Earth Syst Sci Data 10 711ndash725 2018httpsdoiorg105194essd-10-711-2018copy Author(s) 2018 This work is distributed underthe Creative Commons Attribution 40 License

A new bed elevation model for the Weddell Seasector of the West Antarctic Ice Sheet

Hafeez Jeofry12 Neil Ross3 Hugh F J Corr4 Jilu Li5 Mathieu Morlighem6 Prasad Gogineni7 andMartin J Siegert1

1Grantham Institute and Department of Earth Science and EngineeringImperial College London South Kensington London UK

2School of Marine Science and Environment Universiti Malaysia TerengganuKuala Terengganu Terengganu Malaysia

3School of Geography Politics and Sociology Newcastle UniversityClaremont Road Newcastle Upon Tyne UK

4British Antarctic Survey Natural Environment Research Council Cambridge UK5Center for the Remote Sensing of Ice Sheets University of Kansas Lawrence Kansas USA

6Department of Earth System Science University of California Irvine Irvine California USA7Department of Electrical and Computer Engineering The University of Alabama

Tuscaloosa Alabama 35487 USA

Correspondence Hafeez Jeofry (hjeofry15imperialacuk)and Martin J Siegert (msiegertimperialacuk)

Received 11 August 2017 ndash Discussion started 26 October 2017Revised 26 October 2017 ndash Accepted 5 February 2018 ndash Published 9 April 2018

Abstract We present a new digital elevation model (DEM) of the bed with a 1 km gridding of the WeddellSea (WS) sector of the West Antarctic Ice Sheet (WAIS) The DEM has a total area of sim 125 000 km2 coveringthe Institute Moumlller and Foundation ice streams as well as the Bungenstock ice rise In comparison with theBedmap2 product our DEM includes new aerogeophysical datasets acquired by the Center for Remote Sensingof Ice Sheets (CReSIS) through the NASA Operation IceBridge (OIB) program in 2012 2014 and 2016 Wealso improve bed elevation information from the single largest existing dataset in the region collected by theBritish Antarctic Survey (BAS) Polarimetric radar Airborne Science Instrument (PASIN) in 2010ndash2011 fromthe relatively crude measurements determined in the field for quality control purposes used in Bedmap2 Whilethe gross form of the new DEM is similar to Bedmap2 there are some notable differences For example theposition and size of a deep subglacial trough (sim 2 km below sea level) between the ice-sheet interior and thegrounding line of the Foundation Ice Stream have been redefined From the revised DEM we are able to betterderive the expected routing of basal water and by comparison with that calculated using Bedmap2 we are ableto assess regions where hydraulic flow is sensitive to change Given the potential vulnerability of this sector toocean-induced melting at the grounding line especially in light of the improved definition of the Foundation IceStream trough our revised DEM will be of value to ice-sheet modelling in efforts to quantify future glaciologicalchanges in the region and from this the potential impact on global sea level The new 1 km bed elevation productof the WS sector can be found at httpsdoiorg105281zenodo1035488

Published by Copernicus Publications

712 H Jeofry et al A new bed elevation model for the Weddell Sea sector

1 Introduction

The Intergovernmental Panel on Climate Change (IPCC)concluded that global sea level rise may range from 026 to082 m by the end of the 21st century (Stocker 2014) Therising oceans pose a threat to the socio-economic activitiesof hundreds of millions of people mostly in Asia living atand close to the coastal environment Several processes drivesea level rise (eg thermal expansion of the oceans) but thelargest potential factor comes from the ice sheets in Antarc-tica The West Antarctic Ice Sheet (WAIS) which if meltedwould raise sea level by around 35 m is grounded on a bedwhich is in places more than 2 km below sea level (Bamberet al 2009a Ross et al 2012 Fretwell et al 2013) allow-ing the ice margin to have direct contact with ocean waterOne of the most sensitive regions of the WAIS to potentialocean warming is the Weddell Sea (WS) sector (Ross et al2012 Wright et al 2014) Ocean modelling studies showthat changes in present ocean circulation could bring warmocean water into direct contact with the grounding lines atthe base of the FilchnerndashRonne Ice Shelf (FRIS) (Hellmer etal 2012 Wright et al 2014 Martin et al 2015 Ritz et al2015 Thoma et al 2015) which would act in a manner sim-ilar to the ocean-induced basal melting under the Pine IslandGlacier ice shelf (Jacobs et al 2011) Enhanced melting ofthe FRIS could lead to a decrease in the buttressing supportto the upstream grounded ice causing enhanced flow to theocean A recent modelling study using a general ocean cir-culation model coupled with a 3-D thermodynamic ice-sheetmodel simulated the inflow of warm ocean water into theFilchnerndashRonne Ice Shelf cavity on a 1000-year timescale(Thoma et al 2015) A second modelling study this timeusing an ice-sheet model only indicated that the Instituteand Moumlller ice streams are highly sensitive to melting at thegrounding lines with grounding-line retreat up to 180 kmpossible across the Institute and Moumlller ice streams (Wrightet al 2014) While the Foundation Ice Stream was shownto be relatively resistant to ocean-induced change (Wright etal 2014) a dearth of geophysical measurements of ice thick-ness across the ice stream at the time means the result maybe inaccurate

The primary tool for measurements of subglacial topog-raphy and basal ice-sheet conditions is radio-echo sounding(RES) (Dowdeswell and Evans 2004 Bingham and Siegert2007) The first topographic representation of the land sur-face beneath the Antarctic ice sheet (Drewry 1983) was pub-lished by the Scott Polar Research Institute (SPRI) Univer-sity of Cambridge in collaboration with the US National Sci-ence Foundation Office of Polar Programs (NSF OPP) andthe Technical University of Denmark (TUD) following mul-tiple field seasons of RES surveying in the late 1960s and1970s (Drewry and Meldrum 1978 Drewry et al 1980Jankowski and Drewry 1981 Drewry 1983) The compila-tion included folio maps of bed topography ice-sheet surfaceelevation and ice thickness The bed was digitized on a 20 km

grid for use in ice-sheet modelling (Budd et al 1984) How-ever only around one-third of the continent was measured ata line spacing of less than sim 100 km making the elevationproduct erroneous in many places with obvious knock-onconsequences for modelling Several RES campaigns werethenceforth conducted and data from them were compiledinto a single new Antarctic bed elevation product namedBedmap (Lythe et al 2001) The Bedmap digital elevationmodel (DEM) was gridded on 5 km cells and included anover 14 million km and 250 000 km line track of airborne andground-based radio-echo sounding data respectively Sub-glacial topography was extended north to 60 S for purposesof ice-sheet modelling and determination of icendashocean inter-actions Since its release Bedmap has proved to be highlyuseful for a wide range of research yet inherent errors withinit (eg inaccuracies in the DEM and conflicting groundinglines compared with satellite-derived observations) restrictedits effectiveness (Le Brocq et al 2010) After 2001 severalnew RES surveys were conducted to fill data gaps revealed byBedmap especially during and after the fourth InternationalPolar Year (2007ndash2009) These new data led to the most re-cent Antarctic bed compilation named Bedmap2 (Fretwell etal 2013) Despite significant improvements in the resolutionand accuracy of Bedmap2 compared with Bedmap a numberof inaccuracies and poorly sampled areas persist (Fretwell etal 2013 Pritchard 2014) preventing a comprehensive ap-preciation of the complex relation between the topographyand internal ice-sheet processes and indeed a full apprecia-tion of the sensitivity of the Antarctic ice sheet to ocean andatmospheric warming

The WS sector was the subject of a major aerogeophys-ical survey in 2010ndash2011 (Ross et al 2012) revealing thesim 2 km deep Robin Subglacial Basin immediately upstreamof present-day grounding lines from which confirmation ofthe ice-sheet sensitivity from ice-sheet modelling was deter-mined (Wright et al 2014) Further geophysical surveyingof the region has been undertaken since Bedmap2 (Ross etal 2012) which has provided an enhanced appreciation ofthe importance of basal hydrology to ice flow (Siegert et al2016b) and complexities associated with the interaction ofbasal water flow bed topography and ice-surface elevation(Lindbaumlck et al 2014 Siegert et al 2014 Graham et al2017) emphasizing the importance of developing accurateand high-resolution DEMs both for the bed and the surfacein glaciology

In this paper we present an improved bed DEM for the WSsector based on a compilation of new airborne radar surveysThe DEM has a total area of sim 125 000 km2 and is griddedto 1 km cells From this dataset we reveal changes for therouting of subglacial melt water and discuss the differencesbetween the new DEM and Bedmap2 Our new bed DEMcan also be easily combined with updated surface DEMs toimprove ice-sheet modelling and subglacial water pathways

Earth Syst Sci Data 10 711ndash725 2018 wwwearth-syst-sci-datanet107112018

H Jeofry et al A new bed elevation model for the Weddell Sea sector 713

2 Study area

The WS sector covers the Institute Moumlller and Foundationice streams as well as the Bungenstock ice rise (Fig 1)The region covered by the DEM extends 135 km south ofthe Bungenstock ice rise 195 km east of the Foundation IceStream over the Pensacola Mountains and 185 km west ofthe Institute Ice Stream In comparison with Bedmap2 ournew DEM benefits from several new airborne geophysicaldatasets (eg NASA Operation IceBridge OIB 2012 2014and 2016) In addition the new DEM is improved by the in-clusion of ice thickness picks derived from synthetic apertureradar (SAR)-processed RES data from the British AntarcticSurvey (BAS) aerogeophysical survey of the Institute andMoumlller ice streams conducted in 2010ndash2011 This has im-proved the accuracy of the determination of the icendashbed in-terface in comparison to Bedmap2 We used the DifferentialInterferometry Synthetic Aperture Radar (DInSAR) ground-ing line (Rignot et al 2011c) to delimit the ice shelf-facingmargin of our grid

3 Data and methods

The RES data used in this study were compiled fromfour main sources (Fig 1a) first SPRI data collectedduring six survey campaigns between 1969 and 1979(Drewry 1983) second the BAS airborne radar survey ac-complished during the austral summer 2006ndash2007 (knownas GRADESIMAGE Glacial Retreat of Antarctica andDeglaciation of the Earth SystemInverse Modelling ofAntarctica and Global Eustasy) (Ashmore et al 2014) thirdthe BAS survey of the Institute and Moumlller ice streams un-dertaken in 2010ndash2011 (InstitutendashMoumlller Antarctic FundingInitiative IMAFI) (Ross et al 2012) fourth flights con-ducted by the Center for Remote Sensing of Ice Sheets (CRe-SIS) during the NASA OIB programme in 2012 2014 and2016 (Gogineni 2012) supplement the data previously usedin the Bedmap2 bed elevation product to accurately charac-terize the subglacial topography of this part of the WS sector

31 Scott Polar Research Institute survey

The SPRI surveys covered a total area of 696 million km2

(sim 40 of continental area) across West and East Antarc-tica (Drewry et al 1980) The data were collected using apulsed radar system operating at centre frequencies of 60 and300 MHz (Christensen 1970 Skou and Soslashndergaard 1976)equipped on an NSF LC-130 Hercules aircraft (Drewry andMeldrum 1978 Drewry et al 1980 Jankowski and Drewry1981 Drewry 1983) The 60 MHz antennas built by theTechnical University of Denmark comprised an array offour half-wave dipoles which were mounted in neutral aero-foil architecture of insulating components beneath the star-board wing The 300 MHz antennas were composed of fourdipoles attached underneath a reflector panel below the port

wing The purpose of this unique design was to improve thebackscatter acquisition and directivity The returned signalswere archived on 35 mm film and dry-silver paper by a fibreoptic oscillograph Aircraft navigation was assisted using anLTN-51 inertial navigation system giving a horizontal posi-tional error of around 3 km Navigation and other flight datawere stored on magnetic and analogue tape by an AirborneResearch Data System (ARDS) constructed by the US NavalWeapons Center The system recorded up to 100 channelsof six-digit data with a sampling rate of 303 Hz per channelNavigation ice thickness and ice-surface elevation recordswere recorded every 20 s corresponding to around 16 kmbetween each data point included on the Bedmap2 productThe data were initially recorded on a 35 mm photographicfilm (ie Z-scope radargrams) and were later scanned anddigitized as part of a NERC Centre for Polar Observationand Modelling (CPOM) project in 2004 Each film recordwas scanned separately and reformatted to form a singleelectronic image of a RES transect The scanned image wasloaded into an image analysis package (ie ERDAS Imagine)to trace the internal and the icendashbed interface which werethen digitized The digitized dataset was later standardizedwith respect to the ice surface

32 BAS GRADESIMAGE surveys

The GRADESIMAGE project was conducted during theaustral summer of 2006ndash2007 and acquired sim 27 550 km ofairborne RES data across the Antarctic Peninsula EllsworthMountains and FilchnerndashRonne Ice Shelf The BAS Polari-metric radar Airborne Science Instrument (PASIN) operatesat a centre frequency of 150 MHz has a 10 MHz bandwidthand a pulse-coded waveform acquisition rate of 3125 Hz(Corr et al 2007 Ashmore et al 2014) The PASIN systeminterleaves a pulse and chirp signal to acquire two datasetssimultaneously Pulse data are used for imaging layering inthe upper half of the ice column whilst the more powerfulchirp is used for imaging the deep ice and sounding the ice-sheet bed The peak transmitted power of the system is 4 kWThe spatial sampling interval of sim 20 m resulted in sim 50 000traces of data for a typical 45 h flight The radar system con-sisted of eight folded dipole elements four transmitters onthe port side and four receivers on the starboard side The re-ceiving backscatter signal was digitized and sampled using asub-Nyquist sampling technique The pulses are compressedusing a matched filter and side lobes are minimized using aBlackman window Aircraft position was recorded by an on-board carrier-wave global positioning system (GPS) The ab-solute horizontal positional accuracy for GRADESIMAGEwas 01 m (Corr et al 2007) Synthetic aperture radar pro-cessing was not applied to the data

wwwearth-syst-sci-datanet107112018 Earth Syst Sci Data 10 711ndash725 2018

714 H Jeofry et al A new bed elevation model for the Weddell Sea sector

Figure 1 (a) Location of the study area overlain with InSAR-derived ice-surface velocities (Rignot et al 2011d) (b) Aerogeophysical flightlines across the WS sector superimposed over RADARSAT (25 m) satellite imagery mosaic (Jezek 2002) SPRI airborne survey 1969ndash1979(orange) BAS GRADESIMAGE survey 2008 (green) BAS Institute Ice Stream survey in 2010ndash2011 (IMAFI) (yellow) OIB 2012 (red)OIB 2014 (blue) and OIB 2016 (brown) (c) Map of the WS sector based on MODIS imagery (Haran et al 2014) with grounding lines su-perimposed and denoted as follows ICESat laser altimetry (blue hydrostatic point orange ice flexure landward limit green break-in-slope)grounding line (Brunt et al 2010) Antarctic Surface Accumulation and Ice Discharge (ASAID) grounding line (red line) (Bindschadler etal 2011) Mosaic of Antarctica (MOA) grounding line (black line) (Bohlander and Scambos 2007) and Differential Interferometry Syn-thetic Aperture Radar (DInSAR) grounding line (yellow line) (Rignot et al 2011a) Annotations are as follows KIR ndash Korff ice rise HIR ndashHenry ice rise SIR ndash Skytrain ice rise HR ndash Heritage Range HVT ndash Horseshoe Valley Trough IT ndash Independence Trough ET ndash EllsworthTrough ESL ndash Ellsworth Subglacial Lake E1 ndash Institute E1 Subglacial Lake E2 ndash Institute E2 Subglacial Lake A1 ndash Academy 1 SubglacialLake A2 ndash Academy 2 Subglacial Lake A3 ndash Academy 3 Subglacial Lake A4 ndash Academy 4 Subglacial Lake A5 ndash Academy 5 SubglacialLake A6 ndash Academy 6 Subglacial Lake A7 ndash Academy 7 Subglacial Lake A8 ndash Academy 8 Subglacial Lake A9 ndash Academy 9 SubglacialLake A10 ndash Academy 10 Subglacial Lake A11 ndash Academy 11 Subglacial Lake A12 ndash Academy 12 Subglacial Lake A13 ndash Academy 13Subglacial Lake A14 ndash Academy 14 Subglacial Lake A15 ndash Academy 15 Subglacial Lake A16 ndash Academy 16 Subglacial Lake A17 ndashAcademy 17 Subglacial Lake W1 ndash Institute W1 Subglacial Lake W2 Institute W2 Subglacial Lake F1 ndash Foundation 1 Subglacial LakeF2 ndash Foundation 2 Subglacial Lake F3 ndash Foundation 3 Subglacial Lake PAT ndash Patriot Hills IH ndash Independence Hills MH ndash Marble HillsPIR ndash Pirrit Hills MNH ndash MartinndashNash Hills RSB ndash Robin Subglacial Basin WANT ndash West Antarctica EANT ndash East Antarctica

33 BAS InstitutendashMoumlller Antarctic Funding Initiativesurveys

The BAS data acquired during the IMAFI project consistof sim 25 000 km of aerogeophysical data collected during 27flights from two field camps A total of 17 flights were flownfrom C110 which is located close to Institute E2 Subglacial

Lake and the remaining 10 flights were flown from PatriotHills (Fig 1b) Data were acquired with the same PASINradar used for the GRADESIMAGE survey The data rate of13 Hz gave a spatial sampling interval of sim 10 m for IMAFIThe system was installed on the BAS de Havilland Twin Ot-ter aircraft with a four-element folded dipole array mountedbelow the starboard wing used for reception and the identi-

Earth Syst Sci Data 10 711ndash725 2018 wwwearth-syst-sci-datanet107112018

H Jeofry et al A new bed elevation model for the Weddell Sea sector 715

cal array attached below the port wing for transmission Theflights were flown in a stepped pattern during the IMAFI sur-vey to optimize potential field data (gravity and magnetics)acquisition (Fig 1b) Leica 500 and Novatel DL-V3 GPS re-ceivers were installed in the aircraft corrected with two Leica500 GPS base stations which were operated throughout thesurvey to calculate the position of the aircraft (Jordan et al2013) The positional data were referenced to the WGS 84ellipsoid The absolute positional accuracy for IMAFI (thestandard deviation for the GPS positional error) was calcu-lated to be 7 and 20 cm in the horizontal and vertical dimen-sions (Jordan et al 2013) Two-dimensional focused SARprocessing (Heacuteliegravere et al 2007) (see Sect 41) was appliedto the IMAFI data

34 NASA OIB CReSIS surveys

The OIB project surveyed a total distance of sim 32 693sim 52 460 and sim 53 672 km in Antarctica in 2012 2014 and2016 respectively using the Multichannel Coherent RadarDepth Sounder (MCoRDS) system developed at the Univer-sity of Kansas (Gogineni 2012) The system was operatedwith a carrier frequency of 195 MHz and a bandwidth of 10and 50 MHz in 2012 (Rodriguez-Morales et al 2014) and2014 onwards (Siegert et al 2016b) The radar consistedof a five-element antenna array housed in a customized an-tenna fairing which is attached beneath the NASA DCndash8aircraft fuselage (Rodriguez-Morales et al 2014) The fiveantennas were operated from a multichannel digital directsynthesis (DDS) controlled waveform generator enabling theuser to adjust the frequency timing amplitude and phase ofeach transmitted waveform (Shi et al 2010) The radar em-ploys an eight-channel waveform generator to emit eight in-dependent transmit chirp pulses The system is capable ofsupporting five receiver channels with an Analog DevicesAD9640 14 bit analogue-to-digital converter (ADC) acquir-ing the waveform at a rate of 111 MHz in 2012 (Gogineni2012) The system was upgraded in 2014 and 2016 utilizingsix-channel chirp generation and supports six receiver chan-nels with a waveform acquisition rate at 150 MHz Multiplereceivers allow array processing to suppress surface clutterin the cross-track direction which could potentially concealweak echoes from the icendashbed interface (Rodriguez-Moraleset al 2014) The radar data are synchronized with the GPSand inertial navigation system (INS) using the GPS times-tamp to determine the location of data acquisition

4 Data processing

We assume rain waves propagate through ice at a constantwave speed of 0168 m nsminus1 which presupposes the ice tobe homogenous (Gogineni et al 2001 2014 Lythe andVaughan 2001 Plewes and Hubbard 2001 Dowdeswelland Evans 2004) Low-density firn ice chemistry andor iceanisotropy (Diez et al 2014 Fujita et al 2014 Picotti et

al 2015 Shafique et al 2016) violate this assumption typ-ically resulting in a depth bias of the order of sim 10 m Theradar pulse travels through a medium until it meets a bound-ary of differing dielectric constant which causes some of theradio wave to be reflected and subsequently captured by thereceiver antenna The time travelled by the radar pulse be-tween the upper and lower reflecting surface is measuredand converted to ice thickness with reference to WGS 84(Fig 2) The digitized SPRIndashNSFndashTUD bed picks data areavailable through the BAS web page (httpsdatabasacukmetadataphpid=GBNERCBASAEDC00326) The two-dimensional SAR-processed radargrams in SEG-Y formatfor the IMAFI survey are provided at httpsdoiorg1052858a975b9e-f18c-4c51-9bdb-b00b82da52b8 whereas the icethickness datasets in comma-separated value (CSV) formatfor both GRADESIMAGE and IMAFI are available viathe BAS aerogeophysical processing portal (httpssecureantarcticaacukdataaerogeo) The ice thickness data forIMAFI are provided in two folders (1) the region of thin-ner ice (lt 200 m) picked from the pulse dataset and (2) theoverall ice thickness data derived from picking of SAR-processed chirp radargrams The data are arranged accordingto the latitude longitude ice thickness values and the pulserepetition interval radar shot number that is used to indexthe raw data The OIB SAR images (level 1B) in MAT (bi-nary MATLAB) format and the radar depth sounder level 2(L2) data in CSV format are available via the CReSIS web-site (httpsdatacresiskuedu) The L2 data include mea-surements for GPS time during data collection latitude lon-gitude elevation surface bottom and thickness For moreinformation on these data we refer the reader to the ap-propriate CReSIS guidance notes for each field season (iehttpsdatacresiskuedudatardsrds_readmepdf)

41 GRADESIMAGE and InstitutendashMoumlller AntarcticFunding Initiative data processing

The waveform was retrieved and sequenced according to itsrespective transmit pulse type The modified data were thencollated using MATLAB data binary files Doppler filtering(Heacuteliegravere et al 2007) was used to remove the backscatter-ing hyperbola in the along-track direction (Corr et al 2007Ross et al 2012) Chirp compression was then applied tothe along-track data Unfocused synthetic aperture (SAR)processing was used for the GRADESIMAGE survey byapplying a moving average of 33 data points (Corr et al2007) whereas two-dimensional SAR (ie focused) process-ing based on the Omega-K algorithm was used to processthe IMAFI data (Heacuteliegravere et al 2007 Winter et al 2015) toenhance both along-track resolution and echo signal noiseThe bed echo was depicted in a semi-automatic manner us-ing ProMAX seismic processing software All picking forIMAFI was undertaken by a single operator (Neil Ross) Anominal value of 10 m is used to correct for the firn layerduring the processing of ice thickness which introduces an

wwwearth-syst-sci-datanet107112018 Earth Syst Sci Data 10 711ndash725 2018

H Jeofry et al A new bed elevation model for the Weddell Sea sector 713

2 Study area

The WS sector covers the Institute Moumlller and Foundationice streams as well as the Bungenstock ice rise (Fig 1)The region covered by the DEM extends 135 km south ofthe Bungenstock ice rise 195 km east of the Foundation IceStream over the Pensacola Mountains and 185 km west ofthe Institute Ice Stream In comparison with Bedmap2 ournew DEM benefits from several new airborne geophysicaldatasets (eg NASA Operation IceBridge OIB 2012 2014and 2016) In addition the new DEM is improved by the in-clusion of ice thickness picks derived from synthetic apertureradar (SAR)-processed RES data from the British AntarcticSurvey (BAS) aerogeophysical survey of the Institute andMoumlller ice streams conducted in 2010ndash2011 This has im-proved the accuracy of the determination of the icendashbed in-terface in comparison to Bedmap2 We used the DifferentialInterferometry Synthetic Aperture Radar (DInSAR) ground-ing line (Rignot et al 2011c) to delimit the ice shelf-facingmargin of our grid

3 Data and methods

The RES data used in this study were compiled fromfour main sources (Fig 1a) first SPRI data collectedduring six survey campaigns between 1969 and 1979(Drewry 1983) second the BAS airborne radar survey ac-complished during the austral summer 2006ndash2007 (knownas GRADESIMAGE Glacial Retreat of Antarctica andDeglaciation of the Earth SystemInverse Modelling ofAntarctica and Global Eustasy) (Ashmore et al 2014) thirdthe BAS survey of the Institute and Moumlller ice streams un-dertaken in 2010ndash2011 (InstitutendashMoumlller Antarctic FundingInitiative IMAFI) (Ross et al 2012) fourth flights con-ducted by the Center for Remote Sensing of Ice Sheets (CRe-SIS) during the NASA OIB programme in 2012 2014 and2016 (Gogineni 2012) supplement the data previously usedin the Bedmap2 bed elevation product to accurately charac-terize the subglacial topography of this part of the WS sector

31 Scott Polar Research Institute survey

The SPRI surveys covered a total area of 696 million km2

(sim 40 of continental area) across West and East Antarc-tica (Drewry et al 1980) The data were collected using apulsed radar system operating at centre frequencies of 60 and300 MHz (Christensen 1970 Skou and Soslashndergaard 1976)equipped on an NSF LC-130 Hercules aircraft (Drewry andMeldrum 1978 Drewry et al 1980 Jankowski and Drewry1981 Drewry 1983) The 60 MHz antennas built by theTechnical University of Denmark comprised an array offour half-wave dipoles which were mounted in neutral aero-foil architecture of insulating components beneath the star-board wing The 300 MHz antennas were composed of fourdipoles attached underneath a reflector panel below the port

wing The purpose of this unique design was to improve thebackscatter acquisition and directivity The returned signalswere archived on 35 mm film and dry-silver paper by a fibreoptic oscillograph Aircraft navigation was assisted using anLTN-51 inertial navigation system giving a horizontal posi-tional error of around 3 km Navigation and other flight datawere stored on magnetic and analogue tape by an AirborneResearch Data System (ARDS) constructed by the US NavalWeapons Center The system recorded up to 100 channelsof six-digit data with a sampling rate of 303 Hz per channelNavigation ice thickness and ice-surface elevation recordswere recorded every 20 s corresponding to around 16 kmbetween each data point included on the Bedmap2 productThe data were initially recorded on a 35 mm photographicfilm (ie Z-scope radargrams) and were later scanned anddigitized as part of a NERC Centre for Polar Observationand Modelling (CPOM) project in 2004 Each film recordwas scanned separately and reformatted to form a singleelectronic image of a RES transect The scanned image wasloaded into an image analysis package (ie ERDAS Imagine)to trace the internal and the icendashbed interface which werethen digitized The digitized dataset was later standardizedwith respect to the ice surface

32 BAS GRADESIMAGE surveys

The GRADESIMAGE project was conducted during theaustral summer of 2006ndash2007 and acquired sim 27 550 km ofairborne RES data across the Antarctic Peninsula EllsworthMountains and FilchnerndashRonne Ice Shelf The BAS Polari-metric radar Airborne Science Instrument (PASIN) operatesat a centre frequency of 150 MHz has a 10 MHz bandwidthand a pulse-coded waveform acquisition rate of 3125 Hz(Corr et al 2007 Ashmore et al 2014) The PASIN systeminterleaves a pulse and chirp signal to acquire two datasetssimultaneously Pulse data are used for imaging layering inthe upper half of the ice column whilst the more powerfulchirp is used for imaging the deep ice and sounding the ice-sheet bed The peak transmitted power of the system is 4 kWThe spatial sampling interval of sim 20 m resulted in sim 50 000traces of data for a typical 45 h flight The radar system con-sisted of eight folded dipole elements four transmitters onthe port side and four receivers on the starboard side The re-ceiving backscatter signal was digitized and sampled using asub-Nyquist sampling technique The pulses are compressedusing a matched filter and side lobes are minimized using aBlackman window Aircraft position was recorded by an on-board carrier-wave global positioning system (GPS) The ab-solute horizontal positional accuracy for GRADESIMAGEwas 01 m (Corr et al 2007) Synthetic aperture radar pro-cessing was not applied to the data

wwwearth-syst-sci-datanet107112018 Earth Syst Sci Data 10 711ndash725 2018

714 H Jeofry et al A new bed elevation model for the Weddell Sea sector

Figure 1 (a) Location of the study area overlain with InSAR-derived ice-surface velocities (Rignot et al 2011d) (b) Aerogeophysical flightlines across the WS sector superimposed over RADARSAT (25 m) satellite imagery mosaic (Jezek 2002) SPRI airborne survey 1969ndash1979(orange) BAS GRADESIMAGE survey 2008 (green) BAS Institute Ice Stream survey in 2010ndash2011 (IMAFI) (yellow) OIB 2012 (red)OIB 2014 (blue) and OIB 2016 (brown) (c) Map of the WS sector based on MODIS imagery (Haran et al 2014) with grounding lines su-perimposed and denoted as follows ICESat laser altimetry (blue hydrostatic point orange ice flexure landward limit green break-in-slope)grounding line (Brunt et al 2010) Antarctic Surface Accumulation and Ice Discharge (ASAID) grounding line (red line) (Bindschadler etal 2011) Mosaic of Antarctica (MOA) grounding line (black line) (Bohlander and Scambos 2007) and Differential Interferometry Syn-thetic Aperture Radar (DInSAR) grounding line (yellow line) (Rignot et al 2011a) Annotations are as follows KIR ndash Korff ice rise HIR ndashHenry ice rise SIR ndash Skytrain ice rise HR ndash Heritage Range HVT ndash Horseshoe Valley Trough IT ndash Independence Trough ET ndash EllsworthTrough ESL ndash Ellsworth Subglacial Lake E1 ndash Institute E1 Subglacial Lake E2 ndash Institute E2 Subglacial Lake A1 ndash Academy 1 SubglacialLake A2 ndash Academy 2 Subglacial Lake A3 ndash Academy 3 Subglacial Lake A4 ndash Academy 4 Subglacial Lake A5 ndash Academy 5 SubglacialLake A6 ndash Academy 6 Subglacial Lake A7 ndash Academy 7 Subglacial Lake A8 ndash Academy 8 Subglacial Lake A9 ndash Academy 9 SubglacialLake A10 ndash Academy 10 Subglacial Lake A11 ndash Academy 11 Subglacial Lake A12 ndash Academy 12 Subglacial Lake A13 ndash Academy 13Subglacial Lake A14 ndash Academy 14 Subglacial Lake A15 ndash Academy 15 Subglacial Lake A16 ndash Academy 16 Subglacial Lake A17 ndashAcademy 17 Subglacial Lake W1 ndash Institute W1 Subglacial Lake W2 Institute W2 Subglacial Lake F1 ndash Foundation 1 Subglacial LakeF2 ndash Foundation 2 Subglacial Lake F3 ndash Foundation 3 Subglacial Lake PAT ndash Patriot Hills IH ndash Independence Hills MH ndash Marble HillsPIR ndash Pirrit Hills MNH ndash MartinndashNash Hills RSB ndash Robin Subglacial Basin WANT ndash West Antarctica EANT ndash East Antarctica

33 BAS InstitutendashMoumlller Antarctic Funding Initiativesurveys

The BAS data acquired during the IMAFI project consistof sim 25 000 km of aerogeophysical data collected during 27flights from two field camps A total of 17 flights were flownfrom C110 which is located close to Institute E2 Subglacial

Lake and the remaining 10 flights were flown from PatriotHills (Fig 1b) Data were acquired with the same PASINradar used for the GRADESIMAGE survey The data rate of13 Hz gave a spatial sampling interval of sim 10 m for IMAFIThe system was installed on the BAS de Havilland Twin Ot-ter aircraft with a four-element folded dipole array mountedbelow the starboard wing used for reception and the identi-

Earth Syst Sci Data 10 711ndash725 2018 wwwearth-syst-sci-datanet107112018

H Jeofry et al A new bed elevation model for the Weddell Sea sector 715

cal array attached below the port wing for transmission Theflights were flown in a stepped pattern during the IMAFI sur-vey to optimize potential field data (gravity and magnetics)acquisition (Fig 1b) Leica 500 and Novatel DL-V3 GPS re-ceivers were installed in the aircraft corrected with two Leica500 GPS base stations which were operated throughout thesurvey to calculate the position of the aircraft (Jordan et al2013) The positional data were referenced to the WGS 84ellipsoid The absolute positional accuracy for IMAFI (thestandard deviation for the GPS positional error) was calcu-lated to be 7 and 20 cm in the horizontal and vertical dimen-sions (Jordan et al 2013) Two-dimensional focused SARprocessing (Heacuteliegravere et al 2007) (see Sect 41) was appliedto the IMAFI data

34 NASA OIB CReSIS surveys

The OIB project surveyed a total distance of sim 32 693sim 52 460 and sim 53 672 km in Antarctica in 2012 2014 and2016 respectively using the Multichannel Coherent RadarDepth Sounder (MCoRDS) system developed at the Univer-sity of Kansas (Gogineni 2012) The system was operatedwith a carrier frequency of 195 MHz and a bandwidth of 10and 50 MHz in 2012 (Rodriguez-Morales et al 2014) and2014 onwards (Siegert et al 2016b) The radar consistedof a five-element antenna array housed in a customized an-tenna fairing which is attached beneath the NASA DCndash8aircraft fuselage (Rodriguez-Morales et al 2014) The fiveantennas were operated from a multichannel digital directsynthesis (DDS) controlled waveform generator enabling theuser to adjust the frequency timing amplitude and phase ofeach transmitted waveform (Shi et al 2010) The radar em-ploys an eight-channel waveform generator to emit eight in-dependent transmit chirp pulses The system is capable ofsupporting five receiver channels with an Analog DevicesAD9640 14 bit analogue-to-digital converter (ADC) acquir-ing the waveform at a rate of 111 MHz in 2012 (Gogineni2012) The system was upgraded in 2014 and 2016 utilizingsix-channel chirp generation and supports six receiver chan-nels with a waveform acquisition rate at 150 MHz Multiplereceivers allow array processing to suppress surface clutterin the cross-track direction which could potentially concealweak echoes from the icendashbed interface (Rodriguez-Moraleset al 2014) The radar data are synchronized with the GPSand inertial navigation system (INS) using the GPS times-tamp to determine the location of data acquisition

4 Data processing

We assume rain waves propagate through ice at a constantwave speed of 0168 m nsminus1 which presupposes the ice tobe homogenous (Gogineni et al 2001 2014 Lythe andVaughan 2001 Plewes and Hubbard 2001 Dowdeswelland Evans 2004) Low-density firn ice chemistry andor iceanisotropy (Diez et al 2014 Fujita et al 2014 Picotti et

al 2015 Shafique et al 2016) violate this assumption typ-ically resulting in a depth bias of the order of sim 10 m Theradar pulse travels through a medium until it meets a bound-ary of differing dielectric constant which causes some of theradio wave to be reflected and subsequently captured by thereceiver antenna The time travelled by the radar pulse be-tween the upper and lower reflecting surface is measuredand converted to ice thickness with reference to WGS 84(Fig 2) The digitized SPRIndashNSFndashTUD bed picks data areavailable through the BAS web page (httpsdatabasacukmetadataphpid=GBNERCBASAEDC00326) The two-dimensional SAR-processed radargrams in SEG-Y formatfor the IMAFI survey are provided at httpsdoiorg1052858a975b9e-f18c-4c51-9bdb-b00b82da52b8 whereas the icethickness datasets in comma-separated value (CSV) formatfor both GRADESIMAGE and IMAFI are available viathe BAS aerogeophysical processing portal (httpssecureantarcticaacukdataaerogeo) The ice thickness data forIMAFI are provided in two folders (1) the region of thin-ner ice (lt 200 m) picked from the pulse dataset and (2) theoverall ice thickness data derived from picking of SAR-processed chirp radargrams The data are arranged accordingto the latitude longitude ice thickness values and the pulserepetition interval radar shot number that is used to indexthe raw data The OIB SAR images (level 1B) in MAT (bi-nary MATLAB) format and the radar depth sounder level 2(L2) data in CSV format are available via the CReSIS web-site (httpsdatacresiskuedu) The L2 data include mea-surements for GPS time during data collection latitude lon-gitude elevation surface bottom and thickness For moreinformation on these data we refer the reader to the ap-propriate CReSIS guidance notes for each field season (iehttpsdatacresiskuedudatardsrds_readmepdf)

41 GRADESIMAGE and InstitutendashMoumlller AntarcticFunding Initiative data processing

The waveform was retrieved and sequenced according to itsrespective transmit pulse type The modified data were thencollated using MATLAB data binary files Doppler filtering(Heacuteliegravere et al 2007) was used to remove the backscatter-ing hyperbola in the along-track direction (Corr et al 2007Ross et al 2012) Chirp compression was then applied tothe along-track data Unfocused synthetic aperture (SAR)processing was used for the GRADESIMAGE survey byapplying a moving average of 33 data points (Corr et al2007) whereas two-dimensional SAR (ie focused) process-ing based on the Omega-K algorithm was used to processthe IMAFI data (Heacuteliegravere et al 2007 Winter et al 2015) toenhance both along-track resolution and echo signal noiseThe bed echo was depicted in a semi-automatic manner us-ing ProMAX seismic processing software All picking forIMAFI was undertaken by a single operator (Neil Ross) Anominal value of 10 m is used to correct for the firn layerduring the processing of ice thickness which introduces an

wwwearth-syst-sci-datanet107112018 Earth Syst Sci Data 10 711ndash725 2018

714 H Jeofry et al A new bed elevation model for the Weddell Sea sector

Figure 1 (a) Location of the study area overlain with InSAR-derived ice-surface velocities (Rignot et al 2011d) (b) Aerogeophysical flightlines across the WS sector superimposed over RADARSAT (25 m) satellite imagery mosaic (Jezek 2002) SPRI airborne survey 1969ndash1979(orange) BAS GRADESIMAGE survey 2008 (green) BAS Institute Ice Stream survey in 2010ndash2011 (IMAFI) (yellow) OIB 2012 (red)OIB 2014 (blue) and OIB 2016 (brown) (c) Map of the WS sector based on MODIS imagery (Haran et al 2014) with grounding lines su-perimposed and denoted as follows ICESat laser altimetry (blue hydrostatic point orange ice flexure landward limit green break-in-slope)grounding line (Brunt et al 2010) Antarctic Surface Accumulation and Ice Discharge (ASAID) grounding line (red line) (Bindschadler etal 2011) Mosaic of Antarctica (MOA) grounding line (black line) (Bohlander and Scambos 2007) and Differential Interferometry Syn-thetic Aperture Radar (DInSAR) grounding line (yellow line) (Rignot et al 2011a) Annotations are as follows KIR ndash Korff ice rise HIR ndashHenry ice rise SIR ndash Skytrain ice rise HR ndash Heritage Range HVT ndash Horseshoe Valley Trough IT ndash Independence Trough ET ndash EllsworthTrough ESL ndash Ellsworth Subglacial Lake E1 ndash Institute E1 Subglacial Lake E2 ndash Institute E2 Subglacial Lake A1 ndash Academy 1 SubglacialLake A2 ndash Academy 2 Subglacial Lake A3 ndash Academy 3 Subglacial Lake A4 ndash Academy 4 Subglacial Lake A5 ndash Academy 5 SubglacialLake A6 ndash Academy 6 Subglacial Lake A7 ndash Academy 7 Subglacial Lake A8 ndash Academy 8 Subglacial Lake A9 ndash Academy 9 SubglacialLake A10 ndash Academy 10 Subglacial Lake A11 ndash Academy 11 Subglacial Lake A12 ndash Academy 12 Subglacial Lake A13 ndash Academy 13Subglacial Lake A14 ndash Academy 14 Subglacial Lake A15 ndash Academy 15 Subglacial Lake A16 ndash Academy 16 Subglacial Lake A17 ndashAcademy 17 Subglacial Lake W1 ndash Institute W1 Subglacial Lake W2 Institute W2 Subglacial Lake F1 ndash Foundation 1 Subglacial LakeF2 ndash Foundation 2 Subglacial Lake F3 ndash Foundation 3 Subglacial Lake PAT ndash Patriot Hills IH ndash Independence Hills MH ndash Marble HillsPIR ndash Pirrit Hills MNH ndash MartinndashNash Hills RSB ndash Robin Subglacial Basin WANT ndash West Antarctica EANT ndash East Antarctica

33 BAS InstitutendashMoumlller Antarctic Funding Initiativesurveys

The BAS data acquired during the IMAFI project consistof sim 25 000 km of aerogeophysical data collected during 27flights from two field camps A total of 17 flights were flownfrom C110 which is located close to Institute E2 Subglacial

Lake and the remaining 10 flights were flown from PatriotHills (Fig 1b) Data were acquired with the same PASINradar used for the GRADESIMAGE survey The data rate of13 Hz gave a spatial sampling interval of sim 10 m for IMAFIThe system was installed on the BAS de Havilland Twin Ot-ter aircraft with a four-element folded dipole array mountedbelow the starboard wing used for reception and the identi-

Earth Syst Sci Data 10 711ndash725 2018 wwwearth-syst-sci-datanet107112018

H Jeofry et al A new bed elevation model for the Weddell Sea sector 715

cal array attached below the port wing for transmission Theflights were flown in a stepped pattern during the IMAFI sur-vey to optimize potential field data (gravity and magnetics)acquisition (Fig 1b) Leica 500 and Novatel DL-V3 GPS re-ceivers were installed in the aircraft corrected with two Leica500 GPS base stations which were operated throughout thesurvey to calculate the position of the aircraft (Jordan et al2013) The positional data were referenced to the WGS 84ellipsoid The absolute positional accuracy for IMAFI (thestandard deviation for the GPS positional error) was calcu-lated to be 7 and 20 cm in the horizontal and vertical dimen-sions (Jordan et al 2013) Two-dimensional focused SARprocessing (Heacuteliegravere et al 2007) (see Sect 41) was appliedto the IMAFI data

34 NASA OIB CReSIS surveys

The OIB project surveyed a total distance of sim 32 693sim 52 460 and sim 53 672 km in Antarctica in 2012 2014 and2016 respectively using the Multichannel Coherent RadarDepth Sounder (MCoRDS) system developed at the Univer-sity of Kansas (Gogineni 2012) The system was operatedwith a carrier frequency of 195 MHz and a bandwidth of 10and 50 MHz in 2012 (Rodriguez-Morales et al 2014) and2014 onwards (Siegert et al 2016b) The radar consistedof a five-element antenna array housed in a customized an-tenna fairing which is attached beneath the NASA DCndash8aircraft fuselage (Rodriguez-Morales et al 2014) The fiveantennas were operated from a multichannel digital directsynthesis (DDS) controlled waveform generator enabling theuser to adjust the frequency timing amplitude and phase ofeach transmitted waveform (Shi et al 2010) The radar em-ploys an eight-channel waveform generator to emit eight in-dependent transmit chirp pulses The system is capable ofsupporting five receiver channels with an Analog DevicesAD9640 14 bit analogue-to-digital converter (ADC) acquir-ing the waveform at a rate of 111 MHz in 2012 (Gogineni2012) The system was upgraded in 2014 and 2016 utilizingsix-channel chirp generation and supports six receiver chan-nels with a waveform acquisition rate at 150 MHz Multiplereceivers allow array processing to suppress surface clutterin the cross-track direction which could potentially concealweak echoes from the icendashbed interface (Rodriguez-Moraleset al 2014) The radar data are synchronized with the GPSand inertial navigation system (INS) using the GPS times-tamp to determine the location of data acquisition

4 Data processing

We assume rain waves propagate through ice at a constantwave speed of 0168 m nsminus1 which presupposes the ice tobe homogenous (Gogineni et al 2001 2014 Lythe andVaughan 2001 Plewes and Hubbard 2001 Dowdeswelland Evans 2004) Low-density firn ice chemistry andor iceanisotropy (Diez et al 2014 Fujita et al 2014 Picotti et

al 2015 Shafique et al 2016) violate this assumption typ-ically resulting in a depth bias of the order of sim 10 m Theradar pulse travels through a medium until it meets a bound-ary of differing dielectric constant which causes some of theradio wave to be reflected and subsequently captured by thereceiver antenna The time travelled by the radar pulse be-tween the upper and lower reflecting surface is measuredand converted to ice thickness with reference to WGS 84(Fig 2) The digitized SPRIndashNSFndashTUD bed picks data areavailable through the BAS web page (httpsdatabasacukmetadataphpid=GBNERCBASAEDC00326) The two-dimensional SAR-processed radargrams in SEG-Y formatfor the IMAFI survey are provided at httpsdoiorg1052858a975b9e-f18c-4c51-9bdb-b00b82da52b8 whereas the icethickness datasets in comma-separated value (CSV) formatfor both GRADESIMAGE and IMAFI are available viathe BAS aerogeophysical processing portal (httpssecureantarcticaacukdataaerogeo) The ice thickness data forIMAFI are provided in two folders (1) the region of thin-ner ice (lt 200 m) picked from the pulse dataset and (2) theoverall ice thickness data derived from picking of SAR-processed chirp radargrams The data are arranged accordingto the latitude longitude ice thickness values and the pulserepetition interval radar shot number that is used to indexthe raw data The OIB SAR images (level 1B) in MAT (bi-nary MATLAB) format and the radar depth sounder level 2(L2) data in CSV format are available via the CReSIS web-site (httpsdatacresiskuedu) The L2 data include mea-surements for GPS time during data collection latitude lon-gitude elevation surface bottom and thickness For moreinformation on these data we refer the reader to the ap-propriate CReSIS guidance notes for each field season (iehttpsdatacresiskuedudatardsrds_readmepdf)

41 GRADESIMAGE and InstitutendashMoumlller AntarcticFunding Initiative data processing

The waveform was retrieved and sequenced according to itsrespective transmit pulse type The modified data were thencollated using MATLAB data binary files Doppler filtering(Heacuteliegravere et al 2007) was used to remove the backscatter-ing hyperbola in the along-track direction (Corr et al 2007Ross et al 2012) Chirp compression was then applied tothe along-track data Unfocused synthetic aperture (SAR)processing was used for the GRADESIMAGE survey byapplying a moving average of 33 data points (Corr et al2007) whereas two-dimensional SAR (ie focused) process-ing based on the Omega-K algorithm was used to processthe IMAFI data (Heacuteliegravere et al 2007 Winter et al 2015) toenhance both along-track resolution and echo signal noiseThe bed echo was depicted in a semi-automatic manner us-ing ProMAX seismic processing software All picking forIMAFI was undertaken by a single operator (Neil Ross) Anominal value of 10 m is used to correct for the firn layerduring the processing of ice thickness which introduces an

wwwearth-syst-sci-datanet107112018 Earth Syst Sci Data 10 711ndash725 2018

H Jeofry et al A new bed elevation model for the Weddell Sea sector 715

cal array attached below the port wing for transmission Theflights were flown in a stepped pattern during the IMAFI sur-vey to optimize potential field data (gravity and magnetics)acquisition (Fig 1b) Leica 500 and Novatel DL-V3 GPS re-ceivers were installed in the aircraft corrected with two Leica500 GPS base stations which were operated throughout thesurvey to calculate the position of the aircraft (Jordan et al2013) The positional data were referenced to the WGS 84ellipsoid The absolute positional accuracy for IMAFI (thestandard deviation for the GPS positional error) was calcu-lated to be 7 and 20 cm in the horizontal and vertical dimen-sions (Jordan et al 2013) Two-dimensional focused SARprocessing (Heacuteliegravere et al 2007) (see Sect 41) was appliedto the IMAFI data

34 NASA OIB CReSIS surveys

The OIB project surveyed a total distance of sim 32 693sim 52 460 and sim 53 672 km in Antarctica in 2012 2014 and2016 respectively using the Multichannel Coherent RadarDepth Sounder (MCoRDS) system developed at the Univer-sity of Kansas (Gogineni 2012) The system was operatedwith a carrier frequency of 195 MHz and a bandwidth of 10and 50 MHz in 2012 (Rodriguez-Morales et al 2014) and2014 onwards (Siegert et al 2016b) The radar consistedof a five-element antenna array housed in a customized an-tenna fairing which is attached beneath the NASA DCndash8aircraft fuselage (Rodriguez-Morales et al 2014) The fiveantennas were operated from a multichannel digital directsynthesis (DDS) controlled waveform generator enabling theuser to adjust the frequency timing amplitude and phase ofeach transmitted waveform (Shi et al 2010) The radar em-ploys an eight-channel waveform generator to emit eight in-dependent transmit chirp pulses The system is capable ofsupporting five receiver channels with an Analog DevicesAD9640 14 bit analogue-to-digital converter (ADC) acquir-ing the waveform at a rate of 111 MHz in 2012 (Gogineni2012) The system was upgraded in 2014 and 2016 utilizingsix-channel chirp generation and supports six receiver chan-nels with a waveform acquisition rate at 150 MHz Multiplereceivers allow array processing to suppress surface clutterin the cross-track direction which could potentially concealweak echoes from the icendashbed interface (Rodriguez-Moraleset al 2014) The radar data are synchronized with the GPSand inertial navigation system (INS) using the GPS times-tamp to determine the location of data acquisition

4 Data processing

We assume rain waves propagate through ice at a constantwave speed of 0168 m nsminus1 which presupposes the ice tobe homogenous (Gogineni et al 2001 2014 Lythe andVaughan 2001 Plewes and Hubbard 2001 Dowdeswelland Evans 2004) Low-density firn ice chemistry andor iceanisotropy (Diez et al 2014 Fujita et al 2014 Picotti et

al 2015 Shafique et al 2016) violate this assumption typ-ically resulting in a depth bias of the order of sim 10 m Theradar pulse travels through a medium until it meets a bound-ary of differing dielectric constant which causes some of theradio wave to be reflected and subsequently captured by thereceiver antenna The time travelled by the radar pulse be-tween the upper and lower reflecting surface is measuredand converted to ice thickness with reference to WGS 84(Fig 2) The digitized SPRIndashNSFndashTUD bed picks data areavailable through the BAS web page (httpsdatabasacukmetadataphpid=GBNERCBASAEDC00326) The two-dimensional SAR-processed radargrams in SEG-Y formatfor the IMAFI survey are provided at httpsdoiorg1052858a975b9e-f18c-4c51-9bdb-b00b82da52b8 whereas the icethickness datasets in comma-separated value (CSV) formatfor both GRADESIMAGE and IMAFI are available viathe BAS aerogeophysical processing portal (httpssecureantarcticaacukdataaerogeo) The ice thickness data forIMAFI are provided in two folders (1) the region of thin-ner ice (lt 200 m) picked from the pulse dataset and (2) theoverall ice thickness data derived from picking of SAR-processed chirp radargrams The data are arranged accordingto the latitude longitude ice thickness values and the pulserepetition interval radar shot number that is used to indexthe raw data The OIB SAR images (level 1B) in MAT (bi-nary MATLAB) format and the radar depth sounder level 2(L2) data in CSV format are available via the CReSIS web-site (httpsdatacresiskuedu) The L2 data include mea-surements for GPS time during data collection latitude lon-gitude elevation surface bottom and thickness For moreinformation on these data we refer the reader to the ap-propriate CReSIS guidance notes for each field season (iehttpsdatacresiskuedudatardsrds_readmepdf)

41 GRADESIMAGE and InstitutendashMoumlller AntarcticFunding Initiative data processing

The waveform was retrieved and sequenced according to itsrespective transmit pulse type The modified data were thencollated using MATLAB data binary files Doppler filtering(Heacuteliegravere et al 2007) was used to remove the backscatter-ing hyperbola in the along-track direction (Corr et al 2007Ross et al 2012) Chirp compression was then applied tothe along-track data Unfocused synthetic aperture (SAR)processing was used for the GRADESIMAGE survey byapplying a moving average of 33 data points (Corr et al2007) whereas two-dimensional SAR (ie focused) process-ing based on the Omega-K algorithm was used to processthe IMAFI data (Heacuteliegravere et al 2007 Winter et al 2015) toenhance both along-track resolution and echo signal noiseThe bed echo was depicted in a semi-automatic manner us-ing ProMAX seismic processing software All picking forIMAFI was undertaken by a single operator (Neil Ross) Anominal value of 10 m is used to correct for the firn layerduring the processing of ice thickness which introduces an

wwwearth-syst-sci-datanet107112018 Earth Syst Sci Data 10 711ndash725 2018

716 H Jeofry et al A new bed elevation model for the Weddell Sea sector

Figure 2 Two-dimensional synthetic aperture radar (SAR)-processed radargrams of (a) BAS IMAFI data and (b) OIB data

error of the order ofplusmn3 m across the survey field (Ross et al2012) This is small relative to the total error budget of the or-der of plusmn1 Finally the GPS and RES data were combinedto determine the ice thickness ice-surface and bed elevationdatasets Elevations are measured with reference to WGS 84The ice-surface elevation was calculated by subtracting ter-rain clearance from the height of the aircraft whereas thebed elevation was computed by subtracting the ice thicknessfrom the ice-surface elevation

42 OIB data processing

The OIB radar adopts SAR processing in the along-track di-rection to provide higher-resolution images of the subglacialprofile The data were processed in three steps to improvethe signal-to-noise ratio and increase the along-track reso-lution (Gogineni et al 2014) The raw data were first con-verted from a digital quantization level to a receiver voltagelevel The surface was captured using the low-gain data mi-crowave radar or laser altimeter A normalized matched filterwith frequency-domain windowing was then used for pulsecompression Two-dimensional SAR processing was used af-ter conditioning the data which is based on the frequency-wavenumber (F-K) algorithm The F-K SAR processing re-

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H Jeofry et al A new bed elevation model for the Weddell Sea sector 717

quires straight and uniformly sampled data however whichin the strictest sense are not usually met in the raw data sincethe aircraftrsquos speed is not consistent and its trajectory is notstraight The raw data were thus spatially resampled alongtrack using a sinc kernel to approximate a uniformly sampleddataset The vertical deviation in aircraft trajectory from thehorizontal flight path was compensated for in the frequencydomain with a time-delay phase shift The phase shift waslater removed for array processing as it is able to accountfor the non-uniform sampling the purpose is to maintain theoriginal geometry for the array processing Array processingwas performed in the cross-track flight path to reduce sur-face clutter as well as to improve the signal-to-noise ratioBoth the delay-and-sum and minimum variance distortion-less response (MVDR) beamformers were used to combinethe multichannel data and for regions with significant sur-face clutter the MVDR beamformer could effectively mini-mize the clutter power and pass the desired signal with opti-mum weights (Harry and Trees 2002)

43 Quantifying ice thickness bed topography andsubglacial water flow

The new ice thickness DEM was formed from the RES datausing the ldquoTopo to Rasterrdquo function in ArcGIS based on theAustralian National University DEM (ANUDEM) elevationgridding algorithm (Hutchinson 1988) This is the same al-gorithm employed by Bedmap2 The ice thickness DEM wasthen subtracted from the ice-sheet surface elevation DEM(from European Remote Sensing Satellite-1 (ERS-1) radarand Ice Cloud and land Elevation Satellite (ICESat) lasersatellite altimetry datasets Bamber et al 2009b) to derivethe bed topography The ice thickness ice-sheet surface andbed elevations were then gridded at a uniform 1 km spacingand referenced to the polar stereographic projection (Sny-der 1987) to form the new DEMs The Bedmap2 bed ele-vation product (Fretwell et al 2013) was transformed fromthe gl04c geoid projection to the WGS 1984 Antarctic PolarStereographic projection for comparison purposes A differ-ence map between the new DEM and the Bedmap2 prod-uct was computed by subtracting the Bedmap2 bed elevationDEM from the new bed elevation DEM Crossover analysisfor the 2006ndash2007 data onwards (including data acquired onflight lines beyond the extent of our DEM) shows the RMSerrors of 91 m (GRADESIMAGE) 158 m (IMAFI) 459 m(CRESIS 2012) 237 m (CRESIS 2014) and 203 m (CRE-SIS 2016)

Subglacial water flow paths were calculated based on thehydraulic potentiometric surface principle in which basalwater pressure is balanced by the ice overburden pressure asfollows

ϕ = g(ρwy+ ρih) (1)

where ϕ is the theoretical hydropotential surface y is the bedelevation h is the ice thickness ρw and ρi are the density of

water (1000 kg mminus3) and ice (920 kg mminus3) assuming ice tobe homogenous respectively and g is the gravitational con-stant (981 m sminus2) (Shreve 1972) Sinks in the hydrostaticpressure field raster were filled to produce realistic hydro-logic flow paths The flow direction of the raster was then de-fined by assigning each cell a direction to the steepest downs-lope neighbouring cell Sub-basins less than 200 km2 wereremoved due to the coarse input of bed topography and icethickness DEMs

44 Inferring bed elevation using mass conservation(MC) and kriging

Relying on the conservation of mass (MC) to infer the bedbetween flight lines (Morlighem et al 2011) we were ableto investigate how the bed can be developed further in fast-flowing regions using a new interpolation technique To per-form the MC procedure we used InSAR-derived surface ve-locities (Fig 1a) from Rignot et al (2011b) surface massbalance from RACMO 23 (Regional Atmospheric ClimateModel Van Wessem et al 2014) and assumed that the icethinningndashthickening rate and basal melt are negligible Weconstrained the optimization with ground-penetrating radarfrom CReSIS GRADES IMAFI and SPRI described aboveand used a mesh horizontal resolution of 500 m

5 Results

51 A new 1 km DEM of the WS sector

We present a new DEM of the WS sector of West Antarc-tica (Fig 3a) The Bedmap2 bed elevation and the differ-ence map are shown in Fig 3b and c respectively Thenew DEM contains substantial changes in certain regionscompared with Bedmap2 whereas in others there are con-sistencies between the two DEMs for example across theBungenstock ice rise where there are little new data Themean error between the two DEMs isminus8645 m indicating aslightly lower bed elevation in the new DEM data comparedto Bedmap2 which is likely the result of deep parts of the to-pography (ie valley bottoms) not being visible in the field-work non-SAR-processed quality-control (QC) radargramsfrom the IMAFI project (eg Horseshoe Valley near PatriotHills in the Ellsworth Mountains Winter 2016) The bed el-evation upstream of the Bungenstock ice rise and across theRobin Subglacial Basin shows a generally good agreementwith Bedmap2 with only small areas across the Moumlller IceStream and Pirrit Hills significantly different from Bedmap2with differences in bed elevation typically ranging betweenminus109 and 172 m (Fig 3c) There is however large disagree-ment between the two DEMs in the western region of In-stitute Ice Stream across the Ellsworth Mountains (eg inthe Horseshoe valley) the Foundation Ice Stream and to-wards East Antarctica where topography is more rugged Itis also worth noting the significant depth of the bed topogra-

wwwearth-syst-sci-datanet107112018 Earth Syst Sci Data 10 711ndash725 2018

718 H Jeofry et al A new bed elevation model for the Weddell Sea sector

Figure 3 (a) The new bed DEM for the WS sector (b) Bedmap2 bed elevation product (Fretwell et al 2013) pathways of subglacial waterare superimposed in (a) and (b) (c) Profiles AndashAprime BndashBprime CndashCprime and DndashDprime overlain by a map showing differences in bed elevation betweenthe new DEM and Bedmap2 (d) bed DEM inferred using mass conservation and kriging and (e) subglacial water pathways calculated withthe new DEM (blue) and Bedmap2 (red)

phy beneath the trunk of the Foundation Ice Stream wherea trough more than sim 2 km deep is located and delineated(Fig 3a) The trough issim 38 km wide andsim 80 km in lengthwith the deepest section sim 23 km below sea level The newDEM shows a significant change in the depiction of Founda-tion Trough we have measured it to be sim 1 km deeper andfar more extensive relative to the Bedmap2 product

In order to further quantify the differences betweenBedmap2 and our new DEM we present terrain profiles ofboth DEMs relative to four RES flight lines (Fig 3c) The

new DEM is consistent with the bed elevations from the RESdata picks compared to Bedmap2 (Fig 4a and b) The newDEMs show a correlation coefficient of 096 and 092 forProfile A and B respectively This is higher compared withBedmap2 which is 094 (Profile A) and 091 (Profile B) withrelative errors of 2 and 1 for Profile A and B respectivelyAlthough inaccuracies of the bed elevation persist across theFoundation Ice Stream for both DEMs the gross pattern ofthe bed elevation for the new DEM is more consistent withthe RES transects relative to Bedmap2 (Fig 4c and d) with

Earth Syst Sci Data 10 711ndash725 2018 wwwearth-syst-sci-datanet107112018

H Jeofry et al A new bed elevation model for the Weddell Sea sector 719

correlation coefficients of 097 and 094 for Profile C andD respectively These values contrast with correlation co-efficients from Bedmap2 of 087 for Profile C and 083 forProfile D with relative errors of 12 and 13 respectively

The new DEM can be refined further to deal with bumpsand irregularities associated with interpolation effects fromalong-track data in otherwise data-sparse regions We usedthe (MC) technique to infer the bed elevation (Fig 3d) be-neath the fast-moving ice similar to that employed in Green-land (Morlighem et al 2017) In general the bed elevationderived from MC and kriging is consistent with our newDEM However using MC significant changes in the bedmorphology beneath fast-flowing ice occur For example us-ing MC the tributary of the Foundation Ice Stream has beenextended for sim 100 km further inland relative to our newDEM

52 Hydrology

Computing the passageway of subglacial water beneath theice sheet is critical for comprehending ice-sheet dynamics(Bell 2008 Stearns et al 2008 Siegert et al 2016a) Thedevelopment of subglacial hydrology pathways is highly sen-sitive to ice-surface elevation and to a lesser degree to bedmorphology (Wright et al 2008 Horgan et al 2013) Fig-ure 3e shows a comparison of subglacial hydrology path-ways between our new bed DEM and the Bedmap2 DEMThe gross patterns of water flow are largely unchanged be-tween the two DEMs especially across and upstream of In-stitute and Moumlller ice streams The similar water pathwaypattern between both DEMs in these regions is also consis-tent with the small errors in bed topography (Fig 3c) De-spite large differences in bed topography across the Foun-dation Ice Stream and the Ellsworth Mountains region thelarge-scale patterns of water flow are also similar betweenboth DEMs due to the dominance of the ice-surface slopein driving basal water flow in these regions (Shreve 1972)Nonetheless there are several local small-scale differences inthe water pathways (Fig 3e) which highlight hydraulic sen-sitivity The subglacial water network observed in the newDEM across the Foundation Ice Stream appears to be morearborescent than that derived from Bedmap2 This is due tothe introduction of new data resulting in a better-defined bedacross the Foundation Trough (Fig 3a) The subglacial waterpathway observed in the new DEM adjacent to the ground-ing line across the Moumlller Ice Stream is in good agreementwith the position of sub-ice-shelf channels which have beendelineated from a combination of satellite images and RESdata (Le Brocq et al 2013)

Subglacial lakes discovered across the WS sector form anobvious component of the basal hydrological system (Wrightand Siegert 2012) These lakes exist due to sufficient amountof geothermal heating (50ndash70 m W mminus2) which allows thebase of the ice sheet to melt especially in areas of thick iceIn addition the pressure exerted on the bed by the over-

lying ice causes the melting point to be lowered West ofthe Ellsworth Mountains lies a body of the subglacial wa-ter known as Ellsworth Subglacial Lake The lake measures289 km2 with a depth ranging between 52 and 156 m capa-ble of carrying a water body volume of 137 km3 (Woodwardet al 2010) In some cases ice-surface elevation changeshave been linked with subglacial lake hydrological change(Siegert et al 2016a) ndash referred to as ldquoactiverdquo subglaciallakes There are four known active subglacial lakes dis-tributed across the Institute Ice Stream (Wright and Siegert2012) Institute W1 is located close to the Robin SubglacialBasin A whereas Institute W2 is located to the northeast ofPirrit Hills Institute E1 and E2 are located to the southwestof Robin Subglacial Basin B and near the field camp of C110respectively There are three active subglacial lakes in theFoundation Ice Stream catchment Foundation 1 Foundation2 and Foundation 3 (Wright and Siegert 2012) East of theFoundation Ice Stream there are 16 active subglacial lakesdistributed along the main trunk of Academy Glacier (Wrightand Siegert 2012)

53 Geomorphological description of the bedtopography

The WS sector of WAIS is composed of three major ice-sheetoutlets (Fig 1a and b) the Institute Moumlller and Foundationice streams feeding ice to the FRIS the second largest iceshelf in Antarctica Geophysical data in this area reveal fea-tures such as steep reverse bed slopes similar in scale to thatmeasured for upstream Thwaites Glacier close to the Insti-tute and Moumlller Ice Stream grounding lines The bed slopesinland to asim 18 km deep basin (the Robin Subglacial Basin)which is divided into two sections with few obvious signif-icant ice-sheet pinning points (Ross et al 2012) Elevatedbeds in other parts of the WS sector allow the ice shelf toground causing ice-surface features known as ice rises andrumples (Matsuoka et al 2015)

The Institute Ice Stream has three tributaries within oursurvey grid to the south and west of the Ellsworth Moun-tains occupying the Horseshoe Valley Independence andEllsworth troughs (Fig 1c) (Winter et al 2015) The Horse-shoe Valley Trough around 20 km wide and 13 km belowsea level at its deepest point is located downstream of thesteep mountains of the Heritage Range A subglacial ridge islocated between the mouth of the Horseshoe Valley Troughand the main trunk of the Institute Ice Stream (Winter et al2015) The Independence Trough is located subparallel to theHorseshoe Valley Trough separated by the 14 km high Inde-pendence Hills The trough is sim 22 km wide and is 11 kmbelow sea level at its deepest point It is characterized bytwo distinctive plateaus (sim 6 km wide each) on each side ofthe trough aligned alongside the main trough axis Ice flowseastward through the Independence Trough for sim 54 km be-fore it shifts to a northward direction where the troughwidens to 50 km and connects with the main Institute Ice

wwwearth-syst-sci-datanet107112018 Earth Syst Sci Data 10 711ndash725 2018

720 H Jeofry et al A new bed elevation model for the Weddell Sea sector

Figure 4 Bed elevations for RES transects (black) DEM (blue) and Bedmap2 (red) for (a) Profile AndashAprime (b) Profile BndashBprime (c) Profile CndashCprime

and (d) Profile DndashDprime

Stream The Ellsworth Trough is aligned with the Indepen-dence Trough and both are orthogonal to the orientation ofthe AmundsenndashWeddell ice divide dissecting the EllsworthSubglacial Highlands northwest to southeast The EllsworthTrough measures sim 34 km in width and is sim 2 km belowsea level at its deepest point and is sim 260 km in length Itis considered to be the largest and deepest trough-controlledtributary in this region (Winter et al 2015) The EllsworthTrough is intersected by several smaller valleys aligned per-pendicularly to the main axis which are relic landforms from

a previous small dynamic ice mass (predating the WAIS inits present configuration) (Ross et al 2014) The EllsworthTrough contains the sim 15ndash20 km long Ellsworth SubglacialLake (Siegert et al 2004a 2012 Woodward et al 2010Wright and Siegert 2012) It is worth noting that the lake isoutside the grid of our DEM

Satellite altimetry and imagery are able to estimate thegrounding line that separates the grounded ice sheet from thefloating ice shelf based on surface changes due to tidal os-cillations and the subtle ice-surface features Such analysis is

Earth Syst Sci Data 10 711ndash725 2018 wwwearth-syst-sci-datanet107112018

H Jeofry et al A new bed elevation model for the Weddell Sea sector 721

Table 1 Data files and locations

Products Files Location DOIURL

1 km bed elevation DEM 1 km bed elevation DEM Zenodo Data Repository httpsdoiorg105281zenodo1035488

1 km ice thickness DEM Digitized SPRIndashNSFndashTUD bed picks data

UK Polar Data Centre(UKPDC)

httpsdatabasacukmetadataphpid=GBNERCBASAEDC00326

BAS GRADESIMAGEice thickness data

UK Polar Data Centre httpssecureantarcticaacukdataaerogeo

BAS IMAFI ice thicknessdata

UK Polar Data Centre httpssecureantarcticaacukdataaerogeo

NASA Operation Ice-Bridge radar depthsounder level 2 (L2) data

Center for Remote Sens-ing of Ice Sheet (CReSIS)

httpsdatacresiskuedu

1 km ice-sheet-surfaceDEM

ERS-1 radar and ICESatlaser satellite altimetry

National Snow and IceData Center (NSIDC)

httpsnsidcorgdatadocsdaacnsidc0422antarctic1kmdem

Two-dimensional syntheticaperture radar (SAR)-processed radargrams

BAS IMAFI airborne sur-vey

UK Polar Data Centre httpsdoiorg1052858a975b9e-f18c-4c51-9bdb-b00b82da52b8

NASA Operation Ice-Bridge airborne survey

Center for Remote Sens-ing of Ice Sheet

httpsdatacresiskuedu

Ice velocity map of centralAntarctica

MEaSUREs InSAR-based ice velocity

National Snow and IceData Center

httpsdoiorg105067MEASURESCRYOSPHEREnsidc-0484001

Ice-sheet-surface satelliteimagery

MODIS Mosaic ofAntarctica (2008ndash2009)(MOA2009)

National Snow and IceData Center

httpsdoiorg107265N5KP8037

RADARSAT (25 m) satel-lite imagery

Byrd Polar and ClimateResearch Center

httpsresearchbpcrcosuedurslradarsatdata

prone to uncertainty however There are currently four pro-posed grounding-line locations based on different satellitedatasets andor methods of analysis (Bohlander and Scam-bos 2007 Bindschadler et al 2011 Brunt et al 2011 Rig-not et al 2011c) Each of the grounding lines were delin-eated from satellite images but without direct measurementof the subglacial environment This results in ambiguities forthe grounding-line location (Jeofry et al 2017a) In addi-tion RES data have demonstrated clear errors in the positionof the grounding line with a large hitherto unknown sub-glacial embayment near the Institute Ice Stream groundingline The subglacial embayment issim 1 km deep and is poten-tially open to the ice shelf cavity causing the inland ice sheetto have a direct contact with ocean water Our RES analysisalso reveals a better-defined Foundation Trough in which thegrounding line is perched on very deep topography around2 km below sea level

A previous study revealed a series of ancient large sub-parallel subglacial bed channels between Moumlller Ice Streamand Foundation Ice Stream adjacent to the marginal basins(Fig 1b) (Rose et al 2014) While these subglacial channelsare likely to have been formed by the flow of basal water theyare presently located beneath slow-moving and cold-basedice It is thought therefore that the channels are ancient andwere formed at a time when surface melting was prevalent inWest Antarctica (eg the Pliocene)

The bed topography of the WS appears both rough (overthe mountains and exposed bedrock) and smooth (across the

sediment-filled regions) (Bingham and Siegert 2007) Stud-ies of bed roughness calculated using the fast Fourier trans-form (FFT) technique based on the relative measurement ofbed obstacle amplitude and frequency of the roughness ob-stacles have indicated that the Institute Ice Stream and MoumlllerIce Stream are dominated by relatively low roughness val-ues less than 01 (Bingham and Siegert 2007 Rippin etal 2014) which was suggested as being the result of theemplacement of marine sediments as in the Siple Coast re-gion (Siegert et al 2004b Peters et al 2005) Radar-derivedroughness analysis has evidenced a smooth bed across theRobin Subglacial Basin where sediments may exist (Rippinet al 2014) The deepest parts of the Robin Subglacial Basinare anomalously rough marking the edge of a sedimentarydrape where the highest ice flow velocities are generated(Siegert et al 2016b) As such the smooth basal topogra-phy of the Institute and Moumlller Ice Stream catchments is lessextensive than proposed by Bingham and Siegert (2007) Thesubglacial topography of the region between the Robin Sub-glacial Basin and the Pirrit and MartinndashNash Hills is rela-tively flat smooth and gently sloping and has been inter-preted as a bedrock planation surface (Rose et al 2015)Although the exact formation process of the planation sur-face is unknown it is thought that this geomorphological fea-ture formed due to marine andor fluvial erosion (Rose et al2015)

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H Jeofry et al A new bed elevation model for the Weddell Sea sector 717

quires straight and uniformly sampled data however whichin the strictest sense are not usually met in the raw data sincethe aircraftrsquos speed is not consistent and its trajectory is notstraight The raw data were thus spatially resampled alongtrack using a sinc kernel to approximate a uniformly sampleddataset The vertical deviation in aircraft trajectory from thehorizontal flight path was compensated for in the frequencydomain with a time-delay phase shift The phase shift waslater removed for array processing as it is able to accountfor the non-uniform sampling the purpose is to maintain theoriginal geometry for the array processing Array processingwas performed in the cross-track flight path to reduce sur-face clutter as well as to improve the signal-to-noise ratioBoth the delay-and-sum and minimum variance distortion-less response (MVDR) beamformers were used to combinethe multichannel data and for regions with significant sur-face clutter the MVDR beamformer could effectively mini-mize the clutter power and pass the desired signal with opti-mum weights (Harry and Trees 2002)

43 Quantifying ice thickness bed topography andsubglacial water flow

The new ice thickness DEM was formed from the RES datausing the ldquoTopo to Rasterrdquo function in ArcGIS based on theAustralian National University DEM (ANUDEM) elevationgridding algorithm (Hutchinson 1988) This is the same al-gorithm employed by Bedmap2 The ice thickness DEM wasthen subtracted from the ice-sheet surface elevation DEM(from European Remote Sensing Satellite-1 (ERS-1) radarand Ice Cloud and land Elevation Satellite (ICESat) lasersatellite altimetry datasets Bamber et al 2009b) to derivethe bed topography The ice thickness ice-sheet surface andbed elevations were then gridded at a uniform 1 km spacingand referenced to the polar stereographic projection (Sny-der 1987) to form the new DEMs The Bedmap2 bed ele-vation product (Fretwell et al 2013) was transformed fromthe gl04c geoid projection to the WGS 1984 Antarctic PolarStereographic projection for comparison purposes A differ-ence map between the new DEM and the Bedmap2 prod-uct was computed by subtracting the Bedmap2 bed elevationDEM from the new bed elevation DEM Crossover analysisfor the 2006ndash2007 data onwards (including data acquired onflight lines beyond the extent of our DEM) shows the RMSerrors of 91 m (GRADESIMAGE) 158 m (IMAFI) 459 m(CRESIS 2012) 237 m (CRESIS 2014) and 203 m (CRE-SIS 2016)

Subglacial water flow paths were calculated based on thehydraulic potentiometric surface principle in which basalwater pressure is balanced by the ice overburden pressure asfollows

ϕ = g(ρwy+ ρih) (1)

where ϕ is the theoretical hydropotential surface y is the bedelevation h is the ice thickness ρw and ρi are the density of

water (1000 kg mminus3) and ice (920 kg mminus3) assuming ice tobe homogenous respectively and g is the gravitational con-stant (981 m sminus2) (Shreve 1972) Sinks in the hydrostaticpressure field raster were filled to produce realistic hydro-logic flow paths The flow direction of the raster was then de-fined by assigning each cell a direction to the steepest downs-lope neighbouring cell Sub-basins less than 200 km2 wereremoved due to the coarse input of bed topography and icethickness DEMs

44 Inferring bed elevation using mass conservation(MC) and kriging

Relying on the conservation of mass (MC) to infer the bedbetween flight lines (Morlighem et al 2011) we were ableto investigate how the bed can be developed further in fast-flowing regions using a new interpolation technique To per-form the MC procedure we used InSAR-derived surface ve-locities (Fig 1a) from Rignot et al (2011b) surface massbalance from RACMO 23 (Regional Atmospheric ClimateModel Van Wessem et al 2014) and assumed that the icethinningndashthickening rate and basal melt are negligible Weconstrained the optimization with ground-penetrating radarfrom CReSIS GRADES IMAFI and SPRI described aboveand used a mesh horizontal resolution of 500 m

5 Results

51 A new 1 km DEM of the WS sector

We present a new DEM of the WS sector of West Antarc-tica (Fig 3a) The Bedmap2 bed elevation and the differ-ence map are shown in Fig 3b and c respectively Thenew DEM contains substantial changes in certain regionscompared with Bedmap2 whereas in others there are con-sistencies between the two DEMs for example across theBungenstock ice rise where there are little new data Themean error between the two DEMs isminus8645 m indicating aslightly lower bed elevation in the new DEM data comparedto Bedmap2 which is likely the result of deep parts of the to-pography (ie valley bottoms) not being visible in the field-work non-SAR-processed quality-control (QC) radargramsfrom the IMAFI project (eg Horseshoe Valley near PatriotHills in the Ellsworth Mountains Winter 2016) The bed el-evation upstream of the Bungenstock ice rise and across theRobin Subglacial Basin shows a generally good agreementwith Bedmap2 with only small areas across the Moumlller IceStream and Pirrit Hills significantly different from Bedmap2with differences in bed elevation typically ranging betweenminus109 and 172 m (Fig 3c) There is however large disagree-ment between the two DEMs in the western region of In-stitute Ice Stream across the Ellsworth Mountains (eg inthe Horseshoe valley) the Foundation Ice Stream and to-wards East Antarctica where topography is more rugged Itis also worth noting the significant depth of the bed topogra-

wwwearth-syst-sci-datanet107112018 Earth Syst Sci Data 10 711ndash725 2018

718 H Jeofry et al A new bed elevation model for the Weddell Sea sector

Figure 3 (a) The new bed DEM for the WS sector (b) Bedmap2 bed elevation product (Fretwell et al 2013) pathways of subglacial waterare superimposed in (a) and (b) (c) Profiles AndashAprime BndashBprime CndashCprime and DndashDprime overlain by a map showing differences in bed elevation betweenthe new DEM and Bedmap2 (d) bed DEM inferred using mass conservation and kriging and (e) subglacial water pathways calculated withthe new DEM (blue) and Bedmap2 (red)

phy beneath the trunk of the Foundation Ice Stream wherea trough more than sim 2 km deep is located and delineated(Fig 3a) The trough issim 38 km wide andsim 80 km in lengthwith the deepest section sim 23 km below sea level The newDEM shows a significant change in the depiction of Founda-tion Trough we have measured it to be sim 1 km deeper andfar more extensive relative to the Bedmap2 product

In order to further quantify the differences betweenBedmap2 and our new DEM we present terrain profiles ofboth DEMs relative to four RES flight lines (Fig 3c) The

new DEM is consistent with the bed elevations from the RESdata picks compared to Bedmap2 (Fig 4a and b) The newDEMs show a correlation coefficient of 096 and 092 forProfile A and B respectively This is higher compared withBedmap2 which is 094 (Profile A) and 091 (Profile B) withrelative errors of 2 and 1 for Profile A and B respectivelyAlthough inaccuracies of the bed elevation persist across theFoundation Ice Stream for both DEMs the gross pattern ofthe bed elevation for the new DEM is more consistent withthe RES transects relative to Bedmap2 (Fig 4c and d) with

Earth Syst Sci Data 10 711ndash725 2018 wwwearth-syst-sci-datanet107112018

H Jeofry et al A new bed elevation model for the Weddell Sea sector 719

correlation coefficients of 097 and 094 for Profile C andD respectively These values contrast with correlation co-efficients from Bedmap2 of 087 for Profile C and 083 forProfile D with relative errors of 12 and 13 respectively

The new DEM can be refined further to deal with bumpsand irregularities associated with interpolation effects fromalong-track data in otherwise data-sparse regions We usedthe (MC) technique to infer the bed elevation (Fig 3d) be-neath the fast-moving ice similar to that employed in Green-land (Morlighem et al 2017) In general the bed elevationderived from MC and kriging is consistent with our newDEM However using MC significant changes in the bedmorphology beneath fast-flowing ice occur For example us-ing MC the tributary of the Foundation Ice Stream has beenextended for sim 100 km further inland relative to our newDEM

52 Hydrology

Computing the passageway of subglacial water beneath theice sheet is critical for comprehending ice-sheet dynamics(Bell 2008 Stearns et al 2008 Siegert et al 2016a) Thedevelopment of subglacial hydrology pathways is highly sen-sitive to ice-surface elevation and to a lesser degree to bedmorphology (Wright et al 2008 Horgan et al 2013) Fig-ure 3e shows a comparison of subglacial hydrology path-ways between our new bed DEM and the Bedmap2 DEMThe gross patterns of water flow are largely unchanged be-tween the two DEMs especially across and upstream of In-stitute and Moumlller ice streams The similar water pathwaypattern between both DEMs in these regions is also consis-tent with the small errors in bed topography (Fig 3c) De-spite large differences in bed topography across the Foun-dation Ice Stream and the Ellsworth Mountains region thelarge-scale patterns of water flow are also similar betweenboth DEMs due to the dominance of the ice-surface slopein driving basal water flow in these regions (Shreve 1972)Nonetheless there are several local small-scale differences inthe water pathways (Fig 3e) which highlight hydraulic sen-sitivity The subglacial water network observed in the newDEM across the Foundation Ice Stream appears to be morearborescent than that derived from Bedmap2 This is due tothe introduction of new data resulting in a better-defined bedacross the Foundation Trough (Fig 3a) The subglacial waterpathway observed in the new DEM adjacent to the ground-ing line across the Moumlller Ice Stream is in good agreementwith the position of sub-ice-shelf channels which have beendelineated from a combination of satellite images and RESdata (Le Brocq et al 2013)

Subglacial lakes discovered across the WS sector form anobvious component of the basal hydrological system (Wrightand Siegert 2012) These lakes exist due to sufficient amountof geothermal heating (50ndash70 m W mminus2) which allows thebase of the ice sheet to melt especially in areas of thick iceIn addition the pressure exerted on the bed by the over-

lying ice causes the melting point to be lowered West ofthe Ellsworth Mountains lies a body of the subglacial wa-ter known as Ellsworth Subglacial Lake The lake measures289 km2 with a depth ranging between 52 and 156 m capa-ble of carrying a water body volume of 137 km3 (Woodwardet al 2010) In some cases ice-surface elevation changeshave been linked with subglacial lake hydrological change(Siegert et al 2016a) ndash referred to as ldquoactiverdquo subglaciallakes There are four known active subglacial lakes dis-tributed across the Institute Ice Stream (Wright and Siegert2012) Institute W1 is located close to the Robin SubglacialBasin A whereas Institute W2 is located to the northeast ofPirrit Hills Institute E1 and E2 are located to the southwestof Robin Subglacial Basin B and near the field camp of C110respectively There are three active subglacial lakes in theFoundation Ice Stream catchment Foundation 1 Foundation2 and Foundation 3 (Wright and Siegert 2012) East of theFoundation Ice Stream there are 16 active subglacial lakesdistributed along the main trunk of Academy Glacier (Wrightand Siegert 2012)

53 Geomorphological description of the bedtopography

The WS sector of WAIS is composed of three major ice-sheetoutlets (Fig 1a and b) the Institute Moumlller and Foundationice streams feeding ice to the FRIS the second largest iceshelf in Antarctica Geophysical data in this area reveal fea-tures such as steep reverse bed slopes similar in scale to thatmeasured for upstream Thwaites Glacier close to the Insti-tute and Moumlller Ice Stream grounding lines The bed slopesinland to asim 18 km deep basin (the Robin Subglacial Basin)which is divided into two sections with few obvious signif-icant ice-sheet pinning points (Ross et al 2012) Elevatedbeds in other parts of the WS sector allow the ice shelf toground causing ice-surface features known as ice rises andrumples (Matsuoka et al 2015)

The Institute Ice Stream has three tributaries within oursurvey grid to the south and west of the Ellsworth Moun-tains occupying the Horseshoe Valley Independence andEllsworth troughs (Fig 1c) (Winter et al 2015) The Horse-shoe Valley Trough around 20 km wide and 13 km belowsea level at its deepest point is located downstream of thesteep mountains of the Heritage Range A subglacial ridge islocated between the mouth of the Horseshoe Valley Troughand the main trunk of the Institute Ice Stream (Winter et al2015) The Independence Trough is located subparallel to theHorseshoe Valley Trough separated by the 14 km high Inde-pendence Hills The trough is sim 22 km wide and is 11 kmbelow sea level at its deepest point It is characterized bytwo distinctive plateaus (sim 6 km wide each) on each side ofthe trough aligned alongside the main trough axis Ice flowseastward through the Independence Trough for sim 54 km be-fore it shifts to a northward direction where the troughwidens to 50 km and connects with the main Institute Ice

wwwearth-syst-sci-datanet107112018 Earth Syst Sci Data 10 711ndash725 2018

720 H Jeofry et al A new bed elevation model for the Weddell Sea sector

Figure 4 Bed elevations for RES transects (black) DEM (blue) and Bedmap2 (red) for (a) Profile AndashAprime (b) Profile BndashBprime (c) Profile CndashCprime

and (d) Profile DndashDprime

Stream The Ellsworth Trough is aligned with the Indepen-dence Trough and both are orthogonal to the orientation ofthe AmundsenndashWeddell ice divide dissecting the EllsworthSubglacial Highlands northwest to southeast The EllsworthTrough measures sim 34 km in width and is sim 2 km belowsea level at its deepest point and is sim 260 km in length Itis considered to be the largest and deepest trough-controlledtributary in this region (Winter et al 2015) The EllsworthTrough is intersected by several smaller valleys aligned per-pendicularly to the main axis which are relic landforms from

a previous small dynamic ice mass (predating the WAIS inits present configuration) (Ross et al 2014) The EllsworthTrough contains the sim 15ndash20 km long Ellsworth SubglacialLake (Siegert et al 2004a 2012 Woodward et al 2010Wright and Siegert 2012) It is worth noting that the lake isoutside the grid of our DEM

Satellite altimetry and imagery are able to estimate thegrounding line that separates the grounded ice sheet from thefloating ice shelf based on surface changes due to tidal os-cillations and the subtle ice-surface features Such analysis is

Earth Syst Sci Data 10 711ndash725 2018 wwwearth-syst-sci-datanet107112018

H Jeofry et al A new bed elevation model for the Weddell Sea sector 721

Table 1 Data files and locations

Products Files Location DOIURL

1 km bed elevation DEM 1 km bed elevation DEM Zenodo Data Repository httpsdoiorg105281zenodo1035488

1 km ice thickness DEM Digitized SPRIndashNSFndashTUD bed picks data

UK Polar Data Centre(UKPDC)

httpsdatabasacukmetadataphpid=GBNERCBASAEDC00326

BAS GRADESIMAGEice thickness data

UK Polar Data Centre httpssecureantarcticaacukdataaerogeo

BAS IMAFI ice thicknessdata

UK Polar Data Centre httpssecureantarcticaacukdataaerogeo

NASA Operation Ice-Bridge radar depthsounder level 2 (L2) data

Center for Remote Sens-ing of Ice Sheet (CReSIS)

httpsdatacresiskuedu

1 km ice-sheet-surfaceDEM

ERS-1 radar and ICESatlaser satellite altimetry

National Snow and IceData Center (NSIDC)

httpsnsidcorgdatadocsdaacnsidc0422antarctic1kmdem

Two-dimensional syntheticaperture radar (SAR)-processed radargrams

BAS IMAFI airborne sur-vey

UK Polar Data Centre httpsdoiorg1052858a975b9e-f18c-4c51-9bdb-b00b82da52b8

NASA Operation Ice-Bridge airborne survey

Center for Remote Sens-ing of Ice Sheet

httpsdatacresiskuedu

Ice velocity map of centralAntarctica

MEaSUREs InSAR-based ice velocity

National Snow and IceData Center

httpsdoiorg105067MEASURESCRYOSPHEREnsidc-0484001

Ice-sheet-surface satelliteimagery

MODIS Mosaic ofAntarctica (2008ndash2009)(MOA2009)

National Snow and IceData Center

httpsdoiorg107265N5KP8037

RADARSAT (25 m) satel-lite imagery

Byrd Polar and ClimateResearch Center

httpsresearchbpcrcosuedurslradarsatdata

prone to uncertainty however There are currently four pro-posed grounding-line locations based on different satellitedatasets andor methods of analysis (Bohlander and Scam-bos 2007 Bindschadler et al 2011 Brunt et al 2011 Rig-not et al 2011c) Each of the grounding lines were delin-eated from satellite images but without direct measurementof the subglacial environment This results in ambiguities forthe grounding-line location (Jeofry et al 2017a) In addi-tion RES data have demonstrated clear errors in the positionof the grounding line with a large hitherto unknown sub-glacial embayment near the Institute Ice Stream groundingline The subglacial embayment issim 1 km deep and is poten-tially open to the ice shelf cavity causing the inland ice sheetto have a direct contact with ocean water Our RES analysisalso reveals a better-defined Foundation Trough in which thegrounding line is perched on very deep topography around2 km below sea level

A previous study revealed a series of ancient large sub-parallel subglacial bed channels between Moumlller Ice Streamand Foundation Ice Stream adjacent to the marginal basins(Fig 1b) (Rose et al 2014) While these subglacial channelsare likely to have been formed by the flow of basal water theyare presently located beneath slow-moving and cold-basedice It is thought therefore that the channels are ancient andwere formed at a time when surface melting was prevalent inWest Antarctica (eg the Pliocene)

The bed topography of the WS appears both rough (overthe mountains and exposed bedrock) and smooth (across the

sediment-filled regions) (Bingham and Siegert 2007) Stud-ies of bed roughness calculated using the fast Fourier trans-form (FFT) technique based on the relative measurement ofbed obstacle amplitude and frequency of the roughness ob-stacles have indicated that the Institute Ice Stream and MoumlllerIce Stream are dominated by relatively low roughness val-ues less than 01 (Bingham and Siegert 2007 Rippin etal 2014) which was suggested as being the result of theemplacement of marine sediments as in the Siple Coast re-gion (Siegert et al 2004b Peters et al 2005) Radar-derivedroughness analysis has evidenced a smooth bed across theRobin Subglacial Basin where sediments may exist (Rippinet al 2014) The deepest parts of the Robin Subglacial Basinare anomalously rough marking the edge of a sedimentarydrape where the highest ice flow velocities are generated(Siegert et al 2016b) As such the smooth basal topogra-phy of the Institute and Moumlller Ice Stream catchments is lessextensive than proposed by Bingham and Siegert (2007) Thesubglacial topography of the region between the Robin Sub-glacial Basin and the Pirrit and MartinndashNash Hills is rela-tively flat smooth and gently sloping and has been inter-preted as a bedrock planation surface (Rose et al 2015)Although the exact formation process of the planation sur-face is unknown it is thought that this geomorphological fea-ture formed due to marine andor fluvial erosion (Rose et al2015)

wwwearth-syst-sci-datanet107112018 Earth Syst Sci Data 10 711ndash725 2018

718 H Jeofry et al A new bed elevation model for the Weddell Sea sector

Figure 3 (a) The new bed DEM for the WS sector (b) Bedmap2 bed elevation product (Fretwell et al 2013) pathways of subglacial waterare superimposed in (a) and (b) (c) Profiles AndashAprime BndashBprime CndashCprime and DndashDprime overlain by a map showing differences in bed elevation betweenthe new DEM and Bedmap2 (d) bed DEM inferred using mass conservation and kriging and (e) subglacial water pathways calculated withthe new DEM (blue) and Bedmap2 (red)

phy beneath the trunk of the Foundation Ice Stream wherea trough more than sim 2 km deep is located and delineated(Fig 3a) The trough issim 38 km wide andsim 80 km in lengthwith the deepest section sim 23 km below sea level The newDEM shows a significant change in the depiction of Founda-tion Trough we have measured it to be sim 1 km deeper andfar more extensive relative to the Bedmap2 product

In order to further quantify the differences betweenBedmap2 and our new DEM we present terrain profiles ofboth DEMs relative to four RES flight lines (Fig 3c) The

new DEM is consistent with the bed elevations from the RESdata picks compared to Bedmap2 (Fig 4a and b) The newDEMs show a correlation coefficient of 096 and 092 forProfile A and B respectively This is higher compared withBedmap2 which is 094 (Profile A) and 091 (Profile B) withrelative errors of 2 and 1 for Profile A and B respectivelyAlthough inaccuracies of the bed elevation persist across theFoundation Ice Stream for both DEMs the gross pattern ofthe bed elevation for the new DEM is more consistent withthe RES transects relative to Bedmap2 (Fig 4c and d) with

Earth Syst Sci Data 10 711ndash725 2018 wwwearth-syst-sci-datanet107112018

H Jeofry et al A new bed elevation model for the Weddell Sea sector 719

correlation coefficients of 097 and 094 for Profile C andD respectively These values contrast with correlation co-efficients from Bedmap2 of 087 for Profile C and 083 forProfile D with relative errors of 12 and 13 respectively

The new DEM can be refined further to deal with bumpsand irregularities associated with interpolation effects fromalong-track data in otherwise data-sparse regions We usedthe (MC) technique to infer the bed elevation (Fig 3d) be-neath the fast-moving ice similar to that employed in Green-land (Morlighem et al 2017) In general the bed elevationderived from MC and kriging is consistent with our newDEM However using MC significant changes in the bedmorphology beneath fast-flowing ice occur For example us-ing MC the tributary of the Foundation Ice Stream has beenextended for sim 100 km further inland relative to our newDEM

52 Hydrology

Computing the passageway of subglacial water beneath theice sheet is critical for comprehending ice-sheet dynamics(Bell 2008 Stearns et al 2008 Siegert et al 2016a) Thedevelopment of subglacial hydrology pathways is highly sen-sitive to ice-surface elevation and to a lesser degree to bedmorphology (Wright et al 2008 Horgan et al 2013) Fig-ure 3e shows a comparison of subglacial hydrology path-ways between our new bed DEM and the Bedmap2 DEMThe gross patterns of water flow are largely unchanged be-tween the two DEMs especially across and upstream of In-stitute and Moumlller ice streams The similar water pathwaypattern between both DEMs in these regions is also consis-tent with the small errors in bed topography (Fig 3c) De-spite large differences in bed topography across the Foun-dation Ice Stream and the Ellsworth Mountains region thelarge-scale patterns of water flow are also similar betweenboth DEMs due to the dominance of the ice-surface slopein driving basal water flow in these regions (Shreve 1972)Nonetheless there are several local small-scale differences inthe water pathways (Fig 3e) which highlight hydraulic sen-sitivity The subglacial water network observed in the newDEM across the Foundation Ice Stream appears to be morearborescent than that derived from Bedmap2 This is due tothe introduction of new data resulting in a better-defined bedacross the Foundation Trough (Fig 3a) The subglacial waterpathway observed in the new DEM adjacent to the ground-ing line across the Moumlller Ice Stream is in good agreementwith the position of sub-ice-shelf channels which have beendelineated from a combination of satellite images and RESdata (Le Brocq et al 2013)

Subglacial lakes discovered across the WS sector form anobvious component of the basal hydrological system (Wrightand Siegert 2012) These lakes exist due to sufficient amountof geothermal heating (50ndash70 m W mminus2) which allows thebase of the ice sheet to melt especially in areas of thick iceIn addition the pressure exerted on the bed by the over-

lying ice causes the melting point to be lowered West ofthe Ellsworth Mountains lies a body of the subglacial wa-ter known as Ellsworth Subglacial Lake The lake measures289 km2 with a depth ranging between 52 and 156 m capa-ble of carrying a water body volume of 137 km3 (Woodwardet al 2010) In some cases ice-surface elevation changeshave been linked with subglacial lake hydrological change(Siegert et al 2016a) ndash referred to as ldquoactiverdquo subglaciallakes There are four known active subglacial lakes dis-tributed across the Institute Ice Stream (Wright and Siegert2012) Institute W1 is located close to the Robin SubglacialBasin A whereas Institute W2 is located to the northeast ofPirrit Hills Institute E1 and E2 are located to the southwestof Robin Subglacial Basin B and near the field camp of C110respectively There are three active subglacial lakes in theFoundation Ice Stream catchment Foundation 1 Foundation2 and Foundation 3 (Wright and Siegert 2012) East of theFoundation Ice Stream there are 16 active subglacial lakesdistributed along the main trunk of Academy Glacier (Wrightand Siegert 2012)

53 Geomorphological description of the bedtopography

The WS sector of WAIS is composed of three major ice-sheetoutlets (Fig 1a and b) the Institute Moumlller and Foundationice streams feeding ice to the FRIS the second largest iceshelf in Antarctica Geophysical data in this area reveal fea-tures such as steep reverse bed slopes similar in scale to thatmeasured for upstream Thwaites Glacier close to the Insti-tute and Moumlller Ice Stream grounding lines The bed slopesinland to asim 18 km deep basin (the Robin Subglacial Basin)which is divided into two sections with few obvious signif-icant ice-sheet pinning points (Ross et al 2012) Elevatedbeds in other parts of the WS sector allow the ice shelf toground causing ice-surface features known as ice rises andrumples (Matsuoka et al 2015)

The Institute Ice Stream has three tributaries within oursurvey grid to the south and west of the Ellsworth Moun-tains occupying the Horseshoe Valley Independence andEllsworth troughs (Fig 1c) (Winter et al 2015) The Horse-shoe Valley Trough around 20 km wide and 13 km belowsea level at its deepest point is located downstream of thesteep mountains of the Heritage Range A subglacial ridge islocated between the mouth of the Horseshoe Valley Troughand the main trunk of the Institute Ice Stream (Winter et al2015) The Independence Trough is located subparallel to theHorseshoe Valley Trough separated by the 14 km high Inde-pendence Hills The trough is sim 22 km wide and is 11 kmbelow sea level at its deepest point It is characterized bytwo distinctive plateaus (sim 6 km wide each) on each side ofthe trough aligned alongside the main trough axis Ice flowseastward through the Independence Trough for sim 54 km be-fore it shifts to a northward direction where the troughwidens to 50 km and connects with the main Institute Ice

wwwearth-syst-sci-datanet107112018 Earth Syst Sci Data 10 711ndash725 2018

720 H Jeofry et al A new bed elevation model for the Weddell Sea sector

Figure 4 Bed elevations for RES transects (black) DEM (blue) and Bedmap2 (red) for (a) Profile AndashAprime (b) Profile BndashBprime (c) Profile CndashCprime

and (d) Profile DndashDprime

Stream The Ellsworth Trough is aligned with the Indepen-dence Trough and both are orthogonal to the orientation ofthe AmundsenndashWeddell ice divide dissecting the EllsworthSubglacial Highlands northwest to southeast The EllsworthTrough measures sim 34 km in width and is sim 2 km belowsea level at its deepest point and is sim 260 km in length Itis considered to be the largest and deepest trough-controlledtributary in this region (Winter et al 2015) The EllsworthTrough is intersected by several smaller valleys aligned per-pendicularly to the main axis which are relic landforms from

a previous small dynamic ice mass (predating the WAIS inits present configuration) (Ross et al 2014) The EllsworthTrough contains the sim 15ndash20 km long Ellsworth SubglacialLake (Siegert et al 2004a 2012 Woodward et al 2010Wright and Siegert 2012) It is worth noting that the lake isoutside the grid of our DEM

Satellite altimetry and imagery are able to estimate thegrounding line that separates the grounded ice sheet from thefloating ice shelf based on surface changes due to tidal os-cillations and the subtle ice-surface features Such analysis is

Earth Syst Sci Data 10 711ndash725 2018 wwwearth-syst-sci-datanet107112018

H Jeofry et al A new bed elevation model for the Weddell Sea sector 721

Table 1 Data files and locations

Products Files Location DOIURL

1 km bed elevation DEM 1 km bed elevation DEM Zenodo Data Repository httpsdoiorg105281zenodo1035488

1 km ice thickness DEM Digitized SPRIndashNSFndashTUD bed picks data

UK Polar Data Centre(UKPDC)

httpsdatabasacukmetadataphpid=GBNERCBASAEDC00326

BAS GRADESIMAGEice thickness data

UK Polar Data Centre httpssecureantarcticaacukdataaerogeo

BAS IMAFI ice thicknessdata

UK Polar Data Centre httpssecureantarcticaacukdataaerogeo

NASA Operation Ice-Bridge radar depthsounder level 2 (L2) data

Center for Remote Sens-ing of Ice Sheet (CReSIS)

httpsdatacresiskuedu

1 km ice-sheet-surfaceDEM

ERS-1 radar and ICESatlaser satellite altimetry

National Snow and IceData Center (NSIDC)

httpsnsidcorgdatadocsdaacnsidc0422antarctic1kmdem

Two-dimensional syntheticaperture radar (SAR)-processed radargrams

BAS IMAFI airborne sur-vey

UK Polar Data Centre httpsdoiorg1052858a975b9e-f18c-4c51-9bdb-b00b82da52b8

NASA Operation Ice-Bridge airborne survey

Center for Remote Sens-ing of Ice Sheet

httpsdatacresiskuedu

Ice velocity map of centralAntarctica

MEaSUREs InSAR-based ice velocity

National Snow and IceData Center

httpsdoiorg105067MEASURESCRYOSPHEREnsidc-0484001

Ice-sheet-surface satelliteimagery

MODIS Mosaic ofAntarctica (2008ndash2009)(MOA2009)

National Snow and IceData Center

httpsdoiorg107265N5KP8037

RADARSAT (25 m) satel-lite imagery

Byrd Polar and ClimateResearch Center

httpsresearchbpcrcosuedurslradarsatdata

prone to uncertainty however There are currently four pro-posed grounding-line locations based on different satellitedatasets andor methods of analysis (Bohlander and Scam-bos 2007 Bindschadler et al 2011 Brunt et al 2011 Rig-not et al 2011c) Each of the grounding lines were delin-eated from satellite images but without direct measurementof the subglacial environment This results in ambiguities forthe grounding-line location (Jeofry et al 2017a) In addi-tion RES data have demonstrated clear errors in the positionof the grounding line with a large hitherto unknown sub-glacial embayment near the Institute Ice Stream groundingline The subglacial embayment issim 1 km deep and is poten-tially open to the ice shelf cavity causing the inland ice sheetto have a direct contact with ocean water Our RES analysisalso reveals a better-defined Foundation Trough in which thegrounding line is perched on very deep topography around2 km below sea level

A previous study revealed a series of ancient large sub-parallel subglacial bed channels between Moumlller Ice Streamand Foundation Ice Stream adjacent to the marginal basins(Fig 1b) (Rose et al 2014) While these subglacial channelsare likely to have been formed by the flow of basal water theyare presently located beneath slow-moving and cold-basedice It is thought therefore that the channels are ancient andwere formed at a time when surface melting was prevalent inWest Antarctica (eg the Pliocene)

The bed topography of the WS appears both rough (overthe mountains and exposed bedrock) and smooth (across the

sediment-filled regions) (Bingham and Siegert 2007) Stud-ies of bed roughness calculated using the fast Fourier trans-form (FFT) technique based on the relative measurement ofbed obstacle amplitude and frequency of the roughness ob-stacles have indicated that the Institute Ice Stream and MoumlllerIce Stream are dominated by relatively low roughness val-ues less than 01 (Bingham and Siegert 2007 Rippin etal 2014) which was suggested as being the result of theemplacement of marine sediments as in the Siple Coast re-gion (Siegert et al 2004b Peters et al 2005) Radar-derivedroughness analysis has evidenced a smooth bed across theRobin Subglacial Basin where sediments may exist (Rippinet al 2014) The deepest parts of the Robin Subglacial Basinare anomalously rough marking the edge of a sedimentarydrape where the highest ice flow velocities are generated(Siegert et al 2016b) As such the smooth basal topogra-phy of the Institute and Moumlller Ice Stream catchments is lessextensive than proposed by Bingham and Siegert (2007) Thesubglacial topography of the region between the Robin Sub-glacial Basin and the Pirrit and MartinndashNash Hills is rela-tively flat smooth and gently sloping and has been inter-preted as a bedrock planation surface (Rose et al 2015)Although the exact formation process of the planation sur-face is unknown it is thought that this geomorphological fea-ture formed due to marine andor fluvial erosion (Rose et al2015)

wwwearth-syst-sci-datanet107112018 Earth Syst Sci Data 10 711ndash725 2018

H Jeofry et al A new bed elevation model for the Weddell Sea sector 719

correlation coefficients of 097 and 094 for Profile C andD respectively These values contrast with correlation co-efficients from Bedmap2 of 087 for Profile C and 083 forProfile D with relative errors of 12 and 13 respectively

The new DEM can be refined further to deal with bumpsand irregularities associated with interpolation effects fromalong-track data in otherwise data-sparse regions We usedthe (MC) technique to infer the bed elevation (Fig 3d) be-neath the fast-moving ice similar to that employed in Green-land (Morlighem et al 2017) In general the bed elevationderived from MC and kriging is consistent with our newDEM However using MC significant changes in the bedmorphology beneath fast-flowing ice occur For example us-ing MC the tributary of the Foundation Ice Stream has beenextended for sim 100 km further inland relative to our newDEM

52 Hydrology

Computing the passageway of subglacial water beneath theice sheet is critical for comprehending ice-sheet dynamics(Bell 2008 Stearns et al 2008 Siegert et al 2016a) Thedevelopment of subglacial hydrology pathways is highly sen-sitive to ice-surface elevation and to a lesser degree to bedmorphology (Wright et al 2008 Horgan et al 2013) Fig-ure 3e shows a comparison of subglacial hydrology path-ways between our new bed DEM and the Bedmap2 DEMThe gross patterns of water flow are largely unchanged be-tween the two DEMs especially across and upstream of In-stitute and Moumlller ice streams The similar water pathwaypattern between both DEMs in these regions is also consis-tent with the small errors in bed topography (Fig 3c) De-spite large differences in bed topography across the Foun-dation Ice Stream and the Ellsworth Mountains region thelarge-scale patterns of water flow are also similar betweenboth DEMs due to the dominance of the ice-surface slopein driving basal water flow in these regions (Shreve 1972)Nonetheless there are several local small-scale differences inthe water pathways (Fig 3e) which highlight hydraulic sen-sitivity The subglacial water network observed in the newDEM across the Foundation Ice Stream appears to be morearborescent than that derived from Bedmap2 This is due tothe introduction of new data resulting in a better-defined bedacross the Foundation Trough (Fig 3a) The subglacial waterpathway observed in the new DEM adjacent to the ground-ing line across the Moumlller Ice Stream is in good agreementwith the position of sub-ice-shelf channels which have beendelineated from a combination of satellite images and RESdata (Le Brocq et al 2013)

Subglacial lakes discovered across the WS sector form anobvious component of the basal hydrological system (Wrightand Siegert 2012) These lakes exist due to sufficient amountof geothermal heating (50ndash70 m W mminus2) which allows thebase of the ice sheet to melt especially in areas of thick iceIn addition the pressure exerted on the bed by the over-

lying ice causes the melting point to be lowered West ofthe Ellsworth Mountains lies a body of the subglacial wa-ter known as Ellsworth Subglacial Lake The lake measures289 km2 with a depth ranging between 52 and 156 m capa-ble of carrying a water body volume of 137 km3 (Woodwardet al 2010) In some cases ice-surface elevation changeshave been linked with subglacial lake hydrological change(Siegert et al 2016a) ndash referred to as ldquoactiverdquo subglaciallakes There are four known active subglacial lakes dis-tributed across the Institute Ice Stream (Wright and Siegert2012) Institute W1 is located close to the Robin SubglacialBasin A whereas Institute W2 is located to the northeast ofPirrit Hills Institute E1 and E2 are located to the southwestof Robin Subglacial Basin B and near the field camp of C110respectively There are three active subglacial lakes in theFoundation Ice Stream catchment Foundation 1 Foundation2 and Foundation 3 (Wright and Siegert 2012) East of theFoundation Ice Stream there are 16 active subglacial lakesdistributed along the main trunk of Academy Glacier (Wrightand Siegert 2012)

53 Geomorphological description of the bedtopography

The WS sector of WAIS is composed of three major ice-sheetoutlets (Fig 1a and b) the Institute Moumlller and Foundationice streams feeding ice to the FRIS the second largest iceshelf in Antarctica Geophysical data in this area reveal fea-tures such as steep reverse bed slopes similar in scale to thatmeasured for upstream Thwaites Glacier close to the Insti-tute and Moumlller Ice Stream grounding lines The bed slopesinland to asim 18 km deep basin (the Robin Subglacial Basin)which is divided into two sections with few obvious signif-icant ice-sheet pinning points (Ross et al 2012) Elevatedbeds in other parts of the WS sector allow the ice shelf toground causing ice-surface features known as ice rises andrumples (Matsuoka et al 2015)

The Institute Ice Stream has three tributaries within oursurvey grid to the south and west of the Ellsworth Moun-tains occupying the Horseshoe Valley Independence andEllsworth troughs (Fig 1c) (Winter et al 2015) The Horse-shoe Valley Trough around 20 km wide and 13 km belowsea level at its deepest point is located downstream of thesteep mountains of the Heritage Range A subglacial ridge islocated between the mouth of the Horseshoe Valley Troughand the main trunk of the Institute Ice Stream (Winter et al2015) The Independence Trough is located subparallel to theHorseshoe Valley Trough separated by the 14 km high Inde-pendence Hills The trough is sim 22 km wide and is 11 kmbelow sea level at its deepest point It is characterized bytwo distinctive plateaus (sim 6 km wide each) on each side ofthe trough aligned alongside the main trough axis Ice flowseastward through the Independence Trough for sim 54 km be-fore it shifts to a northward direction where the troughwidens to 50 km and connects with the main Institute Ice

wwwearth-syst-sci-datanet107112018 Earth Syst Sci Data 10 711ndash725 2018

720 H Jeofry et al A new bed elevation model for the Weddell Sea sector

Figure 4 Bed elevations for RES transects (black) DEM (blue) and Bedmap2 (red) for (a) Profile AndashAprime (b) Profile BndashBprime (c) Profile CndashCprime

and (d) Profile DndashDprime

Stream The Ellsworth Trough is aligned with the Indepen-dence Trough and both are orthogonal to the orientation ofthe AmundsenndashWeddell ice divide dissecting the EllsworthSubglacial Highlands northwest to southeast The EllsworthTrough measures sim 34 km in width and is sim 2 km belowsea level at its deepest point and is sim 260 km in length Itis considered to be the largest and deepest trough-controlledtributary in this region (Winter et al 2015) The EllsworthTrough is intersected by several smaller valleys aligned per-pendicularly to the main axis which are relic landforms from

a previous small dynamic ice mass (predating the WAIS inits present configuration) (Ross et al 2014) The EllsworthTrough contains the sim 15ndash20 km long Ellsworth SubglacialLake (Siegert et al 2004a 2012 Woodward et al 2010Wright and Siegert 2012) It is worth noting that the lake isoutside the grid of our DEM

Satellite altimetry and imagery are able to estimate thegrounding line that separates the grounded ice sheet from thefloating ice shelf based on surface changes due to tidal os-cillations and the subtle ice-surface features Such analysis is

Earth Syst Sci Data 10 711ndash725 2018 wwwearth-syst-sci-datanet107112018

H Jeofry et al A new bed elevation model for the Weddell Sea sector 721

Table 1 Data files and locations

Products Files Location DOIURL

1 km bed elevation DEM 1 km bed elevation DEM Zenodo Data Repository httpsdoiorg105281zenodo1035488

1 km ice thickness DEM Digitized SPRIndashNSFndashTUD bed picks data

UK Polar Data Centre(UKPDC)

httpsdatabasacukmetadataphpid=GBNERCBASAEDC00326

BAS GRADESIMAGEice thickness data

UK Polar Data Centre httpssecureantarcticaacukdataaerogeo

BAS IMAFI ice thicknessdata

UK Polar Data Centre httpssecureantarcticaacukdataaerogeo

NASA Operation Ice-Bridge radar depthsounder level 2 (L2) data

Center for Remote Sens-ing of Ice Sheet (CReSIS)

httpsdatacresiskuedu

1 km ice-sheet-surfaceDEM

ERS-1 radar and ICESatlaser satellite altimetry

National Snow and IceData Center (NSIDC)

httpsnsidcorgdatadocsdaacnsidc0422antarctic1kmdem

Two-dimensional syntheticaperture radar (SAR)-processed radargrams

BAS IMAFI airborne sur-vey

UK Polar Data Centre httpsdoiorg1052858a975b9e-f18c-4c51-9bdb-b00b82da52b8

NASA Operation Ice-Bridge airborne survey

Center for Remote Sens-ing of Ice Sheet

httpsdatacresiskuedu

Ice velocity map of centralAntarctica

MEaSUREs InSAR-based ice velocity

National Snow and IceData Center

httpsdoiorg105067MEASURESCRYOSPHEREnsidc-0484001

Ice-sheet-surface satelliteimagery

MODIS Mosaic ofAntarctica (2008ndash2009)(MOA2009)

National Snow and IceData Center

httpsdoiorg107265N5KP8037

RADARSAT (25 m) satel-lite imagery

Byrd Polar and ClimateResearch Center

httpsresearchbpcrcosuedurslradarsatdata

prone to uncertainty however There are currently four pro-posed grounding-line locations based on different satellitedatasets andor methods of analysis (Bohlander and Scam-bos 2007 Bindschadler et al 2011 Brunt et al 2011 Rig-not et al 2011c) Each of the grounding lines were delin-eated from satellite images but without direct measurementof the subglacial environment This results in ambiguities forthe grounding-line location (Jeofry et al 2017a) In addi-tion RES data have demonstrated clear errors in the positionof the grounding line with a large hitherto unknown sub-glacial embayment near the Institute Ice Stream groundingline The subglacial embayment issim 1 km deep and is poten-tially open to the ice shelf cavity causing the inland ice sheetto have a direct contact with ocean water Our RES analysisalso reveals a better-defined Foundation Trough in which thegrounding line is perched on very deep topography around2 km below sea level

A previous study revealed a series of ancient large sub-parallel subglacial bed channels between Moumlller Ice Streamand Foundation Ice Stream adjacent to the marginal basins(Fig 1b) (Rose et al 2014) While these subglacial channelsare likely to have been formed by the flow of basal water theyare presently located beneath slow-moving and cold-basedice It is thought therefore that the channels are ancient andwere formed at a time when surface melting was prevalent inWest Antarctica (eg the Pliocene)

The bed topography of the WS appears both rough (overthe mountains and exposed bedrock) and smooth (across the

sediment-filled regions) (Bingham and Siegert 2007) Stud-ies of bed roughness calculated using the fast Fourier trans-form (FFT) technique based on the relative measurement ofbed obstacle amplitude and frequency of the roughness ob-stacles have indicated that the Institute Ice Stream and MoumlllerIce Stream are dominated by relatively low roughness val-ues less than 01 (Bingham and Siegert 2007 Rippin etal 2014) which was suggested as being the result of theemplacement of marine sediments as in the Siple Coast re-gion (Siegert et al 2004b Peters et al 2005) Radar-derivedroughness analysis has evidenced a smooth bed across theRobin Subglacial Basin where sediments may exist (Rippinet al 2014) The deepest parts of the Robin Subglacial Basinare anomalously rough marking the edge of a sedimentarydrape where the highest ice flow velocities are generated(Siegert et al 2016b) As such the smooth basal topogra-phy of the Institute and Moumlller Ice Stream catchments is lessextensive than proposed by Bingham and Siegert (2007) Thesubglacial topography of the region between the Robin Sub-glacial Basin and the Pirrit and MartinndashNash Hills is rela-tively flat smooth and gently sloping and has been inter-preted as a bedrock planation surface (Rose et al 2015)Although the exact formation process of the planation sur-face is unknown it is thought that this geomorphological fea-ture formed due to marine andor fluvial erosion (Rose et al2015)

wwwearth-syst-sci-datanet107112018 Earth Syst Sci Data 10 711ndash725 2018

720 H Jeofry et al A new bed elevation model for the Weddell Sea sector

Figure 4 Bed elevations for RES transects (black) DEM (blue) and Bedmap2 (red) for (a) Profile AndashAprime (b) Profile BndashBprime (c) Profile CndashCprime

and (d) Profile DndashDprime

Stream The Ellsworth Trough is aligned with the Indepen-dence Trough and both are orthogonal to the orientation ofthe AmundsenndashWeddell ice divide dissecting the EllsworthSubglacial Highlands northwest to southeast The EllsworthTrough measures sim 34 km in width and is sim 2 km belowsea level at its deepest point and is sim 260 km in length Itis considered to be the largest and deepest trough-controlledtributary in this region (Winter et al 2015) The EllsworthTrough is intersected by several smaller valleys aligned per-pendicularly to the main axis which are relic landforms from

a previous small dynamic ice mass (predating the WAIS inits present configuration) (Ross et al 2014) The EllsworthTrough contains the sim 15ndash20 km long Ellsworth SubglacialLake (Siegert et al 2004a 2012 Woodward et al 2010Wright and Siegert 2012) It is worth noting that the lake isoutside the grid of our DEM

Satellite altimetry and imagery are able to estimate thegrounding line that separates the grounded ice sheet from thefloating ice shelf based on surface changes due to tidal os-cillations and the subtle ice-surface features Such analysis is

Earth Syst Sci Data 10 711ndash725 2018 wwwearth-syst-sci-datanet107112018

H Jeofry et al A new bed elevation model for the Weddell Sea sector 721

Table 1 Data files and locations

Products Files Location DOIURL

1 km bed elevation DEM 1 km bed elevation DEM Zenodo Data Repository httpsdoiorg105281zenodo1035488

1 km ice thickness DEM Digitized SPRIndashNSFndashTUD bed picks data

UK Polar Data Centre(UKPDC)

httpsdatabasacukmetadataphpid=GBNERCBASAEDC00326

BAS GRADESIMAGEice thickness data

UK Polar Data Centre httpssecureantarcticaacukdataaerogeo

BAS IMAFI ice thicknessdata

UK Polar Data Centre httpssecureantarcticaacukdataaerogeo

NASA Operation Ice-Bridge radar depthsounder level 2 (L2) data

Center for Remote Sens-ing of Ice Sheet (CReSIS)

httpsdatacresiskuedu

1 km ice-sheet-surfaceDEM

ERS-1 radar and ICESatlaser satellite altimetry

National Snow and IceData Center (NSIDC)

httpsnsidcorgdatadocsdaacnsidc0422antarctic1kmdem

Two-dimensional syntheticaperture radar (SAR)-processed radargrams

BAS IMAFI airborne sur-vey

UK Polar Data Centre httpsdoiorg1052858a975b9e-f18c-4c51-9bdb-b00b82da52b8

NASA Operation Ice-Bridge airborne survey

Center for Remote Sens-ing of Ice Sheet

httpsdatacresiskuedu

Ice velocity map of centralAntarctica

MEaSUREs InSAR-based ice velocity

National Snow and IceData Center

httpsdoiorg105067MEASURESCRYOSPHEREnsidc-0484001

Ice-sheet-surface satelliteimagery

MODIS Mosaic ofAntarctica (2008ndash2009)(MOA2009)

National Snow and IceData Center

httpsdoiorg107265N5KP8037

RADARSAT (25 m) satel-lite imagery

Byrd Polar and ClimateResearch Center

httpsresearchbpcrcosuedurslradarsatdata

prone to uncertainty however There are currently four pro-posed grounding-line locations based on different satellitedatasets andor methods of analysis (Bohlander and Scam-bos 2007 Bindschadler et al 2011 Brunt et al 2011 Rig-not et al 2011c) Each of the grounding lines were delin-eated from satellite images but without direct measurementof the subglacial environment This results in ambiguities forthe grounding-line location (Jeofry et al 2017a) In addi-tion RES data have demonstrated clear errors in the positionof the grounding line with a large hitherto unknown sub-glacial embayment near the Institute Ice Stream groundingline The subglacial embayment issim 1 km deep and is poten-tially open to the ice shelf cavity causing the inland ice sheetto have a direct contact with ocean water Our RES analysisalso reveals a better-defined Foundation Trough in which thegrounding line is perched on very deep topography around2 km below sea level

A previous study revealed a series of ancient large sub-parallel subglacial bed channels between Moumlller Ice Streamand Foundation Ice Stream adjacent to the marginal basins(Fig 1b) (Rose et al 2014) While these subglacial channelsare likely to have been formed by the flow of basal water theyare presently located beneath slow-moving and cold-basedice It is thought therefore that the channels are ancient andwere formed at a time when surface melting was prevalent inWest Antarctica (eg the Pliocene)

The bed topography of the WS appears both rough (overthe mountains and exposed bedrock) and smooth (across the

sediment-filled regions) (Bingham and Siegert 2007) Stud-ies of bed roughness calculated using the fast Fourier trans-form (FFT) technique based on the relative measurement ofbed obstacle amplitude and frequency of the roughness ob-stacles have indicated that the Institute Ice Stream and MoumlllerIce Stream are dominated by relatively low roughness val-ues less than 01 (Bingham and Siegert 2007 Rippin etal 2014) which was suggested as being the result of theemplacement of marine sediments as in the Siple Coast re-gion (Siegert et al 2004b Peters et al 2005) Radar-derivedroughness analysis has evidenced a smooth bed across theRobin Subglacial Basin where sediments may exist (Rippinet al 2014) The deepest parts of the Robin Subglacial Basinare anomalously rough marking the edge of a sedimentarydrape where the highest ice flow velocities are generated(Siegert et al 2016b) As such the smooth basal topogra-phy of the Institute and Moumlller Ice Stream catchments is lessextensive than proposed by Bingham and Siegert (2007) Thesubglacial topography of the region between the Robin Sub-glacial Basin and the Pirrit and MartinndashNash Hills is rela-tively flat smooth and gently sloping and has been inter-preted as a bedrock planation surface (Rose et al 2015)Although the exact formation process of the planation sur-face is unknown it is thought that this geomorphological fea-ture formed due to marine andor fluvial erosion (Rose et al2015)

wwwearth-syst-sci-datanet107112018 Earth Syst Sci Data 10 711ndash725 2018

H Jeofry et al A new bed elevation model for the Weddell Sea sector 721

Table 1 Data files and locations

Products Files Location DOIURL

1 km bed elevation DEM 1 km bed elevation DEM Zenodo Data Repository httpsdoiorg105281zenodo1035488

1 km ice thickness DEM Digitized SPRIndashNSFndashTUD bed picks data

UK Polar Data Centre(UKPDC)

httpsdatabasacukmetadataphpid=GBNERCBASAEDC00326

BAS GRADESIMAGEice thickness data

UK Polar Data Centre httpssecureantarcticaacukdataaerogeo

BAS IMAFI ice thicknessdata

UK Polar Data Centre httpssecureantarcticaacukdataaerogeo

NASA Operation Ice-Bridge radar depthsounder level 2 (L2) data

Center for Remote Sens-ing of Ice Sheet (CReSIS)

httpsdatacresiskuedu

1 km ice-sheet-surfaceDEM

ERS-1 radar and ICESatlaser satellite altimetry

National Snow and IceData Center (NSIDC)

httpsnsidcorgdatadocsdaacnsidc0422antarctic1kmdem

Two-dimensional syntheticaperture radar (SAR)-processed radargrams

BAS IMAFI airborne sur-vey

UK Polar Data Centre httpsdoiorg1052858a975b9e-f18c-4c51-9bdb-b00b82da52b8

NASA Operation Ice-Bridge airborne survey

Center for Remote Sens-ing of Ice Sheet

httpsdatacresiskuedu

Ice velocity map of centralAntarctica

MEaSUREs InSAR-based ice velocity

National Snow and IceData Center

httpsdoiorg105067MEASURESCRYOSPHEREnsidc-0484001

Ice-sheet-surface satelliteimagery

MODIS Mosaic ofAntarctica (2008ndash2009)(MOA2009)

National Snow and IceData Center

httpsdoiorg107265N5KP8037

RADARSAT (25 m) satel-lite imagery

Byrd Polar and ClimateResearch Center

httpsresearchbpcrcosuedurslradarsatdata

prone to uncertainty however There are currently four pro-posed grounding-line locations based on different satellitedatasets andor methods of analysis (Bohlander and Scam-bos 2007 Bindschadler et al 2011 Brunt et al 2011 Rig-not et al 2011c) Each of the grounding lines were delin-eated from satellite images but without direct measurementof the subglacial environment This results in ambiguities forthe grounding-line location (Jeofry et al 2017a) In addi-tion RES data have demonstrated clear errors in the positionof the grounding line with a large hitherto unknown sub-glacial embayment near the Institute Ice Stream groundingline The subglacial embayment issim 1 km deep and is poten-tially open to the ice shelf cavity causing the inland ice sheetto have a direct contact with ocean water Our RES analysisalso reveals a better-defined Foundation Trough in which thegrounding line is perched on very deep topography around2 km below sea level

A previous study revealed a series of ancient large sub-parallel subglacial bed channels between Moumlller Ice Streamand Foundation Ice Stream adjacent to the marginal basins(Fig 1b) (Rose et al 2014) While these subglacial channelsare likely to have been formed by the flow of basal water theyare presently located beneath slow-moving and cold-basedice It is thought therefore that the channels are ancient andwere formed at a time when surface melting was prevalent inWest Antarctica (eg the Pliocene)

The bed topography of the WS appears both rough (overthe mountains and exposed bedrock) and smooth (across the

sediment-filled regions) (Bingham and Siegert 2007) Stud-ies of bed roughness calculated using the fast Fourier trans-form (FFT) technique based on the relative measurement ofbed obstacle amplitude and frequency of the roughness ob-stacles have indicated that the Institute Ice Stream and MoumlllerIce Stream are dominated by relatively low roughness val-ues less than 01 (Bingham and Siegert 2007 Rippin etal 2014) which was suggested as being the result of theemplacement of marine sediments as in the Siple Coast re-gion (Siegert et al 2004b Peters et al 2005) Radar-derivedroughness analysis has evidenced a smooth bed across theRobin Subglacial Basin where sediments may exist (Rippinet al 2014) The deepest parts of the Robin Subglacial Basinare anomalously rough marking the edge of a sedimentarydrape where the highest ice flow velocities are generated(Siegert et al 2016b) As such the smooth basal topogra-phy of the Institute and Moumlller Ice Stream catchments is lessextensive than proposed by Bingham and Siegert (2007) Thesubglacial topography of the region between the Robin Sub-glacial Basin and the Pirrit and MartinndashNash Hills is rela-tively flat smooth and gently sloping and has been inter-preted as a bedrock planation surface (Rose et al 2015)Although the exact formation process of the planation sur-face is unknown it is thought that this geomorphological fea-ture formed due to marine andor fluvial erosion (Rose et al2015)

wwwearth-syst-sci-datanet107112018 Earth Syst Sci Data 10 711ndash725 2018

722 H Jeofry et al A new bed elevation model for the Weddell Sea sector

6 Data availability

The new 1 km bed elevation product of the WS sectorcan be found at httpsdoiorg105281zenodo1035488 Weused four radar datasets to construct the 1 km ice thicknessDEM as follows (1) digitized radar data from the 1970sSPRIndashNSFndashTUD surveys in which the bed was picked ev-ery 15ndash20 s (1ndash2 km) recorded here in an Excel 97ndash2003Worksheet (XLS) which can be obtained from the UK Po-lar Data Centre (UKPDC) website at httpsdatabasacukmetadataphpid=GBNERCBASAEDC00326 (2) BASGRADESIMAGE and (3) BAS IMAFI airborne surveysboth available from the UKPDC Polar Airborne GeophysicsData Portal at httpssecureantarcticaacukdataaerogeoand (4) NASA Operation IceBridge radar depth sounder level2 (L2) data available from the Center for Remote Sensing ofIce Sheet (CReSIS) website at httpsdatacresiskuedu

The 1 km ice-sheet surface elevation DEM was derivedfrom a combination of ERS-1 surface radar and ICESat laseraltimetry which is downloadable from the National Snowand Ice Data Center (NSIDC) website at httpsnsidcorgdatadocsdaacnsidc0422_antarctic_1km_dem Newer sur-face elevation models (ie Helm et al 2014) can easily becombined with our improved bed DEM

Two-dimensional SAR-processed radargrams in SEG-Yformat for the BAS IMAFI airborne survey and the NASAOperation IceBridge SAR images (level 1B) in MAT (bi-nary MATLAB) format are provided at httpsdoi1052858a975b9e-f18c-4c51-9bdb-b00b82da52b8 and httpsdatacresiskuedu respectively

Ancillary information for the MEaSUREs InSAR-based ice velocity map of central Antarctica can befound at httpsdoi105067MEASURESCRYOSPHEREnsidc-0484001 and the MODIS Mosaic of Antarctica 2008ndash2009 (MOA 2009) ice-sheet-surface image map is avail-able at httpsdoiorg107265N5KP8037 The RADARSAT(25 m) ice-sheet-surface satellite imagery is accessible fromthe Byrd Polar and Climate Research Center website athttpsresearchbpcrcosuedurslradarsatdata (Byrd Polarand Climate Research Center 2012) A summary of the dataused in this paper and their availability is provided in the Ta-ble 1

7 Conclusions

We have compiled airborne radar data from a number of geo-physical surveys including the SPRIndashNSFndashTUD surveys ofthe 1970s the GRADESIMAGE and IMAFI surveys ac-quired by BAS in 2006ndash2007 and 2010ndash2011 respectivelyand new geophysical datasets collected by CReSIS from theNASA OIB project in 2012 2014 and 2016 From thesedata we produce a bed topography DEM which is griddedto 1 km postings The DEM covers an area ofsim 125 000 km2

of the WS sector including the Institute Moumlller and Founda-tion ice streams as well as the Bungenstock ice rise Large

differences can be observed between the new and previousDEMs (ie Bedmap2) most notably across the FoundationIce Stream where we reveal the grounding line to be restingon a bed sim 2 km below sea level with a deep trough imme-diately upstream as deep as 23 km below sea level In ad-dition improved processing of existing data better resolvesdeep regions of bed compared to Bedmap2 Our new DEMalso revises the pattern of potential basal water flow acrossthe Foundation Ice Stream and towards East Antarctica incomparison to Bedmap2 Our new DEM and the data usedto compile it are available to download and will be of valueto ice-sheet modelling experiments in which the accuracy ofthe DEM is important to ice flow processes in this particu-larly sensitive region of the WAIS

Author contributions HJ carried out the analysis created thefigures and compiled the database HJ NR and MS wrote the pa-per All authors contributed to the database compilation analysisand writing of the paper

Competing interests The authors declare that they have no con-flict of interest

Acknowledgements The data used in this project are availableat the Center for the Remote Sensing of Ice Sheets data portalhttpsdatacresiskuedu and at the UK Airborne GeophysicsData Portal httpssecureantarcticaacukdataaerogeo PrasadGogineni and Jilu Li acknowledge funding by NASA for CReSISdata collection and development of radars (NNX10AT68G) MartinJ Siegert and Neil Ross acknowledge funding from the NERCAntarctic Funding Initiative (NEG0130711) and the IMAFI datacollection team consist of Hugh F J Corr Fausto FerraccioliRob Bingham Anne Le Brocq David Rippin Tom Jordan CarlRobinson Doug Cochrane Ian Potten and Mark OostlanderHafeez Jeofry acknowledges funding from the Ministry of HigherEducation Malaysia and the Norwegian Polar Institute for theQuantarctica GIS package All authors gratefully acknowledgethe anonymous reviewers for their insightful suggestions to thismanuscript

Edited by Reinhard DrewsReviewed by two anonymous referees

References

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Bamber J L Gomez-Dans J and Griggs J Antarctic 1 km digi-tal elevation model (DEM) from combined ERS-1 radar and ICE-

Earth Syst Sci Data 10 711ndash725 2018 wwwearth-syst-sci-datanet107112018

H Jeofry et al A new bed elevation model for the Weddell Sea sector 723

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724 H Jeofry et al A new bed elevation model for the Weddell Sea sector

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Ross N Jordan T A Bingham R G Corr H F Ferraccioli FLe Brocq A Rippin D M Wright A P and Siegert M JThe Ellsworth subglacial highlands inception and retreat of theWest Antarctic Ice Sheet Geol Soc Am Bull 126 3ndash15 2014

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Wright A P Le Brocq A M Cornford S L Bingham R GCorr H F J Ferraccioli F Jordan T A Payne A J RippinD M Ross N and Siegert M J Sensitivity of the WeddellSea sector ice streams to sub-shelf melting and surface accumula-tion The Cryosphere 8 2119ndash2134 httpsdoiorg105194tc-8-2119-2014 2014

wwwearth-syst-sci-datanet107112018 Earth Syst Sci Data 10 711ndash725 2018