Constraining the age of the last geomagnetic reversal from geochemical and magnetic analyses of Atlantic, Indian, and Pacific Ocean sediments

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Constraining the age of the last geomagnetic reversal

from geochemical and magnetic analyses of Atlantic,

Indian, and Pacific Ocean sediments

Jean-Pierre Valet, Franck Bassinot, Quentin Simon, Tatiana Savranskaia,

Nicolas Thouveny, Didier Bourlès, Anouk Villedieu

To cite this version:

Jean-Pierre Valet, Franck Bassinot, Quentin Simon, Tatiana Savranskaia, Nicolas Thouveny, et al..

Constraining the age of the last geomagnetic reversal from geochemical and magnetic analyses of

Atlantic, Indian, and Pacific Ocean sediments. Earth and Planetary Science Letters, Elsevier, 2019,

506, pp.323-331. �10.1016/j.epsl.2018.11.012�. �hal-02629188�

Contents lists available atScienceDirect

Earth

and

Planetary

Science

Letters

www.elsevier.com/locate/epsl

Constraining

the

age

of

the

last

geomagnetic

reversal

from

geochemical

and

magnetic

analyses

of

Atlantic,

Indian,

and

Pacific

Ocean

sediments

Jean-Pierre Valet

a

,

Franck Bassinot

b

,

∗

,

Quentin Simon

c

,

Tatiana Savranskaia

a

,

Nicolas Thouveny

c

,

Didier

L. Bourlés

c

,

Anouk Villedieu

b

a_{InstitutdePhysiqueduGlobedeParis,UniversitéParisDiderot,SorbonneParis-Cité,UMR7154CNRS,1rueJussieu,75238ParisCedex05,France} b_{LaboratoiredesSciencesduClimatetdel’Environnement(CEA-CNRS-UVSQ),DomaineduCNRS,AvenuedelaTerrasse,91198Gif-sur-Yvette,France} c_{CEREGEUM34,AixMarseilleUniv.,CNRS,IRD,INRA,CollFrance,Aix-en-Provence,France}

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory:

Received9February2018

Receivedinrevisedform2September2018 Accepted4November2018

Availableonline20November2018 Editor: B.Buffett Keywords: magneticreversal Brunhes–Matuyama Be-ratio fieldintensity stratigraphy

WestudiedfourmarinesedimentrecordsoftheMatuyama–Brunhesgeomagneticreversalfromfoursites locatedintheIndian,AtlanticandwesternPacificoceans.Theresultscombinepaleomagnetic,cosmogenic nuclideberyllium(10_{Be)andoxygenisotopeanalysesthatwereperformedonthesamesamplesinorder}

toavoidanystratigraphicbias.The threerecordsfromtheequatorialIndianOceanandNorthAtlantic Oceandidnotrevealanyoffsetbetweentheauthigenic10_Be/9_{Beratio(Be-ratio)peakandtheintervalof}

lowrelativepaleointensity(RPI)thatcharacterizesthereversal.Thelowerandupperlimitsoftransitional directions arealsoconcomitant withtheincreasedatmospheric10Beproductionthataccompaniedthe geomagnetic dipole intensitydecrease. In contrast, the record from western equatorial Pacific Ocean sedimentswasfound18cmbelowtheBe-ratiochangesasaresultoflatemagnetizationacquisition.At allfoursites,maximum10BeproductionoccurredatthesameperiodsoonafterthemaximumofMarine IsotopeStage19(MIS19)and,therefore,indicatesitssynchronousworldwidecharacter.Suchfeaturesare effectivelyobservedfromtheBe-ratiosignalsandtheirrelationshipwiththemagnetictransitioninterval, whichfurtherconfirmsthesynchronouscharacterofthetransition.Takingalldatinguncertainties(1.6 ka betweensitesand5kafortheagemodel)intoconsideration,ourrecordssuggestameanreversalage of772.4 _±6.6ka. Theageofthetransition intheAtlanticOceanrecordiscloser to774kabutthis differenceiswithinthelimitofsignificance.

©2018TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).

1. Introduction

The age of the last reversal is constrained by orbitally tuned oxygen isotope records of marine sediments and by radiomet-ric ages that have been mostly obtained from volcanic mate-rial. In the first case, there is a large consensus for a 773 ka transition,whereas some recent controversialradiometric datings of tephra layers (Singer and Pringle, 1996; Singer et al., 2002; Marketal., 2017) concludeinfavorofa 10kyrolderage.A dat-ing range of nearly 12 kyr exists for the last reversal between studiesthatpresentsyounger agescommonlyaveragedat773ka (e.g.Channelletal., 2010; Valetetal.,2014) andolderages cen-teredat785ka(e.g.Sagnottietal., 2016;Marketal.,2017).Such differencesmay seem relatively insignificant and without conse-quence for our knowledge of the geodynamo or forthe mecha-nismsforreversals.However, theaccuracy ofage determinations

*

Correspondingauthor.

E-mailaddress:Bassinot@lsce.ipsl.fr(F. Bassinot).

constrains our ability to reconstruct properly the phase relation-shipamongrecords(e.g.geomagneticandclimatic)fromdifferent locations. Precisiononreversalagesmayalsoaffectvarious stud-ies such as statistical analyses of the geomagnetic polarity time scaleorresearchonpossiblelinksbetweenEarth’sorbital parame-tersandgeomagneticchanges(Worm,1997; Courtillotetal.,2007; Thouvenyetal.,2008).

Twoimportantaspectsmustbeconsideredbeforeattemptingto provideanaccurateageestimateforthelastreversal.Thefirst con-cerns the synchronism of polarity changes at different locations, especiallyatdifferentlatitudes.Bydefinition,anyevolutionofthe dipole field, specificallyits pre-transitional decay andsubsequent growthintheopposite polarity,occurssynchronouslyall overthe Earth andcan, thus, be described by records of relative paleoin-tensity(RPI)and/orby productionofcosmogenicisotopes. Thisis notthecasewhenreferringtotransitionaldirectionsbetweenthe two polarities that emerge when thedipole field intensitydrops toathresholdlimit(about20%ofthepresentgeomagnetic inten-sity (see e.g., Valet and Plenier,2008; Balbas etal., 2018). There is an overall consensus that transitional directions are associated

https://doi.org/10.1016/j.epsl.2018.11.012

324 J.-P. Valet et al. / Earth and Planetary Science Letters 506 (2019) 323–331

Fig. 1. Locationmapofthecoresites.Redlabelsindicatethefoursitesprimarily in-volvedinthisstudy.AlsoshownisthepositionofcoreMD90-940thatwasusedfor themicrotektitestudy.(Forinterpretationofthecolorsinthefigure(s),thereader isreferredtothewebversionofthisarticle.)

with a multipolar field geometry (see Valet and Fournier, 2016; Balbas et al., 2018). Such phases might, therefore, not be per-fectly synchronous at the Earth’s surface. Scarcity of transitional directionsinvolcanicreversalrecordsandestimatesderived from high-resolution sedimentary recordsindicate thatthe transitional period does not exceed a few thousand years (Channell, 2017; Valet etal., 2012), withcontroversialestimates(Evans and Mux-worthy, 2018) of only a few years (Sagnotti et al., 2016), and representsonly 10to 20% ofthefull reversalprocess in contrast tofieldintensitychanges.

A Bayesian inversion model of the last reversal suggests that theonsetofthetransitionmayhavenotbeensynchronousaround the Earth (Leonhardt and Fabian, 2007). However, despite the pertinence of the method, the conclusions of the analysis are controversialbecause several records on which thesimulation is based are demonstrably wrong (see Valet and Fournier, 2016; Channell,2017).Numericalsimulationshavealsoshownapossible time lag between records that basically depends on the location withintheEarth’sliquidcorewherereversedfluxstartsto propa-gate (Olson etal., 2011). Recently, Kong etal. (2014) mentioned a discrepancy between the reversal positions in Chinese loess– paleosolsequences(ZhaoandRoberts,2010;WangetLovlie,2010) andmarinesedimentcores,butseveralstudies(Wangetal.,2014; Zhouet al., 2014) emphasize that post-depositional remagnetiza-tionsruleoutthepertinenceofloesssequencesforstudying mag-netizationvariationsoversuchshorttimeperiods.

Contrastingtransitionalbehaviorcausedbycomplexfield mor-phologyleadsustothesecondaspectthatconcernsthedefinition oftransitionboundariesandthechoice oftheparameter usedto describe geomagnetic changes. As discussed above, the reversal process isconstrained by locallyvarying non-dipolar fieldswhile dipolarvariations mustbeseenasuniversal.Therefore,the rever-sal age can change if we refer to the onset of the field decline comparedto theonsetof directionalchanges, ortoanyindicator usedtodefinethemid-pointofthetransition.

Basedontheseconsiderationsweselectedfoursedimentcores (Fig.1)thatrecordedthelastreversalfromtheAtlantic,Indianand Pacific oceans(Fig. 1). Magnetic recordshave beenpublished for someofthesecores(Valetetal.,2014,2016),buttheyhavebeen revisitedinordertoreducestratigraphicuncertainty.Amajor char-acteristicofthepresentstudyisthatwe relyonmagnetic, authi-genic 10Be/9Be ratio(Be-ratiohereafter),andoxygenisotopedata that were obtainedfromsamples fromtheexact same depths in thefoursedimentaryrecordstoinvestigatethephaserelationship amongthe parameters and, therefore, to determine preciselythe limitsofthereversalphase.WealsoidentifiedtheAustralasian mi-crotektitelayerthatdefinesan instantaneousstratigraphicmarker intwo IndianOcean cores.Combined,thesemeasurements make it possible to discuss the synchronism of the signals worldwide,

within limits provided by astronomical tuning strategies, and to constrain the transition age for sediments deposited in different environments.

2. Samplingsites

Two equatorial cores MD90-949 (2◦06.90N, 76◦06.50E) and MD90-961 (5◦03.71N, 73◦52.57E) were collected in the Indian Ocean(Fig.1)duringtheSeymamacruiseoftheFrenchR/VMarion DufresneeastoftheMaldives platform(Indian Ocean)at3600 m and2446 m waterdepth, respectively.Sediment fromboth cores hassimilarbiogeniccarbonateconcentrationsof67

±

5.5%and64

±

6%, respectively, within the intervals that contain the last re-versal. Note,however,that the meansediment accumulationrate atthedeepersiteMD90949washalfthatatsiteMD90-961over thepast800 kyr.Inaddition,wepresent

δ

18O,magnetic,and mi-crotektite recordsfromathird southernequatorialsediment core (MD90-940,5.35◦S;61.40◦E)fromtheIndianOcean(Meynadieret al., 1994) thatwas takenata3189mwaterdepthandis charac-terizedbyhighbiogeniccarbonatecontents withameanvalueof 81

±

6.5%.

Core MD95-2016 is a 33 m core that was taken at 2318 m

water depth (57.4◦N,29.2◦W)on Reykjanes Ridge (Fig. 1), North AtlanticOcean,duringtheIMAGE1cruiseofR/VMarionDufresne. This high-latitude core can be compared with the other low-latituderecords.Itiscomposedofcalcareousnanofossil-oozewith anaverage21%biogeniccarbonatecontent.

Core MD98-2183 (2◦00.82N, 135◦01.26E) was taken by the R/VMarion Dufresneduring theIMAGEIVcruise at4600m wa-terdepth(Fig.1),closetothecarbonatecompensationdepth.The sediment consistsofhemipelagicclay withcalcareousnanofossils andmore abundantsiliceous microfossils.Thiscorewas formerly studied by Yamazaki andOda (2004, 2005) to confirm the exis-tence of100-kyrinclination cyclesthat were initially reportedin thenearbycoreMD98-2185.

All cores were sampledusing 8 cm3 cubesforstable isotopic analyses, 10_{Be measurements, andmagnetic parameters. In} addi-tion, U-channels were taken almost systematically. In all cases, it has beenpossible to obtain a continuous composite record. In somecases,thelargecorediametersmadeitpossibletoduplicate measurementsatthesamestratigraphiclevels.Thissampling pro-tocol restrained uncertainties concerning the respective positions oftheduplicatesamplestoafewmillimeters atmost.

3. Experiments

3.1. Magnetics

Themeasurementscanbedividedinthreecategories.Magnetic analyses were performed on U-channels and/or on adjacent sin-gle samples.Alternatingfield (AF)demagnetizationofthenatural remanent magnetization (NRM) was carried out with at least 8 steps on single samples from 2 cm stratigraphic intervals in all cores within and outside the transition intervals as well as on U-channel samplesusinga 2G-Enterprises cryogenic magnetome-ter.Rockmagnetic parameterswere measuredatthesame strati-graphic levels as the NRM. They included acquisition and step-wise AF demagnetizationof anhystereticremanentmagnetization (ARM),down-corechangesinlow-fieldmagneticsusceptibility,and S-ratiodetermination.Thesemagneticparametersundergono ma-jor changes overthe transitioninterval(Valet etal.,2016) which supports thestandard qualitycriteriaforRPIdetermination(King et al., 1983; Tauxe, 1993). Relativepaleointensity (RPI) was esti-mated fromtheslopeofthe bestfittinglinepassingthrough the NRM versus ARM data measured atthe same successive demag-netizationlevels.FinalRPIdeterminationswerecalculatedfromat least6successivedemagnetizationlevels.

3.2.Beryllium

The10Beconcentrationsmeasuredinmarinesedimentsdonot only depend on atmospheric 10_{Be production rates but also on} environmental conditions that affect the chemical and granulo-metric composition of sediments (Bourlès et al., 1989; Robinson et al., 1995). Absolute 10_{Be concentrations of marine sediments} are inversely proportional to carbonate content (Henken-Mellies etal., 1990) and proportional to the surface of the settling par-ticles(Simonetal.,2016a).Thus,total10_{Beconcentrationmustbe} correctedforenvironmental effectsby normalizingthe 10_Be con-centrationtotheconcentrationofthestable9Beisotopeassociated withcontinental erosion.The 10_Be/9_{Be ratiois subsequently} ref-eredasBe-ratio.

Samplesof

∼

1gofdrysedimentwereprocessedforBeisotope analyses following the chemical procedure described by Bourlès etal. (1989) and revisitedby Simonet al. (2016a). Both cosmo-genic 10Be and stable 9Be isotopes have different sources, but their soluble form is homogenized in the water column before beingscavengedanddepositedby sinkingparticles. Thisrequires a leaching procedure that extracts the authigenic phase of both isotopes –i.e.the fractiondue toadsorption or chemical precip-itation onto particles from sediments. The Be-ratio measured in this authigenic phase is equal to that of the overlying seawater atthetime ofdeposition. The readerisreferred to Bourlèsetal. (1989);Ménabréazetal.(2011,2012);Valetetal. (2014);Simon etal. (2016b,2018a) forthoroughdescriptionsoftheexperimental protocol used for these experiments and results. Natural authi-genic9_{Be concentrationsweremeasuredusinga graphite-furnace} atomicabsorptionspectrophotometer(AAS)withdouble-beam cor-rection while 10Be concentration measurements were performed after chemical preparation at the French accelerator mass spec-trometer(AMS)national facilityASTER(CEREGE). Authigenic10_Be concentrationsaredecay-correctedusingthe10Behalf-lifeof1.387

±

0.012 Myr(Chmeleffetal.,2010; Korschineketal.,2010). 3.3.Oxygenisotopes

The stableoxygen isotopic composition (

δ

18O)of benthic and planktonic foraminifer specieswas determined at high-resolution (2–4 cmsamplingintervals)usingisotoperatiomassspectrometry (IR-MS)toderivebottomandsurfacewateroceanographic/climatic signalsandtodevelop aglobalstratigraphicframework linked to thewaxing/waningofhigh-latitudeicecaps.

For all cores, we analyzed the benthic species Cibicides wuellestorfi picked from the

>

150 μm size-fraction. Planktonic foraminiferspecieswerepickedinthenarrower250–315 μm size-fractiontoavoidanysize-dependentvitaleffects.Wemeasuredthe isotopic composition of Globigerinabulloides in the high-latitude coreMD95-2016.Inlow-latitudeIndianOceancores,weanalyzed the tropical shallow-dwelling species Globigerinoides ruber (core MD90-961) and Globigerinoides trilobus (more resistant to disso-lution than G.ruber and thus preferentially preserved in deep coresMD90-940andMD90-949).IncoreMD98-2183,onlya ben-thicisotopic record could be obtained. Thislatter siteis situated closeto the carbonatecompensation depth where dissolution of foraminifershellsduringinterglacialperiodshasremovedmostof themorefragileplanktonicforaminifershells,andleftmostly ben-thicshellsthataremoreresistanttodissolution.

Priorto isotopic analyses, the foraminifer shells were cleaned fromfineparticles throughultrasonicationin methanolandwere lettodryovernight atroom temperatureaftertheexcessalcohol hadbeenremoved.Dependingontheamountofavailable carbon-ate,analyseswereperformedonVG-OptimaorElementarIsoprime dual-inletmassspectrometers.Allresultsareexpressedas

δ

18Ovs V-PDB (in

h

). The external analytical reproducibility determined

from replicate measurements of a laboratory carbonatestandard is

±

0.05

h

(1

σ

).Thesingularvalueobtainedfortheinternational calcitestandardNBS-18is

−

23.27

h

.

4. Synchronismbetween10Beandpaleointensitychanges 10_{Be is produced in the upper atmospherefrom spallation of} oxygen and nitrogen by incoming charged and highly energetic galacticcosmicrayparticles.Productioniscontrolled bythe posi-tionofthecut-offrigiditywhichisthelimitthatcosmicraysmust exceedtoreachthetopoftheatmosphere.Thecut-offrigidity de-pends upon latitude and upon the geomagnetic dipole moment. Modulationof10Bebythegeomagneticfieldintensityisnegligible closeto thepolesbecause thesamequantity ofincident charged particlesisalwaystrappedbythefieldlines.Itis,however,strong atlowlatitudeswherethepositionofthecut-offrigiditychanges withfield intensity.Therefore,variableamountsofincident galac-tic andcosmic protons are captured atlow latitudes. Altogether, atmospheric mixing averages latitudinal 10_{Be variations over the} 1–2 yrresidencetimeofthiselementintheatmosphere(Heikkilä etal.,2009),sothatthedipolefieldstrengthistheprimaryfactor thatcontrolsglobalvariationsofatmosphericcosmogenic produc-tion rates.Thus, thesame amount of 10Be overproductionis ex-pectedworldwideatthesametimeduringfield reversalsthatare characterized by dipole moment collapse. This observation holds by definition for a dipolar field, but is also valid for multipolar fieldssincespatial 10_{Be productionvariabilityshouldbe} homoge-nizedbyatmosphericcirculation.

Differencesin10Bedistributionsamongsedimentcorescan re-sultfromhow10_{Be isincorporatedwithin sediments.Larger} am-plitude production peaks are expected in sediments with lower deposition rates, and conversely a wider distribution of values andsmaller peak amplitude are expected forrapid accumulation rates. However, the same total amount of 10Be is incorporated during the sametime interval and, therefore,the maximum pro-duction peak should occur at the same time at all sites. Strati-graphic offsets between peaks of 10Be maximum production are unlikely tobe generatedbychanges inadsorptionprocessesonto sediment particles or by mixing within the sediment. Bioturba-tionsmears the10Berecord, butitdoesnotmove thesignal fur-ther down within the sediment column (Suganuma et al., 2010; Valet etal., 2014). Consequently,the stratigraphicpositionof the maximum10Beproductionpeakwithinasedimentshouldcoincide withtheperiodoflowestgeomagnetic intensity,whichisevident asaworldwidetemporalmarkerofthelastpolarityreversal.

Asdiscussedabove,geomagneticreversalsmaynotbeglobally synchronous.Aslongasthefieldisdipolarthereisworldwide syn-chronismof dipole field intensitychanges withshorttime scales during areversalandmagneticrecordsmustcoincideduring this period.Therefore,offsetsordelayscanonlyresultfrom magnetiza-tion processeswithin thesediment, particularlypost-depositional reorientationofmagnetic particles(Verosub, 1977; DeMenocalet al.,1990;RobertsandWinklhofer,2004).Forthisreason,itis cru-cial to combine 10Be andmagnetic data from the same samples todeterminepotentialoffsetsbetweenthetwosignalsand under-standtheirsignificances(Valetetal.,2014).

5. Resultsanddiscussion 5.1. Be-ratioanddirectional changes

In Fig.2a, b,e, fwe show the evolutionof theBe-ratio mea-sured inthe four coresasa function of depth. Ineach case, the initialBe-ratiorecordsareplottedalongwiththeresultsofaslight smoothingbyweightedaveragethatinvolves5%ofthedatapoints. This approach avoids uncertainties generated by high-frequency changes andconstrains further the positionof the Be-peaks.The

326 J.-P. Valet et al. / Earth and Planetary Science Letters 506 (2019) 323–331

Fig. 2. (a,b,e,f)Be-ratiowithassociatederrorbars(greenlines)andreversalangle(blacklines)asafunctionofdepthacrossthelastreversalinthefourstudiedcores. ThickgreencurvescorrespondtoslightsmoothingoftheBe-ratiobyweightedaverage.Verticaldashedlinesindicatethepositionofmaximumberylliumproductionduring thelastreversal.(c,d,g,h)RelativepaleointensityacrossthesamedepthintervalasfortheBe-ratio.Verticalreddashedlinesshowthepositionofthemid-transitional direction.Notethe20cmoffsetbetweentheBe-ratiopeakandthemagneticmid-reversalpositionincoreMD98-2183.

verticalgreen dashed line indicates the positionof each Be-ratio peak.Inthesameplotsarealsoshowntheevolutionofthe rever-salanglemeasured fromthe directionofthe axialdipole ateach site(Valetetal.,2016).Thisrepresentationdepictstheactualfield evolutionateach siteanddoesnotrequireassumptions required forcalculationofvirtual geomagneticpoles. Giventhelarge scat-terandoveralllowqualityofdemagnetizationdiagramsassociated withtransitionalperiods(discussed inValetetal.,2016)wehave littleconfidenceindetailedinterpretationoftransitionaldirections. However,allreverseandnormalpolaritydirectionsareclearly de-fined and constrain the boundaries of the transition. The record fromcoreMD90-949 hasa regular Be-ratiopeak andasharp di-rectionaltransition. This is expected giventhe low sediment ac-cumulationrate(v

=

2

.

0 cm/ka).The nearbycore MD90-961has a twofoldlarger deposition rate(4.2 cm/ka) withsimilar charac-teristics for the Be-ratio peak, and a longer magnetic transition. Itis significant thatin both coresthere isno offsetbetweenthe Be-ratio peak and the transition. The samesituation prevails for core MD95-2016 (north Atlantic Ocean), which is characterized by asimilar Be-ratio peak anda regular evolutionof transitional directions. Not surprisingly, this record has similarities with re-sultsfromcoreMD90-961becausebothhaveequivalentresolution (3.1 cm/ka).

RPIvariationswithineachcorehavealsobeenplottedinFig.2c, d, g, h on the same depth scale. In all cases (except for core MD98-2183)theRPIminimumcoincides withthetransitional in-tervalaswellaswiththeBe-ratiomaximum.Outsidethereversal we note, however, that the Be-ratio and RPI signals do not al-waysmatcheachother.Thiscomplexsituationdependsonseveral factors that go beyondthe scope of thispaper and that will be presentedanddiscussed elsewhere.The fact thatin all casesthe Be-ratio and RPI records have the largest amplitude changes in concordancewith each other during the reversal is a strong in-dicationoftheirgeomagneticorigin.

CoreMD98-2183istheuniquecaseinwhichthereisa signif-icant offsetof about20 cmbetweenthe Be-ratio peak and both directional andRPI variations that characterize the polarity tran-sition(Fig. 2). Similar offsets have alsobeen observed elsewhere incoresfromthePortuguesemargin(Carcailletetal.,2003,2004), western Philippine Sea (Simon et al., 2018b) and western equa-torial Pacific Ocean (Suganuma et al., 2010). In contrast to the other reversal records, demagnetization diagrams for transitional samplesare ofgoodquality(Valetetal., 2016). Successive transi-tionaldirectionsundergoa progressivedeclination rotationwhile inclination remains close to 0◦ in accordance withthe site lati-tude.This behavior results fromsmearing ofmagnetic directions induced by progressive post-depositional orientation of magnetic grainsimposedbystrongerfieldsinthesubsequentstablenormal polaritystate.

Summarizing,sedimentsfromthetwoequatorialIndianOcean coresandfromthe northAtlantic Oceancoredonotprovide any indication of delayed magnetization, but rather they record syn-chronous10_{Beproduction,RPIanddirectionalchangesateachsite.} OnlythesedimentfromtheEquatorialWestern PacificOceanhas amagnetization acquisition thatis offsetbyabout20 cm, thatis about5kyrdelay,withrespecttothemaximum10_{Beproduction.} 5.2.Microtektitelayers

Another stratigraphic marker is the microtektite layer associ-atedwith the Australasian microtektiteevent whichis well doc-umentedin Indochina, southern China, the Philippines, Malaysia, Indonesia, and Australia. The source crater remains unknown, but the microtektites were produced about 12–15 kyr prior to the last reversal (Glass et al., 1979; Schneider and Kent, 1990; Suganumaetal.,2011). Microtektiteanalysishasbeencarriedout

forcoreMD90-961 andtheresultshavebeenpresented and dis-cussed in comparison with previous studies from the Sulu and Celebesseas(Valetetal.,2014; Suganumaetal.,2011).The ampli-tudeofthedepthoffsetbetweenthelastreversalandtheBe-ratio peakiscorrelatedwithdepositionrate.Thissituationisunlikelyin presenceoflatemagnetizationacquisitionbecauseitwouldchange thedepths independentlyoftheaccumulation rate.Here,we add newresults(Fig.3a, c)fromcoreMD90-949 andfromthe 32 m-long coreMD90-940(61◦40.12E, 05◦33.53S) fromMadingleyRise (Fig. 3b). We did not find microtektites in core MD98-2183 de-spitetheirpresenceinnearbycoreMD98-2187(YamazakiandOda,

2005; Suganumaet al., 2011). The number of counted microtek-tites in the three layers are compared in Fig. 3a and plotted as percentage of their maximum numberin each corein Fig. 3b,c, d.Inallcases,bioturbationhasgeneratedan asymmetricaltektite distributionovera20cmsedimentthicknesswithasharpincrease andaprogressiveupwarddecrease.

InFig. 3b,c,dare showntheoxygen isotoperecords,the po-sitions of the Be-ratio peak, andangular deviationas a function ofdepth ineach core. Inthethree IndianOcean coresthe maxi-mumoccurrenceofmicrotektites(verticalreddashedlines)occurs atthesamestratigraphicpositionatthe beginningofthemarine isotopestage(MIS)20/19transition.Themid-reversalposition de-rivedfromthe Be-ratiopeak position(verticalgreendashed line) incoresMD90-949(Fig.3c)andMD90-961(Fig.3d)coincideswith thelocationofthedirectionalchangesaswellaswiththeRPI min-ima(Fig. 2c,d).We,therefore,inferthat thereis nostratigraphic ambiguityforthereversalpositionduring MIS19fromthese ma-rinesedimentarysequences.

5.3. 10Beandmarineisotopestages

The observedsynchronism betweencosmogenicandmagnetic changes clearly indicates the coherence of these recordsthat al-lows to define an accurate mid-reversal position fromtwo inde-pendent proxies. We can now compare the position of each Be-ratiopeakwiththe

δ

18_{Orecordobtainedatthesamesite.The} re-sultsareplottedasafunctionofdepthforeachofthefourstudied cores(Fig.4a,b,c,d).Theresultsindicatethatmaximum10_Be pro-duction was incorporatedin all sediments immediately afterthe warmest periodofMIS19,i.e.justafterthe MIS19cinterglacial. Thisperiodcannotbeidentifiedinthe

δ

18OrecordofMD98-2183 dueto poorforaminiferaabundance during thisperiod.However, the MIS 20 signature is preservedand is found prior to the Be-ratiopeak,consistentwiththeotherthreerecords.We,thus,infer thatthemid-reversalpositionoccurredaftertheMIS19maximum atallfoursiteswithinthelimitsofcorrelation.

An agemodel was constructed by assigning a commondepth derived from correlation of individual

δ

18O curves to the com-posite recordofsistercoresMD90-961/963(Bassinotetal., 1994; Valet etal., 2014). The MD90-961/963 record was chosen as our internal referencerecord becauseofitshighersedimentationrate andits higher resolutionisotopic record. Onceall of the individ-ual cores hadbeen depth-correlated to core MD90-961/963, the agemodelwasdevelopedbytuningthebenthic

δ

18Orecordfrom core MD90-961/963 to the LR04 global benthic stack of Lisiecki andRaymo (2005).Usingthisstrategy, wehaveasetofcoresfor whichthe agesoftheBe-ratio peaksandRPIminimacanbe de-terminedfromacoherentstratigraphicframework.

Byapplying such datingstrategy, we assume that oxygen iso-tope changes are synchronous worldwide. Over the last decade, however, several works have proven such assumption to be in-valid, revealingpotential offsetsofafewthousand yearsbetween

δ

18Orecords fromdifferentoceanic basinsor fromdifferent wa-termasseswithinthesame basin(SkinnerandShackleton,2005; Lisiecki and Raymo, 2009; Waelbroeck et al., 2011; Stern and

328 J.-P. Valet et al. / Earth and Planetary Science Letters 506 (2019) 323–331

Fig. 3. (a)NumberofcountedmicrotektitesasafunctionofdepthincoresMD90-940,MD90-949andMD90-961fromtheequatorialIndianOcean.(b–c–d)Changesinδ18O (bluelines),reversalangle(darklines)andpercentageofmicrotektites(redlines)inthesamecores.VerticalgreendashedlinesshowthepositionofmaximumBe-ratio. Verticalreddashedlinescorrespondtomaximumcountedmicrotektitesnumber.

Fig. 4. Be-ratio(greenlines)andδ18_{Ocurves(bluelines)asafunctionofdepthincoresMD90-949(a),MD90-961(b),MD95-2016(c)andMD98-2183(d).Verticalgreen}

Fig. 5. Be-ratioasafunctionofageincoresMD90-949(a),MD90-961(b),MD95-2016(c)andMD98-2183(d).OriginalBe-ratiovalues(blackdots)havebeenslightly smoothedbyweightedaverage(greenlines).VerticalblacklinesgivetheageofthecorrespondingmaximumBe-ratios.RedlinesindicatetheBe-ratiobaselineineach core.HorizontalblacklinesshowtheBe-ratiomid-peakvalueswhiledashedverticalgreenlinesindicatetheirlowerandupperages.Green verticallinesgivetheageof theintervalmid-peakassumingasimilarGaussianshapeforallBe-ratiorecords.Thedifferencebetweenthetwotechniques(maximumBe-ratioandmid-peak)isourbest approximationoferrorontheageofmaximumatmospheric10_{Beproduction.}

Lisiecki,2014).Thoseworkshavebeenfueledbytheseminalstudy ofSkinnerandShackleton (2005),whichshowed–basedon high-resolutionplanktonic14Cagemodels–thatthemiddleofthelast terminationdeepPacific benthic

δ

18O laggedthe deepNorth At-lanticby3.9 krduetoadelayedtemperatureresponse.Ithasbeen concluded,however, that such regional time offsets are only im-portantduring glacialterminationsduetomajorreorganizationof ocean circulation and complex interplay of temperature changes anddeglacial

δ

18O transit time in the ocean water masses(e.g., Lisieki and Stern, 2016). The B/M reversal did not occur during the deglaciation from MIS20 to MIS19, but well into the MIS19, after the full interglacial MIS19c. It is our contention that re-gional offsets between

δ

18O records were likely minimal during late MIS19and theprogressive transitionto MIS18. Thus, poten-tialageerrorsintroducedbyassumingthesynchronismofbenthic

δ

18O records are much lessimportant than uncertainties associ-ated withthe graphical alignment of

δ

18Ocurves obtained from ourlow-sedimentationratedeep-seacores.

Be-ratio changes measured between 740 ka and 810 ka are plottedin Fig. 5. As in the previous figures, theBe-ratio records have been smoothed slightly (green lines) to eliminate high-frequencychanges.Alsoshownineachcaseisareferencebaseline (redhorizontalline)that isusefulforcomparingthe relative am-plitude of 10Be production between each site. This baseline was alsohelpfulto definethe centerofthe mid-peakleveland, thus, to check the consistency between two different age determina-tions derived from the position ofthe Be-ratio peak (black lines inFig. 5) and the positionof the mid-height of thepeak (green lines in Fig. 5), respectively. We assumed that the shape of the Be-ratioscanberoughlyapproximatedbyaGaussiandistribution, whichallowustoestimateanerrorontheagesofthepeaks.

In all cases, the ages of the Be-ratio peaks are very close or coincide with those derived fromthe mid-peak. The Indian and Pacific Ocean resultsare consistently found at772.5

±

1 kaand 771.5

±

0.2 ka forthe two Indian Ocean cores, 771

±

1 ka for thewestern Pacific,while theAtlanticdatasetis characterizedby aslightlyolderageat774.6 ka.Theseresultsyieldameanageof 772.4

±

1.6kaforallrecords.Whentakingintoaccountadditional uncertaintiesassociatedwiththeagemodelofLR04andour align-menttothistargetcurve(i.e.

∼ ±

5 ka),ourmeanageestimatefor theB-Mtransitionis772.4

±

6.6ka.

Is the difference with the Atlantic record significant? Coinci-dence ornot, an ageof 773.1 kawas obtainedfrom five nearby northAtlanticOceancoreswithrapidaccumulationrates(Channell et al., 2010). In Fig. 6a the upper part of all Be-ratio peaks has been plottedas a function of ageafter rescaling with respectto the maximum peak values. The vertical bars indicate the lower (771.8 ka)andupper(774ka)agelimitsthatwere definedabove forthe Be-ratio peak. Wefollowed the sameapproach inFig. 6b forthe RPIminima normalizedwithrespect to their meanvalue over the same time interval. As expected and discussed above, latemagnetizationacquisitionprocessinherenttocoreMD98-2183 generated a 15 kyr delay of the RPI minimum with respect to the Be-ratio peak. InFig. 6c themagnetic data ofthis corehave beenre-plottedaftercorrectingforthe20 cmoffsetbetweenthe twosignals.Theresultsshowthatthepaleointensityminimaofall coresliewithintheagelimitsof772.4

±

1.6kadefinedaboveand furthermorewithin the time interval765.8–779 kawhen consid-eringthe uncertaintyof6.6 kaassociatedwith theagemodel of LR04.

We inferthatthe relativeuncertaintyon themid-reversal age doesnot exceed

±

6.6 ka.Causes of errorare linked to the

mor-330 J.-P. Valet et al. / Earth and Planetary Science Letters 506 (2019) 323–331

Fig. 6. Enlargedviewof(a)theupperpartoftheauthigenic10_Be/9_Beratio

(normal-ized)changesand(b)lowerpartofthepaleointensityminimumduringthereversal asafunctionofagewiththeageoffsetoftheRPIsignalincoreMD98-2183.(c) sameas(b)aftercorrectingtheRPIsignalofMD98-2183for20cmstratigraphic delaywithrespecttotheBe-ratiopeak.

phologyof the Be-ratio recordsand to the quality ofthe depth-agecorrelation constrainedbytheoxygen isotoperecords. There-fore,thedifferencesamongthe Atlanticandother recordscannot be considered as significant in the present state of the art and, mostimportantly,wecannotreconcilethepotentialerrorswithca 10 kyr olderageasrecentlyinferred fromchronologiesusing in-terpolationbetweenradiometricdating(Sagnottietal.,2016;Mark etal.,2017).

6. Conclusion

Inthisstudy,wepresentdetailedandindependentanalysesof Be-ratio,paleomagneticdirections,relativepaleointensity,and oxy-gen isotopes from four marine sediment cores covering the last geomagnetic reversal, i.e.,the Matuyama–Brunhes transition. This is likelythe mostcomplete dataset obtainedso far forthis time period.Those measurements werecomplemented by microtektite analysesforthreeIndianOceancorespriortothereversal.Atthree sites,weobservesynchronismbetweenthemagneticand

cosmo-genic records,whichdemonstratesthepotentialofthisdatasetto definethemostaccuratereversalpositionfromindependent prox-ies.TheequatorialwesternPacificOceanistheonlyrecordstudied herethatrevealedan offsetbetweentheRPIminimaandthe Be-ratiopeak,whichiscausedby adelayedmagnetizationlock-inof 20 cm which representsabout5 kyr. The ageof theMatuyama– Brunhes reversal was derived from Be-ratio peaks and is fully consistentwiththeintensityminimaanddirectionalchanges.The depth-timemodelderivedfromthecorrelationofthe

δ

18_Orecords withtheLisieckiandRaymo (2005) LR04globalbenthicstack in-dicates that the field reversed polarity at 772.4

±

1.6 ka within ourcommonchronostratigraphicframe,and772.4

±

6.6kawhen taking into account a potential age uncertainty of

±

5 ka associ-ated withthetuning strategyusedforderiving the agemodelof LR04 andouralignmentto thistargetcurve. Thisageestimate is consistent withall recentdatesthat havebeenobtainedfrom or-bitallytunedpaleomagneticrecords.Theydifferfromtheca10 ka olderage recentlyderived fromradiometric datingoftephra lay-ersfoundinamarinesedimentcore(Marketal.,2017).However, we cannot exclude that the reversalposition was not accurately defined in the ODP 758 sediment used for that study (Channell and Hodell, 2017), as this reversal clearly occursbefore the op-timum of MIS19in ODP-758,whereas we observe it consistently after MIS19cin all ourrecords.There can be variable definitions of reversal boundaries that contribute to the repartition of dif-ferent ages for this reversal. For levels that mark the onset of the Be-ratio peak we obtain an age of 786

±

2 ka(786

±

7 ka when taking into account uncertainties associated to LR04 and the alignment to this target curve) that is consistent with the older radiometric age. Determining the middle of the transition can be difficult when referring to poorly defined transitional di-rections, except for rapidlydeposited records(Valet et al., 2016; Channell, 2017). Therefore,it is justifiedto determine the ageof thereversalusingtheperiodoflowestfieldintensity,whichshould coincide withthe periodofmaximum10Beproduction.This con-ditionwas metinthepresentstudy,whichenablesthedefinition ofreversalposition.

Acknowledgements

The authors thank F. Dewilde(LSCE) for oxygen isotope anal-yses and G. Isguder for foraminifer controls, Anojh Thevarasan (IPGP) for magnetic measurements and LaetitiaGacem and San-drine Choy (CEREGE) for chemical preparation before Be analy-ses. This research was funded by theEuropean ResearchCouncil (ERC)undertheEuropeanUnion’sSeventhFrameworkProgramme (FP7/2007–2013)/ERC advanced grantagreement GA339899. This isIPGPcontribution#3991andLSCEcontribution#6521.

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