The recent rapid growth of rifts in the Brunt Ice Shelf appears to
signal the onset of its largest calving event since records began in 1915.
The aim of this study is to determine whether this calving event will lead to
a new steady state in which the Brunt Ice Shelf remains in contact with the bed,
or an unpinning from the bed, which could predispose it to accelerated flow
or possible break-up. We use a range of geophysical data to reconstruct the
sea-floor bathymetry and ice shelf geometry, to examine past ice sheet
configurations in the Brunt Basin, and to define the present-day geometry of
the contact between the Brunt Ice Shelf and the bed. Results show that during
past ice advances grounded ice streams likely converged in the Brunt Basin
from the south and east. As the ice retreated, it was likely pinned on at
least three former grounding lines marked by topographic highs, and
transverse ridges on the flanks of the basin. These may have subsequently
formed pinning points for developing ice shelves. The ice shelf geometry and
bathymetry measurements show that the base of the Brunt Ice Shelf now only
makes contact with one of these topographic highs. This contact is limited to
an area of less than 1.3 to 3 km
Compilations of marine geophysical data have shown that the Coats Land ice shelves bordering the Weddell Sea in East Antarctica have historically retreated towards the grounding line once detached from pinning points on the seabed (Hodgson et al., 2018).
The Brunt Ice Shelf (BIS) is the only large ice shelf that remains intact in
Coats Land south of 74
Study location map over Landsat 8 satellite images.
The BIS is bounded to the north by the Stancomb-Wills Glacier Tongue (SWGT,
Fig. 1a) and to the southwest by the Dawson-Lambton Glacier Tongue (DLGT;
Figs. 1a, 2b). Together they form a continuous floating ice mass of
approximately 33 000 km
The historical records show cyclical changes of both the SWGT and the DLGT over the last 100 years. These have included an advance, then substantial calving of the SWGT sometime between 1915 and 1955 (Thomas, 1973), and the formation and loss of several > 10 km long ice tongues formed by the Dawson-Lambton Ice Stream since 1958 (Admiralty Charts, British Antarctic Survey Archives). In both cases, these glacier tongues have been of insufficient extent or thickness to re-establish contact with the topographic highs mapped at the distal ends of their glacial troughs (Hodgson et al., 2018). This failure to reconnect with the bed means that they remain un-buttressed and have been described as “failed ice shelves” (Hodgson et al., 2018) (Fig. 2b).
In contrast, the BIS has only experienced relatively small episodic calving
events along its ice front since 1915 (Anderson et al., 2014). This
relative stability has been the result of the base of the ice shelf
maintaining contact with a topographic high on a ridge of glacial sediments
(Seismic profile 5; Elverhøi and Maisey, 1983) known as the
McDonald Bank (75
Images of key ice shelf features. Approximate locations of a–c in
Fig. 1.
Although in contact with the bed at the MIR, the BIS has experienced
substantial changes in its velocity during the instrumental period, including
periods of fast flow between the late 1970s and 2000 and from 2012 to the present.
In the late 1970s, the acceleration in ice shelf velocities from 400 to
> 700 m a
Recent GPS measurements of ice velocity have revealed an ongoing and
spatially heterogeneous acceleration in the flow of the BIS since 2012
(Gudmundsson et al., 2016). The rapid propagation of a new rift
immediately upstream of the MIR in October 2016 (Fig. 1b) has led to further
changes in the velocity of the ice shelf. Known as the “Halloween Crack”
this feature now extends to the northeast, partly decoupling the BIS from
the SWGT (De Rydt et al., 2018). At the same time, an
existing rift in the ice shelf, known as “Chasm 1” has been widening from a
few centimetres per day at the tip to > 20 cm day
The first aim of this study was to describe past changes in ice sheet configurations in the Brunt Basin based on geomorphological interpretations of the bathymetry and subglacial topography. The second was to combine these data with surveys of ice shelf geometry to examine the present-day contact between the BIS and the McDonald Bank, to evaluate a range of future ice shelf configurations following the calving event including (1) a new steady state of the BIS in which it remains in contact with McDonald Bank; (2) a loss of contact followed by accelerated ice shelf flow and re-grounding; (3) the formation of an unpinned, but structurally viable, glacier tongue resulting from (temporary, or longer-term) loss of contact with the McDonald Bank; or (4) a catastrophic break-up resulting from the removal of buttressing coupled with a loss of structural integrity. These four different outcomes depend initially on the direction of propagation of Chasm 1, more precisely, whether Chasm 1 propagates to a point at or downstream of the MIR, thereby maintaining contact between the ice shelf and the bed, or joins the Halloween Crack upstream of the MIR, potentially unpinning the BIS from the McDonald Bank. Model predictions based on the present-day stress distribution of the ice shelf suggest a future pathway that leads directly to the centre of the MIR (De Rydt et al., 2018).
Critical to differentiating between these outcomes is determining the nature and geometry of the contact between the sea floor and the base of the ice shelf. Here we use a range of geophysical data to build an understanding of the regional sea-floor bathymetry and subglacial topography, including under the floating ice of the BIS and SWGT and their grounded ice sheet catchments. We combine this with measurements of current ice shelf geometry. This includes digital elevation models (DEMs) to define the area of the MIR and derive estimates of the ice shelf draft, focusing on ice shelf flow lines upstream of the MIR. These provide the basis for an analysis of the current interaction between the ice shelf and the bed, and of the future development of the ice shelf following the calving event.
In open water areas, the seabed bathymetry was derived from compilations of ship-based multibeam and single-beam bathymetric data (presented in Hodgson et al., 2018) combined with the International Bathymetric Chart of the Southern Ocean (IBSCO; Arndt et al., 2013).
In inaccessible areas that are covered by the ice shelf, the seabed bathymetry was derived from bathymetric measurements from historical ship tracks inland of the present ice front, which is currently at its most advanced position since 1915 (Anderson et al., 2014). Further bathymetric control was provided by 38 seismic data points acquired from the surface of the ice shelf described in Hodgson et al. (2018). These were combined with new estimates of bathymetry from gravity and magnetic data from aerogeophysical surveys in 2017 (Fig. 3). Where the ice sheet is grounded, airborne radio echo-sounding depth data from BEDMAP2 (Fretwell et al., 2013) and the 2017 aerogeophysical survey were used. The revised directly measured bathymetric and topographic dataset used in this study is available online (Hodgson et al., 2019).
Data locations overlain on MODIS Mosaic of Antarctica 2008–2009 (MOA2009) satellite image (Haran et al., 2014). Yellow lines indicate 2017 aerogeophysical flights collecting radar, gravity, and magnetic data. Red shading indicates swath bathymetry data coverage. Blue lines mark radar depth determinations from BEDMAP2 (Fretwell et al., 2013). Orange lines mark the location of ICEGRAV 2013 radar and gravity data (Forsberg et al., 2017). Black dots mark seismic determinations of water column thickness under the Brunt Ice Shelf (Hodgson et al., 2018). The white dots mark soundings from historical ship tracks acquired when the ice front of the Stancomb-Wills Glacier Tongue was less-advanced. The white line marks the grounding line/ice shelf edge from the 2013 Antarctic Digital Database. The red square marks the approximate location of Halley VIa Station.
Inversion of gravity data reveals the sub-ice shelf bathymetry based on the
large density contrast at the water–rock interface (Cochran and Bell,
2012). However, shallow geological factors such as sedimentary basins or
dense intrusions can give rise to gravity anomalies with the same amplitude
and wavelength as the bathymetry, making direct inversion challenging
(Brisbourne et al., 2014). By integrating gravity and
aeromagnetic data to constrain the subsurface geology, we have developed a
procedure to provide the most reasonable estimate of the sub-ice shelf
bathymetry in otherwise un-surveyed areas. Gravity data are from a
“strapdown” type of sensor which provides a resolution of
Revised topography beneath the Brunt Ice Shelf and Stancomb-Wills
Glacier Tongue.
Estimation of the sub-ice shelf bathymetry from the gravity data used a
four-step procedure (for details see Supplement Sect. S1). First, to initiate the estimation we
used interpolated grids of ice surface, sub-surface topography, and bathymetry
from direct observations (Figs. 3 and 4a). The gravity effect of these
surfaces was calculated and subtracted from the compiled free-air gravity
anomaly (Fig. 4b) to give an estimated Bouguer anomaly. The second stage was
to isolate the gravity signatures in the Bouguer anomaly due to bathymetry
not described by direct observation. A low-pass (150 km) filter isolated
signatures due to crustal thickness variations. A
Uncertainties arising from unknown and unmodelled geology are hard to
quantify, as step 4 of the estimation procedure means the model always
matches the direct bathymetric observations. One estimate of the errors due
to geological factors can be made by looking at the difference between the
initial gravity estimation of topography (step 3) and the direct
observations. This reveals a symmetrical error distribution with
In regions where both direct topographic–bathymetric observations and high-resolution gravity data are available (Fig. 4a and b respectively) major
topographic structures, including the deep onshore basin and the trough
beneath the BIS, are resolved as significant lows in the gravity data. This
supports the use of gravity data to fill the intervening areas where no
direct bathymetric measurements are available. Aeromagnetic data across the
study area (Sect. S2) show a clear high-frequency signal
beneath the main survey area. This suggests the geological basement is close
to the surface, and a major thick sedimentary basin which could distort the
results is not present. In addition, no clear
The surface topography of the ice shelf was measured using a high-resolution
surface DEM derived from WorldView stereo imagery
(De Rydt et al., 2018). A total of seven individual WorldView
tiles with a horizontal resolution of 3 m and varying timestamps (20 October 2012
to 30 March 2014) were collated using (1) a surface velocity field from June 2015 (De Rydt et al., 2018) to shift individual
tiles to a common datum, (2) ground control points to fix the floating vertical coordinate, and (3) tidal
corrections to correct vertical offsets. The data were subsampled onto a 30 m
The subglacial topography and bathymetry show that the grounded ice occupies a complex bedrock terrain (Fig. 4c). There is a 1900 m deep trough beneath the Stancomb-Wills Glacier with subglacial catchments to the north, northeast, and south, each fed by multiple tributary valleys. In contrast, the terrain beneath the glaciers that discharge into the BIS consists of a series of small northwest-oriented troughs that are on a bedrock surface, which generally lies < 200 m below sea level.
A steep coastal slope marks the transition between grounded and floating ice
masses (the black and white contour in Fig. 4c is the predicted grounding
line). Downstream of this grounding line, the trough originating under the
BIS is oriented south to north, whilst the trough originating beneath the
SWGT is oriented east to west (orientations indicated by white arrows, Fig. 4c). Both troughs reach depths of 600–1200 m and merge into the Brunt Basin
forming a single north-northwest-oriented 400–800 m deep basin that extends
> 120 km into the southern Weddell Sea at 74
A number of topographic highs occur in the Brunt Basin. Two of these are high enough to make contact with floating ice at the base of the SWGT around the Lyddan Ice Rise and at the base of the BIS forming the McDonald Ice Rumples (Fig. 1). Other topographic highs are present in the NNW-oriented part of the Brunt Basin under the SWGT, including a series of at least three transverse ridges on the flanks of the trough (marked as inferred grounding lines 1–3 in Fig. 4c). Our gravity-derived topography predicts these ridges are in contact with the base of the SWGT, which is not indicated by ice velocity or satellite imagery. Instead, we suggest that these ridges fall just short of the base of the ice. Two similar transverse ridges are seen at the present-day grounding line in the east–west-oriented part of the trough beneath the SWGT (marked as inferred grounding line 4 in Fig. 4c). We interpret these topographic highs as likely contact points, perhaps for both grounded ice retreating from its maximum extent (grounding line positions) and stabilisation points for subsequently floating ice (pinning points).
The McDonald Bank is well resolved, particularly off shore by the swath bathymetry and single-beam surveys (Fig. 4d). The east face of the McDonald Bank rises steeply from the Brunt Basin (Figs. 4d and 6). The upper surface of the bank is relatively flat but has a number of smaller-scale topographic highs, reaching a minimum depth of ca. 212 m below sea level. Some of these appear to be crescent-shaped (indicated in green, Fig. 4d).
The satellite images (Fig. 1) and DEM (Fig. 5) show the heterogeneous nature
of the ice shelf. The main distinction is between those parts of the ice
shelf supplied by higher-velocity glaciers and ice streams and those
supplied by the low-velocity ice of the inland ice sheet. The former supply
the ice shelf with closely packed bands of incorporated icebergs, and the
latter with icebergs that are more widely spaced. These icebergs are
typically oriented with their long axis at 90
The draft of the BIS was examined in detail along three flow lines (northern, central, and southern) upstream of the MIR (Fig. 6). The three flow lines follow the southern edge of a moderately closely packed band of incorporated icebergs within the ice shelf (Fig. 5). All three lines show an increase in ice shelf draft from 35 to 22 km upstream of the McDonald Bank. From 22 to 0 km the draft remains relatively constant. The base of the ice shelf is highly irregular, as a result of the keels of icebergs incorporated into the ice (King et al., 2018; Thomas, 1973).
High-resolution surface digital elevation model (DEM) from
WorldView surface height above the geoid (subsampled at 250 m
The highest point of the McDonald Bank was 212 m below sea level, measured on the southern flow line (Fig. 6). The precise ice shelf thickness at the McDonald Bank is difficult to resolve from the radar data as there was no clear expression of the bed due to interference from the complex topography of the MIR. However, the maximum draft of the iceberg keels along the three flow lines was 214 (northern), 214 (central), and 237 m (southern) below sea level, so the maximum potential overlap between the depth of the incorporated ice shelf keels and the depth of bed along all flow lines ranged from 2 to 25 m.
The DEM analysis of the MIR shows that the area of deformation caused by
contact between the base of the ice shelf and the McDonald Bank is limited
to an area of less than 1.3 to 3 km
Cross section of the McDonald Bank and the Brunt Ice Shelf showing
ice shelf draft and bathymetry along three flow lines upstream of the MIR
(the position of these flow lines is marked in Fig. 4d). The flow lines are
plotted as a function of both
The bathymetry and subglacial topography provide sufficient evidence to propose a range of past ice configurations in the study area. The substantially overdeepened south-to-north-oriented trough under the BIS and east-to-west-oriented trough under the SWGT are the likely products of grounded ice streams which converged in the Brunt Basin (white arrows, Fig. 4c) and presumably discharged westwards towards the Filchner Trough at glacial maxima. At these times, glacial depositional processes operating between the ice streams occupying Filchner Trough and the Brunt Basin likely formed the layered glacial sediments of the McDonald Bank interpreted from seismic surveys (Elverhøi and Maisey, 1983). Although seismic lines at different orientations are required to fully characterise the internal architecture and processes forming this deposit, the line drawings based on sparker seismic source data presented by Elverhøi and Maisey (1983, Profile 5, p. 486) suggest the layered glacial sediments dip westwards into the Weddell Sea. At least two of these layers (Units 1 and 2) were interpreted as being of glacial origin, although whether they are glaciomarine sediments, till and/or glacially compacted glaciomarine sediments cannot be determined. These layers have subsequently been truncated along the steep eastern slopes of the McDonald Bank, presumably by erosive ice advances along the south-to-north-oriented trough under the BIS. The relatively flat top of the McDonald Bank may be the product of glacial planation processes by ice shelves during, and either side of, peak glacial conditions.
During the development of interglacial conditions, it is reasonable to assume that the ice stream occupying the south-to-north-oriented trough under the BIS was starved of ice. We attribute this to the relatively small northwest-oriented glaciers in its catchment being largely < 200 m below sea level and therefore susceptible to progressive isolation from the deep troughs of the inland ice sheet. Instead, we suggest that most of the regional ice flow progressively followed the deep trough under the Stancomb-Wills Glacier, channelling ice from several north-, northeast-, and south-oriented subglacial catchments before discharging it west into the Brunt Basin and then north-northwest towards the southern Weddell Sea.
The topographic highs and ridges at, and just south of, the 74
Following grounding line retreat, floating ice likely occupied the Brunt Basin, as it does today. In this configuration, the various topographic highs associated with the highest parts of the transverse ridges and former grounding lines and the McDonald Bank would have formed potential pinning points for advancing ice shelves and glacier tongues. Most of these features fall just short of the base of the ice. Although the McDonald Bank has a relatively flat top, contact between the ice and the bed can be inferred from evidence of smaller-scale surface topography, including the poorly resolved crescent-shaped features (Fig. 4d). The latter may be ice-push moraine complexes formed where dense aggregations of deep keels from icebergs incorporated into the ice shelf from upstream glacial troughs have grounded on the bank.
As the BIS continued to thin, the topographic highs on the McDonald Bank would have provided the last potential pinning points and frontal buttressing of advancing ice. In its present configuration, only one of these is high enough (ca. 212 m below sea level) to maintain contact with the base of the BIS at the MIR. In contrast, the SWGT is neither thick nor extensive enough to ground on the McDonald Bank but may benefit from lateral buttressing from the Lyddan Ice Rise to the northeast and from the McDonald Bank during periods when it is coupled with the BIS (it is presently decoupled by the Halloween Crack; De Rydt et al., 2018). The DLGT is also not thick enough to ground on the grounding zone wedge at the seaward end of the Dawson-Lambton trough, although its internal architecture suggests grounding in the past and the presence of a frontally buttressed ice shelf (Hodgson et al., 2018).
Satellite images and aerial photographs show that the MIR have changed in their extent and morphology at different times in the recent past. This can be interpreted as an indicator of periodic thinning and at least partial loss of contact with the bed resulting from changes in ice shelf draft and minor calving events (see Bindschadler, 2002). The BIS has significant local thickness variations. This means that along any flow line the ice shelf draft varies considerably. This is due to a combination of the presence of incorporated iceberg keels, initial ice thickness close to the grounding line, changes in mass balance from accumulation of snow on the surface, basal accretion of marine ice, compression along the flow line from the McDonald Bank, ice velocity, and local flow divergence. Below, we consider how these influence the draft of ice shelf approaching the MIR.
Changes in mass balance from snow accumulation and accretion of marine ice
can be estimated from meteorological records and published over-snow radar
surveys of the internal structure of the ice shelf (King et al.,
2018). Meteorological records show a mean snow accumulation rate (Halley
Station data from 1972 to 2017) of 90 cm yr
The ice shelf flow line analysis shows no evidence of upstream thickening of
the ice shelf resulting from buttressing (Fig. 6), although the modelling
suggests that the compressive regime is felt at least 70 km upstream of the
MIR (De Rydt et al., 2018). Analysis of the strain rate
patterns using ice velocities shows that outside of the local area of
compression, most of the ice shelf is moving in similar directions at
similar velocities with thinning rates of less than 1 m yr
The flow line analysis also shows that the base of the ice shelf is highly heterogeneous as a result of the incorporated iceberg keels. Maximum keel depths along our flow lines ranged from 214 (northern and central) to 237 m (southern). This supports ice-penetrating radar analysis of the internal architecture elsewhere on the ice shelf that shows keels ranging between 175 and 250 m in depth interspersed with thinner sections of ice shelf formed by accumulated sea ice and seawater-saturated firn, snowfall, and drift (Fig. 4 in King et al., 2018). At least some of the cracks and chasms downstream of the MIR may be the expression of the differential strain experienced during the grounding then release of the incorporated icebergs, versus the intervening brine and firn sections of ice shelf (this is most apparent downstream of the MIR on the northern side; see Fig. 2 in King et al., 2018).
Collectively the ice shelf geometry analyses suggest that the distribution and depth of the incorporated iceberg keels are key to determining the future grounding potential of the ice shelf it flows towards and around the MIR. Their distribution can be inferred from the surface topography and ICESat data (Fig. 5), as well as Sentinel-1 satellite radar images (King et al., 2018). This shows areas on flow lines upstream of the MIR where the icebergs are more widely spaced or have shallower keels that could fail to make contact with the McDonald Bank (Fig. 6).
The future integrity of the BIS depends not only on the physical properties
of the ice, which are poorly understood, but also on whether it remains
grounded on the McDonald Bank. In the immediate future, this will be
determined by the direction of propagation of the tip of Chasm 1, the
dynamics and geometry of the expected calving event, and the subsequent
response of the remaining ice shelf. We consider four scenarios below.
Chasm 1 propagates to, or downstream of, the MIR and the BIS remains
pinned to the McDonald Bank following the calving event. In this scenario, a
new steady state will persist as long as the ice draft and keels are deep
enough to maintain contact with the bed. This situation has persisted since
the establishment of Halley Research Station on the ice shelf in 1956 and
has included the short-term loss and reestablishment of mechanical contact
with the bed, following a small local calving event in 1971, However, the
DEM surface topography (and ICESat data) shows large variations in the
distribution of the incorporated icebergs (Fig. 5) and the ice shelf draft
upstream of the MIR (Fig. 6), which can be used to determine if the grounding
will be maintained in future. For example, there are currently portions of
the ice shelf, due to reach the MIR 4–12 years from 2017 that may have
insufficient draft to ground, with no keels extending below Chasm 1 propagates to, or upstream of, the MIR, and the BIS temporarily loses
contact with the bed, but then rapidly increases in velocity and re-stabilises. If Chasm 1 joins the Halloween Crack at or upstream of the
McDonald Bank, then contact with the bed will be reduced or lost following
the calving event. In this scenario an immediate increase in the velocity in
the ice shelf might be anticipated as a result of the removal of buttressing
(Gudmundsson et al., 2016). This “surge” could result in a
re-grounding on the McDonald Bank and re-stabilisation of the ice shelf. Chasm 1 propagates upstream of the MIR, the BIS loses contact with the
bed, and this un-grounded state persists for a sufficient period that the ice
shelf essentially becomes an unpinned glacier tongue, which may or may not
extend beyond the McDonald Bank, but does not re-ground. The analogue for
this scenario is the SWGT, which has maintained a degree of structural
integrity in the absence of frontal buttressing (Fig. 1a) but experiences
large calving events at the ice front. The SWGT extends more than 200 km
from the grounding line; if BIS were to follow this scenario it could
advance forward, and potentially re-ground on topographic highs elsewhere on
the McDonald Bank (including at the location of the MIR), but the timescale
for a re-grounding remains highly uncertain.
Under scenarios 1–3 the presence of the large iceberg calved from the BIS
poses an additional threat to the integrity of the remaining parts of the
ice shelf through iceberg collision. This hypothesis was considered by
Anderson et al. (2014) with respect to a calving event on the SWGT.
In this scenario, Chasm 1 propagates upstream of the MIR, the BIS loses contact with the bed, and the subsequent increase in velocity is not extensive enough for the ice shelf to reconnect with the bed but is sufficient to increase strain rates to a level at which further fractures across the ice shelf are likely. This more widespread collapse would likely be influenced by the internal structural properties inherited at the grounding line (King et al., 2018). Observations show that these have resulted in occasional episodes of fast crack propagation (De Rydt et al., 2018), and inverse and forward modelling results suggest vulnerability to destabilisation by relatively rapid changes in the ice mélange properties, resulting from the interaction of its marine ice component with ocean water, or by the further propagation of a frontal rift (Khazendar et al., 2009). The local analogues for a more widespread collapse scenario are the DLGT, which is a glacier tongue that is subject to frequent calving events (Fig. 2c), and the ice margin south of the BIS that calves directly into the Weddell Sea (Fig. 2d).
Although the outcome of the calving of the ice shelf is not yet known, these four potential scenarios all show the importance of understanding the geometry of the ice shelf and the bed. Which of the four scenarios will transpire depends, at least in part, on whether the ice shelf is able to remain in contact with one of the topographic highs on the McDonald Bank. Whilst the depth of the topographic highs is fixed, the contact between the ice shelf and the bed depends on a number of variables. These include the short-term development of the ice shelf flow regime following calving and its impact on ice shelf draft (e.g. lateral spreading) and structural integrity (e.g. development of further rifts resulting from changes in the strain rate). They also include the long-term influences of processes at the grounding line, such as the thickness and velocity of ice flowing across the Brunt Icefalls, which determine the spacing and depth of the iceberg keels.
The ice dynamics of the BIS and SWGT glacier catchments have evolved from widespread occupation by grounded ice streams (during glacial periods) to a retreat of grounded ice through several inferred grounding line positions (during deglaciation), followed by the development of floating ice forming both buttressed ice shelves (BIS) and glacier tongues (SWGT, DLGT). In the former case, buttressing has occurred where the ice has been thick enough to maintain contact with topographic highs in the seabed.
The BIS has maintained its overall structural integrity since it was first observed in 1915, despite experiencing several periods of fast flow (Simmons and Rouse, 1984), fracturing, and episodic calving events along its ice front (Anderson et al., 2014). This relative stability can be attributed to its (at least intermittent) grounding on the McDonald Bank.
Although situated in a region of Antarctica that is not presently experiencing a rapid increase in atmospheric temperature or a known intrusion of circumpolar deep water, the BIS has nonetheless entered a period of rapid change (De Rydt et al., 2018), marked by the rapid propagation of rifts that will likely result in the largest calving event since observations began. Being composed of icebergs fused together by sea ice and accumulated snow, its internal structure differs from most other Antarctic ice shelves, and hence its dynamics are more difficult to predict. Specifically, it is not yet known if the ice shelf will lose contact with the bed and how it will respond after the calving event. Based on the history of different ice sheet configurations and the geometry of the ice shelf and the seabed, we have outlined four scenarios that might occur following the expected calving event, which will occur as Chasm 1 progresses north. These scenarios range from a re-stabilisation of the BIS to a more widespread collapse.
Priorities for future work on the BIS include (1) continued assessment of changes in the flow velocity and compressive regime in the ice shelf resulting from un-grounding at the MIR; (2) use of radar data to examine changes in ice shelf draft resulting from compression and changes in mass balance (firn and snow accumulation and marine ice accretion); specifically, modelling the balance between accumulation and lateral spreading under different grounding scenarios; (3) calibration and refinement of the geometry data by direct measurements of sea-floor bathymetry in new areas exposed by iceberg calving and measurements of ice shelf thickness and depth to the bed where the BIS meets the McDonald Bank (with access through sea ice in the Halloween Crack and Chasm 1); (4) further measurements to assess the different material properties of incorporated icebergs versus the intervening areas of sea ice and accumulated snow (temperature, viscosity, fracture toughness, etc.); and (5) an assessment of the future evolution of the ice thickness and draft from transient model runs with assumptions about the present and future calving events.
The data sets used in this paper are Hodgson et al. (2019) and Becker et al. (2018) which are available at the
NERC Polar Data Centre (
The supplement related to this article is available online at:
All authors contributed to the writing of the paper. Bathymetry data were acquired and processed by DH, TJ, PF, KH, AS, SS, and DB. Data on ice shelf geometry were acquired and processed by JDR, TJ, and PF. DAH and AJ contributed equally to this work.
The authors declare that they have no conflict of interest.
We thank the British Antarctic Survey (BAS) Air Operations Team and airborne survey specialists including Hugh Corr, Carl Robinson, and Ian Potten. We thank Hilmar Gudmundsson (BAS and Northumbria University) for management of data acquisition on the Brunt Ice Shelf and Ed King (BAS) for discussions on its internal structure. Steve Colwell (BAS) provided snow accumulation and temperature data. Laura Gerrish (BAS) prepared Fig. 1. This research contributes to NERC grant NE/K003674/1 “Reducing the uncertainty in estimates of the sea level contribution from the westernmost part of the East Antarctic Ice Sheet”. Sarah Greenwood, the anonymous reviewer, and the editor Joe MacGregor are thanked for their constructive advice and suggestions. Edited by: Joseph MacGregor Reviewed by: Sarah Greenwood and one anonymous referee