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Water--leveleffectsonbenthicforaminiferaloxygenisotoperatiosin
theAtlanticOceanduringtheLGM
11121
ölpel,,,-
1MARUM-CenterforMarineEnvironmentalSciencesandFacultyofGeosciences,
UniversityofBremen,Bremen,Germany
2
SchoolofEarthandAtmosphericSciences,GeorgiaInstituteofTechnology,Atlanta,GA,
USA
Correspondingauthor:AndréPaul(******@)
KeyPoints:
Benthicδ18Oglacial-interglacialchangesoffNorthWestAfricaandintheWest
AtlanticOceanwereinvestigatedwithanoceanmodel
18
Modelresultssuggestsmallerglacial-interglacialδOdifferencesaroundthe
thermoclinelayerresultduetoglacialsea-levellowering
TheexpansionofcoldsouthernsourcewaterduringtheLGMledtolargerglacial-
18
interglacialδOdifferencesinthedeepAtlanticOcean
Thisarticlehasbeenacceptedforpublicationandundergonefullpeerreviewbuthasnot
beenthroughthecopyediting,typesetting,paginationandproofreadingprocesswhichmay
doi:
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Abstract
DepthtransectsofbenthicforaminiferaloxygenisotopesfromtheAtlanticOceanshowthat
glacial-interglacialchangesarelargeratdeep(>~2000m)thanatintermediatewaterlevels.
Ourmodelresultssuggestthatthesmallerchangesintheupper1000mofthewatercolumn
arearesultoftheglacialsea-levelloweringofabout120m,leadingtowarmertemperatures
ofaround1°Candhenceasmallerglacial-
contrast,ashoalingofthewater-massboundaryto~2000mwaterdepthbetweenthe
northernsourceandsouthernsourcewaterisaccompaniedbytheexpansionofacold(close
tothefreezingpoint)southernsourcewaterintheabyssalocean,increasingtheoxygen
explainthedifferentamplitudesofglacial-interglacialstableoxygenisotopedifferencesin
theupperanddeeperwatercolumnoftheAtlanticOcean.
1Introduction
TheclimateoftheLastGlacialMaximum(LGM,19,000-23,000yearsbefore
present(BP)-.,2001)wascharacterizedbyareductioninsea-surface
temperaturesof~2°C(MARGOProjectMembers,2009),tightlylinkedtolowatmospheric
CO2concentrationsof~190ppm(Monninetal.,2001)andassociatedwithlargenorthern
hemispherecontinentalicesheetsleadingtoasea-levelloweringof~120m(Clarketal.,
2009;Fairbanks,1989).Thus,theLGMconstitutesaclimatethatdifferedsignificantlyfrom
,inparticularthe
AtlanticMeridionalOverturningCirculation(AMOC),whichplaysakeyroleinregulating
heattransportespeciallyintheAtlanticrealmandalteringglacial-interglacialglobal
atmosphericCO2concentrations(Brovkinetal.,2007,Ganopolski&Brovkin,2017)by
storingcarboninthedeepoceanforlongtimeperiods.
Sofar,modelstudiesandproxyreconstructionshaveproducedconflictingresults
regardingthestrengthoftheglacialAMOC:someestimateaweakerandshallower,othersa
strongeranddeeperAMOC(Lippoldetal.,2012;LynchStieglitzetal.,1999,2006;
McManusetal.,2004;Otto-Bliesneretal.,2007;Yuetal.,1996).Arecurringhypothesis
emergingfrompaleoceanographicreconstructionsistheshoalingoftheglacialwater-mass
boundarybetweentheNorthAtlanticDeepWater(NADW)andAntarcticBottomWater
(AABW)toapproximately2000mwaterdepth,basedonthedistributionoftheratioof
13
stablecarbonisotopes(δC)inbenthicforaminiferalcarbonate(Curry&Oppo,2005;
Duplessyetal.,1988;Sarntheinetal.,1994).Thisshoalingissuggestedtobeaccompanied
bytheexpansionofacold(closetothefreezingpoint)southernsourcewaterinthedeep
abyssalocean(Duplessyetal.,2002;Labeyrieetal.,1992).Theprocessesbehindthe
rearrangementofthewatermassdistributionduringtheLGMarecontroversial,butmaybe
linkedtotheexpansionofsummerseaicearoundAntarctica(.,2014;
Mackensenetal.,1996;Paul&Schäfer-Neth,2003).Thiswouldalsoexplainthesalt
stratificationaswellasthereversedmeridionalgradientofsaltintheglacialoceanas
demonstratedbyporefluidmeasurements(Adkinsetal.,2002),becausethesoutherndeep
waterupwelledtothesurfaceunderseaice,leadingtohighsalinitiesduetostrongbrine
-massboundaryawayfromthezoneofintensemixingnearthe
seafloormighthaveledtoareducedverticalmixingofthesouthernsourcewaterwiththe
overlyingnorthernsourcewater,whichmayhaveenhancedtheocean´sabilitytostore
carbon(Adkins,2013;Ferrarietal.,2014;Lundetal.,2011).
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However,theinterpretationofthewatermassdistributionbasedonδ13Ccanbe
complicated,duetothenon-
circulation,
1618
theindividualconcentrationsofH2OandH2Oaswellastemperatureareconservativein
calculatetheratioofstableoxygenisotopesinbenthicforaminiferalcarbonate(δ18O)may
c
beamoreaccurateattempttoprovideinformationaboutthewatermassdistribution.
FortheglacialwesternAtlanticOcean,Lundetal.(2011)andKeigwin(2004)
18
showedthatbelow2000mwaterdepththeδOcdifferencesbetweentheLGMandHolocene
18
aredrivenbyalargeincrease(~‰)inglacialδOc,whichmaybelinkedtoa
,becausesimilar
observationshavebeenreportedbeforefromtheIndian(Kalleletal.,1988)andPacific
Oceans(Hergueraetal.,1992).
18
Hereweinvestigatewhethertheincreasedglacial-interglacialδOcanomaliesbelow
2000mwaterdeptharealsorecognizableintheeasternAtlanticOceanandmayaswell
correspondtotheglacialwater-massboundaryconsistentwithotherproxyreconstructions.
Therefore,weanalyzedbenthicδ18OvaluesoffivesedimentcoresoffNorthWestAfrica
c
(NWAfrica)andcomparedtheirdepthprofiletooceanmodelresultsobtainedfromthe
MassachusettsInstituteofTechnologygeneralcirculationmodel(MITgcm)includingstable
waterisotopesaspassivetracers(Völpeletal.,2017).Inthisway,weevaluatedtheeffectof
water-massandsea-levelchangesontheglacial-
c
modelsimulation,wefurtheranalyzedtheLGMoceanstateandassessedthestrengthofthe
glacialAMOC.
2MaterialsandMethods
–MITgcm
ForthereconstructionoftheLGMocean,theMassachusettsInstituteofTechnology
generalcirculationmodel(MITgcm)wasemployedtosolvetheBoussinesqformofthe
hydrostaticNavier-Stokesequationsinconjunctionwithanonlinearfreesurface(Adcroftet
al.,2004b;Marshalletal.,1997).Usingacubed-spheregridthatconsistedof6faceswith
32x32horizontalgridcells,anearlyuniformresolutionof~°wasestablishedandpole
singularitieswereavoided(Adcroftetal.,2004a).Theoceanwasdividedinto15vertical
levelsofincreasingthickness,
relativelycoarseresolutionprovidedanefficienttooltostudythelarge-scalecirculation
*toavoidthepossible
vanishingoftheupperlayerassociatedwiththenonlinearfree-surface(Adcroft&Campin,
2004)andthepartialcellformulationofAdcroftetal.(1997)torepresentthebathymetry.
Adynamic-thermodynamicseaicemodelwithviscous-plasticrheology(Loschetal.,
2010)(Gentand
McWilliams,1990;Redi,1982),isopycnaldiffusionandeddy-inducedmixingwere
-5
·10(in
m2s-1)andusedthepolynomialapproximationofJackettandMcDougall(1995)forthe
-orderadvectionwith
directspace-timetreatment(Hundsdorfer&Trompert,1994).
1618
Inthismodel-setup,thestablewaterisotopesH2O,H2OandHDOwere
surface,themodelwasforcedwiththeisotopiccontentofprecipitationandwatervapor,
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descriptionoftheimplementationofthestablewaterisotopesandtheevaluationofthemodel
resultsunderPIconditionsisgiveninVölpeletal.(2017).ThisPIsimulationwillalsobe
usedasareferenceforthenewLGMsimulationwhenanalyzingtheanomaliesbetweenthose
andtheisotopiccontentofriverrunoffwasonlyestimatedbasedontheisotopiccomposition
ofthelocalprecipitationattherivermouth,agoodagreementbetweenthemodeledδ18O
w
valuesandobservationaldata(theNASAGISSGlobalSeawaterOxygen-18Database,
Schmidtetal.,1999),bothattheseasurfaceandinthedeepocean,,the
comparisonwithplankton-towdata(.,2003)agreedquantitativelywellwith
offoraminiferalδ18OduringtheLGMwithrespecttoPIconditionsusingthesame
c
numericalmodel.
Becauseofthebuild-upoflargecontinentalicesheetsanditsassociatedsea-level
loweringofapproximately120mduringtheLGM,themodelbathymetrywasadjustedby
remappingtheICE-5Gtopography(Peltier,2004),boththePIand
LGMsimulationsusedthesameverticalgrid(15levelswiththesameresolution),butthe
land-seamaskandbottomtopographydiffered,resultinginasmallervolumeoftheglacial
ocean.
Themodelwasforcedwithclimatologicalmonthly-meanatmosphericfields(air
temperature,specifichumidity,zonalandmeridionalwindvelocity,windspeed,(snow-)
precipitation,incomingshortwaveradiationaswellasriverrunoff)derivedfromanLGM
simulationwiththefullycoupledCommunityClimateSystemModelVersion3(CCSM3)
(Merkeletal.,2010)followingthePaleoclimateModelingIntercomparisonProject2
(Braconnotetal.,2007a,2007b),butmodifiedbyKurahashi-Nakamuraetal.(2017)tobe
consistentwithproxyreconstructionsofannual-meanSST(MARGOProjectMembersetal.,
1813
2009),benthicδOc(Marchal&Curry,2008)andbenthicδCc(Hesseetal.,2011)usinga
dataassimilationtechnique(Errico,1997;Giering&Kaminski,1998;Heimbachetal.,
2005).Theseoptimizedatmosphericfieldswerebasedonamodelsimulationwithoutthe
,wealsoexcludedtheMediterraneanSea.
TotestwhetherthismayhavehadaneffectonthesubtropicalNorthAtlanticOcean,we
performedanadditionalsimulationthatincludedtheMediterraneanSea.
Climatologicalmonthly-meansoftheisotopiccontentofprecipitationandwater
vaporwereavailablefromtheNationalCenterforAtmosphericResearchCommunity
AtmosphereModelincludingawaterisotopescheme(NCARIsoCAM)(Tharammaletal.,
2013).SinceTharammaletal.(2013)usedtheSSTfromMerkeletal.(2010)asaboundary
condition,aconsistencyofourforcingfieldsfromtwodifferentsourceswasobtained.
Besidestheabove-mentionedatmosphericfields,theMITgcmalsoneededincoming
-2
longwaveradiationǪL↓(inWm)asanexternalforcingfieldforcalculatingthenetsurface
(inWm-2)
L*
weestimatedǪL↓following
Ǫ=(1−A)∙ε∙σ∙SST4+A∙ε∙σ∙IST4−Ǫ∗(1)
L↓iceOiceIL
whereAicewastheice-coveredareafraction,εO=1andεI=
-8-2-4
oceanandice,respectively,σ=·10(WmK)wastheStefan-Boltzmannconstant
andSSTandIST(inK)werethesurfacetemperaturesoftheoceanandice,
variablesweretakenfromtheCCSM3toreproduceǪL↓asaccuratelyaspossible.
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Boththefreshwaterandisotopicair-seafluxeswereinternallycomputedinthemodel
followingthebulkformulaebyLargeandYeager(2004).Furthermore,toprevent
uncontrolleddriftsinsalinityandtracerconcentration,acorrectionfactorfortheprecipitation
field/tracerspecificprecipitationfieldhasbeenimplemented,wherebytheglobalfreshwater
flux/isotopicfluxwasannuallybalanced(ölpeletal.,2017).
Theoceanwasinitializedwiththesalinity,temperatureandtracerdistributionsfrom
thePIsimulation(Völpeletal.,2017).Toaccountforthelowersealevelduetothe
continentalices
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