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Continental carbonate facies of a Neoproterozoic panglaciation, 1
NE Svalbard 2
IANJ.FAIRCHILD*,EDWARDJ.FLEMING*†‡,HUIMINGBAO§,DOUGLASI.BENN†¶,IAN3
BOOMER*,YURIV.DUBLYANSKY�,GALENP.HALVERSON◊,MICHAELJ.HAMBREY⌂,CHRIS4
HENDY☺,EMILYA.MCMILLAN*,CHRISTOPHSPÖTL�,CARLT.E.STEVENSON*ANDPETERM.5
WYNN●6
7
*SchoolofGeography,EarthandEnvironmentalSciences,UniversityofBirminghamB152TT,8
UK.(E-mail:[email protected])9
†DepartmentofGeology,TheUniversityCentreinSvalbard(UNIS),N-9171Longyearbyen,10
Norway11
‡Presentaddress:CASP,WestBuilding,181AHuntingdonRoad,Cambridge,CB30DH,UK12
§DepartmentofGeologyandGeophysics,E235Howe-RussellComplex,LouisianaState13
University,BatonRouge,LA70803,USA14
¶SchoolofGeographyandGeosciences,UniversityofStAndrews,StAndrewsKY168YA,15
Scotland,UK16
◊DepartmentofEarth&PlanetarySciences/Geotop,3450UniversitySt.Montreal,QuebecH3A17
0E8,Canada18
⌂DepartmentofGeographyandEarthSciences,AberystwythUniversity,Aberystwyth,19
CeredigionSY233DB,UK20
☺DepartmentofChemistry,UniversityofWaikato,PrivateBag3105,Hamilton3240,New21
Zealand.22
�InstitutfürGeologie,Leopold-Franzens-UniversitätInnsbruck,Innrain52,23
A-6020Innsbruck,Austria24
2
●LancasterEnvironmentCentre,UniversityofLancaster,LancasterLA14YQ,UK25
RevisionsubmittedtoSedimentologyAugust31st2015 26
3
ABSTRACT27
TheMarinoanpanglaciation(?650-635Ma)isrepresentedinNESvalbardbytheWilsonbreen28
Formationwhichcontainssyn-glacialcarbonatesintheupper100mofthe130-175m-thick29
formation.ThesesedimentsarenowknowntohavebeendepositedunderaCO2-rich30
atmosphere,lateintheglaciation,andglobalclimatemodelsfacilitatetestingofproposed31
analogues.Precipitatedcarbonatesoccurinfourofsevenfaciesassociations:FluvialChannel32
(includingstromatoliticandintraclasticlimestonesinephemeralstreamdeposits);Dolomitic33
Floodplain(dolomite-cementedsandandsiltstonesandmicrobialdolomites);CalcareousLake34
Margin(intraclasticdolomiteandwave-rippledoraeoliansiliciclasticfacies)andCalcareousLake35
(slump-foldedandlocallyre-sedimentedrhythmic/stromatoliticlimestonesanddolomites36
associatedwithice-raftedsediment).Thereisnostrongcyclicityandmodernanaloguessuggest37
thatsuddenchangesinlakelevelmaybecharacteristic.38
Bothcalciteanddolomiteinstromatolitesandrhythmitesdisplayeitherprimaryorearly39
diageneticreplacivegrowth.Oxygenisotopevalues(-12to+15‰VPDB)broadlycovarywithδ13C.40
Highδ13Cvaluesof+3.5to+4.5‰correspondtoequilibrationwithanatmospheredominated41
byvolcanicallydegassedCO2withδ13Cof-6to-7‰.Limestoneshaveconsistentlynegative42
δ18Ovalues,whilst,rhythmicandplayadolomitespreserveintermediatecompositions,and43
dolocretespossessslightlynegativetostronglypositiveδ18Osignatures,reflectingsignificant44
evaporationunderhyperaridconditions.Meltwatercompositionsinferredas-8to-15.5‰45
couldreflectsmallerRayleighfractionationrelatedtomorelimitedcoolingthaninmodernpolar46
regions.Acommonpseudomorphmorphologyisinterpretedasareplacementofikaite47
(CaCO3.H2O),whichmayalsohavebeentheprecursorforwidespreadreplacivecalcitemosaics.48
Localdolomitizationoflacustrinefaciesisinterpretedtoreflectmicroenvironmentswith49
fluctuatingredoxconditions.Althoughdifferingin(palaeo)latitude,tectonicsetting,and50
4
carbonateabundance,theWilsonbreencarbonatesprovideauniquepre-Cenozoicanaloguefor51
theMcMurdoDryValleysofAntarctica.52
53
Keywords:Cryogenian,oxygenisotopes,carbonisotopes,lacustrine,ikaitepseudomorphs,54
SnowballEarth 55
5
INTRODUCTION56
57
ThesecondoftwoNeoproterozoicpanglaciations,inwhichicesheetsreachedsealevelinthe58
tropics,terminated635Myagoatthebaseofatransgressivecapcarbonatedefiningthe59
Cryogenian-EdiacaranSystemboundary(Table1).DepositsofCryogenianiceagesarepreserved60
onmostcontinentsandareofteninterpretedasglacimarine(Arnaudetal.,2011).However,in61
NESvalbard,theMarinoan-aged130-175mthickWilsonbreenFormation(Halverson,2011)62
uniquelycontainsnon-marinecarbonatesaswellassubglacialtillites(Fig.1).Theseevince63
hyperaridterrestrialenvironments(Fairchildetal.,1989)andanatmosphererichincarbon64
dioxideduringglaciation(Baoetal.,2009),thelatterconclusionfulfillingapredictionofthe65
SnowballEarthhypothesis(Kirschvink,1992;Hoffmanetal.,1998).BecauseWilsonbreen66
Formationoutcropsarerestrictedtoremoteicefieldnunataks(Fig.2),theyhaverarelybeen67
visited,andhenceourknowledgeofthesedimentaryarchitecturehasbeenincomplete.The68
WilsonbreenFormationcarbonatescontainprobablythehighestcarbonateandsulphateδ18O69
valuesandlowestsulphateΔ17Osignaturessofardiscoveredinthegeologicalrecord,features70
whichevokeoneofthemostextremeclimaticeventsinEarthhistory(Baoetal.,2009,Bennet71
al.,2015).Herewecharacterizearangeofnon-marineenvironmentsinwhichthecarbonates72
wereprecipitatedusingacombinationoffield,petrographicandstableisotopeevidence,and73
scrutinizeaclaim(Fairchildetal.,1989)thatthesedepositsareananalogueoftheextreme74
terrestrialenvironmentsofthemodernMcMurdoDryValleyregionofAntarctica,albeitformed75
atmuchlowerpalaeolatitudes.76
ThestudyareaintheSvalbardmainlandofSpitsbergen(Figs.1,2)andthebasin77
continuationtotheNEhavelongbeenrecognizedasclassicareasforlatePrecambrian78
glaciation(Kulling1934).ThefirstdetaileddescriptionoftheWilsonbreenFormationwasby79
6
Wilson&Harland(1964),althoughcarbonateswerediscussedonlyintermsofitsbounding80
dolomitesasstratigraphicmarkers.Alatersedimentologicalsynthesis(Hambrey,1982;Fairchild81
&Hambrey,1984)showedthatevidenceofglacialactivitywasconfinedtotwoglacialunitsin82
thePolarisbreenGroup:theWilsonbreenFormation,andanewlydiscovered,thin,olderunitin83
theElbobreenFormation(PetrovbreenMemberalsoknownasE2)(Table1).Severaldistinctive84
formsofcarbonateinassociationwiththeglacialdeposits.Thefirstoftheseisdolomiticglacial85
rockflour,demonstratedinultra-thinsections(Fairchild,1983).Subsequently,stableisotope86
studiesdemonstratedthepresenceofglacimarineprecipitatesinthePetrovbreenMemberand87
glacilacustrinedepositsintheWilsonbreenFormation(Fairchild&Hambrey,1984;Fairchild&88
Spiro,1987;Fairchildetal.,1989).Itwasalsoshownthatthesecarbonatescontrastwitha89
distinctivemarinetransgressive‘capcarbonate’(followingWilliams,1979)overthe90
WilsonbreenFormation.Halversonetal.(2004)providedamuchmoredetailed91
chemostratigraphicframeworkforthePolarisbreenGroupandpostulatedthatbothdiamictite92
unitsbelongedtothesame,Marinoanglaciation.However,newchemostratigraphicdataledto93
areversiontotheprevioustwo-foldglaciationinterpretationofolderliterature(Halverson,94
2006;Halversonetal.,2007).NodirectgeochronologicalconstraintsexistonSvalbard,butthe95
low87Sr/86SrvaluesatthebaseofthePolarisbreenGroupcorrelatewellwiththoseassociated96
withthefirstevidenceofNeoproterozoicglaciationelsewhere(Halversonetal.,2010).Also,the97
majormarinetransgressionsuccessionabovetheWilsonbreenFormationcloselyresemblesthe98
basalEdiacaranfacieselsewheredatedat635Ma(Rooneyetal.,2015).Palaeomagnetic99
constraintssuggestthatNESvalbardlayinthesubtropicsthroughouttheCryogenian(Lietal.,100
2013)aspartoftheequatoriallycentred,fragmentingRodiniasupercontinent.101
ThecloselysimilarstratigraphyincentralEGreenland(nowofficiallyredesignatedas102
NortheastGreenland)(Hambrey&Spencer,1987;Moncrieff&Hambrey,1990)indicatesthatit103
7
representsasouthwesternbasincontinuation(Hambrey,1983;Knolletal.,1986;Fairchild&104
Hambrey,1995),subsequentlyoffsettothesouthbyleft-lateralstrike-slipfaulting(Harland,105
1997).TheNeoproterozoicsuccessioninwesternSvalbardisquitedifferentandwasprobably106
notdepositedinthesamebasin(Harlandetal.,1993).107
108
Cryogenianeventsandpanglaciation109
110
TheconceptofNeoproterozoiclow-latitudeglaciationwaschampionedbyHarland(1964)who111
arguedfromthewidespreadoccurrenceoftillitesthattheyweregloballydistributedandhence112
shouldbeusedincorrelation.Subsequentlyittookanenormouseffortfromthegeological113
communitytoestablishthatthediamictiteunitsarepredominantlyglacigenicoratleast114
glaciallyinfluenced,thatconvincingevidenceoflow-latitudeglaciationexists,andthatthe115
correlationsaresupportedbyageochronologicalframework(Fairchild&Kennedy,2007).116
Despitethecomplexityofindividualsuccessions,summarizedinArnaudetal.(2011),itisnow117
widelyrecognizedthattwopanglacialeventsoccurredduringtheCryogenianperiod(c.720–635118
Ma),referredtoastheSturtian(orearlyCryogenian)andMarinoan(orlateCryogenian)119
glaciations(Halversonetal.,2007;Macdonaldetal.,2010;Hoffmanetal.,2012;Calveretal.,120
2013;Rooneyetal.2014,2015;Lanetal.,2014).Thetraditionalgeologicalapproachrecognized121
theroleoflowatmosphericCO2levelsintriggeringglaciation,buttherewasalackof122
understandingofwhole-Earthbehaviouruntilinsightsfromclimatemodellingofapossible123
future“nuclearwinter”(e.g.Budyko,1969)ledtotherealizationthatafrozen,highalbedo124
planetrepresentsastableclimaticstate.Kirschvink(1992),incoiningthetermSnowballEarth,125
suggestedthatlowpalaeolatitudesofcontinentsinNeoproterozoictimescouldhavehadarole126
intriggeringpanglaciation,andthattoescapetheSnowballstateabuild-upofvolcanically127
8
derivedCO2wasneeded(Caldeira&Kasting,1992)toovercomethedominanceofthealbedoof128
Earth’sicysurfaceonitsenergybalanceandtriggerdeglaciation.Hoffmanetal.(1998)and129
Hoffman&Schrag(2002)subsequentlyexpandedupontheSnowballmodel,incorporating130
insightsfromplanetarymodellingtoenlightennewstratigraphic,sedimentologicaland131
geochemicaldata.SnowballEarthisbestregardedasatheory,notmerelyahypothesis,because132
itisbuiltaroundaseriesofpropositionswhichare,toagreaterorlesserextent,interacting133
(Fairchild&Kennedy,2007).Thetheoryhasstimulatedahugeaccelerationindata-gathering134
andmodellingwhichhassignificantlyclarifiedtheextenttowhichagivenphenomenonis135
essentialtoorcharacteristicofaSnowballEarth.Initially(Hoffmanetal.,1998),therewasa136
focusontheassociatednegativecarbonisotopeanomaly,apparentlyencompassingtheglacial137
period,whichwasregardedasacharacteristicofloworganicproductivityinanoceanwhich138
becameisolatedfromtheatmosphereoncetheSnowballstatehadbeenestablished.139
Additionally,thepost-glacialcapcarbonatesandtheassociatedtransientsinthenegative140
carbonisotopeanomalieswereregardedasapositivetestofthehypothesis,reflectingrapid141
meltdownandsea-levelriseunderapost-Snowball,ultra-greenhouseenvironment.However,142
afterfurtherscrutinyallofthesepropositionshaveemergedasflawedorambiguous(Kennedy143
etal.,2001a.b;Hoffman&Schrag,2002;Halversonetal.,2002;Schragetal.,2002;Trindadeet144
al.,2003;Hoffmanetal,2007;LeHiretal.,2008,2009;Kennedy&Christie-Blick,2011;Hoffman145
etal.,2012).146
IntheearlyyearsoftheSnowballhypothesis,missingwasanaccountofhowtheglacial147
formationsthemselvescouldbeusedtoprovidesupportforthetheory.Inthe“hard”Snowball,148
theconceptisofauniversallythickicecoverovertheoceans,composedofanupperzoneof149
glaciericeandlowerzoneoffrozenseawater(Pierrehumbert2005;Pierrehumbertetal.,2011).150
Ifso,sealevelshouldbegreatlyloweredandmarineicemarginsshouldoccuratmuchgreater151
9
depthsthantheshallow-waterpre-glacialsediments.Evidencetosupportthishypothesiswas152
foundinNamibia.Onplatformtops,glacigenicdepositsarerare,whereasonplatformmargins153
tidewater-glaciergrounding-linephenomenacanbedemonstrated,inferredtobesome154
hundredsofmetrestopographicallylower(Hoffman,2011;Domack&Hoffman,2013).Onthe155
otherhand,mostglacialsedimentologistswerehostiletoSnowballtheory,insistingthatthereis156
evidenceofrepeatedadvancesandretreatsoficeinmarineenvironmentsduringglaciation,157
andalsothatlocallywave-andstorm-generatedstructuresarepresent,indicatingopenwater158
(Xiaoetal.,2004;Etienneetal.,2007;Allen&Etienne,2008;LeHeronetal.,2011,2013).159
Apparentsupportfrommodelsshowingequatorialopenwater(e.g.Hydeetal.,2000)facedthe160
problemthatthesimulatedclimatesolutionswerenotstable.Hence,theresponseofHoffman161
(2011)wasthat“counterarguments[totheSnowballmodel]basedontemperate-typeglacial162
sedimentologyfailtograspthatthepreservedglacialsedimentaryrecordreflectstheendofthe163
SnowballEarth,whenmeltingwasboundtoemergetriumphant”.Afurthertwisthoweverwas164
thatasimulationofalong-livedmarineice-freeequatorialfringewasachievedbyAbbotetal165
(2011),inwhattheytermtheJormangundstate.Hoffmanetal.(2012)notedthatwhereasthe166
Jormangundstatepreservedthepatternofmodernlow-latitudeclimatebelts,withamoister167
equatorialregion,theSnowballclimaticpattern(Pierrehumbertetal.,2011)wouldresultin168
higherprecipitationminusevaporationinthesubtropicsandanextremelyaridequatorialzone.169
Thisdrawsattentiontotheneedforstudyofcontinentalglacialdepositssuchasthe170
WilsonbreenFormationwheremoredirectevidenceofclimaticconditionscanbeobtained.171
ThesurvivingessentialpredictionsofSnowballEarththeorycanbesummarizedas172
follows:1)glaciationsmustoccursynchronouslyglobally,2)theymustbelong-lasting(>1Ma)173
toallow3)thebuildupofatmosphericCO2tohighlevelswhen4)sedimentationoccursina174
briefperiodpriortotermination.Althoughearliergeochronologicalcompilationshadlegitimate175
10
doubtsabout1)and2)(AllenandEtienne,2008),theyarenowmorefirmlyestablished(Rooney176
etal.,2015),althoughthedurationoftheMarinoan(?5-15My)isimpreciselyknown.The177
WilsonbreenFormationhasnowpermittedpositivetestsof3)and4).178
ComparedwithCenozoicstrata,thereareveryfewapproachestodeterminationof179
atmosphericCO2concentrationsintheNeoproterozoic.Aboldnewapproachwasappliedby180
Baoetal.(2008)basedonprocessesoccurringduringstratosphericozoneformationwhich181
resultsinanenrichmentintheisotope17Oinozoneandcarbondioxideanddepletioninoxygen182
(O2).Thisisanonmass-dependenteffectwhichdoesnotinfluence18Oabundances.The17O183
signalcanbepreservedintheoxygenatomsofsulphateinrocksifatmosphericoxygenisused184
tooxidizesulphidesonthelandsurface.Sulphatehasthepeculiarpropertyofnotexchanging185
oxygenatomswithotherspeciesover1000Mytimescalesatsurfaceconditions,providedthere186
isnomicrobialredoxcyclingofsulphur-bearingions.Baoetal.(2008)testedtheideathatat187
timeperiodswhenatmosphericPCO2wasenhanced,therewouldbe17O-depletedsulphatein188
thegeologicalrecord,andindeedfoundthemostsignificantanomalythathadatthattimebeen189
discovered,recordedinbaritecrystalfansoccurringinthecarbonatesuccessionoverlying190
MarinoanglacialdiamictitesinSouthChina.191
Baoetal.(2009)thenfocusedonlacustrinecarbonatesofthecentralpart(memberW2)192
oftheWilsonbreenFormationandfoundmoreprofound17O-deficienciesincarbonate-193
associatedsulphate(CAS)inlimestones,consistentwithveryhighatmosphericPCO2during194
glaciation.Morerecentstudiesinothergeographicregions,coupledwithprocessmodelling195
approaches,havesupportedthisapproachinMarinoancapcarbonates(Baoetal.,2012;Cao&196
Bao,2013;Killingsworthetal.,2013;Bao,2014),buttheWilsonbreenFormationremainsthe197
onlyunitwherePCO2canbeestimatedduringglaciation.198
11
Subsequently,Bennetal.(2015)haveusedthesameisotopesystematicsofCASona199
muchlargerdatasetoflimestonesfrommembersW2andW3toarguethatsimilarhighPCO2200
values(estimatedatbetween1and10%atmosphericCO2)occurredthroughoutthedeposition201
oftheWilsonbreenFormation.SinceitwouldhavetakenalongtimetoaccumulateCO2,the202
inferencewasthatthebulkoftheWilsonbreenFormationwasdepositedinarelativelyshort203
periodneartheendoftheglaciation.Inturn,thisimpliesanextendedhiatusearlyinthe204
glaciationwhichwasidentifiedwithapermafrostedhorizonatthebaseoftheWilsonbreen205
Formation.206
CoupledicesheetandatmosphericgeneralcirculationmodelresultsinBennetal.(2015)207
usingSnowballEarthboundaryconditionsdemonstratethatat2%atmosphericPCO2thick208
glaciersexistonthecontinentsalongwithextensiveareasofbaregroundandthathyperaridity209
iswidespread.Precessionalforcinggeneratesmovementsoficemarginsbyatleasthundredsof210
kilometresandwaslinkedtothepresenceofdistincticeadvanceswithintheWilsonbreen211
Formation(Fig.1).AlthoughconclusionsbasedontheMarinoanglaciationshouldnot212
necessarilyapplytothemuchlongerSturtianglaciation,thenewresultsprovideapossible213
routetoreconciletheopposedpositionsstatedinAllen&Etienne(2008),oftemperateglacial214
conditionsduringpanglaciation,andHoffman(2011),ofsedimentdepositionoccurringrapidly215
duringmeltdown.Itisthepurposeofthecurrentpapertoprovideadetailedsedimentological216
analysistounderpinthisnewsynthesisandinparticulartodemonstratetheplausibilitythatthe217
Wilsonbreencarbonatesweredepositedwithinacoherentgeomorphic-climaticsystem.Asa218
firststep,weneedtoexaminethepossiblemodernanalogue.219
220221
TheAntarcticMcMurdoDryValleysasanaloguesfortheWilsonbreenFormationcarbonates222
223
12
Fairchildetal.(1989)previouslyusedtheDryValleysregionasanexemplarforthecarbonates224
inmemberW2oftheWilsonbreenFormation.Conversely,leadingworkersonthemodern225
environment(Lyonsetal.,2001)commentedthatthisfrigid,drymodernenvironmentmightbe226
avaluableanalogueforProterozoicSnowballEarthenvironments.TheDryValleyshavealso227
beenusedasanaloguesfortheMartiansurface(Marchant&Head,2007;Dicksonetal.,2013).228
Thelocal“alpine”glaciersoftheDryValleysarecold-based,butoutletglacierssuchasthe229
TaylorGlacierarewarm-based.Giventhecriteriaforrecognitionofglacialthermalregime230
presentedbyHambrey&Glasser(2012),warm-basedanaloguesareneededtounderstand231
muchoftheglacigenicfaciesoftheWilsonbreenFormation(Flemingetal.,2016).Nevertheless,232
forthecarbonatefacies,thecontextoftheDryValleyregion,uniquelyforpresent-day233
environments,providesparallelsforthefollowingevidenceprovidedbyFairchildetal.(1989):234
(1)arecordofextremeevaporationwithpotentiallyupto20‰differenceinδ18Obetween235
inputwatersandthoseresponsiblefortheprecipitationofthecarbonateswithheaviestδ18O236
signaturesand(b)alternatingdepositionofglacialsedimentandcontinentaldepositsindicating237
iceadvanceandretreat,specificallyincludingrhythmicandmicrobial,presumedlacustrine238
carbonatesandevidenceofformerevaporites.239
Fig.3illustratesthecontextofthemodernsettingwhichhasbeenextensivelystudied240
overseveraldecadesbyAntarcticgroupsledfromNewZealand,JapanandtheUSA,withTaylor241
valleybeingthefocussince1993oftheMcMurdoDryValleysLong-termEcosystemResearch242
(LTER)ProgramoftheNationalScienceFoundation.TheDryValleysisa4800km2region243
occupyingpartoftheTransantarcticMountainsbetweentheEastAntarcticIceSheet(EAIS)and244
theRossSea.TheindividualvalleysthemselvesmostlytrendE-Wandareupto80kmlongand245
upto15kmwideandareinternallydrained,oftenwithseveraldiscretelakebasinsineach246
valley.TwooutletglaciersfromtheEAIS(WrightUpperandTaylorglaciers)onlyjustcrossthe247
13
regionalbedrockaltitudedivideintotheeastward-drainingcatchments,whilsttheFerrarglacier248
flowsallthewaytothecoast(Fig.3).Localvalleyglaciersextendfromthemountainsintodry249
valleys,e.g.CanadaGlacierinTaylorValley(insetinFig.3)andephemeralstreamsdevelopfor250
shortperiodsinsummer.Avarietyoflakesoccur,includingthoseoccupiedbyicefrozentothe251
bedintherelativelyhigh-altitudeVictoriaValley,highlysalinelakeswithnoicecover(e.g.Don252
JuanPond,UpperWrightValley)andfinallylakeswitha3-5micecover(e.g.Lakes253
BrownsworthandVandainWrightValleyandLakesBonney,HoareandFryxellinTaylorValley),254
whichmeltonlyinamarginalmoatinsummer(Green&Lyons,2009;Dicksonetal.,2013).255
Microbe-dominatedbiotasflourishwhereverandwheneverthereisliquidwaterintheregion,256
includingmatsonstreamandlakebeds,andphotosynthesizingalgaeareimportantinlakes257
duringthespringseason(Fountainetal.,1999).Thesematstakeupnutrientsrapidlyfrom258
streamwaterandthebiogeochemicalpatternsarestronglyinfluencedbyexchangeof259
hyporheicwaterswithstreamwater(McKnightetal.,1999).260
Thereisaspatialmicroclimaticzonationcomprisingacoastalzonewithjust-thawedsoils261
insummerandastablecolduplandzonewithparticularlylowrelativehumidity(Marchantand262
Head,2007;Marchantetal.,2013).Theinterveningvalleyshaveameanannualtemperatureof263
-16to-21°Candthemaximumdailytemperatureisbelowzeroonaveragethroughouttheyear264
(Fountainetal.,1999).Thevalleysreceivelessthan10mmwater-equivalentofprecipitation265
peryear,almostalwaysassnow.Becauseoftheprevailinglowrelativehumidities(e.g.between266
50and60%inTaylorValley,Fountainetal.,1999),ablation(mostlyassublimation)greatly267
exceedsprecipitation.Wilson(1981)describedtheconsequencesofthisgeographic268
configurationandclimatologyusingphysicalandchemicalprinciples.Precipitationriseswith269
altitude,butfallsinlandfurtherfromtheRossSea.Thesnowlinemarkstheboundarywhere270
ablationexceedsprecipitationanditrisesinlandasprecipitationdeclines.Wilson(1981)271
14
attemptedtoexplainthedistributionofsaltsbasedonanunderstandingofdownslope-272
increasingaridity,butitseemsthatvariablemeteorologyconfoundsthepredictionsindetail.273
Nevertheless,itisthecasethatdeliquescentsaltsflowdownhillinthesub-soilabovea274
permanentlyfrozenlayer.Lakeswithalidoficedisplayabalancebetweenablationatthe275
surfaceandfreeze-onoflakewaterbeneaththelid,replenishedseasonallybystreaminflow.276
Oncesnowhasbeenremovedbyablation,sunlightcanpenetratethroughverticalicecrystals277
andsignificantsolarheatingoflakescanoccur.Thepresentconfigurationofsaltsallows278
deductionsofbothlong-andshort-termhistory.Specifically,spatialvariabilityinsaltsrequiresa279
long-term(>105-106years)stabilityoftheice-freesubaerialvalleysides.Ontheotherhand,280
somelakeshaveabasalbrinelayerwhichdiffusionmodellingshowsoriginatedfromaperiod281
around1200yearsagowhensomelakeswereice-freeshallowbrines,beforebeingre-filledby282
freshmeltwater.283
OneaspectnottreatedbyWilson(1981)istheeffectofwind.Animportant284
reinforcementmechanismforaridityisprovidedbytheregionaldevelopmentofkatabatic285
windsflowingofftheEastAntarcticIceSheet.Asthisairwarmsadiabatically,humidity286
decreases,particularlyinwinter(Nylenetal.,2004).Itisnowclearthattheepisodicallystrong287
summerwindsareactuallywarmfoehnwinds,whicharisefromstrongpressuregradients288
duringtheoccurrenceofcyclonesovertheRossSea,theincidenceofwhichdependson289
hemisphericclimaticanomalies.Thesetopographicallyenhancedandchannelledwindsflowat290
typically>5ms-1westerlyalongtheDryValleysandcauseverylargeintra-annualandinter-291
annualincreasesinmeltwaterproductionandstreamflow(Doranetal.,2008;Speirsetal.,292
2013).Thehighincidenceofthesewindsinsummer2001/2,whenpositivedegreedays293
increasedbyanorderofmagnitude,ledtorisesinlakelevelof0.5-1m,effectivelywipingout294
theprevious14yearsofloweringoflakelevelinaperiodofthreemonths(Doranetal.,2008).295
15
Thesedetailseffectivelydemonstratethesensitivityoftheenvironmenttoclimaticchangesthat296
willstronglyinfluencethefaciesdeposited.297
Barrett(2013)recentlyreviewedthecontroversiallong-termhistoryoftheMcMurdo298
region.EvidenceforlandscapeevolutionbasedonthepositionofMiocenevolcanicashdeposits299
clearlydemonstratesthataftertheestablishmentofthecurrentlargeEastAntarcticIceSheetin300
theMiocene,onlysurficiallandscapemodificationhasoccurred(Sugdenetal.,1995;Lewiset301
al.,2007).Importantly,thecold,dryclimatesoftheDryValleysremainedstable,evenduring302
significantwarmingeventsrecordedintheRossSeaduringthePliocene.Thislong-termclimatic303
stasiscanbecomparedwiththepredictedlong-termhydrologicalinactivityanticipatedona304
SnowballEarthascarbondioxidelevelsslowlyrose.305
LargelakesformedduringtheLastGlacialMaximuminallthemajorDryValleys.This306
requiredtwoconditions:(1)expansionoftheRossSeaIcesheettoblockthemarinemarginsof307
theDryValleys(Hendy,1980;Halletal.,2013)and(2)increasedmeltwaterproductioninthe308
valley(bywind-inducedmelting,Doranetal.,2008),despitesignificantlycolderconditions.309
Conversely,datingofaragoniticlacustrinedepositsshowsthat,ininterglacialperiods,therewas310
anexpansionofTaylorGlacier,notedforcharacteristicallylightoxygenisotopevalues,in311
additiontoexpansionoflocalvalley-sideglaciers,withheavierisotopevalues(Hendy1979,312
1980).ThesephenomenapresumablyreflecthighersnowaccumulationontheEAIS,enhanced313
meltwaterproduction,andpartialblockageofthevalleybyice.Importantly,theseinferences314
drawattentiontothepotentialanti-phasingofglobaltemperatureandlocalglacieradvanceand315
thegreaterimportanceofregionalhumiditycontrolsonMilankovitchtimescales.316
Inthecurrentpaper,awealthofnewdataontheWilsonbreenFormationcarbonates317
arepresentedwhichallowstheanalogiespreviouslymadetobetestedandevaluatedinmuch318
moredepth.Theinterpretationofthesedataisassistedbymodernanaloguesandnew319
16
computersimulationstudiesofNeoproterozoicclimates(Pierrehumbertetal.,2011,Bennetal.,320
2015).321
322
METHODS323
324
Fieldwork(2010-11)wassupportedbyhelicopterandbyskidoo.Sections,includingsixentirely325
newlocations,weremeasuredby30mtape,orientatedbycompassandAbneylevel,andlinked326
tobeddingdips,withtotalthicknesscheckedbyGPSwithuncertaintyofaround5%.Thebest327
availablesectionsineachregionwerelogged(Figs.1,2),althoughtheyvaryinqualityfrom328
almostperfect,tointermittentgoodoutcropseparatedbyground-levelfrost-shatteredregolith.329
Thecarbonaterocksarechemicallyfresh,butlaboratorystudyofcutorsectionedsampleswas330
necessarytoidentifyfaciesinmanycases.Over350samplesweresawn,210ofwhichwere331
thin-sectionedofwhich60werestainedwithAlizarinRed-Sandpotassiumferricyanideand332
over30polishedsectionsstudiedbycold-cathodecathodoluminescence(CL)at15kV.333
Carbonandoxygenstableisotopedataarepresentedhereasδ13Candδ18Oinpartsper334
thousandwithrespecttotheVPDBstandard.Differencesbetweenlaboratoriesareinsignificant335
inrelationtothewiderangeofisotopevalues(29‰inδ18Oand8‰inδ13C)inthisstudy.336
SupplementarydatainBaoetal.(2009)includedmethodsandalldatacollectedtothatdate.337
NewdatawasobtainedattheUniversityofBirminghamusingacontinuous-flowIsoprimeIRMS,338
withamultiflowpreparationsystem.Samplesofbetween80-250µgpowderedcarbonatewere339
reactedwithphosphoricacidat90°Cfor90minutes.ResultswerecalibratedusingIAEA340
standardsNBS-18andNBS-19.Afluidinclusionstudyisreportedinthesupplementary341
information.SulphateoxygenandsulphurisotopesarepresentedinBennetal.(2015).Wedraw342
onpreviouslypresentedtraceelementdatafromthecarbonatefractionsolubleindilutenitric343
17
acid(Fairchild&Spiro,1987;Fairchildetal.,1989;Baoetal.,2009),whilstnewtraceelement344
andotherisotopeanalyseswillbepresentedelsewhere.345
Infigurecaptions,thelocationandstratigraphicpositionaregiveninastandardformat.346
ForexampleW2,Dracoisen,70m,referstoasamplefrommemberW2fromtheDracoisen347
section,70mabovethebaseoftheformation(orthebaseofthesectionwherethebaseisnot348
seen).Thecontextandoxygenisotopecompositionofthecarbonatefromthathorizoncanbe349
foundinthestratigraphicsectiondiagrams:Fig.4forDracoisenandsupportingFigs.1-6forthe350
othersections.CarbonatesinmembersW1andW3aresummarizedonlyinTable2,butfull351
samplelogsaregiveninBennetal.(2015).352
353
WILSONBREENFORMATIONARCHITECTURE,COMPOSITIONANDPOST-DEPOSITIONAL354
HISTORY355
356
TheWilsonbreenFormation,dominatedlithologicallybysandydiamictites,containsawealthof357
evidenceindicatingthatitislargelyglacigenic,anddepositedinbothaqueousandsubglacial358
settings(Hambrey1982;Fairchild&Hambrey,1984;Dowdeswelletal.,1985;Harlandetal.,359
1993).Newwork(Bennetal.,2015;Flemingetal.,2016),includingfivepreviouslyundescribed360
sections,hasresultedinacoherentstratigraphic-faciesreconstruction(Fig.1).Thispaper361
focusesonbeds(sandstones,rhythmites,mudrocks)containingprecipitatedcarbonate;the362
presenceofsuchstratawasoriginallyusedtodefinememberW2(Hambrey,1982).Inthe363
northernmostsection(Dracoisen),W2isreadilydistinguishedbythreesuchcarbonate-bearing364
bedsseparatedbydiamictites(Fig.2A),andoverallasimilarpatternappliesinothersections365
(Fig.1).HoweverthinrhythmiteunitsalsooccurinmemberW1,onecontainingprecipitated366
carbonate,andmostofthethinnon-diamictite(sandstoneandrhythmites)bedsinmemberW3367
18
containsuchcarbonatefacies(Table2). AlthoughglacigenicrockscontinueNNEtothecoast368
andtoNordaustlandet,thesewerenotincludedinourstudysincediamictitesarethinnerand369
lesswellexposed,andprecipitatedcarbonatesareabsent(Halversonetal.,2004;Hoffmanet370
al.,2012).NeitherdosuchcarbonatesoccurintheequivalentStoreelvFormationofNE371
Greenland,representingtheSWcontinuationofthebasin(Hambrey&Spencer,1987;Moncrieff372
&Hambrey,1990;Hoffmanetal.2012,Fleming,2014).373
374
Terrigenousdetritus375
376
Harlandetal.(1993)summarizedinformationfromfivesites,determiningthatcarbonateclasts377
makeup40-85%,igneousandmetamorphicclasts5-33%andsandstonesandquartzites10-20%378
ofstones(i.e.gravel-sizeddebris);thebasementlithologiesareprimarilygranitoidsand379
gneisses,withsomebasaltinwhichferromagnesianmineralsarecommonlychloritized.Inthe380
widercontextofterranesaffectedbytheCaledonianorogen,detritalzirconstudiesindicate381
provenancefromArchaeanandPalaeoproterozoicrocksofthecratonicinteriorand382
MesoproterozoicdetritusderivedfromtheerodedremnantsoftheGrenville-Sveconorwegian383
orogen(Cawoodetal.,2007).Asystematicstudyofcarbonateclastcompositionswillbe384
presentedelsewhere,butinsummary80%ofpebblesaredolomiterockand20%arelimestone;385
manyofthesecarbonatesresembleunitsfromtheunderlyingsuccession(Table1)inlithology386
andisotopecomposition;nostratigraphictrendsinclasttypeorisotopecompositionwere387
found.Thesandfractionisdominatedbyquartzandfeldsparwithsubordinatedolomiteand388
limestoneandthemudfractionlikewisebyquartzo-feldspathicdebrisanddolomite(Fig.5).The389
dominanceofsiltinthemudfractionisshowningradedrhythmites(Fig.5B,C);notablydetrital390
calciteoccursinthemudfractiononlylocallyandtheproportionofclaymineralsislikewisevery391
19
low.ThedolomitematrixofPolarisbreenGroupdiamictitesispredominantlytheresultofglacial392
transportandcomminutionasshownbythepresenceofsub-micronrelicrockflour(Fairchild,393
1983)inwhichclaymineralsarevirtuallyabsent(Fig.5D).Thisdetritalmatrixcontainsa394
complexmixtureofbothbrightanddullyluminescentreddolomitewhenviewedinCL(Fig.5C).395
Themeancarbonandoxygenisotopecompositionsofdolomiticmatrixindiamictitesand396
wackes(n=43)are+2.4±1.3and-3.5±1.8‰respectively,slightlylowervaluesthanfor397
dolomitepebbles.Thismatrixcompositionformsausefulreferencepointforcomparisonwith398
precipitatedcarbonatesintheWilsonbreenFormation.399
400
Burialdiageneticmodification401
402
TheWilsonbreenFormationisoverlainbyamarinetransgressivesuccession,butmost403
Ediacaranstrata(Table1)arenon-marineplayalakefacies,inturnoverlainby950mofCambro-404
Ordovicianplatformcarbonates(Harland,1997)indicatingaminimumburialof1.5km.Late405
SilurianCaledonianfoldingandlocalthrustingfollowed(Fig.2),butsmall-scalefoldingis406
generallyabsent,asispenetrativedeformation.TheWilsonbreenFormationliesoutsidethe407
thermalaureolesofDevoniangraniteplutonsinthefoldbelt(Harland,1997).Poresinthecap408
carbonatearefilledwithbitumen,indicatingthatthesedimentspassedthroughtheoilwindow,409
butgoodpreservationisindicatedbytheabilitytoremoveindividualstriatedclastsfrom410
diamictitematrix(Hambrey,1982)andsomeunusuallyhighoxygenisotopecompositionsof411
dolomites(Fairchildetal.,1989;Baoetal.,2009).Afluidinclusionstudy(seesupporting412
information)indicatesthatinitialporefluidswereadvectivelyreplacedbyatypicalmeteoric413
fluidwithδ2Hintherange-60to-100‰andthat18Oisslightlyenrichedbyexchangewiththe414
solidphase,butfluidinclusionvolumesareordersofmagnitudetoosmalltohaveaffectedthe415
20
bulksolidcomposition.Physicalcompactioneffectsarenotnoticeablebecauseofthelowclay416
contentofthesedimentsandearlycementationofthecarbonates,butverylocallylamina417
boundariesarestyolitic.Uncementedsandstonesexhibitstraighttoconcavo-convexboundaries418
betweenquartzgrains.419
ManyWilsonbreenFormationoutcropsarereddened,andeveninunreddenedfacies,420
organiccarboncontentsarelow(belowdetectionlevelsof0.2%onthreeW2carbonates).421
Fairchild&Hambrey(1984)regardedthehaematitepigmentaspost-depositionalbasedonthe422
occurrenceofLiesegangbandstructuresandongoingpalaeomagneticstudiesmaythrow423
furtherlightonitstiming.Reddeningislesspervasiveincleansandstones,implyingthatit424
requiresasourceofironinthefinefractionorincarbonates.Thelowpreservationoforganic425
carbonmayreflectlowabundanceofclay(Kennedyetal.,2006)aswellasalow-productivity426
continentalsetting,contrastingwiththedark-colouredglacimarinedepositsoftheearly427
Cryogenianglacialdepositsinthestudyarea(MemberE2,Table1).428
Limitedquartzovergrowthsarecommoninsandstonesandonsandgrainsfloatingin429
dolomites,whilstsomesandstonesdisplaypartialpoikilotopiccalcitecement.Ferroansaddle430
dolomiteandferroancalciteoccludelargerprimarypores,indolomitesandlimestones431
respectively.Thesecarbonatephasesalsooccur,locallytogetherwithquartzandwhitemicain432
crystalpseudomorphs.Saddledolomiteaverages-12.2±1.5‰inδ18Oand-1.8±1.5‰inδ13C433
andcalcitesparlikewise-10.5±1.6‰and+2.0±1.5‰(Baoetal.,2009).434
435
FACIESANALYSIS436
437
FaciesAssociations438
439
21
FollowingBennetal.(2015)sevenfaciesassociations(FAs)arerecognized;Table3presents440
summarydescriptionsandinterpretations,whilstFig.7Aillustratesinferredenvironments441
diagrammatically.Theglacialandperiglacialfaciesassociations(FA1,6,7)arediscussedin442
Hambrey(1982),Fairchild&Hambrey(1984),Bennetal.(2015)andFlemingetal.(2016),and443
onlyafewsalientpointsarementionedhere.Twodistinctglacieradvancesfromthesouthare444
recognized,justbelowandabovememberW2,byoccurrencesofFA1:sheareddiamictite,445
sandstonesandgravels,locallyrestingonhighlydeformedrhythmites(glacitectonites)or446
striatedclastpavements(Fig.1,labelledFA1subglacial).ThebulkoftheWilsonbreenFormation447
ishowevercomposedofweaklystratifieddiamictites(FA6)withdecimetretometre-scale448
lensesofrhythmites,commonlywithprecipitatedcarbonateinmembersW2andW3in449
particular.Localpresenceofdropstonesorisolatedgravelclasts(lonestones),tillpellets,450
stratification,andabsenceofsubglacialshearingphenomena,pointtoasubaqueousice-rafted451
origin.Locallensingconglomeratesareinterpretedassediment-gravityflowswhich,at452
Dracoisen,compriseaconspicuouslow-anglecross-stratifiedunit(Fig.1,labelledFA6,453
proximal),interpretedasagrounding-linefan(Bennetal.,2015;Flemingetal.,2016). 454
GravelwithventifactsoverlyingshattereddolomiterepresentsthePeriglacialFacies455
Association(FA7)atthebaseoftheWilsonbreenFormationandBennetal.(2015)interpretthis456
asamulti-millionyearcontinentalhiatus.FA7alsooccursatthebaseofW2whereaperiglacial457
exposurehorizonwithsandstonewedgespenetratingsubglacialtillisfound(Fig.1)atSouth458
Ormen(ORM),SouthKlofjellet(KLO)andDitlovtoppen(DIT)andalsoattheGolitsynfjellet(GOL)459
section(Fairchildetal.,1989),whichisbetweenMcDonaldryggen(McD)andthe460
Backlundtoppen-KvitfjellaRidge(BAC).Theperiglacialhorizonisoverlainbysandstones461
correlatedwiththoseatthebaseofW2atDracoisen(Dracoisen).462
22
TheotherfourFaciesAssociationscontainprecipitatedcarbonateandaredescribedin463
detailbelow,complementingthetabularanddiagrammaticsummaries(Table3;Fig.6).Eachof464
theFaciesAssociationsoccursasunits0.1to4minthicknessaspresentedinthestratigraphic465
logs(Fig.4,supportinginformationFigs.S1-S6).466
467
FA2(Facies2Sand2T)468
469
Description470
Thisfaciesassociationisrepresentedbyerosionally-based,fining-upwardstabularsandstone471
beds,0.5-4minthickness.Facies2Ttypicallycomprisesmoderatelysortedveryfinetomedium-472
grainedsandstonewithabasalerosionalsurfacecappedbyagravellag.Internalcross-473
stratificationistabular,typicallywithlow-angleaccretionsurfacesandsetthicknessof0.5-1m.474
SuchfaciesareseenatthebaseoftheW2sectionatDitlovtoppen(Supp.Fig.1),SouthOrmen475
(Supp.Fig.2),EastAndromedafjellet(Supp.Fig.3),andReinsryggen(Supp.Fig.4),andarealso476
prominentnearthetopofthememberatDracoisen(Fig.8H)andatDitlovtoppen,wherethin477
siltstonebedsdefinelow-angleaccretionsurfaces(Fig.8G).478
Facies2SisdistinguishedfromFacies2Tbythepresenceoflimestonelaminaeand479
intraclasts,andreducedsiltcontent.Thebest-exposedexampleisa4m-thickunitthatforms480
thebasalpartofmemberW2atDracoisen(Figs.2A,4),restingonmassivediamictitewith481
decimetre-scaleerosionalrelief.Athinbasalpebbleconglomeratepassesupinto0.7mof482
pebblysandstone,whilstthemain,centralpartofthebedisveryfine-tomedium-grained483
sandstonewithlocaltabularcross-stratification,withsetthicknessupto15cmandcurrent484
ripples(Fig.8D).Atbothlevels,thereareabundant(ca.25-50%)layersoflimestone,with485
individuallaminaetypically1-3mmthick(Fig.8A).Universalfeaturesareadomedgrowth486
23
morphology(Fig.8A)anddifferentiatedmicrostructures(Fig.8B)withwelldevelopedslightly487
irregularlaminaeofmicrite,microspar,anddetritus-richcarbonatewithregularmm-scale488
fenestrae(Fairchild,1991;Riding,2000).Verylocallyathrough-goingverticalstructure489
reminiscentofcyanobacterialoralgalfilamentmoulds(Fairchildetal.,1991)isobserved(Fig.490
9B).Alltheselimestonesarelaterallydiscontinuous,breakingdowntotrainsofintraclaststhat491
arelocallystackedathighangles.Inplaces,extensivecrusts,upto0.5-6mmthick,ofradiaxial492
calcitecementdevelop,alsobrokentoformintraclasts(Fig.8C).493
UnderCL,thecalcitefabricsthatarebrokenintointraclastsfluoresceuniformly494
brightly,whereaslaterveincementsshowmorevariableCL(Fig.9A).Chemically,thisis495
reflectedinhighMncontentsofseveralthousandppmandexceptionallyhighMn/Feof1(Bao496
etal.,2009).Strontiumcontentsare150-300ppm,whilstMgis4000-6000ppm(Baoetal.,497
2009),equivalenttoaround2mole%MgCO3.Theradiaxialfabricsappearpristineand498
microdolomiteinclusionsareabsent.Sulphatecontentishigh(2000-5000ppm,Baoetal.,499
2009),whilstthepreservationofanegativeΔ17Oanomalydemonstratesthatthesulphatehas500
notundergonereduction(Baoetal.,2009;Bennetal.,2015).Thestableisotopecompositionof501
stromatoliticlimestonesdefinesacoherentfield(Fig.7A)withδ18Orangingfrom-10.5to-3.4502
‰andδ13Cfrom+0.9to+4.6‰,weightedtowardshighervalues,withoverallisotopic503
covariation(Fig.7B).Amicromilltraversethroughsyndepositionalcalciterevealslaminato504
laminavariationsinδ18Oof2‰andinδ13Cof0.5‰withoutstrongcovariation(Fig.8E,F).The505
clearpetrographicdistinctionbetweensyn-depositionalandlatercalcitesparcementis506
reflectedalsointhelowδ18Osignatureofthesparof-10to-12‰(Fig.8F).507
508
Interpretation509
24
FA2isnotableforadominanceoftractionalsedimenttransport.Theconsistentpresenceof510
laterallyextensivebasalerosionsurfacesimplyachannelcontextforthisfaciesassociation,511
whilstforFacies2Tthelow-angleaccretionsurfaceswiththinsiltbedsisreminiscentofpoint-512
bardepositswhich,althoughnotunexpectedinPrecambriansediments,wouldrequiremore513
extensiveexposurestoconfirm(Davies&Gibling,2010).Thepresenceofhigh-anglecross-514
stratification,goodsorting,anddisruptedintraclastsinFacies2Sarecharacteristicsfoundin515
eithertidalsandflatsorinlowsinuosityfluvialchannels.Thelimestonescontaintwofeatures516
typicalofNeoproterozoicmicrobialdeposits:macroscopicallydomedlaminaeanddifferentiated517
microstructuresformedbyperiodicvariationsinphenomenasuchasedimenttrapping,gas518
generationandcarbonateprecipitation(Fairchild,1991;Riding,2000)andsocanbereferredto519
asstromatolitic.Limestoneswerelithifiedbeforeerosionsincecementcrustsarebrokenand520
derivedintraclastsaredispersedandstacked,forexamplealongforesets,asexpectedfor521
significanttractionalflowsinshallowwater.Thefactthatallofthethinlimestonesdisappear522
laterallytobereplacedbytrainsofintraclasts,orelsearecompletelyerodedindicateshighly523
variableflowconditions.Further,thelocallyhighlyregularmicrobiallamination,includingclastic524
layers,pointstoaperiodiccontrolonflowrates.525
Whentakeninisolation,amarineoriginforfacies2Scouldbepostulatedonageneral526
similaritywithtidalsandflatdepositsandtheoccurrenceofwell-preservedradiaxialcements527
withrelativelyhighδ18Ocompositions(Fairchild&Spiro,1987).However,tidalsandflatdeposits528
inNeoproterozoicsuccessionstypicallyshowwell-developedherringbonecross-stratification529
(Fairchild,1980;FairchildandHerrington,1989).Thereisaclearcontrastwiththemoreregular530
macrostructuresofmarinestromatoliteselsewhereinthebasin(Fairchild&Herrington,1989;531
Knoll&Swett,1990;Halversonetal.,2004)andNeoproterozoicdepositsmoregenerally532
(Grotzinger&Knoll,1999).Thismaybeattributabletoamorehostileenvironment,withhighly533
25
variableratesofsedimentation.ThemostsignificantpointmaybethatNeoproterozoicmarine534
peritidaldepositsareinvariablydolomitized(KnollandSwett,1990),probablyafeatureofhigh535
Mg/Cainseawater(HoodandWallace,2012).536
Thealternativeisafreshwater,fluvialcontext.Wenowknowthatradiaxialfabricsare537
notdiagnosticofmarinewaters,butalsooccurinspeleothems(Neuser&Richter,2007)and538
alsothatsuchfabricsareprimaryandconsistentwiththerelativelowSrcontentofthecalcite539
(Fairchild&Baker,2012).Hence,weinterprettheradiaxialcalciteasthepristineoriginallow-540
Mgcalcitephase.Micriteandmicrosparfabricshavesimilarchemistriesandarealsoconsidered541
toreflectdepositionalconditions.TheimplicationisthattheMncontentofFA2calcitesisalso542
primaryandreflectiveoflowcontemporaryatmosphericPO2(cf.Hood&Wallace,2014),but543
notanoxicconditionselsesulphatereductionwouldhaveoccurredandthedistinctnegative544
Δ17Oanomalywouldhavebeenerased(Baoetal.,2009).Reassuringly,theMgcompositionof545
facies2SlimestoneissimilartomodernspeleothemdepositsinacoolScottishcavedepositing546
fromwaterswithMg/Cacontrolledbydolomitedissolution(Fairchildetal.,2001).Thelamina547
thicknessofthestromatolitesisverysimilartothoseofmodernfluvialmicrobialtufaswhichare548
complexdepositscontainingbothbiologicallymediatedandinorganicprecipitatesjustlike549
Facies2S(AndrewsandBrasier,2005;Andrews,2006).Interestinglythelaminationisannual550
andreflectsastrongannualvariationindischargeandFacies2Spossessesphysical551
sedimentologicalcharacteristicsconsistentwiththoseofephemeralstreams.Inthis552
interpretation,thestableisotopecompositions,whicharesimilarforradiaxialandmicrosparry553
calcites,canalsobeinterpretedasprimary,inwhichcasetheisotopiccovariation(Fig.7B)554
wouldhavetobeinterpretedasanevolutionarytrendtowardsmoreevaporatedequilibrated555
solutions(Talbot,1990),ratherthanthelightervaluesreflectinghigher-temperature556
recrystallization.ModernAntarcticstreamslackthecalcitemineralization,butmicrobialmats557
26
arewell-developedandareadaptedtoephemeralflowconditions,readilyreactivatingeven558
afterbeingdryformanyyears(McKnightetal.,2007).Thefluvialinterpretationwillbe559
developedlaterinthelightoftherelationshipofFA2tootherfaciesassociations.560
561
FaciesAssocation3(Facies3Dand3S)562
563
Description564
Facies3Dismarkedbydiscretezonesofpronounceddolomitecementationwithinasandstone565
orsiltstone;insomecasesthesedimentispervasivelycemented.Wheredolomiteismost566
abundant,detritusfloatsinadisplacivemassofdolomitecrystals(Fig.10B,D,E,F),butother567
dolomite-cementedsiltysandsarestillclast-supported(Fig.10C).Locally,highlydistinct568
dolomite-cementednodulesarevisible(Fig.10A)orastructurelessdolomitebedcanbe569
encounteredwithalowcontentoffloatingsiltandsand.Themostcharacteristicstructuresare570
mm-scalenodulardolomicritestructureswithinmassivedolomite-cementedlayersand571
associatedwithcalcite-filledfractures.Thesephenomenaarefoundatonehorizoninmember572
W3(Fig.10D),aswellasinseverallocationsinW2(e.g.Fig.10F).Ararerphenomenonisthe573
presenceofequantcentimetre-scalecauliflower-likepseudomorphs,filledbyferroansaddle574
dolomitecement(Fig.11A)occurringatthetopofaconglomerate-basedfining-upwardscycle575
(Fig.8G).Sincesaddledolomiteisaburialphase(RadkeandMathis,1980),theimplicationis576
thatthepseudomorphswereoccupiedwithsolublecrystalsthatdissolvedduringburialpriorto577
cementation.578
Facies3Sreferstodolomiticlaminites,withbroadcm-scaledomedmacrostructure,with579
anaspectratiooftypically10:1.Theyarefounduniquelyinacomplexbed,inassociationwith580
Facies3D,andoverlyingfacies2S(at70m,Dracoisen,Fig.4).Ithasbeenstudiedonthe581
27
“Multikolorfjellet”cliffsandthe“Tophatten”nunatak1kmtothenorth.Thebedisarounda582
metreinthickness,butwithvariabilityinitsinternalstructure.Mostcommonlythereisminor583
erosionofunderlyingdiamictiteatthebaseofthebed,overlainbycrudelylaminatedveryfine-584
tomedium-grainedgreensandstone,locallywithmm-scalelimestonelayersthatarepartly585
disruptedintointraclasts(Facies2S).Inplacesthiscanbeseentopassupwardsintointensively586
dolomite-cementedsandinwhichtherockfabricappearstohaveexpanded,associatedwith587
corrosionofquartzdetritusbydolomiteandtheformationofcavities,linedwithdolomicrospar,588
andoccludedbycalcite.Atthe“Tophatten”locality,achaoticbrecciaunitafewdmthickis589
locallyfoundatthebaseofthebedinsteadofsandstone.Everywhere,thetopofthebedis590
markedby10-20cmofdololaminiteswithacomplexmicrostructure,whichalternateonacm-591
scalewithdisplacivelycementedsands(Fig.10E).Thelaminaecanbecomposedofdolomicrite592
ordolomicrosparandcontaincommonfenestrae(Fig.9E,H),whilstsurfaceexposurereveala593
finelytexturedmicrotopography(Fig.10G).Locally,slightlylowerinthebed,limestone594
laminites(FA5)forma10cmhorizonoverlyinga20-30cmchaoticcarbonatebreccia(Fig.10I)595
andgraduallybecomedisrupteddownwards.596
DolomitefromFA3ischaracteristicallybrightunderCL(Fig.11B-D).Infacies3S,597
dolomicriteclotsareuniformlybright,whilstadjacentdolomicrospardisplaysdullergrowth598
fillingsmallfenestraewhilstlargerfenestraearefilledbydolosparwithmorevariableproperties599
(Fig.11B).Manganese(3000-4000ppm),Fe(10000-15000),Na(2000)andSr(250-350)ppm600
valuesareallunusuallyhigh(Baoetal.,1989)andourunpublishedelectronmicrosopeimages601
andmicroanalysesshowenrichmentsalsoinmanytransitionmetalsandrareearthsanda602
consistentchemicalzonationwithincrystalsofdolomicritemosaics.Infacies3D,adifferencein603
meanCLbrightness,andhencetimingofgrowth,cansometimesbeobservedbetweennodules604
andsurroundingmatrix(Fig.11D),whilstlocallyzonationwithinindividualcrystalsgrowing605
28
betweensiliciclasticsandgrainscanbeobserved(Fig.11C).Sulphateconcentrationsarehigh606
(4000ppm);thereisnoΔ17Oanomalybuthighδ18Oinsulphate(Baoetal.,2009;Bennetal.,607
2015).608
Carbonandoxygenisotopevaluesarecorrelated(Fig.7),butFig.12illustratesthatthe609
twoanalysesfrommemberW3lieabout1‰higherinδ13Cthanexpectedfromthistrend.610
Facies3Dhasarangeofδ18Ofrom-1.9to+11.4‰,butthemeanvalueisbiasedupwardsby611
multipleanalysesfromasamplewhichpassesupintofacies3Swhichtendstohavehigher612
values(Fig.12).Thelatterfaciesisnotableforpossessingpossiblytheheaviestoxygenisotope613
valuesofcarbonaterockssofarrecordedinthegeologicalrecord(Baoetal.,2009),withvalues614
upto+14.7‰(VPDB)beingfound(Fig.12).Amicromilltraverse(Fig.10H,J)demonstratesthat615
theseextremehighvaluesaremaintainedonthemm-scale,butthatoverpetrographic616
boundaries,δ18Ovaluescanvarybyasmuchas6‰.617
618
Interpretation619
ForFacies3D,thedolomicritic,syndepositional,passivetodisplacivegrowthwithnodular620
structureandcracksischaracteristicofcalcretesinwhichprecipitationoccursasaresponseto621
evaporativelossesatoraboveawatertable.Althoughrare,thespar-filledpseudomorphs(Fig.622
11A),interpretedasafteranhydrite(Fairchildetal.,1989),providefurtherevidenceof623
evaporativeconditions.Specificallytheevidencefordisplacivegrowth,nodulesandcracks624
servedtoidentifyalphacalcretes(Wright,1990)whichinPhanerozoicexamplestendtooccur625
onnon-carbonatesubstratesandinmorearidconditionsthanthemorecommonbetacalcretes626
containingstructuresresultingfromhigherplants(Wright&Tucker,2009).Thehighδ18Ovalues627
andcovariationwithcarbonisotopesrequireevaporation(Fairchildetal.,1989)whichatthe628
highendofthespectrumreachesextremeproportionsandhencerequiresanextremelyarid629
29
environment.Dolocretesareratherlesscommonthancalcareouscalcretesandtendtobe630
betterdevelopedwhenoriginatingfromgroundwaterthanwhenpedogenic,asinTriassicstrata631
oftheParisBasin(Spötl&Wright,1992).Inthisexample,pedogenicandgroundwater632
dolocreteshadasimilarrangeofstableisotopecompositionstoeachother,buttheir633
covarianceslope(1:1)wassteeperthaninFA3.Overalltheabsenceofanysignaloflightcarbon634
fromoxidationoforganicmatterinFA3isnotable,butconsistentwiththeundetectablylow635
organiccarboncontentsoftherocks.InPhanerozoicrocks,thepresenceofsomebiological636
features(e.g.rootstructures)canservetoidentifypedogeniccalcrete,butthisisnotpossiblein637
theProterozoic.638
TheWilsonbreenFormationdolocretesareinterpretedaspedogenicprimarilybecause639
theextremelyhighδ18Ovalueswouldrequiregroundsurfaceconditionsforsuchextremely640
effectiveevaporationtooccur.Aswillbediscussedlater,thisinterpretationisalsoconsistent641
withtheverticalfaciesrelationships.642
RegardingFacies3S,thedifferentiatedmicrostructuresareagaintypicalofmicrobial643
deposits.Suchlaminitesarefoundinassociationwithsoilsandintermittentlyfloodedsubaerial644
surfaces(Alonso-Zarza,2003),althoughyoungerexamplesincluderootmatsfromhigherplants645
thatareclearlyinapplicablehere.Klappa(1979)ascribedcm-scalelaminateddeposits“hard646
pan”oncalcretizedlimestonesubstratesasoriginatingfromtheactivitiesoflichenwhich647
colonize,boreintoandformaccretionarydepositsonsurfaces.Thelichen-formeddepositsdo648
exhibitfeaturessuchasfenestrae,sedimentincorporationandvariablecrystalsizewhichare649
consistentwiththeWilsonbreenFormationexample.However,noevidenceofalterationof650
underlyingcementedmaterialhasbeenfoundandtheWilsonbreenFormationmicrobial651
laminaearemuchmoredistinctandarenoticeablydomed,contrastingwithlaminarcalcretes.652
Infact,themicrobiallyinfluencedlayeringisindicativeofactiveupwardaccretion,ratherthan653
30
slowpedogeneticalteration.Theaccretiontooktheformbothofgrowthofcarbonate-654
mineralizedmicrobialmats,butalsosandlaminae.Ashallowdepressiononafloodplain/playa655
marginseemsapposite.Acombinationofahighwatertablefromwhichevaporationcould656
occur,orveryshallowwaterinundationfollowedbydryingoutandsedimentaddition,perhaps657
byaeolianactionisindicated.AtDracoisen(Fig.9I),thegradationalrelationshipbetween658
laminatedcarbonateandunderlyingchaoticbrecciaisaclassiccharacteristicofevaporite659
dissolutionbreccias.Thecalcite-cementednatureofthebrecciaisconsistentwithremovalof660
oneormorehorizonsofcalciumsulphateevaporiteseitherduringdepositionofthebed,or661
soonafterwardsfollowingresumptionofglacialconditions.ApossiblysimilarMesoproterozoic662
exampleisprovidedbyBrasier(2011)fromOntarioinwhichstromatolitesareassociatedwith663
collapsebrecciasandcalcretes,andinferredtoformataplayalakemargin.Likewise,the664
modernMcMurdoDryValleyscontainarecordofmanyshallowsalinelakesandsaltpans665
(Wilson,1981).666
Dolomiteisknowntobecapableofprecipitatingasaprimaryphaseorbyreplacement667
ofaCaCO3precursorinarangeofsurfaceenvironments(Warren,2000),althoughtheinitial668
crystals(protodolomite)maylackwell-developedorderingreflectionsandthesecanincrease669
overtime(Greggetal.,1992).ThepetrographiccharacteristicsofFA3dolomicriteareconsistent670
withveryearlydiageneticreplacementofaprecursorcarbonateorofprimarygrowthof671
(proto)dolomiteandthelatterisclearlythecaseforzoneddolomicrosparcavity-linings(cf.672
HoodandWallace,2012).Thepresenceofeuhedralcrystalswithindisplacivefabrics(Fig.11C)is673
distinctive.AlthoughTandon&Friend(1979)interpretedeuhedralgrowthzonesindisplacive674
calciteincalcretesasevidenceofrecrystallizationitismorelogicaltoseeitasaprimarygrowth675
fabric,asarguedfordolocretesbySpötl&Wright(1992).676
31
Theextremelyhighδ18Ovaluesruleoutpost-depositionalmodificationandan677
interpretationofthevaluesasreflectiveofthedepositionalenvironmentisalsoconsistentwith678
thetraceelementchemistryandpreservedcrystalgrowthzones.ThehighMncontentand679
absenceofpyriteimplieslowpO2,butnotanoxia,althoughthesulphateoxygen-isotope680
systematicsareindicativeofmoreredoxvariabilitythaninFacies2S.Specificallybacterially681
mediatedelectronshuttlingbyMn-speciescancatalyzerepeatedtransitionsbetweensulphate682
andsulphitecanpermittheerasureofaΔ17Osignatureandcreationofahighδ18Oinsulphate683
(Baoetal.,2009).Suchprocessescouldcatalyzedolomitenucleationgiventheevidencefrom684
otherfieldandexperimentalstudiesonthecatalyticroleofsulphatereduction(Vasconceloset685
al.,2005;Zhangetal.,2012).Theinferredredoxvariationsmayberelatedtoasupplyofbrine686
primarilyfromwithinthesediment,contrastingwiththesurfacewatersfromwhichFacies2S687
precipitated.Theoccurrenceofthehighestδ18OvaluesinlaminateddolomitesofFacies3Sis688
consistentwiththeirformationbyverynear-surfaceevaporation,whilstabruptvariationsin689
δ18O(Fig.10J)aresuggestiveofoccasionalinundationsbylessevolvedwaters.Insummary,FA3690
providesexamplesoffaciesthatstretchtheboundariesofearthsurfacephenomenaand691
indicatedepositioninunusuallyaridterrestrialenvironments.692
693
FaciesAssociationFA4(Facies4I,4Rand4S)694
695
Description696
Facies4Risthemostcommonfaciesinthisassociationandconsistsofrhythmicalternationsof697
carbonateandsortedterrigenoussediment,whichoccurinassociationwithstructuressuchas698
waveripplelaminationordesiccationstructures.Thefinecarbonatelayersareusually699
dolomitic,ormixeddolomitic-calcitic,butincludesomelimestone(Figs.7A,12).Universally,the700
32
coarsersedimentlayers,composedofsedimentinthesizerangecoarsesilttofinesand,show701
signsoftractionalsorting,whichisakeydiscriminantfromFA5.Whereverlaminaeare702
sufficientlythick,undulatorycross-laminationisdisplayed(Fig.13B)whichcanbeconfidently703
identifiedaswave-generated.Locally,symmetricalripplesarepreservedincross-section(Fig.704
13A)oronbeddingplanes(Fig.13C).Dryingoutiscommonlyindicatedbydesiccationstructures705
withassociatedsmallintraclasts(Fig.13B,H)orsaltpseudomorphs(Fig.13D),althoughsuch706
structuresarenotpresentinthemajorityofsamples.Fourexamplesofapparentlynon-707
evaporiticcrystalpseudomorphshavebeenfound,butthesearemuchbetterdevelopedinFA5708
andaredescribedinthatsection.Carbonatelaminaearemicriticintextureandtypically709
uniform,althoughdifferentiatedclottedmicrostructuresalsooccur,similartothosedescribed710
belowinFA5,consistentwithprecipitationbeneathbenthicmicrobialmats(Riding,2000).This711
facieswaslocallyhighlyaffectedbysubsequentglacitectonicdeformationatthetopofW2at712
Ditlovtoppen,asdescribedbyFlemingetal.(2016).713
TheisotopetraverseofFig.13Jrevealsashiftinisotopesfromthesandylayersinto714
dolomicriteconsistentwithanauthigenicoriginforfinedolomite,whichisconfirmedbyCL715
observations(Fig.14A,B).Thelimestonelaminaeinthisfaciesassociationdisplayarangeof716
δ18Ovaluesfrom-11.9to-3.2withameanof-8.1‰,whereasthedolomiterangesfrom-5.3to717
+1.4withameanof-1.9‰(Fig.7B).Thedifferenceof6‰inmeanvalue,comparedwith718
inferredandobserveddifferencesof2.6-3‰forcalciteand(proto)dolomiteprecipitatingfrom719
equivalentfluids(Land,1980;Vasconcelosetal.,2005)impliesthatthedolomitesprecipitated720
onaveragefromwaterswithhigheroxygenisotopevaluesandthedolomitesdisplayisotope721
covariance(Fig.7B).Traceelementdatawillbepresentedelsewhere,butFA4andFA5722
dolorhythmitesalsoshowacovariationofSr(from100to200ppm)withδ18Oandsomewhat723
33
higherSrvaluesincalcites,andlikeotherWilsonbreenprecipitatedcarbonates,theyareMn-724
rich(>1000ppm).725
Facies4Ioccurstypicallyasdiscretebeds,normally10-20cmthick,ofsharp-to726
erosionallybasedintraclasticsandstonewithwave-generatedlamination.Thesandmatrixis727
moderatelysortedcoarsesilttomedium-grainedsandandintraclastsaresometimesconfined728
tothelowerhalfofthebed.SeveralsuccessivebedsareshowninFig.13Hinasection729
transitionalupwardsfromFaciesAssociation5,andanexampleofthisfaciesinthinsectionis730
illustratedinFig.13G.Twooccurrencesofooids(Fig.13E)havebeenfound.AtNorthKlofjellet,731
highinagenerallypoorlyexposedW2section,isa1mbedofindistinctlycross-laminatedsands732
alternatingwithcm-scaledesiccatedlimestonebedscontainingscatteredsandgrains.Thisisthe733
lateralequivalentoffluvialfacies(FA2)1kmawayatSouthKlofjellet.Ooidsarefoundnearthe734
topoftheunit,butmakeuplessthan5%ofthesandfraction.Arangeofcorticesfrom735
superficialcoatingsthroughtofullydevelopedooidswithnovisiblecorearedeveloped.Fabrics736
aremicriticandmicrospariticwithacrudeconcentricstructure.Themeanoxygenisotopevalue737
ofthelimestone(>90wt.%CaCO3)is-7.6‰.Thesecondexampleistheoccurrenceofasmall738
proportionofooidswithinthin(0.1m)cross-laminatedgreensandstoneunderlyingFA5739
sedimentsinmemberW3atOrmen(Table3).740
Facies4SisrepresentedbydistinctsandstonebedsintheDitlovtoppenandDracoisen741
sections.Thesesandstonesarehighlyuniform,consistingofwell-sortedfine-tomedium-742
grainedsandstone,withverywell-roundedgrains.Beddingstructuresareconfinedtoan743
indistinct,discontinuousparallelstratification.TheDitlovtoppenexamplepresentsasatabular3744
m-thickbedoverthe200mwidthoftheoutcrop.Itslowerfewdecimetresarelocallythinly745
laminatedsandanddolomite,changinglaterallytouniformsandstone.Twothinbedsof746
dolomitewithfloatinggrainsarealsofoundatthetopoftheunit.Grainsareverywell-rounded747
34
andrangefromveryfinetocoarse-grained,butmostoftherockvolumeiscomposedof748
mediumtocoarsesandgrains(Fig.13F).Theonlysedimentarystructuredisplayedisan749
indistinctcm-scalehorizontallaminationwithslightvariationsingrainsize,orlocallywithmm-750
scalelaminaewithsiltydolomiticmatrix.Locallythelaminationdisplayssedimentary751
deformation,suggestiveofupwardfluidescape.Oxygenisotopesinseveralsamplesareslightly752
heavierthanexpectedfordetritalmatrix,consistentwithadditionofprecipitateddolomite.753
Thereisatransitiondownwards(Fig.10H)toFacies4Rinwhichmm-cmscalesandlaminae754
betweenrhythmiccarbonatesdevelopcross-laminationandwave-ripplemorphology,and755
within20-30cmoftheboundaryoccursathinrepresentativeofFacies4I(intraclasticflake756
breccias)anddesiccationstructuresindicativeofemergence.757
758
Interpretation759
Facies4Rand4Idisplayevidenceofsortingandreworkingofthesedimentsbywaveaction.This760
indicatesdepositioninawaterbodythatwasunfrozenatthetimeofdepositionofthecoarser761
layerswhichrepresentdistincttimeperiodswithmorepronouncedwaveaction.Thesharp-762
basedintraclasticbeds(Facies4I)appeartorepresentdistinctstormeventsinwhich763
considerabledisruptionandtransportationofcementedcarbonatelayersoccurred,althoughat764
leastinsomecasestheselayerswerealreadydisruptedbydesiccation.765
Thegrainsandstructuresinfacies4Rand4Iareconsistentwitheitheramarineora766
lacustrineorigin,althoughitisnotedthatthereisanabsenceofdemonstrabletide-related767
phenomenon(cf.Fairchild&Herrington,1989)andalthoughthereisinsufficientinformation768
availabletoprovideaquantitativedescriptionofwaveclimate(Allen,1984),nowave769
phenomenawereseenrequiringoceanicconditions.Althoughooidsarebest-knownfrom770
marineenvironmentsandthickooliticunitswereusedasacriterionforwarmclimatesinthe771
35
NeoproterozoiccontextbyFairchild(1993),ooidshavebeendescribedfromQuaternary772
sedimentsreworkedintoAntarcticmoraines(Raoetal.,1998)andinvariousmodernalkalineor773
hypersalinelakes.Lacustrineooidsforminwaterdepthsof1-5m,withthebestdevelopmentin774
shallowestwater.TheWilsonbreenFormationexamplesdonotresemblehypersalinearagonite775
ooidswithradialstructure(e.g.Halley,1977),consistentwiththeoxygenisotopecomposition776
whichdoesnotindicateanyevidenceforhypersalinity.Ooidsinsmallermodernlakestendto777
besuperficialwithrelativelyirregularoutlines,whereasfullydevelopedooidsarefoundonthe778
largeLakeTanganyikainBurundi(Cohen&Thouin1987)correlatingwithstrongerwaveaction.779
Chemicalargumentsfavouralacustrineoriginforthecarbonates.Theyprobablyarose780
assomecombinationofwatercolumnprecipitatesorwithinthesediment,e.g.asmicrobially781
influencedprecipitates.Dolomitecouldbeprimaryor,giventheCLevidence,beaveryearly782
diageneticreplacementofCaCO3,althoughnotonewithhighSrcontent.Giventheconsistent783
chemicalcharacteristicofMn-enrichment,itseemshighlyimprobablethatburialdiagenetic784
recrystallizationtookplaceandsoitismoststraightforwardtointerpretthestableisotope785
valuesasprimary,inwhichcasethewiderangeofδ18Ocompositionsisnotablebecauseitis786
muchgreaterthanexpectedfrommarinewaters.Inthecaseofthedolomites,thisismuch787
greaterthantherelativesmallchanges(1-2‰)thatmightbeexpectedtoaccompanyincreased788
orderingfromaninitialprotodolomitetoanordereddolomite(Greggetal.,1992;Kaczmarek&789
Sibley,2014).Theformationofdolomitefrommore18O-rich,evaporatedwaters,isconsistent790
withthestandardparageneticmodelinplayalakes(Dutkiewiczetal.,2000),althoughchanges791
insourcewatercompositionaswellasevaporationarelikelytohaveoccurred.792
Thewell-roundedsandgrainsfoundinFacies4Sareconsistentwithaeoliantransportasinthe793
McMurdoDryValleysofAntarctica(Fig.3C;Calkin&Rutford,1974;Hambrey&Fitzsimons,794
2010).Howevergrainswithsuchatransporthistorycanfinallybedepositedinaeolian,fluvialor795
36
lacustrinesettings.Theconsistentgrain-sizecharacteristicsofindividuallaminaeandgood796
sortingofthecoarselaminaeinsteadpointtotractionalflows,butlackofcross-stratification797
rulesoutaeolianorsubaqueousdunes.Hendyetal.(2000)developedtheice-conveyormodel798
toaccountforsandydepositsonthefloorsofcertainice-coveredAntarcticlakeswithfloating799
glacier-icemargins.Wind-blownsandmeltsitswaythroughtheiceincontrasttogravelwhich800
remainsonthesurfacewhereitistransportedbyiceflowtothedistallakemargins.Suchan801
environmentcandeveloptheindistinctparallelstratificationobservedinthisfacies,buttwo802
featuresofthemodernsystemsnotobservedaremoundedbeddingandupwardgradationinto803
coarsegravel(Halletal.,2006).TheDitlovtoppenbedistabular,whereasinthemodernlakes804
sandtransmissiontotheiceisfocused,leadingtomoundsandridgesonthelakefloor.805
AplausiblealternativeforFacies4Iisaninter-duneenvironment,asettingwherewind806
ripplemigrationwouldbeexpected(Lancaster&Teller,1988).Suchphenomenacouldgiverise807
totranslatentsub-horizontallaminae,representingthesetboundaries,withoutinternalripple808
cross-laminationbeingpreserved(Mountney&Thompson,2002).Awatertablethatwasat809
leastseasonallyhighisrequiredtoaccountfordewateringstructuresandtheprecipitationof810
dolomite(incipientdolocrete).Insuchmodernenvironmentsseasonalfloodingbysurfacewater811
oremergentgroundwatermightoccur,accountingforoccasionaldolomicritelaminae.Thestyle812
ofstratificationisinconsistentwithdunedeposition,butisthatexpectedonasandflatorplaya813
withahighwatertableandfitswiththetransitiontoFacies4Rand4IobservedinFig.10H.814
OverallFA4representsshallow-waterandexposedsedimentsassociatedwithawave-815
dominatedshorelineandsusceptibletowindreworking.Althoughdistinctionofmarinefrom816
lacustrinecoastalsettingsisnevereasy,thewiderangeofoxygenisotopecompositionsof817
micriticMn-richcarbonatesfavoursalacustrineorigin.818
819
37
FaciesAssociation5(Facies5Dand5R)820
821
Description822
Thisfaciesassociationisdominatedbyrhythmicalternationsofcarbonatesandpoorlysorted823
clasticsediment(Facies5R).Locally,gradationsareseentobrecciatedrhythmiteswhichare824
distinguishedasFacies5D.FaciesAssociation5isonlyaminorconstituentofmostoftheW2825
sections,beingmoreprominentintheSKlofjelletandBacklundtoppen-KvitfjellaRidge(BAC)826
sections(Figs.S5andS6),butitisthedominantcarbonate-bearingfaciesassociationinmember827
W3whereitalternateswithice-rafteddiamictitesofFA6.828
Fig.11illustratesvariantsofFacies5RfoundinmemberW2.Fig.11Aexhibitshighly829
regularmillimetre-scalealternationsoflimestoneandwacke.Theclasticsedimentincludes830
manymicroscopicdiamictitepellets(arrowed)whichareanormalfeaturefoundinthisfacies,831
whereasthesmallpseudomorphscrossinglaminaboundariesarefoundmorelocally.Pebble-832
sizedfragmentsinclasticlayersareseeninFig.11Cand11D,thelatterdisplayingadropstone833
textureassociatedwithdisruptionoflimestonelaminae.Anindicatorofinstabilityisshownby834
theminorfaultinFig.11Cacrosswhichthenumberoflimestonelaminaechanges,indicating835
erosionontheupthrownsideandhencethatthisisasedimentarygrowthfault.Larger-scale836
disturbanceisshownbythefoldsinFig.11Finwhichcarbonatelaminaedisplaysomeplasticity,837
butarealsofractured,indicatingpartialcementationandasedimentaryoriginforthefolds.838
Abovethis,thesedimentsarevisiblydisruptedandtransitionaltoFacies5D.Thethickest839
exampleofafacies5Dobservedwasa0.4mbedatDitlovtoppen,containingbothintraclastic840
andterrigenoussediment,andwhichdisappearedlaterallywithin100m.Itclearlywasderived841
bylocalizedresedimentationofFacies5R.842
38
InmemberW3,Facies5Rispresentasisolatedbedsupto1mthickexhibitingsimilar843
alternationsofcarbonatelaminaeandwacke/diamictiteasinmemberW2(Fig.16).Fig.16A844
illustratesthebaseofonesuchbedwithclearalternationsofthickdiamictitelaminaeand845
carbonatepassingupwardsintomorecarbonate-dominatedfacieswithonlythinclastic846
laminae.ThedominanceofprecipitatedcarbonateinthisfaciesisshowninFigs.16C,D,E,the847
latterbeingthesoleexampleofprecipitatedcarbonateinmemberW1.Disturbancebysoft-848
sedimentfoldingisnearlyuniversalandthesamecombinationofductileandbrittlebehaviour849
ofcarbonatelayersisshown(Fig.16D,E)asinmemberW2.Fig.16Billustratesthedevelopment850
ofaresedimentedbed(Facies5D)dominatedbyintraclasts,butwithsomepoorlysorted851
sedimentmaterial,overahorizonwithsoft-sedimentfolds.852
ItiscommonforcarbonatelayersinFacies5Rtoshowirregularlaminationordomal853
structures.Therecanbeupwarddomingoflayertops(Figs.15C,16C)uptocentimetre-scale854
(Fig.15F).Topographycanbeinheritedfromunderlyinglayers(Figs.15F,16C),whereas855
sometimesthebaseaswellasthetopofthelayerisdomedupwards(Fig.15C).Fig.15Bdisplays856
boththesefeaturesinlayersunderlainbycomplexcementcrustswithsomeremainingporosity.857
Thecrustsshowneitherabotryoidalnoreuhedralmorphologyandarecomposedof858
polycrystallinecalcitemosaicsinwhicheachcalciteshowsthesamezonationinCL.Thereare859
transitionsthroughbedswithonlyminorclasticdebris(Fig.15F)tomoremassivelimestones860
withcomplexmicrostructures.861
Petrographically,carbonatelaminaecanberegularly(rhythmically)developed,millimetre-scale862
inthickness.Laminaeareoftenheterogeneous,andmaybeeitherdolomitic,calcitic(Fig.17A,B,863
D,F)ormixedmineralogyincomposition(Fig.17C).Whereheterogeneous,laminaemayshow864
anincreaseincrystalsizeupwards(Fig.17A)ordisplaymoreorlessevidentclottedtexturesof865
micritewithinmicrospar.UnderCL,bothcalciteanddolomitepresentcoherentreplacivefabrics866
39
(e.g.Fig.17F),inwhichcrystalswithidenticalzonesgrowthroughoutthefabricandenlargeinto867
fenestrae.Rareexamplesofmicriticrodsaround10-20µmindiameter,withminorassociated868
pyrite,arereminiscentofcalcifiedsheathssuchasfoundinthepre-CryogenianDraken869
Formationinthestudyarea(cf.Knolletal.,1993),butarenotasdistinct.Millimetre-scale870
convexitiesontheupperbedsurfaceappearasthromboliticintexture(Riding,2000)with871
irregularfenestrae(Fig.17B).Clasticlaminaetendtolevelthemicrotopography(Fig.17A),872
whilstindividualsandgrains,dropstonesordiamictitepelletscanoccuranywherewithinthe873
carbonatefabrics(Fig.17D).874
TraceelementcompositionsofcarbonatearesimilartoFA4.Thelimestonelaminaein875
thisfaciesassociationdisplayarangeofδ18Ovaluesfrom-5.6to-12.8‰withameanof-9.2‰876
andtheδ13Cvaluesalsodisplayalargerangefrom-2.1to+3.5‰.Althoughthefullrangein877
δ13CisshownbymemberW2,valuesinmemberW3tendtobelower(Fig.12).FA5dolomite878
rangesfrom-10.3to+3.7‰,withameanof-3.2‰(Fig.7B)andasforFA4,thedolomites879
displayisotopiccovariance.880
CommonexamplesofcrystalpseudomorphsoccurinFA5,usuallyassubhedralto881
euhedralcrystals,variablyjoinedintoconfluentmassesembeddedwithinorapparentlycutting882
acrosslamination(Fig.18E,F,G).Insomecases,trainsofcrystalsarealignedalongor883
concentratedwithinparticularlaminae.Sizeofindividualcrystalsissimilarwithinsamplesand884
rangesfrom0.1-0.2mm(Fig.18F)to1-3mm(Fig.18B,19A),themostcommonsizebeing0.5-1885
mm(Fig.186E,G).Therangeofcross-sectionsisdominatedbyfour-sidedorsix-sidedfiguresof886
crystalswithequanttocolumnarhabit.Pseudomorphsareequallylikelytobedevelopedin887
rhythmiteswithcomplexmicrostructuresasinrhythmiteswithuniformmicrite.Limestone888
hostsforpseudomorphs(n=15)hadameanδ18Ocompositionof-8.8(range-6.2to-12.7)‰889
40
anddolomiteslikewise(n=4)mean=-1.92,(range-6.4to+1.7)‰,thatissimilartoFA5asa890
whole.891
Whilstthewithin-sedimentmodeofoccurrenceisfoundinbothmembersW2andW3,892
themostspectacular,upward-growingcrystalshaveonlybeenseenattwolevelsintheS.893
KlofjelletsectionofW3.Apolishedhandspecimen(Fig.19A)displaysthreedistinctcrystal894
horizonsofwhichtheuppertwoareshowninthephotomicrographofFig.18B.Thecrystals895
evidentlygrewfreelyupwardsandcrystalterminationsarestronglydrapedbyoverlying896
sedimentlayersindicatingthatthecrystalsformedatthesediment-waterinterface.Indifferent897
cases,theyaredrapedeitherbycarbonate(e.g.formingroundedmassesonthelowerhorizon898
ofFig.18B),orpoorlysortedsediment(wackeordiamictite).Inthelattercase,crystalfacesare899
variablycorrodedatthecontact(Fig.18B,D).Measurementoftheinternalcrystalangles(=180°900
minustheapparentinterfacialangle)incutsectionsofthissampleyielded75measurements901
assignedto10°bins.Theresultsdisplaytwomodescentredaround40-50°and90-110°(Fig.902
18C).Inspectionofthecrystalpseudomorphswhichgrewwithinsediment(Fig.18E,G)is903
consistentwiththeseresults.904
Thepreservationofthepseudomorphsistypicallyinthesamemineralasthehost905
carbonate,dolomiteorcalciteasappropriate.Insomecases,theinfillingphaseiswholly906
cementing(Fig.18E),whilstinothersthevariableinternalfabricspointtoadominantly907
replaciveorigin(Fig.18G).Suchanoriginisveryclearfortheupward-growingcrystalswhere908
eachispseudomorphedinamosaicof20-100µmcalcitecrystals,whichinstainedthinsection909
showanon-ferroancoreandaferroanperiphery(Figs.18D,20A).910
911
Interpretation912
41
Thelackofsize-sortinginFA5sedimentsindicatestheywerelaiddowninawaterbody913
lackingsignificantcurrentactivity,butonunstableslopesassuggestedbythesoft-sediment914
folds,interpretedasslumpstructures.Alltransitionsoccurfromdisturbedandslump-folded915
Facies5Rtoresedimenteddiamictites(Facies5D,interpretedasdebrisflows)composedlargely916
ofFacies5Fblockswithsomeexoticclasts,areseen.TheclasticsedimentinFacies5Risclearly917
glaciallyderivedbecauseitisinvariablyverypoorlysorted,containsdiamictitepelletslikelyto918
bederivedbyicerafting(tillpellets)andlocalice-raftedclasts(dropstones),andgravelis919
presentinthickerlaminae.AlsoFA5transitionsupwardsanddownwardsintostratified920
diamictitesinterpretedbyFlemingetal.(2016)asrepresentingmorecontinousdepositionfrom921
floatingice.Incontrast,nodistinctfine-grainedsediment-gravityflowswereobserved,implying922
lackofproximitytofluvialinputtothewaterbody.Thecarbonatelaminaecommonlydisplay923
evidence(thromboliticdomalgrowthmorphologyandcomplexmicrostructures)ofamicrobial924
origin(Fairchild,1991;Riding,2000),includingmuchevidenceforin-situcarbonate925
precipitation,coupledwithsomesiliciclasticsedimentincorporation.Acombinationof926
photosynthesisandfavourablenucleationofcarabonatewithinextracellularpolymeric927
substanceanddeadcellularmaterialpresentinmicrobialmatscanbeenvisagedaspromoting928
carbonateprecipitation(Riding,2000;Bosak&Newman,2003).However,insomecases,there929
isnoclearmicrobialstructurewithincarbonatelayersandwecannotruleoutsettlingofwater-930
columnprecipitatesinthesecases.931
Thehighlyregularnatureofthemm-scalecarbonate-siliciclasticcouplets,particularlyin932
thethicknessofthecarbonatelayers,isstriking(Figs.15,16)andtherearemanymodernand933
lateQuaternarylacustrineanaloguesinwhichsuchcoupletsareannual,i.e.varves.Although934
thereisnobarriertosuchsedimentsformingundermarineconditions,itistruethat935
photosynthesisraisescarbonatesaturationfasterinlowionicstrengthwatersthaninseawater936
42
(Fairchild,1991)andnomodernmarineanalogueshavebeendescribed.Thedominantprocess937
creatingcarbonatesinAlpinelakesiswater-columnphotosynthesisfromalgalblooms(Kelts&938
Hsü,1978),andthishasguidedmanyinterpretationsoflateQuaternaryvarves,allowing939
inferenceofasuccessionofeventsthroughtheyear(e.g.Neugebaueretal.,2012).However940
lakesvarygreatlyintheirhydrology,internalstructure,salinityandstateofcarbonate941
saturationandtherearemanydifferentpatterns,e.g.carbonatemineralproductionmay942
continuethroughtotheautumn(Shanahanetal.,2008)ormaypartlydependonperiodswhen943
thereisinputofionsfromriverineinput(Stockheckeetal.,2012),ormayinpartrelateto944
winterfreezingconditions(Kaluginetal.,2013).Variationsinredoxconditionsovertime945
influencethepreservationofvarvesinmodernlakesandinthecaseoftheGotlanddeepofthe946
brackishBalticSeaepisodicoxygenationepisodesmaytriggerachainofeventsleadingto947
characteristicMn-carbonatelayerssuperimposedontheannualpattern(Virtasoloetal.,2011).948
TheabsenceofburrowingorganismsinCryogeniantimesledtocontinuouspreservationof949
varvestructure,buttheMn-chemistryofthesecarbonatesisquiteconsistent(Fairchildetal.,950
1989;Baoetal.,2009)implyinglessstrongredoxfluctuationsinthewaterbody.951
Thecrystalpseudomorphsalsopresentimportantenvironmentalevidence.Thefacies952
occurrenceofthesepseudomorphsarguesagainstanevaporativeorigin.Althoughdolomite953
sampleswithhighisotopevaluesindicatessomeevaporation,theoxygenisotopecomposition954
ofthedominantlimestoneanddolomiteoccurrencesistypicalforfaciesassociation5andlacks955
evidenceforincreasedsalinity.This,togetherwiththecarbonatecomposition,impliesa956
carbonateprecursor.Theveryregularmodeofreplacementbycalcitewithinternalcrystal-957
growthzonation(Fig.17E)indicatesthattheprecursorwasmoresolublethancalcite,butstill958
capableofformingeuhedra,andhencecrystallineratherthanamorphous.Vateriteand959
monohydrocalcitecanberuledoutbecausetheyinvariablyformspherulitesormicrocrystalline960
43
precipitates(Dahl&Buchardt,2006;Pollocketal.,2006;Rodriguez-Blancoetal.,2014).961
Althougharagonitetypicallyformsmosaicsofmicrocrystallineorthorhombicfibres,itiscapable962
offormingradiatingpseudo-hexagonaltwinned“ray”crystalsofsimilarsizetothoseseeninthe963
presentstudy(Fairchildetal.,1990,Riccionietal.,1996).However,wheresix-sidedcross-964
sectionsareseenintheWilsonbreenFormation(Fig.16E),theyareoftenelongatedratherthan965
equant,andthemostelongatedcrystalsincross-sectionshowapairofterminatingfaces,nota966
basalpinacoidcharacteristicofaragonite.StrontiumcontentofWilsonbreenrhythmitesisalso967
low,whereasitistypicallyhighinformerlyaragoniticlimestones(e.gFairchildetal.,1990).968
Ikaiteisahigh-pressurephase,metastableatEarthsurfaceconditions,butbecomes969
relativelymorestableatcoldtemperatures(Kawanoetal.,2009),andnearlyalwaysforms970
naturallyatcooltemperatures(-1.9to+7°C,Huggettetal.,2005),onthesedimentsurfacein971
spring-fedalkalinelakesandfjordsandwithinmarinesediment(Buchardtetal.,2001).Itreadily972
disintegratestoformcalciteunlessthesolutioncontainsaninhibitorforcalciteprecipitation.973
Bischoffetal.(1993)foundthatphosphatewasmosteffectiveinthisrespectandindeed974
significantphosphatelevelsaretypicalofmodernikaiteoccurrences(Huggettetal.,2005;975
Sellecketal.,2007).Itisacknowledgedthatikaitemayhavebeenmuchmorewidelypresentas976
aprimaryphasethanhadbeenrealized(Shearman&Smith,1985;Bischoffetal.,1993;977
Buchardtetal.,2001).Thisisbeingborneoutbynewdiscoveriessuchasinseaice(Fig.19C,978
Nomuraetal.,2013)andasmillimetre-scalecrystalsincoldlakes(Fig.19D,E;fromthe979
PatagonianArgentinianLagunaPotrokAlke,Oehlerichetal.,2013).980
Inthediscussionabove,ikaiteastheprecursorphaseforthepseudomorphsintheFA5981
wasdeducedbyaprocessofelimination,butitisimportanttodemonstratethattheobserved982
propertiesareconsistentwiththisidentification.Ikaiteisamonoclinicmineralthatvaries983
considerablyinhabit,fromequant(Sekkal&Zaoui,2013)toelongateprismatic(Buchardtetal.984
44
2001;Lastetal.,2013)andthedominantcrystallographicformsvarygreatly,andmaybe985
steppedorcurved(Shearman&Smith,1985),whichmakeitspositiveidentificationdifficult.986
Mostofourknowledgeofitslikelymorphologycomesfrompseudomorphs,includingthe987
“bipyramidal”aggregatesofcrystalsknownasglendonites(Davidetal.,1905;Fig.18A),foundin988
shalesassociatedwithPermianglacialdepositsandothercool-waterenvironmentsandthe989
“thinolites”ofQuaternarylakesofthewesternGreatBasin,USA,figuredbyDana(1884)[Fig.990
19B]andinterpretedasikaitepseudomorphsbyShearman&Smith(1985)andShearmanetal.991
(1989).Swainson&Hammond(2001)reinforcedthisidentificationfollowingdeterminationof992
refinedlatticecellparametersofa=8.8,b=8.3andc=11.0Åwithangleβbetweenaandcof993
110°.994
ThepseudomorphsoftheWilsonbreenFormationareinterpretedtorepresenta995
combinationofforms.Prismandpinacoidformsmeetatinternalanglesof90°and110°when996
cutatahighangletothefaces.Thiswouldaccountforthehighermodeinthecrystalangle997
distribution(Fig.18C)andthecommon“rhomb”shapesinsection,similartothecrystalsinFig.998
19C.Secondlythe30-60°modeisinterpretedasrepresentingthejunctionsbetweenprismatic999
facessuchasthoseseentoterminatecrystalsinFigs.18A,19B,E.Itisnotablethatikaiteisonly1000
one-thirdasdenseascalciteandsopseudomorphsincalcitewouldbeexpectedtobeinitially1001
highlyporouseveniftheCaCO3wasprecipitatedlocally.Thisisconsistentwiththestylesof1002
preservationobserved(Fig.16),theexampleofFigs.17Eand18Dcomparingwellwithexamples1003
inLarsen(1994),Huggettetal.(2005),Sellecketal.(2007)andFranketal.(2008).Animportant1004
corollaryisthatthereplacivecalcitemosaicsobservedinFA5(andFA4)limestonesmore1005
generally(Figs.15B,17F)arelikewisealsolikelytobeafterikaite.1006
Identificationofadiageneticprocess,ikaitereplacement,thatisexpectedtobe1007
syngenetic,andpreservationofthecrystalgrowthzonesinCL,lendsweighttothepreservation1008
45
ofprimaryisotopicchemistryinFA5(andFA4)carbonates.Furthermore,forFA5thealternation1009
withice-raftedsedimenttakentoindicatetemperaturesconsistentlyclosetofreezingand1010
hencegiventhelowδ18Osignatures,thewaterbodymusthavebeenfresh.ApplicationofKim&1011
O’Neil’s(1997)experimentallydeterminedfractionationfactorsextrapolatedto0°C,indicates1012
thatwatercompositionsontheVSMOWscaleareapproximately2.7‰higherthancalcite1013
compositionsontheVPDBscale,i.e.-8.3to-15.5‰,arangewhichislikelytoreflectmixingof1014
differentwatersources.Dolomitefacieshavevaluesonaverage6‰higher,implyingformation1015
fromwatersofonaveragehighersalinityandhighermeanMg/Ca(Mülleretal.,1972).1016
Thereareplentyofmodernanaloguesforcalcareousmicrobiallaminaeincoldlakes,1017
eveninsuchextremeenvironmentstodayastheice-coveredlakesoftheMcMurdoDryValleys1018
ofAntarctica(Parkeretal.,1981),orreducingsolutionhollowsbeneaththeGreatLakes1019
(Voorhuisetal.,2012).OnecuriousphenomenonarethickcementcrustsfoundinbothFA5and1020
FA2.ApossibleoriginfortheseisthephenomenonobservedincertainAntarcticlakesofthe1021
localizedliftingofmatsbygasgeneration(Parkeretal.,1981).Earlycementationofthemat1022
wouldallowamoregradualfilloftheresultingfenestraeasfoundinmodernAntarcticlakes1023
(Whartonetal.,1982).1024
Inrespectofseasonality,manywell-studiedmodernlakesshowaturnoverassociated1025
withcoolinginwinterandmayhaveafrozensurfaceinthatseason.Inthatcase,winter1026
sedimentationistypicallydominatedbyclayandorganicmatter(e.g.Lauterbachetal.,2011,1027
Kaluginetal.,2013),butboththesecomponentsarescarceintheWilsonbreenFormation1028
examples.InAntarctica,carbonateprecipitationislinkedtopeakwatercolumnormicrobialmat1029
photosynthesisinlateSpringtoearlysummer(e.g.Whartonetal.,1982;Lawrence&Hendy,1030
1985),limitedalsobynutrientavailability.Highersedimentinputwouldbeexpectedinthelate1031
summertoautumnwhenicecoverwasataminimum.1032
46
Acluetotheoverallsimilarityofthemicrobialfabricsinthedifferentfaciesassociations1033
maybediscernedintheworkonAntarcticstromatolites.Thefirstmajormat-formertobe1034
identified(PhormidiumfrigidumFritsch)wasknowntobepre-adaptedtocoldenvironments1035
andlowlightconditions(Parkeretal.,1981)andtoleratesconditionsfromfreshtosalineand1036
anoxictooxygen-saturated.Simmonsetal.(1993)amplifiedthatthisspeciesisfoundnotonly1037
inlakes,butalsoinglacialmeltstreams,soilsandcryoconiteholesonablatedglaciersurfaces,1038
thatiseasilyspanningtherangeofenvironmentsencounteredformicrobialdepositsinthe1039
WilsonbreenFormation.Voorhuisetal.(2012)foundthataspeciesofPhormidiumalso1040
dominatedthematcommunityatlowoxygenlevelsina23m-deepGreatLakessinkholeand1041
demonstratedthatthegenushasgeneticpropertiesthatfacilitatetolerationofsulphideor1042
utilizationofitforanoxygenicphotosynthesis.Itthereforeseemsthatitislikelythatthe1043
WilsonbreenFormationmicrobialcommunitieswereoflowdiversity,asexpectedfor1044
extremophiles,withPhormidiumorasimilarancestor,dominatingthebiota.1045
1046
1047
Verticalandlateralfaciesrelationships1048
1049
Verticalfaciestransitionscanbeusedtoestablishwhethersedimentarysuccessions1050
havepredictablycyclic,Markovproperties(Powers&Easterling,1982).Atransitionmatrix1051
derivedfromalltheloggedW2sections(Fig.S7)demonstratesthatthereareverticaltransitions1052
betweenallofthefaciesassociations,exceptFA1/FA7whichareonlyfoundboundingW2,not1053
withinit.Thisindicatesastochasticelementtothefaciesaccumulation,buttherearealso1054
clearlypreferredtransitions.Althoughdataareinsufficientforformalstatisticalanalysis,since1055
totaloccurrencesofeachFaciesAssociationaresimilar,therelativenumberoftransitionsisa1056
usefulguide(Fig.22).Twosetsofthemostcommontransitionsconcurwiththeinterpretations1057
47
madeearlier.Firstly,fluvialchannels(FA2)mostcommonlypassupintofloodplainsediments1058
(FA3)andvice-versa.SuchacloserelationshipcouldnotbeanticipatedifFA2weremarine.1059
Secondly,calcareouslakesediments(FA5)passintoFA6(glacilacustrine)andvice-versa(andthis1060
isalsothedominantpatterninmemberW3).Thisrelationshipemphasizesthetransitional1061
natureofchangebetweenmajoramountsoffloatingiceandreductioninicesufficientto1062
permitcarbonateaccumulationinmicrobialmats.Othertransitions,suchasthosebetweenice-1063
raftedsedimentation(FA6)andfloodplainandlake-marginalsediments(FA3and4),would1064
requiremoresuddenchangesinlakelevel.1065
Overall,theNortheastGreenland-NESvalbardNeoproterozoicsedimentarybasinis1066
envisagedaselongate,withbasementexposedinthefarSWinGreenland(Fairchild&Hambrey,1067
1995)andaxialglacierflowtotheNNEinNESvalbard(Flemingetal.,2016).IntheFormationas1068
awhole(Fig.1)theinferredpalaeogeographyisreflectedinsourcingthesubglacialadvances1069
fromtheSSWcorrelatedwithgrounding-linefaciesinW3onlyinthenorthernsections.Within1070
memberW2however,thefaciesmosaicsdonotillustratethisoverallpatternsoclearly.Fig.221071
illustratesthecumulativethicknessofeachofthefaciesassociationsanddemonstratesthat1072
thereisnosimplespatialtrendinfacieswithinW2,exceptperhapsforthehighincidenceof1073
fluvialfacies(FA2)inthesouthernmostsection.Suchacomplexfaciesmosaicimpliesthatwater1074
andsedimentarederivedfrommultiplesources.1075
1076
1077
Integrationofsedimentological,geochemicalandmodellingevidence1078
1079
InthissectionweextendtheargumentsastowhytheWilsonbreenFormationenvironments1080
canbeconsideredasacoherentwholeandhencebeconsideredbyamodernanalogueina1081
48
singleclimaticsetting.Consideringfirstthesedimentologicalevidence,FA5containsclear1082
evidenceofsyn-glacialdeposition(ice-raftedsedimentandikaiteformation)andFacies4Rand1083
4Ishowsimilartypesofcarbonateaccumulation,andlocalizeddropstonedeposition,butmore1084
evidenceofreworkingbywavesinshallowwater.FA3evincesahyperaridterrestrial1085
environmentthatneverthelessbordersaccumulationofmicrobialtufasinstreamswith1086
regularlyfluctuatingdischarge(Facies2S).Facies4Sevokesalandscapewithsignificantaeolian1087
sedimenttransport.Allofthesefaciesareconsistentwithacold,aridterrestrialenvironment,1088
butfewresemblestandardNeoproterozoicfaciesbetweenglaciations(Fairchild,1993).1089
Secondly,thegeochemicalevidencefromsulphateisotopesystematics(Baoetal.,2009;1090
Bennetal.,2015)demonstratesthatthroughoutthedepositionofallthecarbonatefaciesin1091
W2andW3,atmosphericPCO2wasveryhigh(probablyoftheorderof0.1bar)andyettheseare1092
icehouse,notgreenhousesediments.1093
Thirdly,avarietyofmodellingconstraintsindicatethatglaciationandhighPCO2canonly1094
coincideinthecaseofaSnowballEarth,thatisaplanetthatisundergoingahysteresisofCO21095
andclimateinwhichinitiallowPCO2triggersglaciationwhichatacriticalstageofdevelopment1096
becomesgloballydistributedbecauseofice-albedofeedbacks.Anecessaryconditiontoescape1097
fromsuchanextremeiceageisbuild-upofhighPCO2.Modellingofcontinentalenvironments1098
withtheWilsonbreencaseinminddemonstratesthatatsufficientlyhighPCO2,glacialadvances1099
andretreatscanbetriggeredbyprecessionalforcing(Bennetal.,2015),butthattheclimate1100
remainsuniformlycold,changingprimarilyintermsoftheaccumulationofsnowandablationof1101
snowandice.Continentalicevolumeschangelittleandsosignificantsealevelchangewould1102
notbeexpected.Deglaciationtowarm,ice-freeconditionsisnotreversiblewithouta1103
necessarilyslow(multi-millionyear)fallinPCO2(LeHiretal.,2009).Inthisperspective,onceone1104
hasdemonstratednon-marineconditionsforpartoftheWilsonbreenFormation,thenthe1105
49
expectationwouldbethatitwouldallbecontinentalincharacter.Marinetransgressionwould1106
notbeexpecteduntilglaciationisover,andindeedsuchadistincttransgressionisfound1107
truncatingtheWilsonbreenFormation(Fairchild&Hambrey,1984;Halversonetal.,2004).1108
OverallthisimpliesthatinSvalbard,isostaticdepressionbyneighbouringicesheetswasmuch1109
smallerthaneustaticsealevelfallduetobuild-upofice(Bennetal.,2015).Insummary,the1110
Wilsonbreenfaciesarelikelytoreflectamosaicofaridterrestrialenvironmentssubjectedto1111
glaciation,whichbringsusbacktothemodernanalogue.1112
1113
ComparisonwiththeMcMurdoDryValleysandsynthesis1114
1115
Theaimofthissectionisbothtoassessthedegreeofsimilarityofthemodernand1116
NeoproterozoicsettingsandtogainfurtherinsightsfrommodernandQuaternaryphenomena1117
intothelikelycontrolsonWilsonbreendeposition.Westartbyexaminingthreetopicswhere1118
differencesmightbeexpected:tectonicsetting,(palaeo)latitude,andthecompositionof1119
bedrocksundergoingweatheringwithimplicationsforcarbonatemineralogy,andthenuse1120
insightsfromtheDryValleystoaidsynthesizingtheinterpretationofstableisotopefieldsofthe1121
carbonates.1122
TheWilsonbreenFormationrepresentsasmallpartofalargelymarinedepositional1123
basinthatpersistedformorethan300Myanddepositedapileofsediments(HeclaHoek1124
Supergroup)manykilometresthick.TheoutcropbelthasaNNE-SSWtrend,andtheoriginal1125
basinmayhavebeenanintracratonicriftwithasimilartrend(Cawoodetal.,2007)leadingto1126
thetabularnatureoftheFormationfromnorthtosouth.TheDryValleysregionoccupiesa1127
hingebetweentheupliftingTransantarcticMountainsandthesubsidingMcMurdoSound1128
(Etienneetal.,2007).Whilstuplandareasnotcoveredbyicehaveneithererodednor1129
50
accumulatedsediment,significantQuaternarysedimentaccumulationshavebeendrilledin1130
LowerTaylorValley(McKelvey,1981).Meltwaterissuppliedbothfromoutletglaciersandlocal1131
valleyglaciers,andicesheetadvancefromtheadjacentRossEmbaymenthasoccurred1132
repeatedly.Incontrast,sincetheSvalbardareawasalreadyatsealevelpriortoglaciation,anda1133
panglaciationthenensuedwithnothickicecoverlocally,apronouncedeustaticsealevelfall1134
shouldhaveisolatedthebasinfromthesea.Theproximityofuplandareastosourceglaciersis1135
alsounknown,butthisbasinconfigurationoffersthepossibilityofglacieradvancebothalong1136
andacross-strike,leadingtocreationanddestructionoflakes,ashasbeenthecaseinthelate1137
QuaternaryintheDryValleys.Suchexternallyimposedchangesinlakelevelorglacialadvances1138
andretreatswouldbesuperimposedonanyintrinsictendencyforfaciesmigrationandarethe1139
mostlikelyreasonforthemyriadverticalfaciestransitionsandcomplexlateralgeometriesin1140
theWilsonbreenFormation.1141
ThehighpalaeolatitudesoftheDryValleysensuresahighseasonalityoftemperature1142
suchthatmeltwaterproductionislimitedtoacoupleofmonthsinsummer.TheWilsonbreen1143
Formationisthoughttohavelaininthesub-tropics(Lietal.,2013)whereseasonalitytoday1144
primarilyrelatestomoisturebalance.ThisalsoappliesinthelatestagesofaSnowballEarth,1145
sincegeneralcirculationmodellingindicatesthatalternatingseasonsinwhichprecipitation1146
exceedsablationinthesummerandvice-versainthewinterwouldhavebeenastrongfeature1147
oftheclimateinlatitudes20-30°fromtheequator(Bennetal.,2015).TheSnowballclimatealso1148
hasgreatlyenhanceddiurnalandannualtemperaturevariabilityatagivenlatitudecompared1149
withtoday(Pierrehumbert,2005)sincetemperaturerespondsdirectlytoinsolation,withless1150
effectivesmoothingthantodaybylaterallatentheattransportbymoisture.Theimplications1151
remaintobefullyexplored,butBennetal.(2015)givesanindicativeplot(theirFig.S15)1152
demonstratingthat,inthemonthofApril,althoughmeantemperaturesremainbelowzero,1153
51
lowlandareasinthetropicsandsub-tropicsshouldexperienceasignificantnumberofpositive1154
degreedayswhenmeltingcanoccur.Theoveralleffectisthat,openwaterinWilsonbreenlakes1155
wasclearlyextensive,atleastseasonally,contrastingwiththeperennialicecoverintheDry1156
Valleys,apartfrommarginalmoats.1157
Anotherlatitude-relatedfactoristherelativesensitivitytoclimaticchangerelatedto1158
orbitalforcing.WhilsttheDryValleyregionissusceptibletofacieschangesrelatedtosubtle1159
environmentalchangesoninter-annualtomultimillennialscales,themostsignificant1160
environmentalchangesrelatedtoglacialadvancesandretreatsareonorbitaltimescales(Hendy1161
1980).Fig.1illustratesthatfortheWilsonbreenFormationasawhole,thereareaboutten1162
periodsofcessationofglacialdeposition,withuncertaintiesbecauseofthethinandlensing1163
(possiblyeroded)intervalsinW3inparticular.Accumulationratesofannuallylaminated1164
carbonatefacies,bothfluvially(FA2)andinlakes(FA4/FA5)isoftheorderof1mm/year:hence1165
suchretreatphasesrepresentminimumperiodsof103-104yearssinceperiodsofnon-1166
depositionduetolackoflocalaccommodationspaceorsedimentsupplyarelikely.Such1167
timescalesareapproachingthepresent-dayMilankovitchscaleofclimaticfluctuation,an1168
observationwhichstimulatedthemodellingofprecessionalcyclesinBennetal.(2015).Given1169
thatprecession(ontheca.20kytimescale)isgreatlyenhancedduringhighereccentricity(on1170
the100katimescale),aplausibleinterpretationofmembersW2andW3isthattheyrepresent1171
oneortwoeccentricitycycles,withineachofwhichseveralprecession-relatedfluctuationsare1172
recorded.1173
ModernAntarcticahasverylittlecarbonatebedrock.Nevertheless,alkalinityis1174
generatedbothbycarbonateandsilicateweathering,permittingcarbonateprecipitation,1175
particularlyinlakes,controlledbyseasonalphotosynthesis.Bycontrast,theWilsonbreen1176
Formationreflectstheerosionofcarbonate-richcatchmentsandcalciteprecipitationis1177
52
recognizedalsoinstreams.Althoughlimestonemakesup20%ofthegravelfraction,dolomiteis1178
theonlycarbonateinthefinefractionofdetritusbecauseofpreferentialcalcitedissolution,1179
implyingthatunmodifiedmolarMg/Caratioswillonaveragebe<1,althoughwillincreaseas1180
calciteisreprecipitatedwithinthebasin.DatafromstreamsandmostlakesintheDryValleys1181
alsohaveMg/Ca<1,althoughextensiveCaCO3precipitationinLakeFryxellleadstoenhanced1182
Mg/Ca(Greenetal.,1988)andpresumablysuchwaterswereresponsibleforprecipitationof1183
aragoniteinmanylateQuaternarylakes(Hendy,1979).Ratherthanaragonite,ikaiteis1184
recognizedasaprecursortocalciteintheWilsonbreencase.Modernexamplesofikaite1185
formationareassociatedwithhighaqueousphosphatewhichactsasaninhibitortocalcite1186
precipitation(Bischoffetal.,1993)andyetmodernAntarcticlakesareoligotrophic.However,1187
Burton(1993)stressedthattheroleofinhibitorsiscomplexbecausetherearemanypotential1188
inhibitingspecies(e.g.magnesium,phosphate,sulphate,organicradicalsandmanyother1189
speciesdependingonconcentration)andtheycaninteract.Onepossiblefactoristhatacircum-1190
neutralpHwouldbeexpectedinWilsonbreenwatersbecauseofhighatmosphericPCO2which1191
hastheeffectofincreasingthebalanceofHPO42-toPO4
3-andstrengtheningthephosphate-1192
inhibitioneffectoncalcite;thiseffectisalsoaccentuatedbyhighsulphate(Burton,1993).1193
ModerndolomitehasnotyetbeenrecognizedinAntarctica,yetitiscommoninthe1194
Wilsonbreenfacies,apparentlyaidedbyfactorssuchasevaporationandfluctuatingredox,and1195
possiblyalsothewidespreadoccurrenceofdolomiterockflourasnuclei.1196
Intheprevioustext,extensiveevidencehasbeenpresentedforthedevelopmentof1197
stablemineralogyintheenvironmentbyprimaryprecipitationorearlydiagenesis,astability1198
favouredbylowenvironmentaltemperatures.Inturnthisarguesfortreatingthestableisotope1199
dataasdirectindicatorsofdepositionalconditions.Giventheverywiderangeofδ18Ovalues,1200
theeffectsofvariationindepositionaltemperature,orminorpost-depositionalexchange,are1201
53
minimized.Carbonisotopesareinanycaseresilienttosecondaryalteration(Banner&Hanson,1202
1990).AsynthesisoftheWilsonbreencarbonatesinrelationtothestableisotopefieldsisgiven1203
inFig.23.Theδ13Csignatureislinkedtoacquisitionofcarbonfrombedrockandoxidationof1204
organicsourcesandvariableequilibrationwithatmosphericcarbondioxide,whilstvariabilityin1205
δ18Oisinterpretedasduetomixingofmeltwatersordifferentcompositionsofprimary1206
meltwaterscoupledwithevaporation.WenowdiscusstheseasfactorsintheDryValley1207
context.1208
Regardingcarbonisotopes,intheDryValleystheδ13Ccompositionofacarbonatesource1209
isnotwelldefined,butapotentialorganicsourceisfoundintheformofdark-coloured1210
cryoconiteholes,whichperiodicallyareflushed,providingnutrientsforephemeralstreamsand1211
lakebasins(Bagshawetal.,2013).Additionofrespiredcarbonfromthissourcecanbeseenin1212
thechemistryofsomeAntarcticstreamswithδ13Cvaluesaslightas-9.4‰(Lyonsetal.,2013).1213
Mostvaluesare-3to+2‰,possiblyreflectingcarboncontributionfromacarbonatesource1214
rock(Leng&Marshall,2004)andrangingupto5‰whichLyonsetal.(2013)attribute1215
qualitativelytoprogressiveequilibrationwithatmosphericCO2.Chemicalequilibrationbetween1216
atmosphericCO2andanAlpinemeltstreamwasshowntobeattainedwithinafewhundred1217
metresofflow(Fairchildetal.,1999),althoughthissituationfallsshortofisotopicequilibration,1218
requiringmoreextensiveexchangeuntilthelargegaseoussourceofcarbondominates1219
(Fairchild&Baker,2012,chapter5).Moreefficientprocessesformodifyingδ13Carefoundin1220
DryValleylakes,whereverticaltrendscausedbywater-columnphotosynthesisatshallow1221
depths(leadingtopositivevaluesofδ13C),organicmatteroxidation(causingadecreaseinδ13C1222
withdepth),andlocalmethanogenesis(releasingCO2withcomplementaryhighδ13Cvaluesat1223
depth)havebeendescribed(Lawrence&Hendy,1985;Neumannetal.,2004).Coldlakesin1224
54
othersettingscansometimesdevelopverylowδ13Cvaluesthroughreleaseofmethanefrom1225
solidhydrates(PropenkoandWilliams,2005).1226
1227 Foroxygenisotopesinstreams,Gooseffetal.(2006)gathereddataonδ18Ovariationfor1228
glacierice,snow,streamsandlakes,althoughinterpretationwascomplicatedbyevidentstrong1229
inter-annualvariations.InTaylorValleythemeancompositionofglaciericeineachsub-basin1230
feedingaspecificlakevariedfrom-21to-33‰andsomesampleswereaslightas-45‰.1231
Streamsamplesvariedfrom-42to-22‰andlayclosetothemeteoric-waterlineupto-32‰,1232
butdivergedfromitathighervalues,reflectingtheconsequencesofevaporation.This1233
correspondedalsotostreamlengthsofgreaterthan2km.Inadditiontosimplesurface1234
evaporation,mixingwithisotopicallyheavyshallowsubsurface(hyporheic)waterwasalsoa1235
factor,indicativeoftheeffectivenessofevaporationofsubsurfacewateratashallowwater1236
table.Furtherisotopefractionationoccurswherewaterresideslongerinsalinelakes.1237
Matsubayaetal.(1979)measuredandmodelledthe18O-enrichmentsinthemostsalineponds1238
intheDryValleyareaandfoundupto20‰highervaluesintheunfrozenDonJuanPondand1239
theeastlobeofLakeBonneycomparedwiththesource.Likewise,Nakaietal.(1975)1240
documenteda23‰variationinδ18Ocompositionofcalciteswiththehighestvalues1241
representingevaporativedepositsonlandsurfaces.Suchpronounced18Oenrichmentsalong1242
streamcoursesandtheca.20‰totalvariationareonlypermittedbecauseofthe1243
meteorologicalfactorsleadingtopersistentlylowrelativehumiditiesof50-60%.1244
Fortheancientcarbonates,itisusefultoconsiderthepresenceorabsenceofcovariance1245
ofδ18Oandδ13C(Figs.7,23).Talbot(1990)compileddatafromavarietyofmodernandancient1246
lakesandfoundthatalackofisotopiccovariationistypicalofopenlakesinwhichcontrolling1247
factorsforthevariationinthetwoisotopesaredecoupled.Thiscanbecomparedwiththe1248
limestonesofFA4andFA5(Figs.7b,23).Conversely,covaryingtrendsinlacustrinecarbonates1249
55
(FA4andFA5dolomites)wereidentifiedasacharacteristicfeatureofclosedlakesandreflected1250
acombinationofevaporationtocauseincreaseinδ18O,andresidencetime,permitting1251
equilibrationwithatmosphericCO2asafirst-ordercontrolonultimateδ13Cvalues.Although1252
δ13Candδ18OdataonAntarcticstreams(Gooseffetal.,1006;Lyonsetal.,2013)wasobtained1253
separatelyandsohasnotbeencross-plotted,theseenvironmentstoocanbeanticipatedto1254
displaycovariation.TheslopesofcovariationinTalbot’s(1990)datawerefoundtobequite1255
varied,withlowerslopesinterpretedasreflectingbroad,shallowlakesinwhichevaporation1256
wouldplayamoreprominentrole.Thedatawereprimarilyfromlimestoneandwater-column1257
precipitates,althoughexampleswereshownwherebenthiccarbonateanddolomitesfittedthe1258
trends.Apartfromattaininghighabsolutevaluesforδ18OinFA3,theWilsonbreenFormation1259
carbonatesshowcovaryingslopesandrangeswithinthosepresentedbyTalbot(1990),1260
providingaconfirmationthattheevaporation-equilibrationexplanationfordatatrendsis1261
reasonable.1262
Firstweconsiderδ13Cdata:wherecovariationsareabsentandatthestartingpointsfor1263
covaration.Theinitialcarbonisotopecompositionofaglacialmeltstreammaynotreflectany1264
atmosphericinfluencebecausemanymeltwatersderivefromicewithlowaircontentandhave1265
PCO2valueswellbelowatmospheric(Fairchildetal.,1994).Themeanδ13Cofdetritaldolomitein1266
theWilsonbreenFormationis+2.4‰(Fig.6B),butwithsignificantlocalvariations(±1.3‰)and1267
additionaluncertaintybecauselimestoneappearstohavepreferentiallydissolvedfromthe1268
matrix.Acombinationofmoderatelypositiveδ13Cfromdetritusandorganiccarboncouldhave1269
fixedthestartingδ13Cvalueofaround+1‰fortheFA2covaryingtrend.Thehigherstarting1270
pointforFA3reflectsthemoreevolvednatureofthesefluidswhichareconsistentlyforming1271
dolomite.ThetrendforFA4andFA5dolomitesstartsatlowervaluesof0to+1‰,withinthe1272
rangeofcalcitesintheselacustrinefacies.Carbonisotopesignatures<+1‰,thatislowerthan1273
56
thoseinfluvialfacies,presumablyreflectsadditionofcarbonfromanorganicsource,butoverall1274
variationsarenotlargeenoughtosuggestaroleformethanogenesis.Intheopenlakesimplied1275
bythelackofisotopecovariationinFA4andFA5calcites,thehighδ13Cvaluescouldreflect1276
eithertheeffectsofphotosynthesisorgreaterCO2-equilibrationwithoutevaporation.Themost1277
extremehighδ13C-lowδ18OlimestonesinFA4arefrommemberW3(Fig.12);theseare1278
intraclasticbrecciasimplyingpossibleexposurewhichwouldhaveaidedatmospheric1279
equilibration.Nodifferenceisnotedbetweenobviouslymicrobialandotherlimestones,a1280
commonpatternintheNeoproterozoic(Fairchild,1991).Lowδ13Cvaluesoccurineachsection1281
studiedlocationandarealmostalwaysstratigraphicallyclosetovaluesthataremuchhigher,1282
possiblyindicativeofchanginglakelevels.TheFA5sedimentsfromW3allhaverelativelylow1283
valueswhichmightreflectarelativelydeepwatersetting,consistentwiththedominanceofice-1284
raftedsedimentationinthismemberinthesouthernsectionsandtheuniversaldisruptionof1285
FA5sedimentsbyslumping.1286
Nowweconsiderthetheoreticalδ13Cend-pointresultingfromequilibration.Carbonate1287
precipitatedfromasolutioninisotopicequilibriumwithatmosphericCO2isexpectedtodisplay1288
δ13Cvaluesheavierthantheatmosphereby10.4‰at0°C(fallingto9.1‰at20°C),as1289
calibratedbytheexperimentalworkofMooketal.(1974).Thelong-term(>108year)δ13C1290
compositionoftheatmosphereshouldshowvariationlargelyinparallelwithoceanwaterwith1291
whichittendstoequilibrate,andoceanwaterinturnhasacompositionconstrainedbythe1292
proportionalburialofisotopicallylightorganiccarbon.Asaresult,short-termvariationscanbe1293
expectedbecauseoffluxvariabilities,asdemonstratedbydirectmeasurementofpast(pre-1294
industrialHolocene)atmospheresfromicecoresshowingarangefrom-6.3to-6.6‰(Elsiget1295
al.,2009).Afurthermass-balanceconstraintisthebulkEarthmeancompositionofcarbon1296
(Berner,2004).Thelattercanbeestimatedfrommantleandmeteoritesamplesasaround-7‰,1297
57
butvolcanicgasesaretypicallysomewhatheavier(Javoyetal.,1986).Anatmospherewitha1298
compositionaround-6‰wouldbeinequilibriumatzerodegreeswithcarbonatesaround+4.41299
‰.1300
ThecarbondioxidelevelintheSnowballEarthatmosphereshouldhaveprogressively1301
risenbecauseofsustainedinputfromvolcanicsourcesandlimitedremoval,mainlyby1302
dissolutionintheoceanwherevergapsintheicecoveroccurred(LeHiretal.,2008).Thelimited1303
opportunitiesforback-exchangefromtheoceansimplythattheatmosphereshouldhave1304
providedagoodsampleofthecarbonisotopecompositionofvolcanicemissions.FortheW21305
data,eachofthefaciesassociationfields(Figs.12,23)topsoutataround3.5to4.5‰whichis1306
closetotheexpectedvaluesforequilibrationwiththeatmospheredominatedbyvolcanic1307
emissionsasdiscussedabove.MemberW3dolocreteshaveδ13Cvalues1‰higherthan1308
consideredsofar(Fig.12),butthisdifferenceisdifficulttointerpretwithoutanoveralldata1309
trend.Onepossibilityisthatthereisalocalcontributionbyfreezing,whichLyonsetal.(2013)1310
invoketoexplainδ13Cvaluesintherange+5to+12‰oncarbonate-encrustedrocksontheDry1311
Valleys’landsurface.1312
Wenowfocusattentiononδ18O,startingwiththerangeofvaluesincovaryingtrends.1313
Therangeofδ18OinFA2limestonesisratherlessthanobservedintheDryValleystreams,1314
demonstratingthefeasibilityofevaporationasadriverforvariability,althoughsomevariation1315
insourcewatercompositionisalsopossible.SimilarremarksapplytothedolomitesofFA4and1316
FA5,althoughlargelyopenlacustrineenvironmentssuchasthesearenotfoundintheDry1317
Valleys.TheslopeofcovariationismuchlowerfortheFA3thanfortheotherfaciesassociations,1318
whichisconsistentwithasurficialoriginleadingtomoreefficientevaporation(Talbot,1990).1319
Allowingfora3‰offsetbetweendolomiteandcalcite(Land,1980),theevaporativetrend1320
(Figs.7B,12)extendsbeyondthecompositionoffluviallimestonesbyafurther11‰for1321
58
dolocretes(Facies3D)and15‰forstromatoliticlaminites(Facies3S).Evaporationofcoastal1322
seawater,undertypicalhighhumidityconditionswouldcauseanincreaseinδ18Oofatmost61323
‰,whereas17‰increasefroma-10‰startingpointwasobservedinafreshwaterTexan1324
pondunderconditionsoflessthan50%humidity(Lloyd,1966).TheextremeenrichmentsinFA31325
stromatoliticcrusts,requiresimilarlylowhumiditiestothisexampleandtheDryValleys,and1326
couldonlybepossibleforfaciesdevelopedatthelandsurface.1327
TheFA4andFA5lacustrinecalciteshavelowerδ18OcompositionsthanthoseofFA21328
fluviallimestones.Byanalogywithmodernenvironments,thisislikelytoreflectalocal,low1329
altitudesourceofwaterforFA2,whereasthevariabilityinδ18OwithinFA4andFA5couldreflect1330
varyingmeltwatersources,includinglargeglacierswithlowδ18O.Therangeofδ18Oisactually1331
ratherlessinFA2thanintheDryValleystreamsandgiventhatthevaluesaretypicallyheavier1332
thanthelakes,arelativelylocal,low-altitudesourceformeltwaterisimplied.FortheCarbonate1333
LakeMarginFA4,theδ18OvaluesaresimilartoCarbonateLakeFA5andthereisalackof1334
isotopiccovariationwithbothsetsofdata.Thiswouldimplyanopenlake(Talbot,1990),but1335
careisneededwithsuchaninterpretationbecausethedatarepresentseveraldifferentlakes1336
thatformedsuccessively.Asderivedearlier,therangeofδ18Owatervaluesimpliedfromcalcite1337
precipitationat0°Cisaround-8.5to-15.5‰ontheV-SMOWscale.Thisrangemightbe1338
explainedbyvariablemixingoflocalsnow,localglaciericeandmeltfromlargerorhigher1339
glaciers,byanalogywiththeDryValleyregion.1340
TheultimatedriverforRayleighfractionationintheatmosphere,whichleadsto18O-1341
depletedvaluesinatmosphericprecipitation,ispartialcondensationandremovalofvapourasa1342
resultofafallintemperatureoftheairmass.InferredWilsonbreenmeltwaterδ18Ovalues1343
higherthanthoseinthemodernDryValleysimpliesalesserdegreeoffractionation,as1344
originallynotedbyFairchildetal.(1989).Acomparablemodernglacialareaforthe1345
59
Wilsbonbreenintermsofδ18OistheVatnajökullicecapofIcelandbelowwhichRobinsonetal.1346
(2009)foundarepresentativeice-andsnowmeltcompositionof-12‰.Notethatindirect1347
evidenceformoreisotopicallylightNeoproterozoicmeltwaterhasbeenfoundintworecords1348
fromSouthChina(Zhao&Zheng,2010;Pengetal.,2013),althoughthesedonotnecessarily1349
relatetoapanglacialandintheformercaseisayounger,Ediacaranglaciation.Amorepertinent1350
recordisthatofKennedyetal.(2008)incalcite-cementedMarinoantidalsiltstonesofSouth1351
Australiawherecalciteaslightas-25‰wasanalyzed,andinterpretedtoreflectinputof1352
meltwaterfromhighlyfractionatedlow-latitudeicesheets,althoughtheremaybeother1353
possibleinterpretationsofthesedatabearinginmindtheactivedecompositionofclathratesat1354
thissite.Insummary,thedistinctivelyheavyisotopicsignatureofWilsonbreenmeltwatermay1355
beacharacteristicfeatureoflow-latitudeglaciationandclearlyrequiresstudybyanisotope-1356
enabledgeneralcirculationmodel.1357
13581359
CONCLUSIONS1360
1361
1.CarbonateintheWilsonbreenFormationconfirmsthenon-marineenvironmentof1362
depositioninaglaciatedbasinsuppliedwithdebrisfromerosionofplatformcarbonates.The1363
consistentlydolomiticnatureofthedetritalmatrixofWilsonbreenFormationsediments1364
contrastswiththecommonpresenceoflimestoneincoarserdebrisanddemonstratesthe1365
preferentialdissolutionofcalciteoverdolomite.Carbonate-richmeltwaterswerethusableto1366
precipitateCaCO3oncesupersaturationwasachievedbyprocessessuchasphotosynthesisor1367
evaporation.1368
2.Evidencefromphysicalsedimentarystructuresallowsfourfaciesassociationstobe1369
distinguishedinwhichcarbonatewasprecipitated,distinctfromthreefaciesassociations1370
60
dominatedbyglacialandperiglacialprocesses.TheFluvialChannelFaciesAssociation(FA2)1371
commonlycontainsmicrobiallimestonelaminaewithmm-scalelaminationandnotablesyn-1372
depositionalradiaxialcalcitecements.Thesecomparephysicallywithmodernmicrobialmatsin1373
ephemeralAntarcticstreams.TheDolomiticFloodplainFaciesAssociation(FA3)consistsofsoil1374
zone/playasurficialdolocretesanddolomiticstromatolitesinwhichdolomitewasprobablya1375
primaryprecipitate.TheCarbonateLakeMarginFaciesAssociation(FA4)typicallydisplays1376
micriticandlocallymicrobiallaminaeinterlaminatedwithwave-sortedsiltsandsandswithlocal1377
developmentofintraclasticbreccias.Finally,theCarbonateLakeFaciesAssociation(FA5)1378
displaysmm-scale(varved)alternationsofmicriticcarbonateorlaminatedmicrobialcarbonate1379
(includingmicrospariticandfenestrallaminae)withpoorlysortedsedimentcontaining1380
recognizableice-rafteddebris.LocallyinmemberW2,andpervasivelyinmemberW3,partly1381
lithifiedsedimentsweredisturbedbyslumpfoldingandlocallytransformedtocarbonate-rich1382
debrisflows.1383
3.InbothFA4andFA5,therearetexturalindicatorsofmineralogicalreplacements.1384
DolomitecanbeseentoreplaceaCaCO3precursor.Althoughsomecalciteislikelytobe1385
primary,calcitepseudomorphsafterikaite(CaCO3.6H2O)arecommon.Theikaiteformed1386
individualcrystalswithinthesediment,formedcrustswhichgrewcentripetallyintopores,and1387
locallygrewupwardsatthesediment-waterinterface.Thisparagenesisisnowbecomingbetter1388
knowninmoderncoldlakes.1389
4.Stableisotopedatademonstratethatcarbonatesinthedifferentfaciesassociations1390
formdistinctfieldswhichareallinterpretedasconsistentwithprimarydepositionalconditions.1391
LimestonesinFA4andFA5lackδ13C-δ18Ocovariationandwereprimarilyinfluencedbymixing1392
ofmeltwatersources,thevariableadditionoflightcarbonfromorganicdecomposition,and1393
somere-equilibrationwiththeatmosphere.Othersub-setsdemonstratecovariation,1394
61
interpretedasacombinationofevaporationandequilibrationwiththeatmosphere.Thisallows1395
theδ13CcompositionofCO2releasedfromvolcanismduringtheglaciationtobeconstrainedto-1396
6to-7‰.Directevidenceofprimaryfluidcompositionsisunavailablebecauseofsecondary1397
fluidmigrationintoinclusions,despitethepresenceofprimarytraceelementgrowthzones.1398
Nevertheless,theverywiderangeofδ18Ovaluesmustbeprimarilyrelatedtochangesinwater1399
composition,giventheconsistentlycooldepositionalconditions.Theexceptionallyhighδ18O1400
signaturesofFA3dolomites,upto+14.7‰VPDB,attesttothehyperaridityoftheenvironments.1401
Conversely,theinferredδ18Ocompositionsoftheinputmeltwaters(-8to-15‰VSMOW)aremore1402
comparabletomodernIcelandthantopresent-daypolarregions.Thisislikelytoreflect1403
relativelylimitedRayleighfractionationintheatmospherebecauseofitsrelativewarmthlinked1404
toenhancedabsorptionofinfra-redradiationfromhighCO2levels.1405
5.Althoughpreferredfaciestransitionsoccur,thereisnodevelopmentoverallofmulti-1406
faciescyclicity.Thestrongisotopiccovariationsassociatedwithclosedlakesandstreams,and1407
rhythmiccarbonatelaminaearestrongmotifsofnon-marinefacies.Intheseenvironments,the1408
landscapewasrepeatedlytransformedbydamminganddrainingoflakesastheglaciers1409
advancedandretreated.Inturnthesearelikelytohaverepresentedamplifiedgeomorphic1410
responsestosubtleclimaticshiftsinapersistentlyhyper-aridsettingforwhichtheMcMurdo1411
DryValleysprovidearichmodernanalogue.AlthoughWilsonbreenlakeswerenotperennially1412
ice-coveredasinthemodernenvironment,manypointsofsimilaritybetweenthemodern1413
Antarcticandtheancientenvironmentshavebeendrawninthisstudy,specificallyincludingthe1414
ratesandstylesofsedimentdeposition,biogeochemistry,andextreme18O-enrichmentrelated1415
tothehyperaridclimate.1416
6.ThecarbonatesoftheWilsonbreenFormationaredistinctiveandincludeunique1417
faciesandrecord-breakingisotopecompositions.Theyrepresent,alongwithinterbedded1418
62
diamictites,thecomplexenvironmentalresponsetochangingratesofaccumulationand1419
ablationforcedbyaseriesofprecessionalcycleslateintheevolutionofaSnowballEarth.1420
1421
1422
ACKNOWLEDGEMENTS1423
1424
ThefieldworkandsubsequentanalyseswerefundedbyNaturalEnvironmentResearchCouncil1425
grantGR3/NE/H004963/1withinprojectGAINS(GlacialActivityInNeoproterozoicSvalbard).1426
WearegratefultotheNorwegianauthoritiesinSvalbardandthelogisticaldepartmentofthe1427
UniversityinSvalbardinfacilitatingourfieldwork,andothermembersoftheGAINSteamfor1428
theircollegialityandscientificzest.TheseniorauthorrecordshispersonalappreciationtoPaul1429
Hoffmanforhisinspiration,andsupportforthisprojectatacriticalstage,toMartinKennedyfor1430
hisfriendship,andrigorouslycriticalfeedbackonNeoproterozoicdataandideas,andtoTony1431
Spencerforhisinsight,andcarefuleditorialcommentsonadraft.Journalreferees,including1432
AlexBrasier,gaveinvaluableadvicethatledtoarestructuringofthepaperandsharpeningofits1433
arguments.1434
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73
Table1:NeoproterozoicchronostratigraphyandNESvalbardlithostratigraphy(afterHalversonetal.,2007,Halverson,2011,updatedbyunpublisheddata).Glacialunitshighlightedinred.ThispaperdealswiththeWilsonbreenFormation,representingtheyoungerofthetwoCryogenianglaciations.
GeologicalSystem Group Formation Member
Thick-ness(m)
Lithologies Interpretedenvironment
Ediacaran
Polarisbreen
DracoisenD4toD7 265
Sandstonesandmudstones(D4andD6);dolomiteD5andD7 Playas(D4toD6);Coastal(D7)
D1toD3 200Capcarbonate(D1)transitional(D2)toblack
shaleD3) Transgressivecoastal(D1)tooffshore(D2-D3)
Cryogenian
Wilson
breen
W3(Gropbreen) 65-95
Diamictitesandsandstoneswithminorlimestone Glacilacustrine;subglacialatbase
W2(MiddleCarbonate)
20-30
Threemainintervalsofcarbonate-bearingsandstonesandsiltstoneswithintervening
diamictites
Carbonateintervalsarefluvialandlacustrinewithglacialinfluence;diamictitesare
glacilacustrinerainoutdeposits
W1(Ormen)
55-85
Brecciatedunderlyingdolomitelocallyoverlainbysandstonesandconglomeratespassingupintodiamictitesandsandstones
withlocalrhythmites
Basalperiglaciatedsurface,locallysucceededbyfluvialdeposits,thenglacilacustrinerainout
depositsandsedimentgravityflows
Elbobreen
E4(Slangen) 20-30 Ooliticdolomite Regressiveperitidal
E3(Macdonaldryggen)
200 Finelylaminateddolomiticsiltyshale Offshoremarine
E2(Petrovbreen) 10-20 Dolomiticdiamictites,rhythmitesandconglomerates
Glacimarine
(basetobedefined)
E1(Russøya) 75-170 Dolomitesoverlainbylimestonewithmolartoothstructure,blackshaleanddolomite
Shallowmarine
Tonian
Akadem
iker-
breen
Backlund-toppen
530 Limestoneanddolomite Carbonateplatform
Draken 350 Intraclasticdolomite Peritidal
Svanberg-fjellet
425 Limestoneanddolomite Carbonateplatform
Grusdiev-breen
700 Limestoneanddolomite Carbonateplatform
74
75
Table2.SummaryofcharacteristicsofcarbonatesinmembersW1andW3.Analysesareofcalciteexceptwhereitalicized(dolomite).s=standarddeviation;n=numberofsamples.
Section
Member,mabovememberbase(frommembertop)
Faciesassociation Lithology
δ18O δ13C
mean s mean s n
AND W330(-15)5
Intraclasticstromatoliticrhythmites(carbonatelayers5-10mmthick)withinterveningdiamictite.Somecalcite-filledpseudomorphs -9.22 0.20 0.27 0.33 5
AND W338(-7)5
Stromatoliticlimestone(laminae5-10mmthick)withdiamictitelaminae,variablybrokenupwithindiamictite.Observedover100mlaterally. -8.16 0.55 0.23 0.70 6
AND W340.5(-4.5) 5 Brecciatedlimestone - - - - -REIN W356(-6.5) 5 Limestonewithscatteredsand -8.92 - -0.44 - 1
KLO W325(-18) 5 Carbonatesinterlaminatedwithdiamictiteatbaseofbed.EFK15closelyresemblesthemiddlecarbonatelayeratsectionAND.
-7.97 - 1.83 - 1-5.03 - 1.04 - 1
KLO W328(-15) 5 Laminatedcarbonatesindiamictite.Somecrystalpseudomorphsbothwithinsedimentandgrowingupwards. -8.86 0.4 -0.38 0.1 3
-4.04 - 1.16 - 1
KLO W331(-12)5
Laminatedcarbonatesinice-raftedsedimentswithprominentupwardgrowingcrystalpseudomorphs(indolomite)ofikaite. -9.44 2.38 0.31 1.09 4
KLO W335(-8) 5 Similartohorizon4mlowerinsection. - - - - -
PIN W3n/a(-26)5
Slumpfoldedandbrecciatedthicklylaminatedstromatoliticlimestoneinredsiltysandydiamictites.Traceablelaterallyfor30m
-12.53 0.28 -0.04 0.29 3
ORM W13.5(-20.5)5
Slump-foldedrhythmiteswithdistinctmm-scalestromatoliticlimestonelaminae,somewithbulboustops,separatedbydiamictite. -12.5 - 0.32 - 2
ORM W328(-30)3
Sandstonewithconvolutebeddingwithnodulardolocrete(floatingquartz,micro-nodulesandcracks)alongspecificlaminae. -9.78 - -5.60 - 2
ORM W333(-25) 4Diamictiterestsonmassivestromatoliticlimestonewithpseudomorphsandslumpfolds
orintraclastsovergreenishcross-laminatedsandstonewithooids. -10.76 1.12 2.20 1.56 6
ORM W346.5(-11.5)5
Red,thicklylaminatedandslump-foldedstromatoliticlimestoneindiamictite.Locallymassivewithlotofearlycement. -9.44 2.38 0.31 1.09 4
76
Table3.FaciesandfaciesassociationsoftheWilsonbreenFormation.Thefaciesassociationsare1numberedtorepresentanenvironmentalcontinuumasdepictedinFig.7A.23FaciesAssociation ConstituentFacies EnvironmentalInterpretation1:DeformedDiamictiteDetailedfaciesanalysisinFlemingetal.(2016)
Dominatedbymassivediamictites,locallywithdeformedstratificationwithlenticularsandandgravelbodies(1-5mthick)withdisruptedmargins.Locallyassociatedwithupwards-increasingshearofunderlyingsedimentandstriatedboulderpavements.
Subglacialtillandchannelbodies;glacitectonites.
2:FluvialChannel 2S:Bedsof0.5-4mveryfinetomedium-grained,locallycross-stratifiedsandstonewitherosionalconglomeraticbase.Somevariantshavelow-angleaccretionsurfaceswithsilt-dominatedbeds.Stromatoliticlimestoneiseitherabsentorabundant,formingdmpackageswithmm-cm-scalelaminaeseparatedbysandlaminaeandlaterallyerodedintotrainsofintraclasts.2T:Asabove,butlackingcarbonateprecipitates
Ephemeralstreamchannelsandswithstronglyseasonalflows.Insomecasesthereisawidespreadcarpetofcalcifiedmicrobialmats.
3:DolomiticFloodplain
Allfaciescontaindolomitewithapositiveδ18Ocompositionthatactivelycements,displacesand/orupwardlyaccretesthesediment.3D:Nodules,cm-todm-scaleofdolomite-cementedsiltsandsandstransitionaltostructurelessdm-tom-scalebedswithfloatingsiltandsandindolomite,internalnodulesandcalcite-cementedfractures3S:Stromatoliticdololaminites,dm-scaleandmm-laminated
Dolocretesrepresentingsoilhorizons,typicallyabovefluvialdeposits,withlocallyraisedwatertable3N:Nodulardolocrete3D:Beddeddolocrete3S:Subaerialstromatolites
4:CalcareousLakeMargin
4R:Rhythmiteswithmm-scaledolomite,limestoneormixedmineralogylaminae,commonlydesiccated,alternatingwith1-10mmwavycross-laminatedsiltysandstones.Carbonatelaminaeareoftenstromatolitic.Almostinvariablyreddened.4I:Intraclastic,dm-scale,veryfine-tomedium-grainedsandstones.Verylocallycontainooids.4S:Indistinctlyhorizontallystratifiedwell-sortedfine-tomedium-grainedsandstone,withwell-roundedgrains,locallywithcm-scaledolomitelaminae.
3R:Shallowlaketoplayaenvironmentwithwave-reworkedsedimentalternatingwithmicrobialmataccretion.3I:Discretestormhorizonsinshallowlake/playareworkingsandandstromatoliticcarbonate3S:Aeoliansandflatdeposits
5:CarbonateLake 5R:Rhythmiteswithmm-scalelimestone,dolomiteormixedmineralogylaminae,usuallywithstromatoliticmicrostructureandlocallybuildingdmstromatoliteswithcm-scalerelief.Alternatewith0.1-5mmlaminaeofdiamictite/wacke,commonlywithtillpelletsandoccasionallonestones.Slumpfoldingandbrecciationcommon.5D:Discretegraveltodiamictiteunitswithsignificantintraclasticdebrisinadditiontoterrigenousgravel.
5R:Carbonatemicrobialmatdepositsinlacustrineenvironmentsubjecttoseasonalice-raftingandslopeinstability.5D:Slope-relatedresedimentation.
6:GlacilacustrineDetailedfaciesanalysisinFlemingetal.(2016)
Dominatedbymassiveandstratifieddiamictiteswithcommonlonestonesandtillpellets.Decimetre-tom-scaleintervalsofsiltyrhythmitesoccurlocally,especiallyinmemberW1.Maycontainlensesofconglomerate,sometimeschannelled,andlensesorthinbedsofsandstoneformingpackageswhichcanbeinclinedatupto20°.
Ice-raftedglacilacustrinesedimentlocallyreworkedassediment-gravityflows.Inclinedpackagesformgrounding-linefans(termedproximalglacilacustrineonFig.1)andrhythmiteslikewisereflectice-marginretreat.
7:PeriglacialSeeFairchild&Hambrey(1984)andBennetal.(2015)
Decimetre-widesandstonewedgespenetratingupto2mintounderlyingsedimentfromadiscretesurfacethatmayhaveagravellag.AtbaseofWilsonbreenFormation,gravelwithventifactsoverlyingshattereddolostone.
Exposedperiglaciatedbedrockandsandwedgesfromexposedperiglacialenvironment.
4 5
77
6FigureCaptions7
8
Fig.1.Reconstructionofthesedimentaryarchitectureandpalaeoenvironmentsofthe9
WilsonbreenFormationanditsconstituentmembers(W1toW3).Precipitatedcarbonateis10
presentinthepalaeoenvironmentalgroupcalled“CarbonateLacustrineandFluvial”throughout11
W2(exceptlocallyatthetopandbase),alsoinW3andatoneofthelocationsinW1aslistedin12
Table2.TheSvalbardarchipelagoisshownbottomrightandSpitsbergenisthemainislandofthe13
group,whilstNordaustlandetistheislandtotheNE.TherectangleontheSvalbardmapshowsthe14
studyareaasenlargedupperrightwithWilsonbreenFormationoutcrops(red)withinnunataks15
(grey)risingfromthehighlandsnowfield.Fromnorthtosouth,studylocationsare:DRA16
(Dracoisen);herethemainsectionislocatedonanunatakinformallyknownasMultikolorfjellet17
withsomeadditionalsamplingfromW2atasecondnunatakwetermTophatten1kmtothe18
north;DIT(Ditlovtoppen);AND(EastAndromedafjellet);REIN(aridgeonSouthAndromedafjellet19
informallyknownasReinsryggen);KLO(SouthKlofjellet);withsomeadditionalobservationsfrom20
apartialW2section1kmaway,atNorthKlofjellet;McD(MacDonaldryggen);GOL(Golitsynfjellet,21
intermediatebetweenMcDandBAC)–apartialW2sectionwasillustratedbyFairchildetal.22
(1989);BAC(Backlundtoppen-Kvitfjelletridge);PIN(anunnamednunatakinformallytermed23
Pinnsvinryggen);SLA(SoutheastSlangen)andORM(SouthOrmen).24
25
Fig.2.Examplesofstudiedsectionoutcrops.A.MemberW2atMultikolorfjellet,Dracoisen(DRA)26
illustratingthethreegroupsofglacialretreatfaciesbeds(numbered1to3)separatedby27
diamictiteswhichalsomakeupmembersW1andW3.B.MemberW2atORM(SouthOrmen)with28
palesand-dominatedunitsanddarkredfinerunits.Beddingisinvertedanddipssteeplyaway29
fromthephotographer.C.REIN(Reinsryggen)sectiononthesouthflankofAndromedafjellet30
78
illustratingWilsonbreenFormationmembersoverlainbycapcarbonatememberD1.D.KLO31
(SouthKlofjellet)sectionoftheentireWilsonbreenFormationonsteepslopescutbyminorfaults,32
oneofwhichishighlighted.E.BAC(Backlundtoppen-Kvitfjelletridge)photomontagefrom33
helicopterhoveringabovetheglacierWilsonbreen.Thevisiblesectionisvertical(thrustfault34
shown)andtheaccessiblepartdefinesanarrowridgebetweenthecliffandasnowbankonthe35
ridgecrest.36
37
Fig.3.A.ObliqueaerialviewoftheMcMurdoDryValleys(1/1/1999imageryfromUSGeological38
SurveyviaGoogleEarth)withlocationarrowedininsetofAntarctica(upperright).EAIS=East39
AntarcticIceSheet.LGM=LastGlacialmaximum.Abbreviationsonupperrightinset:EA=East40
Antarctica,WA=WestAntarctica,RIS=RossIceShelf.Thelowerleftinsetshowsanobliqueaerial41
photographlookingwestupTaylorValleywithcold-basedvalleyglaciersonhillsideonleft,ice-42
coveredLakeBonney(valleyfloor,right)withthetipoftheTaylorGlacierbeyond.B.Despitesub-43
zerotemperatures,runoffoccursfromtheLowerWrightGlacier(anoutletglacierofthecoastal44
WilsonPiedmontGlacier)andfeedstheOnyxRiver.Thisstreamflowsinlandtothewest,45
eventuallytoLakeVanda,visibleinA.inthecentralpartofthevalley.Notetheaeoliansands46
bankedagainstheglacier.C.ObliqueaerialviewofVictoriaLowerGlacier(EendofVictoriaValley)47
whichfeedsastreamflowinginland,westwardstoLakeVida,seeninthedistance.Aeoliandunes48
tranversetothestreamarevisibleonright-sideofthethevalley.49
50
Fig.4.ProfileofmemberW2atDracoisen.Thelithologicallogemphasizesphysicalcharacteristics51
(seekey,upperright)whilsttheassignmenttofaciesassociations(centre)alsodrawson52
petrologicalandstableisotopeinformation.FA1toFA6representanenvironmentalcontinuum53
(cf.Fig.6)andFA7isgroupedwithFA1astheterrestrialglacialend-member.Eachdatapoint54
79
representsthestratigraphicpositionofastudiedsampleorobservedlithologicalboundaryinthe55
field.Theoxygenisotopecompositionofprecipitatedcalcitesanddolomites(thelatteradjusted56
by-3‰)areshown.Mixed-mineralogysamples(<90%calciteordolomiteincalcite-dolomite57
mixtures)arenotplotted.Thesameconventionsareusedforprofilesoftheotherstudied58
sections,whicharepresentedassupportingfigures(Figs.S1-S6).59
60
Fig.5.Detritaltexturesandminerals.A.andB.PairedimagesofapolishedthinsectionunderCL61
andintransmittedlightrespectivelyofthematrixofasiltysandstone(W2,81.2m,Dracoisen)62
illustratingtheabundanceoffaintblue-luminescingfeldsparandvariedfragmentsofbothbright63
anddullred-luminescingdolomiteinthemudfraction.C.Thinsection,transmittedlight.Siltstone64
gradedrhythmites(W3,60.4m,EastAndromedafjellet)displayingseveralsand-sizedtillpellets.D.65
Crossedpolars.Matrixatthetopofagradedsiltlayerillustratingquartzo-feldspathicdebrisand66
micron-sizeddolomite,butnoclayminerals(W1,10mSlangen).67
68
Fig.6.Summarycartoonoffaciesassociations.FaciesAssociations2to5arecolour-codedhere69
andinisotopeplots.Themarginofanicesheetisdepicted,terminatingpartlyonlandand,inthe70
foreground,inthelake.FA1isshowninasubglacialenvironment(subglacialsedimentsare71
colouredbrown);periglacialphenomenaarealsogroupedinthisfaciesassociation.FA2andFA372
bothoccurinafluvialsetting(sedimentscolouredyellow),whilstFA5andFA6weredepositedin73
lakes(sedimentscolouredlightbrown).FA4includesbothshallowlacustrineandcoastal74
sediments.75
76
Fig.7.A.Summaryofstableisotopecompositionsofprecipitatedcarbonate,togetherwiththe77
meancompositionofdolomiticdetritus.Dolomiteandcalcitegroupingsareseparatedandonly78
80
sampleswith>90%ofeitherdolomiteorcalciteinthecarbonatefractionareplotted.B.Stable79
isotopefieldsillustratingdegreeofcovarianceofthesamplegroups.80
81
Fig.8.Faciesassociation2(FluvialChannel).A-FareFacies2S(W2,Dracoisen,ataroundthe60m82
level),whereasGandHarefacies2T.A.Sandstonewithmicrobiallaminitesandlow-domed83
stromatolitesvariablybrokenintointraclasts.B.Stainedthinsectionintransmittedlightof84
stromatolitemicrostructurewithmicrite(M),microspar(S),fenestral(F)anddetrital(D)laminae.85
C.Stainedthinsectionintransmittedlightshowingthebrokenedgesofstromatoliticintraclasts86
withradiaxialcalcitecementcrustsinacalcareoussandstonematrix.D.Cross-stratifiedsandstone87
(set30cmhigh)withstromatoliteintraclastsoverlainbycurrentrippleforms.E.Polishedrock88
slicewitharrowdenotingmicromilledtraverseshowninF.F.Micromillisotopetraverseacross89
twomicrite/microsparlaminaandacentralzoneofcalcitesparwhichhasamuchlowerisotope90
signature.G.Sandstonebody(withpebblybase)showingaccretionarysurfaces(e.g.dashedline).91
Palaeohorizontalshownbysolidblackline.(W2,Ditlovtoppen,119m).H.Photomontageof92
tabularsandstoneunitofFA2withapebblehorizonnearitstop(arrowed).Itrestserosivelyon93
floodplain(FA3)siltsandisoverlainbyredlakemargin(FA4)sediments;ruleris25cmlong(W2,94
Dracoisen,83m).95
96
Fig.9.PhotomicrographsofstromatoliticlimestonesfromFA2.A.Pairedtransmittedlight97
(left)andCL(right)micrographs.Stromatoliticlaminaeoforange-luminescingmicrospar(MS)with98
subhedralauthigenicquartzandclasticlayer(M)includingquartzandfeldspargrains(thelatter99
luminescesdarkblue).Largefenestraisfilledbyradiaxialcalcite(R)seeninbothtransverseand100
basalsectionsanddisplayingbrighterearliergrowthanddullerlatergrowth.Thesefabricsarecut101
byavein(V)filledwithbrighttodullluminescingcalcite.W2,Dracoisen,58.5m.B.Stainedthin102
81
section,transmittedlight.Alternatingmicrite,microsparandclasticlaminaewithprominent103
irregularvertical“filamentous”structureofclearcalcite.W2,Dracoisen,58.5m.104
105
Fig.10.FA3(Dolomiticfloodplain).Facies3DisshowninA-D,Facies3Sintheothers,andboth106
faciesinE.AlloftheFacies3SimagescomefromW2,Dracoisen,70m.A.Nodulardolocretewith107
calcite-linedvugsinsiltstonewithscaleinmm(W2,EastAndromedafjellet,35m).B.Stainedthin108
sectioninplanepolarizedlightshowingmatrix-supportedfabricofdolomitecementationofsandy109
siltstone.Dolomicrosparlinesafenestrawhichisoccludedbycalcite.W2,Dracoisen,70m.C.Thin110
sectioninplanepolarizedlight.Grain-supporteddolomicritecementofsiltysandstone.The111
dolomitehasaδ18Ocompositionof+2.7‰andhasauniformtextureincontrasttoclastic112
dolomiteofFig.4D.W2,SouthKlofjellet,57m.D.Displacivedolomitecementsupportingsiltand113
sandgrains.Well-developedstructureofdarknoduleswhichshowdifferentCLcharacteristics114
fromsurroundingdolomitefromwhichtheyareseparatedbycurvedcracks.W3,SouthOrmen,78115
m.E.Stainedthinsectionofinterlaminateddolomite-cementedsandandmicrobiallaminaewith116
fenestrae,someoccludedbyferroandolomite(turquoisearrow)orferroancalcite(purplearrow).117
F.Stainedthinsectionillustratingsimilarfabricto(D.),butwithcalcitecementationofcracksand118
largerpores(W2,Dracoisen,83m)G.Fieldphotographoftexturedbeddingsurfaceof119
dololaminiteidentifiedasmicrobialmattexture(W2,Dracoisen,70m).H.Polishedrockchipof120
microbialdololaminiteswitharrowmarkingpositionof5.2mmmicromilltraverse(showninJ.121
below).I.Microbiallaminitesdrapingdownwardsintounderlyinglaminatewhosebrecciationis122
attributedtoevaporitedissolutioncollapse.Outlinedruleris20cmlong.J.Stableisotopeprofile123
(in‰withrespecttoV-SMOW)ofmicrobialdololaminitesalonglineillustratedinH.Theisotopes124
covaryoveramagnitudeof6‰forδ18Oand1‰forδ13C.125
126
82
Fig.11.FA3(dolomiticfloodplain).A.Transmittedlight,stainedthinsection.Sandydolocrete(FA3)127
containingequantnodulecementedbyferroansaddledolomite(turquoise),withlocallatecalcite128
(red),interpretedasafillofasmallanhydritenodule.W2,Backlundtoppen-Kvitfjelletridge,74.7129
m.B.Facies3Sstromatolitewithfenestrae.Pairedtransmittedlight(left)andCL(right)images.130
Brightlyluminescingdolomitemaybeprimaryoranearlyreplacementofaprecursor.W2,131
Dracoisen69.95m.C.Pairedtransmittedlight(left)andCL(right)images.Dolocreteshowingvery132
fine-grainedquartzsandgrainsfloatingindolo(micro-)sparwithcrystalsdisplayingacommon133
zonationofbrighttodullCL.Displaciveprimarydolomitegrowthisthepreferredinterpretation.134
W2,Ditlovtoppen,118.5m.D.Pairedtransmittedlight(left)andCL(right)images.Dolocrete,135
similartoFig.10D,F,withsparsefloatingquartzandfeldspar(blackandbluerespectivelyinCL)136
andcalcite-filledcracks(centre)andpores(base).Uniformlyluminescingdolomicritecrystals,differ137
inbrightnesswithinnodulespresumablyformingatdifferentstages.Calcite-filledporesshowCL138
zonation(base)ornoCL(cracks,centre).W2,Dracoisen,82.9m.139
140
Fig.12.Stableisotopeplot,differentiatingfacieswithinFA3and(inpurpleandlargersymbols)141
samplesfrommemberW3.142
143
Fig.13.FA4(CalcareousLakeMargin).ImagesA,B,DandH-JareFacies4R;C,EandGareFacies4I144
andFisFacies4S.A.Laminatedrhythmiclimestonesandsiltysandstoneswithconspicuous145
isolatedwaveripplestructureincentreofview.W2,Dracoisen,90m.B.Sand-richexampleof146
facies4Rwithcross-laminatedsiltysandsanddolomiticrhythmites,inpartdesiccatedoreroded147
toformintraclasts.Whiteareasneartoparemineral(probablesalt)pseudomorphs.W2,148
Ditlovtoppen,109m.C.Waverippleswith15cmwavelengthonbedtopW2,Dracoisen149
(Tophatten),approximatelyequivalenttothe87mlevelonFig.4.D.Carbonaterhythmitesurface150
83
cutbydesiccationcracksandbearingsaltpseudomorphs.W2,Reinsryggen,82.5m.E.Stainedthin151
section,planepolarizedlightofooliticintraclasticsandstone.Ooidsarebimineralic(calciteand152
dolomite)withdominantconcentricstructure.W3,SouthOrmen,84m.F.Partofscannedthin153
sectionintransmittedlight.Well-sortedquartzosesandstonewithverywell-roundedgrainsand154
low-anglelaminationmarkedbysubordinateinterstitialdolomite.W2,Ditlovtoppen,114.2m.G.155
Stainedthinsectionintransmittedlight.Sandstonewithconspicuousstromatoliticrhythmite156
limestoneintraclasts.W2,Backlundtoppen-Kvitfjelletridge,76.5m(supportingFig.6).H.Two-157
metrehighsectionatofDitlovtoppen(108.5-110.5onsuppl.Fig.1)showingpoorlystratified158
sandstonebed(Facies4S)overlyingFacies4Rwithseveraldiscretegradedintraclasticsandstone159
beds(Facies4I).I.Polishedslabofdolomiticrhythmiteswithdesiccationcracks(C).The3.2mm-160
longmicromillisotopetraverseofJisindicated.W2,SouthOrmen,31.3m.J.Isotoperesultsfrom161
themicromilltraversewithlighterisotopevaluescorrespondingtodetritaldolomitematrixand162
theheaviestvalues(atright)indicativeofprecipitateddolomitecomposition.163
164
Fig.14.FA4illustratingcontrastbetweendetritalandreplacivedolomite.AandB.Paired165
transmittedlightandCLimagesrespectively.Enlargementoftheboundarybetweenadolomicrite166
lamina(below)andadetritallamina(top)ofthesamesampleasinFig.13I,J.Thedetritallayer167
showsquartz(blackinCL),feldspar(blue),adolomiteclast(brightring)whilstthedolomicrite168
showsaconsistentzonationofcrystalswithabrightcoreaswellassomefinesilt-sizedsiliciclastic169
debris.Thedolomicriteisinterpretedasanearlydiageneticreplacementofanearlycarbonate170
phase.W2,SouthOrmen,31.3m.171
172
Fig.15.FA5(CalcareousLake)inmemberW2.AllillustrateFacies5R,buttransitionstoFacies5D173
areshowninDandE.A.Thinsectionintransmittedlight.Calcareousrhythmiteswithsubordinate174
84
clasticsedimentincludingtillpellets(e.g.yellowarrows).Whiteareas,1-2mmacross,aremineral175
pseudomorphs.Reinsryggen,81m.B.Calcareousrhythmiteswithirregularlaminatopsindicative176
ofmicrobialstructureandgrowthdomesabovevuggyareaswithsyn-depositionalcalcitecements.177
Notescaleinmm.Reinsryggen,80.5m.C.Sawnandpolishedhandspecimenillustratingagrowth178
faultacrosswhichthestratigraphyofstromatoliticrhythmitelayerschanges.Notescaleincm.179
EastAndromedafjellet,30.3m–notethisisathinrhythmiteoccurrencewithindiamictite180
(supportingFig.1).D.Stainedthinsectionintransmittedlight.Dolomitedropstone(d)deforms181
underlyinglimestonerhythmiteandliesatthebaseofacoarserresedimentedlayerincluding182
limestoneintraclasts.Ditlovtoppen,108m.E.Lowerhalfofadiscrete40cmdiamictitedebrisflow183
unitwithrhythmitesdeformedbyslumpingatthebase.Numerals1cmapartontape,lowerright184
corner.Ditlovtoppen,109m(wedgingoutover100mtothesectionshowninsupportingFig.1).F.185
Stromatoliticrhythmites,alternatelypurewhiteandimpuresediment-bearinglimestone.186
Backlundtoppen-Kvitfjelletridge,77m.LocationofmicromilltraverseofGillustrated.G.Micromill187
traverseasinFillustratingasystematicshiftinδ18O,butwithinarelativelynarrowrangeof1‰,188
similartorangeofuncorrelatedvariationinδ13C.189
190
Fig.16.FA5(calcareouslake)inmembersW3andW1.AllshowFacies5Rwithvarioustransitions191
toFacies5D.A.Diamictitesoffacesassociation6becominginterlaminated(yellowarrows)with192
limestonerhythmites.Slumpfoldsinupperleft.Lenscapforscale.W3,SouthKlofjellet,116.5m.193
B.Limestonerhytmitesdevelopingslumpfoldsupwardsandtransitioningtoanintraclastic194
diamictite.W3,SouthKlofjellet,116.5m.C.Polishedhandspecimenillustratingerosional195
truncationofstromatoliticrhythmitelaminaebydiamictitewithprominentcm-scalepebbles.W3,196
SouthKlofjellet,120m.D.Thinsectionintransmittedlightofpartlybrecciatedrhythmitewith197
laminaeuptocmscalewithinterstitialice-raftedsediment.Equantwhiteareasafewmmacross198
85
aremineralpseudomorphs.W3,EastAndromedafjellet,63m.E.Slump-folded1-3mmlimestone199
rhythmitelayerswithinterstitialgreenice-raftedsediment.Scaleisincm.Thisistheonly200
precipitatedcarbonatehorizoninW1(SouthOrmen,3.5m).201
202
Fig.17.StromatoliticfabricsinFA5.A.Transmittedlight.Millimetre-scalecalcitelaminites203
separatedbythinnerice-raftedlaminaeandlocallycontainingtillpellets(P).Calcitelaminites204
displaypeloidalclotsandlocalfenestraeandhavevariablybulboustops.W2,Reinsryggen,79.8m.205
B.Transmittedlight.SimilarhorizontoC.displayingclasticlaminaoverlainbyclottedandfenestral206
microbiallaminawithbulboustop.W2,EastAndromedafjellet,13.5m.C.Stainedthinsection,207
transmittedlight.Dolomiticmicrobiallaminatecontainingdolomicriteanddolomicrosparlaminae208
andfloatingdetritus(white).Conspicuousfenestraearefilledbypink-stainedcalcite.W2,South209
Ormen,31.4m.D.Transmittedlight.W2,Reinsryggen,79.8m.Faintlyclotted(peloidal)micrite210
(examplesarrowed)andmicrosparlaminaewithinterveningcalcite-filledfenestra.Darkpatches211
aremicro-tillpellets(somearelabelledP),nowpartlysilicified.212
213
Fig.18.Crystalpseudomorphs.B-GareallFA5.A.PermianglendonitefromSouthAustralia:ikaite214
pseudmorphsthatoriginalgrewinglacimarinemudrocks.Pencilforscale.Sampleprovideby215
MalcolmWallace.B.Transmittedlightviewofinterlaminateddiamictitesandlaminated216
limestones,microbialinpart,anddisplayingtwodistincthorizonsofupward-growingcrystals.217
Crystalsinfluencesubsequentsedimentationpatternandhencegrewintothewatercolumn.W3,218
SouthKlofjellet,116.5m.C.Histogramofapparentinterfacialanglesfromthinsectionsofsamples219
showninB.Modesaremostconsistentwithanikaiteprecursor(seetext)D.Transmittedlight,220
stainedthinsectionofsamesampleasB.Pseudomorphsarecompositeofmosaicsofzoned,221
mostlyferroan(bluish)calcitecrystals.Outeredgesofcrystalshaveinpartbeendissolvedandin222
86
partsilicified.W3,SouthKlofjellet,116.5m.E.Transmittedlight.Calcite-cementedcrystal223
pseudomorphs,morphologyconsistentwithikaite,instromatoliticlimestone.W3,SouthOrmen,224
85m.F.Transmittedlight.Indistinctcalcite-cementedprobablyikaitepseudomorphs,morphology225
probablyconsistentwithikaite,instromatoliticlimestone.W2,Reinsryggen,81.2m.G.226
Transmittedlight.Dolorhythmiteshostingdolomitepseudomorphs,inferredtobeafterikaite,227
withvaryingmicrite-microspar-sparreplacivetextures.W2,NorthKlofjellet,67m.228
229
Fig.19.Wilsbonbreencrystalpseudomorphs(A)comparedwithikaitecrystals(C)and230
pseudomorphsofdifferentagesandcontexts(B,D,E).A.Polishedhandspecimen(samesample231
asFig.16B.Notesmallpinkpebbletoleftindiamictitelayeroverlyingtopcrystallayer.Crystals232
grewupwardsatthreehorizonsandweredrapedbyoverlyingsedimentsbeforebeingreplacedby233
calciteasillustratedinFigs.18Dand20A.W3,SouthKlofjellet,116.5m.B.Examplesof“thinolite”234
crystalsfromDana(1884),interpretedasikaitepseudomorphsbyShearmanetal.(1989).Scale235
notgivenintheoriginal,butcrystalsaretypicallycm-dm-scale.C.Profilesofikaitecrystals236
recoveredfromArcticseaicebypartialmelting(Nomuraetal.,2013).D.andE.ikaite237
pseudomorphsfromaPatagonianlake(Oeherlichetal.,2013).D,illustratescrystalswithequant238
habitandsteppedfacesasseenbyscanningelectronmicroscopy.E.illustratespseudomorphs239
attachedtomossfilaments.240
241
Fig.20.CalcitefabricsofFA5.A.Transmittedlight,stainedthinsection.Enlargementoftheikaite242
pseudomorphsofFig.15Ashowingreliccrystaloutlinesindarkmicriteandreplacivecalcite243
(micro-)sparmosaicofzonedeuhedralnon-ferroancalciteovergrowthbyferroancalcite.W3,244
SouthKlofjellet,116.5m.B.Pairedtransmittedlight(left)andCL(right)images.Fenestral245
microbialfabricsimilartoFig.17Ddisplayingconsistentcrystalzonation:brightertodullerin246
87
micriticmatrixandsharperzoneswithinfenestralcements.Fabricisconsistenteitherwithprimary247
calcitegrowthwithinextracellularpolymericsubstanceorreplacementofikaite,followedby248
cementationoffenestrae.W2,Reinsryggen,80.6m.249
250
Fig.21.Vertical(upward)faciestransitionsinmemberW2.ForthispurposeFA1andFA7are251
conflated.Thewidthofthearrowsisproportionaltothenumberoftransitionsminusone.The252
transitionmatrixshowninsupportingFig.8indicatesthatatleastoneverticaltransitionoccurs253
betweeneachofthefaciesassociationsexceptFA1.Seetextfordiscussion.254
255
Fig.22.Relativethicknessofstratabelongingtothedifferentfaciesassociationsinthefive256
sectionswithcomparablythickcarbonatefaciespreservedinW2,fromsouthtonorth.Seetextfor257
discussionandabbreviationsinFig.1.258
259
Fig.23.Diagrammaticsummaryoftheprocessesresponsibleforvariationinisotopecomposition260
oftheWilsobreencarbonateand,interpretationsoftheirparageneseswith(atbase)acartoon261
environmentalprofile.LargenumbersrefertoFaciesAssociations2to5.262
263
N.B.SupportingInformationissuppliedasafree-standingpdfandanExceldocument;supporting264
figurecaptionsarenotrepeatedhere.265
266