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The Ellsworth Subglacial Highlands: Inception and Retreat of the West Antarctic Ice Sheet

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The Ellsworth Subglacial Highlands: Inception and Retreat of the West Antarctic Ice Sheet Downloaded from gsabulletin.gsapubs.org on January 28, 2014 The Ellsworth Subglacial Highlands: Inception and retreat of the West Antarctic Ice Sheet Neil Ross1,†, Tom A. Jordan2, Robert G. Bingham3, Hugh F.J. Corr2, Fausto Ferraccioli2, Anne Le Brocq4, David M. Rippin5, Andrew P. Wright4, and Martin J. Siegert6 1School of Geography, Politics and Sociology, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK 2British Antarctic Survey, Cambridge CB3 0ET, UK 3School of Geosciences, University of Edinburgh, Edinburgh EH8 9XP, UK 4School of Geography, University of Exeter, Exeter EX4 4RJ, UK 5Environment Department, University of York, York YO10 5DD, UK 6Bristol Glaciology Centre, School of Geographical Sciences, University of Bristol, Bristol BS8 1SS, UK ABSTRACT landscape predates the present ice sheet and these uplands were the main seeding grounds of was formed by a small dynamic ice fi eld(s), West Antarctic Ice Sheet growth, while Bamber Antarctic subglacial highlands are where similar to those of the present-day Antarctic et al. (2009) hypothesized them as pinning points the Antarctic ice sheets fi rst developed and Peninsula, at times when the marine sections of a retreating ice sheet. Testing these hypoth- the “pinning points” where retreat phases of of the West Antarctic Ice Sheet were absent. eses requires evidence of former ice dynamics. the marine-based sectors of the ice sheet are The Ellsworth Subglacial Highlands repre- Several investigations in East Antarctica have impeded. Due to low ice velocities and limited sent a major seeding center of the paleo–West demonstrated the utility of radio-echo sounding present-day change in the ice-sheet interior, Antarctic Ice Sheet, and its margins repre- (RES) in mapping ancient glacial geomorphic West Antarctic subglacial highlands have sent the pinning point at which future retreat features from which ice-sheet reconstructions been overlooked for detailed study. These re- of the marine-based West Antarctic Ice Sheet can be based (Bo et al., 2009; Young et al., 2011). gions have considerable potential, however, would be arrested. Here, we present geophysical data on the mor- for establishing the locations from which phology of the Ellsworth Sub glacial Highlands, the West Antarctic Ice Sheet originated and INTRODUCTION gained during three seasons of ground-based and grew, and its likely response to warming cli- airborne RES measurements around Ellsworth mates. Here, we characterize the subglacial The West Antarctic Ice Sheet rests largely on Subglacial Lake (Woodward et al., 2010) and over morphology of the Ellsworth Subglacial High- a bed several hundred meters below sea level. the Institute and Möller ice streams (see Fig. 1; lands, West Antarctica, from ground-based As a marine-based ice sheet, it may be inher- Ross et al., 2012). These surveys have revealed a and aerogeophysical radio-echo sounding ently unstable due to its sensitivity to ocean tem- deep (>2000 m below sea level [bsl]), broad (up (RES) surveys and the Moderate-Resolu- peratures and upstream deepening of its bed in a to 25 km across), and >300-km-long sub glacial tion Imaging Spectroradiometer (MODIS) number of key areas (Weertman, 1974; Bamber trough, named the Ellsworth Trough, which Mosaic of Antarctica. We document well- et al., 2009a; Joughin and Alley, 2011). How- dissects the Ellsworth Subglacial Highlands preserved classic landforms associated with ever, despite it being critical to our evaluation of northwest to southeast (Fig. 2). Aligned roughly restricted, dynamic, marine-proximal alpine West Antarctic Ice Sheet inception, stability, and parallel to linear topographic trends in the Ells- glaciation, with hanging tributary valleys the likelihood of future sea-level change from worth Mountains, and bounded on both sides by feeding a signifi cant overdeepened trough ice-sheet loss (Bentley, 2010), the glacial his- rugged mountainous subglacial topography, the (the Ellsworth Trough) cut by valley (tide- tory of West Antarctica is not well constrained. Ellsworth Trough is one of a series of NW-SE– water) glaciers. Fjord-mouth threshold bars The recognition that the subglacial highlands trending subglacial valleys extending from the down-ice of two overdeepenings defi ne both of East Antarctica acted as critical nucleation core of the Ellsworth Subglacial Highlands into the northwest and southeast termini of paleo- sites for the East Antarctic Ice Sheet has driven the Bentley Subglacial Trench (Fig. 1), several of outlet glaciers, which cut and occupied the much recent research (e.g., Bo et al., 2009; Bell which contain subglacial lakes (Vaughan et al., Ellsworth Trough. Satellite imagery reveals et al., 2011; Ferraccioli et al, 2011). In compari- 2006, 2007). The Ellsworth Trough lies roughly numerous other glaciated valleys, terminat- son, however, the subglacial upland areas of the orthogonal to the Amundsen-Weddell ice divide ing at the edge of deep former marine basins West Antarctic Ice Sheet have been little investi- (Ross et al., 2011), currently contains Ellsworth (e.g., Bentley Subglacial Trench), throughout gated. Two dominant upland regions, which may Subglacial Lake (Woodward et al., 2010; Siegert the Ellsworth Subglacial Highlands. These be instrumental in the growth and decay of the et al., 2012), and hosts an enhanced-fl ow tribu- geomorphic data can be used to reconstruct ice sheet, exist beneath the West Antarctic Ice tary of Institute Ice stream (Joughin et al., 2006; the glaciology of the ice masses that formed Sheet: the Ellsworth Subglacial Highlands, and Rignot et al., 2011), which connects to the deep the proto–West Antarctic Ice Sheet. The the coastal mountain ranges of Marie Byrd Land, marine basin (the “Robin Subglacial Basin”) centered around the Executive Committee Range that underlies the coastal parts of the Institute Ice †E-mail: [email protected] (Fig. 1A). Bentley et al. (1960) hypothesized that stream and Möller Ice stream (Ross et al., 2012). GSA Bulletin; January/February 2014; v. 126; no. 1/2; p. 3–15; doi: 10.1130/B30794.1; 9 fi gures. For permission to copy, contact [email protected] 3 © 2013 Geological Society of America Downloaded from gsabulletin.gsapubs.org on January 28, 2014 Ross et al. A B Figure 1. (A) Subglacial topography of West Antarctica (BEDMAP2) (m WGS84) (Fretwell et al, 2013), showing areas of Figures 1B, 6A–6C, 7A (large white box), and 3C (small white box). Inset shows location of study area. Thin black lines are bed topography contours at 1000 m intervals. (B) Subglacial topography of the Ellsworth Subglacial Highlands (BEDMAP2) (m WGS84) (Fretwell et al., 2013). Filled white polygons represent areas of exposed bedrock of the Sentinel and Heritage Ranges (from SCAR Antarctic Digital Database [ADD] (http:// www.add.scar.org/terms.isp). The white in-fi lled box delimits the area of the 2007–2009 ground-based RES survey of Ellsworth Subglacial Lake (Fig. 3A). Thick white lines show bed elevation measurements made during the 2010–2011 aerogeophysical survey of the Institute and Möller ice streams. Location of Figure 2 (large black rectangle) is shown. Thin black lines are bed topography contours at 250 m intervals. In this paper, we fi rst describe and interpret using half-dipole lengths of 40 m with the sys- GPS processing used daily precise point position- (1) high-resolution ground-based radar data tem towed behind a snowmobile traveling at ~12 ing (PPP)–processed positions of a GPS receiver acquired over the northwestern parts of the km h–1. Measurements were stacked 1000 times, located over the center of Ellsworth Subglacial Ellsworth Trough; and (2) airborne radar data giving along-track measurements (traces) every Lake as the fi xed reference station. Roving data acquired over the southeastern parts of the Ells- 2–5 m. Roving global positioning system (GPS) were corrected for the ~90 m offset between the worth Trough. We then combine the bed topo- data, to locate the x-y-z positions of individual GPS receiver and the midpoint between the radar graphic data with ice-surface remote-sensing radar traces, were acquired using a GPS antenna transmitter and receiver. Radar data processing, data to place our interpretation of these data into secured on the radar receiver sledge. Differential undertaken using REFLEXW processing the broader context of the Ellsworth Subglacial Highlands and West Antarctica. METHODS Figure 2 (on following page). (A) Moderate-Resolution Imaging Spectroradiometer (MODIS ) Mosaic of Antarctica (MOA) digital image mosaic of the surface morphology of Radio-Echo Sounding, the Northwestern the Antarctic ice sheet (Haran, 2005; Scambos et al., 2007) over and around the Ellsworth Ellsworth Trough Trough (ET) (see Fig. 1B for location). Small white box (in A–C) delimits area of ground- based survey around Ellsworth Subglacial Lake. Filled white polygons (in A–D) represent Ground-based RES data were acquired subaerially exposed bedrock (from SCAR Antarctic Digital Database [ADD] (http://www over and around the Ellsworth Trough during the .add.scar.org/terms.isp). (B) Profi le curvature analysis of MODIS MOA data around the 2007–2008 and 2008–2009 Antarctic fi eld sea- Ellsworth Trough. Cells with curvature <–0.05 and >0.05 are colored black; cells with cur- sons (Fig. 1B; Siegert et al., 2012). Data were vature between –0.05 and 0.05 are colored light gray. (C) Ice-sheet surface velocity (Rignot acquired with the low-frequency (~2 MHz) et al., 2011) superimposed on profi le curvature map. Color scale of velocity data is on a log DELORES (DEep-LOok Radio Echo Sounder) scale and is saturated at 130 m yr–1. Black contours are in 25 m yr–1 intervals. (D) Bedrock radar (further details of the system are provided elevation (m WGS84) from surveys of Ellsworth Subglacial Lake and Institute and Möller in King, 2009). Acquisition was undertaken ice streams superimposed on map of profi le curvature. 4 Geological Society of America Bulletin, January/February 2014 Downloaded from gsabulletin.gsapubs.org on January 28, 2014 Ellsworth Subglacial Highlands A B C 25 m yr –1 25 m yr –1 –1 50 m yr –1 –1 75 m yr r 75 m y –1 –1 50 m yr 25 m yr D Figure 2.
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