Union Glacier Please Refer to the Corresponding ﬁnal Paper in TC If Available

icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion The Cryosphere Discuss., 8, 1227–1256, 2014 Open Access www.the-cryosphere-discuss.net/8/1227/2014/ The Cryosphere TCD doi:10.5194/tcd-8-1227-2014 Discussions © Author(s) 2014. CC Attribution 3.0 License. 8, 1227–1256, 2014

This discussion paper is/has been under review for the journal The Cryosphere (TC). Union Glacier Please refer to the corresponding ﬁnal paper in TC if available. A. Rivera et al. Union Glacier: a new exploration gateway for the West Antarctic Ice Sheet Title Page Abstract Introduction A. Rivera1,2, R. Zamora1, J. A. Uribe1, R. Jaña3, and J. Oberreuter1 Conclusions References 1Centro de Estudios Cientíﬁcos, P.O. Box 5110466, Valdivia, Chile 2Departamento de Geografía, Universidad de Chile, P.O. Box 3387, Santiago, Chile Tables Figures 3Instituto Antártico Chileno, Punta Arenas, Chile

Received: 31 December 2013 – Accepted: 24 January 2014 – Published: 20 February 2014 J I Correspondence to: A. Rivera ([email protected]) J I

Published by Copernicus Publications on behalf of the European Geosciences Union. Back Close

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1227 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Abstract TCD Union Glacier (79◦460 S/83◦240 W) in the West Antarctic Ice Sheet (WAIS), has been used by the private company Antarctic Logistic and Expeditions (ALE) since 2007 for 8, 1227–1256, 2014 their landing and commercial operations, providing a unique logistic opportunity to per- 5 form glaciological research in a vast region, including the Ice divide between Institute Union Glacier and Pine Island glaciers and the Subglacial Lake Ellsworth. Union glacier is ﬂowing into the Ronne Ice Shelf, where future migrations of the grounding line zone (GLZ) in A. Rivera et al. response to continuing climate and oceanographic changes have been modelled. In order to analyse the potential impacts on Union glacier of this scenario, we installed an Title Page 10 array of stakes, where ice elevation, mass balance and ice velocities have been mea-

sured since 2007, resulting in near equilibrium conditions with horizontal displacements Abstract Introduction between 10 and 33 myr−1. GPS receivers and three radar systems have been also used to map the subglacial topography, the internal structure of the ice and the pres- Conclusions References ence of crevasses along surveyed tracks. The resulting radar data showed a subglacial Tables Figures 15 topography with a minimum of 858 m below sea level, much deeper than estimated before. The below sea level subglacial topography conﬁrms the potential instability of the glacier in foreseen scenarios of GLZ upstream migration during the second half of the J I

XXI century. J I

Back Close 1 Introduction Full Screen / Esc

20 WAIS, the West Antarctic Ice sheet (Fig. 1) has been considered potentially unsta- ble because its bedrock is located well below sea level (Humbert, 2012), and its total Printer-friendly Version disintegration could contribute up to 4.3 m (Fretwell et al., 2013) to global sea-level Interactive Discussion rise. In many cases, the topography underneath WAIS is inversed (ice is deeper upstream) and steeper than at the present grounding line zones (GLZ) (Ross et al., 2012). 25 This topographic condition, in a context of ongoing global changes, especially oceanic warming in areas of the Southern Ocean, is leading to GLZ upstream migration, as ob- 1228 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion served for example at the Amundsen Sea Embayment Area (ASEA) glaciers, like Pine Island (PIG) and Thwaites (Rignot et al., 2002). This migration process in a context TCD of inversed subglacial topographies, has an impact on glacier dynamics, since bottom 8, 1227–1256, 2014 melting at the GLZ is higher in deeper waters, provoking higher ice fluxes, ice thinning 5 and, in general, a more negative mass balance (Rignot and Jacobs, 2002). However, there are WAIS areas where the changes are not as dramatic as observed at ASEA, Union Glacier with some regions thickening rather than thinning (Joughin and Bamber, 2005). One of the WAIS areas where glaciological changes are not strong at present is the A. Rivera et al. Weddell Sea sector (Rignot and Thomas, 2002), where the Ronne Ice Shelf (RIS) is 10 located (Fig. 1). This floating platform has been relatively stable in recent decades, Title Page with different behaviours of the GLZ and very small net mass balance changes (Rig- not et al., 2011b). The relative stability of the RIS is partly explained by the Weddell Abstract Introduction Seas oceanographic and atmospheric conditions, and the associated sea ice extension (Mayewski et al., 2009). The sea ice volume in this area has been stable (Cavalieri and Conclusions References

15 Parkinson, 2008), but recent studies (Hellmer et al., 2012) forecast a sea ice volume Tables Figures reduction for the XXI century resulting in incremental circulation of warm water beneath the Filchner ice shelf toward the GLZ of the Slessor, Support Force, Moeller and Foun- J I dation ice streams. The intrusion of warmer waters will certainly increase the basal melt rate of the ice shelf, and possibly lead to retreat of the GLZ. This, in turn, would J I 20 favour higher ice fluxes along tributary glaciers, and thinning that will spread upstream Back Close along deep channels (Rignot et al., 2011a). Between two of the main ice streams in the region (Rutford and Institute), there Full Screen / Esc are two smaller glaciers also draining into the RIS; Union glacier (79◦460 S/83◦240 W) ◦ 0 ◦ 0 flowing into the Constellation inlet and the Horseshoe valley (80 18 S/81 22 W) flowing Printer-friendly Version 25 into the Hercules inlet. The study of these glaciers could provide an important clue about ongoing changes taking place in the region, especially considering that local Interactive Discussion glacier conditions can be significantly affected by RIS changes.

1229 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion In this context, the main aim of this paper is to present recent glaciological results obtained at the Union glacier and nearby areas that provide a base line for possible ice TCD dynamic responses to ongoing and modelled future changes of RIS. 8, 1227–1256, 2014

2 Study area Union Glacier ◦ 0 ◦ 0 5 The Horseshoe valley (80 18 S/81 22 W) has been studied since the 1990s (Wendt A. Rivera et al. et al., 2009), when Chilean Air Force C-130 airplanes landed on wheels at the Patriot Hills Blue Ice Area (BIA). BIAs have long been recognized as suitable places for landing airplanes (Mellor and Swithinbank, 1989). Since the 1980s, the private company Title Page Antarctic Logistics and Expeditions (ALE) also operated at this part of the Ellsworth Abstract Introduction 10 Mountains using the Patriot Hills BIA for their commercial operations. However, the

landing of heavy airplanes has been frequently disrupted due to strong prevailing cross Conclusions References winds. In order to increase the number of airplane operational days and improve access Tables Figures for heavy cargo airplanes, in 2007/8 ALE moved to Union Glacier (79◦460 S/83◦240 W) 15 (Fig. 2), where the prevailing wind direction is in line with the landing strip (tail-head J I katabatic winds), helping airplanes to operate even with strong gusts. Thanks to ALE logistic support, four scientiﬁc campaigns have been conducted on Union glacier since J I 2008, where a glaciological program was established, including ice dynamic, mass Back Close balance and geophysical surveys (Rivera et al., 2010). 2 Full Screen / Esc 20 Union glacier has an estimated total area of 2561 km , with a total length of 86 km from the ice divide with Institute ice stream down to the grounding line of the Constella- tion Inlet on the RIS. The glacier has several glacier tributaries, the main trunks being Printer-friendly Version

located in the Union and Schanz valleys (Figs. 1 and 2), which are feed through narrow Interactive Discussion glacial valleys (9 km wide) ﬂowing from the interior plateau until they join together at the 25 Union “gate”. This gate or narrowest section of the glacier (7 km wide), has a central moraine line comprised of clasts and small-sized debris. The central moraine is visible

1230 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion along the local BIA that is used for landing Ilyushin IL76 airplanes on wheels under a contract with ALE llc. TCD An array of 21 stakes was installed at the gate in 2007 for ice dynamic and surface 8, 1227–1256, 2014 mass balance studies. The Union gate has been re-surveyed by using GPS and radar 5 systems installed onboard convoys pulled by Camoplast tractors travelling along pre- designated routes. Results of the ﬁrst campaigns of 2007 and 2008 were published by Union Glacier Rivera et al. (2010). A. Rivera et al.

3 Methods Title Page 3.1 Meteorological data Abstract Introduction

10 The meteorological data analysed here was collected at the Union Glacier base camp Conclusions References (Fig. 2) Automatic Weather Station (AWS), which has been operating since 2008. The AWS includes air temperature, humidity, air pressure, and wind speed and direction Tables Figures sensors. The AWS data series show several data gaps due to malfunction or low power

supply problems during the winter. In spite of the above, the available data are good J I 15 enough for weather forecast during landing operations in the summer, and for describ- ing general temperature conditions in the area, as well as predominant winds and di- J I

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Mid-summer ASTER satellite images acquired between 2002 and 2013 were analyzed Printer-friendly Version 20 and classiﬁed via manual digitalization in order to delineate the local Union Glacier main BIA. This task allowed areal changes in snow cover to be quantiﬁed and analysed Interactive Discussion for possible relationships with local meteorological data. The ASTER images were geometrically corrected using the internal parameters of each scene. A true colour composition (Band 1, 2 and 3N) for each year was pro-

1231 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion duced and the contrast and brightness modified by means of histogram equalization to facilitate the delimitation of the blue ice area. TCD The BIA outline mapping was based upon a similar procedure established by Rivera 8, 1227–1256, 2014 et al. (2013), where the boundary between snow and ice is manually identified assum- 5 ing the maximum extent criterion of identification. This is easily done thanks to the spectral differences between snow and ice. Union Glacier

3.3 Glaciological mass balance A. Rivera et al.

The protruding heights of every stake were measured yearly between 2007 and 2011 (Figs. 1 and 2). The annual heights were compared and local mass balance was sub- Title Page 10 sequently calculated. In order to convert these values into water equivalent, a mean Abstract Introduction density of 910 kgm−3 (Paterson, 1994) was used for stakes drilled on ice. In those cases where the stakes were located at snow surfaces, the densities were measured Conclusions References by using a Mount Rose snow probe with penetration capacity of near 50 cm of snow- ﬁrn. Tables Figures

15 3.4 GPS J I

In December 2007, the initial observation network of 21 bamboo-stakes was installed J I

on a Blue Ice Area on the Union Glacier (Fig. 2). For each of these stakes GPS mea- Back Close surements were made using Topcon GR3 dual frequency GPS receivers with measurement times of less than 30 s per stake, yielding metre scale errors. In December 2008, Full Screen / Esc 20 2009 and 2010, the same network, as well as 35 new stakes were surveyed with dual frequency Javad GPS model Lexon GGD receivers. In order to apply a diﬀerential cor- Printer-friendly Version rection procedure, a similar GPS receiver was continuously collecting data at a rock Interactive Discussion location (RX1 in Fig. 2). The precision obtained by these surveys increased to less than 10 cm, by measuring between 15–30 min per stake. Additionally, a dual frequency 25 Javad GPS receiver was installed on snow (Bstat in Fig. 2) collecting data between 11 December 2009 and 31 January 2010 for high resolution studies (each 30 s), with 1232 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion the aim of detecting possible tidal modulated ice velocities. In January 2011, a Real Time Kinematic procedure was applied to data collected at the Union glaciar gate by TCD a Leica SR 9500 receiver, with estimated decimetre accuracy. All GPS data collected 8, 1227–1256, 2014 were processed using the commercial software GrafNav 8.20.

5 3.5 Surface elevation changes and geodetic mass balance estimation Union Glacier

The data obtained by the GPS measurements were compared in order to calculate the A. Rivera et al. local mass balance and the absolute surface elevation changes at the BIA. Using the measured height diﬀerences obtained by GPS and considering the surface Title Page topographic slope eﬀect, the submergence/emergence velocity (we) was calculated:

Abstract Introduction 10 we = ws − us tanα (1) Conclusions References where ws is the vertical ice velocity, us is the ice velocity along the surface ﬂow direction and α is the slope. The emergence velocity represents the vertical ﬂow of ice relative Tables Figures to the glaciers surface and allows the estimation of the net balance if it is assumed that

density does not change with depth during the period (Hooke, 2005). J I 15 By subtracting the emergence velocity of the measured specific balance (stake height difference) the surface elevation change with time at a fixed position can be J I

obtained. Back Close ∂h = b − w (2) Full Screen / Esc ∂t e If the glacier is in a steady state, there would be no surface changes since accumulation Printer-friendly Version 20 and ablation equals out the submergence and emergence velocities so that the surface Interactive Discussion proﬁle remains unchanged (Hooke, 2005). In this case b = we, however most glaciers are not perfectly in steady state. Only the stake array measured at the Union gate was considered for this analysis.

1233 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 3.6 Radar TCD A pulse compression radar depth sounder designed at CECS, was used to measure ice thickness. The radar operates at a central frequency of 155 MHz, a bandwidth of 8, 1227–1256, 2014 20 MHz, and 200 W of peak power. Yagi antennae were used for both the transmitter 5 and receiver. The radar includes a digital waveform generator, up converter transmitter, Union Glacier a timing and synchronization controller and an 80 dB variable gain digital receiver. A Frequency Modulated – Continuous Wave (FM-CW) radar, was also used to mea- A. Rivera et al. sure surface snow accumulation (up to 450 m thick), with high vertical resolution (1 m). This radar works at a frequency from 550 MHz to 900 MHz using two separated log pe- Title Page 10 riodic antennae for the transmitter and receiver. The transmit power was 21 dBm, the

intermediate frequency (IF) ampliﬁer gain was 70 dB and the whole system operated at Abstract Introduction a PRF (Pulse Repetition Frequency) 10 kHz. A Direct Digital Synthesis (DDS) system was used to generate an extremely linear frequency sweep transmitted signal. Conclusions References The third radar system used along these tracks was a commercial Ground Penetrat- Tables Figures 15 ing Radar (GPR), a Geophysical Survey Systems Inc. (GSSI) model SIR-3000, working at 400 MHz. The digital control unit of the radar triggers pulses at 100 kHz repetition rate. The range was set between 190 and 300 ns to record subsurface reﬂection with J I

the aim to ﬁnd crevasses in real time. The GPR antennae were mounted on a 7 m long J I rod which was attached to a tractor and had rubber car tire tube installed at the opposite Back Close 20 end. The radar data were analysed in real time while the tractor was moving, in order

to alert the driver to the presence of hyperbolae, potentially related to crevasses. This Full Screen / Esc system was used particularly in areas previously detected on ASTER satellite imagery as being highly crevassed. Printer-friendly Version Post processing and data analyses were carried out using Reflex-Win V5.6 (Sand- 25 meier Scientific Software) for the three radar systems. A background removal, dewow Interactive Discussion filter and adjustment of the gain function, among other procedures, were applied to the raw data. For crevasses analysis, migration correction procedures were also applied to collapse the hyperbolic diffractions to their proper point origins (Plewes and Hub-

1234 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion bard, 2001). In order to convert travel time to ice thickness in metres, it was assumed that the electromagnetic wave travelled through the ice at 0.168 mns−1, a mean value TCD representative of cold ice (Glen and Paren, 1975) and at 0.194 mns−1 assumed to be 8, 1227–1256, 2014 representative for snow/ﬁrm (Woodward and King, 2009).

Union Glacier 5 4 Results A. Rivera et al. 4.1 Meteorological data and BIA area changes

The meteorological data collected at Union Glacier between 2008 and 2013 contains Title Page several gaps and non valid records, but in general provide a good idea of the local conditions, which are especially useful for landing operations. In terms of climatological Abstract Introduction 10 analysis, the series is too short and noisy, but is the only one available in the region. Conclusions References The mean daily air temperature at this location (Fig. 3) from 2008 to 2012 was ◦ ◦ −20.6 C, with an absolute minimum of −42.7 C recorded on 12 August 2008 at 2 a.m. Tables Figures and an absolute maximum of 0.5 ◦C registered on 15 January 2010 at 8 p.m. Daily air

temperatures have maximums in December–January following direct solar radiation, J I 15 however between April and August (included) when the Sun is below the horizon, the lowest temperatures have a mean of −25.5±3.5 ◦C. Temperatures close to the melting J I

point have only been observed in three summer events: 7–8 January 2008, 14–15 Jan- Back Close uary 2010 and 25–26 December 2010, but no surface melting has been observed on this site. The mean air temperature in January is −10.3 ◦C. Full Screen / Esc 20 Regarding wind speed and direction, the data are very consistent between 2008– ◦ 2012 with a mean value 16.3 knots and predominant direction from 224 . The maximum Printer-friendly Version wind speed recorded on site was near 60 knots and the predominant wind speeds are Interactive Discussion higher than 25 knots (Fig. 4). This predominant wind direction explains the extension of the main BIA which has experienced less than 10 % area changes between 2002 25 and 2013. The series is however too short to detect a trend (Table 1).

1235 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 4.2 Mass balance TCD Snow densities measured at stakes drilled on snow surfaces have a mean value of 400 ± 3 kgm−3, with small variability between all surveyed stakes. No fresh and soft 8, 1227–1256, 2014 snow was detected due to the lack of precipitation in the region during our surveys. 5 Table 2 indicates the details of each stake, the type of surface and period of measure- Union Glacier ments. The mass balance estimations between 2007 and 2011 at the stakes installed on the A. Rivera et al. BIA gate (V00 to V21, Table 2), are typically negative (Fig. 5), with a mean inter annual mass balance of −0.097 m w.eq. a−1. Title Page 10 The stakes installed along the traverse between Patriot Hills and Union Glacier

(Fig. 1) were only measured between 2008 and 2009 and showed diﬀering results Abstract Introduction depending on their location. Stakes installed on snow surfaces (B1–B12) have positive mass balances with a mean snow accumulation of 0.3 ma−1 (0.12 m w.eq. a−1). Conclusions References Just outside the BIA, stake B12 showed the maximum net balance (0.2 m w.eq. a−1), Tables Figures 15 indicating that snow drift coming from the BIA, due to the predominant katabatic wind direction (Fig. 4) is having a positive downstream accumulation eﬀect. The only negative mass balance along the track Patriot Hills-Union Glacier (excluding the Union J I

glacier BIA) was detected at stake B5, which is located at the edge of the local BIA J I of the Plumber glacier (Fig. 1). Here a modest −0.07 m w.eq. a−1 (Table 2) was ob- Back Close 20 tained between 2008 and 2009. Stakes B13–B19 (Table 2, Fig. 2), also had negative −1 values with a mean surface balance of 0.1 m w.eq. a , coincident with the other stakes Full Screen / Esc located at a local BIA.

4.3 Surface ice velocity Printer-friendly Version Interactive Discussion Surface ice velocities were obtained for 19 stakes (B1–B19) located along the track 25 between Patriot Hills and Union Glacier base camp (Figs. 3 and 6), with measurements periods of near 1 yr, and resulting ice velocities between 0.1 and 34.6 ma−1. Minimum values were observed at local ice divides (B2–B8) while the maximum velocity was 1236 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion observed at stake B10 located at the steepest area of Union Glacier, between two crevasse fields (Figs. 2 and 6). TCD Between 11 December 2009 and 31 January 2010 a dual frequency GPS receiver 8, 1227–1256, 2014 was attached to stake Bstat, taking continuous measurements every 15 s. The receiver 5 was powered by batteries and solar panels, providing a high detailed record of daily ice dynamic during the spanned period of time. The main aim of this survey was to test Union Glacier the hypothesis that the ice dynamics of Union glacier are affected by Ronne Ice Shelf tides, as has been observed at the Rutford ice stream where a modulated ice flow A. Rivera et al. was detected (Gudmundsson, 2006). This stake (Bstat, Table 2) is located ∼ 38 km 10 upstream the local grounding line zone of the Constellation inlet at the Ronne Ice Shelf Title Page (Figs. 1 and 2). The resulting ice velocity was 33 ma−1, and thanks to the analysis carried out by H. Gudmunsson (personal communication, 2010), no tidal effect was Abstract Introduction detected. The neighbouring stake to Bstat (B9, located within a distance of 10 m) was only Conclusions References

15 measured for 30 min in December 2009 and in December 2010, resulting in a an- Tables Figures nual velocity of 33.26 ± 0.7 ma−1. The good correspondence with the Bstat results (33 ma−1) indicates that Union Glacier does not show important seasonal ice veloc- J I ity changes. At the Union Glacier gate stake array (Fig. 2), 22 stakes were drilled into its Blue J I 20 Ice Area in 2007 and have been annually resurveyed up until 2011. Unfortunately, Back Close not all the original stakes have survived, or were re-measured every year. However, −1 from the remaining stake network a mean velocity of 20 ma was estimated for the Full Screen / Esc measurement period (Fig. 6). The orientation of the overall ice movement shows an ◦ angle of 65.42 at the gate, similar to the main wind direction (Fig. 4) observed at the Printer-friendly Version ◦ ◦ 25 BIA (from 224 to 44 ). No temporal velocity changes were observed during the studied period. Interactive Discussion

1237 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 4.4 Surface elevation change TCD Considering the surface velocities obtained for the stakes (Table 2) and by applying Eq. (1), a mean vertical velocity for the blue ice of −0.07±0.007 ma−1 was found at the 8, 1227–1256, 2014 Union Gate. For a steady state glacier this would be equal to the local mass balance, 5 however this is not the case for the studied area (there is a negative mass balance Union Glacier at the BIA) and a mean local elevation change of −0.012 ma−1. was found. In any case, this result is close to the estimated error of the measurements and indicates A. Rivera et al. near equilibrium conditions.

4.5 Glacier thickness and internal structure Title Page

Abstract Introduction 10 More than 450 km long radar tracks were measured between 2008 and 2010 at the

Union Glacier area, including oversnow traverses between Patriot Hills, Union Glacier Conclusions References and the high Antarctic plateau. The ∼ 80 km long proﬁle between the Antarctic Plateau and Union Glacier (Fig. 7 Tables Figures upper), including the transit along Balish, Schneider, Schanz and Driscoll glaciers, was 15 surveyed with a FM-CW radar for snow accumulation data, a compression pulse radar J I for ice thickness data and a GPR system for crevasse detection purposes. The deep ice of the plateau is seen at the beginning of the survey proﬁle (A in Fig. 7), J I then is followed by a passage characterized by shallow ice (B) where the steepest Back Close section of the whole traverse was crossed. After this section, the thickness increases Full Screen / Esc 20 sharply until a local maximum thickness of 1120 m was observed at Balish Glacier. The local divide between the Balish and Schneider glaciers (C in Fig. 7) has a prominent subglacial peak (C in Fig. 7) where the mountain range dividing the two glaciers is also Printer-friendly Version

visible underneath the ice. The maximum thickness measured at Schneider Glacier Interactive Discussion was 900 m, 1050 m at Schanz Glacier, 1510 m in Driscoll Glacier and 1540 m in Union 25 Glacier, close to the base camp. At the beginning of the radar profile (A and B in Fig. 7), it is possible to see the shallowest ice at the Gifford Peaks pass (passage), with a thickness between 45 and 1238 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 140 m (Fig. 8) obtained with the low gain channel. This section (B in Fig. 7 bottom) corresponds to a ∼ 5 km long FM-CW radargram where 400 m of dynamic range were TCD processed detecting the snow/firm stratigraphy with a resolution of 1 m. 8, 1227–1256, 2014 All along the whole A–F profile (Fig. 7 bottom), a detailed internal stratigraphy was 5 detected in the upper hundred metres, comprised of isochronous layers that are particularly prominent at local ice divides (e.g. at C in Fig. 7 bottom). In those areas, the Union Glacier internal structures are characterized by typical minimum and divergent flow and the presence of Raymond Bumps (Paterson, 1994) and other internal inflections that can A. Rivera et al. lead to the understanding of long-term changes in the ice flow. 10 The collected ice thickness data at Union glacier has a mean ice thickness of 1450 m. Title Page The subglacial topography in the valley is smooth, with “U” shape flanks. The FM-CW radar detected multiple internal snow-firn layers and the snow-ice boundary layer with Abstract Introduction a high resolution (Fig. 8). Also, the FM-CW radar was capable of detecting the bedrock up to 350 m, improving the bedrock detection from the pulse-compression radar when Conclusions References

15 surveying shallow ice (B at Fig. 7 bottom). Tables Figures The GPR data collected along the track from Union Glacier to the Antarctic Plateau allowed the detection of many crevasses both near the runway area, and at the Gifford J I Peaks passage, just before reaching the plateau. Along the survey route from the Union Glacier to the Antarctic Plateau the GPR system was able to detect the upper 20 m of J I 20 the internal structure of the ice. Within this upper layer annual snow/firn layers suffered Back Close (in places) discontinuities due to crevasses that appeared as hyperbolae on the radar traces. Full Screen / Esc In general, the route from Union glacier to the Antarctic plateau is almost totally free of crevasses. An exception to this was the Gifford passage (B in Fig. 7) where Printer-friendly Version 25 a minor system of crevasses was detected corresponding to steepening slopes. These crevasses have widths between 1 and 5 m, with snow bridges between 1.5 and 3 m Interactive Discussion thick. The Gifford passage is a narrow and steep valley, where the longitudinal stresses and velocities are probably much higher than the rest of the area. In spite of these crevasses, the Gifford passage is the shortest (235 km) gateway for oversnow tra-

1239 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion verses from Union glacier to the Subglacial Lake Ellsworth (Vaughan et al., 2007) at the Antarctic plateau, and is therefore the preferred route. The alternative route to this sub- TCD glacial lake is near 520 km long, starting at Union glacier, passing along Patriot Hills, 8, 1227–1256, 2014 then travelling around the Tree Sails and ﬁnally turning west toward the Subglacial lake 5 Ellsworth (Fig. 1 bottom). Union Glacier

5 Discussion A. Rivera et al.

At Union glacier, the local BIA is shaped by strong snowdrift caused by katabatic winds accelerated by the slope at the main junction between the two glacial valley arms feed- Title Page ing Union glacier from the upper Antarctic plateau. As a result, the net mass balance Abstract Introduction 10 at this BIA is negative, the driving factor explaining the mass loses being sublimation

of ice during summer months, as no melting event has been observed since 2007. Conclusions References However, melting events have been observed in the region, especially at Patriot Hills, where a small water pound was formed at the boundary between the BIA and the Tables Figures nunatak during a very warm 1997 summer (Carrasco et al., 2000). 15 In spite of this negative surface mass balance, the resulting ice elevation changes J I between 2008–2011 at the Union gate are very close to the error of the measurements (combined error of 0.01 m), with a mean of −0.012 ma−1 and high spatial variability J I −1 (±0.044 ma ) among the gate stakes. Accordingly, the glacier must be considered Back Close in equilibrium without significant changes compared with previous data (Rivera et al., Full Screen / Esc 20 2010). The ice velocities at the BIA fluctuate between 11 and 24 ma−1 without significant changes between 2007 and 2011. However, downstream of the Union gate, the veloc- Printer-friendly Version −1 ities increase up to 33 ma at the continuous GPS stake (Bstat), where no seasonal Interactive Discussion variations or tidal modulated variability were detected. 25 The GPR survey allowed the detection of many more crevasses than were previously mapped with the ASTER imagery. The 400 MHz GPR is capable of identifying in real time surface and buried crevasses. The 400 MHz GPR crevasses data were compared 1240 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion to the 500–900 MHz FMCW records and in most of the cases the wider crevasses (2 to 4 m width) could be detected in both radars. However, the FMCW radar did not TCD provide the best information in regards to snow bridge thicknesses (less than 1 m), its 8, 1227–1256, 2014 resolution being insufficient to detect the first meters of the snow pack. 5 Comparisons of surface and bedrock topography along transect A–F (Fig. 9), resulted in a maximum ice thickness difference of 1447 m with a mean difference of Union Glacier 477 m (standard deviation: 348 m) between the data presented here and BEDMAP2 (Fretwell et al., 2013). The surface topography of the surveyed traverse was found to A. Rivera et al. differ from BEMAP2 and IceSat (2003–2005) by a mean of 91 m and 169 m, respec- 10 tively. These differences are understandable due to the coarse resolution of BEDMAP2 Title Page and the footprint of IceSat. The subglacial topography (Fig. 9) at the main trunk of the Union glacier toward Abstract Introduction the local GLZ (Segment E–G in Fig. 7), showed a subglacial topography well below sea level (−858 m), much deeper than previously thought. However, between Union Conclusions References

15 glacier and the local GLZ, the subglacial topography shows a maximum altitude of Tables Figures −190 m at F in Fig. 9. The subglacial topography then deepens toward the GLZ, where the bedrock is estimated to be 1050 m below sea level (Fretwell et al., 2013). This J I subglacial condition implies that an upstream migration of the GLZ until point F in Fig. 9 will not have a strong eﬀect on Union glacier. However, the glacier can respond J I 20 in a more dynamic way if this possible migration is aﬀecting Union glacier upstream this Back Close point. Full Screen / Esc 6 Conclusions Printer-friendly Version

Several glaciological oversnow campaigns have been undertaken to Union Glacier and Interactive Discussion nearby areas since 2007, where the surface and subglacial topographies were mapped 25 in detail. These results were compared to the Bedmap dataset, showing much deeper bedrock and a much more complex subglacial topography than previously estimated. The obtained results determined a maximum ice thickness of 1540 m in Union Glacier, 1241 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion with a maximum snow ice boundary layer at 120 m. The internal structure of the ice was also mapped, including the detection of isochronous layers and crevasses, allowing TCD logistic operators and scientists to work along safer routes. 8, 1227–1256, 2014 Ice dynamics were also recorded thanks to the measurement of ice velocities and ice −1 5 thickness. Maximum ice velocity values of 34.6 ma were obtained. Near equilibrium conditions were calculated at the BIA, where mean velocities of 20 ma−1 were mea- Union Glacier sured. The snow accumulation among the studied stakes outside BIAs showed values of up to 0.2 m w.eq. a−1 (near 0.5 ma−1 of snow). At the BIA, a local negative mass A. Rivera et al. balance was detected as expected, with mean ablation rates of 0.1 m w.eq. a−1. 10 Due to the below sea level subglacial topography upstream the local GLZ, it is ex- Title Page pected that Union Glacier can experience some thinning and acceleration in future scenarios of GLZ migration. However these impacts will not aﬀect the whole glacier, Abstract Introduction because at near 55 km upstream the present GLZ, the bedrock topography is close to sea level. Conclusions References

15 Overall the collected data allowed a ground route from Union glacier to the upper Tables Figures Antarctic plateau to be satisfactory mapped. The route is important as it traverses the Subglacial lake Ellsworth and the main ice streams flowing into the Amundsen J I Sea Embayment area (Pine Island Glacier) and toward the Weddell Sea (Institute and Rutford Ice stream). This new 235 km long route toward Subglacial Lake Ellsworth is J I 20 shorter than half of the previously traversed track of 517 km, providing a much direct Back Close and short gateway into inner Antarctica. Full Screen / Esc Acknowledgements. This research was supported by Antarctic Logistic Expeditions (ALE) Ltd. Special thanks to M. Sharp, M. and P. McDowell, C. Jacobs, Eddy, Boris and all ALE personnel at Union. CECs is funded by the Basal fund of CONICYT, among other grants. J. A. Neto col- Printer-friendly Version 25 laborated with GPS data collection in 2011. A. Wendt, C. Rada, T. Kohoutek and M. Barandun helped with data processing. H. Gudmundsson analysed the GPS data at Bstat in order to de- Interactive Discussion tect possible tidal modulated effects on the ice flow. J. Carrasco and F. Burger helped with the met data. D. Carrión helped with figures and satellite image analysis. R. Wilson edited the text. A. Rivera is a Guggenheim fellow.

1242 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion References TCD Carrasco, J., Casassa, G., and Rivera, A.: A warm event at Patriot Hills, Antarctica: an ENSO related phenomena?, in: Sixth International Conference on Southern Hemisphere Meteorol- 8, 1227–1256, 2014 ogy and Oceanography, 240–241, Santiago, Chile, 3–7 April, 2000. 1240 5 Cavalieri, D. and Parkinson, C.: Antarctic sea ice variability and trends, 1979–2006, J. Geophys. Res., 113, C07004, doi10.1029/2007JC004564, 2008. 1229 Union Glacier Fretwell, P.,Pritchard, H. D., Vaughan, D. G., Bamber, J. L., Barrand, N. E., Bell, R., Bianchi, C., A. Rivera et al. Bingham, R. G., Blankenship, D. D., Casassa, G., Catania, G., Callens, D., Conway, H., Cook, A. J., Corr, H. F. J., Damaske, D., Damm, V., Ferraccioli, F., Forsberg, R., Fujita, S., 10 Gim, Y., Gogineni, P., Griggs, J. A., Hindmarsh, R. C. A., Holmlund, P., Holt, J. W., Jaco- bel, R. W., Jenkins, A., Jokat, W., Jordan, T., King, E. C., Kohler, J., Krabill, W., Riger- Title Page Kusk, M., Langley, K. A., Leitchenkov, G., Leuschen, C., Luyendyk, B. P., Matsuoka, K., Mouginot, J., Nitsche, F. O., Nogi, Y., Nost, O. A., Popov, S. V., Rignot, E., Rippin, D. M., Abstract Introduction Rivera, A., Roberts, J., Ross, N., Siegert, M. J., Smith, A. M., Steinhage, D., Studinger, M., Conclusions References 15 Sun, B., Tinto, B. K., Welch, B. C., Wilson, D., Young, D. A., Xiangbin, C., and Zirizzotti, A.: Bedmap2: improved ice bed, surface and thickness datasets for Antarctica, The Cryosphere, Tables Figures 7, 375–393, doi:10.5194/tc-7-375-2013, 2013. 1228, 1241, 1256 Glen, J. and Paren, J.: The electrical properties of snow and ice, J. Glaciol., 15, 15–38, 1975. 1235 J I 20 Gudmundsson, G.: Fortnightly variations in the ﬂow velocity of Rutford Ice Stream, West Antarc- tica, Nature, 444, 1063–1064, 2006. 1237 J I Hellmer, H., Kauker, F., Timmermann, R., Determann, J., and Rae, J.: Twenty-ﬁrst-century Back Close warming of a large Antarctic ice-shelf cavity by a redirected coastal current, Nature, 485, 225–228, 2012. 1229 Full Screen / Esc 25 Hooke, R.: Principles of Glacier Mechanics, 2nd Edn., Cambridge University Press, Camb- dridge, UK, 2005. 1233 Printer-friendly Version Humbert, A.: Vulnerable ice in the Weddell Sea, Nat. Geosci., 5, 370–371, 2012. 1228 Jezek, K., and RAMP Product Team: RAMP AMM-1 SAR Image Mosaic of Antarctica. Version Interactive Discussion 2. Fairbanks, AK: Alaska Satellite Facility, in association with the National Snow and Ice Data 30 Center, Boulder, CO, Digital media, 2002. 1254

1243 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Joughin, I. and Bamber, J.: Thickening of the ice stream catchments feeding the Filchner-Ronne Ice Shelf, Antarctica, Geophys. Res. Lett., 32, L17503, doi:doi:10.1029/2005GL023844, TCD 2005. 1229 Mayewski, P., Meredith, M., Summerhayes, C., Turner, J., Worby, A., Barrett, P., Casassa, G., 8, 1227–1256, 2014 5 Bertler, N., Bracegirdle, T., Naveira, A., Bromwich, D., Campbell, H., Hamilton, G., Lyons, W., Maasch, K., Aoki, S., Xiao, C., and van Ommen, T.: State of the Antarctic and Southern Ocean climate system, Rev. Geophys., 47, RG1003, doi:10.1029/2007RG000231, 2009. Union Glacier 1229 A. Rivera et al. Mellor, M. and Swithinbank, C.: Airfields on Antarctic glacier ice, CRREL Rep. 89–21, US Army 10 Cold Regions Research and Engineering Laboratory, USA, 97 pp., 1989. 1230 Paterson, W.: The Physics of Glaciers, 2nd Edn., Pergamon Press, Exeter, Great Britain, 1994. 1232, 1239 Title Page Plewes, A. and Hubbard, B.: A review of the use of radio-echo sounding in glaciology, Prog. Phys. Geog., 25, 203–236, 2001. 1234 Abstract Introduction 15 Rignot, E. and Jacobs, S.: Rapid bottom melting widespread near Antarctic Ice Sheet grounding Conclusions References lines, Science, 296, 2020–2023, 2002. 1229 Rignot, E. and Thomas, R.: Mass balance of polar ice sheets, Science, 297, 1502–1506, 2002. Tables Figures 1229 Rignot, E., Vaughan, D., Schmeltz, M., Dupont, T., and MacAyeal, D.: Acceleration of Pine J I 20 Island and Thwaites Glaciers, West Antarctica, Ann. Glaciol., 34, 189–194, 2002. 1229 Rignot, E., Mouginot, J., and Scheuchl, B.: Ice flow of the Antarctic Ice Sheet, Science, 333, J I 1427–1430, 2011a. 1229 Rignot, E., Mouginot, J., and Scheuchl, B.: Antarctic grounding line mapping from differential Back Close satellite radar interferometry, Geophys. Res. Lett., 38, L10504, doi:10.1029/2011GL047109, 25 2011b. 1229 Full Screen / Esc Rivera, A., Zamora, R., Rada, C., Walton, J., and Proctor, S.: Glaciological investigations on Union Glacier, Ellsworth Mountains, West Antarctica, Ann. Glaciol., 51, 91–96, 2010. 1230, Printer-friendly Version 1231, 1240 Rivera, A., Cawkwell, F., Wendt, A., and Zamora, R.: Mapping blue ice areas and crevasses Interactive Discussion 30 in West Antarctica using ASTER images, GPS and radar measurements, in: Global Land Ice Measurements from Space, edited by: Kargel, J., Leonard, G., Bishop, M., Kääb, A., and Raup, B., Springer-Praxis, Heidelberg, 743–757, 2013. 1232

1244 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Ross, N., Bingham, Corr, H., Ferraccioli, F., Jordan, T., Le Brocq, A., Rippin, D., Young, D., Blankenship, D., and Siegert, J.: Steep reverse bed slope at the grounding line of the Weddell TCD Sea sector in West Antarctica, Nat. Geosci., 5, 393–396, 2012. 1228 Scambos, T., Haran, T., Fahnestock, M., Painter, T., and Bohlander, J.: MODIS-based Mosaic of 8, 1227–1256, 2014 5 Antarctica (MOA) data sets: continent-wide surface morphology and snow grain size, Remote Sens. Environ., 111, 242–257, 2007. 1248 Vaughan, D., Rivera, A., Woodward, J., Corr, H., Wendt, J., and Zamora, R.: Topographic and Union Glacier hydrological controls on Subglacial Lake Ellsworth, West Antarctica, Geophys. Res. Lett., 34, A. Rivera et al. L18501, 2007. 1240 10 Wendt, A., Casassa, G., Rivera, A., and Wendt, J.: Reassessment of ice mass balance at Horseshoe Valley, Antarctica, Antarct. Sci., 21, 505–513, 2009. 1230 Woodward, J. and King, E.: Radar surveys of the Rutford Ice Stream onset zone, West Antarc- Title Page tica: indications of ﬂow (in)stability?, Ann. Glaciol., 50, 57–62, 2009. 1235 Abstract Introduction

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Title Page Table 1. BIA extension based upon ASTER images collected since 2002. Abstract Introduction 2 Date Area (km ) Conclusions References 24 Nov 2002 95.6 9 Dec 2004 104.7 Tables Figures 27 Jan 2007 112.0 17 Jan 2009 111.8 J I 11 Feb 2012 88.2 1 Feb 2013 96.7 J I

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H. Velocity Mass Balance 8, 1227–1256, 2014 Stakes Lat◦ Lon◦ Period Type of surface (m.a.e) (m a−1) (m w.eq. a−1) B1 −80.211 −81.230 803.12 12.7 0.18 B2 −80.124 −81.113 813.51 1.8 0.19 snow B3 −80.045 −80.937 759.07 0.6 0.04 Union Glacier B4 −79.985 −81.080 573.57 1.8 0.06 2008–2009 B5 −79.922 −81.211 538.16 2.0 −0.07 snow/BIA B6 −79.840 −81.298 669.59 0.1 0.03 A. Rivera et al. B7 −79.801 −81.348 553.87 1.2 0.04 B8 −79.757 −81.965 459.99 2.9 0.14 B9 −79.709 −82.455 531.04 33.3 0.14 snow 2009/11/12 Bstat −79.708 −82.454 531.34 33.0 - 2010/31/01 Title Page B10 −79.717 −82.643 639.79 34.6 0.17 B11 −79.757 −82.803 672.46 20.5 0.13 B12 −79.760 −82.931 690.56 20.9 0.20 Abstract Introduction B13 −79.764 −83.091 703.53 22.9 −0.09 B14 −79.768 −83.266 738.39 22.4 −0.06 2008–2009 ice B15 −79.798 −83.281 735.19 21.6 −0.16 Conclusions References B16 −79.796 −83.316 742.20 23.2 −0.13 B17 −79.765 −83.343 739.36 17.0 −0.08 Tables Figures B18 −79.760 −83.370 747.68 14.0 −0.11 ice/crevasses B19 −79.764 −83.372 743.72 11.1 −0.10 V00 −79.769 −83.370 739.47 18.1 −0.08 2008–2010 V01 −79.770 −83.370 738.44 19.0 −0.08 2007–2011 J I V02 −79.772 −83.368 738.13 20.2 −0.04 2007–2010 V03 −79.774 −83.365 739.69 21.7 −0.11 J I V04 −79.775 −83.363 736.40 22.3 −0.07 2007–2011 V05 −79.777 −83.361 733.57 23.0 −0.22 V06 −79.779 −83.358 735.12 23.5 −0.11 Back Close 2007–2010 V07 −79.781 −83.355 736.61 23.9 −0.08 V08 −79.784 −83.352 739.05 24.3 −0.13 2007–2009 ice Full Screen / Esc V09 −79.787 −83.343 738.60 24.3 −0.11 2007–2011 V10 −79.790 −83.334 737.63 24.3 −0.13 V11 −79.792 −83.325 738.82 24.0 −0.07 2007–2010 V12 −79.771 −83.337 738.51 21.9 −0.09 Printer-friendly Version V13 −79.773 −83.335 735.87 22.3 −0.08 2007–2011 V14 −79.774 −83.332 733.20 22.4 −0.08 V15 −79.777 −83.327 731.27 22.7 −0.09 Interactive Discussion V16 −79.779 −83.323 731.72 23.1 −0.10 2007–2009 V17 −79.782 −83.316 731.16 23.4 −0.11 V18 −79.784 −83.313 730.33 23.5 −0.13 V19 −79.786 −83.309 729.48 23.6 −0.07 2007–2011 V20 −79.788 −83.303 728.57 23.3 −0.11 V21 −79.791 −83.298 733.66 23.0 −0.07 1247 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion TCD 8, 1227–1256, 2014

2 Rivera et al (2013): Union Glacier; a new exploration gateway for the West Antarctic Ice Sheet Union Glacier

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J I Fig. 1. Left:Fig. 1. WAISLeft: WAIS Location Location map. map.Right: Union Right: glacier basin union (lightglacier blue area). In basin yellow, the (light main ice blue divides inarea). the region. In In yellow,dashed white the main the 2005 track to Subglacial lake Ellsworth (517 km). In green, the new track surveyed in 2010 (235 km). Stakes B1 to B9 are described in Back Close ice dividesthe text. in The the black region. box is shown In at dashed Figure 2. Background white image: the MODIS 2005 mosaic track of Antarctic to Subglacial (Scambos et al., 2007). lake Ellsworth (517 km). In green, the new track surveyed in 2010 (235 km). Stakes B1 to B9 are described in the text. Full Screen / Esc The black65 turn, box would is favour shown higher iceat fluxes Fig. along2. tributaryBackground glaciers, 90 image:operations. MODIS However, the mosaic landing of heavy of Antarctic airplanes has been (Scambos and thinning that will spread upstream along deep channels frequently disrupted due to strong prevailing cross winds. et al., 2007(Rignot). et al., 2011a). In order to increase the number of airplane operational Between two of the main ice streams in the region days and improve access for heavy cargo airplanes, in 2007/8 Printer-friendly Version (Rutford and Institute), there are two smaller glaciers also ALE moved to Union Glacier (79◦46’S/83◦24’W) (Figure ◦ ◦ 70 draining into the RIS; Union glacier (79 46’S/83 24’W) 95 2), where the prevailing wind direction is in line with the Interactive Discussion flowing into the Constellation inlet and the Horseshoe valley landing strip (tail-head katabatic winds), helping airplanes (80◦18’S/81◦22’W) flowing into the Hercules inlet. The to operate even with strong gusts. Thanks to ALE logistic study of these glaciers could provide an important clue support, four scientific campaigns have been conducted on about ongoing changes taking place in the region, especially Union glacier since 2008, where a glaciological program 75 considering that local glacier conditions can be significantly 100 was established, including ice dynamic, mass balance and affected by RIS changes. geophysical surveys (Rivera et al., 2010). In this context, the main aim of this paper is to present Union glacier has an estimated total area of 2561 km2, recent glaciological results obtained at the Union glacier and with a total length of 86 km from the ice divide with Institute nearby areas that provide a base line for possible ice dynamic1248ice stream down to the grounding line of the Constellation 80 responses to ongoing and modelled future changes of RIS. 105 Inlet on the RIS. The glacier has several glacier tributaries, the main trunks being located in the Union and Schanz valleys (Figure 1 and 2), which are feed through narrow 2 Study Area glacial valleys (9 km wide) flowing from the interior plateau until they join together at the Union “gate”. This gate or ◦ ◦ The Horseshoe valley (80 18’S/81 22’W) has been studied 110 narrowest section of the glacier (7 km wide), has a central since the 1990s (Wendt et al., 2009), when Chilean Air Force moraine line comprised of clasts and small-sized debris. The C-130 airplanes landed on wheels at the Patriot Hills Blue central moraine is visible along the local BIA that is used for 85 Ice Area (BIA). BIAs have long been recognized as suitable landing Ilyushin IL76 airplanes on wheels under a contract places for landing airplanes (Mellor and Swithinbank, 1989). with ALE llc. Since the 1980s, the private company Antarctic Logistics and 115 An array of 21 stakes was installed at the gate in 2007 for Expeditions (ALE) also operated at this part of the Ellsworth ice dynamic and surface mass balance studies. The Union Mountains using the Patriot Hills BIA for their commercial gate has been re-surveyed by using GPS and radar systems Rivera et al (2013): Union Glacier; a new exploration gateway for the West Antarctic Ice Sheet 3 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion TCD 8, 1227–1256, 2014

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Fig. 2.Fig.Yellow 2. Yellow dots dots show show the stakes the described stakes in described the text (Figure 1 in and the Table text 2). RX1 (Fig. is the1 locationand of Table the static2 GPS). RX1 on rock. is Crevasses the location Interactive Discussion are shown in red. The Union Glacier “gate” is a transversal profile between B18 and B15 (inset) where the ice runway is shown in purple. of theMean static ice velocities GPS onarea shownrock. as arrows.Crevasses Summer base are camp shown is located nearin red. B11. The The background Union image Glacier is an ASTER “gate” false composite is a transver- 321 sal profileacquired between on February 1, B18 2013. and B15 (inset) where the ice runway is shown in purple. Mean ice velocities area shown as arrows. Summer base camp is located near B11. The background imageinstalled is an ASTER onboard convoys false pulled composite by Camoplast 321 tractors acquired3 on Methods 1 February 2013. travelling along pre-designated routes. Results of the first 120 campaigns of 2007 and 2008 were published by Rivera et al. 3.1 Meteorological data (2010). 1249 The meteorological data analysed here was collected at the 125 Union Glacier base camp (Figure 2) Automatic Weather Station (AWS), which has been operating since 2008. The AWS includes air temperature, humidity, air pressure, and wind speed and direction sensors. The AWS data series show several data gaps due to malfunction or low power icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion TCD 8, 1227–1256, 2014 Rivera et al (2013): Union Glacier; a new exploration gateway for the West Antarctic Ice Sheet 5 Union Glacier

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Back Close ◦ Fig. 3. Mean daily temperature ( C) during 2008 (black triangles) and modeled solar direct Full Screen / Esc −2 Fig. 3. Mean dailyradiation temperature (red line) (◦C) atduring the 2008AWS (black site (kW triangles) m ). and modeled solar direct radiation (red line) at the AWS site (kW/m2).

Printer-friendly Version radar works at a frequency from 550 MHz to 900 MHz using 1975) and at 0.194 m/ns assumed to be representative for Interactive Discussion two separated log periodic antennae for the transmitter and 260 snow/firm (Woodward and King, 2009). receiver. The transmit power was 21 dBm, the intermediate frequency (IF) amplifier gain was 70 dB and the whole 230 system operated at a PRF (Pulse Repetition Frequency) 10 4 Results kHz. A Direct Digital Synthesis (DDS) system was used to generate an extremely linear frequency sweep transmitted1250 4.1 Meteorological data and BIA area changes signal. The third radar system used along these tracks was a The meteorological data collected at Union Glacier between 235 commercial Ground Penetrating Radar (GPR), a Geophysical Survey Systems Inc. (GSSI) model SIR-3000, working at 2008 and 2013 contains several gaps and non valid records, 400 MHz. The digital control unit of the radar triggers pulses 265 but in general provide a good idea of the local conditions, at 100 kHz repetition rate. The range was set between 190 which are especially useful for landing operations. In terms and 300 ns to record subsurface reflection with the aim to find of climatological analysis, the series is too short and noisy, but is the only one available in the region. 240 crevasses in real time. The GPR antennae were mounted on a The mean daily air temperature at this location (Figure 3) 7 m long rod which was attached to a tractor and had rubber ◦ car tire tube installed at the opposite end. The radar data were 270 from 2008 to 2012 was -20.6 C, with an absolute minimum of -42.7◦C recorded on August 12, 2008 at 2 AM and analysed in real time while the tractor was moving, in order ◦ to alert the driver to the presence of hyperbolae, potentially an absolute maximum of 0.5 C registered on January 15, 2010 at 8 PM. Daily air temperatures have maximums in 245 related to crevasses. This system was used particularly in areas previously detected on ASTER satellite imagery as December-January following direct solar radiation, however being highly crevassed. 275 between April and August (included) when the Sun is below Post processing and data analyses were carried out the horizon, the lowest temperatures have a mean of -25.5 ± ◦ using Reflex-Win V5.6 (Sandmeier Scientific Software) for 3.5 C. Temperatures close to the melting point have only been observed in three summer events: January 7-8 250 the three radar systems. A background removal, dewow filter and adjustment of the gain function, among other 2008, January 14-15 2010 and December 25-26, 2010, but 280 no surface melting has been observed on this site. The mean procedures, were applied to the raw data. For crevasses ◦ analysis, migration correction procedures were also applied air temperature in January is -10.3 C. to collapse the hyperbolic diffractions to their proper point Regarding wind speed and direction, the data are very consistent between 2008-2012 with a mean value 16.3 255 origins (Plewes and Hubbard, 2001). In order to convert ◦ travel time to ice thickness in metres, it was assumed that the knots and predominant direction from 224 . The maximum electromagnetic wave travelled through the ice at 0.168 m/ns, 285 wind speed recorded on site was near 60 knots and the a mean value representative of cold ice (Glen and Paren, predominant wind speeds are higher than 25 knots (Figure 4). This predominant wind direction explains the extension of the main BIA which has experienced less than 10% area 6 Rivera et al (2013): Union | Paper Discussion | Glacier; Paper Discussion | Paper Discussion | a Paper Discussion new exploration gateway for the West Antarctic Ice Sheet TCD 8, 1227–1256, 2014 320 4.3 Surface ice velocity

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SurfaceA. Rivera ice et velocities al. were obtained for 19 stakes (B1-B19) located along the track between Patriot Hills and Union

GlacierTitle base Page camp (Figure 3 and 6), with measurements

periodsAbstract of nearIntroduction 1 year, and resulting ice velocities between

325 0.1Conclusions and 34.6References m/a. Minimum values were observed at local ice

dividesTables (B2-B8)Figures while the maximum velocity was observed at stake B10 located at the steepest area of Union Glacier, betweenJ two crevasseI ﬁelds (Figure 2 and 6). BetweenJ DecemberI 11, 2009 and January 31, 2010 a dual Back Close 330 frequency GPS receiver was attached to stake Bstat, taking continuousFull Screen / measurements Esc every 15 seconds. The receiver

wasPrinter-friendly powered Version by batteries and solar panels, providing a high Fig. 4. Predominant wind speed and direction 2008–2012 at the Union Glacier Automatic detailedInteractive record Discussion of daily ice dynamic during the spanned Weather Station. period of time. The main aim of this survey was to test Fig. 4. Predominant wind speed and direction 2008-2012 at the 335 the hypothesis that the ice dynamics of Union glacier are Union Glacier Automatic Weather Station. affected by Ronne Ice Shelf tides, as has been observed 1251 at the Rutford ice stream where a modulated ice ﬂow was detected (Gudmundsson, 2006). This stake (Bstat, Table 2) is located ∼38 km upstream the local grounding line zone changes between 2002 and 2013. The series is however too 340 of the Constellation inlet at the Ronne Ice Shelf (Figure 1 290 short to detect a trend (Table 1). and 2). The resulting ice velocity was 33 m/a, and thanks to the analysis carried out by H. Gudmunsson (personal 4.2 Mass balance communication), no tidal effect was detected. The neighbouring stake to Bstat (B9, located within a 345 distance of 10 m) was only measured for 30 minutes in Snow densities measured at stakes drilled on snow surfaces December 2009 and in December 2010, resulting in a annual 3 have a mean value of 400 ± 3 kg/m , with small variability velocity of 33.26 ± 0.7 m/a. The good correspondence with between all surveyed stakes. No fresh and soft snow was the Bstat results (33 m/a) indicates that Union Glacier does 295 detected due to the lack of precipitation in the region during not show important seasonal ice velocity changes. our surveys. Table 2 indicates the details of each stake, the 350 At the Union Glacier gate stake array (Figure 2), 22 stakes type of surface and period of measurements. were drilled into its Blue Ice Area in 2007 and have been The mass balance estimations between 2007 and 2011 at annually resurveyed up until 2011. Unfortunately, not all the the stakes installed on the BIA gate (V00 to V21, Table 2), original stakes have survived, or were re-measured every 300 are typically negative (Figures 5), with a mean inter annual year. However, from the remaining stake network a mean mass balance of -0.097 m w.eq./a. 355 velocity of 20 m/a was estimated for the measurement period The stakes installed along the traverse between Patriot (Figure 6). The orientation of the overall ice movement Hills and Union Glacier (Figure 1) were only measured shows an angle of 65.42◦ at the gate, similar to the main between 2008 and 2009 and showed differing results wind direction (Figure 4) observed at the BIA (from 224◦ 305 depending on their location. Stakes installed on snow to 44◦). No temporal velocity changes were observed during surfaces (B1-B12) have positive mass balances with a mean 360 the studied period. snow accumulation of 0.3 m/a (0.12 m w.eq./a). Just outside the BIA, stake B12 showed the maximum net balance (0.2 m w.eq./a), indicating that snow drift coming from the BIA, 4.4 Surface elevation change 310 due to the predominant katabatic wind direction (Figure 4) is having a positive downstream accumulation effect. The only Considering the surface velocities obtained for the stakes negative mass balance along the track Patriot Hills-Union (Table 2) and by applying Equation 1, a mean vertical Glacier (excluding the Union glacier BIA) was detected at velocity for the blue ice of -0.07 ± 0.007 m/a was found at stake B5, which is located at the edge of the local BIA 365 the Union Gate. For a steady state glacier this would be equal 315 of the Plumber glacier (Figure 1). Here a modest -0.07 m to the local mass balance, however this is not the case for the w.eq./a (Table 2) was obtained between 2008 and 2009. studied area (there is a negative mass balance at the BIA) Stakes B13-B19 (Table 2, Figure 2), also had negative values and a mean local elevation change of -0.012 m/a. was found. with a mean surface balance of 0.1 m w.eq./a, coincident with In any case, this result is close to the estimated error of the the other stakes located at a local BIA. 370 measurements and indicates near equilibrium conditions. icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion TCD 8, 1227–1256, 2014

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Back Close Fig. 5. Mass balance (m w.eq. a−1) 2007–2011 at stake located at the local BIA. Stakes loca- Fig. 5. Masstions balance are (m shown w.eq./a) in 2007-2011 Fig. 2. at stake located at the local BIA. Stakes locations are shown in Figure 2. Full Screen / Esc

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Fig. 6. Ice velocities (m/a) at the Union Glacier gate by indicated years. Stakes location is shown in Figure 2.

4.5 Glacier thickness and internal structure 390 mountain range dividing the two glaciers is also visible underneath the ice. The maximum thickness measured at More than 450 km long radar tracks were measured between Schneider Glacier was 900 m, 1050 m at Schanz Glacier, 2008 and 2010 at the Union Glacier area, including oversnow 1510 m in Driscoll Glacier and 1540 m in Union Glacier, traverses between Patriot Hills, Union Glacier and the high close to the base camp. 375 Antarctic plateau. 395 At the beginning of the radar profile (A-B in Figure 7), it The ∼80 km long profile between the Antarctic Plateau is possible to see the shallowest ice at the Gifford Peaks pass and Union Glacier (Figure 7 upper), including the transit (passage), with a thickness between 45 and 140 m (Figure 8) along Balish, Schneider, Schanz and Driscoll glaciers, was obtained with the low gain channel. This section (B in Figure surveyed with a FM-CW radar for snow accumulation data, 7 bottom) corresponds to a ∼5 km long FM-CW radargram 380 a compression pulse radar for ice thickness data and a GPR 400 where 400 m of dynamic range were processed detecting the system for crevasse detection purposes. snow/firm stratigraphy with a resolution of 1 m. The deep ice of the plateau is seen at the beginning of the All along the whole A-F profile (Figure 7 bottom), a survey profile (A in Figure 7), then is followed by a passage detailed internal stratigraphy was detected in the upper characterized by shallow ice (B) where the steepest section hundred metres, comprised of isochronous layers that are 385 of the whole traverse was crossed. After this section, the 405 particularly prominent at local ice divides (e.g. at C in thickness increases sharply until a local maximum thickness Figure 7 bottom). In those areas, the internal structures are of 1120 m was observed at Balish Glacier. The local divide characterized by typical minimum and divergent flow and between the Balish and Schneider glaciers (C in Figure 7) the presence of Raymond Bumps (Paterson, 1994) and other has a prominent subglacial peak (C in Figure 7) where the Rivera et al (2013): Union Glacier; a new exploration gateway for the West Antarctic Ice Sheet 7 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion TCD 8, 1227–1256, 2014

Fig. 5. Mass balance (m w.eq./a) 2007-2011 at stake located at the local BIA. Stakes locations are shown in Figure 2. Union Glacier A. Rivera et al.

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Fig. 6. Ice velocities (m a−1) at the Union Glacier gate by indicated years. Stakes location is Back Close shown in Fig. 2. Fig. 6. Ice velocities (m/a) at the Union Glacier gate by indicated years. Stakes location is shown in Figure 2. Full Screen / Esc

4.5 Glacier thickness and internal structure 390 mountain range dividing the two glaciers is also visible Printer-friendly Version underneath the ice. The maximum thickness measured at Interactive Discussion More than 450 km long radar tracks were measured between Schneider Glacier was 900 m, 1050 m at Schanz Glacier, 2008 and 2010 at the Union Glacier area, including oversnow 1510 m in Driscoll Glacier and 1540 m in Union Glacier, traverses between Patriot Hills, Union Glacier and the high close to the base camp. 375 Antarctic plateau. 395 At the beginning of the radar profile (A-B in Figure 7), it The ∼80 km long profile between the Antarctic Plateau is possible to see the shallowest ice at the Gifford Peaks pass and Union Glacier (Figure 7 upper), including the transit1253(passage), with a thickness between 45 and 140 m (Figure 8) along Balish, Schneider, Schanz and Driscoll glaciers, was obtained with the low gain channel. This section (B in Figure surveyed with a FM-CW radar for snow accumulation data, 7 bottom) corresponds to a ∼5 km long FM-CW radargram 380 a compression pulse radar for ice thickness data and a GPR 400 where 400 m of dynamic range were processed detecting the system for crevasse detection purposes. snow/firm stratigraphy with a resolution of 1 m. The deep ice of the plateau is seen at the beginning of the All along the whole A-F profile (Figure 7 bottom), a survey profile (A in Figure 7), then is followed by a passage detailed internal stratigraphy was detected in the upper characterized by shallow ice (B) where the steepest section hundred metres, comprised of isochronous layers that are 385 of the whole traverse was crossed. After this section, the 405 particularly prominent at local ice divides (e.g. at C in thickness increases sharply until a local maximum thickness Figure 7 bottom). In those areas, the internal structures are of 1120 m was observed at Balish Glacier. The local divide characterized by typical minimum and divergent flow and between the Balish and Schneider glaciers (C in Figure 7) the presence of Raymond Bumps (Paterson, 1994) and other has a prominent subglacial peak (C in Figure 7) where the 8 Rivera et al (2013): Union Glacier; a new exploration gateway for the West Antarctic Ice Sheet icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion TCD 8, 1227–1256, 2014

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Printer-friendly Version Fig.Fig. 7. 7. Upper:Upper: Location location map of the map radar andof GPS the survey radar measured and between GPS the survey Antarctic measured Plateau (A) and thebetween Union Glacier the (F) Antarctic in 2010 Plateau(green line). (A) In and blue, the the available Union data Glacier between F and(F) the in local 2010 Grounding (green Line Zoneline). at the In Constellation blue, the Inlet available (G). The background data between image Interactive Discussion is the RAMP AMM-1 SAR Image Mosaic of Antarctica (Jezek and RAMP Product Team, 2002). Bottom: Surface (red line) and subglacial F andtopography the (bluelocal line) Grounding interpreted from Line the radar Zone data collected at the in 2010 Constellation along the A-F track. Inlet (G). The background image is the RAMP AMM-1 SAR Image Mosaic of Antarctica (Jezek and RAMP Product Team, 2002). Bottom: surface (red line) and subglacial topography (blue line) interpreted from the radar data internal inflections that can lead to the understanding of 420 The GPR data collected along the track from Union 410collectedlong-term in changes 2010 in along the ice flow. the A–F track. Glacier to the Antarctic Plateau allowed the detection of The collected ice thickness data at Union glacier has a many crevasses both near the runway area, and at the Gifford mean ice thickness of 1450 m. The subglacial topography in1254Peaks passage, just before reaching the plateau. Along the the valley is smooth, with “U” shape flanks. The FM-CW survey route from the Union Glacier to the Antarctic Plateau radar detected multiple internal snow-firn layers and the 425 the GPR system was able to detect the upper 20 m of the 415 snow-ice boundary layer with a high resolution (Figure 8). internal structure of the ice. Within this upper layer annual Also, the FM-CW radar was capable of detecting the bedrock snow/firn layers suffered (in places) discontinuities due to up to 350 m, improving the bedrock detection from the crevasses that appeared as hyperbolae on the radar traces. pulse-compression radar when surveying shallow ice (B at In general, the route from Union glacier to the Antarctic Figure 7 bottom). 430 plateau is almost totally free of crevasses. An exception to this was the Gifford passage (B in Figure 7) where a Rivera et al (2013): Union Glacier; a new exploration gateway for the West Antarctic Ice Sheet 9 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion TCD 8, 1227–1256, 2014

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Fig. 8. FM-CW data obtained at Union glacier. Upper: location of the surveyed track near Fig. 8. FM-CW data obtained at Union glacier. Upper: location of the surveyed track near the base camp of Union glacier. The background Printer-friendly Version imagethe is base an ASTER camp false of composite Union 321 glacier. acquired on The March background 14, 2013. Bottom: image Radargram is showing an ASTER the surface false topography composite (red line), internal 321 annual snow/firn layers and the firn/ice boundary (blue line). Letters A-B indicates the base camp where the snow/firn layers are near 30-40 acquired on 14 March 2013. Bottom: radargram showing the surface topography (red line), Interactive Discussion minternal thick in total. annual Letter C, snow/firn corresponds layersto the central and moraine the firn/ice line of the boundary glacier. Letter (blueD, indicates line). theappearance Letters A ofthe and BIA B at indicate the surface. the base camp where the snow/firn layers are near 30–40 m thick in total. Letter C, corresponds minorto the system central of crevasses moraine was line detected of the corresponding glacier. Letter to D,5 indicates Discussion the appearance of the BIA at the steepeningsurface. slopes. These crevasses have widths between 1 and 5 m, with snow bridges between 1.5 and 3 m thick. At Union glacier, the local BIA is shaped by strong snowdrift 435 The Gifford passage is a narrow and steep valley, where1255caused by katabatic winds accelerated by the slope at the the longitudinal stresses and velocities are probably much main junction between the two glacial valley arms feeding higher than the rest of the area. In spite of these crevasses, 450 Union glacier from the upper Antarctic plateau. As a result, the Gifford passage is the shortest (235 km) gateway for the net mass balance at this BIA is negative, the driving factor oversnow traverses from Union glacier to the Subglacial explaining the mass loses being sublimation of ice during 440 Lake Ellsworth (Vaughan et al., 2007) at the Antarctic summer months, as no melting event has been observed since plateau, and is therefore the preferred route. The alternative 2007. However, melting events have been observed in the route to this subglacial lake is near 520 km long, starting 455 region, especially at Patriot Hills, where a small water pound at Union glacier, passing along Patriot Hills, then travelling was formed at the boundary between the BIA and the nunatak around the Tree Sails and finally turning west toward the during a very warm 1997 summer (Carrasco et al., 2000). 445 Subglacial lake Ellsworth (Figure 1 bottom). In spite of this negative surface mass balance, the resulting ice elevation changes between 2008-2011 at the Union gate 460 are very close to the error of the measurements (combined error of 0.01 m), with a mean of -0.012 m/a and high icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion TCD 8, 1227–1256, 2014

10 Rivera et al (2013): Union Glacier; a new exploration gateway for the West Antarctic Ice Sheet Union Glacier A. Rivera et al.

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J I Fig.Fig. 9. 9.ComparisonsComparisons of surface of surface and bedrock and topography bedrock of different topography data sources along of di transectfferent A - data G (see Figuresources 8 upper along for location transect of transect). CECs surface and bedrock from year 2010. IceSat surface from 2003-2005, Bedmap 2 surface and Bedrock from Fretwell et al. A–G (see Fig. 8 upper for location of transect). CECs surface and bedrock from year 2010. Back Close IceSat(2013). surface Bedmap2 bottom from dashed 2003–2005, line is interpolated. Bedmap 2 surface and Bedrock from Fretwell et al. (2013). Bedmap2 bottom dashed line is interpolated. Full Screen / Esc spatial variability (± 0.044 m/a) among the gate stakes. in Figure 7), showed a subglacial topography well below Accordingly, the glacier must be considered in equilibrium 495 sea level (-858 m), much deeper than previously thought. without significant changes compared with previous data However, between Union glacier and the local GLZ, the Printer-friendly Version 465 (Rivera et al., 2010). subglacial topography shows a maximum altitude of -190 The ice velocities at the BIA fluctuate between 11 and m at F in Figure 9. The subglacial topography then deepens 24 m/a without significant changes between 2007 and 2011. toward the GLZ, where the bedrock is estimated to be 1050 Interactive Discussion However, downstream of the Union gate, the velocities 500 m below sea level (Fretwell et al., 2013). This subglacial increase up to 33 m/a at the continuous GPS stake (Bstat), condition implies that an upstream migration of the GLZ 470 where no seasonal variations or tidal modulated variability until point F in Figure 9 will not have a strong effect on were detected. Union glacier. However, the glacier can respond in a more The GPR survey allowed the detection of many more dynamic way if this possible migration is affecting Union crevasses than were previously mapped with the ASTER 505 glacier upstream this point. imagery. The 400 MHz GPR is capable of identifying in1256 475 real time surface and buried crevasses. The 400 MHz GPR crevasses data were compared to the 500 - 900 MHz FMCW 6 Conclusions records and in most of the cases the wider crevasses (2 to 4 m width) could be detected in both radars. However, the Several glaciological oversnow campaigns have been FMCW radar did not provide the best information in regards undertaken to Union Glacier and nearby areas since 2007, where the surface and subglacial topographies were mapped 480 to snow bridge thicknesses (less than 1 m), its resolution being insufficient to detect the first meters of the snow pack. 510 in detail. These results were compared to the Bedmap Comparisons of surface and bedrock topography along dataset, showing much deeper bedrock and a much more transect A - F (Figure 9), resulted in a maximum ice thickness complex subglacial topography than previously estimated. difference of 1447 m with a mean difference of 477 m The obtained results determined a maximum ice thickness of 1540 m in Union Glacier, with a maximum snow ice 485 (standard deviation: 348 m) between the data presented here and BEDMAP2 (Fretwell et al., 2013). The surface 515 boundary layer at 120 m. The internal structure of the ice was topography of the surveyed traverse was found to differ from also mapped, including the detection of isochronous layers BEMAP2 and IceSat (2003-2005) by a mean of 91 m and and crevasses, allowing logistic operators and scientists to 169 m, respectively. These differences are understandable work along safer routes. Ice dynamics were also recorded thanks to the 490 due to the coarse resolution of BEDMAP2 and the footprint of IceSat. 520 measurement of ice velocities and ice thickness. Maximum The subglacial topography (Figure 9) at the main trunk ice velocity values of 34.6 m/a were obtained. Near of the Union glacier toward the local GLZ (Segment E-G equilibrium conditions were calculated at the BIA, where mean velocities of 20 m/a were measured. The snow