RESEARC H LETTER Ra dar ‐Detected E nglacial Debris i n t he West 10.1029/2019 GL084012 A ntarctic Ice S heet

Key Poi nts: Kate Wi nter 1 , John Wood ward 1 , Neil Ross 2 , St uart A. D u n ni ng 2 , A ndre w S. Hei n 3 , • I c e ‐pe netrati ng radar reveals basal 1 4 3 4, 5 sedi me nt re fl ectors i n t he West Matt he w J. Westoby , Riley C ulberg , S hasta M. Marrero , Dusti n M. Sc hroeder , A ntarctic Ice S heet David E. S ugde n 3 , a nd Marti n J. Siegert 6 • Basal ther mal regi mes pro mote sedi ment entrain ment 1 Depart ment of Geography and Environ mental Sciences, Faculty of Engineering and Environ ment, Northu mbria • O nce e ntrai ned, basal sedi me nts are University, Ne wcastle upon Tyne, U K, 2 School of Geography, Politics and Sociology, Ne wcastle University, tra nsported by regio nal ice fl o w Ne wcastle upon Tyne, U K, 3 School of Geosciences, University of Edinburgh, Edinburgh, U K, 4 Depart ment of Electrical Engineering, Stanford University, Stanford, C A, US A, 5 Depart ment of Geophysics, Stanford University, Supporti ng I nfor matio n: 6 • Supporting Infor mation S1 Stanford, C A, US A, Grantha m Institute and Depart ment of Earth Science and Engineering, I mperial College London, London, U K

Correspo nde nce to: K. Wi nt er, A bstract Str uct ural glaci ‐geological processes can entrain basal sedi ment into ice, leading to its k. winter @northu mbria.ac.uk tra nsportatio n a nd depositio n do w nstrea m. Sedi me nts pote ntially ric h i n esse ntial n utrie nts, like silica and iron, can thus be transferred fro m continental sources to the ocean, where deposition could enhance

Cit ati o n: marine pri mary productivity. Ho wever, a lack of data has li mited our kno wledge of sedi ment entrain ment, Wi nt er, K., W o o d w ar d, J., R oss, N., tra nsfer, a nd distrib utio n i n A ntarctica, u ntil no w. We use ice ‐pe netrati ng radar to exa mi ne e nglacial D u n ni ng, S. A., Hei n, A. S., Westoby, M. sedi me nts i n t he sector of t he West A ntarctic Ice S heet. Radargra ms reveal e nglacial re fl ect ors J., et al. (2019). Ra dar ‐detecte d e nglacial debris i n t he West A ntarctic o n t he leeside of n u nataks a nd s ubglacial hig hla nds, w here Mie scatteri ng a nalysis of t he re fl ect ors Ice S heet. Geophysical Research Letters , suggests particle sizes co nsiste nt wit h surface morai ne sedi me nts. We hypot hesize t hat t hese sedi me nts are 4 6 , 10,454 –10,462. htt ps:// d oi. org/ entrained at the ther mal boundary bet ween cold and war m ‐based ice. Co nservative esti mates of >130 × 10 9 10.1029/ 2019 GL084012 kg of e nglacial sedi me nt i n Horses hoe Valley alo ne s uggest t hat t he ice s heet has sig ni fi cant entrain ment

Received 5 J U N 2019 pote ntial u nappreciated previo usly. Accepted 21 A U G 2019 Accepted article online 26 A U G 2019 Plain Language Su m mary Glaciers can acquire and transport sedi ments rich in essential P u blis h e d o nli n e 1 0 S E P 2 0 1 9 n utrie nts, like silica a nd iro n, fro m t he la nd to t he ocea n, w here depositio n may e n ha nce biological productivity. Our ability to detect a nd map sedi me nt ‐ric h ice i n A ntarctica has bee n hi ndered by tec h nical li mitatio ns of geop hysical eq uip me nt a nd data availability, u ntil no w. I n t his paper ice ‐pe netrati ng ra dar is used to detect sedi me nts i n t he Weddell Sea sector of t he West A ntarctic Ice S heet. We ide ntify sedi me nt near t he glacial surface, a nd furt her do w n t he ice colu m n, alo ng partially buried mou ntai n ra nges a nd bedrock hills be neat h t he ice. Our co nservative esti mates reveal >130 millio n metric to n nes of e nglacial sedi me nt i n a si ngle valley. T hese sedi me nts are e ntrai ned at t he ice/bed i nterface a nd tra nsported to mo u ntai n fro nts a nd t he coast by local a nd regio nal ice fl o w.

1. I ntrod uctio n O nce e ntrai ned i n glacier ice, sedi me nts, i ncl udi ng t hose ric h i n esse ntial n utrie nts like silica a nd iro n, ca n be tra nsferred fro m co nti ne ntal so urces to t he ocea n by ice fl o w, a nd t he n iceberg productio n a nd tra nsport ( Deat h et al., 2014; Ha w ki ngs et al., 2014, 2018; Nic holls et al., 2012). T his ca n create layers of ice ‐rafte d de b- ris i n d e e p ‐sea deposits ( Leve nter et al., 2006; Pierce et al., 2011), s uc h as t he Nort h Atla ntic Hei nric h layers ( Hei nric h, 1998). I n A ntarctica, t hese offs hore deposits are used to reco nstr uct i nla nd sedi me nt so urces, a nd assess past c ha nges i n t he locus of glacial erosio n ( Lic ht & He m mi ng, 2017; Wilso n et al., 2018). Offs hore sedi ments have also been used to exa mine glacially derived bioavailable iron concentrations (Shoenfelt

et al., 2018), which can i mpact pri mary productivity and C O 2 dra wdo wn in the ( Death et al., 2014; Ha wki ngs et al., 2014, 2018). W hile t he sig ni fi cance of sedi ment entrain ment, transfer, and ©2019. T he Aut hors. deposition are understood, observations of englacial sedi ment in large ice sheets, which can corroborate T his is a n ope n access article u nder t he ter ms of the Creative Co m mons i nterpretatio ns of mari ne sedi me ntary records, are rare. T his is a fu nctio n of bot h ice ‐pe netrati ng radar Attrib utio n Lice nse, w hic h per mits use, (IP R) resolution and data availability. Recent i mprove ments in the collection and processing of IP R data distrib utio n a nd reprod uctio n i n a ny allo w us to investigate mechanis ms of sedi ment entrain ment and transfer in the Weddell Sea sector of the mediu m, provided t he origi nal work is pro perly cite d. West A ntarctic Ice S heet ( W AIS).

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Fig ure 1. Horseshoe Valley, West . (a) Landsat I mage Mosaic of Antarctica ( LI M A) ( US GS: http://li ma.usgs. gov/) contextualizing the Patriot and in Horseshoe Valley ( marked on the inset map of Antarctica) a nd t he locatio n of airbor ne radio ec ho so u ndi ng ( R ES) tra nsects 1 – 9. Note t hat airbor ne R ES tra nsect 2 is marked i n red, to differe ntiate its red uced le ngt h i n Fig ure 2d fro m t he w hole fl ig ht li ne (tra nsect 4) used i n Fig ure 3a. (b) Bed elevatio n plot ( Fret well et al., 2013) s uperi mposed over R A D A RS A T ( Li u et al., 2001) to s ho w t he relief of Horses hoe Valley, its deli miting mountains, and the neighboring Independence Trough.

2. St udy Area We a nalyze e nglacial re fl ectors, recorded by both regional airborne radio echo sounding ( R ES) and local ground ‐based IP R, in and around Horseshoe Valley (80°018 ′S, 81°122 ′W), which is in the southern of ( Figure 1). These mountains separate the ~20 ‐k m ‐wide a nd >2,000 ‐m ‐ t hick Horses hoe Glacier ( Wi nter et al., 2015), w hic h fl o ws at 8 – 12 m/a ( Rig not et al., 2017), fro m neig hbori ng i c e fl o w, and channelize ice in Horseshoe Valley to ward the Filchner ‐Ronne Ice Shelf, ~45 k m a way at Hercules Inlet. Sharp peaks, ridges, and bedrock spurs constrain topographic e mbay ments on the lee ward slopes of the Liberty, Marble, Independence, and Patriot Hills of Horseshoe Valley (Figure 1) where enhanced subli mation by katabatic winds cause ice and basal debris to fl o w to ward the mountain front, for mi ng exte nsive blue ice areas a nd associated blue ice morai nes ( Fog will et al., 2012; Hei n et al., 2016; S ugde n et al., 2017; Fig ure 2). T hese glacial a nd at mosp heric processes ca use previo usly horizo ntal e nglacial layers t o fl o w up to ward the mountain front ( Winter et al., 2016), exhu ming glacially entrained sedi ment clasts t hat ofte n ex hibit glacial abrasio n, i n t he for m of suba ngular to subrou nded s hapes a nd milli meter ‐ scale striations (Figures 2b and 2c) and exposing inclined isochrone sequences at the glacial surface ( Tur ney et al., 2013). T he existe nce of subglacially tra nsported clasts a nd exotic lit hologies i n Horses hoe Valley blue ice morai nes a nd historical i nterpretatio n of morai ne sedi me nt i n local IP R tra nsects ( Doake,

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Fig ure 2. Characterization of Independence Hills Blue Ice Moraine. (a) Sedi ment e merging fro m blue ice in Horseshoe Valley. (b) Sedi me nt clast wit h s ubro u nded s hape a nd straig ht, parallel striatio ns. (c) Sedi me nt clast wit h s uba ng ular s hape a nd m ultiple striatio ns, i n a variety of orie ntatio ns. (d) Airbor ne R ES tra nsects across a valley glacier a nd I ndepe ndence Hills (s ho w n i n p hotograp h f) reveal e nglacial re fl ectors w hic h exte nd fro m t he ice/bed i nterface to Bl ue Ice Morai ne ( BI M) deposits i n t he lee of I ndepe ndence Hills. (e) Hyperbolic re fl ectors i n a n I P R tra nsect (collected across the BI M sequence at the base of Independence Hills) reveal seven linear features beneath the sedi ment ‐stre w n s urface (sho wn in photograph f).

1981) make Horseshoe Valley well suited for investigation of basal sedi ment entrain ment and transfer processes i n A ntarctica.

3. Met hods 3.1. Gro u nd Based Ice ‐Pe netrati ng Radar A sl e d g e ‐mounted Pulse Ekko 1000 syste m, operating at 200 M Hz, was used to acquire high ‐resol utio n I P R data fro m Horses hoe Valley at 0.1 ‐m i ntervals i n t he a ustral s u m mer of 2012/2013 ( Wi nter et al., 2016). I P R survey antenna were co ‐polarized a nd orie ntated perpe ndicular to t he survey li nes, wit h t heir broadsides parallel to eac h ot her. Differe ntial G PS was collected to e nable topograp hic correctio n to deci meter acc uracy. R e fl ex W (2012) radar processi ng soft ware (versio n 6.1.1; Sa nd meier Scie nti fi c Soft ware, 2012) was used to process IP R data using standard processing steps ( King, 2011; Welch & Jacobel, 2005; Wood ward & King, 2009), i ncl udi ng ti me zero correctio n, ba nd ‐p ass fi lteri ng, a nd applicatio n of a n e nergy decay gai n. As poi nt r e fl ectors are more apparent in un migrated data, migration was not applied. To convert travel ti me to ice thickness a standard electro magnetic wave propagation speed in ice of 0.168 m/ns was applied. 3.2. Airbor ne Radio Echo Sou ndi ng T he Britis h A ntarctic Survey Polari metric ‐radar Airborne Science Instru ment ice ‐sounding radar syste m (Jeofry et al., 2018) was used to collect R ES data in and around Horseshoe Valley in the austral su m mer of 2010/2011 for t he Britis h A ntarctic S urvey I nstit ute Möller A ntarctic F u ndi ng I nitiative s urvey ( https:// doi:10.5285/8a975b9e ‐f18c ‐4c51 ‐9bdb ‐b00b82da52b8). P ASI N operates at a frequency of 150 M Hz, using a p uls e ‐coded wavefor m acquisitio n rate of 312.5 Hz a nd ba nd widt h of 12 M Hz. Aircraft positio n was obtai ned fro m differe ntial GPS, w hile terrai n cleara nce was measured usi ng radar/laser alti meters. Re moval of radar ‐ scatteri ng hyperbolae i n t he alo ng ‐track direction was achieved through t wo ‐di mensional focused Doppler

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(sy nt hetic apert ure radar) processi ng. T his tec h niq ue also e n ha nced basal feat ures. As a f u nctio n of t his pro- cessing, the upper most 200 sa mples (~150 – 200 m) of synthetic aperture radar ‐processed radargra ms are ofte n poorly resolved, so no n t wo ‐di mensional S A R pulse data were used to investigate near ‐s urface re fl e c- ti o ns ( R E S li n es 1 – 3; Figure 2d). An electro magnetic wave propagation speed of 0.168 m/ns and a 10 ‐m fi r n correctio n was applied to all R ES tra nsects ( Ross et al., 2012). Tec h nical details of t he Polari metric ‐ra dar Airborne Science Instru ment syste m and additional details on the acquisition and processing of the Institute Möller Antarctic Funding Initiative R ES data are available in Jeofry et al. (2018), and refere nces t herei n.

3.3. Mie Scatteri ng A nalysis To assess t he li keli hood t hat e nglacial re fl ectors in R ES data are entrained sedi ments, we apply Mie theory ( Aglya mov et al., 2017) to qua ntify t he sedi me nt grai n sizes required to produce t he observed ec ho po wers a nd co mpare t hese esti mates to surface sedi me nt grai n sizes fro m Patriot Hills, recorded by Westoby et al. (2015). U nder t his model, e ntrai ned sedi me nt is treated as a ra ndo m collectio n of sp herical particles of so me mea n radius occupyi ng so me fractio n of t he ice volu me. T he total po wer received by t he radar is t he n t he su m of the po wer backscattered by every particle in the illu minated volu me ( Ulaby & Long, 2014). Kno wing the echo po wer, radar syste m para meters, and fl ight geo metry, we set the fractional volu me and solve the volu metric radar equation ( Ulaby & Long, 2014) for the Mie backscatter coef fi cie nt, a f u nctio n of particle size a nd material. A n ope n ‐so urce Mie solver ( Mätzler, 2002) was used to i nvert for t he esti mated particle radi us. F urt her details of t hese met hods, partic ularly t he radio metric calibratio n of t he radargra ms a nd associated u ncertai nties, are described i n t he s upporti ng i nfor matio n.

4. Res ults O ur geop hysical observatio ns reveal steeply dippi ng e nglacial re fl ectors, exte ndi ng fro m t he ice/bed i nter- face up to blue ice morai ne co mplexes i n t he foregrou nd of exposed bedrock alo ng t he margi ns of t he sout h- er n Heritage Ra nge ( Fig ure 2). Si milar re fl ectors are recorded in Horseshoe Valley Glacier beneath >1.4 k m of ice ( Fig ure 3). T hese re fl ectors are explored i n t he follo wi ng paragrap hs.

4.1. E nglacial Re fl ect ors I n t he lee of I ndepe nde nce Hills, e nglacial re fl ectors are recorded i n several airbor ne R ES tra nsects, be neat h expa nsive blue ice morai nes ( Figure 2). R ES fl ig hts 1 – 3 ( Fig ure 2d) record re fl ectors which extend through the ice colu mn (spanning 400 – 1,000 m), fro m t he ice/bed i nterface, to s urface sedi me nt deposits. T he detail a nd co mplexity of t hese re fl ectors are recorded by IP R. A n IP R tra nsect across I ndepe nde nce Hills blue ice moraine ( Figure 2e) reveals nu merous hyperbolic returns beneath the sedi ment ‐stre w n s urface, eac h repre- se nti ng poi nt feat ures i maged by t he radar at differe nt a ngles ( Da niels, 2004). T he steepe ned li mbs a nd crests of stacked hyperbolic ret ur ns help to reveal seve n li near feat ures wit hi n t he ice i n Fig ure 2e. Airborne R ES transects also reveal steeply dipping englacial re fl ectors in the main fl o w of Horseshoe Glacier, alo ng t he s ubglacial exte nsio n of Patriot Hills ( Fig ure 3a). I n tra nsects 4 – 6 e nglacial feat ures exte nd fro m t he ice/bed i nterface, to ward t he ce nter of Horses hoe Valley, be neat h >1.4 k m of ice (~50 m belo w sea level; Fig ure 3a). Alt ho ug h re fl ectors i n tra nsects 4 a nd 5 are co n nected to t he basal s ubstrate, t he dippi ng r e fl ectors i n tra nsect 6 are co mpletely surrou nded by ice. I n t his last tra nsect (tra nsect 6), t wo disti nct pac kages of re fl ectors are recorded, ~100 a nd 240 m a way fro m t he valley side wall. T hese re fl ectors exte nd to ward t he ce nter of Horses hoe Valley at a red uced i ncli natio n to t hose hig hlig hted i n tra nsects 4 a nd 5. Airbor ne R ES tra nsects 7 – 9 ( Fig ure 3c), collected j ust o utside t he co n fi nes of Horses hoe Valley ( Fig ure 1; w here ice fl o ws fro m Horseshoe Valley's neighboring Independence Trough, to ward the Institute Ice Strea m a nd t he Filc h ner Ro n ne Ice S helf ( Wi nter et al., 2015)), reveal si milar e nglacial re fl ectors to t hose recorded i n R ES tra nsects 4 – 6 ( Fig ure 3a). I n Fig ure 3c i ncli ned re fl ectors are recorded alo ng t he edge of a subglacial hig hla nd, w here t he e nglacial features exte nd up, a nd i nto t he ice fl o w at a ngles of ~55° to 63° (fro m vertical). I n tra nsects 7 a n d 8, t hese re fl ectors exte nd fro m t he raised basal s ubstrate be neat h b uckled a nd disr upted e nglacial stratigrap hic layers. Seq ue ntial radargra ms s ho w ho w t he spatial a mplit ude a nd fre- q ue ncy of t hese e nglacial layers decrease do w n fl o w, u ntil largely straig ht a nd parallel layers are recorded i n tra nsect 9, w here t he bed elevatio n is lo wer. I n t his fi nal radargra m dippi ng e nglacial re fl ectors are prese nt near t he ice/bed i nterface b ut t hey are not co n nected to t he bed.

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Fi g ure 3. E n gl a ci al r e fl ectors i n Horses hoe Valley, detected more t ha n 1.4 k m belo w t he ice s heet s urface. (a) Airbor ne R ES fl ig hts 4 – 6 r e c or d r e fl e ct ors, t h o u g ht t o have derived fro m debris ‐ri c h i c e. R e fl ectors exte nd fro m t he subglacial mou ntai n side to t he ce nter of Horses hoe Glacier. Note t hat t he upper most 200 sa mples ( ~500 m) of t he ice col u m n are poorly resolved as a f u nctio n of 2 ‐D sy nt hetic apert ure radar (S A R) processi ng (see sectio n 3). (b) A sc he matic represe ntatio n of (a), highlighting sedi ment entrain ment along the local ther mal boundary (and in co mpressive BI As where katabatic winds subli mate the ice surface). When s ubglacial topograp hy c ha nges a nd sedi me nts are no lo nger available for e ntrai n me nt at t he elevatio n of t he local t her mal bo u ndary (e.g., R ES tra nsect 6), regio nal i c e fl o w will passively tra nsport glacially e ntrai ned clasts do w n fl o w, to ward t he local grou ndi ng li ne. (c) Radargra ms 7 – 9 r e v e al si mil ar e n gl a ci al r e fl ect ors t hat exte nd fro m a subglacial hig hla nd regio n. We hypot hesize t hat t hese re fl ectors evide nce basal sedi me nts e ntrai ned be neat h buckled e nglacial layers i n R ES tra nsects 7 – 8. When the subsurface topography changes do wn fl o w i n R ES li ne 9 t hese e nglacial sedi me nts are detac hed fro m t he basal substrate a nd passively transported do wn fl o w.

T h e st e e pl y di p pi n g r e fl ectors we hig hlig ht i n airbor ne R ES tra nsects are recorded i n a variety of radargra ms, which have been collected along fl ig ht pat hs of varyi ng orie ntatio n. I n t hese radargra ms, re fl ector s ha pe, si z e, a n d r e fl ectivity vary with depth through the ice colu mn, and with increasing distance do wn fl o w. W e t herefore i nterpret t hese re fl ectors as arising fro m sedi ment entrained within the basal layers of the W AIS. This hypothesis is supported by basal clasts in local blue ice moraine sequences ( Figures 2b and 2c) a nd t he occ urre nce of hyperbolically de fi ne d poi nt ‐li ke feat ures i n a n I P R tra nsect ( Fig ure 2e). We reject alter native i nterpretatio ns t hat t he re fl ectors co uld represe nt off ‐li n e r e fl e ct ors fr o m v all e y si d e w alls ( e. g., Holt et al., 2006) or c ha nges i n t he crystal fabric of t he ice (e.g., Eise n et al., 2007) beca use seq ue ntial ice ‐ pe netrati ng radargra ms reveal re fl ectors at differe nt dept hs, a ngles, a nd dista nces fro m t he mo u ntai n fro nt. These t wo processes also fail to account for the occurrence of subangular and subrounded clasts with milli m et er ‐scale striatio ns i n local bl ue ice morai ne sedi me nts ( Fig ures 2b a nd 2c). Mie scatteri ng a nalysis allo ws us to explore deep s ubs urface ec hoes mat he matically. A nalysis prod uces p hy- sically pla usi ble particle sizes, i n t he regio n of 3 – 10 c m in R ES transects 5, 6, 8, and 9, when we assu me an ice/debris ratio of 90:10 ( Figure 4 a nd supporti ng i nfor matio n), w hic h is co nsiste nt wit h observed sedi me nt co nce ntratio ns i n A ntarctic ice cores of 0.3 – 3.4 % ( Tiso n et al., 1993) a nd 12 – 15 % ( Go w et al., 1979). T hese particle sizes correlate wit h meas ured grai n sizes wit hi n t he Patriot Hills bl ue ice morai ne ( Westoby et al., 2015). We repeat Mie scatteri ng a nalysis for a ra nge of atte nuatio n values to accou nt for t he u ncertai nty i n radio metric calibratio n a nd fi nd that esti mated particle sizes re main co mparable with measured sedi ment grai n sizes as lo ng as t he ice/debris ratio is eq ual to or greater t ha n 90:10 (see s upporti ng i nfor matio n for f urt her descriptio n of met hods a nd res ults). T his a nalysis s upports o ur i nterpretatio n t hat t hese deep re fl e c- tors are i ndicative of e ntrai ned sedi me nts.

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Fi g ure 4. Sedi me nt sizes required to produce t he recorded po wer fro m deep re fl ectors i n vario us airbor ne R ES tra nsects as a fu nctio n of t he e nglacial atte nuatio n rate at a 10 % fractio nal volu me of sedi me nt. T he blue das hed li ne is t he mea n modeled atte n uatio n rate ( Mats uoka et al., 2012). Black sq uares s ho w t he opti mal esti mate of sedi me nt size for eac h deep r e fl ector a nd error bars give t he variatio n i n particle size for fractio nal vol u mes fro m 3 to 20 %. T he w hite recta ngle hig hlig hts t he ra nge of p hysically pla usible sedi me nt grai n sizes a nd atte n uatio n rates. Eve n co nsideri ng u ncertai nty i n atte n uatio n a nd fractio nal vol u me, t he observed ec ho po wers i n tra nsects 5, 6, 8, a nd 9 are co nsiste nt wit h scatteri ng fro m e ntrai ned sedi me nts (a nd observed sedi me nt sizes i n local blue ice morai ne deposits).

4.2. E nglacial Debris Vol u me To esti mate the area, volu me, and mass of englacial debris in and around Horseshoe Valley, polygons are use d to de fi ne t he exte nt of debris re fl ector ba nds wit hi n zo nes of off ‐li n e r e fl ectors a nd hyperbolic radar returns in the nine airborne R ES transects we exa mine. Approxi mate area calculations are provided in Table 1. To convert area to volu me, kno wn distances bet ween R ES fl ig ht li nes were fed i nto eq uatio n 1,

w hic h de notes a sta ndard eq uatio n, developed to calc ulate t he vol u me of a fr ust u m ( V f) fro m t wo give n areas (A 1 , A 2 ).

V f ¼ ðdista nce bet wee n tra nsects = 3 Þ × A 1 þ A 2 þ √ ðA 1 × A 2 Þ ( 1)

T his calc ulatio n yields esti mates of ~506 × 10 6 m 3 of de bris ‐ric h ice bet wee n airbor ne R ES tra nsects ( w hic h cover <10 % of Horseshoe Valley Glacier). Assu ming a standard high li mestone density of 2,560 kg/ m 3

Ta ble 1 Approxi mate Debris Calculations Fro m Sedi ment Re fl ectors i n Ni ne Airbor ne R ES Tra nsects R E S fl i g ht Are a Dista nce bet wee n Debris volu me Mass 100 % Mass 10 % Mass 20 % nu mber ( ×10 3 m 2 ) R ES tra nsects ( m) ( ×10 6 m 3 ) ( ×10 9 k g) ( ×10 9 k g) ( ×10 9 k g)

1 9 2 ‐ ‐ ‐ ‐ ‐ 2 5 5 1, 6 9 0 1 2 3 3 1 5 3 2 6 2 3 2 1 1, 5 9 6 5 9 1 5 0 1 5 3 0 4 8 ‐ ‐ ‐ ‐ ‐ 5 8 3, 4 2 6 2 7 7 0 7 1 4 6 (r e fl ect or 1) 1 5 1 2, 6 1 1 1 4 3 3 6 5 3 7 7 4 6 (r e fl ect or 2) 2 6 6, 0 0 0 a 9 7 2 4 8 2 5 5 0 7 2 3 ‐ ‐ ‐ ‐ ‐ 8 3 0 1, 1 0 4 2 9 7 5 7 1 4 9 1 2 1, 3 9 6 2 8 7 3 7 1 4 T ot al: 290 × 10 3 m 2 506 × 10 6 m 3 1296 × 10 9 kg 130 × 10 9 kg 258 × 10 9 k g N ote . Mass of debris = total volu me of debris × 2,560 (represe nti ng t he de nsity of li mesto ne i n kg/ m 3 (Ingha m, 2010)). Mass 100 % assu mes 100 % debris block. Mass 10 % approxi mates 90 % ice a nd 10 % debris, etc. a Esti mated dista nce d ue to lack of 3 ‐D d at a.

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(I ng ha m, 2010), w hic h re fl ects regio nal lit hology (S ugde n et al., 2017), t hese approxi matio ns s uggest co nser- vative esti mates of >130 millio n metric to n nes of e nglacial sedi me nt i n a nd arou nd Horses hoe Valley (usi ng a n ice/debris ratio of 90:10). I n co ntext, t his esti mate eq uates to a sedi me nt yield of ~75 kg/ m 2 were t his mass to be distrib uted across t he s urface of Horses hoe Glacier.

5. Disc ussio n Basal t her mal regi mes co ntrol subsurface erosio n, melt water productio n, basal slip, a nd sedi me nt e ntrai n- me nt i n glacial syste ms ( Boulto n, 1972; Weert ma n, 1961, 1966). I n areas w here bed topograp hy favors co n- verge nt ice fl o w, ice velocities will increase and produce more heat by internal defor mation and/or basal sliding (Sugden, 1978). These t wo funda mental concepts govern sedi ment entrain ment and sedi ment trans- p ort i n large ice s heets.

5.1. Sedi me nt E ntrai n me nt a nd Tra nsfer i n Blue Ice Areas We propose t hat basal sedi me nts i n Horses hoe Valley are e ntrai ned wit hi n glacial ice t hro ug h repeated melt- i ng a nd freezi ng processes at t he glacial bed, c haracteristic of war m ‐based ice ( Alley et al., 1997; H ubbard & Sharp, 1993). When clasts are englacially entrained along the margins of Horseshoe Valley, ice fl o w will transfer these sedi ments up ward, with ice fl o w, to ward blue ice surfaces. On the lee ward side of I n d e p e n d e n c e Hills, t his fl o w trajectory is evide nced by stacked hyperbolic re fl ectors a n d s heets of e nglacial sedi me nts. We hypot hesize t hat t hese re fl ectors represe nt pla nes of co nce ntrated sedi me nt i n t he ice, for med in response to the co mpression i mposed by ice fl o w to ward the mountain front.

5.2. Sedi me nt E ntrai n me nt a nd Tra nsfer at Depth Regio ns of freeze o n a nd melt i n A ntarctica will c ha nge spatially a nd te mporally as t he local glacial t her mal bo u ndary reacts to ice fl o w across varied and co mplex topography and glacial ‐t o ‐i nterglacial c ha nges i n ice t hick ness. T he i mpact of varyi ng relief is appare nt i n tra nsects 4 a nd 5 ( Figure 3a), w here sedi me nts are e ntrai ned t hro ug h regelatio n processes alo ng t he s ubglacial exte nsio n of Patriot Hills. We hypot hesize t hat entrain ment occurs at the ther mal boundary bet ween war m ‐ a nd cold ‐based ice. W he n subglacial topogra- p hy c ha nges i n tra nsect 6, a nd co nditio ns for e ntrai n me nt are no lo nger met ( w he n t he local t her mal bou nd- ary is likely to be elevated with respect to the lo wer subglacial topography), sedi ments cannot be incorporated into the glacier. These processes are sho wn sche matically in Figure 3b. This fi gure de mon- strates sedi me nt e ntrai n me nt alo ng t he margi ns of Horses hoe Valley w he n t he local t her mal bou ndary i nter- sects the ice/bed interface, and passive transportation of previously entrained debris ‐rich ice when co nditio ns for e ntrai n me nt are no lo nger met. Duri ng t his passive p hase, previously e ntrai ned sedi me nt is “ s meared out ” i n t he directio n of fl o w (Stokes & Clark, 2002). T his do w n ‐fl o w atte nuatio n eve ntually lo wers t he t hick ness of e nglacial sedi me nt layers beyo nd t he resol utio n of c urre nt radar syste ms, w hic h acco u nts for the reduction and eventual loss of sedi ment derived re fl ectors i n airbor ne R ES tra nsects collected f urt her do wn fl o w, at ever ‐i ncreasi ng dista nces fro m sedi me nt sources. Si milar met hods of e ntrai n me nt a nd tra nsfer are recorded i n R ES tra nsects 7 – 9 w he n e nglacial re fl ect ors are i maged alo ng t he side of a topograp hic upla nd ( Figure 3c), situated just outside t he co n fi nes of Horses hoe Valley. I n t hese radargra ms, re fl ectors wit h radar scatteri ng i ndicative of sedi me nt particles i n t he ra nge of 3 – 10 c m (see Figure 4 a nd supporti ng i nfor matio n) are recorded be neat h buckled e nglacial stratigrap hic layers. T hese layers develop as a res ult of co mpressio n d ue to ice fl o w around the bedrock obstacle, which initiates and maintains a local ther mal boundary bet ween slo w (colder) fl o w around the bedrock obstacle, and faster ( war mer) ice fl o w above the obstacle ( Boulton, 1979; Hind marsh & Stokes, 2008; Stokes & Clark, 2002). Sedi me nt is e ntrai ned alo ng t his i nterface t hroug h melti ng a nd freeze ‐o n. J ust like sedi me nt e ntrai n me nt a nd tra nsfer i n margi nal bl ue ice areas, t he prese nce of sedi me nt particles i n t he fl o w fi el d of t he wider ice s heet s ho w t hat sedi me nt is still available for e ntrai n me nt, a nd t hat sedi me nts co nti nue to be excavated, entrained, and transported in West Antarctica. Ho wever, as sedi ment entrain ment and trans- portatio n processes will vary spatially a nd te mporally as a res ult of s ubglacial topograp hy, sedi me nt avail- a bilit y, i c e fl o w, and ice te mperature, changing conditions (for exa mple, in ice thickness) will alter sedi ment abundance and distribution in Antarctica, i mpacting Southern Ocean sedi ment delivery.

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6. Co ncl usio ns Steeply dippi ng e nglacial re fl ectors i n regio nal airbor ne R ES a nd hyperbolic re fl ectors i n local gro u n d ‐base d I P R tra nsects are evide nce of debris ‐rich ice in the W AIS. In Horseshoe Valley katabatic winds pro mote and mai ntai n exte nsive bl ue ice areas i n t he lee of n u nataks, w here basal sedi me nts are e ntrai ned at t he ice/bed interface by repeated melting and refreezing of war m ‐based ice. These sedi ments are transported through t he ice col u m n to t he glacial s urface by co mpressive ice fl o w, enabling basal sedi ment exhu mation in debris ba nds a nd blue ice morai nes. Glacial t her mal regi mes also facilitate sedi me nt e ntrai n me nt t hroug h regela- tion at greater depths (beneath >1.4 k m of ice), at the junction bet ween war m ‐ a nd cold ‐base d ice. Co mbined, these englacial sedi ment re fl ectors yield co nservative esti mates of 130 × 10 9 kg of sedi me nt i n i c e fl o wing in and around Horseshoe Valley. Given si milar glaciological settings in the Weddell Sea Sector, and else where in the W AIS, these fi ndi ngs s uggest t hat t he ice s heet has a sig ni fi cant entrain ment pote ntial u nappreciated previo usly.

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