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Recent advances in understanding Antarctic subglacial lakes and hydrology

For ReviewJournal: Philosophical Only Transactions A Manuscript ID: RSTA-2014-0306.R1

Article Type: Review

Date Submitted by the Author: n/a

Complete List of Authors: Siegert, Martin Ross, Neil; Newcastle University, Le Brocq, Anne; University of Exeter,

Issue Code: Click here to find the code for your issue.:

Glaciology < EARTH SCIENCES, Geophysics < Subject: EARTH SCIENCES, Hydrology < EARTH SCIENCES

Keywords: Ice sheet, Basal processes, Ice flow, Antarctica

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1 2 3 Recent advances in understanding Antarctic subglacial lakes and hydrology 4 5 Martin Siegert 1, Neil Ross 2, Anne Le Brocq 3 6 7 1. Grantham Institute and Department of Earth Science and Engineering, Imperial College London, South Kensington, 8 London SW7 2AZ, UK 9 2. School of Geography, Politics and Sociology, Newcastle University, Claremont Road, Newcastle Upon Tyne, NE1 7RU, UK. 10 3. Geography, College of Life and Environmental Sciences, University of Exeter, Exeter EX4 4RJ, UK. 11 12 13 100 word summary : Knowledge of the flow and storage of water beneath the Antarctic Ice Sheet 14 has improved substantially in recent years. We now have evidence for more than 400 subglacial 15 lakes and have identified how basal water flows from the ice-sheet interior to the margin. We also 16 appreciate that ‘active’ subglacial lakes, discrete ice-surface areas that lower or rise due to 17 presumed volume changes in a lake beneath, are not commonly associated with expected subglacial- 18 lake records from ice-penetratingFor Review radar. Uncertainty in what Only ‘active’ subglacial lakes are, together 19 with absence of information concerning subglacial groundwater, demonstrates our incomplete 20 21 understanding of Antarctic subglacial hydrology. 22 23 24 It is now well documented that over 400 subglacial lakes exist across the bed of the Antarctic Ice 25 Sheet. They comprise a variety of sizes and volumes (from the ~250 km long Lake Vostok to bodies 26 of water < 1km in length), relate to a number of discrete topographic settings (from those 27 contained within valleys to lakes that reside in broad flat terrain) and exhibit a range of dynamic 28 29 behaviours (from ‘active’ lakes that periodically outburst some or all of their water, to those 30 isolated hydrologically for millions of years). Here we critique recent advances in our 31 understanding of subglacial lakes, in particular since the last inventory in 2012. We show that 32 within three years our knowledge of the hydrological processes at the ice sheet base has advanced 33 34 considerably. We describe evidence for further ‘active’ subglacial lakes, based on satellite 35 observation of ice surface changes and discuss why detection of many ‘active’ lakes is not resolved 36 in traditional radar methods. We go on to review evidence for large-scale subglacial water flow in 37 Antarctica, including the discovery of ancient channels developed by former hydrological 38 39 processes. We end by predicting areas where future discoveries may be possible, including the 40 detection, measurement and significance of groundwater (i.e., water held beneath the ice-bed 41 interface). 42 43 44 45 Introduction and background 46 47 Antarctic subglacial lakes were first detected using ice-penetrating radar in the late 1960s (Robin et 48 49 al., 1969), from a geophysical survey of the Antarctic Ice Sheet that took place between 1968 and 50 1979 (Turchetti et al., 2008). Subglacial lakes are easily identifiable in radar data, as the bright, flat 51 specular reflections from an ice-water interface are distinct from the rough, variable power 52 reflections from subglacial bedrock or sediment. The first inventory of 17 lakes (Oswald and Robin, 53 54 1973) revealed that the centre of the East Antarctic Ice Sheet supported a widespread collection of 55 large water bodies. This was followed by further discoveries, including the first detection of Lake 56 Vostok, the largest Antarctic subglacial lake (Robin et al., 1977). Subglacial lakes exist because ice is a 57 particularly good insulator. With only background levels of geothermal heating, if the ice is 58 59 60 1

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1 2 3 sufficiently thick, the temperature at the bed can reach the pressure melting value. Subglacial water 4 will flow under gravity and ice overburden pressure and pool where the hydrological potential is at a 5 minimum. Such minima can form within discrete topographic basins and over flatter topography as a 6 7 consequence of a localised shallow ice thickness gradient resulting from ice flow variations. As ice 8 surface slopes have an order of magnitude greater influence on water flow than basal slopes 9 (Shreve, 1972) a phenomenon can occur in which basal water flows uphill provided basal slopes are 10 less than ten times (and in opposite direction to) the ice surface. 11 12 Subsequent to the discovery of subglacial lakes over 40 years ago, the consensus among glaciologists 13 14 at the time was that water flowed very slowly at the Antarctic Ice Sheet bed and, therefore, had 15 minimal glacial dynamical impact. As a consequence, little research on subglacial lakes was 16 conducted in the 1980s, bar an unpublished chapter in a PhD thesis (McIntyre, 1983). This lack of 17 research coincided with the cessation of large-scale long-range radar sounding (Dean et al., 2008; 18 For Review Only 19 Naylor et al., 2008), in favour of smaller-scale hypothesis-driven data acquisition. 20 21 Interest in subglacial lakes was renewed in the early 1990s, when high precision ERS-1 satellite radar 22 altimetry revealed that the 1970s radar data from Lake Vostok coincided with a notably flat ice 23 surface (Ridley et al., 1993); the flat surface being caused by the change in ice dynamics from 24 grounded ice shearing to lateral extension when afloat. The association between the two datasets 25 26 implied that the lake was >200 km long and ~50 km wide. Through a remarkable episode of 27 serendipity, the satellite data also revealed that Vostok Station, the site of the deep palaeoclimate 28 ice core, was located at the southern extreme margin of the lake. Seismic investigations at Vostok 29 Station had been conducted in the early 1960s by Russian scientists to determine the ice thickness, 30 31 so it was possible that these old data contained a record of the lake floor. A meeting was convened 32 in Cambridge in 1994, with the (now retired) scientists responsible for the data acquisition and those 33 familiar with the satellite observations, to investigate what the multiple datasets collectively 34 revealed about the lake. Thus, the first multinational collaborative work on Lake Vostok was formed 35 36 leading to the discovery that it is at least 500 m deep (Kapitsa et al., 1996). This breakthrough paper 37 – the first to measure the water depth of a subglacial lake – led to awareness that this and other 38 lakes, were extreme yet viable habitats for microbial life and holders of ancient climate records (Ellis 39 Evans and Wynn Williams, 1996). 40 41 In the twenty years that followed, our appreciation of Antarctic subglacial lakes developed 42 43 considerably and allowed a wider appreciation of hydrological processes beneath the ice. We now 44 know that over 400 lakes exist at the ice sheet bed. The last full inventory of subglacial lakes 45 provided information on 379 features (Wright and Siegert., 2012); this was updated later in the same 46 year with the publication of a further 19 lakes across Dome C and the Aurora Subglacial Basin 47 48 (Wright et al., 2012) and more recently by the discovery of three lakes in the upper catchment of 49 Byrd Glacier (Wright et al., 2014) and one at the West Antarctic ice divide (Rivera et al., 2015), taking 50 the tally of published Antarctic subglacial lakes to 402, with several additional lakes noted in as yet 51 unpublished records. 52 53 Some subglacial lakes are prone to sudden discharges of water (‘active’ subglacial lakes), which can 54 55 flow hundreds of kilometres and also connect with other lakes (Wingham et al., 2006). Such water 56 flow can influence the short-term flow of ice above (Stearns et al., 2008) and many lakes are 57 associated with the onset of enhanced flow acting as potential regulators of ice sheet dynamics 58 59 60 2

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1 2 3 (Siegert and Bamber, 2000; Bell et al., 2007). We also know that Greenland contains a low number 4 (currently four) of subglacial lakes (Palmer et al., 2013; Willis et al., 2014; Howat et al. 2015) toward 5 the edges of the ice sheet. 6 7 The last review on the state of knowledge of subglacial lakes and their impact on ice sheet hydrology 8 9 was undertaken by Wright and Siegert (2011). An excellent recent review of subglacial hydrology is 10 provided in Ashmore and Bingham (2014). Here, we concentrate on discoveries since 2011 that 11 further add to our view of Antarctica subglacial lakes as a diverse, dynamic and intriguing system, on 12 which further exciting research awaits. 13 14

15 16 17 Satellite Altimetric Detection of Active Subglacial Lakes 18 For Review Only 19 Satellite altimetry techniques have been used on numerous occasions to identify the presence of 20 ‘active’ subglacial lakes, their connectivity and their impact on ice dynamics (Gray et al., 2005; 21 Wingham et al., 2006; Fricker et al., 2007; Stearns et al., 2008). The assumption of these studies is 22 that any short-term focused variation in the surface elevation of the ice sheet is an expression of the 23 24 movement of water beneath the ice sheet (i.e. the draining or filling of subglacial water bodies). 25 Along-track measurements of ice surface elevation using the ICESat satellite, operational between 26 2003 and 2009, were used to produce the first continent-scale map of ‘active’ subglacial lakes in 27 Antarctica (Smith et al., 2009). The distribution of ‘active’ subglacial lakes is distinct from that of the 28 29 deep interior continental lakes (Wright and Siegert 2012), with the majority of ‘active’ lakes located 30 between ice stream onset zones and the ice sheet margin (Siegert et al., this volume). 31 32 One of the most remarkable ‘active’ subglacial lakes identified by Smith et al., (2009) is Lake Cook, 33 located in Victoria Land, East Antarctica. A suite of remote sensing methods have recently been used 34 to monitor and investigate this subglacial lake, including ICESat, CryoSat-2, (McMillan et al., 2013) 35 36 and ASTER and SPOT5 satellite imagery (Flament et al., 2014). The data show the drainage of Lake 37 Cook led to drawdown of the ice sheet surface of ~70 m between 2007 and 2008 and the formation 38 of a 260 km 2 surface depression. Between 2009 and 2012 the lake re-filled slowly, causing the ice 39 sheet surface to slowly rise again (McMillan et al., 2013; Flament et al., 2014). The Lake Cook 40 3 41 outburst represents the largest single subglacial drainage event (5.2 ± 1.5 km ) yet reported from 42 Antarctica (Flament et al., 2014). Analysis of radar altimetry down-ice of the lake demonstrated the 43 impact its drainage had on subsequent downstream lakes, providing evidence for a 500-km long 44 connected hydrological system in the Wilkes Basin (Flament et al., 2014). 45 46 Since the publication of Smith et al. (2009), several radar surveys have targeted ‘active’ subglacial 47 48 lakes (e.g. Welch et al., 2009; Langley et al., 2011; Siegert et al., 2014; Wright et al., 2014). A 49 significant proportion of these have found little or no evidence for the presence of significant 50 volumes of subglacial water beneath the observed ice surface anomalies. This may be for several 51 reasons: (i) the change in ice surface elevation was not caused by the movement of subglacial water 52 53 (i.e. they do not represent ‘lakes’); (ii) the lakes were empty at the time of the RES survey; (iii) the 54 central coordinates of the mapped surface area of the ‘active’ lakes targeted by the RES surveys are 55 offset from the locality which experienced the greatest range of surface elevation change observed 56 by satellite altimetry (i.e. the subglacial water body was more limited than the area identified and 57 reported by Smith et al. (2009) and the RES surveys did not target the correct locality); or (iv) the 58 59 60 3

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1 2 3 surface elevation expressions are artefacts of the ICESat data and its analysis. The latter is most likely 4 to occur where the surface elevation change is negligible (i.e. within or close to the uncertainties 5 associated with the data), or where the ‘active’ lakes were inferred from a small number of ICESat 6 7 tracks (i.e. 1 or 2 passes). 8 9 To investigate the nature of the subglacial environment directly beneath and around a zone of 10 known ice-surface elevation change, Siegert et al. (2014) undertook a targeted airborne survey of 11 the ‘active’ subglacial lake named Institute E2, located in the upper catchment of the Institute Ice 12 Stream in West Antarctica. The survey included a polarimetric set of profiles, centred on the 13 14 published coordinate for the middle of the ‘active’ lake (Smith et al., 2009), as well as numerous 15 transects surrounding the region. The result was a high-resolution image of the ice sheet topography 16 around the ‘active’ lake and radar reflection data collected at a variety of orientations over the lake 17 (Figure 1). Two main observations were made from the data, which help us understand the nature of 18 For Review Only 19 Institute E2. 20 21 The first thing to note about the resulting dataset is that no obvious radar reflection from a deep- 22 water subglacial lake was identified (Figure 2). Normally, one might speculate this could be because 23 of the orientation of the profile (as subglacial corrugation is possible over subglacial lakes, which 24 would lead to scattering of radar reflections). However, as a polarimetric survey was conducted, this 25 26 possibility can effectively be ruled out. Reflected power is also a good indication of subglacial water, 27 as those from an ice-water interface as generally 10-20 dB greater than that from ice and rock 28 (Oswald and Robin, 1973). The data for Institute E2 show a high degree of variability in reflected 29 power, with some level of spatial coherence, but not consistent with the expected outline of the 30 31 ‘active’ lake from surface observations (Figures 1 and 2). The first conclusion from analysis of radar 32 data is that there is no evidence for Institute E2 being a deep-water subglacial lake with dimensions 33 described in Smith et al. (2009). 34 35 The second observation is that Institute E2 does coincide, at least in part, with a small minimum in 36 the subglacial hydrological potential, meaning that basal water is expected to accumulate at some 37 38 level. The location of this potential minimum is downstream of a subglacial hill. The implication is 39 that Institute E2 is an ephemeral, likely shallow, lake that exists because water pools on the lee side 40 of a subglacial hill; the potential minimum being a result of the ice thickness gradient rather than the 41 basal topography. One possible explanation, given the spatially restricted nature of the hydrological 42 43 minimum, is that its dimensions are far smaller than those proposed by Smith et al. (2009). 44 45 Clearly the difference between Institute E2 and Lake Vostok are substantial and they possibly 46 represent end-members of the spectrum of Antarctic subglacial lakes. To see whether other ‘active’ 47 subglacial lakes conformed to observations at Institute E2, Wright et al. (2014) undertook an analysis 48 of airborne radar data across a selection of ‘active’ subglacial lake locations within the Byrd Glacier 49 50 catchment in East Antarctica, the region where Stearns et al. (2008) had noticed an association 51 between subglacial lake discharges and ice-sheet flow enhancement. As per Siegert et al. (2014), no 52 direct evidence for subglacial lakes was found for any of the potential ‘active’ lake sites in the radar 53 data. The radar used was certainly capable of detecting subglacial lakes, as three new lakes were 54 55 discovered in the same survey (the unique ‘three-tier’ lake system, in which subglacial water 56 cascades up a subglacial hill while pooling on three occasions). Across the whole region, considerable 57 topographic relief was measured, with an abundance of subglacial hills up to 500 m in height obvious 58 59 60 4

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1 2 3 in the data. As with Institute E2, it seems likely that the flow of subglacial water in the Byrd Glacier 4 catchment is influenced heavily by subglacial topography and that ephemeral pooling of the water is 5 possible in the lee side of hills. It also seems likely that the horizontal dimensions of ‘active’ lakes 6 7 derived from satellite observations are far greater than that of the water change responsible for the 8 changes detected. If this is true, for conservation of volume to be upheld, the vertical change in the 9 water level must be greater than the elevation change observed at the ice surface, with the 10 implication being that ‘active’ lakes are relatively deep when filled. 11 12

13 14 Ice Shelf Channels and Evidence of Organised Subglacial Flow of Water 15 16 17 The dynamic behaviour of ‘active’ lakes demonstrates that there are hydrological pathways and 18 connections beneathFor the ice sheet. Review Wright et al. (2012) showed Only that these connections are capable 19 of transporting subglacial water generated in the centre of the ice sheet to the margins. They used a 20 combination of radar analysis and numerical ice sheet modelling to reveal that the ice sheet bed was 21 continuously wet between the subglacial lakes at Dome C in central East Antarctica and the coast at 22 23 Totten Glacier via the Aurora subglacial basin. As a consequence they concluded there was nothing 24 to stop water flowing from the ice sheet centre to the margin (Figure 3). Modelling of ice and water 25 flow in the Siple Coast in West Antarctica also indicates that subglacial water drains through a 26 subglacial lake system and exits at the grounding line (Carter et al. 2012).The nature of the 27 28 hydrological system, driven by both periodic lake drainage and a background hydrological flow, is 29 still in question however and this uncertainty has large implications for our appreciation of the 30 interaction between subglacial water and the ice flow above it. 31 32 Wingham et al. (2006) first evaluated the nature of the connections between subglacial lakes, 33 suggesting an R-channel, incised upwards into the ice, was likely to be a plausible mechanism for 34 35 draining the volumes of water in the ‘active’ lakes. Schroeder et al. (2013) provide further evidence 36 for channelised water flow through R-channels, from an analysis of the specularity of radar returns 37 from the ice sheet base beneath the Thwaites Glacier in West Antarctica (see also Young et al., this 38 volume). The question of whether the periodic draining of lakes or the background hydrological 39 40 system is capable of supporting an R-channel type channelised drainage is still under investigation 41 (Fricker et al., this volume). Carter et al. (in review) suggests it is more likely that the lakes are 42 drained through channels cut into the sediment, for example. Improved inferences of geothermal 43 heat flux (Schroeder et al., 2014) suggest that the heat flux beneath Thwaites Glacier may be more 44 -2 45 than three times the value (~200 compared with ~60 mW m ) used in most ice sheet models that 46 produce basal melt estimates. Using the correct (greater) geothermal heat flux may produce higher 47 meltrates than previously considered, which may be capable of supporting persistent R-channels 48 beneath the Thwaites Glacier. 49 50 Le Brocq et al. (2013) provide further evidence for water flowing in focussed units beneath the ice 51 52 sheet, though they draw no conclusions about whether the channels are incised into the ice or the 53 sediment. Observations of often sinuous channels on the surface of ice shelves indicate the presence 54 of channels incised into the underside of the ice shelf (Figure 4). These sub-ice-shelf channels line up 55 with flow routes predicted to be taken by subglacial water drainage, indicating that the ice shelf 56 57 channels are cut by meltwater plumes generated by buoyant subglacial water exiting the grounding 58 line in a focussed manner. It is not clear whether the background hydrological system is causing 59 60 5

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1 2 3 these features, or whether lake drainage leads to elevated volumes of meltwater that are capable of 4 forming an organised drainage system. In some cases, the ice shelf channels show a level of sinuosity 5 that is only explicable through lateral migration of the basal water exiting the ice sheet base, 6 7 suggesting changes to the grounding line position and/or temporal and spatial modifications to the 8 subglacial hydrology system upstream (Figure 4; Le Brocq et al. 2013). 9 10 11 12 Channels at the Bed of the Ice Sheet 13 14 Geomorphologic evidence for channels associated with former subglacial flow of water and its 15 connectivity is well documented from the areas of former Northern Hemisphere glaciation (e.g 16 17 Kristensen et al., 2007), from the Antarctic Dry Valleys (Lewis et al., 2006) and from the offshore 18 Antarctic continentalFor shelf (e.g. LoweReview and Anderson, 2003). WeOnly have limited comparable evidence 19 beneath the present-day Antarctic Ice Sheet, however. 20 21 RES data from the Weddell Sea Embayment in East Antarctica revealed a 100 km long, 70 km wide 22 system of major incised channels (5 km wide and >200 m deep) and smaller-scale canyon-like 23 24 structures (250 m wide and <40 m deep) (Figure 5) (Rose et al., 2014). Equivalent geomorphic 25 features found in the Wilkes Basin, in East Antarctica, were interpreted as subglacial meltwater 26 channels formed during the drainage of a large (850 km 3) palaeo-subglacial lake in the upper Wilkes 27 Basin in the middle Miocene (Jordan et al., 2010). Characterisation of such landforms is often 28 29 restricted by the line spacing of RES surveys, preventing continuous along-track identification and 30 measurement. Such limitations can be overcome if satellite-derived ice surface imagery and radar 31 backscatter data can be used to infer bed properties between RES data. Rose et al. (2014) utilised 32 this approach to reveal the presence of a system of ancient large sub-parallel subglacial channels 33 beneath a slow flowing sector of the West Antarctic Ice Sheet between the Foundation and Möller 34 35 Ice Streams. These subglacial channels (average width 2.6 km, average depth 158.9 m) extend for at 36 least 250 km. They are thought to be caused by flow of water at the bed of the ice sheet, rather than 37 by surface (i.e. fluvial) water flow, as they lie below present-day sea level (average elevation of -623 38 m) and have undulating longitudinal profiles. The present-day ice sheet over them is cold based, 39 -1 40 meaning they are inactive as water conduits at present, and slow flowing (i.e. <5 ma ), meaning the 41 channels have clear surface expressions (i.e. each channel is associated with a narrow elongated 42 depression on the ice sheet surface, which is apparent in optical and radar mosaics of the Antarctic 43 Ice Sheet, e.g. Scambos et al., 2007). Combining the ice-surface imagery with RES observations of the 44 45 channels, Rose et al. (2014) were able to map the geomorphic form and extent of the channels in 46 detail. The direction of the channels, at an angle to the ice-sheet surface slope, indicates that even if 47 the ice sheet were warm based, water would not be routed along them. This implies that a former 48 ice sheet, with different configuration than today, was responsible for their formation. The channels 49 50 are not thought to be the result of the rapid drainage of a former subglacial lake, due to the lack of a 51 potential site upstream for such a lake. Instead, Rose et al. (2014) interpreted their formation as 52 resulting from the transfer of surface meltwaters to the ice sheet bed, during a time when the West 53 Antarctic Ice Sheet was temperate and subject to significant amounts of seasonal surface melt. Such 54 55 meltwater may have acted to fill the Marginal Basins (Rose et al., 2014), which are located upstream 56 of the channels, and which may have acted to focus the meltwater from upstream. Rose et al. (2014) 57 58 59 60 6

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1 2 3 postulated that the Pliocene was the most recent date at which this could have occurred, suggesting 4 a highly dynamic West Antarctic Ice Sheet at this time. 5 6 While channels at the ice-sheet bed have only been identified across a few locations, it is entirely 7 possible that other regions of the ice sheet bed contain both relic and ‘active’ subglacial drainage 8 9 features. By combining satellite and airborne geophysical datasets, using techniques established in 10 Rose et al. (2014) and Jordan et al. (2010), the importance of extensive connected subglacial 11 drainage systems to the Antarctic Ice Sheet through geological time and their long-term impact as 12 agents of landscape evolution, may be evaluated more fully. 13 14

15 16 17 Summary and Future Work 18 For Review Only 19 Research undertaken in the last four years has expanded our knowledge of subglacial hydrology 20 considerably and has also allowed a number of new research questions to be framed. Wright and 21 Siegert (2012), Wright et al. (2012) and Wright et al. (2014) list evidence for 402 Antarctic subglacial 22 lakes. Wright and Siegert (2012) also plotted the timeframe over which lake discoveries had been 23 24 published, demonstrating an apparent exponential growth in lake numbers with time. Clearly, the 25 number of actual subglacial lakes is finite, hence the curve must flatten at some stage. If we consider 26 that very few subglacial lakes have been discovered in the past few years, this transition in the time 27 series of discoveries may have already started. 28 29 The notion of ‘active’ subglacial lakes has provoked speculation that subglacial lake water at the 30 31 centre of the ice sheet is able to flow to the margin. While direct evidence for such flow will 32 obviously be difficult to acquire, Wright et al. (2012) have shown that there is little impediment to 33 such flow at least in the Aurora basin and Totten Glacier region of East Antarctica. Evidence for 34 subglacial water emerging at the margin is compelling (Le Brocq et al., 2013) and, while there is 35 36 debate on whether such water derives from subglacial lake discharge, it must be fed from an 37 upstream source. This water is likely to be composed of a combination of local basal melting and 38 more distant sources, the proportions of which and the influence of ‘active’ lakes in driving water to 39 the margin, are as yet poorly understood. 40 41 Another unknown is the size and nature of the basal water bodies that are responsible for ice- 42 43 surface elevation changes. Research to date points to much smaller water bodies than originally 44 depicted, but more work is needed to fully appreciate the hydrological processes responsible for the 45 surface observations. 46 47 The traditional view of Antarctic subglacial hydrology is to consider the ice-sheet bed as being 48 impermeable. This is almost certainly not the case and there is every possibility that groundwater 49 50 exists across large portions of the continent. Indeed ice-sheet modelling has pointed to groundwater 51 as being critical to maintaining the flow of ice streams in the Siple Coast (Christoffersen et al., 2014). 52 Further, Wadham et al. (2012) postulated that biogeochemical processes within deep water- 53 saturated sediments may have led to substantial stores of methane within a microbially-active 54 55 wetland system beneath the ice. Despite the likelihood of its existence and its potential significance, 56 Antarctic groundwater has never been detected, apart from at the edges of the continent, such as in 57 the Dry Valleys, where highly-concentrated brine has been detected using airborne electromagnetic 58 59 60 7

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1 2 3 methods (Mikucki et al., 2015). The reason groundwater has yet to be detected is that the 4 geophysical experiment needed has yet to be designed and undertaken. Indeed, sounding beneath 5 the ice-bed interface, which itself is beneath several kilometres of ice, is conceptually challenging. 6 7 The solution will probably lie in a combination of geophysical techniques, such as 3-D seismics, ice- 8 penetrating radar, electro seismics and electro magnetics, acquiring data simultaneously at small 9 scale and in high resolution. Several sites where deep basal sediments are likely are known (e.g. the 10 Aurora Basin, Siple Coast and Institute Ice Stream). Future targeted geophysical research here may 11 12 lead to the discovery of a further element at the scientific frontier of Antarctic subglacial research. 13 14 15 16 Competing Interests 17 18 We have no competingFor interests toReview declare. Only 19 20 21 22 Author contributions 23 24 All authors planned the paper’s structure and content; MJS led the writing with NR and ALB 25 contributing specific sections; all authors contributed to revising the paper in light of referees’ 26 advice. 27 28

29 30 Acknowledgements 31 32 We wish to thank two anonymous referees for their helpful and supportive comments on an earlier 33 draft. 34 35 36 Funding statement 37 38 We acknowledge funding from UK NERC grants NE/G013071/1, NE/D003733/1 and 39 NE/G00465X/1/2/3. 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 8

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1 2 3 References 4 5 Ashmore, D. W., & Bingham, R. G. Antarctic subglacial hydrology: an overview of current knowledge 6 and forthcoming scientific challenges. Antarctic Science , 26(6), 758-773. Doi: 7 10.1017/S0954102014000546 , (2014). 8 9 Bell, R.E., Studinger, M., Shuman, C.A., Fahnestock, M.A., and Joughin, I. Large subglacial lakes in 10 East Antarctica at the onset of fast-flowing ice streams, Nature , 445, 904–907 (2007). 11 Carter, S.P. and Fricker, H.A. The supply of subglacial meltwater to the grounding line of the Siple 12 Coast, West Antarctica. Annals of Glaciology 53(60) 267-280. (2012) 13 14 Christoffersen, P., Bougamont, M., Carter, S.P., Fricker, H.A. and Tulaczyk, S. Significant groundwater 15 contribution to Antarctic ice streams hydrologic budget. Geophys. Res. Lett. 41, 2003-2010, (2014). 16 17 Dean, K., Naylor, S. and Siegert, M. Data in Antarctic Science and Politics. Social Studies of Science , 18 38/4, 571–604. (2008).For Review Only 19 Ellis-Evans, J.C., and Wynn-Williams, D., 1996. Antarctica: A great lake under the ice. Nature , 381: 20 644-646. 21 22 Flament, T., Berthier, E., and Rémy, F.: Cascading water underneath Wilkes Land, East Antarctic ice 23 sheet, observed using altimetry and digital elevation models, The Cryosphere, 8, 673-687, 24 doi:10.5194/tc-8-673-2014 (2014). 25 26 Fretwell, P., Pritchard, H.D., Vaughan, D.G., Bamber, J.L., Barrand, N.E., Bell, R., Bianchi, C., Bingham, 27 R.G., Blankenship, D.D., Casassa, G., Catania, G., Callens, D., Conway, H., Cook, A.J., Corr, H.F.J., 28 Damaske, D., Damm, V., Ferraccioli, F., Forsberg, R., Fujita, S., Gim, Y., Gogineni, P., Griggs, J.A., 29 Hindmarsh, R.C.A., Holmlund, P., Holt, J.W., Jacobel, R.W., Jenkins, A., Jokat, W., Jordan, T., King, 30 E.C., Kohler, J., Krabill, W., Riger-Kusk, M., Langley, K.A., Leitchenkov, G., Leuschen, C., Luyendyk, 31 B.P., Matsuoka, K., Mouginot, J., Nitsche, F.O., Nogi, Y., Nost, O.A., Popov, S.V., Rignot, E., Rippin, 32 D.M., Rivera, A., Roberts, J., Ross, N., Siegert, M.J., Smith, A.M., Steinhage, D., Studinger, M., Sun, B., 33 Tinto, B.K., Welch, B.C., Wilson, D., Young, D.A., Xiangbin, C. and Zirizzotti, A.: Bedmap2: improved 34 ice bed, surface and thickness datasets for Antarctica, The Cryosphere, 7, 375-393, 2013. 35 36 Fricker, H., Scambos, T., Bindschadler, R., Padman, L. An Active Subglacial Water System in West 37 Antarctica Mapped from Space. Science, 315, 1544-1548. DOI: 10.1126/science.1136897 (2007). 38 39 Fricker, H., Siegfried, M., Carter. S. A decade of progress in observing and modelling Antarctic 40 subglacial water systems. Phil. Trans. R. Soc. A , this volume. 41 Gray, L., Joughin, I., Tulaczyk, S., Spikes, V. B., Bindshadler R., and Jezek, K.: Evidence for subglacial 42 water transport in the West Antarctic Ice Sheet through three-dimensional satellite radar 43 interferometry, Geophys. Res. Lett., 32, L03501, doi:10.1029/2004GL021387, 2005. 44 45 Haran, T., Bohlander, J., Scambos, T., Painter, T. and Fahnestock, M., 2005, updated 2013, MODIS 46 Mosaic of Antarctica (MOA) Image Map, Map [NSIDC_0280, MOA125_2004]: Boulder, Colorado USA: 47 National Snow and Ice Data Center, http://dx.doi.org/10.7265/N5ZK5DM5. 48 49 Howat I.M. et al . 2015. Sudden drainage of a subglacial lake beneath the Greenland Ice Sheet. The 50 Cryosphere 9, 103-108; doi: 10.5194/tc-9-103-2015 51 Jordan, T.A., Ferraccioli, F., Corr, H., Graham, A., Armadillo, E. and Bozzo, E., Hypothesis for mega- 52 outburst flooding from a palaeo-subglacial lake beneath the East Antarctic Ice Sheet. Terra Nova, 22, 53 283-289. DOI:10.1111/j.1365-3121.2010.00944.x (2010). 54 55 Kapitsa, A., Ridley, J.K., Robin, G. de Q., Siegert, M.J., and Zotikov, I., 1996. Large deep freshwater 56 lake beneath the ice of central East Antarctica. Nature , 381, 684-686. 57 58 59 60 9

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1 2 3 Kristensen, T.B., Huuse, M, Piotrowski, J.A. and Clausen, O.R. A morphometric analysis of tunnel 4 valleys in the eastern North Sea based on 3D seismic data. J. Quaternary Sci ., 22. 801–815. ISSN 5 0267-8179. (2007). 6 7 Langley, K., Kohler, J., Matsuoka, K., Sinisalo, T., Scambos, T., Neumann, T., Muto, A., Winther, J.-G., 8 and Albert, M. Recovery Lakes, East Antarctica: Radar assessment of sub-glacial water extent, 9 Geophys. Res. Lett., 38, L05501, doi:10.1029/2010GL046094, 2011. 10 Le Brocq, A., Ross, N., Griggs, J., Bingham, R., Corr, H., Ferroccioli, F., Jenkins, A., Jordan, T., Payne, 11 A., Rippin, D., Siegert, M.J. Evidence from ice shelves for channelized meltwater flow beneath the 12 13 Antarctic Ice Sheet. Nature Geoscience , 6, 945-948. doi:10.1038/ngeo1977 (2013). 14 Lowe, A.L., and Anderson, J.B. Evidence for abundant subglacial meltwater beneath the paleo-ice 15 sheet in Pine Island Bay, Antarctica: Journal of Glaciology, v. 49, p. 125–138, doi: 16 10.3189/172756503781830971. (2003). 17 18 McIntyre, N.F., 1983.For The topography Review and flow of the Antarctic Only ice sheet. Unpublished PhD thesis, 19 University of Cambridge, England. 20 McMillan, M., Corr, H., Shepherd, A., Ridout, A., Laxon, S., and Cullen, R.: Three-dimensional 21 22 mapping by CryoSat-2 of subglacial lake volume changes, Geophys. Res. Lett., 40, 4321–4327, 23 doi:10.1002/grl.50689, 2013. 24 Mikucki, G. et al., Deep groundwater and potential subsurface habitats beneath an Antarctic Dry 25 Valley. Nature Communications , 6, 6831, doi: 10.1038/ncomms7831 (2015). 26 27 Naylor, S.K., Siegert, M.J., Dean, K. and Turchetti, S. Science, geopolitics and the governance of 28 Antarctica. Nature Geosciences , 1, 143-145. (2008). 29 Oswald, G.K.A, and Robin, G. de Q., 1973. Lakes beneath the Antarctic Ice Sheet. Nature , 245: 251- 30 31 254. 32 Palmer, S., Dowdeswell, J.A., Christoffersen, P., Young, D., Blankenship, D.D., Greenbaum, J., 33 Bamber, J., Siegert, M.J. Greenland subglacial lakes detected by radar. Geophysical Research Letters , 34 40, doi:10.1002/2013GL058383 (2013). 35 36 Ridley, J.K., Cudlip, W., and Laxon, S.W., 1993. Identification of subglacial lakes using ERS-1 radar 37 altimeter. Journal of Glaciology , 39: 625-634. 38 39 Rivera, A., Uribe, J., Zamora, R., and Oberreuter, J. Subglacial Lake CECs: Discovery and in situ survey 40 of a privileged research site in West Antarctica, Geophys. Res. Lett. , 42 , 3944–3953, 41 doi:10.1002/2015GL063390. (2015). 42 Robin, G. de Q., 1969. Antarctic RES. Philosophical Transactions of the Royal Society of London A. 265 43 (1166), 437-505. 44 45 Robin, G. de Q., Drewry, D.J., and Meldrum, D.T., 1977. International studies of ice sheet and 46 bedrock. Philosophical Transactions of the Royal Society of London A , 279, 185-196. 47 48 Rose, K.C., Ross, N., Bingham, R.G., Corr, H.F.J., Ferraccioli, F., Jordan, T.A., Le Brocq, A., Rippin, D.M. 49 and Siegert, M.J. A temperate former West Antarctic ice sheet suggested by an extensive zone of 50 bed channels. Geology , 42, 971-974. dri: 2014343, doi:10.1130/G35980.1 (2014). 51 Scambos, T. A., Haran, T. M., Fahnestock M.A., Painter, T.H., Bohlander, J. 2007 MODIS-based Mosaic 52 of Antarctica (MOA) data sets: Continent-wide surface morphology and snow grain size. Remote 53 Sensing of Environment 111(2-3): 242-257 54 55 Scambos, T.A., Haran, T.M., Fahnestock M.A., Painter T.H., Bohlander J. MODIS-based Mosaic of 56 Antarctica (MOA) data sets: Continent-wide surface morphology and snow grain size. Remote 57 Sensing of Environment 111 242–257. doi:10.1016/j.rse.2006.12.020 (2007). 58 59 60 10

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1 2 3 Schroeder D.M., Blankenship D.D., Young D.A. (2013) Evidence for a water system transition beneath 4 Thwaites Glacier, West Antarctica. PNAS 110 (30) 12225–12228. 5 6 Schroeder, D.M., Blankenship, D.D., Young, D.A. and Quartini, E. (2014) Evidence for elevated and 7 spatially variable geothermal flux beneath the West Antarctic Ice Sheet. PNAS 111 (25) 9070–9072. 8 Shreve, R.L., 1972. Movement of water in glaciers. Journal of Glaciology , 11, 205-214. 9 10 Siegert, M.J., Priscu, J., Alekhina, I., Wadham., J. and Lyons, B. Antarctic subglacial lake exploration: 11 first results and future plans. Phil. Trans. R. Soc. A , this volume. 12 Siegert, M.J., Ross, N., Corr, H., Smith, B., Jordan, T., Bingham, R., Ferraccioli, F., Rippin, D. and Le 13 Brocq, A. Boundary conditions of an active West Antarctic subglacial lake: implications for storage of 14 15 water beneath the ice sheet. The Cryosphere , 8, 15-24 doi:10.5194/tc-8-15-2014. (2014). 16 Smith, B.E., H.A. Fricker, I.R. Joughin, and S. Tulaczyk (2009), An inventory of active subglacial lakes in 17 Antarctica detected by ICESat (2003-2008), Journal of Glaciology , 55, 573-595. 18 For Review Only 19 Stearns, L.A., Smith, B.E. and Hamilton, G.S. 2008. Increased flow speed on a large East Antarctic 20 outlet glacier caused by subglacial floods. Nature Geosci , 1(12): 827-831Siegert, M.J. & J.L. Bamber. 21 Subglacial water at the heads of Antarctic ice-stream tributaries. Journal of Glaciology , 46, 702-703. 22 (2000). 23 24 Turchetti, S., Dean, K., Naylor, S. and Siegert, M. Accidents and Opportunities: A History of the Radio 25 Echo Sounding (RES) of Antarctica, 1958-1979. British Journal of the History of Science , 41, 417-444 26 (2008). 27 Wadham, J.L. et al. Potential methane reservoirs beneath Antarctica. Nature 488, 633–637, (2012). 28 29 Welch, B. C., Jacobel, R. W., and Arcone, S.: First results from radar profiles collected along the US- 30 ITASE traverse from Taylor Dome to South Pole (2006–2008), Ann. Glaciol., 50, 35–41, 2009. 31 32 Willis, M.J. et al . Recharge of a subglacial lake by surface meltwater in northeast Greenland. Nature , 33 published online January 21, 2015; doi: 10.1038/nature14116 34 Wingham, D.J., Siegert, M.J., Shepherd, A.P. and Muir, A.S. Rapid discharge connects Antarctic 35 subglacial lakes. Nature , 440, 1033-1036 (2006). 36 37 Wright, A., Siegert, M.J. The identification and physiographical setting of Antarctic subglacial lakes: 38 an update based on recent geophysical data. In, Subglacial Antarctic Aquatic Environments (M. 39 Siegert, C. Kennicutt, B. Bindschadler, eds.). AGU Geophysical Monograph 192. Washington DC. 9-26. 40 (2011). 41 42 Wright, A.P. and Siegert, M.J. A fourth inventory of Antarctic subglacial lakes. Antarctic Science , 24, 43 659–664. doi:10.1017/S095410201200048X (2012). 44 Wright, A.P., Young, D.A., Bamber, J.A., Dowdeswell, J.A., Payne, A.J., Blankenship, D.D. and Siegert, 45 M.J. Subglacial hydrological connectivity within the Byrd Glacier catchment, East Antarctica. Journal 46 of Glaciology 60 (220), 345-352 doi: 10.3189/2014JoG13J014. (2014). 47 48 Wright, A.P., Young, D.A., Roberts, J.L., Schroeder, D.M., Bamber, J.L., Dowdeswell, J.A., Young, N., 49 LeBrocq, A.M., Warner, R.C., Payne, A.J., Blankenship, D.D., van Ommen, T. and Siegert, M.J. 50 Evidence for a hydrological connection between the ice divide and ice sheet margin in the Aurora 51 Subglacial Basin, East Antarctica. Journal of Geophysical Research. Earth Surface 117, F01033, 15pp, 52 doi:10.1029/2011JF002066 (2012). 53 54 Young et al. The distribution of basal water between Antarctic subglacial lakes from radar sounding. Phil. 55 Trans. R. Soc. A , this volume 56 57 58 59 60 11

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1 2 3 Zwally, H.J., Schutz, R., Bentley, C., Bufton, J., Herring, T., Minster, J., Spinhirne, J. and Thomas, R. 4 GLAS/ICESat L2 Antarctic and Greenland Ice Sheet Altimetry Data version 31, 20th February 2003 to 5 8th April 2009. (2011). 6 7 8 9 10 11 12 13 14 15 16 17 18 For Review Only 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 12

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1 2 3 FIGURES 4

5 6 7 Figure 1. Geophysical investigations of ‘active’ subglacial lake Institute E2. (a) Subglacial topography 8 of the Institute Ice Stream region (with insert for study region in West Antarctica) (Fretwell et al., 9 10 2013). The grounding line is provided in white. Elevations are in meters above WGS84. (b) Ice- 11 surface elevation with RES (grey lines) (Ross et al., 2012) and ICESat transects (black dotted lines) 12 13 over and around Institute E2 (dashed line), as delineated by Smith et al. (2009). RES lines in black 14 15 and labelled A-A’, B-B’ and C-C’ refer to RES transects provided in Figure 2. Elevations are in meters 16 above WGS84. Taken from Siegert et al. (2014). 17 18 For Review Only 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 13 http://mc.manuscriptcentral.com/issue-ptrsa Submitted to Phil. Trans. R. Soc. A - Issue Page 14 of 22

1 2 3 Figure 2 4 5 6 7 Figure 2. RES transects centred on Institute E2. The locations of the transects are provided in Figure 8 9 1b. (a) Transect A-A’. (b) Transect B-B’. (c) Transect C-C’. For each transect, the coverage of the 10 ICESat-derived lake extent (after Smith et al., 2009) is shown as a white bar on the radargram. 11 12 Beneath the radargrams graphs of ice surface elevation (m WGS84), bed elevation (m WGS84), basal 13 14 hydropotential and basal reflectivity are provided. Taken from Siegert et al. (2014). 15 16 17 18 For Review Only 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 14 http://mc.manuscriptcentral.com/issue-ptrsa Page 15 of 22 Submitted to Phil. Trans. R. Soc. A - Issue

1 2 3 Figure 3 4 5 6 7 Figure 3. The subglacial hydrology of the Aurora Subglacial Basin area, including the locations of 8 subglacial lakes (triangles) and all sites of surface height change interpreted as subglacial water 9 movements (purple circles). Sites that do not resemble substantial lakes in radar data, but are 10 identified by the automated algorithm for detecting subglacial water, are also shown. The extent of 11 the predicted flow paths of subglacial water is limited to the areas of the subglacial topography at 12 the pressure melting point. Subglacial lakes with numbers related to new lakes discovered since the 13 last full inventory (Wright and Siegert, 2012) increasing the tally of Antarctic subglacial lakes to 398. 14 15 Taken from Wright et al. (2012). 16 17 18 For Review Only 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 15 http://mc.manuscriptcentral.com/issue-ptrsa Submitted to Phil. Trans. R. Soc. A - Issue Page 16 of 22

1 2 3 Figure 4 4 5 6 7 Figure 4. Ice-shelf surface channels visible on the MODIS Mosaic of Antarctica, overlain by 8 calculations of subglacial meltwater flux. (a) Filchner-Ronne Ice Shelf, the arrowed features are 9 downstream of (left to right) Institute, Moller, Foundation and Support Force Ice Streams. (b) 10 MacAyeal Ice Stream, flowing into the Ross Ice Shelf. (c-d) A series of small East Antarctic ice shelves. 11 (e) Lambert Glacier, which flows into the Amery Ice Shelf. Black arrows highlight ice-shelf surface 12 channel features. Orange circles indicate evidence of migration of the exit point of subglacial 13 channels. Green line is the MODIS-derived ice-sheet grounding line. Dashed lines on (a) are airborne 14 15 radar flightlines. Taken from Le Brocq et al. (2013). 16 17 18 For Review Only 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43

44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 16 http://mc.manuscriptcentral.com/issue-ptrsa Page 17 of 22 Submitted to Phil. Trans. R. Soc. A - Issue

1 2 3 Figure 5 4 5 6 Figure 5. (a) MODIS satellite imagery (Haran et al. 2013) revealing the location of the inner Weddell 7 Sea Embayment, West Antarctica. Black box marks the extent of panels (b) and (c). (b) Linear surface 8 features identified in MODIS imagery. (c) Subglacial topography overlain with channel locations 9 (white lines) observed in (b). White diamonds identify subglacial channels that are visible in both the 10 MODIS imagery and in ice-penetrating radar data. Black diamonds mark channels only visible in 11 radar data. The black dashed line is the 0 m elevation contour. Solid black lines denote the Marginal 12 13 Basins (-650 m contour). Annotations are provided as follows: BI – Berkner Island; BIR – Bungenstock 14 Ice Rise; DIR – Doake Ice Rumples; FlP – Fletcher Promontory; FoP – Fowler Peninsula; FIS – 15 Foundation Ice Stream; HIR – Henry Ice Rise; KIR – Korff Ice Rise; MIS – Möller Ice Stream; PN – 16 Pagano Nunatak; SIR – Skytrain Ice Rise; SH – Stewart Hills; TB – Transitional Basins; TT – Thiel 17 Trough (southern edge). Taken from Rose et al. (2014). 18 For Review Only 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 17 http://mc.manuscriptcentral.com/issue-ptrsa Submitted to Phil. Trans. R. Soc. A - Issue Page 18 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 For Review Only 19 20 21 22 23 24 25 26 27 Figure 1. Geophysical investigations of ‘active’ subglacial lake Institute E2. (a) Subglacial topography of the 28 Institute Ice Stream region (with insert for study region in West Antarctica) (Fretwell et al., 2013). The 29 grounding line is provided in white. Elevations are in meters above WGS84. (b) Ice-surface elevation with 30 RES (grey lines) (Ross et al., 2012) and ICESat transects (black dotted lines) over and around Institute E2 31 (dashed line), as delineated by Smith et al. (2009). RES lines in black and labelled A-A’, B-B’ and C-C’ refer 32 to RES transects provided in Figure 2. Elevations are in meters above WGS84. Taken from Siegert et al. (2014). 33 159x94mm (220 x 220 DPI) 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 http://mc.manuscriptcentral.com/issue-ptrsa Page 19 of 22 Submitted to Phil. Trans. R. Soc. A - Issue

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 For Review Only 19 20 21 22 23 24 25 26 Figure 2. RES transects centred on Institute E2. The locations of the transects are provided in Figure 1b. (a) 27 Transect A-A’. (b) Transect B-B’. (c) Transect C-C’. For each transect, the coverage of the ICESat-derived 28 lake extent (after Smith et al., 2009) is shown as a white bar on the radargram. Beneath the radargrams 29 graphs of ice surface elevation (m WGS84), bed elevation (m WGS84), basal hydropotential and basal 30 reflectivity are provided. Taken from Siegert et al. (2014). 31 159x90mm (220 x 220 DPI) 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 http://mc.manuscriptcentral.com/issue-ptrsa Submitted to Phil. Trans. R. Soc. A - Issue Page 20 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 For Review Only 19 20 21 22 23 24 25 26 27 28 29 Figure 3. The subglacial hydrology of the Aurora Subglacial Basin area, including the locations of subglacial 30 lakes (triangles) and all sites of surface height change interpreted as subglacial water movements (purple 31 circles). Sites that do not resemble substantial lakes in radar data, but are identified by the automated 32 algorithm for detecting subglacial water, are also shown. The extent of the predicted flow paths of subglacial 33 water is limited to the areas of the subglacial topography at the pressure melting point. Subglacial lakes 34 with numbers related to new lakes discovered since the last full inventory (Wright and Siegert, 2012) 35 increasing the tally of Antarctic subglacial lakes to 398. Taken from Wright et al. (2012). 36 349x233mm (96 x 96 DPI) 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 http://mc.manuscriptcentral.com/issue-ptrsa Page 21 of 22 Submitted to Phil. Trans. R. Soc. A - Issue

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 For Review Only 19 20 21 22 23 24 25 26 27 28 29 30 Figure 4. Ice-shelf surface channels visible on the MODIS Mosaic of Antarctica, overlain by calculations of subglacial meltwater flux. (a) Filchner-Ronne Ice Shelf, the arrowed features are downstream of (left to 31 right) Institute, Moller, Foundation and Support Force Ice Streams. (b) MacAyeal Ice Stream, flowing into 32 the Ross Ice Shelf. (c-d) A series of small East Antarctic ice shelves. (e) Lambert Glacier, which flows into 33 the Amery Ice Shelf. Black arrows highlight ice-shelf surface channel features. Orange circles indicate 34 evidence of migration of the exit point of subglacial channels. Green line is the MODIS-derived ice-sheet 35 grounding line. Dashed lines on (a) are airborne radar flightlines. Taken from Le Brocq et al. (2013). 36 265x178mm (96 x 96 DPI) 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 http://mc.manuscriptcentral.com/issue-ptrsa Submitted to Phil. Trans. R. Soc. A - Issue Page 22 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 For Review Only 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 Figure 5. (a) MODIS satellite imagery (Haran et al. 2013) revealing the location of the inner Weddell Sea Embayment, West Antarctica. Black box marks the extent of panels (b) and (c). (b) Linear surface features 48 identified in MODIS imagery. (c) Subglacial topography overlain with channel locations (white lines) 49 observed in (b). White diamonds identify subglacial channels that are visible in both the MODIS imagery and 50 in ice-penetrating radar data. Black diamonds mark channels only visible in radar data. The black dashed 51 line is the 0 m elevation contour. Solid black lines denote the Marginal Basins (-650 m contour). Annotations 52 are provided as follows: BI – Berkner Island; BIR – Bungenstock Ice Rise; DIR – Doake Ice Rumples; FlP – 53 Fletcher Promontory; FoP – Fowler Peninsula; FIS – Foundation Ice Stream; HIR – Henry Ice Rise; KIR – 54 Korff Ice Rise; MIS – Möller Ice Stream; PN – Pagano Nunatak; SIR – Skytrain Ice Rise; SH – Stewart Hills; 55 TB – Transitional Basins; TT – Thiel Trough (southern edge). Taken from Rose et al. (2014). 78x119mm (300 x 300 DPI) 56 57 58 59 60 http://mc.manuscriptcentral.com/issue-ptrsa