Tabular Icebergs: Implications 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 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.

Download Full-text

Tabular Icebergs: Implications from Geophysical Studies of Ice Shelves

Journal of Glaciology ◽

10.1017/s0022143000005037 ◽

1982 ◽

Vol 28 (100) ◽

pp. 413-430 ◽

Cited By ~ 45

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.

Download Full-text

Characteristics of ice rises and ice rumples in Dronning Maud Land and Enderby Land, Antarctica

Journal of Glaciology ◽

10.1017/jog.2020.77 ◽

2020 ◽

Vol 66 (260) ◽

pp. 1064-1078

Author(s):

Vikram Goel ◽

Kenichi Matsuoka ◽

Cesar Deschamps Berger ◽

Ian Lee ◽

Jørgen Dall ◽

...

Keyword(s):

Mass Balance ◽

Ice Cores ◽

Ice Thickness ◽

Ice Shelf ◽

Ice Dynamics ◽

Surface Mass ◽

Ice Shelves ◽

The Past ◽

Dronning Maud Land ◽

The Antarctic

AbstractIce rises and rumples, locally grounded features adjacent to ice shelves, are relatively small yet play significant roles in Antarctic ice dynamics. Their roles generally depend upon their location within the ice shelf and the stage of the ice-sheet retreat or advance. Large, long-stable ice rises can be excellent sites for deep ice coring and paleoclimate study of the Antarctic coast and the Southern Ocean, while small ice rises tend to respond more promptly and can be used to reveal recent changes in regional mass balance. The coasts of Dronning Maud Land (DML) and Enderby Land in East Antarctica are abundant with these features. Here we review existing knowledge, presenting an up-to-date status of research in these regions with focus on ice rises and rumples. We use regional datasets (satellite imagery, surface mass balance and ice thickness) to analyze the extent and surface morphology of ice shelves and characteristic timescales of ice rises. We find that large parts of DML have been changing over the past several millennia. Based on our findings, we highlight ice rises suitable for drilling ice cores for paleoclimate studies as well as ice rises suitable for deciphering ice dynamics and evolution in the region.

Download Full-text

Ice-thickness Patterns and the Dynamics of the Ross Ice Shelf, Antarctica

Journal of Glaciology ◽

10.1017/s0022143000014805 ◽

1979 ◽

Vol 24 (90) ◽

pp. 287-294 ◽

Cited By ~ 81

Author(s):

Charles R. Bentley ◽

John W. Glough ◽

Kenneth C. Jezek ◽

Sion Shabtaie

Keyword(s):

Large Scale ◽

Rapid Change ◽

Thickness Variation ◽

Ice Thickness ◽

Ice Shelf ◽

Mountain Glacier ◽

Ross Ice Shelf ◽

Scale Turbulence ◽

Thickness Variations ◽

Ice Water

AbstractAs 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.

Download Full-text

Constraining variable density of ice shelves using wide-angle radar measurements

The Cryosphere ◽

10.5194/tc-10-811-2016 ◽

2016 ◽

Vol 10 (2) ◽

pp. 811-823 ◽

Cited By ~ 9

Author(s):

Reinhard Drews ◽

Joel Brown ◽

Kenichi Matsuoka ◽

Emmanuel Witrant ◽

Morgane Philippe ◽

...

Keyword(s):

Mass Balance ◽

Propagation Speed ◽

Radar Data ◽

Temporal Variations ◽

Variable Density ◽

Ice Thickness ◽

Wide Angle ◽

Ice Shelf ◽

Surface Mass ◽

Ice Shelves

Abstract. The thickness of ice shelves, a basic parameter for mass balance estimates, is typically inferred using hydrostatic equilibrium, for which knowledge of the depth-averaged density is essential. The densification from snow to ice depends on a number of local factors (e.g., temperature and surface mass balance) causing spatial and temporal variations in density–depth profiles. However, direct measurements of firn density are sparse, requiring substantial logistical effort. Here, we infer density from radio-wave propagation speed using ground-based wide-angle radar data sets (10 MHz) collected at five sites on Roi Baudouin Ice Shelf (RBIS), Dronning Maud Land, Antarctica. We reconstruct depth to internal reflectors, local ice thickness, and firn-air content using a novel algorithm that includes traveltime inversion and ray tracing with a prescribed shape of the depth–density relationship. For the particular case of an ice-shelf channel, where ice thickness and surface slope change substantially over a few kilometers, the radar data suggest that firn inside the channel is about 5 % denser than outside the channel. Although this density difference is at the detection limit of the radar, it is consistent with a similar density anomaly reconstructed from optical televiewing, which reveals that the firn inside the channel is 4.7 % denser than that outside the channel. Hydrostatic ice thickness calculations used for determining basal melt rates should account for the denser firn in ice-shelf channels. The radar method presented here is robust and can easily be adapted to different radar frequencies and data-acquisition geometries.

Download Full-text

Heating and Melting of Floating Ice Shelves

Journal of Glaciology ◽

10.1017/s0022143000023741 ◽

1960 ◽

Vol 3 (27) ◽

pp. 626-645 ◽

Cited By ~ 49

Author(s):

H. Wexler

Keyword(s):

Steady State ◽

Sea Water ◽

Freezing Point ◽

Atmospheric Environment ◽

Constant Thickness ◽

Cold Region ◽

Ice Shelf ◽

Ice Shelves ◽

Ross Ice Shelf ◽

Steady State Solutions

Abstract Based on an observed temperature profile through the Ross Ice Shelf at Little America and partial profiles in the Maudheim Is-shelf and the Filchner Ice Shelf near the Ellsworth I.G.Y. station, various models of heating with and without melting from below are analysed to find the residence time of the respective ice shelves over the ocean. Estimated movements are compared with observed shelf movements seaward. 100 and 200 yr. melting rates for an ice shelf initially 20.5° C. below the freezing point of seawater are found as functions of the deviation of sea-water from its freezing point and the eddy conductivity of the ocean below the ice shelf. Steady-state solutions based on constant accumulation and sinking in an ice shelf of constant thickness are discussed. The effect of heating of an ice shelf from above and below as it moves to a warmer atmospheric environment is described and it is concluded that the decreasing temperature with depth found only in the Ellsworth ice is a result of its rapid motion from the cold region to the south-east of the station (Coats Land).

Download Full-text

Estimates of the refreezing rate in an ice-shelf borehole

Journal of Glaciology ◽

10.3189/2013jog12j117 ◽

2013 ◽

Vol 59 (217) ◽

pp. 938-948 ◽

Cited By ~ 6

Author(s):

Kenneth G. Hughes ◽

Pat J. Langhorne ◽

Michael J.M. Williams

Keyword(s):

Sea Water ◽

Salt Water ◽

Heat Fluxes ◽

Water Interface ◽

Ice Shelf ◽

Ross Ice Shelf ◽

Flux Model ◽

Thick Region ◽

Ice Water ◽

Heat Flux Model

AbstractThe refreezing rate of a borehole drilled through a 252 m thick region of the Ross Ice Shelf, Antarctica, is determined using oceanographic measurements over two periods of a day. We first use a method based on the conservation of salt in the supercooled salt water of the borehole. This is compared to a model using a numerical solution of the heat equation to find the temperature distribution in the host ice, allowing ice growth to be calculated from the balance of heat fluxes at the ice/water interface. This second method broadly confirms the refreezing rates deduced from salinity measurements, giving confidence in the generalization of this simple heat-flux model to predict refreezing rates of other boreholes. Predictions from both are subject to uncertainty due to the poorly defined value of the solid fraction of ice that freezes in a supercooled volume of sea water. This is taken to be 0.5 ± 0.1 throughout this study. The predicted rates are also strongly dependent on the initial and boundary conditions chosen, but results show the initial diameter of 600 mm decreases at a rate of ∼3–5 mm h−1 in an ice shelf with a minimum temperature of −22°C.

Download Full-text

Ice-thickness Patterns and the Dynamics of the Ross Ice Shelf, Antarctica

Journal of Glaciology ◽

10.3189/s0022143000014805 ◽

1979 ◽

Vol 24 (90) ◽

pp. 287-294 ◽

Cited By ~ 2

Author(s):

Charles R. Bentley ◽

John W. Glough ◽

Kenneth C. Jezek ◽

Sion Shabtaie

Keyword(s):

Large Scale ◽

Rapid Change ◽

Thickness Variation ◽

Ice Thickness ◽

Ice Shelf ◽

Mountain Glacier ◽

Ross Ice Shelf ◽

Scale Turbulence ◽

Thickness Variations ◽

Ice Water

AbstractAs 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.

Download Full-text

Investigation of Bottom Mass-Balance Rates by Electrical Resistivity Soundings on the Ross Ice Shelf, Antarctica

Journal of Glaciology ◽

10.1017/s0022143000014854 ◽

1979 ◽

Vol 24 (90) ◽

pp. 331-343 ◽

Cited By ~ 21

Author(s):

Sion Shabtaie ◽

Charles R. Bentley

Keyword(s):

Activation Energy ◽

Electrical Resistivity ◽

Steady State ◽

Mass Balance ◽

Apparent Resistivity ◽

Ice Thickness ◽

Seasonal Temperature ◽

Ice Shelf ◽

Near Surface ◽

Ross Ice Shelf

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.

Download Full-text

Heating and Melting of Floating Ice Shelves

Journal of Glaciology ◽

10.3189/s0022143000023741 ◽

1960 ◽

Vol 3 (27) ◽

pp. 626-645 ◽

Cited By ~ 1

Author(s):

H. Wexler

Keyword(s):

Steady State ◽

Sea Water ◽

Freezing Point ◽

Atmospheric Environment ◽

Constant Thickness ◽

Cold Region ◽

Ice Shelf ◽

Ice Shelves ◽

Ross Ice Shelf ◽

Steady State Solutions

AbstractBased on an observed temperature profile through the Ross Ice Shelf at Little America and partial profiles in the Maudheim Is-shelf and the Filchner Ice Shelf near the Ellsworth I.G.Y. station, various models of heating with and without melting from below are analysed to find the residence time of the respective ice shelves over the ocean. Estimated movements are compared with observed shelf movements seaward. 100 and 200 yr. melting rates for an ice shelf initially 20.5° C. below the freezing point of seawater are found as functions of the deviation of sea-water from its freezing point and the eddy conductivity of the ocean below the ice shelf. Steady-state solutions based on constant accumulation and sinking in an ice shelf of constant thickness are discussed.The effect of heating of an ice shelf from above and below as it moves to a warmer atmospheric environment is described and it is concluded that the decreasing temperature with depth found only in the Ellsworth ice is a result of its rapid motion from the cold region to the south-east of the station (Coats Land).

Download Full-text