Millennial-scale variability in Late Pleistocene ice-rafting to the South Atlantic Ocean

146 Pages·2002·6.1 MB·English

by  Kanfoush, Sharon L., 1968-

MILLENNIAL-SCALEVARIABILITYINLATEPLEISTOCENEICE-RAFTING
TOTHESOUTHATLANTICOCEAN
By
SHARONL.KANFOUSH
ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL
OFTHEUNIVERSITYOFFLORIDAIN
PARTIALFULFILLMENTOFTHEREQUIREMENTS
FORTHEDEGREEOFDOCTOROFPHILOSOPHY
UNIVERSITYOFFLORIDA
2002
ACKNOWLEDGMENTS
Iwishtothankseveralpeoplefortheirhelpinthisproject,withoutwhomitwouldnot
havebeenpossible.First,IamgratefultoDr.DavidHodellforgenerouslysharinghisknowledge
ofthesubjectmatterandforteachingmearigorous,hencefruitful,approachtoscience.Iamalso
gratefultoDr.PaulCiesielskiandDr.JimChannellforservingonmycommitteeandproviding
insightfulfeedback.IthankDr.ClaireSchelskeandDr.JoannMossaforservingonmy
committeeandprovidingsupportandencouragement.IamgratefultoDr.MikePerfitandDr.
PaulMuellerforthemanyteachingassignmentsthatprovidedinvaluableexperienceaswellas
financialsupport.Iamverythankfultomyparentsforthesupporttheyhavegivenmethroughout
mylifeandespeciallyinacademicendeavors.IamespeciallythankfultoDanforeverything.
IamindebtedtonumerousfriendsandcolleagueswhofacilitatedfieldworkintheSouth
Atlanticandwillinglysharedsamplesanddataaswellastheirexpertise.FeedbackfromDr.
ChristopherCharlesregardingmyinterpretationsofmydataprovedinvaluable.IthankDr.Dietz
WamkeforsharinginsightgarneredfromhisvastexperienceinIRDanalysesandinterpretation.
IthankDr.JosephStonerforthemanyinterestingdiscussionsoutofwhichgrewmanynew
researchideas.IthankKathyVenzCurtis,Dr.EllenMartinandDr.JayeCableforproviding
suchinspirationalrolemodelsforwomeninscienceandfortheirencouragementandsupport
throughoutthisproject.Thefollowingindividualsprovidedassistanceaswellasgoodcompany
inthelaboratory:MikeRosenmeier,Dr.JasonCurtis,RhondaCabanilla,GiannaBrownand
KendallFountain.SamplesforthisstudywereprovidedbytheOceanDrillingProgramonLeg
177andtheprecedingsitesurveycruisewithsponsorshipfromNSF.Thisresearchwassupported
byU.S.ScienceSupportProgramgrantF000849toDavidHodellandme;andNSFgrant
OCE-9907036toDavidHodellandChristopherCharles.
n
TABLEOFCONTENTS
Page
ACKNOWLEDGMENTS ii
ABSTRACT v
CHAPTER
1 INTRODUCTION 1
StatementofObjectives 1
MillennialClimateVariability 1
AntarcticIceSheet 2
PreviousWork 6
Ice-RaftedDebris 6
IRDasanIndicatorofIce-SheetChange 7
SouthAtlanticIRD 10
StudyArea 11
2 THELASTGLACIAL-INTERGLACIALCYCLE 15
Introduction 15
MaterialsandMethods 17
Results 23
LastGlacialPeriod(MarineIsotopeStages2-4) 23
Termination1 29
Holocene(MarineIsotopeStage1) 31
Discussion 34
InterpretationofSouthAtlanticIRD 34
RelationshiptoNADWFlux 44
InterhemisphericLinkage 46
Conclusions 47
3 RELATIONSHIPTOENVIRONMENTALANDCLIMATICPROXIES 49
Introduction 49
MaterialsandMethods 50
Results 51
PlankticOxygenIsotopes 51
BenthicOxygenIsotopes 53
BenthicCarbonIsotopes 54
iii
Discussion 54
Marine-IceCorrelationsUsingStableIsotopeRecords 54
TestingMarine-IceCorrelationsUsingPaleointensity 66
InterhemisphericRelationships 72
Conclusions 79
4 THELASTFOURCLIMATICCYCLES 81
Introduction 81
MaterialsandMethods 82
Results 85
GlacialPeriods 88
InterglacialPeriods 90
GlacialTerminations 94
Discussion 95
Whole-CorePhysicalPropertiesasProxiesofIRD 95
Glacial-to-InterglacialVariability 99
SuborbitalVariability 101
Marine-IceCorrelation 104
Conclusions 109
5 SYNTHESIS 112
SynthesisofLatePleistoceneIce-RaftingandAISDynamics 112
FutureWork 114
InterpretationofIRDLayers 114
IdentificationofSourceAreas 116
PotentialCausesofIncreasedIceDischarge 118
REFERENCES 120
BIOGRAPHICALSKETCH 139
IV
AbstractofDissertationPresentedtotheGraduateSchool
oftheUniversityofFloridainPartialFulfillmentoftheRequirementsfortheDegreeofDoctorof
Philosophy
MILLENNIAL-SCALEVARIABILITYINLATEPLEISTOCENEICE-RAFTING
TOTHESOUTHATLANTICOCEAN
By
SharonL.Kanfoush
August2002
Chairman:DavidA.Hodell
MajorDepartment:GeologicalSciences
VariabilityoficedischargetotheSouthAtlanticOceanhasbeenreconstructedonthe
basisofice-rafteddebris(IRD)concentrationwithinmarinesedimentsatthreesitesalonga
transectfrom41°to53°S,spanningthePolarFrontalZone(PFZ).DiscreteIRDlayersrecuron
millennialtimescalesduringthelastglaciation(marineisotopestages(MIS)2-4).PeaksinIRD
aresynchronousnorthandsouthofthePFZ.Therefore,suchlayersarenottheresultof
temperature-controlledshiftsinthelocusoficebergmelting.Rathertheirsynchroneityimplies
thattheIRDlayersrepresentincreasesinAntarcticicedischarge.
Downcorephaserelationshipsandmarine-icecorecorrelationssuggestthatmillennial
SouthAtlantic(SA)IRDeventsoccuronthecoolinglimbaftermillennial-scalewarmingevents
onAntarctica.SAIRDeventscoincidewithnear-peakwarmthofinterstadialeventsrecordedin
Greenlandice,indicatingananti-phaserelationshipcomparedtoAntarcticice.ComparisonofSA
eventswithsea-levelrecordssuggestsexistenceofsea-levellinkagewherebymeltingof
GreenlandiceassociatedwithenhancedNorthAtlanticDeepWater(NADW)formationand
interstadial-warmthelevatedsealeveland"un-pinned"WeddellSeaiceshelves.
v
Themillennial-scalepulsesinIRDdeliverytotheSouthAtlanticthatmarkedthelast
glaciationarecharacteristicofthelastfourglacialperiods(MIS2-4,MIS6,MIS8andMIS10)
atSite1094(53°S).Suborbitalvariabilityiniceraftingalsocharacterizeseachofthelastfive
interglacialperiods(theHolocene,MIS5,MIS7,MIS9andMIS11).Anearlyhypsithermal
period,characterizedbylowIRDandhighforaminiferalabundance,isfollowedbyareversalof
theseconditionsinterpretedasaneoglaciation.MillennialIRDvariabilitybetweenglacialand
interglacialperiodsisfundamentallydifferentwithrespecttocharacter(magnitudeand
recurrenceinterval)aswellascause.WhereasmillennialIRDeventsofglacialperiodsare
interpretedtorepresenttimesofincreasedAntarcticicedischarge,IRDdepositionduring
interglacialperiodsisinsteadamanifestationofchangesinicebergandseaicesurvivability
controlledbyfluctuationsinsea-surfacetemperatureresultingfromnorth-southmigrationofthe
PFZandwintersea-icemargin.
vi
CHAPTER
1
INTRODUCTION
StatementofObjectives
MillennialClimateVariability
Thediscoveryofrapid(1000-,1450-,and3000-year)climaticoscillationsinthe
Greenlandicecores,theso-called“Dansgaard-Oeschger”(D-O)Events(Johnsenetah,1992;
Dansgaardetah,1993;Grootesetah,1993),markedoneofthemostimportantrecentfindingsin
climateresearch.TherecognitionandcorrelationofHeinrichandhigher-frequencyice-rafting
eventsintheNorthAtlanticprovidedevidenceformillennial-scale,quasi-periodic(-7000to
10000-year,2000to3000-yearand1450-year)instabilityoftheLaurentideIceSheetduringthe
lastglacialperiod(Heinrich,1988;AndrewsandTedesco,1992;Bondetah,1992;Broecker,
1994;Fronvaletah,1995).TheirsubsequentassociationwiththeD-0cyclesspurredanardent
searchforevidenceofsimilarhigh-frequencycyclicitiesinotherclimaticrecordsfromaroundthe
globe(Bondetah,1993;BondandLotti,1995).
Sincetheseinitialfindingsmorethanadecadeago,evidenceofmillennialvariabilityhas
beenfoundinnumerousice-core,marineandterrestrialrecordsfromboththenorthernand
southernhemispheresatperiodicitiessimilartothoseoftheD-0events(PorterandAn,1995;
BehlandKennett,1996;Charlesetah,1996;Blunieretah,1998;Schulzetah,1998)andto
thoseoftheHeinrichevents(Grimmetah,1993;KeigwinandLehman,1994;Lowelletah,
1995)revealingtheirglobalsignature(seeLeuschnerandSirocko,2000forareview).Inaddition
tothevastspatialscaleoftheinferredclimatechanges,whichistoolargetobeexplainedby
localoceanographicormeteorologicalchanges,theapparentmagnitudeofchangeinmany
recordsisalarminglylargeandtherateofchangeoccursextremelyrapidly,ontheorderof
1
2
centennialandperhapsevendecadalscales(Bondetal.,1997,2001;Broecker,1997;Tayloret
al.,1997;Thompsonetal.,1997,1998;Adkinsetal.,1998;SachsandLehman,1999;
SeveringhausandBrook,1999;Petersonetal.,2000;Hendyetal.,2002).
Thetriggeroftheseseeminglyglobalclimateoscillationsremainsunknownandthe
subjectofintenseinterestandspeculation.Onehypothesisisthatchangesinthestrengthof
thermohalinecirculation,changesintherateofformationofNorthAtlanticDeepWater
(NADW),affectedthedeliveryofheattohighlatituderegionsofbothhemispheres(Broecker
andDenton,1989).Alternatively,instabilitiesinherenttolargeicesheetsimpactlocal
oceanographyand/ormeteorologyandsubsequentlypropagateglobally(MacAyeal,1993).Yet
anotherpotentialmechanismisthatchangesinsurfaceoceancirculationmayleadtoincreasesor
decreasesintropicalseasurfacetemperaturesandincreaseordecrease,respectively,watervapor
fluxtotheoverlyingatmosphere(Broecker,1995;Webbetal.,1997)thusinitiatingclimatic
impactsthatarethenpropagatedgloballybysuchmeansasredirectingjetstreamsineither
hemisphere(Cane,1998).
Southernhemisphereclimaterecordscapturingmillennialvariabilityincludebothmarine
sedimentrecordsandterrestrialclimatearchives.AnalysisofSouthAtlanticcoreRC11-83
revealedoscillationsinbenthiccarbonisotopesthatboreastrikingresemblancetoD-0events
recordedinGreenlandice(Charlesetal.,1996).GlacialadvancesinalpineglaciersofChileand
NewZealandwereshowntobecoevalwithpulsesinice-raftingwithintheNorthAtlanticOcean
(Lowelletal.,1995).Theglobalnatureofmillennialvariabilityand,specifically,itspresence
withinsouthernhemisphererecordsraisedthequestionofhow,orif,similarvariabilitywas
manifestedinthedynamicsoftheAntarcticIceSheet(AIS).
AntarcticIceSheet
TheAntarcticIceSheetcomprisestwomorphologicallyandbehaviorallydistinctice-
sheets:thethicker,land-basedEastAntarcticIceSheet(EAIS)andthethinner,marine-based
WestAntarcticIceSheet(WAIS)(Figure1-1).Presentlygroundedwelloverakilometerbelow
3
sealevel,theWA1Sispotentiallysusceptibletosmallperturbationsinsealevel(Hollin,1962;
DentonandHughes,1983;Dentonetal.,1986;AndersonandThomas,1991).Thisinherent
instabilitywasalsoacharacteristicoftheLaurentideIceSheetduringthelastglaciation,which
waspartiallygroundedbelowsealevelintheHudsonBayregion(AndrewsandTedesco,1992;
Marshalletah,2000).Underfullglacialconditions,theWAISwasabouttwo-thirdslargerthan
thepresent-dayWAIS,sealevelwas-120mlower,andgroundediceshelvesinmarginalseas
aroundAntarcticaweremoreextensive(DentonandHughes,1983;Bindschadleretah,1998).
Consequently,analogywiththeLISsuggeststhatthePleistocenehistoryoftheWAISalso
includesmillennial-scaleepisodesofinstabilityandsurging.Similarly,analogywiththeNorth
AtlanticimpliesthatmarinesedimentsfromoffshoreAntarcticashouldrecordsuchepisodesof
ice-sheetinstabilityaslayersenrichedinice-rafteddetritus(IRD).
LatePleistocenehistoryoftheA1Shasimportantimplicationstomodemsociety.Ofthe
modemworldtotaloffrozenfreshwater,morethan90%liesinAntarctica(Meier,1983).The
WAISalonepresentlycontainsenoughicetoelevateglobalsealevelby-5to6mifitwereto
suffercompletecollapse(IPCC,1996).Anthropogenicglobalwarmingduringthetwentieth
centuryisestimatedtobeabout0.6±0.2°Candincreasedheatcontenthasbeendetectedinthe
atmosphere,theworldoceanandthecryosphere(IPCC,1996;Levitusetal.,2001).Inresponseto
increasingtemperature,mountainglaciersaroundtheworldarereportedtohavealreadysuffered
dramaticrecessionandarecontributingtoglobalsea-levelriseatanestimatedrateofabout
0.4±0.2mm/yr(Meier,1984,1993;Dowdeswelletal.,1997;DyurgerovandMeier,2000;Irion,
2001).Anegativemassbalancehasalsobeenreportedforsomemodempolaricecaps.Whether
theresultofmeltingorincreasedbasalcreep,rapidthinninghasbeendetectedinsouthern
Greenland(Davisetal.,1998;Krabilletal.,1999).Likewise,numerouspronouncedchanges
occurredrecentlyintheAntarcticregion,capturingpublicaswellasscientificattention.Recent
changesintheAISincludelargecalvingevents,particularlyfromtheRonne-FilchnerandRoss
iceshelvesandthesmallericeshelvesoftheAntarcticPeninsula(Rottetal.,1996;Frezzotti
4
30°W 0° 30°E
60°
90°
120°
Figure1-1.MapoftheAntarcticIceSheetshowingthelocalitiesandfeaturesmentionedinthe
text.TheTransantarcticMountains(approximatedbythedottedline)separatetheEast
AntarcticIceSheet(EAIS)fromtheWestAntarcticIceSheet(WAIS).Numerousice
shelvesareshown(stippledpattern),thelargestofwhicharetheFilchner-RonneIce
ShelfoftheWeddellSearegionwithintheSouthAtlanticsectoroftheSouthern
OceanandtheRossIceShelfoftheRossSearegionwithinthePacificsector.The
locationsareindicatedoftheVostokIceCoreofEastAntarcticaandtheByrdIce
CoreofWestAntarctica.