The Occurrence, Analysis, and Future Exploration of Lake Vostok and Other Antarctic Subglacial

38 Pages·2005·1.1 MB·English

by  Martin J. Siegert

18Mar2005 11:44 AR AR233-EA33-07.tex XMLPublishSM(2004/02/24) P1:JRX
10.1146/annurev.earth.33.092203.122725
Annu.Rev.EarthPlanet.Sci.2005.33:215–45
doi:10.1146/annurev.earth.33.092203.122725
Copyright(cid:1)c 2005byAnnualReviews.Allrightsreserved
FirstpublishedonlineasaReviewinAdvanceonJanuary11,2005
LAKES BENEATH THE ICE SHEET: The Occurrence,
Analysis, and Future Exploration of Lake Vostok
and Other Antarctic Subglacial Lakes
g
s.or Martin J. Siegert
w
vie BristolGlaciologyCenter,SchoolofGeographicalSciences,UniversityofBristol,
e
alr BristolBS81SS,UnitedKingdom;email:[email protected]
u
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m arjournals.nal use only. bK■eenyAeabWtshtortrhadecsticeAAsinhrtebaeortcrsntioecfag,Aesonuptbahgryclastiiccciasa.lhTeanhsveibrleoaenrgnmeuestns,etsLd,aetkoxepiVldoeornsatttoiifokyn,misomreortehathna1n02050lakkems
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aded fror pers igneolethnegrtmhaalnhde1atkinmg,daenedp.gSeunbergaltaecdiamllealktwesatoecrccuorllbeecctsauinsetotphoegicraepbhaicsehiosllkoewpst.wFaorrmlabkye
oF watertobeinequilibriumwiththeicesheet,itsroofmustslopetentimesmorethanthe
wnl05. icesheetsurface.Thisslopecausesdifferentialtemperaturesandmelting/freezingrates
Do18/ acrossthelakeceiling,whichexciteswatercirculation.Theexplorationofsubglacial
245. n 04/ lakes has two goals: to find and understand the life that may inhabit these unique
05.33:215-n Siegert o eTdnihrveeicrtteocnshtmundeoinleotssgoaicnfadslutdobegmvleaelcaoispaumlrleeankthtesesrcmelqiamuyiartteaedkrefeocsroeivrndessriatthluaymteoaecrascsuutorreihnmasepenpdtesinmm.eenatns,ohnolwakeeveflro,tohrast.
ci. 20Marti
net. SProf. 1.INTRODUCTIONANDBACKGROUND
Plaby
h. 1.1.ConditionsBeneaththeIceSheet
art
v. E TheconceptofliquidwaterbeneaththeicesheetsofAntarcticais,tothoseunfa-
Re miliarwithglacialprocesses,somewhatincongruous.Thesurfaceairtemperatures
u. incentralEastAntarcticaoftenreachbelow−60◦C,andthecoldestofficialtem-
n
An perature ever recorded on Earth, −89.2◦C (−128.6◦F), occurred at the Russian
VostokStationonJuly21,1983.Yet,alittlelessthan4kmbelowtheicesurface
atVostokStation,attheicesheetbase,ahugebodyofwaternamedLakeVostok
exists.Thislakeisthelargest(byanorderofmagnitude)ofmorethan100known
lakesthatlayundertheEastandWestAntarcticicesheets.Temperaturescanattain
themeltingvaluebeneathanicesheetbecauseofthreefactors.First,thepressure
beneathanicesheet(i.e.,theweightofice)causesareductioninthetemperatureat
whichicemelts.Beneath4kmofice,thisvalueisapproximately−3◦C.Second,
the ice sheet insulates the base from the ultra cold temperatures at the surface.
Third,heatisgeneratedattheicesheetbasefromtheEarth(geothermalheat)and
0084-6597/05/0519-0215$20.00 215
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216 SIEGERT
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245. n 04/ Figure1 Icesheetthermalconditionsandthemaintenanceofwarmsub-
215-ert o glacialconditionsinEastAntarcticaowingtogeothermalheating.
05.33:n Sieg
ci. 20Marti tFhoeriacnesicheeesthietseetl4f(kfrmicttihoinckh,etahteowheiantgrteoqtuhiereddeftoormmealttiobnaosafliciceeanisdabpapsraolxsilmidaintegl)y.
net. SProf. 50 mW m−2, which is the background geothermal value (Figure 1). Thus, sub-
Plaby glacialwater,andlakes,canoccurbeneaththecenterofalargeicesheetwithout
h. theneedforunusualgeothermalconditions.
art Water flow beneath an ice sheet is controlled by the water pressure gradient
E
v. (a combination of gravity and ice overburden). In simple terms, water may flow
e
u. R uphilliftheslopeoftheicesurfaceexceedsapproximately1/10oftheicesheet
nn base.Inothercases,subglacialwaterflowsdownhill.Theproductionandflowof
A
waterattheicesheetbedleadtoitsaccumulationwithintopographichollowsand,
hence,theformationofsubglaciallakes.
1.2.IdentificationofSubglacialLakes
LakesbeneaththeAntarcticIceSheetwerefirstreportedfromairborneradio-echo
sounding (RES) records in the late 1960s and early 1970s (Robin et al. 1970,
Oswald&Robin1973).ThetechniqueofRESworksthroughtheissuingofVHF
radio waves into the ice sheet, which reflect of boundaries of dielectric contrast
(Figure2).Suchboundariesoccurattheicesurface,withintheicesheet(internal
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ANTARCTICSUBGLACIALLAKES 217
he1970s,mitteranderaircraftpearance.
es.Inttranssmallflatap
alreviews.org ndothersubglaciallakmountedwiththeradarmostradarsurveysuseuniformlystrongand
Annu. Rev. Earth. Planet. Sci. 2005.33:215-245. Downloaded from arjournals.annuby Prof. Martin Siegert on 04/18/05. For personal use only. Thetechniqueofairborneradarsounding,anditsapplicationtoidentifyingLakeVostokaradarsurveyswereundertakenwithaC130Herculestransporteraircraft,withthewingsAircraftnavigationwasaccuratetoapproximately5kminthecenterofAntarctica.Today,tonavigate.Subglaciallakesareeasilyidentifiedonairborneradarrecordsowingtotheirperturbationsarerecordedashyperbolaeinradardata.
e2neer.PSck
rrvGo
FiguairboreceiandBedr
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218 SIEGERT
layering),andfromtheicesheetbase.AirborneRESat60MHzisoftenusedto
penetratetothebaseoficemorethan4kmthickinAntarctica(e.g.,Robinetal.
1977, Drewry 1983, Morse et al. 2002, Studinger et al. 2003a). This is possible
because ice is relatively transparent to radio waves at this frequency (Johari &
Charette1975),especiallywhenitisseveraltensofdegreesbelowfreezing,asis
thecaseformostoftheAntarcticIceSheet.Thestrengthofreflectionfromthebed
dependstoafirstorderonthedifferencebetweenthedielectricpropertiesoftheice
(dielectricconstantε = 3.2)andthedielectricpropertiesofthesubicematerial.
Asthedielectricconstantofwater(ε = 81)isverydifferentfromtypicalbedrock
org (ε = 4to9),amuchstrongerreflectionisobtainedfromanice-waterinterface
s.
w comparedwithanice-rockinterface.Thisdifferenceisincreasedbytherelatively
e
vi rough character of an ice-bedrock interface, which scatters energy and further
e
alr reducesechostrength.ThismakesRESanidealtechniqueforidentifyingwater
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n bodiesbeneathicesheets.Subglaciallakesareidentifiedon60MHzRESrecords
a
om arjournals.onal use only. sbf1rty0ore–mtn2hg0etthhpderBaelisocseentnrgocsnhethgeoeeeftrtrtbtahhaceasknef,o,ialnwldodjhwiacicciaenhtnigvtaecpihcopaefer-aaabrncetdbienrrroitigescthrkiftcarscoeen(flFtehfiicgaltumitoriensrsev3;ce)(o:rbyr()dass)emcsaohtnroodotenhsagoroenrfettflcyhoepecnistcsicatoaalnlnlyest
oaded frFor pers omfatxhiemRuEmSswloapveesletynpgitcha;lalyndap(cp)roaxvimeraytefllayt−an1d0vtiirmtueaslltyhehosurirzfoacnetasllcohpaer.acter,with
wnl05. More than 100 lakes have now been identified beneath several regions of the
Do18/ Antarctic Ice Sheet (Siegert et al. 1996, 2005; Tabacco et al. 2003; Popov &
245. n 04/ Masolov2004).LakesrangeinsizefromlessthanakilometerinlengthtoLake
05.33:215-n Siegert o V(Koastpoiktsa(theetable.s1t9k9n6o)w(nFiogfutrhee4l,akseees)c,owlohricihnsiesrtth).elargestbyanorderofmagnitude
ci. 20Marti
net. SProf. Figure3 60MHzRESdatafor11Antarcticsubglaciallakesandtheirsurroundi→ng
Plaby icesheetbedtopography.(a)TwolakesintheDomeCarea(#33and#34).Themeanice
arth. thicknessabovetheselakesis∼4000m.(b)LakeintheRidgeBarea(#46).Themean
v. E icethicknessabovethelakeis∼3700m.(c)LakeatTitanDomeneartheSouthPole
Re (#52).Themeanicethicknessabovethelakeis∼3070m.(d)LakeintheWhitmore
nu. Mountains area (#68). The mean ice thickness above the lake is ∼2900 m. (e) Two
n
A lakesintheHerculesDomearea(#72and#73).Themeanicethicknessabovethese
lakesis3200m(#72,right-handsideoftheimage)and2800m(#73,left-handside
oftheimage).(f)LakeVostok,EastofRidgeB.Themeanthicknessatthispartof
thelakeis∼4000m.(g)LakeatthemouthoftheAstrolabeSubglacialBasin(#30).
Themeanicethicknessabovethislakeis4000m.(h)LakeattheheadoftheByrd
Glacier(#61).Themeanicethicknessabovethislakeis2580m.(i)Lakebeneaththe
centeroftheWestAntarcticIceSheet(#67).Themeanicethicknessabovethislake
is3200m.(a)–(f)areadaptedfromDowdeswell&Siegert(2002),(g)–(i)arefrom
Siegert(2002).Lakesarelocatedbyawhiteletter“L”ineachRESimage.Numbers
ofsubglaciallakesareasdefinedinSiegertetal.(1996).
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ANTARCTICSUBGLACIALLAKES 219
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220 SIEGERT
1.3.ScientificInterestinSubglacialLakes
There has been a huge degree of scientific and media interest in Lake Vostok
(and subglacial lakes in general) following the discovery that the water depth
of the lake was several hundred meters (Kapitsa et al. 1996). Discussion about
whethertomakeinsitumeasurementsofthelakehasbeendrivenbytwoscientific
hypotheses.Thefirstisthatuniquemicroorganismsinhabitthelake.Thesecond
isthatacompleterecordoficesheethistoryisavailablefromthesedimentsthat
lie across the lake floor. Future exploration of subglacial lakes will be focused
g on testing these hypotheses. If the hypotheses are correct, future investigations
or
s. of subglacial lakes could enable valuable insights into the history of Antarctica,
w
vie detailingitsresponsetoandcontrolonclimatechangeandourunderstandingof
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alr biologicalfunctioningwithinextremeenvironments.
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wnl05. estandtheicesheetiswarm-basedacrosslargeregions.Thesecondiscloserto
Do18/ theicesheetmarginbeneathwarmed-based,enhancediceflowunits.Dowdeswell
245. n 04/ &Siegert(2002)identified10areasofAntarcticawheresubglaciallake-typere-
05.33:215-n Siegert o fl(Dienoccmtlouerd,siSnoogcucLtuhar:kPeDoVloemo,sHetoeCkrc)(,uwDlheosemrDeeomAmoer(,ewtthhheearnWe4oh0nitllmyakotwerseohMlaavokeuesnbtehaeainnvsei,dbteheneetniTfifreoadun)ns;adRn)i,tdaTgricettaiBnc
ci. 20Marti Matothuentmainosut(hneoafrBthyerdAGstlraoclaiebre),SOuabtgeslaLciaanldB,aansidnG).eMorogreeVreLcaenndtly(,inTcalubdaicncgoaeltaakle.
net. SProf. (2003)haveidentifiedafurther14lakesacrosstheDomeCregion,andPopov&
Plaby Masolov(2004)havefoundevidenceof16lakesaroundtheDomeAandDome
h. Fregions(Figure4).
art
E
v. 2.2.LakeSurfaceAreas
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n Approximately75%oflakeshaveobservedlengthsoflessthan5km.OnlyLake
n
A
Vostokislongerthan30km.LakeVostokismorethan250kmlongandasmuch
as 80 km wide, making it unique in terms of surface area (Tabacco et al. 2002,
Studinger et al. 2003a). Around Dome C, Siegert & Ridley (1998) found that a
numberoflakesthoughttoberelativelysmalllaybeneathanoticeablylargeflat
icesurface.AsisthecaseforLakeVostok,Siegert&Ridley(1998)hypothesized
that the spatial extent of these flat surface features mark the actual extent of the
lakesbeneath,whichmakesthesizeofthelakesmuchgreaterthanhadbeenfirst
thought.NewRESdatafromDomeCconfirmsthatrelativelylargelakes(threeof
whichare∼1000km3byarea)existbetweenDomeCandLakeVostok(Tabacco
etal.2002,Tikkuetal.2002).
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ANTARCTICSUBGLACIALLAKES 221
2.3.LakeDepthsandWaterVolumes
Only Lake Vostok has been sounded by seismic methods, which reveal a depth
ofbetween510m(Kapitsaetal.1996)and∼1000m(Siegertetal.2001).Water
depths for a number of other lakes have been inferred, however, from measure-
mentsofthesurroundingbedrocktopography.Thesemeasurementssuggestthat
thedepthsofmanylakesarebetweenapproximately50and250m(Dowdeswell
&Siegert1999).Indeed,severallakesmaybemuchgreaterthanthis,judgingby
theside-wallslopes.AccordingtoDowdeswell&Siegert(1999),morethan50%
g ofthelakesarelikelytocontainlessthan5km3ofwater,andonly10%storemore
or
s. thanapproximately100km3.LakeVostok,bycontrast,isthoughttoholdapprox-
w
vie imately5000km3 ofwater(Studingeretal.2004).Thus,LakeVostokisunique
e
alr intermsofitsvolume,butthismayreflectthefactthatitislargebysurfacearea,
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n ratherthanunusuallydeepcomparedwithothersubglaciallakesinAntarctica.
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245. n 04/ aapbpovroex,itmheatselloyp5e0omf.thFeoricteh-ewlaakteertoinbteerifnacheydmruossttabtiecaepqpuriolixbirmiuamtewlyitthenthteimiceess,haenedt
215-ert o inoppositedirectionto,theicesurfaceslope(Figure5).Thus,theicethicknessof
ci. 2005.33:Martin Sieg tTlhahkeeensor.erltahteironnsehnidpobfeLtwakeeenVoicsteokanisdalpapkreoxsuimrfaatceelys5lo0p0ems igsretrauteerftohranalilnstuhbegsloauctihal.
net. SProf.
Plaby 2.5.TypesofSubglacialLakes
h.
art Dowdeswell&Siegert(2002)characterizedsubglaciallakesintothreemaintypes
E
v. asfollows:(a)lakesinsubglacialbasinsintheice-sheetinterior,(b)lakesperched
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R ontheflanksofsubglacialmountains,and(c)lakesclosetotheonsetofenhanced
u.
n iceflow.
n
A
The majority of Antarctic subglacial lakes are located within 200 km of ice
dividesintheinterioroftheicesheet(Figure4).Thebedrocktopographyofthe
ice sheet interior involves large subglacial basins separated by mountain ranges
(Drewry1983).Thelakesinthiscategoryarethosefoundin,andonthemargins
of,subglacialbasins.Theselakescanbedividedintotwosubgroups.First,there
arethoselocatedwheresubglacialtopographyisrelativelysubdued,oftentoward
thecenterofsubglacialbasins(e.g.,Figures3a,i).Secondly,somelakesoccurin
significanttopographicdepressions,oftenclosertosubglacialbasinmargins,but
stillneartheslow-flowingcenteroftheAntarcticIceSheet.Wherebedtopography
isverysubdued,deepsubglaciallakesareunlikelytodevelop.
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222 SIEGERT
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ANTARCTICSUBGLACIALLAKES 223
Perchedsubglaciallakesarefoundmainlyintheinterioroftheicesheet,onthe
flanksofsubglacialmountainranges(Figure3d).Inseveralcases,small(<10km
long) subglacial lakes have been observed perched on the stoss face of large
(>300mhigh),steep(gradient>0.1)subglacialhills.
At least 16 subglacial lakes occur at locations close to the onset of enhanced
ice flow hundreds of kilometers from the ice sheet crest (Siegert & Bamber
2000). An example is provided by three subglacial lakes near the onset of fast
flow into Byrd Glacier (e.g., Figure 3h). Byrd Glacier is fast flowing and drains
a very large interior ice sheet drainage basin into the Ross Ice Shelf (Drewry
g
or 1983). These subglacial lakes are similar in size and depth to the small and
s.
w probablyshallowlakesfoundinpartsofmajorsubglacialbasinsintheicesheet
e
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a 2.6.TectonicSettingofLakeVostok
m arjournals.nal use only. ILgneaokrpeechVeynostisctyaoelka.sruSsr,tvuaedycinoognfecrethretetedalal.ekfe(f2oa0rn0td3haai)tsssbuleomecnmaleamr(iazFdeiegduttorheem6raaeps,utshleteessfcriozolemoraaninndseeexxrttte)e.nnsTtihvoeef
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wnl05. A major new finding of this survey was that the western half of the lake has
Do18/ distinct gravity and magnetic anomaly compared to the eastern half. The border
245. n 04/ betweenthetwoanomaliesisclearandlinearalongtheeasternmarginofthelake
215-ert o (Figure6b).
05.33:n Sieg ←
ci. 20Marti Figure5 ThedimensionsandtopographicsettingofLakeVostok.(a)ERS-1altime-
net. SProf. tVroysotofkthceanAbnetairdcetinctiIficeedSfhroemettbheetwaneoenmaRliodugseflBataincdesDuorfmaceeCre.gTiohne.lSocPaRtiIo(nScooftLtPakoe-
h. Plaby larResearchInstitute,UniversityofCambridge)radarflightlinesandthelocationof
art all known subglacial lakes around Lake Vostok (denoted as black squares) are pro-
E
v. vided.Thesurfaceicesheetelevation,derivedfromtheERS-1altimeter,isalsoshown.
e
R Thecontourintervalis10m.Arrowsdenotethedirectionofsurfaceflowoficeover
u.
n LakeVostokcalculatedfromInSAR(InterferometricSyntheticApertureRadar)(Kwok
n
A
etal.2000).Itmustbenotedthatrecentanalysisofflowstructureswithintheicesheet
suggeststheInSARdatamaybeinaccurateacrossthesouthofLakeVostok(Tikku
etal.2004).(b)Cross-sectionfromnorthtosouthalongthe200kmlengthofthelake.
(c)Cross-sectionfromWesttoEastalongthe50kmwidthofthelake.Thedepthof
LakeVostokcanbeestimatedby(1)seismicinformation,whichhasrevealedawater
depthof>500mbeneathVostokStation;(2)side-wallbedrockgradientadjacentto
thelakeof0.1,whichindicatesseveralhundredmetersofwaterdepthinthecenterof
thelake;(3)radio-wavereflectionsfromthelakefloor,showingthewaterdepthtobe
between10and20minthenorthofthelake;and(4)bedrockislandsmeasuredby
radar.
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224 SIEGERT
Studinger et al. (2003b) analyzed aerogeophysical and seismological data to
establisha“conceptualtectonicmodel”forLakeVostokanditslocale.Theycon-
cludedthatthetectonicframeworkaroundLakeVostokinvolvesacrustalboundary
(thesourceofthelinearmagneticanomaly).Thecauseofthisboundaryislikely
tobeassociatedwiththeemplacementofathrustsheetontoapreviouslypassive
continentalmargin.TheageofthethrustinghasbeenestimatedasProterozoic(i.e.,
Precambrian,inexcessof600millionyears).Subsequentnormalreactivationof
thethrustsheetthenmayhavecreatedthetroughinwhichLakeVostokislocated
(Studinger et al. 2003b). What remains unknown about Lake Vostok’s trough is
g
or theextenttowhichsubglacialerosionduringtheonsetofglaciationcontributed
s.
w toitsdevelopment.
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a 3.ORIGINANDAGEOFLAKEVOSTOK
m arjournals.nal use only. ManuocrhdearttoefntmioangnhiatsudbeeelnarggievretnhatonLanaykeotVhoesrtosukbagslaacpiaolslsaikbele,LhaabkietaVtofsotrolkifhea.sBbeeineng
oaded froFor perso vetieawl.e2d0b0y3)m. anyastheultimatelong-termtargetforexploratoryresearch(Priscu
wnl05.
Do18/ 3.1.OriginandAgeoftheLake
245. n 04/ TheageandoriginofLakeVostokwillbecriticaltothebiotawithinthelake,and
215-ert o totheageandqualityofthegeologicalrecordsonthelakefloor.Onepublished
05.33:n Sieg tthheisotrhyecooryn,ctehrenilnakgethiseaosrsiugmineodftoLahkaveeVeoxsitsotkediswuinthliikneiltyst(rDouugxhbuprryioerttaolg.l2a0c0ia1t)i.oInn,
ci. 20Marti andremainintactastheicesheetgrewacrossthelaketoitscurrentrelativelystable
net. SProf. cboyngfirgouurnadtieodni.cIendfuarcitn,gthiceeresghieoentbthuailtdiuspn,oewveLnaikfethVeotsrotoukghwaansdplraokbeabwlyeroecpcruepseiendt
h. Plaby prior to glaciation. This is because the margin of the ice sheet would have been
art far closer to the position of the lake during the early stages of ice growth (e.g.,
E
v. DeConto&Pollard2003).Thesurfaceslopesoftheicesheetovertheregionof
e
R Lake Vostok would, therefore, have been significantly greater than those at the
u.
n center of today’s ice sheet. In this situation, water would have been driven out
n
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ofthetroughtotheicesheetmargin.AprobableanalogytoLakeVostokduring
icesheetbuildupistheAstrolabeSubglacialTroughinWilkesLand,whichholds
thethickesticeinAntarctica(4776m).Thistroughhasasmallsubglaciallakeat
itsmouth,whichindicatesthatthewholetroughissubjecttosubglacialmelting,
and that water is driven out of the deepest parts of the trough. As the Astrolabe
SubglacialTroughisunabletoholdalargelakeowingtotheiceoverburden,Lake
Vostok would not have been resident in its trough during the early stages of ice
sheetgrowthinAntarctica.Thus,thelakeismostlikelytopostdatetheformation
of the current ice sheet. The exact age of the East Antarctic Ice Sheet has been
stronglydebatedoverthepastfewdecades.Somebelievethatithasremainedin