Jeofry H, Ross N, Corr HFJ, Li J, Gogineni P, Siegert MJ. A deep subglacial embayment adjacent to the grounding line of Institute Ice Stream, West Antarctica. Exploration of Subsurface Antarctica: Uncovering Past Changes and Modern Processes. Geological Society, London, Special Publications 2017, 461 Copyright: This article is published under the terms of the CC-BY 3.0 license DOI link to article: http://doi.org/10.1144/SP461.11 Date deposited: 10/01/2018 This work is licensed under a Creative Commons Attribution 3.0 Unported License Newcastle University ePrints - eprint.ncl.ac.uk Downloaded from http://sp.lyellcollection.org/ by guest on January 10, 2018 A deep subglacial embayment adjacent to the grounding line of Institute Ice Stream, West Antarctica HAFEEZ JEOFRY1,2*, NEIL ROSS3, HUGH F. J. CORR4, JILU LI5, PRASAD GOGINENI6 & MARTIN J. SIEGERT1 1Grantham Institute and Department of Earth Science and Engineering, Imperial College London, South Kensington, London, UK 2School of Marine Science and Environment, Universiti Malaysia Terengganu, Kuala Terengganu, Terengganu, Malaysia 3School of Geography, Politics and Sociology, Newcastle University, Claremont Road, Newcastle Upon Tyne, UK 4British Antarctic Survey, Natural Environment Research Council, Cambridge, UK 5Center for the Remote Sensing of Ice Sheets, University of Kansas, Lawrence, Kansas, USA 6Electrical and Computer Engineering, University of Alabama, Tuscaloosa, Alabama, USA *Correspondence: [email protected] Abstract: The Institute Ice Stream (IIS) in West Antarctica may be increasingly vulnerable to melt- ing at the grounding line through modifications in ocean circulation. Understanding such change requires knowledge of grounding-line boundary conditions, including the topography on which it rests. Here, we discuss evidence from new radio-echo sounding (RES) data on the subglacial topo- graphy adjacent to the grounding line of the IIS. In doing so, we reveal a previously unknown subglacial embayment immediately inland of the IIS grounding zone which is not represented in the Bedmap2 compilation. We discuss whether there is an open-water connection between the embayment and the ice-shelf cavity. The exact location of the grounding line over the embayment has been the subject of considerable uncertainty, with several positions being proposed recently. From our compilation of data, we are able to explain which of these grounding lines is most likely and, in doing so, highlight the need for accurate bed topography in conjunction with satellite obser- vations to fully comprehend ice-sheet processes in this region and other vulnerable locations at the grounded margin of Antarctica. Gold Open Access: This article is published under the terms of the CC-BY 3.0 license. There is major concern about the long-term stability the Institute Ice Stream (IIS) being most sensitive. of the West Antarctic Ice Sheet (WAIS), the bulk In addition, analysis of englacial layering shows of which rests on a bed below sea level and is thought that the nearby Bungenstock Ice Rise has experi- to be susceptible to ‘marine ice-sheet instabi- enced a major ice-flow change in the last few thou- lity’ (MISI) (Mercer 1978). MISI occurs where the sand years (possibly as recently as a few hundred grounding line, which separates ice resting on its years ago). This would have been associated with bed from that floating, retreats over topography that substantial reorganization of the IIS (Siegert et al. deepens upstream. Such a configuration is thought 2013), with likely modification to the grounding-line to support continual acceleration of grounding-line position. retreat once it commences (Weertman 1974; Schoof Our ability to define the exact location of the 2007; Vaughan & Arthern 2007; Durand et al. grounding line across this sensitive region has been 2009). One region where such change has been pre- inhibited by limited measurements of the subgla- dicted to occur late in the twenty-first century is the cial environment. Furthermore, a large tidal range Weddell Sea sector of the WAIS (Hellmer et al. of c. 8 m in this region and the presence of glacio- 2012). Ice-sheet modelling acknowledges the ice logical phenomena, such as ice plains close to the streams of the Weddell Sea sector as highly vulnera- mouth of the IIS, provide ambiguity as to the land- ble to potential grounding-line melting (Wright et al. ward limit of ice flexure (Scambos et al. 2004; 2014a; Cornford et al. 2015; Ritz et al. 2015), with Fricker & Padman 2006; Lambrecht et al. 2007; From:SIEGERT, M. J., JAMIESON,S.S.R.&WHITE, D. A. (eds) 2018. Exploration of Subsurface Antarctica: Uncovering Past Changes and Modern Processes. Geological Society, London, Special Publications, 461, 161–173. First published online June 13, 2017, updated November 10, 2017, https://doi.org/10.1144/SP461.11 © 2018 The Author(s). Published by The Geological Society of London. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Downloaded from http://sp.lyellcollection.org/ by guest on January 10, 2018 162 H. JEOFRY ET AL. Brunt et al. 2011). As a consequence, several differ- grounding line, with a waveform acquisition rate of ent grounding-line positions have been proposed 150 MHz (Gogineni 2012). The aircraft was posi- (Bohlander & Scambos 2007; Brunt et al. 2010; tioned with GPS and inertial navigation systems Bindschadler et al. 2011; Rignot et al. 2011a) based (Rodriguez-Morales et al. 2014). All radar data solely on satellite data, and without geophysical were SAR-focused (synthetic aperture radar; Hélière measurement of the bed. et al. 2007; Gogineni et al. 2014). Evidence from geophysical surveying in this Ice surface elevation (Fig. 1a, b) was derived from region reveals the IIS grounding line to be resting altimetric data from the European Remote Sensing on a bed c. 1 km below sea level, dipping upstream Satellite-1 (ERS-1) radar and the Ice Cloud and towards the c. 1.8 km deep Robin Subglacial land Elevation Satellite (ICESat) laser satellite altim- Basin (Ross et al. 2012). Ice-sheet models treat the etry (Bamber et al. 2009). Grounding-line positions, grounding interface crudely however, despite recent derived from several satellite products, are illustrated advances in modelling technology (Wright et al. in Figure 1b. Surface ice velocities (Fig. 1c) derived 2014a; Martin et al. 2015; Ritz et al. 2015; Thoma from interferometric SAR (InSAR) data (Rignot et al. 2015) and recognition of the importance of et al. 2011b) allow the flow structure to be quantified grounding-line processes in modulating ice-sheet and the shear margin, separating fast and slow flow- change (Hellmer et al. 2012; Favier et al. 2014; ing ice, to be located. Joughin et al. 2014; Rignot et al. 2014; Khazendar We mapped bed topography over the IIS region et al. 2016; Siegert et al. 2016). Bed topography, using the RES data. The topography was computed basal conditions and subglacial processes are neither by subtracting a new ice thickness digital elevation known precisely nor modelled realistically at this model (DEM) from the satellite-derived ice-sheet critical interface of the ice-sheet system across the surface elevation. The DEM was calculated from whole of the Antarctic ice-sheet margin. Existing the along-track RES datasets and interpolated using observations of grounding lines (e.g. Horgan et al. the ‘Topo to Raster’ function in ArcGIS based on 2013) point to a complex transition between floating the Australian National University Digital Eleva- and grounded ice that needs to be understood fully tion Model gridding algorithm, which creates a and characterized well if we are to predict how ice smooth and hydrologically sound surface (Hutchin- sheets behave in such places. Here, we analyse radio- son 1988). The ice thickness, ice-sheet surface eleva- echo sounding (RES) data from the grounding line tion and bed topography DEMs were then gridded at adjacent to the IIS to highlight the previously a uniform 1 km spacing. The WGS 84 Polar Stereo- unknown complexity of this subglacial environment, graphic projection (Snyder 1987) was used as a and infer how this may influence the future evolution reference. of the ice sheet. Subglacial water pressure was calculated using the hydraulic potentiometric surface, which assumes that the basal water pressure is balanced by the over- Dataset and methods riding ice pressure, expressed as: w =(r × × )+(r × × ) We analyse RES datasets collected by the British w g y i g h (1) Antarctic Survey (BAS) in the austral summer of 2006–07 and the austral summer of 2010–11, sup- where φ is the theoretical hydropotential surface, y plemented with those acquired by the Center for is the bed elevation, h is the ice thickness, ρw and the Remote Sensing of Ice Sheets (CReSIS) dur- −3 ρi are the density of water (1000 kg m ) and ice ing the Operation Ice Bridge (OIB) programme (920 kg m−3), respectively, and g is the gravitational in 2014, which add substantially to data forming constant (9.81 m s−2)(Shreve 1972). the previously published subglacial bed topography (Bedmap2) across the IIS (Fretwell et al. 2013) (Fig. 1a). The BAS Polarimetric Airborne Survey A deep subglacial embayment near the Instrument (PASIN) is a coherent radar system Institute Ice Stream with a carrier frequency of 150 MHz, 12 MHz band- width and a pulse-coded waveform acquisition rate Immediately west of the IIS grounding line, RES of 312.5 Hz. Two global positioning system (GPS) data reveal a previously unknown deep subglacial receivers were installed in the aircraft and corrected embayment (Fig. 2). The embayment measures c. with two Leica 500 GPS base stations to obtain the 10 km (north–south) by c. 23 km (east–west), and position of the aircraft (Jordan et al. 2013). In 2014 is less than c. 10 km from both the grounding line CReSIS adopted a radar system operating with and the southern shear margin of the IIS.