High-resolution 10Be and paleomagnetic recording of the last polarity reversal in the Chiba composite section: Age and dynamics of the Matuyama–Brunhes transition

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High-resolution 10Be and paleomagnetic recording of

the last polarity reversal in the Chiba composite section:

Age and dynamics of the Matuyama–Brunhes transition

Quentin Simon, Yusuke Suganuma, Makoto Okada, Yuki Haneda, Georges

Aumaitre, D.L. Bourles, Karim Keddadouche

To cite this version:

Quentin Simon, Yusuke Suganuma, Makoto Okada, Yuki Haneda, Georges Aumaitre, et al..

High-resolution 10Be and paleomagnetic recording of the last polarity reversal in the Chiba composite

section: Age and dynamics of the Matuyama–Brunhes transition. Earth and Planetary Science Letters,

Elsevier, 2019, 519, pp.92-100. �10.1016/j.epsl.2019.05.004�. �hal-02136716�

High-resolution

10

Be

and

paleomagnetic

recording

of

the

last

polarity

reversal

in

the

Chiba

composite

section:

Age

and

dynamics

of

the

Matuyama–Brunhes

transition

Quentin Simon

a,

∗

,

Yusuke Suganuma

b,c

,

Makoto Okada

d

,

Yuki Haneda

d

,

ASTER

Team

a,1

a_{CEREGEUM34,AixMarseilleUniv,CNRS,IRD,INRA,CollFrance,13545AixenProvence,France} b_{NationalInstituteofPolarResearch,10-3Midoricho,Tachikawa,Tokyo190-8518,Japan} c_{DepartmentofPolarScience,SOKENDAI,10-3Midoricho,Tachikawa,Tokyo190-8518,Japan} d_{DepartmentofEarthSciences,IbarakiUniversity,2-1-1Bunkyo,Mito,Ibaraki310-8512,Japan}

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory:

Received30November2018 Receivedinrevisedform25April2019 Accepted1May2019

Availableonlinexxxx Editor: B.Buffett

Keywords:

authigenic10_Be/9_Beratio relativepaleointensity virtualgeomagneticpoles

Matuyama–Brunhespolaritytransition Chibacompositesection

GSSPstratotype

We presentnew magnetic(directionand relativepaleointensity)andberyllium isotope(10Be,9Beand

10_Be/9_{Be ratio) results covering the lastgeomagnetic reversal, i.e.,the Matuyama–Brunhes transition}

(MBT),fromtheChibacompositesection(CbCS),east-centralJapan.Theveryhighsedimentationrates (>90 cm/ka)ofthe studiedsite,acandidate siteforthe globalboundarystratotypesectionand point (GSSP) of the Lower–Middle Pleistocene boundary, allow the acquisition of a dataset of exceptional resolution.Coupledcosmogenic10_{Beandmagneticresultsmeasuredonthesamesamplesdemonstrate}

thatthemagnetizationacquisitionconservedthetimingofrapidgeomagneticfeatures,allowingaccurate paleomagneticinterpretations.Apolarityswitch(PS)capturing mostoftheangulardeviationoccurring between771.9and773.9kawasfollowedbyadirectionalinstabilityrebound(IC1)until768.5ka.This sequencewith5.4kadurationwascharacterizedbyaweakdipolefieldaslowas2.3±0.3×1022Am2. FourrapidepisodesofdirectionalinstabilitiesprecedingandfollowingthePS-IC1phasecompletedthe reversalsequence.Theasymmetryobservedbetweenthelong-termdipoledecayandsharprecovery,and rapidoscillations(<1ka)demonstratethecomplextransitionalfieldbehaviorduringdipolemomentlow beforereestablishmentofthefullpolaritystate. Ourobservationsreinforce thefactthatmostreversal records do not integrate the full field behavior associated with the geodynamoaction. Althoughthis posesproblemsforunderstandingtheunderlyingphysicalprocessesthatproducereversals,itdoesnot hamperstratigraphiccorrelationsamongmostgeologicrecords.

©2019TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

Geomagnetic polarityreversalsare themostsevereexpression of geomagnetic field variations generated and sustained by geo-dynamo action (Merrill and McFadden, 1999). Despite the large databaseavailable, interpretations ofreversal records are contro-versialduetosporadicvolcanicsequencesandlimitedconstraints on the processes involved in sediment magnetization (Love and Mazaud, 1997; Coe and Glen, 2004; Valet and Fournier, 2016). Paleomagnetic data from transitional lava flows provide impor-tant hints about field characteristics, but these are hardly help-ful for constraining the complete dynamical sequence of rever-sals (e.g., Jarboe et al., 2011; Valet et al., 2012; Balbas et al.,

*

Correspondingauthor.

E-mailaddress:simon@cerege.fr(Q. Simon).

1 _{GeorgesAumaître,DidierL.Bourlès,KarimKeddadouche.}

2018).Forcontinuouslydepositedsediments,low-to-moderate res-olution paleomagnetic records (defined by typical sedimentation rates of

<

10 cm/ka) are associated with (post-)detrital rema-nent magnetization (pDRM) uncertainties andwithsmearingand smoothingofthemagneticsignal,preventingaccuraterecordingof rapid geomagneticvariations (e.g.,Roberts andWinklhofer,2004; Suganumaetal.,2011; Valetetal.,2016).Highsedimentationrate records (10 to 20 cm/ka) reveal complex reversal dynamics but are limitedto afew records intheNorth Atlantic(e.g.,Channell,

2017).Inaddition,therarereversalrecordsfromsiteswithhigher sedimentation rates (

>

20 cm/ka) are oftenassociated with vari-ablesedimentationpatternsorcomplexdiageneticprocesses(e.g., complete or partial remagnetization, chemical transformations of magneticminerals)thatcallintoquestiontheirabilitytoproperly recordgeomagnetic fieldchangesovertransitionalperiods charac-terizedbyweak fields(e.g.,Quidelleur etal., 1992; Robertsetal.,

2010; Sagnotti et al., 2010, 2016; Evans and Muxworthy, 2018).

https://doi.org/10.1016/j.epsl.2019.05.004

Fig. 1. GeographiclocationoftheChibacompositesection(CbCS,reddot)alongwithmarineandglacialrecordsdiscussedinthetext(blackdots).Thesereferencesinclude MontalbanoJonico(Simonetal.,2017;Nomadeetal.,2019);MD95-2016,MD95-0949,andMD98-2183(Simonetal.,2018b; Valetetal.,2019);MD90-0961(Valetetal.,

2014);EpicaDomeC(EDC)(Raisbecketal.,2006);MD97-2143(Suganumaetal.,2010;Simonetal.,2018a);MD05-2930(Ménabréazetal.,2014; Simonetal.,2016a);and Heqingpaleolake(Duetal.,2018).ThepreciselocationoftheCbCSandageologicalmapoftheBosoPeninsulaareprovidedinboxes(a)and(b),respectively.Samplesused inthisstudyarefromtheYoroRiverandYoro-Tabuchisections(highlightedinredinb).(Forinterpretationofthecolorsinthefigure(s),thereaderisreferredtotheweb versionofthisarticle.)

Therefore,sediment recordsprovideinvaluableinformationabout long-termgeomagneticfieldbehavior,butthequestionofwhether theyare capableofrecordingrapidvariations withadequate pre-cision has generated much discussion over the years, restraining accurateinterpretations ofreversaldynamics(i.e.,simpleor com-plex?)andtheir underlyingphysicalprocesses.It alsocastsdoubt onthe ability to makeaccurate comparisons between paleomag-neticreversalrecordsfromlow,moderate,andhighsediment de-positionratesites.

Alongwith transient directions,all reversals are characterized by a collapse of the geomagnetic dipole moment (GDM). This large-scale featureof the dipole field must be capturedcorrectly bymodelstounderstand(andpossiblypredict)itsoccurrenceand detailedbehavior (Buffett,2015; Morzfeldetal., 2017).Paleofield intensitycanbereconstructedfromsedimentsusingarelative pa-leointensity (RPI) index (e.g.,Tauxe, 1993; Valet, 2003) and cos-mogenicnuclideberyllium-10 (10_{Be)measurements (e.g.,Lal and}

Peters,1967; Raisbecketal., 1985). Paleomagnetic measurements allow reconstruction of the total paleomagnetic vector, including directions, but they are associated with DRM uncertainties (e.g., variable lock-in depth) and likely contain intricate contributions ofthe dipole and non-dipolefields during transitions (Leonhardt andFabian,2007; ValetandFournier,2016).Measurementof10Be concentrationsin sedimentary sequences permitsthe reconstruc-tionofpastproductionratesafterenvironmentalcomponentshave beenscreenedout,thusprovidinganindependentproxyforGDM variationsandfieldinstabilities(e.g.,Simonetal.,2016a).Thetwo complementarymethodscanbeemployedsimultaneouslyto eval-uate their respective limitations, obtain reliable reversal records, andhelp addresstwo fundamentalquestions:(1)Is DRMcapable of accurately recording the dynamics of rapid geomagnetic field changesduring reversals?(2)Dogeomagneticfieldrecordsduring polaritytransitionspresentsimilarandglobally synchronized sig-natures?

In this study, we present new paleomagnetic and authigenic

10_Be, 9_{Be, and} 10_Be/9_{Be ratio results covering the last}

geomag-netic reversal, i.e., theMatuyama–Brunhes transition(MBT),from theChibacompositesection(CbCS),acandidatehostoftheglobal boundarystratotypesectionandpoint(GSSP)oftheLower–Middle Pleistocene boundary. The very high sedimentation rates, more than 10 times higher than those of typical marine sediment records, of the studied site allow the acquisition of a dataset of exceptionalresolution.Cosmogenic10Beandpaleomagneticresults obtainedfromthesamesamplesarecomparedtoextractaccurate field values(amplitudes, durations, rates) during the MBT. These features, which may provide constraints on geomagnetic models andassistglobalcorrelationofpaleo-records,arethendiscussed.

2. Geologicalsettingsandchronology

The Yoro-Tabuchi andYoro River sections formthe main part of the CbCSin the Kokumoto Formation ofthe Kazusa Group in thesoutheasternJapaneseislands(Suganumaetal., 2018) (Fig.1). The sections represent an expanded and well-exposed sedimen-tary succession characterized by stable deposition of bioturbated silty beds. Marine oxygen isotope records reveal continuous de-position from Marine Isotope Stage (MIS) 21 to MIS 18 with glacial and interglacial cycles corresponding to sandstone- and siltstone-dominatedunits,respectively(OkadaandNiitsuma,1989; Pickering et al., 1999). Widespread tephra beds originating from regional volcanic activity provide straightforward correlations of distant sections. Among those tephra layers, the Ontake–Byakubi tephra (Byk-E hereafter)bed, consistingof whitepumiceous fine ash with 1 to 5 cm thickness (Kazaoka et al., 2015) originat-ing from the Ontake volcano inthe central partof the Japanese archipelago(Takeshita et al., 2016), provides the reference strati-graphiclevel(i.e.,0m),whichiseasilyidentifiedintheCbCS.

Fig. 2. GeochemicalandmagneticresultsfromtheChibacompositesection(CbCS).StratigraphiccorrelationsbetweentheYoroRiverandYoro-Tabuchisectionsareprovided onthelefthandside(seelegend).SamplesforBemeasurementsareshownbyblackhorizontallines.Thebenthicδ18_{OdataarefromSuganumaetal. (2015) (emptydots)} andOkadaetal. (2017) (fulldots).kARM/kLFandkLFprovideinformationonmagneticmineralogy(seetext).

A

δ

18Ostratigraphyfrombenthicforaminifera was first estab-lished by Suganuma etal. (2015) and subsequentlyimproved by Okada etal. (2017) and Suganuma et al. (2018). The CbCS

δ

18O recordwastunedtothesealevelcurveofElderfieldetal. (2012) in ordertoestablishachronologicaltemplate.Thisastrochronologyis controlledbyaradioisotopicU–PbzircondateoftheByk-Etephra of 772.7

±

7.2ka (2 sigma analytical uncertainty; Suganuma et al., 2015). The resulting agemodel yields an average sedimenta-tion rate of ca. 208 cm/ka for the whole CbCS (from 801 ka to 752 ka)andofca.89cm/kawithintheMBTinterval(Suganumaet al.,2018).

3. Materialsandmethods

Rock samples for paleomagneticand beryllium isotope analy-seswerecollected fromtheYoroRiver andYoro-Tabuchisections, locatedat35◦17.41N,140◦8.48 Eand35◦17.41N,140◦8.49E, re-spectively.AnintegratedstratigraphyoftheCbCSbasedondetailed tephrostratigraphic and stratigraphic investigations (Suganuma et al., 2018) revealedthat the Yoro-Tabuchisection is3.5mshorter than height of Okada et al. (2017). Cores with a diameter of 2.54 cmwerecollectedat310horizonswitha10to50cm strati-graphicinterval andcoveringa52-msuccession across theByk-E tephra bed usinga portable engine drill. Each sample integrates about 30 yrs of field record during the MBT interval and up to 12 yrsusingtheaveragesedimentationrate.Allcoresampleswere orientedwithamagneticcompassbeforebeingremovedfromthe outcropandcutinto2-cm-longspecimens.

Newmagneticmeasurements onsamplesfromthe upperpart ofthe Yoro-Tabuchi section were performed to completethe de-tailedpaleomagneticstudyofOkadaetal. (2017). Low-field mag-netic susceptibilitywas measured on all specimensusing a Kap-pabridgesusceptibilitymeteratIbarakiUniversity.Thenatural re-manent magnetization (NRM) was measured using a three-axis cryogenic magnetometer (2G SRM-760R) installed in a magneti-callyshieldedroomattheNationalInstituteofPolarResearchand also partly measured using a two-axis cryogenic magnetometer (2G SRM-750R) in a magnetically shielded room at Ibaraki Uni-versity.Anhystereticremanentmagnetization (ARM)was acquired

in a 0.03-mT DC field with an 80-mT AF using the SRM-760R

magnetometer. Characteristic remanentmagnetization (ChRM) di-rections were calculated after a hybrid demagnetization method, i.e., 5- to10-mTincrementsup to50–60mTalternating field de-magnetization (AFD) after 300◦C thermal demagnetization (ThD) toremove secondarylow-temperaturecomponents.TheRPIindex was calculated by ARM normalizationusing resultsofthe hybrid demagnetizationmethod(seeOkadaetal.,2017,fordetails).

Samplesforberylliumisotopicanalysisweresub-sampledfrom the paleomagneticsamples followingthecompletion of magnetic measurements to providedirectresultsforcomparisonandavoid any samplingbias. The analyses were carried out at the CEREGE National Cosmogenic Nuclides Laboratory (LN2C, France) accord-ingtothechemicalprocedureestablishedbyBourlèsetal. (1989) andrevised bySimon etal. (2016b).Authigenic 10Be andits sta-bleisotope9_{Bewereextractedfrom}

_∼

_{1gdrysamplesbysoaking}

thesamplesin20mlofleachingsolution(0.04Mhydroxylamine (NH2OH–HCl) and25% acetic acid) at95

±

5◦C for7 h. A2-ml

aliquot of the resulting leaching solution was sampled for mea-surementofthenatural9_{Beconcentrationusingagraphite-furnace}

atomic absorption spectrophotometer (AAS) with a double beam correction (Thermo Scientific ICE3400®_{). The remaining solution}

was spikedwith300 μlofa9.8039

×

10−4 _{g g}−1 9_{Be carrier}

be-foreBepurificationbychromatographytodetermineaccurate10Be sample concentrationsfromaccelerator massspectrometer (AMS) measurements of10_Be/9_{Be ratios at the FrenchAMS national}

fa-cilityASTER(CEREGE).10Besampleconcentrationswerecalculated fromthemeasured spiked 10Be/9Be ratiosnormalizedto theBeO STD-11in-housestandard(1.191

±

0.013

×

10−11_{)(Braucheretal.,} 2015).Authigenic 10Beconcentrationswere decay-correctedusing the10Behalf-life(T1/2)of1.387

±

0.012Ma(Chmeleffetal.,2010;

Korschineketal.,2010).

4. Resultsandinterpretations

4.1. Beryllium

Theauthigenic9Be concentrationsvaryfrom0.9to2.0

×

1016 at g−1,withanaverageandstandarddeviationof1.4

±

0.2

×

1016 at g−1 _{(Fig.2).Overall,higher}9_{Beconcentrationsandlarger}

upwardfrom10m.Theauthigenic10_{Be(decay-corrected)}

concen-trationsvaryfrom1.42to5.26

×

108 at g−1,withanaverageand standarddeviationof2.6

±

0.8

×

108 at g−1.The10Be concentra-tions are higher between

−

1 and24 m, peakingat 12 m above theByK-Elevel. This10Be spikealso corresponds to a major9Be increaselikelyassociatedwithanincrease ofBedeliveryprobably attributabletohigherdenudationratesand/orscavengingrate en-hancements relatedto specific climatic/oceanographic conditions. Theauthigenic10Be/9Beratio(Beratiohereafter)variesfrom0.98 to 3.05

×

10−8_{, with an average and standard deviation of 1.8}

±

0.5

×

10−8_{.The Be ratio profileis very similarwith the}10_Be

record andpresents a high andsignificant correlation coefficient of0.90.Incontrast,nocorrelationexistsbetweentheBeratioand its normalizer 9_{Be (r}

_{= −}

_{0.16). It is also worth noting that the}

peakobservedat12minboth Beisotoperecordsandassociated withanenvironmentalimprintisaveragedoutbytheBeratio. Val-uesabovethemeanoccurbetween

−

2and24m(yellowbanding inFig.2) andare characterizedby 3sub-peaks at1.95, 11.5, and 20m,respectively.ThesharpBe ratiopeakobservedimmediately abovetheByk-Etephralayer(0.55m)correspondstoa9_Beoutlier

(seedottedlineinFig.2)andisnotconsideredassociatedwitha geomagneticorigin.

4.2.Rockmagnetismandpaleomagnetism

Suganumaet al. (2015) and Okada et al. (2017) showed that the Yoro River and Yoro-Tabuchi sections are characterized by homogeneous rock-magneticproperties withreduced mineralogi-cal andgrain-size changes andthat the remanence is carried by magnetitetoTi-poortitanomagnetiteinthepseudo-singledomain (PSD).Thesepropertiesare summarizedinFig.2bythemagnetic susceptibility(kLF)andARMsusceptibilitynormalizedtomagnetic

susceptibility(kARM/kLF)usedasmagneticconcentrationand

grain-sizeproxies,respectively. ThekLF andkARM/kLF recordsshow that

themagneticoscillations aremainly drivenby low-order concen-tration, grain size, or subtle mineralogical changes, fulfilling the criteria established for constructing a robust RPI index (Tauxe,

1993). TheRPIshowsalarge intervalofaverage lowvalues(0.05

±

0.03)spanning thedepth interval of

−

2 to24 m(Fig. 2). Itis significantlylowerthan thesurroundingvalues(0.13

±

0.05)and correspondstothehighestBe ratiovalues(andthushigherglobal productionofatmospheric10_{Be).Bothseriesarehighlycorrelated}

(

r

= −

0

.

81

)

,indicating a similarcontrolling mechanism.This low intervalisonlyinterruptedbytwosharppeaksofhigherRPI cen-teredat 8.2 and9.2 m (black stars in Fig. 2) that donot corre-spondtoanymagneticpropertychanges.Thevirtual geomagnetic poles(VGPs)exhibit transitionaldirectionsandlarge scatter(38.1

±

46.0◦) within this low RPI/high Be ratio interval. Conversely, theydefineantipodaldirectionsbelow(

−

67.5

±

9.9◦;reverse)and above(77.5

±

4.8◦,normal)thisinterval(Fig.2).

5. Discussion

5.1.RecordingofgeomagneticfieldvariationsovertheMBT

OurnewpaleomagneticresultscompletetherecordfromOkada etal. (2017), which was limited toup to 15 m above theByK-E tephra layer. The entire dataset now captures the field intensity recoveryanddirectionalstabilizationin normalpolarityand cov-ersthewhole reversalprocess fromthe late Matuyama(reverse) to the early Brunhes (normal). Complementary to the magnetic results, the Be ratio dataset provides an independent proxy of GDMvariations, which allowsbetter assessmentof therecording ofthegeomagnetic fieldintheCbCS,becausetheproductionrate of atmospheric 10_{Be is controlled primarily by the GDM}

follow-inganestablishedinverserelationship(e.g.,Poluianovetal.,2016).

This theoretical relationship has been verified in sediments and icecoresbytheidentificationandquantificationof10_Be

overpro-duction episodes associated systematically withall reversals and geomagnetic excursions ofthe last 2Ma (e.g.,Franket al., 1997; Carcailletetal.,2004;Raisbecketal.,2006;Thouvenyetal.,2008; Suganumaetal.,2010;Ménabréazetal.,2012; Valetetal., 2014; Horiuchietal.,2016;Simonetal.,2016a,2017,2018a,2018b;Du etal.,2018).

TheexcellentalignmentofRPIdecrease(increase)withBeratio increase (decrease)shows that the magnetic measurements sup-portatrustworthyrecordingofthedipolecomponentofthefield collapse/recoveryassociatedwiththeMBT(Fig. 3a). The standard-izedrecordsareparticularlycoherent,expressingsimilarvariations during theseintervalscontrolled bythedipole field(Fig. 3b).Our resultsfurtherdemonstratethatrecordingofthegeomagneticfield occurredat,orimmediatelyafter,thetimeofsedimentdeposition, withoutanytime delay,giventheveryshortatmosphericjourney of10_{Be anditsrapidscavengingrateatoceanicmargins.Together}

withtheveryhighsedimentationrates,thisevidenceofnoorvery little lock-in depth offset and post-depositional reworking sup-portstheconclusionthatthemagnetizationacquisitionconserved thetimingofrapidgeomagneticfeatures,allowinghigh-resolution andaccurateinterpretationsofthepaleomagneticrecordfromthe CbCS.

Figs. 3c–dshow that the polaritytransition (transitionalVGPs definedusinga conservativecut-off limitof

±

45◦) occurswithin the firststage of the“intensitypit” (i.e., overall lowerRPI values andhigherBe ratiofrom

−

2to 24m, Fig.2). Thesharppolarity switch(PS)between0.25and1.95m(VGPlatitudesshiftingfrom

−

80◦ to 71◦)is followedby a large directionalinstability cluster (IC1,VGPs averaging34

±

19◦) withinthe2–5minterval, before stabilizationtoaroundthecut-offlimitvalue from5to8m(IC2, 45

±

14◦).ThePS–IC1sequencefrom0.25to5mcorresponds to thefirstlowdipoleintensityinterval(Dip1)tracedbyboththeRPI and10Beproxies(Fig.3d).StablenormalVGPlatitudesbetween8 and10m(72

±

5◦)arefollowedbyaminorVGPinstabilitycluster (IC3, 58

±

7◦) that corresponds toa second dipole intensitylow (Dip2)withinthe11–14minterval.From8to19m,theslight dis-crepanciesbetweentheRPIandBeratiodatasets(Fig.3b)couldbe attributabletoeithernon-removedenvironmentalcontentsonone and/or bothproxies orinterplayofdipolarandnon-dipolar fields actingdifferentlyonbothrecords.TheRPIspikesobservedwithin the8–12.5mdepthintervalsuggestadominantnon-dipolar signa-tureonthepaleomagneticrecordduringtheintervalofoveralllow dipole fieldintensity,asshownbyhigherBe ratio.Suchan inter-val dominatedby highernon-dipoleenergies (directly imprinting onthemagneticsignal)andlowerdipolarenergy(maximum10_Be

productionrate)agreeswithgeomagneticmodelsandgeodynamo simulations (Leonhard and Fabian,2007; Olson et al., 2011) that show a similar state during timesof transitional field (produced mainlybythereductionofdipolestrength).Alternatively,theseRPI spikescouldalsobeassociatedwithsecondarymagneticoverprints amplified during theperiodofoverall weak magnetizationofthe primary dipolar component, but no rock-magnetic evidence sup-portssuchascenario(Okada etal., 2017). Between14and19m, lowRPIvaluestogetherwithVGPsindicatingrelativelystable nor-mal polarity andreducedBe ratios suggest that the surface field geometry is dominated by a dipole component of low intensity untilhighdipolemomentrecoversfrom24mupward.

The CbCS record exhibits a VGP sequence represented by a rapid polarity switch followed by three instability clusters (IC1–IC2–IC3) showing progressive diminution of amplitude be-forestabilizationinnormalpolarity(Fig.3d).Priortothereversal, twoVGPdeviationsbarelyexceeding45◦ areobservedwithin the overalllong-termintervalofdecreasingfieldintensity.

Fig. 3. Comparisonbetweenrelativepaleointensity(RPI),Beratio,andvirtualgeomagneticpole(VGP)variations.(a)Beratio(red)andRPI(black)results.(b)Standardized RPI(black)andBeratio(red)records.(c)VGP(gray)andstandardizedBeratio(red)records.(d)Firstcomponent(PC1)oftheprincipalcomponentanalysis(PCA)performed ontheRPIandBeratiorecords(blue)andVGPchanges(gray).(c–d)Theblacklinecorrespondstoa3-pointrunningmeanoftheVGPs.Theverticaldottedblacklines correspondtotheVGPcut-offlimits(±45◦)usedfordefiningthetransitionalfield.TheMBTischaracterizedbytwophases:apolarityswitchwithVGPsshiftingfrom−80◦ to71◦between0.25and1.95m(darkbrownbanding),followedbytransitionalfieldvaluesoscillatingaround39±14◦withinthe1.95to8minterval(lightbrownbanding). Thisintervalischaracterizedbytwoinstabilityclusters(IC1andIC2).Thepolarityswitchandtransitionalfieldstagescorrespondtoafirstintervalofdipoleintensitylow withinthe0.25to5mdepthinterval(Dip1).Athirdinstabilitycluster(IC3)ofloweramplitudecorrespondstoasecondintervalofdipoleintensitylow(11to14m,Dip2). 5.2. DynamicsofgeomagneticvariationsovertheMBT

Fig. 4 compares VGP latitude and dipole moment variations through time using the chronology of Suganuma et al. (2018). Chronologicaluncertainties associatedwithastronomical assump-tionsandagemodelconstructioncan alterestimatesofthe abso-luteage,rate,anddurationofgeomagneticvariationswithinmodel uncertainties(

±

5ka),butnottheirchronostratigraphicorderingor averagemagnitudes(timeandamplitude).ThenormalizedBeratio record wascalibrated intermsofdipole moment(DM)usingthe theoreticalmethodpresentedinSimonetal. (2018b) andbasedon the10_{BeproductionmodelofPoluianovetal. (2016).Overallhigh}

DMabove10

×

1022Am2,whichisproportionallysimilartothose observed during the late Holocene(Pavón-Carrasco et al., 2014), wasreconstructedfromeithersideofthe“intensitypit”(whichis averaged at3.9

±

1.2

×

1022 _Am2 _between

₋

_{2 and24 m) that}

spansa17kaintervalbetween777and760ka.

HighDMvaluesof12

±

1

×

1022_Am2_{prevailuntil790kaand}

thendecreasebeforevaluesstabilizeat6.5

±

0.5

×

1022Am2 be-tween 784and777ka. Thisfirst-stephalvingis characterizedby aratherconstantrateofdecreaseof1.5

×

1022Am2/ka.This esti-mate corresponds to the decayrate observed over thelast three millenniums (Genevey et al., 2008) and prior to the Laschamp excursion by independenthigh-resolution records (LajandKissel,

2015) andisabouttwotimeslessrapidthanthedecayofthe ax-ialcomponentofthedipolefieldobservedinthelasttwocenturies (Finlayetal.,2016).ThesignificantVGPdeviationobservedat784 kaiscoherentwiththeoccurrenceofmillennialscaleinstabilities overthe

∼

33kaperiodprecedingtheMBT,asproposedbyBalbas et al. (2018), andwithan excursionalprecursor recorded inlava flowsinChileandGuadeloupe(Singeretal.,2018).

A second-step field collapseof similar rateobserved between 777 and 772 ka is interrupted by a small break at mid-slope,

Fig. 4. Beratio-deriveddipolemoment(BeDM;greencurve)andVGPlatitude(gray) overtheMBT.Theblacklinecorrespondstoa3-pointrunningmeanoftheVGPs. Thepolarityswitchesthatexplainmostoftheangulardeviationspan2ka(773.9 to771.9ka),butthetransitionalfieldprevailsuntil765.2ka(IC1–IC2).Thelargest VGPdeviationscorrespondtotheDip1interval(773.7to767.4ka).Thegreen load-ingbarshowsDMpercentrelativetopresentvalue.Theresultsarecomparedwith aschematicreversalpath(red)illustratingthethree-phasesuccessionofreversal (precursor,transit,andrebound)describedbyValetetal. (2012) fromreversal vol-canicsequences.

where the VGPs show a millennial deviation at 775 ka. The PS occursduringthesecondhalfofthisfieldintensitydecrease,over the interval of 773.9 to 771.9 ka (2 ka duration), with DM val-ues waning from 4.6

±

0.4 to 2.3

±

0.3

×

1022 Am2. Most of the VGP deviation (128◦ drift) occurs at the early stage of the PS phase within a 30 cm interval (0.25–0.55 m), representing about400

±

200 yrs(773.8–773.4 ka).Such rapidVGP drift cor-respondsto the maindirectional change ofa polarity reversalin a numerical dynamo (Olson et al., 2011) and is consistent with theidentificationofsub-millennial-length fieldoscillationsduring transitionalintervals(Nowaczyketal., 2012; Sagnottietal.,2016; Chou et al., 2018). However, the uncertainty associated withthe actualCbCSchronologyprecludesanyrobustinterpretationatsuch centennialtimescales.Overall,the772–774(

±

5)kaagerange cor-respondstothedatingoftheMBTfromthemostreliablemarine sedimentaryrecords(e.g.,Tauxeetal.,1996; Channelletal.,2010; Channell,2017;Simonetal.,2017,2018b; Valetetal.,2019)and transitionallavaflows (e.g.,Ricci etal., 2018;Singeretal.,2018). Between771.9and768.5ka,largedirectionalinstabilitiesare con-temporaneouswithweakdipolefield,averageof2.7

±

0.2

×

1022 Am2,indicating asignificant non-dipolar contribution.Altogether, the succession of large directional instabilities and low DM val-ues observed between 773.9 and 768.5 ka (PS and IC1, DM

=

3.0

±

0.7

×

1022 Am2; VGP

=

28.3

±

26.7◦) agrees with the transition and rebound phases of the sequential reversal model proposed by Valet etal. (2012) fromdetailedvolcanic recordsof reversals(Fig. 4).Onecanalsotentativelyassociatetheinstability eventat775kawiththeprecursorphaseofthesequential rever-salmodel,butitssmallamplitudedeviationprecludesany defini-tive conclusion.The identificationof thesesuccessivephasesand thenearly perfecttiming agreementbetweenour high-resolution recordandthemodelcanbeseenasaposteriorvalidationofthe commonduration assumptionusedby Valetetal. (2012) for cor-relatingalltransitionallavaflows.Itisalsoworthnotingthatthis well-definedsequentialreversalmodeloccursatthefinalstageof

a long-term unstable geodynamo period characterized by overall lower field intensity and the occurrence of one or several pre-cursor events. These precursor events, defined by paleointensity lowsand/ordirectionalinstabilities,aresignificantlyolderthanthe precursor phase of the sequentialreversal model (e.g.,Hartl and Tauxe,1996;Channelletal.,2009;Singeretal.,2005,2018;Valet etal.,2014).Ourresultssuggestthattheprecursoreventdatedat

∼

784kaispartofacontinuumprocessassociatedwiththedipole fieldstrengthreductionbeforetheMBT(Fig.4).Unfortunately,the limitedtime intervalcoveredbyourrecordpreventsdiscussionof the olderprecursor event, datedat

∼

797ka, andits significance into the long-termgeomagnetic field instability period preceding thereversal(Balbasetal.,2018).

Giventheelementspresentedabove,weare inclinedtodefine theMBTbythePSandIC1eventsduringthelowdipole moment spanning a 5.4ka time interval (Dip1, 773.9–768.5ka). However, the integrationof thelow-amplitudetransitional VGP cluster ob-serveduntil765.2ka(IC2eventat

∼

45◦) extentsthetotal transi-tionalfieldinterval,usingaconservativedefinition,toamaximum 8.7kaduration(Fig.4).WhethertheMBTshouldbedefinedbythe PSphasealone(2 kaorless),theDip1lowdipole intensity inter-val(PS–IC1,5.4ka),orthePS–IC1–IC2succession(8.7ka)depends largelyonthedefinitionofreversal(useofdirectionalflipor inten-sitylow)ontherecordresolutionandisalsolikelysite-dependent becauseoflargenon-dipolecontributionduring weakdipole peri-ods(Clement,2004; Valetetal.,2016).Theseelementsare impor-tantbecausetheyadd uncertaintytointerpretations and chronos-tratigraphic correlations of reversals worldwide. As a matter of fact, rapid geomagnetic features are not recorded similarly be-tween slower/fastersedimentationraterecords,calling into ques-tion proper correlation of VGP paths from distinctive records. In Fig.5a,wecompareourVGP recordtomarine sequencesof mid-to high-sedimentationrates,i.e., MD90-0961: 4.2cm/ka(Valet et al.,2014),IODPSiteU1308:7.3cm/ka(Channelletal.,2008),and ODP983, 13cm/ka(ChannellandKleiven,2000).The VGPrecords fromCbCSandODP983areparticularlywellcorrelated,expressing asimilarsuccessionofinstabilityevents,especiallyforthePSand IC1phases,buttheVGPrecordsfromsitesU1308andMD90-0961 appearlargely smoothed,time-averaging rapidinstabilitiesof the transition sequence. VGP records of reversals from other high sedimentation rate sites (e.g., Okada and Niitsuma, 1989; Tric et al., 1991; Mazaud and Channell, 1999; Hyodo et al., 2006; Mazaudetal.,2009; Kusuetal.,2016; Channell,2017; Kirscheret al.,2018)orlavaflows(Jarboeetal.,2011; Mochizukietal.,2011; Valet et al., 2012) also show a complex transitional field behav-ior with a similar succession of events and durations, whereas low sedimentationraterecords always presentsmoothedprofiles (Valet et al., 2016). The stochastic geomagnetic model of Buffett (2015) interprets such temporal complexity by the strength and structure of convective fluctuationsin the outer core, explaining theincreasedvariationsofdipolegenerationduring polarity tran-sitions.

ImmediatelyaftertheMBT,adipole momentlow(Dip2,2.4

±

0.4

×

1022Am2)andsmall-amplitudeinstabilitiescluster(IC3)are observedbetween763.5and762.5ka.This1kaduration instabil-ityeventconstitutesthelastgeomagneticfieldricochetbeforethe dipolefieldstabilizedinitsnewstate.From762ka,thedipolefield increasesrapidlytoveryhighDMvalues(14.3

±

1.7

×

1022Am2), whiledirectionsare stabilizedinnormalpolarity.Therateofthis fieldrecovery,averageof5.3

×

1022Am2/ka,isthreetimeshigher than the rate offield decay,and DMreaches higher valuesthan before the MBT.Similar field asymmetry betweenincreasing and decreasing dipole strength changes framing reversalshas already beenobservedfromlow-resolutionglobalRPIstacks,sequencesof superposedlava flows, andseafloor magnetization, andhas been computedbygeomagnetic modelsandreproducedby

experimen-Fig. 5. GlobalcomparisonoftheMBTrecord.(a)VGPfromCbCS(black),IODPsite U1308(blue),ODP983(green),andMD90-0961(red).(b)Virtualaxialdipole mo-ment(VADM)seriesfromreferenceRPIstacks(Xuanetal.,2016) and (c)dipole momentseriescalibratedfrom10_{Be-records(BeDM).The}10_{Be-fluxfromEDC(light} blue,Raisbecketal.,2006)hasbeencalibratedintermofDMusingthemethod fromSimonetal. (2018b).(d)BeDMandRPIrecordsfromtheCbCS.

tal dynamo (Fig. 5b; Valet etal., 2005; Guyodoand Valet, 2006; Leonhardt and Fabian, 2007; Pétrélis et al., 2009; Ziegler and Constable, 2011; Avery et al., 2017), supporting different phys-ical control processes characterized by different timescales, i.e., long-termdiffusion associated withfield decayand rapid advec-tionduringrecovery.Overall,thefielddecreasestoitslowest val-ues in about 18 ka,but it recovers through only a 3 ka period. During the transitional field interval, the succession of rapid in-stabilities (

<

1 ka) demonstrates complex field behaviors at the surfaceresultingfromtherapidemergence,growth,andtransport ofreversed flux patches atthe core–mantle boundary associated with strong advective processes in the fluid outer core resulting inbrokenequatorialflowsymmetry(BloxhamandGubbins,1986; Coeetal.,2000; Amitetal.,2010; Olsonetal.,2011).

TheCbCSrecords asequence ofgeomagnetic fieldinstabilities coinciding with periods of low-dipole intensity during the MBT period. The actual record does not allow a comparison of these transitional features with secular variations during stable polar-ityintervals. Futuresamplingatthissitemightholdthe keys for analyzing the two extremes of the geomagnetic field variability continuum, i.e., reversals and secular variations, andallow com-parisonofreversalvalueswithlong-timeaverages.

omagneticdipole momentcollapseandthatthefieldexperienced a sequence ofrapidoscillations beforethe full polaritystate was reestablished. Apolarityswitch (PS)episodefrom771.9–773.9ka capturingmostoftheangulardeviationfollowedby adirectional instability phase(IC1)until768.5 kaoccurredduringweakdipole field.Fourrapidepisodesofdirectionalinstabilitiesprecedingand following the PS–IC1 phase complete the reversalsequence. The asymmetry observed betweenthe long-term dipole decay phase andthesharp recoveryphase, andrapidoscillations (

<

1 ka) evi-dencethecomplextransitional fieldduring theweakdipole field. Therecognitionofthesefeaturesfurtheremphasizesthefactthat mostreversalrecordsdonotintegratethefullfieldbehavior,which isproblematicforreconstructingtheprecisedynamicsand under-standing the underlying physical processes. However, this short-coming does not hamper stratigraphic correlations among most geologic records,especially when using a dipolar intensityproxy astheglobalmarker.

Acknowledgements

QS acknowledges Laëtitia Gacem and Coralie Andreucci for assistance with the beryllium sample preparation. YS and MO were supported by a JSPS Kakenhi Grant (16H04068;15K13581; 17H06321). James E.T. Channell is acknowledged for sharing North Atlantic paleomagnetic data. The ASTER AMS national fa-cility (CEREGE,AixenProvence) issupportedby INSU/CNRS,ANR through the EQUIPEX “ASTER-CEREGE” action, andIRD. The data presented inthisstudyareavailable within the supporting infor-mation.

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