Increased production of cosmogenic 10 Be recorded in oceanic sediment sequences: Information on the age, duration, and amplitude of the geomagnetic dipole moment minimum over the Matuyama–Brunhes transition

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Increased production of cosmogenic 10 Be recorded in

oceanic sediment sequences: Information on the age,

duration, and amplitude of the geomagnetic dipole

moment minimum over the Matuyama–Brunhes

transition

Quentin Simon, Nicolas Thouveny, Didier Bourlès, Franck Bassinot, Tatiana

Savranskaia, Jean-Pierre Valet

To cite this version:

Quentin Simon, Nicolas Thouveny, Didier Bourlès, Franck Bassinot, Tatiana Savranskaia, et al..

Increased production of cosmogenic 10 Be recorded in oceanic sediment sequences:

Informa-tion on the age, duraInforma-tion, and amplitude of the geomagnetic dipole moment minimum over the

Matuyama–Brunhes transition. Earth and Planetary Science Letters, Elsevier, 2018, 489,

pp.191-202. �10.1016/j.epsl.2018.02.036�. �hal-01735837�

Contents lists available atScienceDirect

Earth

and

Planetary

Science

Letters

www.elsevier.com/locate/epsl

Increased

production

of

cosmogenic

10

Be

recorded

in

oceanic

sediment

sequences:

Information

on

the

age,

duration,

and

amplitude

of

the

geomagnetic

dipole

moment

minimum

over

the

Matuyama–Brunhes

transition

Quentin Simon

a

,

b

,

∗

_,

_{Nicolas Thouveny}

a

_,

_Didier

_{L. Bourlès}

a

_,

_{Franck Bassinot}

c

_,

Tatiana Savranskaia

b

,

Jean-Pierre Valet

b

,

ASTER

Team

1

a_{CEREGEUM34,AixMarseilleUniv.,CNRS,IRD,INRA,CollFrance,Aix-en-Provence,France}

b_{InstitutdePhysiqueduGlobedeParis,SorbonneParis-Cité,UniversitéParisDiderot,UMR7154CNRS,Paris,France} c_{LSCE,UMR8212,LSCE/IPSL,CEA–CNRS–UVSQandUniversitéParis-Saclay,Gif-Sur-Yvette,France}

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory:

Received20December2017

Receivedinrevisedform22February2018 Accepted24February2018

Availableonlinexxxx Editor:M.Frank Keywords:

authigenic10_Be/9_Beratio

geomagneticdipolemomentminimum Matuyama–Brunhestransition(MBT) geomagneticpolarityreversal marineisotopestage19 atmospheric10_{Beproductionrates}

Newhigh-resolutionauthigenic10_Be/9_{Beratio(Be-ratio)recordscoveringthelastgeomagneticreversal,} i.e.theMatuyama–Brunhestransition(MBT),havebeenobtainedandsetonatimescaleusingbenthic

δ18O(Cibicideswuellerstorfi)records.Thegeographic distributionofthefourstudiedsitesallowsglobal comparisonbetweenthe NorthAtlantic, Indianand PacificOceans.AllBe-ratiorecordscontain a two-foldincreasetriggeredbythegeomagneticdipolemoment(GDM)collapseassociatedwiththeMBT.The stratigraphicpositionoftheBe-ratiospike,relativetomarine isotopestages,allowsestablishmentofa robustastrochronological frameworkfortheMBT, anchoringitsagebetween778and 766ka(average mid-peaksat772ka),whichisconsistentwithallotheravailable10_{Be-proxyrecordsfrommarine,ice} andloessarchives.Theglobal10Beatmosphericproductiondoublingrepresentsanincreaseofmorethan 300 atoms m−2_s−1 _{that is compatiblewith the increased magnitudeofatmospheric} 10_{Be production} obtainedbysimulationsbetweenthepresentGDM andanull-GDM.Theminimum10_{Be-derivedGDM} average computed for the 776–771 ka interval is 1.7±0.4×1022 _Am2_{, in agreement with model} simulationsandabsolutepaleointensitiesoftransitionallavaflows.

©2018TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

The systematic coupling of polarity reversals with geomag-netic dipole moment (GDM) collapses constitutes the main en-tryforunderstandingfundamentalmechanismsofthegeodynamo (e.g. Amit et al., 2010). Magnetohydrodynamic models are be-ginning to provide insights into these mechanisms (Glatzmaier and Coe, 2015), but high-resolution and high-quality observa-tions are vital to provide accurate constraints on long-term and rapidGDM variations. Paleomagnetic studies of the last geomag-netic reversal, i.e. the Matuyama–Brunhes transition (MBT), pro-vide numerous records of dipole moment variations and virtual geomagnetic pole (VGP) paths of transitional states (e.g.

synthe-*

Correspondingauthorat:CEREGEUM34,AixMarseilleUniv.,CNRS,IRD,INRA, CollFrance,Aix-en-Provence,France.

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

1 _{GeorgesAumaître,KarimKeddadouche.}

sis by Valet and Fournier, 2016), but severalinconsistencies lead to conflicting interpretations on the geometry andchronology of the transitional field (Tauxe et al., 1996; Channell et al., 2010; Valet et al., 2012,2016; Sagnotti etal., 2016; Mark et al., 2017; Channell,2017).Insedimentarypaleomagneticrecords,thesources ofsuch unconformities inconsistencies aremultiple: i) multicom-ponent magnetizations (Roberts, 2015); ii) uncertainties or even ignoranceaboutsedimentmagnetizationlock-inprocesses(Roberts etal., 2013) andthe influenceof strongerpost-transitional fields and(partial-) remagnetization/realignment ofgrains deposited in weakfields(CoeandLiddicoat,1994; Simonetal.,2018);iii) sig-nalsmearingandaveragingduetoinsufficienttemporalresolution (RobertsandWinklhofer,2004; Valetetal.,2016; Channell,2017). Volcanicpaleomagneticrecordssufferfrom:i)spatial heterogene-ityofthermoremanentmagnetizationacquisitionalong/acrosslava flowsections;ii)theinfluenceofthemagnetizationofunderlying lavaflows onthelocalfieldrecordedby theoverlyingflow while cooling (Vellaetal., 2017);iii) discontinuous lavaemission rates. Other biasesthataffectboth typesofrecordsaredueto regional https://doi.org/10.1016/j.epsl.2018.02.036

0012-821X/©2018TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Fig. 1. Geographiclocationsofthefourstudiedcores(blackcircles)alongwithBe-ratioand10_{Be-fluxfrommarineandglacialrecordsdiscussedinthetext(whitecircles).}

Thesereferencesinclude:MontalbanoJonico(Simonetal.,2017);EpicaDomeC(EDC)(Raisbecketal.,2006);MD97-2140(Carcailletetal.,2003);MD97-2143(Simonet al.,2018);MD05-2930(Ménabréazetal.,2014; Simonetal.,2016a),andMD98-2187(Suganumaetal.,2010).(Forinterpretationofthecolorsinthefigure(s),thereaderis referredtothewebversionofthisarticle.)

effects of non-dipole field sources (Leonhardt and Fabian, 2007; Amitetal., 2010) and chronological uncertainties in astrochrono-logical,radiometricandother datingcalibrations(e.g.Lisieckiand Raymo,2009; Jichaetal.,2016; Niespoloetal.,2017).Allofthese factorsrestrictthereliabilityofourunderstandingofthe morphol-ogy(intensityanddirection)andchronology(timingandduration) oftransitionalfields.

Acomplementary andindependentmethod toreconstructthe GDMvariationisprovided byreconstruction oftime variations of theatmosphericproductionofthecosmogenicnuclide Beryllium-10(10_{Be).Pioneeringstudiesshowedthatcosmogenicnuclide}

pro-ductionis inverselyproportionalto theGDM (e.g.LalandPeters,

1967).Thisinverserelationshipwaslaterdemonstratedand quan-tified (e.g. Kovaltsov and Usoskin, 2010 and references therein). The mostdetailed10_{Be recordsfrom marine sediments,loess, or}

ice cores also document large 10Be overproduction episode that correspondsto thedipoleintensitycollapseattheMBT(Raisbeck etal., 1985, 2006; Carcaillet et al., 2003, 2004; Suganumaet al.,

2010; Zhou et al., 2014; Ménabréaz et al., 2014; Valet et al.,

2014; Simon et al., 2017, 2018). Recent studies by Simon et al. (2016a, 2018) further strengthen evidencefor the systematic oc-currenceof10Beoverproductionepisodesassociatedwith geomag-netic dipole minima linked to polarity reversals and excursions overthelast2Ma.Thissupportstheuseof10_{Beforreconstructing}

accurateGDMchanges.

Here,wepresentnewauthigenic10Be/9Beratio(Be-ratio here-after)andbenthic

δ

18OresultsacrosstheMBTintervalfromthree marine sediment corescollected in theIndian, Pacific,andNorth AtlanticOceans,whichareintegratedwithpublished resultsfrom core MD90-0961 (Valet etal., 2014). The 2-cm sampling resolu-tion allows interpretation and comparison of Be-ratio variations during the last reversal from these four distant sites with un-precedentedhigh-resolution.The

δ

18Orecordspermit comparison ofthestratigraphicpositionsofBe-ratiovariationsandenable de-velopmentofareliableastrochronologicalframeworkfortheMBT. Robust reconstruction of dipole moment variations over the last geomagneticreversalprovides newconstraints,independentfrom paleomagnetism, which assists with understanding paleo-records andgeodynamosimulations.

2. Materialsandmethods

2.1. Corelocationsandsamplingstrategy

Themarinesedimentcoresstudiedwereretrievedusingthe Ca-lypso coring systemon boardthe R/VMarion-Dufresne fromthe Indian (MD90-0949 and MD90-0961), Pacific (MD98-2183), and North Atlantic (MD95-2016) Oceans (Fig. 1 and Table 1). They were sampledusing u-channelsandindividual 8cm3 cubic plas-tic boxes forpaleomagnetic measurements. The cubeswere later subsampled forBe and

δ

18Omeasurements followingcompletion ofthepaleomagneticanalyses,whichensuresreliabledepth corre-lations betweenpaleomagneticandgeochemicalresults.Magnetic measurementsperformedonu-channelandindividualsamples in-dicate the depth ofthe MBTin coresMD98-2183 (Yamazakiand Oda,2005;Valetetal.,2016),MD90-0961(Valetetal.,2014),and MD90-0949/MD95-2016 (Valet etal., 2016). We usedthese pale-omagnetic results to sample at high-resolution over stratigraphic intervalsthatcorrespondtotheMBTineachcore.Beresultsfrom core MD90-0961 were presented by Valet etal. (2014) and cor-rectedbySimonetal. (2016a).

2.2. Oxygenisotopes

Stable oxygen isotopic compositions were measured on the benthicforaminiferaspeciesCibicideswuellerstorfi (

>

150 μm size-fraction)at

∼

2to4cmstratigraphicintervals.Analyseswere per-formedwithVG-OptimaorElementarIsoprimedual-inletgasmass spectrometersattheLaboratoiredesSciencesduClimatetde l’En-vironnement(LSCE).Allresultsareexpressedas

δ

18Ovs. VPDB(in

h

)withrespecttoNBS19standard.Theinternalanalytical repro-ducibilitydeterminedfromreplicatemeasurementsofacarbonate standardis

±

0.05%(1

σ

).

2.3. Australasianmicrotektites

Microtektite analysis was carried out on cores MD90-0961, MD90-0949, andMD98-2183 todetect thethickness ofthe sedi-mentarymixedlayerandtoprovidean independentstratigraphic

Table 1

Corelocationsandaveragesedimentationrates. Cores Latitude/longitude (◦) Waterdepth (m) Averagesedimentation rate(cm/ka) #Be samples MD98-2183 2◦00.82N/135◦01.26E 4388 2.0±0.6 138 MD95-2016 57◦42.46N 29◦25.44’W 2318 3.1±1.0 182 MD90-0949 2◦06.90N 76◦06.50E 3600 2.0±0.5 124 MD90-0961 5◦03.71N 73◦52.57E 2446 4.2±0.6 116 MD97-2143a ₁₅◦_87N/124◦₆₅_{E 2989 1.1±0.3 51}

Averagesedimentationratesandsamplenumbersarefromthe∼700–850kaageintervalineachcore(seedata inthesupplementarymaterial,TablesS1–S4).

a _{DatafromSimonetal. (2018).}

marker.Thesemicrotektiteswere producedafterameteoritic

im-pact that preceded the MBT by 12–15 ka, and are found in

marine sediments throughout much of the Indian Ocean and in marginalseasofthewesternPacificOcean(Schneideretal.,1992; LeeandWei,2000; Suganumaetal.,2011; Valetetal.,2014). Fol-lowingtheprocedureestablishedby Schneideretal. (1992), sedi-mentsamplesweretreatedwith0.1MHCl, a20%H2O2 solution,

anda5%sodiumhexametaphosphatesolutiontoremovecarbonate andresidualorganics,andtoaiddisaggregation.Thesampleswere wet-sievedandthe

>

100 μmfractionwasexaminedvisuallyusing abinocular microscope.Onlyobjectsidentified unambiguouslyas microtektiteswerecounted.

2.4.Berylliumprinciplesandmeasurements

The exchangeable-10Be concentration measured in sediments, or authigenic 10_{Be hereafter, corresponds to the fraction that is}

adsorbedorchemicallyprecipitatedonto settling particles.It pri-marilydependson10_{Beatmosphericproductionratebutisalso}

af-fectedbysecondaryenvironmentalcomponents(oceanictransport processesordetritalinputchanges).Thesesecondary environmen-talcomponentsareminimized/removedbyusingauthigenic9_Beas

anormalizer (Bourlèsetal., 1989) inmostrecords,which makes itpossibletoinfergeomagneticpaleointensityvariations fromthe Be-ratio(Simonetal.,2016aandreferencestherein).

AuthigenicBeisotopesanalyseswerecarriedoutattheCEREGE NationalCosmogenicNuclidesLaboratory(LN2C),France.Atotalof 444samples were collected from coresMD90-0949, MD95-2016, and MD98-2183 over the

∼

700–850 ka age interval with vari-ablesampling resolution rangingfrom 10to 1 cm. Theseresults complete previous Be-measurements performed using the same method on core MD90-0961 (Table 1). An average 2-cm sam-pling interval is attained within the MBT interval in each core. The sampleshave beentreatedaccording to thechemical proce-dure established by Bourlès et al. (1989) and revised by Simon et al. (2016b). Authigenic 10Be and its stable isotope 9Be were extracted from

∼

1 g dry samples by soaking them in a 20 ml leaching solution (0.04 M hydroxylamine (NH2OH–HCl) and 25%

acetic acid) at 95

±

5◦C for 7 h. A 2 ml aliquot of the re-sulting leaching solution was sampled for measurement of the natural 9_{Be concentration using a graphite-furnace Atomic}

Ab-sorption Spectrophotometer (AAS) with a double beam correc-tion (Thermo Scientific ICE 3400®_{). The remaining solution was}

spiked with 300 μl of a 9

.

8039

×

10−4 g g−1 9Be-carrier before Be-purification by chromatography to determine accurately 10Be sample concentrationsfromacceleratormassspectrometer (AMS) measurements of 10Be/9Be ratiosat the French AMS national fa-cilityASTER(CEREGE).10_{Besampleconcentrationswerecalculated}

fromthe measuredspiked 10Be/9Be ratiosnormalizedto theBeO STD-11in-housestandard

(

1

.

191

±

0

.

013

×

10−11

)

(Braucheretal.,

2015) forcoresMD95-2016andMD98-2183;andtotheNIST4325 StandardReferenceMaterial

(

2

.

79

±

0

.

03

×

10−11

)

(Nishiizumiet al., 2007) forcores MD90-0961 andMD90-0949.Authigenic 10_Be

concentrationsare decay-corrected using the 10_{Be half-life (T} 1/2)

of 1.387

±

0.012 Ma (Chmeleff et al., 2010; Korschinek et al.,

2010).

3. Resultsandinterpretations

Results are plotted in Fig. 2 as a function of the corrected depthsofeachcore,with

δ

18_{Oandmicrotektiteintheupperpanel,}

and10Be and9BeconcentrationsandBe-ratiosinthemiddleand lower panels, respectively, and are listed in Supplementary Ma-terial (Tables S1 to S4). The polarity change associated with the MBT ismarked by thereversalangle(lower panel ofFig.2) that definestheangulardeviationbetweenthemeasuredmagnetic vec-tor andthedirection ofthe axialdipole field atthe sitelatitude (Valet et al., 2016). It describes the variabilityof the local mag-netic field vector without any a-priori assumption regarding the field geometryinherent to the VGP.The transitional interval cor-respondstothesedimentthicknessbetweenangulardeviationsof 30°and150°.

ThebenthicCibicideswuellerstorfi

δ

18Odatavarybetween

∼

3.2 and4.7

h

attheNorthAtlanticsite(MD95-2016)andoscillate be-tween

∼

2.4and4.2

h

attheIndian(MD90-0949andMD90-0961) andwesternPacificsites(MD98-2183)(Fig.2).Glacial–interglacial cycles are labeled according to the nomenclature proposed by Railsback et al. (2015). A sharp decrease of

∼

1.2 to 1.5

h

char-acterizestermination9 (the deglaciation betweenmarine isotope stage (MIS) 20 and 19c) at each site, while the MIS 19c–MIS 18etransitionismoregradual,withsecondaryoscillationsat sub-stagesMIS19a–b.Theamplitudeofglacial–interglacial

δ

18_O

tran-sitions corresponds to those observed in the LR04 global ben-thic stack (

∼

1.2

h

), which is indicative of recording of a re-liable global ice volume signature (Lisiecki and Raymo, 2005). The record from core MD90-0961 is limited to the MIS 20–19a interval and has been completed by Cibicides wuellerstorfi

δ

18O data from sister coreMD90-0963 retrieved at the same location (Bassinot et al., 1994). Massive carbonate dissolution occurs in core MD98-2183 at the MIS 19 level, which complicates robust interpretation apart from primary glacial–interglacial oscillations. Furthermore,the rapid

δ

18Oincrease at3040cm towarda value closeto glacial onestogether withthefollowing 40cmdata gap precludesidentificationofanyMIS19sub-stagesinthiscore.

Amicrotektitelayerisidentified incoresMD90-0961(Valetet al.,2014) andMD90-0949,butisnotobservedincoreMD98-2183. Themaximumconcentrationcoincideswiththeonsetof termina-tion9inbothcores,andisfollowedbyaprogressivedecreaseover 23 and26 cm in coreMD90-0961 and MD90-0940,respectively. Thisreflectssmearinginducedbybioturbationinthesurficial mix-ing layer (Fig. 2). Similar 20–25 cm depth ranges between the maximum concentration anddisappearance ofmicrotektites have beenobservedatsitesODP758Band769A(Schmidtetal.,1993) andMD97-2142(LeeandWei,2000),whileitis15–20cmincores MD97-2187(Suganumaetal.,2011) andMD97-2143(LeeandWei,

2000).Microtektiteconcentrationsinthesetwocoresare,however, lowerthanthatattheothersites.Althoughlimited,thissite com-parison suggestsa first-order influence ofthe initial microtektite

Fig. 2. Oxygenandberylliumisotoperesults.ThebenthicCibicideswuellerstorfiδ18_{Orecordsandmicrotektitedistributions(orange)arerepresentedintheupperpanel.Marine}

isotopicstage(MIS)andsub-stagenomenclatureisfromRailsbacketal.(2015).Authigenic10_Be(blue)and9_{Be(green)concentrationareshowninthemiddlepanel.The}

Be-ratio(red)andreversalangle(black)foreachcoreisrepresentedinthelowerpanel.NotethatdifferentscalesareusedfortheBe-data.Thereversalangle(Valetetal.,

2016) enablesdefinitionofthetransitionalfielddepthintervalineachcore.Thegreybandshighlightthetransitionalfieldsbetweenfullpolaritystates.

concentration,along withsedimentationrate, onthethicknessof the mixinginterval. This Australasian microtektitelayer was also usedtodeconvolvethe10_{BesignalbySuganumaetal. (2011) and}

Valet etal. (2014), showing nostratigraphic displacement ofthe

10_{Be peak inducedby bioturbationfiltering.Volcanicglassshards}

(tephra)werealsodetectedbetweendepthsof3710and3713cm in core MD90-0961 (Fig. 2) but were not geochemically studied here.

Authigenic 9Be concentrations vary from 0.77 to 1

.

94

×

1016 at g−1 incoreMD90-0961(meanvalue:1

.

34

±

0

.

22

×

1016at g−1) andfrom 0.88to 2

.

19

×

1016 at g−1 (mean value: 1

.

44

±

0

.

26

×

1016at g−1)innearbycoreMD90-0949.Inthetwoothercores,the higherauthigenic9Beconcentrationsvaryoverwiderranges,that isfrom1.45to3

.

36

×

1016at g−1 incoreMD98-2183(meanvalue: 2

.

27

±

0

.

32

×

1016 at g−1) and1.10to 2

.

86

×

1016 at g−1 incore MD95-2016(meanvalue:1

.

92

±

0

.

37

×

1016_{at g}−1_).IntheNorth

AtlanticOceancoreMD95-2016,authigenic9_{Bevariationsseemto}

beinfluencedbytheglacial–interglacialpattern.9_{Beconcentration}

maximacoincidewithglacial

δ

18Ovalues,whichsuggeststhat9_Be

deliveryatthelocationisinfluencedbydenudationratesand clas-ticinputs (e.g.Simon etal., 2016b) and/or hydrographic changes (waterflowspeedandnorthernsourcewater;Kleivenetal.,2011). IncoreMD90-0949,onthecontrary,9_{Bemaximaoccurduring}

in-terglacialMIS 19,while innearby MD90-0961 coreno particular

9_{BetrendisobservedduringMIS19,whichsuggestsdifferent}9_Be

oceanic pathways for these neighboring cores situated at

differ-entwaterdepths(Table1).Theserelationshipsarelikelyrelatedto i) sedimentprovenancechanges influencedby oceaniccirculation and ii)water massstratification changes. The 9_{Be record ofcore}

MD98-2183ischaracterizedbyminoroscillationsbetweenMIS21 and19andtwosuccessiveminimawithintheMIS18b–cinterval andattheMIS17–18atransition.

Theauthigenic10Beconcentrations(decay-corrected)varyfrom 3.30to10

.

64

×

108 at g−1 and4.56to14

.

89

×

108 at g−1 incores MD90-0961(meanvalue:5

.

14

±

1

.

57

×

108at g−1)andMD90-0949 (mean value: 7

.

52

±

2

.

20

×

108 at g−1), respectively. The authi-genic 10Be concentrations in core MD95-2016 (North Atlantic) are similar to the authigenic 10Be concentration values of core MD90-0961, rangingfrom3.13to 8

.

85

×

108 at g−1 (meanvalue: 5

.

15

±

1

.

31

×

108 at g−1). In core MD98-2183 from the Pacific Ocean, the authigenic 10Be concentrations are nearly two times higher andare distributed over a wider range, that is from9.71 to25

.

34

×

108 _{at g}−1_{(meanvalue:15}

_.

₈₂

_±

₃

_.

₄₂

_×

₁₀8 _{at g}−1_).The

mainfeatureinallstudiedcoresisthelargeincreasein10_Be

con-centrationcomparedwiththesurroundingintervalsinMIS19a–b (Fig. 2). A secondary 10_{Be peak is also observed during MIS 17}

while a minor10_{Be increase is observedduring MIS18a ateach}

site.

IncoresMD90-0961andMD90-0949,Be-ratios varyfrom2.26 to 8

.

27

×

10−8 _{(mean value: 3}

_.

₉₃

_±

₁

_.

₃₆

_×

₁₀−8_{) and 4.07 to}

8

.

22

×

10−8_(meanvalue:5

_.

₁₉

_±

₀

_.

₈₄

_×

₁₀−8_{),respectively.Incore}

Fig. 3. Authigenic Be-ratios versus10_{Be and}9_{Be concentrations. The grey-shaded area represent values within the MBT interval. See Table S5 for correlation coefficients.} value:6

.

96

±

1

.

05

×

10−8) andin coreMD95-2016 from 1.53to

5

.

26

×

10−8 (meanvalue:2

.

78

±

0

.

89

×

10−8). Theseaverage Be-ratios correspond to differentocean basin averages estimated by vonBlanckenburgandBouchez (2014) frommodernseawaterand deepoceansurfacesedimentBeconcentrations,therefore support-ing complete homogenization of both Be isotopes in the water column (see Wittmann et al., 2017). The significant relationship between the long-term average Be-ratio and 10Be with site wa-ter depths

(

r

>

0

.

9

)

favors higher scavenging of 10Be at deeper siteslikelyresultingfromlongersinkingparticleresidencetimes,a higher10Bereservoiratthesesites,and/orassociatedwith carbon-ate compensation depth that could have contributed to increase therelativeconcentrationsofbothBeisotopesatsiteMD98-2183. A negative correlation between sedimentation rate and Be-ratio

(

r

= −

0

.

7

)

also suggests higher Be contents at slow deposition sites.Exceptfortheselong-termaverageBe-ratiofeaturesbetween cores,themain Be-ratiochange inall coresischaracterized by a doublinginMIS19a–bcomparedwithsurroundingintervals.

Inthreeofthefourcores,themainBe-ratiopeakcoincideswith transitionalpaleomagneticdirectionsthatidentifytheMatuyama– Brunhes polarity transition(dark grey banding inFig. 2). In core MD97-2183, an 18-cm stratigraphic offset between the Be-ratio mid-peakandthepolarity reversalisobserved.Such offsetshave been identified in clayey and carbonate sedimentary sequences from the Western Pacific Ocean (MD97-2140, Carcaillet et al.,

2003; MD97-2143, Suganuma et al., 2010; Simon et al., 2018) andfromtheNortheast AtlanticOcean(MD95-2040, MD95-2042,

Carcaillet etal., 2004;MD04-2811, Ménabréazetal., 2011). With thicknesses ofafew cmto severaltensofcm,they are compati-blewiththeeffectsofpost-depositionalmagnetizationlock-in(e.g. Robertsetal.,2013)astheresidencetimeofdissolvedBeisotopes in the oceans (200 to 700 years at studied sites; von Blancken-burgetal.,1996)canproduceatbestdepthoffsetsof0.5to2cm, dependingonsedimentationrateofeachsite.

ThefirstorderBe-ratiopeakinallstudiedcorescan,therefore, be related to enhanced cosmogenic nuclide productionproduced by thegeomagneticdipole minimumassociatedwiththelast po-larity reversal. Such a Be-ratio, or 10Be-flux, doubling has been observed in marine sediments and ice core records at the time oftheMBTreversalandduring themaindipole minimalinked to reversals and excursions of the late Matuyama chronand to ex-cursions of the Brunhes chron (e.g. Raisbeck et al., 1985, 2006; Franket al., 1997; Carcaillet etal., 2003; Suganumaet al., 2010; Ménabréazetal.,2014; Horiuchietal.,2016;Simonetal.,2016a,

2018andreferencestherein).

Significant positive correlation coefficients between 10Be and the Be-ratio (r

=

0

.

77 to 0.90) support a strong control of 10_Be

variations on the Be-ratio over the studied interval (Fig. 3 and Table S5). No significant correlation between 9_{Be and Be-ratio is}

noted, exceptin coresMD90-0961 andMD95-2016,where r val-uesaround

−

0.5suggestthatthenormalizationproceduredidnot fully remove possible environmental imprints. A principal com-ponent analysis on the four Be-ratio series provides a PC1 that explains 88% ofthe total variance (Fig. 4C). Three Be-ratio series

Fig. 4. Resultsplottedonacommondepthscale.(A)δ18_{Orecordssynchronizedto}

acommondepthscale(MD90-961/963asatarget)usingminimaltiepoints num-bers(indicatedbydiamonds).(B)Be-ratiosfromeachcorestandardizedoverthe samecompositedepthforthesametimeperiod,i.e.700–850ka.(C)Scoresofthe firstprincipalcomponent(PC1explains88%ofthevariance)fromaprincipal com-ponentanalysisonthefourBe-ratioseriesstudied,andreversalanglesdefiningthe transitionalfielddepthintervalsineachcore(Valetet al.,2016).Thegreyband highlightsthetransitionalfieldsbetweenfullpolaritystates,asindicatedby rever-salanglesbetween30◦and150◦.

are highly correlated

(

r

>

0

.

92

)

with PC1,which supports a ma-jorcommonforcing bydipole momentchanges.Thefact thatthe MD95-2016 Be-ratio record presents a slightly lower correlation withPC1

(

r

=

0

.

82

)

isprobablyduetothesecondary environmen-talinfluenceoraweakstratigraphicoffsetwithintheuncertainty range(seebelow;Fig.4B–C).

If the data associated withthe MBT dipole minimum are re-moved, the correlation between 10Be and Be-ratio decreases sig-nificantly, while the correlation between 9_{Be and Be-ratio data}

increases. This further confirms that the 10_{Be produced during}

thegeomagneticdipoleminimumintervalprovidesthemain con-straintontheBe-ratiovariationandits dependenceonthedipole moment value. The MBT is, therefore, identified accurately by cosmogenic 10_{Be nuclideenhancements innatural archives (grey}

shadedareainFig.3).Furthermore,anddespitetracesofpossible environmentalcomponentsintheBe-ratiooftwoofthefourcores

studied,theamplitudeoftheBe-ratioincreaseattheMBTremains constant between each site, oncestandardized (z-score) over the samecompositedepthinterval(representingthe700to850kaage interval)(Fig.4B). Alloftheseobservations confirmthereliability of the 9Be normalization method to provide a robust identifica-tion of the geomagnetic dipole moment collapseassociated with theMBTineachstudiedcore(Fig.4B).

4. Commondepthscaleandchronologicalframework

The chronological framework required to globally assess the age and duration of the MBT is obtained from radiometric dat-ing and/or astrochronological calibrations (e.g. Coe et al., 2004; Channelletal.,2010).Thesemethodsprovidereliablesolutionsbut contradictory results, andlead to disputed ages (see Mark etal.,

2017andcommentbyChannellandHodell,2017).Thechronology ofthepresentsyntheticrecordwasconstructedbyapplyingan or-bital tuning on thebenthic oxygen isotope records.This strategy providesarelativechronologywithmaximumageerrorsof

±

5ka for ages around the MBT interval due to uncertainties on delays betweentheinsolationsignal andthe globalresponse oficecaps (LisieckiandRaymo,2005).

The two-stepsapproach usedinthisstudyisdescribed as fol-lows. First, all recordswere placed on a commondepth scale by correlating high-resolution

δ

18O benthic records (Cibicides wuel-lerstorfi) (Fig.4A),assuming globalsynchronicitybetweenbenthic

δ

18_{O signals (see below). Site MD90-0961/0963 (Indian Ocean),}

which hasbeenstudiedextensivelyforpaleomagnetismand oxy-genisotopegeochemistry(Bassinot etal.,1994;Valetetal.,2014,

2016),wasselectedasthecommonreferencetarget.Depthscales for all other recordswere rescaled using a minimumnumber of tie-pointstoavoidaccordion-likestratigraphicshifts(Fig.4A).This strategy leads to a clear androbust core-to-core correlation and permitsaccurateassessmentofthepositionofBe-ratiopeaks asso-ciatedwiththeMBTversusclimatostratigraphicmarkers(Fig. 4B). IncoreMD98-2183,benthicforaminiferadissolutionwithinMIS19 (dashedlineinFig.4A)hampersuseoftheisotopicsignalfor pro-viding independent constraints outsideof the primary glacial/in-terglacialoscillations.TheBe-ratiopeaklimitsassociatedwiththe MBT were, therefore, used to adjust the depth scale, providing better alignment that is not independent of Be measurements. In cores MD90-0961 and MD90-0949, perfect alignment of the microtektite peak at 3730 cm (common depth) provides a ro-buststratigraphicmarker,independentoftheadjustmentstrategy (Fig.4A).Moreover,allBe-ratiopeaksandtransitionalreversal an-gle paths arelocated between

∼

3620and3680cmandbetween

∼

3640 and 3690 cm, respectively (Fig. 4B–C). Such successful alignmentofindependentproxyrecordsisconsistentwiththe as-sumptionofglobalaverage synchronicityofbenthic

δ

18Orecords, withinuncertaintylimits,whichsupportsourstrategy. Chronolog-icaloffsetsbetweenbenthicseriesproducestratigraphic misalign-mentsby2to4kabetweenoceanicbasinsatglacialterminations (Lisiecki andRaymo,2009),butsincethey mainlyconcernglacial terminations,theyhaveminorimpactonastrochronologicaldating oftheMBTwithinMIS19.

Second,wederivedacommonchronologicalframeworkby tun-ingfine-scale

δ

18Ofeaturesfromeachrecordplacedonacommon depthscale,withtheglobalbenthic

δ

18OLR04stackofLisieckiand Raymo (2005) (Fig.5).Inparallel,tuningofindividual

δ

18Orecords ontheirowndepthscaleswiththeLR04stackwasalsoperformed to evaluate uncertainties inthe matchingprocedure. Averageage deviationsbetweenthetwoderivedchronologies(commonand in-dividualdepths)liebetween0.4and2ka,i.e.lessthanthe

±

5ka uncertainty associated with the LR04 stack in the 0–1 Ma time interval (Lisiecki and Raymo, 2005). This new chronological re-construction provides an ageof 793–794 ka forthe Australasian

Fig. 5. Chronostratigraphyfortherecordsstudiedhere.BenthicforaminiferaCibicides wuellerstorfi oxygenisotopestratigraphyfromallcores(seelegend)havebeentuned tothe benthicLR04globalstack(LisieckiandRaymo,2005) toderiveacommon astrochronologicalframework.

microtektitelayeridentifiedincoresMD90-0961andMD90-0949. Thisageliesbetweentwo 40_Ar/39_{Arages obtainedforthislayer:}

amaximumageat799.2

±

3.8ka(Smitetal.,1991) anda mini-mumageat786

±

2ka(Marketal.,2017).Furthermore,volcanic glassshards(tephra)detectedinthe3710–3713cmdepthinterval in core MD90-0961 lie at ca 789–790 ka. This is indistinguish-ablefromtheastronomicallytuned ageofAshDatODPSite758: 788.0

±

2.2ka(Leeetal.,2004) andiscoherentwithtwodistinct eruptionsfromthe Tobavolcano(OTTAandOTTB)that aredated radiometricallyat792.4

±

0.6kaand785.6

±

0.8ka(Marketal.,

2017).

The chronological coherency between the astronomical age modelandindependentlydated eventsconfirms the reliabilityof ourageframework.Thenewchronologicalframeworkallows accu-ratecomputationofsedimentationratesfortheMBTintervalinall studiedcores(Table1).The2-cmsamplingspacingthenimpliesan integratedtimeintervalrangingfrom0.5to1ka.Addingtheslight signal smoothinginduced frombioturbation andoceanicBe resi-dencetime, this samplingspacingprovides millennial resolution, whichisthefinestresolutionpossiblefromcoreswithsuchlowto moderatesedimentationrates.Aduration of30 to32 kaforMIS 19isdeducedfromsedimentationrates,inagreementwithother estimates(33ka,Tzedakisetal.,2012;34ka,Giaccioetal.,2015). Similarly, an average duration of 11 kaestimated for MIS 19cis alsocoherentwithrecordsfromMontalbanoJonico(11.2ka,Simon etal.,2017),Sulmonapaleolake(10.8ka,Giaccioetal.,2015),and theNorth Atlantic Ocean (10.5–12.5 ka, Tzedakiset al., 2012). It isworthnotingthataveragesedimentationratescomputedforthe 700–850 kainterval (Table 1) are12 to55% lower than previous estimates by Valet et al. (2016), which considered the complete sedimentthicknessbetweencoretopsandtheMBT.By doingso, theseestimatesneglectedthesignificantstretchingeffectimposed bytheCalypsocoreronthe upper10–15metersofthefirst gen-eration ofMarionDufresnes cores that has beendemonstrated by anisotropyof magneticsusceptibilitymeasurements (Thouvenyet al.,2004).

5. Global10_{BeoverproductionepisodeatthetimeoftheMBT} 5.1. Generalcomments

The occurrence of a single large Be-ratio peak interval is demonstrated in all studied cores (Fig. 4B). Their alignment/co-incidence in the common stratigraphic/chronological framework indicates that they resultfromthesameglobal10Be overproduc-tion episode triggered by the GDM collapse linked to the MBT (Fig.4C).Afterstandardization,theseBe-ratiopeakshavethesame amplitude atall sites, whichconfirms the globalhomogenization anddepositionofatmospheric10BeproducedduringtheMBT.This isfurtherconfirmedbythehighcorrelationofeachBe-recordwith the first principal component PC1 (see section 3) that explains 88%ofthevarianceoftheBe-ratioserieswithabroadpeakwith highestscoresat766–778ka(midpeakat

∼

772ka)(Fig.6).The fact thatthisBe-ratio peak coincideswiththe minimumof sum-merinsolationlinkedwiththeendofinterglacialMIS19c(Fig.6) allowsfixing oftheageoftheMBTbetween770and778ka. Us-ing June 21 as a reference instead of mean summer insolation accounts for a maximum 2 ka age shift toward older ages. This age range is compatible with: 1) astronomical calibration of pa-leomagneticrecords(e.g.Tauxeetal., 1996; Channelletal., 2010; Channell,2017);2)radiometricdatingofthetransitionalHaleakala lava flow (776.0

±

2.0ka; Coe etal., 2004), which hasbeen re-cently revised to 772.0

±

2.0 ka (Singer et al., 2017) using a new generation of multi-collector mass spectrometers (Jicha et al., 2016); 3) radiometric dating at 774.1

±

0.9 ka of a tephra layer in the Montalbano Jonico section (Nomade et al., in revi-sion) within the Be-ratio peak associated with the MBT (Simon et al., 2017) (Fig. 6);4) the ageof the VGP midpoint horizonin theChiba recordcalculated at770.9

±

7.3kafromtheU–Pb zir-con age of a tephra layer deposited immediately below (Okada et al., 2017); and 5) a weighted average age of 779

±

7.5 ka calculated by Mark et al. (2017) from the four transitional lava flows presented by Singer et al. (2005). Furthermore, our new Be-ratio records are remarkably similar to all currently available Be-ratioMBTrecordsfrommarinesedimentsequences(Fig.6). Fi-nally, the duration of the 10_{Be overproduction episode deduced}

from our new Be-ratio records (766–778 ka) is similar to that measured inthe EPICADome C(EDC) icerecord (Raisbeck etal.,

2006) when seton the AICC2012chronology (Bazinetal., 2013) (Fig.6).

Alldatacurrentlyavailablefromdifferentgeologicalreservoirs, thus,confirmtheglobalstratigraphic/chronologiccoherenceofthe

10_{Be overproduction interval associated with the MBT (Fig. 6).}

All of these 10Be data provide compelling arguments in favor of the younger agescenario forthe MBT (i.e.

∼

772 ka) despite the fact that some recent paleomagnetic and radiometric dating re-sults support an overall older age at 783.4

±

0.6 ka (Mark et al., 2017).An accepted(common) definitionofreversalboundary from the continuous transitional process, together with evalua-tion and comparison of the resolution of each proxy will help to better constrain their respective limitations (e.g. pDRM lock-indepths,astronomicalandradiometric calibrations, environmen-talbiases) andto reducechronological disagreement(Valetetal.,

submitted).

The10BeoverproductionepisodetriggeredbytheMBT geomag-neticdipoleminimumprovidesagloballysynchronousmarkerthat is usefulfor inter-correlationof ice-cores and continental (lacus-trine andloess) andmarine sediment records.The ages ofGDM variationsreconstructedeitherbypaleointensityorby10_Berecords

should be the same in different parts of the globe. Therefore, dipole moment proxies should be morereliable than magnetiza-tion directions for providing dating and an inventory of paleo-magneticeventssuchasexcursions andreversals,mostly because

Fig. 6. 10_{Be-proxy comparisonfor the Matuyama–Brunhes transition. (A) Stack}

ofthefour benthic δ18_{Orecordsand meansummerinsolation at65}◦_{N (Laskar}

et al., 2004).(B) Scores ofthe firstprincipal component (PC1),which explains 88%ofthe total variancebetween allseries.Theage ofc.772ka corresponds to the mid-PC1 peak. The colored diamonds correspond to radiometric esti-matesofMBTage(seereferencesinthelegend).(C)All10_{Be-proxyrecordshave}

beenstandardizedoverthesametimeintervalandinclude:theMontalbano Jon-ico(Simon et al.,2017; Nomade et al., in revision); MD97-2143(Simon et al.,

2018);MD98-2187(Suganumaetal.,2010);MD97-2140(Carcailletetal., 2003); MD05-2930(Ménabréazet al., 2014; Simonet al.,2016a); EpicaDomeC(EDC) (Raisbecketal.,2006).

rapiddirectionaltransitionsarenotadequatelyrecordedin low-to-moderatesedimentationratecores(RobertsandWinklhofer,2004; Valetetal., 2016; Channell,2017) andstratigraphic/temporal off-sets,such aspDRM effects, leadto chronological errorsand mis-interpretations. Cosmogenic signatures of reversals or excursions should, thus, be investigated systematically together with paleo-magneticsignatures,especiallywhenhypothesesofpaleomagnetic field–paleoclimateconnectionsarediscussed.

Despiteoverallagreementbetweenpaleomagneticand cosmo-genicexpressionsofdipolefield variations,unresolveddifferences appearorpersist,whichgivesrisetomethodologicalortheoretical discussions. The best example isthe occurrence of a paleointen-sityminimumreportedbeforetheMBTinsomemarinesediments andlavaflows,whichsuggeststheoccurrenceofaprecursorevent priortotheMBT(e.g.HartlandTauxe,1996).Ontheonehand, Be-ratiorecordsfromtheNorthAtlanticOcean(MD95-2016), Mediter-ranean Sea (Montalbano Jonico; Simon et al., 2017), and Indian

Ocean (MD90-0961 and MD90-0949) do not document such an

occurrence (Fig. 6), but onthe other hand,Be-ratio recordsfrom thePacificOceanandAntarcticacontainweak10Beenhancements prior to the MBT. In core MD98-2183, a small Be-ratio increase before the MBTis coherentwith a 10Be-fluxmaximum observed in the nearby core MD98-2187 (Suganuma et al., 2010). In core MD05-2930,suchasignaturecanbeidentifiedatthecorebottom (Fig. 6). IncoresMD97-2143(Simonetal.,2018) andMD97-2140 (Carcaillet etal., 2003), the MBT ispreceded by a two steps en-hancement.TheAntarcticEDCicecorerecordalsocontainsa10Be flux enhancement prior to the MBT signature (Raisbeck et al.,

2006).However, severalcriticalpointsare evidentforthese stud-ies.Forexample,1)theexact10_{Besignatureoftheprecursorinthe}

EDCrecordcanbequestionedbasedonglaciologicalconstraints in-ducedbyhighpressuresatthebottom(below3000mdeep)ofthe EDCcore(Simonetal.,2016a),2)alargefractionofthehigh10 Be-fluxes attributed to the precursor by Suganuma et al. (2010) in coreMD98-2187likelyresultsfromstrongenvironmentalimprints duringtermination9(Simonetal.,2018).Moreover,theprominent minimum inthe PISO-1500stack is mainlyinduced by the IODP Site U1308 record (Channell et al., 2009) and is less obvious in other high-resolutionNorthAtlanticrecords(seeFig.10binXuan etal.,2016).Theprecursormay,therefore,resulteitherfrom pale-omagneticbiasesorfromstillunquantifiedthresholdeffectsofthe dipole momenton magnetosphericshielding, andin turnon cos-mogenicnuclideproduction,orfromamix ofboth. Thisquestion deservestobeclarifiedinnewhigh-resolution paleomagneticand cosmogenicrecordspriortodrawinganyfirmgeomagnetic conclu-sions.

5.2. Cosmogenicproductionmodels

The concurrent increaseof 10_{Be-proxy (i.e.Be-ratio,} 10_Be-flux)

observed inall records,includingpolariceandmid-to-lower lat-itude marine sediment cores, atthe MBT supports a global 10Be production rateincrease. In orderto estimate the amountof ad-ditional atmospheric 10Be produced during this dipole moment minimum, the Be-ratio GDM calibration of Simon et al. (2018) is compared with the global atmospheric 10_{Be production rate}

model ofPoluianov etal. (2016) usinga constant mean modula-tion potential of 650 MV (Fig. 7). Calibration of empirical versus theoreticalproductionmodelsallowstheestimationofpastglobal

10_{Beproductionratechanges(Fig.7B).AnormalizedBe-ratiostack}

wasconstructedbycompilingtheindividualBe-ratiorecordsafter even-spacing andaveraging each seriesover 1ka time windows. Theglobalproductionrateofatmospheric10_{Bechangescalculated}

usingthisfunctionandtheBe-ratiostackincreasestwo-foldfrom thebasetothepeakofthe10Beenhancement(Fig.8).Theperiods that surroundthe main increase interval haveaverage 10Be pro-ductionratessimilartothosemeasuredinyear1980byMonaghan etal. (1986) orsimulatedformodernperiods(seeboxinFig.8or Table 4inSimonetal.,2016b).

Atmospheric 10_{Be production change computed during the}

dipole moment collapse of the last geomagnetic reversal that passes from

∼

300 to

>

600 atoms m−2s−1 is a doubling of the average productionoutside the event, so that global10Be atmo-spheric production increases by more than 300 atoms m−2_s−1

(Fig. 8). This is statistically comparable with the result (

+

272 atoms m−2_s−1_{) ofthe theoretical modelof Heikkilä etal. (2009)}

applied topresentdayproductionconditions(controlrun)andto theso-called“Laschamp-run”,asimulated10Beproductionmodel for a zero geomagnetic field strength (Fig. 8). This comparison supportsaglobal10Beoverproductionperiodinducedby geomag-netic modulation at the MBT, and also indirectly confirms that stratospheric homogenization of 10_{Be is the controlling process}

Fig. 7. Calibrationoftheauthigenic10_Be/9_{Beratiointermsof}10_{Beglobalproductionrates.(A)ComparisonbetweentheempiricalcalibrationsfromSimonetal. (2018) and}

theglobalatmospheric10_{BeproductionratemodelofPoluianovetal. (2016).(B)Calibrationcurve.}

Fig. 8. Globalproduction rate enhancement ofatmospheric10_{Beover the MBT.}

(A) Stackofthefourbenthicδ18_Orecords.(B)10_{Beglobalproductionratecurve}

obtainedfromtheBe-rationormalizedstackofthefourauthigenic10_Be/9_Beratio

recordscalibratedtotheglobal10_{BeproductionmodelofPoluianovetal. (2016).}

Atmospheric10_{Beproductionratesarecomparedtoseveralreferencesfrom}

numer-icalsimulationsandmeasurementestimates: i)globaldistribution(average area-weightedglobalflux)ofHolocene10_{BefluxesfromHeikkiläandvonBlanckenburg}

(2015),ii)modernaverageglobal10_{BeproductionratesfromthePoluianovetal.}

(2016) model,iii)presentdayand“Laschamprun”(zerogeomagneticdipole) ex-perimentoutcomesoftheHeikkiläetal. (2009) simulation,andiv)globalaverage

10_{Beproductionmeasuredandcalculatedforyear1980byMonaghanetal. (1986).}

productionincrease reachesa maximum plateauduring the MBT (Fig.7A).

6. GDMcalibrationofthe10_{Berecordandindependentinsight}

intofieldintensitychanges

Determination and quantification of minimum value of GDM thresholdrequiredtotriggerreversalsandexcursionsareof funda-mentalimportance forunderstanding geodynamo processes. Crit-ical dipole moment values between

∼

1 and 2

.

5

×

1022 _Am2_,

i.e. 10 to 30% of the current dipole moment, have been sug-gestedby simulations (e.g.Glatzmaierand Roberts,1995; Buffett et al., 2013), paleomagnetic studies (e.g. Channell et al., 2009; Valetetal.,2005),andearliercosmogenicnuclideproduction stud-ies (Ménabréazet al., 2012; Simon et al., 2016a). Acombination ofapproachesisrequired toprovideaccurate valuesandto eval-uatethe relevance ofinput parameters forexperimental and

nu-merical geodynamo simulations. OurnormalizedBe-ratio stack is calibratedusingtheglobalatmospheric10_{Beproductionmodelof}

Poluianov et al. (2016) to produce a 10Be-derived GDM record (Fig. 7B). This approach differs from that used in earlier stud-ies by Simon etal. (2016a, 2018), but relieson similar Be-ratios from marine sedimentary sequences to give comparable relative results(amplituderange)andonlyminorchangesforquantitative reconstructions.Themethodalsocomplementsvirtualaxialdipole moment(VADM)andvirtualdipolemoment(VDM)estimates de-rivedfrompaleointensityrecordsthatcanbeaffectedsignificantly bynon-dipolecomponents,particularlywhenthedipolevanishes.

Alongstanding 10_{Be-derivedGDMminimumthatlasted12ka,}

from 778 to 766 ka, corresponds to the broad minimum in the PISO-1500 record (Fig. 9). This period is characterized by values below 2

.

2

×

1022 _Am2 _(i.e.

_∼

_{35% of the time-averaged VADM}

withinthe700–850kaperiodor

∼

30%ofthecurrentVADM).The sharperfield recoveryobservedin thePISO-1500stackcompared to the 10_{Be-derived record might result either from a}

smooth-ing of the Be-ratio signal, or an overprint from the high inten-sitypost-transitionalfieldonmagneticgrainsdepositedduringthe low intensityMBT interval (Coeand Liddicoat, 1994; Fig. 9).The factthatnosimilaroffsetisobservedelsewheresupportsthislast interpretation,butitneedstobecarefullyscrutinizedforother pa-leointensity minima.Theaverage ofminimum 10Be-derivedGDM values between 776 and 771 ka (1

.

7

±

0

.

4

×

1022 _Am2_{) is the}

same as the minimum value for transitions in numerical simu-lations (e.g. Glatzmaier and Roberts, 1995; Buffett et al., 2013; Wicht and Meduri, 2016). It is also the same value as that de-rived fromcalibrated RPIstacks (1.6

±

0.2, 1.7

±

0.2, and 1

.

6

±

0

.

4

×

1022Am2forthePISO-1500,SINT-2000andPADM2Mstacks, respectively), and absolute paleointensities measured from tran-sitional lavaflows: La Palma (1

.

4

±

0

.

5

×

1022 _Am2_{,Valet etal.,} 1999)(1

.

9

±

0

.

6

×

1022 Am2,Brownetal., 2009);Tahiti–Punaruu (1

.

6

±

0

.

5

×

1022Am2,Mochizukietal.,2011).The5katime inter-valbetween776and771kacharacterizedinourrecordbydipole momentminima(highlightedbyadarkgreybaratFigs.8and9), thereforeprovides thebestestimate oftheMBTduration.Similar VADMranges forthe MBT fromdifferentvolcanic measurements andfromdifferentcalibrationmethods/productionsimulations fur-thersupportourevaluationusingtheindependent10Beapproach. Furthermore, calibrations of different RPI stacks used different strategies: forSINT-2000,Valet et al. (2005) averaged VADM val-uesover100katimeintervals, thencalculatedthetime-averaged VADMoverthepast800ka(7

.

46

±

1

.

16

×

1022Am2)andusedthis valuetocalibratethe2MaRPIstack.Channelletal. (2009) scaled their RPIstacktothemeanVADMvalueofValetetal. (2005) for

Fig. 9.10_{Be-derivedGDMreconstruction overthe700–850kainterval.(A) Stack}

ofthefourstudiedbenthicδ18_{Orecords.(B)The}10_{Be-derivedGDMrecord}

con-structedusingthetheoreticalglobal10_{Beproductionrate modelofPoluianovet}

al. (2016) (seeFig.7)iscomparedtothe calibratedrelativepaleointensity PISO-1500stack(Channelletal.,2009) andtoaveragetransitionalVADMvaluesfrom theSINT-2000(Valetet al.,2005) andPADM2M(Ziegleret al.,2011) globalRPI stacks.Thefieldpercentagesarecalculatedrelativetothe modernVADM value. A longstandingdipolemomentminimum(lightgreybanding)occurredat778–766 ka(<2.2×1022Am2)andischaracterizedbyamainlowinterval(averagevalue: 1.7±0.4×1022 Am2_{)associatedwiththeMBTwithinthe771–776ka(darkgrey}

band)timeperiod.

thelast 800 kaandthen assignedan intensityof7.5 μT atIODP SiteU1308(whichservesasthereferencerecordforthestack)for theminimumRPIvalue(attheCobbMountainSubchron)arguing thatthe minimumintensityvaluecorresponds tothe likelyvalue oftheresidualfieldaftertotalcollapseoftheaxialdipoleattimes of reversals (Constable andTauxe, 1996). In the PADM2M stack, a penalized iterative spline model was used for sparse absolute paleointensitydata topredict a PADMvalue atevery time point, thenestimatedthescaleneededtotransformeachRPIseriesinto VADMvalues(Ziegleretal., 2011).Thesethreecalibration proce-duresagreewithour10_{Be-derivedcalculation toprovideaVADM}

thresholdof1

.

7

±

0

.

4

×

1022_Am2 _{fortheoccurrenceoftheMBT,}

which corresponds to about 20% of the present dipole moment. Othercasestudiesneedtobemadebeforegeneralizingthis eval-uationtootherpolarityreversalsorexcursions.

7. Conclusions

Wepresentnewhigh-resolutionauthigenic10_Be/9_{Be ratio}

(Be-ratio) and benthic

δ

18_{O (Cibicideswuellerstorfi) records spanning}

the Matuyama–Brunhestransition. The geographic distribution of thestudiedsites intheNorthAtlantic,Indian,andPacific Oceans allowsglobalscaleassessmentofrelatedcosmogenicnuclide10_Be

productionchanges.Astrochronologicalcalibrationof

δ

18_Obenthic

recordsprovidesarobustchronologicalframeoverthe700–850 ka time interval. All records include two-fold increases of the Be-ratio through the polarity transition, which supports a common originlinked totheGDMcollapseassociatedwiththeMBT.These newBe-ratiorecords together withall other available 10Be-proxy records obtained from other sediment sequences and the EPICA DomeC Antarcticaicecore, confirmtheglobalsynchronizationof this10_{Be overproduction episode. Ourcomputedage of theMBT}

between776and771ka agreeswiththeage derivedfrom high-resolution North Atlantic paleomagnetic records (Channell et al.,

2010).

We used the atmospheric 10_{Be production rate model of}

Poluianov et al. (2016) to calibrate our Be-ratio stack in terms of global10_{Be productionandGDM changesthrough thelast}

ge-omagneticreversal. Thetwo-foldBe-ratioincrease corresponds to a global 10Be atmospheric productionincrease of morethan 300 atoms m−2_s−1_{triggeredbytheGDMcollapse.Thisorderof}

magni-tude iscomparablewiththat obtainedfroman atmospheric10Be production simulation in modern and zero dipole moment con-ditions (Heikkilä et al., 2009). The dynamics of our 10_Be-derived

GDM reconstruction agree well with the best records obtained fromrelative(RPI)andabsolute(PI)paleointensitymeasurements.

Minimum GDM values between 776 and 771 ka averaging to

1

.

7

±

0

.

4

×

1022 Am2 are similar to the minimum value from models and to VADM values calibrated from RPI stacks or de-ducedfromabsolutepaleointensitiesmeasuredontransitionallava flows extrudedduring theMBT.Itis,therefore,areasonable GDM threshold for the occurrence of geomagnetic reversals. Coupling Be-ratio and oxygen isotope studies on sedimentary sequences providesanaccuratemeanstoestimatetheage,duration,and dy-namics of the last polarity reversal, and enables deciphering of geomagnetic dipole momentforreconstructionsofpaleomagnetic fieldvariationsandgeodynamosimulations.

Acknowledgements

TheauthorsparticularlyacknowledgeSandrineChoy(CEREGE), Adrien Duvivier (CEREGE) and Anouk Villedieu (LSCE) for sam-plespreparation. WeacknowledgeStepanPoluianov(Universityof Oulu) for his advice on the cosmogenic production model and code. We thank the editor, MartinFrank, Andrew P. Robertsand two anonymous reviewers for very constructive comments that contributedtoimprovesignificantlythe qualityofthispaper.The ASTER AMS national facility (CEREGE, Aix en Provence) is sup-portedbyINSU/CNRS,ANRthroughtheEQUIPEX“ASTER-CEREGE” action, and IRD. This study is supported by the ERC advanced grant to JPV “GA 339899-EDIFICE” under the ERC’s 7th Frame-work Program (FP7-IDEA-ERC). The data presented in this study are available within the supporting information and archived at thePangaeadatabase(www.pangaea.de).

Appendix A. Supplementarymaterial

Supplementarymaterialrelatedtothisarticlecanbefound on-lineathttps://doi.org/10.1016/j.epsl.2018.02.036.

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