A Stratigraphic Link Across 1100 Km of the Antarctic Ice Sheet Between the Vostok Ice-Core Site and Titan Dome (Near South

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Citation for published version: Siegert, MJ & Hodgkins, R 2000, 'A stratigraphic link across 1100 km of the Antarctic Ice Sheet between the Vostok ice-core site and Titan Dome (near South Pole)', Geophysical Research Letters, vol. 27, no. 14, pp. 2133-2136. https://doi.org/10.1029/2000GL008479

Digital Object Identifier (DOI): 10.1029/2000GL008479

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Download date: 06. Oct. 2021 GEOPHYSICAL RESEARCH LETTERS, VOL. 27, NO. 14, PAGES 2133-2136, JULY 15, 2000

A stratigraphic link across1100 km of the Antarctic Ice Sheet between the Vostok ice-core site and Titan Dome (near South Pole) Martin J. Siegert BristolGlaciology Centre, School of GeographicalSciences, University of Bristol,Bristol, England

RichardHodgkins Departmentof Geography,Royal Holloway, University of London,Egham, Surrey, England.

Abstract. Isochronousinternal ice-sheet layering, (3) crystalorientation fabrics [Fujita et al., 1999]. Internal measuredfrom airborne 60 MHz radar, was traced between layersbelow 800 m observedin 60 MHz radarare believed Lake Vostok and the Titan Ice Dome (100 km from South to be isochronous, and traceable over several hundred Pole Station), Antarctica. Three layers were selected kilometers[Millar, 1981; Siegert et al., 1998; Fujita et al., betweenRidge B and Titan Dome, and betweenRidge B 1999]. Radar layers are readily identifiablein the archive andLake Vostok.This layeringcan be usedto correlatethe of radar data held at the Scott Polar Research Institute existing Vostok ice core across1100 km of the ice sheet (SPRI) in Cambridge.These data cover over 400,000 km of interior. Our correlation is also matched to the new EPICA track across 40% of the East Antarctic Ice Sheet. ice-core site, by using an existing radar link between The Vostok ice core has already been matchedto the Vostok and Dome C stations.Thus, three East Antarctic ice EPICA ice-core site at Dome C through radar layering domesare linked stratigraphically for the first time through [Siegertet al., 1998]. Five internalradar layerswere traced internalice-sheet radar layering.Our resultsindicate that along a flightline between Vostok and Dome C. The the basal layers of ice at Titan Dome are around 165,000 internallayering showed that 300 m more ice was present yearsold suggestingthat this location and, by inference,the at Dome C for the last glacial-interglacialcycle than in the SouthPole Station,are prime sitesfor a high-resolutionice Vostok core. Moreover, the depth-ageprofile at Dome C corefrom the lastglacial-interglacial cycle. (adjusted from Vostok) supportedflow model results indicating that the base of the Dome C core would be significantlyolder thanthe Vostok ice core base[Siegert et Introduction al., 1998]. Thus, internal layer correlationbetween Vostok Deepice coresfrom the centresof largeice massesyield andDome C hasprovided a usefulguide for the EPICA ice detailedpaleoenvironmental information spanning several core and its relationshipto the Vostok core [Hodgkinset hundredthousand years of recent Earth history. For al., in press]. example,the Vostokice core holdsclimate data for the last Titan ice dome is located about 100 km from South Pole. 420,000 years [Petit et al., 1999]. In order to establishas In this paper, a new internal ice-sheetradar layer link is comprehensivea history of palaeoclimateas possible, made betweenTitan Dome and Vostok Station. In doing records from ice cores separatedby several hundred so, three major ice divides (Dome C, Ridge B and Titan kilometersneed to be correlated.To date, Be•ø markers, Dome) in Antarcticaare linked directlyfor the first time. volcanic horizons and dust layers within the ice have provided an effective means to compare ice-core Correlating internal radar layers acrossthe information.The use of radar layeringnot only links ice ice sheet coresstratigraphically, but detailsthe spatialbehaviour of Titan Dome is linked to the Vostok Station by radar the ice column across the ice sheet. flightlinesas follows (Fig. 1). One airborneradar flightline Internalice-sheet radar layering at 60 MHz is causedby betweenRidge B (200 km from Vostok Station) and South electromagnetic-wavereflections from boundaries of Pole was examinedto identify a radar link betweenthese dielectriccontrast. Such boundaries are causedby (1) ice sites(line 121). The radar line at Ridge B is crossedby a densityvariations to an ice depthof 800 m (2) horizonsof furtherradar line (line 009) which is alignedalong the line relativelyhigh acidityformed from the aerosolproduct of of ice flow from Ridge B to Vostok Stationand, therefore, large volcanic events containedwithin ancient snow and the ice core site. We also link an unnamed ice dome, referred to here as 'Dome X', located 100 km from South Pole, to the Vostok ice core by radar data alonglines 133 Copyright2000 by the AmericanGeophysical Union. and009 (Fig. 1). Papernumber 2000GL008479. Because the radar data are in the same format, the 0094-8276/00/2000GL008479505.00 methodused to traceinternal layers in this paperis detailed

2133 2134 SIEGERT AND HODGKINS' RADAR LAYERS BETWEEN VOSTOK & SOUTH POLE

were difficult to trace across about 5% of the transect due to the apparentbreak-up of radar layeringbetween Titan Dome and South Pole Station.Radar-layer buckling in Z- scope mode is an artefact that can be resolved using migration procedures.However, in the analogue data presentedhere, migrationtechniques cannot be used.The occurrenceof radar 'buckling' may be associatedwith a changein the flow of ice, from the slow-movingice sheet interior where layers are continuous,to the faster flowing ice drainagefeatures where the layersdeform [Bell et al., 1998]. Importantly,at South Pole, there is a significant enhancedice flow systemthat is locatedwhere layershave been observed to buckle [Bamber et al., 2000]. However, the bedrock elevation between Titan Dome and South Pole, and thereforethe ice thickness,can be measuredaccurately from our radardata (Fig. 2). Internalradar reflections are not causedby a singlelayer Figure 1. Airborneradar flightlinesbetween Dome C, of ice, but rather a numberof closelyspaced layers which Vostok and South Pole. The area covered by the may contain dielectric properties different to the investigationis indicatedin the inset.Line 136 is dashed 'background'value of the surroundingice. The resolution near to Vostok Station due to a lack of 60 MHz Z-scope data[$iegert et al., 1998],and lines 121 and 133 are dashed of our resultsis limited to the 250 ns pulse lengthof the nearto SouthPole dueto the breakup of internallayers. radar equipment,which equatesto about 40 m in the ice sheet[e.g. Siegertet al., 1998]. Our resultsmust therefore in $iegert et al. [1998]. Specifically,individual layers are be viewed in the context of these limitations. traced across 2 km sections of the ice sheet through separateanalysis of A-scope(single pulse radar data) and Z-scope(time-continuous pseudo-cross sections of the ice Linking Titan Ice Dome and the Vostok ice core site sheet)(Fig. 3). Three internallayers were tracedcontinuously along the The depths of internal layers, and from these the radar flightlines.However, it shouldbe noted that layers normaliseddepths (i.e. the depth of an internal layer as a

South Pole Titan Dome .....,.• • :•...... •...... -,.,? --•,• •-•,-•-,. •: . ...._,--• ...... - • •<,---•:•- , ß ,. <;•, ---, ...... , ,•: ...... •...: :..'...-•-•-'-',,•,-•,.•:•'•-'•*-'---•"•'•'• .• _ -:...... •,- ...... •-•--• ---:-::••• ...... : .. •-•-•••••'-•,•*:• ...... -. .:-:.... -.... . • ...... ,: x ...... ••• •••'•••••••' % "•' '•-"•':t'.:..... :-::,,•:•"•,7•.•:•:•%•½'•.,•------.•:,,,'-<.•::z*•: •..:..•&•%':%•:i•?-•'•-"-•'•]•Ei'J•'is•:" •5';t;'•'%"•::•"•'"'? T::'" .... '" ' •½,:• ':"',:,.:, ,'•:2:; :':"•'••;., •:-•'•-;•: •9:s•.., 8 ...•:4•*;-½'•, '•:•,•,•.•.•:.½:•2•>•:•-:.:•s.•-,*•; ,*•'•**• '-•:---*'•--'• •' ß '- - ... -'' .. .• ;•;.':•:• , .. -...... :;'.- :.:-. - .. ,: :....-...-: - . . .: -..-;...... :...... : , ..-:, .•

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0 100 200 300 400 500 km Ridge B

500 600 700 800 900 1000 km

Figure2. 60 MHz Radardata from line 121between South Pole and Ridge B. Notehow the internal layersare traceable across the ice sheet even in thisheavily vertically-exaggerated image. SIEGERT AND HODGKINS: RADAR LAYERS BETWEEN VOSTOK & SOUTH POLE 2135

Ice depth, km

.....:'::.•::.--:-:'-'"-•:• ;...... •::•'"•7'•::':•:';E.".7.... .'t':•'..'...:: '•:":':::.7-'--' '.:...... :...... •:...... :.....,:... •--t,.x...... •,..: ...... :::•:.• / -.,..:,..•...•.:.•,G.'•..::: :,..:•.Eß.,'.." ..?•...• .... •,..• . .,...•:•,•,,....,.,..•,.•....*•:::..7..•r•,• •...... :-:, ,.,...... •:•...... ,,....•...... ,:•,•.,,..•. ,.,,½•.:,;:•,•...... ,,...... •;•.•.:.:.,:•:....,..•,•.:,. ,.•.•.:-,,..... Internal ': -"•: : •.•;.%..:;"-•:•'""./•.*,.•:::.•".:;,:.-::?```•...••;•?..•:•::.•:•:::.•.;:•.•...:•;:?•;.:`•.•:`•:•.:•:;;.?:•.•:• layering*'.:';':""':'": •"":.':.:.:';;:'"'-:.•::"?•'"••'•";?•":::::'":½ '":"'"':':;""";-: ...... :;...... •:...... •..... '"':'. ' • '"'-'••' ;'"":': .....:": ...... , ;•,,•:;::.,.:.:,--/-.•.._• • •.,,.:,..:.., .-,•...%•...,;-- ....%.;.. ;...t -,, ,-....:.-:•½:..:-.-.- ,.:.;-...... :..::::.:::,: .:.:'.::.:.::;;::.-.-::.,.::,•,-ff•-,: :,•,.:.,?• ...... ::,•..•.. ,;...... :...' .....::..,. :•:; %,'".•:•,,.-...... •::-..: ... ••••4:;'½,.'...... ';;;:•.:%,';:. ;.t-:'*:':•""•"••<'½:•:::.'.•%:,:.,.;.:"' '"j•'•;*,,:•':":'•' '••'•••"••••:7.':%::.'::;%;:?*• • ' "•'••' '•' ...... ' '•"•••••'•'-•%•"''••h '• .".' ' Bedrock:'..,",'..7...•,•½;•)•.t•,..:?•.:.•>':::;::':'•::'.:; '*.--.- ....-"½.'"' ' ....,:,..:::;%,;;:...... -:•.•,..•4•.•:•,::.:-::'--:-..',':;'.",;.•.:4•,•;::;;:.:•..: ....

•.•,...... --•;•• .•..,•.•,•%.••.:.•...... ß...... • ...... ß...... ,.•-•...... -...... •-,.,L•;• ...... •; ...... •,,., ...... 0 5 10 15 20 Distance, km

Figure 3. An exampleof 60 MHz radar,taken from flightline121 betweenRidge B and SouthPole, Continuousradar layeringis easily identifiedand, therefore,traceable across this section.Radar data on thisscale were used to traceinternal layering across the flightlinesin Fig. 1.

fraction of the total ice thickness),were measuredfrom the ice divides can be estimated from dated isochrons because radar data (Fig. 2). The depthslayers are affectedby the the effect of strain thinning via horizontalice-sheet flow flow of ice, the thickness of echo free zones (EFZs, shouldbe negligible.However, in order to date isochrons, referred to below) and the rates of ice accumulationacross they needto be linked to an ice core. the continent.The absolutedepth of internallayers can be EFZs representseveral hundredmeters of the total ice clearlyseen to' increase generally beneath progressively thickness and are thoughtby Fujita et al. [1999] to be deeperice as one movesfrom Ridge B to Titan Dome and caused by enhancedshearing of ice that destroys the Dome X (Figs 4 and 5). The relative accumulationrates at internal layering.Exact identificationof the thicknessof

(a) -.•, Line121 • J (a)

• , 50 ka 1:: ka •'0 : ' '50lOOO • 1000 85 ka 85 ka 107 ka • 107 ka 127 ka ._ • 127ka 2000 ß 2000 165 ka •o ...... 165 ka

3000 C3 3000 4000 , I , , , I , ' , (b) 4OOO (b) Titani Dome Ridge B Vostok -•DomeX RidgeB Vostok• 0 . 0 ß

0.2 ...... ';...... 0.2 ...... : : • ' : c• 0.4 -

• 0.6 0.4.....---• ...... o z 0.8 ......

1 :

1000 800 600 400 200 0 1000 800 600 400 200 0 Distance (km) Distance (km) Figure 4. (a) Internallayering and (b) normaliseddepth of Figure5. (a) Internallayering and (b) normaliseddepth of internal layers, digitised from 60 MHz radar records, internal layers, digitised from 60 MHz radar records, between Titan Dome and Vostok Station. The bold line betweenDome X andVostok Station (Fig. l). The boldline denotes the ice-sheet base. denotes the ice-sheet base. 2136 SIEGERT AND HODGKINS: RADAR LAYERS BETWEEN VOSTOK & SOUTH POLE

EFZs in our 60 MHz radardata is problematicbecause the Acknowledgments.We thankDavid Morse (Universityof appearanceof an echo-freezone could be causedby Texasat Austin) and P. BufordPrice (Universityof Berkeley, reflections below the detection limit of the radar California)for commentsto anearlier draft of themanuscript, and equipment.Thus, in our radar data, the thicknessof the two anonymousreferees for helpful reviews.The airborneradar EFZ may be an overestimate. However, the way in which datapresented in this paperwere collectedin the 1970sby a the thicknessof our EFZs relateto the subglacialrelief is consortium involving the SPRI, the US National Science very similar to that of the calibrated EFZs observed at Foundationand the TechnicalUniversity of Denmark.We thank the Directorof the SPRI, Cambridge,for accessto thesedata and Dome Fuji [Fujita et al., 1998]. In topographiclows the hissupport of ourwork. Funding for thiswork was provided by EFZ is relativelythick whereas over topographic highs the UK-NERC grantGR9/4782 to MJS. zonehas a minimumthickness. There is greaterfolding of internal layers over subglacialbedrock where EFZs are absent(e.g. at ice divides).The presenceof an EFZ causes References a relative decreasein the normaliseddepth of internal layers. At the Titan Dome ice divide there is no EFZ and Bamber,J.L., D.G. Vaughanand I. Joughin,Widespread complex the internallayers found at a normaliseddepth of between flow in the interior of the Antarctic Ice Sheet, Science, 287, 0.4 and 0.6 at Vostok Station are instead located between 1248-1250, 2000. normaliseddepths of 0.5 and0.8 (Fig. 4). Bell, R.E., D.D. Blankenship,C.A. Finn, D.L. Morse, T.A. Scambos,J.M. Brozena and S.M. Hodge, Influence of Estimatesof the depth-ageprofile at Titan subglacialgeology on the onsetof a West Antarcticice stream Dome and, by inference,around South Pole from aerogeophysicalobservations, Nature, 394, 58-62, 1998. Fujita, S., H. Maeno, S. Uratsuka,T. Furukawa,S. Mae, Y. Fujii By matchingisochrons to the establisheddepth-age and O. Watanabe, Nature of radio-echo layering in the relationshipat Vostok [Petit et al., 1999], our results Antarcticice sheetdetected by a two-frequencyexperiment, J. indicatethat the thicknessof ice youngerthan 100 ka at Geophys.Res., 104, 13,013-13,124,1999. Titan Dome is 600 m greaterthan at RidgeB, and400 m Hodgkins,R., M.J. Siegert and J.A. Dowdeswell,Geophysical morethan at Vostok(Fig. 5). Further,the existingradar investigationsof ice-sheetlayering and deformationin the link betweenVostok and Dome C ISlegertet al., 1998] DomeC regionof centralEast Antarctica, J. Glaciol.,in press, 2000. showsthat there is 300 m moreice youngerthan 100 ka at Dome C than at Vostok. Our radar link between three ice Kapitsa,A., J.K. Ridley, G. de Q. Robin, M.J. Siegert,and I. Zotikov, Large deepfreshwater lake beneaththe ice of central divides allows us to infer that the averagerate of East Antarctica, Nature, 381,684-686, 1996. accumulationover the last 100,000years at Titan Dome is Millar, D.H.M., Radio-echolayering in polar ice sheetsand past 75% greaterthan that at Ridge B, and 30% more than at volcanicactivity, Nature, 292, 441-443, 1981. Dome C. The ice accumulation rate at South Pole has Petit, J.R., J. Jouzel, D. Raynaud,N.I. Barkov, J.M. Barnola, I. recentlybeen measured to bearound 70 mmyr" (averaged Basile,M. Bender,J. Chappellaz,M. Davis, G. Delaygue,M. over the last 900 years) [van der Veenet al., 1999], whilst Delmotte,V.M. Kotlyakov,M. Legrand,V.Y. Lipenkov,C. at VostokStation the rate is only27 mmyr 4 [Kapitsaet al., Lorius, L. P6pin, C. Ritz, E. Saltzman and M. Stievenard, 1996]. Our resultssuggest, therefore, that the presentrates Climate and atmospherichistory of the past 420,000 years of ice accumulationin Antarcticamay differ substantially from the Vostok ice core, Antarctica, Nature, 399, 429-436, to thosein pre-Holocenetime. 1999. In contrast, the thickness of ice between isochronsat 100 Price, P.B., K. Woschnaggand D. Chirkin, Age vs depth of and 165 ka is 650 m at Titan Dome; 350 m less than at glacialice at SouthPole, Geophys.Res. Lett. (in press). Vostok and 550 m less than at Dome C. Therefore, the Siegert,M.J., R. Hodgkinsand J.A. Dowdeswell,A chronology for the Dome C deep ice-core site throughradio-echo layer depth-agegradient between 100 and 165 ka mustless steep correlationwith the Vostokice core,Antarctica, Geophys. Res. at Titan Dome than at Vostok or'Dome C. Given that the Lett., 25, 1019-1022, 1998. ice thickness at Titan Dome is around 2200 m, there must van der Veen, C.J., E. Mosely Thompson,A.J. Gow and B.G. be very little ice olderthan 165 ka at Titan Dome. This has Mark, Accumulationat South Pole: comparisonof two 900 implicationsfor the age of ice downstreamat the nearby yearrecords, J. Geophys.Res., 104, 31067-31,076,1999. South Pole Station ice core site [Price et al., in press]. Assuming that the normalised depths of radar layers continue to increase between Titan Dome and South Pole R. Hodgkins,Department of Geography,Royal Holloway, Universityof London,Egham, Surrey, TW20 0EX, England.(e- as they do betweenTitan Dome andRidge B (and likewise mail: [email protected]) for the SouthPole - Dome X - Ridge B data), we would M.J. Siegert, Bristol Glaciology Centre, School of expectthe depthof the 165 ka isochronto be locatedclose GeographicalSciences, University of Bristol, Bristol, BS8 1SS, to the ice base (or EFZ) at South Pole. This meansthat an England.(e-mail: m.j.siegert@bristol. ac.uk) ice core taken from the SouthPole Stationshould yield a very highresolution ice corefor the last glacial-interglacial (ReceivedJanuary 11, 2000; revisedApril 17, 2000; accepted cycle [Price et al., in press]. May 18, 2000.)