geologic interpretation. Only the first four of these objectives are addressed here.About 170 Landsat computer-compatible tapes covering Victoria Land, the coastline of West Antarctica, the Antarctic Peninsula, and other selected areas were assembled into false-color, multi-spectral, digital composites of band 4 (0.5 to 0.6 μm, green), band 5 (0.6 to 0.7 μm, red) and band 7 (0.8 to 1.1 μm, near infra-red). The tapes were subjected to routine image-processing procedures, such as noise removal and radiometric and geometric corrections. Further processing included haze removal and enhancement by linear stretching of individual MSS bands based on inspection of gray-value (digital-number) histograms. Saturation of snow-covered scenes in bands 4, 5, and 6 is a severe problem in Landsat MSS images of Antarctica and makes many images unsuitable for multi-spectral work. We have developed special techniques to restore the saturated snow-and-ice information in these bands to overcome this problem.The Landsat image maps have different formats, depending on their planned applications. An example of a planimetric image map is the one of the McMurdo Sound area; it is based on excellent ground control and processed at full spatial resolution. It comprises five complete and three partial 1 : 250 000 scale topographic quadrangles. One of these is the Convoy Range quadrangle which includes the Allan Hills meteorite-collection site. Blue-ice areas show exceptionally well on this quadrangle, and new information on blue-ice locations and delineations was obtained from it.Thematic maps prepared for the Byrd Glacier area selectively show only rock or ice areas, thus depicting the location of desired features. Synoptic-view maps and mosaics provide information on flow lines associated with ice streams, the location of ice rises, ice rumples, and other possible grounded areas, and the location and extent of buried mountain ranges. Image maps which cover the same area at different times do show changes: a 10-year interval between Landsat images obtained in the Byrd Glacier area shows that crevasses had opened and rifts had drifted within the floating part of the glacier; measurements of the changed positions have yielded average velocities for glacier and ice-shelf movements in that area. The ease with which the dynamics of the coastline can be monitored on Landsat images is particularly useful, because such changes have implications for variations in world climate.

Multi-Spectral Landsat Image Maps of Antarctica (Abstract)

The US Geological Survey is conducting a program to prepare digitally enhanced Multispectral Scanner (MSS) Landsat images of Antarctica. The goal is to furnish accurate planimetric, false-colour composite image maps in Lambert conformal conic projection for the following purposes: (1) to locate and delineate blue-ice areas for the collection of meteorites; (2) to produce special purpose maps showing selected features; (3) to provide synoptic views that aid in the detection and interpretation of glaciological features associated with the inland ice sheet, outlet glaciers, ice streams, and ice shelves; (4) to monitor changes in coastline and glacial features; (5) to enable the superposition and correlation of different types of digital cartographic data; and (6) to furnish spectral and (or) structural information in areas of limited bedrock outcrop to aid in regional geologic interpretation. Only the first four of these objectives are addressed here. About 170 Landsat computer-compatible tapes covering Victoria Land, the coastline of West Antarctica, the Antarctic Peninsula, and other selected areas were assembled into false-color, multi-spectral, digital composites of band 4 (0.5 to 0.6 μm, green), band 5 (0.6 to 0.7 μm, red) and band 7 (0.8 to 1.1 μm, near infra-red). The tapes were subjected to routine image-processing procedures, such as noise removal and radiometric and geometric corrections. Further processing included haze removal and enhancement by linear stretching of individual MSS bands based on inspection of gray-value (digital-number) histograms. Saturation of snow-covered scenes in bands 4, 5, and 6 is a severe problem in Landsat MSS images of Antarctica and makes many images unsuitable for multi-spectral work. We have developed special techniques to restore the saturated snow-and-ice information in these bands to overcome this problem. The Landsat image maps have different formats, depending on their planned applications. An example of a planimetric image map is the one of the McMurdo Sound area; it is based on excellent ground control and processed at full spatial resolution. It comprises five complete and three partial 1 : 250 000 scale topographic quadrangles. One of these is the Convoy Range quadrangle which includes the Allan Hills meteorite-collection site. Blue-ice areas show exceptionally well on this quadrangle, and new information on blue-ice locations and delineations was obtained from it. Thematic maps prepared for the Byrd Glacier area selectively show only rock or ice areas, thus depicting the location of desired features. Synoptic-view maps and mosaics provide information on flow lines associated with ice streams, the location of ice rises, ice rumples, and other possible grounded areas, and the location and extent of buried mountain ranges. Image maps which cover the same area at different times do show changes: a 10-year interval between Landsat images obtained in the Byrd Glacier area shows that crevasses had opened and rifts had drifted within the floating part of the glacier; measurements of the changed positions have yielded average velocities for glacier and ice-shelf movements in that area. The ease with which the dynamics of the coastline can be monitored on Landsat images is particularly useful, because such changes have implications for variations in world climate.

Radio-Echo Investigations of Ronne Ice Shelf

Since 1981, the British Antarctic Survey has flown 27 000 line kilometres over Ronne Ice Shelf, during which radio-echo ice thicknesses were recorded. An earlier map of ice thickness was compiled from about 5000 km of flight lines flown by three different organisations over a period of 12 years A new, detailed map of the thickness of Ronne Ice Shelf has been produced, which agrees with the earlier version, but shows more structure in the north-western part. In particular, major ice streams maintain their idenity from sources such as Evans and Rutford ice streams all the way to the ice front. Regions where the ice shelf is locally grounded over Kershaw Ice Rumples and over more extensive ice rumples between Korff and Henry ice rises play a significant role in controlling the dynamics of the ice shelf. There is a possibility of extensive freezing-on of sea water under the thin, central section of Ronne Ice Shelf, although the radio-echo evidence for a thick layer of saline ice is ambiguous.

Multispectral Digital Image Mapping of Antarctic Ice Features

Landsat multispectral images of the Antarctic ice sheet have been digitally enhanced by the US Geological Survey to show ice surface features not seen in earlier photographic products of the same scenes. Now for the first time it is worthwhile to prepare image maps at scales of up to 1:250 000 of ice sheet areas even where no nunataks are visible. Derivatives of the data can be stretched to bring out glaciologically significant features in smooth areas that traditionally have been described as featureless. Over large tracts of the ice sheet, the direction of ice flow can be revealed as clearly as it is by the medial moraines of an Alpine glacier system. Ice streams, ice divides, ice rises, ice rumples, grounding lines, crevasses, and rifts are seen where none had been identified before. In the same way that Seasat altimetry of the surface of the ocean has much to tell about the bed of the ocean, Landsat has much to tel! about the bed of the ice. Not only major structural features but also many details of the sub-glacial landscape are unmasked by their subtle reflection on the ice sheet surface. Ground control on ice sheets can be obtained by Doppler satellite observations tied to image-identifiable surface features. Because of ice movement, the standard of geodetic control can never approach that of conventional surveys based on rock stations. But the precise standards of conventional surveys are unnecessary for ice sheet maps.

Multispectral Digital Image Mapping of Antarctic Ice Features

Landsat multispectral images of the Antarctic ice sheet have been digitally enhanced by the US Geological Survey to show ice surface features not seen in earlier photographic products of the same scenes. Now for the first time it is worthwhile to prepare image maps at scales of up to 1:250 000 of ice sheet areas even where no nunataks are visible. Derivatives of the data can be stretched to bring out glaciologically significant features in smooth areas that traditionally have been described as featureless. Over large tracts of the ice sheet, the direction of ice flow can be revealed as clearly as it is by the medial moraines of an Alpine glacier system. Ice streams, ice divides, ice rises, ice rumples, grounding lines, crevasses, and rifts are seen where none had been identified before. In the same way that Seasat altimetry of the surface of the ocean has much to tell about the bed of the ocean, Landsat has much to tel! about the bed of the ice. Not only major structural features but also many details of the sub-glacial landscape are unmasked by their subtle reflection on the ice sheet surface. Ground control on ice sheets can be obtained by Doppler satellite observations tied to image-identifiable surface features. Because of ice movement, the standard of geodetic control can never approach that of conventional surveys based on rock stations. But the precise standards of conventional surveys are unnecessary for ice sheet maps.

Radio-Echo Investigations of Ronne Ice Shelf

Since 1981, the British Antarctic Survey has flown 27 000 line kilometres over Ronne Ice Shelf, during which radio-echo ice thicknesses were recorded. An earlier map of ice thickness was compiled from about 5000 km of flight lines flown by three different organisations over a period of 12 years A new, detailed map of the thickness of Ronne Ice Shelf has been produced, which agrees with the earlier version, but shows more structure in the north-western part. In particular, major ice streams maintain their idenity from sources such as Evans and Rutford ice streams all the way to the ice front. Regions where the ice shelf is locally grounded over Kershaw Ice Rumples and over more extensive ice rumples between Korff and Henry ice rises play a significant role in controlling the dynamics of the ice shelf. There is a possibility of extensive freezing-on of sea water under the thin, central section of Ronne Ice Shelf, although the radio-echo evidence for a thick layer of saline ice is ambiguous.