When and why sediments fail to record the geomagnetic field during polarity reversals

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When and why sediments fail to record the geomagnetic

field during polarity reversals

Jean-Pierre Valet, Laure Meynadier, Quentin Simon, Nicolas Thouveny

To cite this version:

Jean-Pierre Valet, Laure Meynadier, Quentin Simon, Nicolas Thouveny. When and why sediments

fail to record the geomagnetic field during polarity reversals. Earth and Planetary Science Letters,

Elsevier, 2016, 453, pp.96-107. �10.1016/j.epsl.2016.07.055�. �hal-01419504�

Contents lists available atScienceDirect

Earth

and

Planetary

Science

Letters

www.elsevier.com/locate/epsl

When

and

why

sediments

fail

to

record

the

geomagnetic

field

during

polarity

reversals

Jean-Pierre Valet

a

,

∗

,

Laure Meynadier

a

,

Quentin Simon

b

,

Nicolas Thouveny

b

a_{InstitutdePhysiqueduGlobedeParis,SorbonneParis-Cité,UniversitéParis-Diderot,UMR7154CNRS,75238ParisCedex05,France} b_{CEREGE,UMR34Aix-MarseilleUniversité,CNRS-IRD,13545AixenProvence,France}

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory:

Received8April2016

Receivedinrevisedform10June2016 Accepted28July2016 Availableonlinexxxx Editor:B.Buffett Keywords: geomagneticreversals magnetizationacquisition rockmagnetism sediment demagnetization

WepresentfournewrecordsoftheMatuyama–Brunhes(M–B)reversalfromsedimentsoftheEquatorial IndianOcean,WestEquatorialPacificandNorthAtlanticOceanswithdepositionratesbetween2cm/kyr and 4.5 cm/kyr.The magneticmeasurements wereperformedusing8 cccubic samples and provided well-defined reverse and normalpolaritydirections priorand after the lastreversal.Inthree records stepwisedemagnetizationofthetransitionalsamplesrevealedasuccessionofscattereddirectionsinstead ofawell-definedcharacteristic component ofmagnetization.Thereisnorelationshipwithchangesin magneticmineralogy,magneticconcentrationandmagneticgrainsizes.Thisbehaviorcouldbecausedby weaklymagnetizedsediment.Howeverthetransitionalsamplesoftwocoreshavealmostthreeordersof magnitudestrongermagnetizationsthanthenon-transitionalsamplesthatyieldedunambiguousprimary directions intheothertwocores.Moreoverasimilarproportionofmagneticgrains wasalignedinall records.Therefore,thelargeamountofmagneticgrainsorientedbytheweaktransitionalfielddidnot contributetoimprovethedefinitionofthecharacteristiccomponent.Weinferthattheweaknessofthe fieldmightnotbeonlyresponsible.Assumingthatthetransitionalperiodisdominatedbyamultipolar field,itislikelythattherapidlymovingnon-dipolecomponentsgenerateddifferentdirectionsthatwere recordedoverthe2cmstratigraphicthicknessofeachsample.Thesecomponentsarecarriedbygrains with similarmagneticpropertiesyieldingscattereddirections duringdemagnetization.Incontrast,the stronglymagnetizedsedimentsofthefourthcorefromtheWestEquatorialPacificOceanwereexemptof problemsduringdemagnetization.Thedeclinationsrotatesmoothlybetweenthetwopolaritieswhilethe inclinationsremainclosetozero.Thisscenarioresultsfrompost-depositionalrealignmentthatintegrated variousamountsofpre- andpost-transitionalmagneticdirectionswithineachsample.Thepresentresults pointouttheimpossibilityofextractingreliableinformationongeomagneticreversalsfromlow-medium deposition rate sediments with the current sampling techniques. Among otherfeatures, they put in questiontherelationshipbetweenreversaldurationandsitelatitude.

©2016TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

Most paleomagnetic records of reversals has been published during thepast 40yearsfromsedimentary sequences with accu-mulationratesmostlyvaryingfrom2to5cm/kyrandinsomevery rare cases up to 10–12 cm/kyr. Despite the existence of a large database, no consensus has emerged yet concerning the struc-ture and geometry of the transitional field, and reversal models remain controversial (Clement and Constable, 1991; Bogue and Merrill, 1992; Jacobs, 1994; Roberts, 1995; Merrill and McFad-den,1999;Constable,2003; Amitetal., 2010; ValetandFournier,

*

Correspondingauthor.

E-mailaddress:valet@ipgp.fr(J.-P. Valet).

2016). Among various issues, a typical example was the contro-versy concerning the apparent tendency of virtual geomagnetic pole(VGP)pathstobeconfinedwithintwopreferredlongitudinal bands.ThelongitudinalconfinementofsomeVGPpathsispuzzling whenfacedtothelargedispersionofthetransitionalVGPsderived from high resolution records.This situation leads usto question the meaning of the magnetic signal recorded by sediments dur-ingreversals.Thisisalsojustifiedbyourlimitedknowledgeofthe processesinvolvedinsediment magnetization(see e.g.Katariand Bloxham,2001; Tauxeetal.,2006; ShcherbakovandSycheva,2010; Spassov and Valet, 2012; Roberts et al., 2013). A first unknown isthatmagnetization dependsontimerequiredforlocking-inthe orientation of magnetic grains presentat the same level. A sec-ond parameter is the fidelity of magnetic alignment in presence of low field intensity and a third one, barely considered, is the http://dx.doi.org/10.1016/j.epsl.2016.07.055

Fig. 1. Geographic locations of the four studied cores and of ODP Hole 664D.

ability of sediment magnetization at accounting for rapid field changes.It ispuzzling thatmosttransitional demagnetization di-agrams are of low quality and in fact rarely discussed in detail, while they may reveal interesting information. The first ques-tion that comes to mind concerns the reproducibility of the re-sults.Reversal recordsfromvery nearby sites(Valet etal., 1988; Van Hoof andLangereis, 1992; VanHoof et al., 1993) havebeen shown to be coherent with each other, but they were recorded insimilar environments, andthereforemay be potentially biased bythesameartifacts.Thenon-dipolefieldisreorganized substan-tiallyover aperiodofapproximately1kyr(Lhuillieretal.,2011). Therefore,temporalandspatialvariabilityofthenon-dipolar field maygeneratedifferenttransitionalfield behavioratrelatively dis-tant sites. Assuming an accumulation rate of 5 cm/kyr, no more thantwo8 cccubic paleomagneticsamplesdescribe a1kyrlong transitionalperiod.Nocriticalevaluationhasbeenproposedsofar concerningthemeaning ofsedimentary reversalrecordswith ac-cumulation rates of the order of 4–5 cm/kyr because emphasis was placed on the importance of building up a large dataset.In summary,we arefacedtoasituationinwhichalargenumberof resultshasbeenpublished,whiletoolsaremissingtoevaluatethe fidelityoftherecords.Infact,we donotreallyknowwhatisthe potentialofsedimentstodocumentpolaritytransitions.

Afirstbasicapproachmightbeto comparedistinctrecordsof thesamereversalfromnearbyanddistantlocations.Thiswould al-lowustocheckfortheirconsistencybycombiningtheirmagnetic characteristicsand their resolution. Thispaper analyzes four dis-tinctrecordsofthelastreversal(Matuyama–Brunhes,M–B).Three recordswereobtainedatequatorialsitesfromsedimentsdeposited indistinctenvironmentalconditions,andthelastone comesfrom ahighlatitudeNorthAtlanticOceansite.

2. Corelocationsandsamplings

ThreecoresweretakenattheequatorintheIndianandPacific oceans.CoresMD90-949andMD90-961werecollectedduringthe Seymama cruise of the French R.V. Marion Dufresne in 1990 at nearbysitesthatdifferby only3◦ inlatitudeandlongitude. Core MD90-949(2◦06.90N76◦06.50E) is28m longandwas takenat 3600m waterdepth. Sediment is dominated by nanofossilooze. ThesiteofcoreMD90-961(05◦03.71N,73◦52.57E)is locatedto theeastofthe MaldivesPlatform(Fig. 1).Beryllium and paleoin-tensitymeasurementshavedocumentedthefieldintensitychanges across the last reversal recorded by this sediment (Valet et al., 2014). Lithology is composed ofcalcareous nanofossil ooze with abundantforaminifera.The37mlongcoreMD97-2183(2◦00.82N, 135◦01.26E) was also taken by the R-V Marion-Dufresne in the western Pacific ocean andconsists of hemipelagic clay with cal-careous and siliceous microfossils. Previous measurements of U-channelstakenfromtheentirecorebyYamazaki andOda (2004)

helpedto determinethe positionofthe last reversal. Lastly,core MD95-2016 was obtained at high latitude (57.42◦N, 370.8◦E) in

theNorthAtlanticocean(Fig. 1) andisdominatedbybrowngray clay. Note that all four cores were obtained usingthe same cor-ing systemon board the R.V Marion-Dufresne, andtherefore no largedifferencebetweenrecordsshouldbeimputedtothecoring technique.Allcoresweresampledusing8 cm3cubicplasticboxes takenadjacenttoeachotherandinafewcaseswitha1cm over-lap.No U-channelhas beenusedto avoidadditionalsmearing of thesignal thatconsiderablybiasthe transitionrecords(Valetand Fournier,2016).

Except in core MD95-2016, previous magnetic measurements performedonU-channels indicated thedepth ofthelast reversal in each core (Yamazaki and Oda,2004; Valet etal., 2014). Sedi-mentthicknessabovethelastreversalgivestheaveragedeposition rateateachsiteduringthepast800kyr.Thelowestaccumulation rate v

=

2

.

8 cm

/

kyr wasfound atsiteMD90-949 andprovidesa resolutionaslowas0.714kyrpersample.TheIndianOcean sedi-mentcoreMD90-961hasthefastestdepositionrateof4.8 cm/kyr sothateachtypical2 cmthick sampleintegrates0.42 kyrof geo-magnetichistory.ItsproximitytocoreMD90-949givesthe oppor-tunity of comparing two records withdifferent resolution.Based on theB–M stratigraphic position,the averageaccumulation rate of the sediment from core MD97-2183 is 4.3 cm/kyr. However, the time-depth correspondence derived from the record of rela-tivepaleointensityproposedbyYamazakiandOda (2004)suggests 3.8 cm/kafortheBrunheschronand7cm/kaforthepast200 ka that, according to the authors,could reflectoversampling during coring.Thesetwovaluesgiveafieldintegrationbetween0.48 kyr and0.52 kyrwithin a2cmsample.Lastly,coreMD95-2016from North Atlantic has an accumulation rate of 4 cm/kyr yielding a maximumresolutionof0.5kyrpersample.Interestingly,theselast threerecordsare characterizedby similarresolution.Rapid varia-tions in deposition rates cannot be excluded, butthey wouldbe withoutmajorrelevancetotheconclusionsandobservationsofthe presentstudy.

3. Demagnetizationcharacteristics

All samples were stepwise demagnetized by alternating fields at small increments of2 to 5 mT (depending upon evolution of their magneticmoment)untilcompletedemagnetizationbetween 60and80mT.Theyweremeasuredinfourdifferentpositions us-ing a 2G cryogenic magnetometer. In order to fullydescribe the evolution ofdemagnetization within each sequence,we show on purposealldemagnetizationdiagramsofthesamplesclosetoand withineachtransitionalinterval.TheyhavebeenplottedinFigs. 2 and3usingthePaleoMacsoftware(Cogné,2003).Further informa-tion isgiveninFigs. S1–S3that show theevolution ofthe direc-tions on stereoplotsand the evolutionof the magneticmoments aftereachdemagnetizationstep.Thenameofeachsampleisgiven in centimeters and corresponds to its stratigraphic depth down-core.Declinationswerecorrectedfromcoreorientation by adjust-ing the mean normal declinations of the Brunhes chron to zero. Despite many demagnetization steps and care taken at isolating thecharacteristiccomponentofmagnetization,most demagnetiza-tion diagramsofsamplescloseorwithin thetransitional interval are of much poorer quality than the normal or reverse polarity samples.Belowwedescribethedemagnetizationcharacteristicsof thesampleswithintermediatedirectionsbetweenthetwo polari-tiesineachcore.TheyarehighlightedinFigs. 2 and3.

The declinationofsamplesbetween2153cmand2150cm in coreMD90-949 (Figs. 2and S1)jumps from southto north. The demagnetization diagrams show that after removing a soft over-printwithpositiveinclination,thesuccessivedirectionsarehighly scattered and located in two opposite quadrants of the demag-netization diagram. There is a scattered succession of directions dominatedbyeitherreverseornormalpolarity.Notransitional

di-Fig. 2. DemagnetizationdiagramsandstereographicprojectionsofthesampledirectionscloseandwithinthetransitionalintervalsfromcoresMD90-949andMD90-961. Samplescorrespondingtothetransitionalintervalsareunderlined.Allsuccessivedirectionsareshownonpurposetoprovideacompleteviewofeachpaleomagneticrecord.

Fig. 3. Sameas

Fig. 2

forthesamplesfromcoresMD95-2016andMD98-2183.Thesampledepthsindicatethecumulativedepthsderivedfromthelengthofeachsection anddonottakeintoaccountthepresenceofvoidsorelongationofthesediment.

Fig. 4. Reversalanglecalculatedfromthedirectionoftheaxialdipoleateachsiteasafunctionofdepthplottedalongwithitsmaximumangulardeviation(MAD)as indicativeofscatterofdirectionsaroundthecharacteristiccomponentforeachtransitionalinterval.

rection has been recorded and therefore no significant smearing hasgeneratedartificialintermediate directions.Ratherthe abrupt polarity change occurs over a maximum thicknessof 3 cm that wouldsuggestaveryshorttransition.

Demagnetization performed on samples from the transitional levelsofcoreMD90-961(Figs. 2andS1)frequentlyfailedtoisolate thecomponentcarryingthecharacteristicremanentmagnetization (ChRM).Problemsstartabove3689cmdepth.Thedirectionsshow asuccessionofnorthpointingdeclinationswithsteepinclinations andthen anerraticevolutionofbothcomponents.Thenext sam-ple(3687 cm)exhibitsaverytiny horizontalcomponentpointing south withrelatively steep inclinationsafterthe first demagneti-zation steps and large scatter of declinations at the final steps. Nexttoit,at3684cmthedeclinationsarestillorientedsouthbut demagnetizationdidnot allowtodeterminean univectorial com-ponentdecreasingtowardtheorigin.Similar erraticbehaviorsare reproduceduptothe3668cmlevelwhereanormalpolarity char-acteristicdirectionwasunambiguouslyindentified.

CoreMD95-2016 was takenathighlatitude.Beforethe rever-sal,the inclinationis steepandthe declinationpoints southward (Figs. 3 and S1). Quality of demagnetization diagrams decreases graduallywhenapproachingthetransition.At3106cmdepth,the declination is oriented south, butthe inclinationsare very shal-low and become scattered after the third demagnetization step. Thenexttwosamplesat3103cmand3105cmshowsuccessions oferraticdirectionsduringdemagnetizationwithoutanyclear po-larity.Theseintervalsarecertainlyrepresentativeofthetransition, butnocharacteristiccomponent(ChRM)canbeisolateduptothe normalpolaritysamplelocatedat3084 cm.Furtherinsightisgiven by the evolution of magnetic moments (supplementary material, Figs. S1–S2)that frequentlystop decreasing beyondfields aslow

as15 mTwithinthetransitionzone.Resistance toa.f. demagneti-zationdisappearsprogressivelyaswemoveawayfromthe transi-tionalintervalandthemagneticmomentsoffullpolaritysamples smoothlydecreaseduringdemagnetization.

The demagnetization diagrams from core MD98-2183 are in

sharpcontrastwiththepreviousones(Figs. 3andS3).Transitional directions are nicely definedfrom a componentthat linearly de-creases tothe origin(orvery closetoit) andthus interpretedas the ChRM.The inclinationsremain closeto zerowhile the decli-nations rotate smoothly from south to north. At all stratigraphic levels the magnetic moments decreased regularly during demag-netization.

The low quality demagnetization diagrams within the transi-tionalzonescanbequantifiedbythemaximumangulardispersion (MAD)foreachsample.Weattemptedtoextractapseudo charac-teristiccomponentandusedeachsuccessivedirectiontocompute theevolutionofthereversalanglethatrepresentstheangular de-viationfromthedirectionoftheaxialgeocentricdipoleatthesite. The reversal angles for each transition are shown as a function of depth in Fig. 4 with the MAD values derived from the de-magnetization diagrams. TheChRMs thatwere obtainedfromthe transitional samples are characterized by a MAD that is at least 20timeslargerthanoutsidethetransitionalintervals.Asexpected fromitshighqualitydemagnetizationdiagrams,thesedimentfrom MD98-2183isanexceptiontothisrule.

To summarize, we identified two distinct magnetic behaviors forthe sampleslocatedwithin thetransitional intervals. Thetwo equatorialIndianOceancoresMD90-949andMD90-961 andcore MD95-2016 from the North Atlantic are characterized by erratic behaviorofthedirectionsduringdemagnetizationwhichprecludes fromisolatingawell-definedcharacteristiccomponent.Incontrast,

Fig. 5. Declinationandinclinationchangesasafunctionofdistancetomid-reversaldepththatcorrespondstothestratigraphiclevelwith90◦reversalangle(see

Fig. 3

). Negativedepthscorrespondtopost-reversalperiod.ThedirectionalrecordfromODPHole664D(Valetetal.,1989) intheEasternEquatorialAtlanticOceanisshownfor comparison.

thesedimentfromtheWestPacificOceancoreMD98-2183shows a nicely univectorial characteristic component that progressively rotateswithinthehorizontalplanefromsouthtonorth.

4. DirectionalchangesandVirtualGeomagneticPoles(VGP)

Declinationswererotatedtoobtaina0◦ meandeclination dur-ingtheBrunhesperiod.Intheabsenceofdetaileddepth–age cor-respondence,therecordswerematchedonacommondepthscale. The stratigraphicpositions were rescaled by relying on the aver-agedeposition ratethatwas derivedfromthepositionofthelast reversalateach site.SedimentfromsiteMD90-949withthe low-est resolution was taken as reference. The sample depths from the other sites were thus multiplied by the ratio between their own deposition rate andthe 2.8 cm/kyr value obtained for core MD90-949.

Thedeclinationandinclinationvariationsderivedfromthe sin-gle samples measured at each site are plotted in Fig. 5 as a function of their distance to the mid-transition point (that cor-responds to the mid-reversal angle). The negative relative depth valuescorrespondtothemostrecentperiod.Thepatternsof vari-ationsduring each reversal differat each site. As expectedfrom thedemagnetizationdiagrams,nolargeinclinationchangeandno intermediatedeclinationawayfromthenorthandsouthpolarities definethepolaritychangeatsiteMD90-949(Fig. 5a–b).The vari-ationsdisplayedby thesediment fromcoreMD90-961(Fig. 5c–d) show a precursory episode marked by large declination changes

followedby a rapidtransition.The inclinationis characterizedby abruptvariationsduringbothepisodes.Thethirdequatorialrecord from core MD98-2183 (Fig. 5e–f)shows a succession of inclina-tionoscillationsthatkeepthesameamplitudewithin andoutside the transition. The declinationrotates regularly betweenthe two polarities whereas there is a 60◦ rapid switch in the middle of the transition.Forcomparison, we haveadded thedatasetof the Atlanticequatorial recordfromODP Hole664D (Fig. 5g–h) (Valet etal., 1989) that showsarather smooth evolutionof declination with an abrupt change in the middle of the transition that re-minds thepatternoftherecord fromMD98-2183ifone excludes thesmoothercharacterofthislast record.Theinclinationdisplays a succession ofoscillations that keep thesame amplitude during thetransition.Finally,thehighlatitudetransitionrecordfromcore

MD95-2016 shows a progressive change from upward to

down-wardpointinginclinations(Fig. 5i–j)witharapidmotionbetween negative and horizontal values. The declination reverses in two successivephaseswiththetransitionanda pronouncedrebound. Inbothcases,fewintermediatevaluesarepresentbetweenthe re-verseandnormalpolarities.

Thefivereversalrecordsarecharacterizedbydifferent trajecto-riesandpatternsoftheirVirtualGeomagneticPoles(VGPs)(Fig. 6).

The VGPs from core MD90-949 (Fig. 6a) confirm the absence

of transitional direction, despite a deposition rateonly 1.7 times lower than atthenearby siteMD90-961(Fig. 6b). The first tran-sitionalVGPs ofthe precursoryphaserecorded atsiteMD90-961 are concentrated in the Southwest Pacific Ocean, andthe

polar-Fig. 6. VGPpathsofthereversalrecords.TheVGPsfromODPHole664D(Valetet al.,1989) arealsoshownforcomparison.

itytransitionisrepresentedbyVGPsthatarescatteredwithinthe NorthAtlanticOcean.Asexpectedfromthedeclinationvariations (Fig. 6c),theVGPsfromcoreMD98-2183followasmoothand lon-gitudinaltrajectorywithaclusterattheeastofSouthEastAfrica, nolowlatitudeVGPsandasuccession ofpositionsbetween north-ern Africa and northern Europe. Similarly, the record from ODP Hole664D(Fig. 6d)doesnotshowanyVGPatlowandequatorial latitudes andtwo mid-southernandmid-northern latitudes clus-terstothewestoftheIndianmeridian.Thesefeaturesareinsharp contrastwiththeworldwidescatterofVGPsderivedfromthehigh latitudesiteMD95-2016(Fig. 6e).

5. Magneticcharacteristicsoftransitionalintervals

5.1. Coercivityandmagneticmineralogy

Several factors can influence magnetic characteristics of the records.Changesinmagneticgrain sizesand/or inmagnetic min-eralogycouldaffectthesedimentwithinthetransitionalintervals. Suchvariationscouldbelinkedtothenatureofthedetrital parti-clesandthereforetoenvironmentalconditionsthatprevailed

dur-Table 1

Meanvaluesoftypicalrockmagneticparametersacrosseachtransitionalinterval.

Core S ratio NRM/ARM IRM/ARM

Outside transition Within transition

MD90 949 0.89±0.1 0.84±0.05 0.17±0.06 58±8 MD90 961 0.97±0.01 0.97±0.01 0.23±0.1 110±15 MD98 2183 0.99±0.08 0.99±0.06 0.12±0.06 40±8 MD95 2016 0.93±008 0.93±0.02 0.3±0.18 10±6

ingthisperiod.Climaticchangesoccurredonaglobalscale,butthe core locationswere associated withdifferent paleoenvironmental conditions.It isdifficultto expectsimilar consequencesat north-ern andequatorial sites.Notwithstanding, thishypothesis cannot be neglected andfurther informationcan be gainedfromseveral parameters.

Anhysteretic Remanent Magnetization (ARM) of most marine sediments governed by magnetitegrainsis on the averageabout ten times larger than NRM, while Isothermal Remanent Magne-tization (IRM) is one order of magnitude larger than ARM. The mean IRM/ARM ratio (Table 1) is comprised between 40and 50 for cores MD90-949 and MD98-2183, while it is around 10 for coresMD90-961andMD95-2016.Thedifferencecould reflectthe presence oflargermagneticgrainswithinthe firsttwocores,but both sediments behave quite differently during demagnetization. Therefore,theIRM/ARM ratioisnot correlatedwiththemagnetic behaviorofthetransitionalsamples,whichmakessensesinceeach transitionwasrecordedbysedimentsofdifferentorigins.

Similarly, it is difficult to consider that changes in magnetic mineralogy were responsible for poor demagnetization of sedi-ment during polarity transitions.The highto low coercivityratio asexpressedbytheSratio(Bloemendaletal.,1992) issimilarfor transitionalandnon-transitionalintervals(Table 1).Sedimentfrom core MD90-949 has a ratio of 0

.

88

±

0

.

09 that indicates larger coercivity material than at the other sites with values between 0

.

94

±

0

.

02 (MD90-961)and0

.

99

±

0

.

1 (MD98-2183)without sig-nificant differencesbetweentransitional andnon-transitional lev-els. Therefore, neither differencesin magnetic grain sizes, nor in magnetic mineralogy can explain the absence of a well-defined characteristiccomponentinthetransitionalsamples.

5.2. Influenceofsecondaryoverprint?

We can alsoenvisage that a resistant overprint (e.g.of chem-ical origin)overlapsthecharacteristictransitionalcomponentand preventsits determination.Resistancetoa.f.demagnetizationand the amplitudeof thesecondarymagnetization componentcanbe evaluatedbyconsideringi)theamountofmagnetizationlostafter completeremovaloftheoverprintii)theafpeakfieldrequiredto reachthisstep.BothindicatorsareplottedinFig. 7a–dasa func-tion of distance to mid-transition. The percent of magnetization lostishighlyvariable,relativelylargevalueswithintransitional in-tervals likely result from the weak primary magnetization. They areaccompaniedbylargerresistancetoafdemagnetizationofthe secondary component, except for core MD98-2183 that exhibits the sameunblocking field of20–25 mT atall stratigraphic levels (Fig. 7c).Overall,nolargeandsystematicchangesincoercivity sug-gest thatasignificantportionofsecondaryoverprint overlapsthe primarycomponent.

Indirectindicationsaboutchangesincoercivitycanbeobtained fromtheevolutionofARMresistancetoa.f.demagnetization.The ARM is mostsensitive to single domain grains and therefore to thefractionofgrainscarryingtheNRM.Ifdispersionofdirections during demagnetizationwas linked to changes inmagnetic coer-civity orintheamountofmagnetic grains,it shouldbe reflected by a similar evolution during ARM demagnetization. In Fig. 7f–i wecomparethedowncoreevolutionsoftheamountsofARMand

Fig. 7. Testingthepotentialinfluenceofthesecondaryoverprint:a–d)NRMlostbetween0 mTand40or50 mT.Evolutionoftheafpeakfieldrequiredtoremovethe overprint.e–h) ComparisonoftheamountofNRMandARMlostbetween20mTand40–50or60 mT.ResistancetodemagnetizationvariesforNRMwhileitisconstantfor ARM.Downcoreevolutionsofallparametersarepresentedasafunctionofdistancetomid-reversaldepth.

NRMlostafterdemagnetizationacross thefourtransitional inter-vals. It is striking that the ARM shows similar resistance to af demagnetization atall levels within and outsidethe transitional intervals, whileNRM resistanceto a.f.demagnetizationis weaker within transitional intervals, except againforthe specific caseof theMD98-2183 Pacificocean core(Fig. 7g).Therefore,nochange inrockmagneticpropertiescanbeconsideredasbeingresponsible fortheanomalousbehaviorofthetransitionalsamples.

5.3.Magneticconcentrationandmagneticmoment

Theweakermagnetizationintensityoftransitionalintervalscan be caused by (i) changes in magnetic mineralogy, (ii) reduction oftheamountofmagneticparticles, or(iii)alternativelymay re-sultfrom weak geomagnetic field intensity. All three factors can alsoacttogether.We alreadydiscussedtheabsence ofsignificant changeinmagnetic mineralogy andgrain sizes. Changesin mag-netic concentration would be reflected by changes in ARM and SIRM, butnoneofthese indicatorsrevealed significant variations betweenthefullpolarityintervalsandthetransitionalzones.

Two possibilities remain. The existence of a threshold

be-low which the number of magnetic moments becomes too low

for statisticalproper alignment of the magnetic particles by the field is discarded by comparing the magnetization intensities of thecores. The transitional samples from siteMD95 2016are

al-most 1000 times more magnetic than the sediment from cores

MD90-961andMD90-949(Fig. 8).Thehighlymagnetizedsamples fromMD95-2016andtheweaklymagnetizedtransitionalsamples fromtheother two sitesare characterizedby complex demagne-tizationbehavior. Moreover,full polaritysamplesfromMD90-961 andMD90-949 that are three orders of magnitude weaker than the transitional interval from MD95-2016 provide unambiguous characteristic components. The sediment from MD98-2183 is an interestingexception. Itis alsostronglymagnetizedbutthistime thecharacteristiccomponentwasverywelldefinedinallsamples. Thedifferencesinmagneticconcentrationsbetweenthefoursites indicatedby theARM ortheIRM valuesare similarforthe NRM (Table 1).Thisobservationfurtherconfirmstheabsenceoflink be-tween magnetic properties anddemagnetization behavior of the transitionalsamples.

Fig. 8. Comparisonofthemagneticmomentswithinthefourtransitionalintervals. Verticalaxisisinlog-scaletoaccountforthreeordersofmagnitudeweaker mag-netizationsofcoresMD90-949and961withrespecttotheothertwoones.

Summarizingthemagneticgrainsdonotappeartobeproperly alignedbythegeomagneticfield andthisfailureisnotcorrelated withchangesinmagneticconcentration.Wearethusleftwiththe second scenario that involves the dominant contribution of the field. A differentmechanism was evidently atwork in the sedi-mentfromcoreMD98-2183.Below,wediscusswhichfactorscan generatesuchdifferentsituations.

5.4. Alignmentofmagneticmoments

Twohypothesescanbeenvisagedtoaccountfortheerratic de-magnetization behavior ofthetransitional samplesthat hasbeen reported in three ofthe four records. A first assumption is that, in presence of low field intensity the magnetic torque was not strongenoughtoalignproperlythemagneticgrainsalongthefield lines.Thisprocessisindependentfrommagneticconcentration be-causethemagnetictorqueactingoneachindividualgraincanonly bechangedby increasingordecreasingitsmagneticmomentand therefore by changing thesize or themineralogy ofthe particle.

Fig. 9. a)Simulationofanequatorialreversalrecordbysedimentswitha4 cm/ka accumulationrateanda1.5 kyrlongtransitionassumingsmearingofthesuccessive directionsover3cmand6cmlargewindows(eachdatapointrepresents120yrs). Reorientationofmagneticgrainswithina3cmwindowisenoughtostrongly af-fecttherecord.b,c)Reversalangleoftheequatorialrecords(includingtheresults fromODPHole664D)asafunctionofdistancetomid-reversaldepth,a)original data,b)afterre-scalingthedepthstothoseofcoreMD90-949usingthesame de-positionrate,c)aftertime-averagingthedirectionsinaccordancetotheresolution ofMD90-949.

The argumentinfavor ofthisinterpretation is obtainedby com-paringthemeanvaluesoftheNRM/ARMratiosoftherecordsfrom coresMD90-949and961withthosefromcoreMD95-2016.Since wearedealingwiththesametimeintervalsthefieldcontribution totheNRM/ARMratiowassimilaratthethreesites.Itisstriking thatdespitethreeordersofmagnitudedifferencebetweenthe mo-mentsofstronglyandweaklymagnetizedtransitionalintervals,the NRM/ARMratioshavesimilarvalues(Table 1),whichindicatesthat asimilarsmallproportionofmagneticgrainshasbeenalignedby thefield atall sites.Insummary,despitesimilarprocessesacting

inall cases,thousandoftimesmoregrainswere orientedincore MD95-2016 than in the other two ones butthislarge difference didnotcontributetoimprovethedemagnetizationofthesamples. We infer that the demagnetization behavior is not linked to the magnetizationprocess,nordoesitdependonthemagnetic charac-teristicsofthesamples,butappearstoberelatedtothemagnetic field itself. The first constraint is the presence of weak geomag-netic intensity during the transition that reduces the magnetic torque acting on each individual magnetic grain. However, one rather expecta betterdefinitionof thecharacteristiccomponents inpresenceofstrongmagnetizationduetothelargepopulationof oriented grainsand thisis not the case. Anotherparameter that warrants attention is field geometry. There are strong evidences fromthedetailedvolcanicrecordsofreversals(Jarboeetal.,2011; Valet and Fournier, 2016) that a complex rapidly changing non-dipole field becomes dominant during the short transitional pe-riod. The timescales of the non-dipole field are associated with a secular variation constant of the order of 500 yr (Lhuillier et al., 2011) which means that the field has been completely reor-ganized overa 1 kyrperiod.This isinconsistent withthe timing inherenttomagnetization lock-inover2cmofsediment.Because ofthesefastvariations,magneticgrainswithsimilar demagnetiza-tionspectrarecorddifferentfielddirectionswithineach2cmthick sedimentslicedepositedduringthetransition.Thechaotic succes-sionofdirectionsduringdemagnetizationiscausedbytheabsence ofadominantcomponentoncethetinysoftsecondary magnetiza-tionhasbeenremoved.WenotedthattheMADparameter(Fig. 4) peaksinthemiddleofthetransitionwhenthenon-dipole compo-nentsaredominant.Itisalsosignificantlylowerinpresenceofthe weak dipolarfieldimmediatelybeforeandafterthetransitioni.e. inpresence ofslowfieldchanges,pointingout theimpactoffast varyingdirections.

This explanation cannot account for the smooth and regular evolutionofthetransitional directionsisolatedfromthesediment ofcoreMD98-2183.Thesecharacteristicscombinedtothoseofthe VGP path shown inFig. 6 could be directly interpreted in terms of rotation of the axial dipole betweenthe two polarities. Inter-estingly, all intermediate directionsmeasuredatall demagnetiza-tion steps in coreMD98-2183 (Fig. S3) are distributed along the western equatorial circle. The declinations rotate smoothly from southtonorth,whiletheinclinationsstayclosetozero.Therefore, both secondary and primary declinations follow the same path. We now haveto explain whythese transitionaldirections are so well-definedwithrespecttotheB–MrecordsfromtheIndianand AtlanticOcean,includingtheequatorialrecords.Toourknowledge, such a smooth evolution of the declinations combined to stable inclinations remaining close to zero was rarely reported in tran-sition studies.Notethat thisbehavior isopposite tothe complex evolutionoftransitionaldirectionsrecordedbyvolcanicsequences (Jarboeetal.,2011; Valetetal.,2012).

Similar behavior of transitional directions has been observed in a few attempts at simulating the consequences of long-term post-depositionalreorientationsofmagneticgrains(Meynadierand Valet, 1996).Typical caseswere reportedby severalauthors(Coe andLiddicoat,1994;VanHoofandLangereis,1992;Langereisetal., 1992)whopointedout thebiasinducedontransitionaldirections by integrating magnetic grains with reversed and normal direc-tions in various amounts. Because reversed and normal polarity components were acquiredin stronger field than thetransitional components, they largely dominate the resultant magnetization. Intermediatedirectionsareartificiallygeneratedbythecoexistence of reverse and normal components that are present in various amountswithineachsample.

We have simulated this process on a 1.5 kyr long equato-rial transition (Fig. 9a) record that would be characterized by a rotation of the declination interrupted by a large swing and a

Fig. 10. a)RelationshipbetweendepositionrateandthicknessofthetransitionalzoneforallB–Mrecordsin

Clement (2004)

database(blackdots)andthepresentdata (reddots).TherecordsfromcoreMD90-949withouttransitionaldirectionandfromlakeTecopa(remagnetized)werebothremoved.Linearfitscorrespondtodatafromlow (blue),medium(red)andhighlatitude(black)sites,respectively.Notetheinfluenceoflowdepositionraterecordsontheslopeofthefits;b)reversaldurationasafunction ofsitelatitudeforthesamerecordsasin(a)afterremovingdatawithaccumulationrateslowerthan3 cm/ka.

gradual field intensity decrease reaching ten percent of its pre-reversalvalue beforerecovering withthenewpolarity. In Fig. 9a

we show that depths windows as small as 3 cm and 6 cm are

large enough to smear out the transitional changes recorded by sediments with a 4 cm/kyr accumulation rate. This process has generatedaprogressiverotationofdeclinationsimilarlytothedata fromMD95-2183.Notethattheresultsremaingloballyunchanged whateverthemagnetizationblockingprofile.Theprocesses gener-ating such heavy smearing remain to understand. We have seen that it is mostlikely not caused by peculiar magnetic grain size and/or mineralogy. The sedimentary fraction and/or the deposi-tionalconditionsplayedcertainlyamajorrole.

6. Constraintsonreversalduration

Failuretoprovidereliable successionsoftransitionaldirections lendsdoubt regardingour abilityatdefining the reversal bound-aries,hencetheapparent duration ofthereversalineach record. Despite this potential limitation, we investigated whether these newrecordscanbe used toprovidefurther constraintson rever-sal duration. We made theassumption that the last reverse and the first normal polarity directions mayhave remained geomag-neticallysignificant.Weevaluatedfirstthedurationsderivedfrom thethreeequatorialreversalrecordstowhichwe addedthedata fromtheequatorialAtlanticOceanODPHole664D.Allrecordsare supposedtoprovidemoreorlesssimilardurations,differencescan resultfromthe configurationofthenon-dipolefield attheir site. Asone data point representsatleast400 yr ofgeomagnetic his-torytheboundariesofthetransitionalinterval arepoorlydefined. InFig. 9bareplottedthereversalanglesasafunction ofdistance to mid-reversal depth. The stratigraphic thickness of each tran-sitional interval incorporates the directions lying more than 30◦ away fromthe directionof the axial dipole, i.e.basically outside therangeofsecularvariation.Therefore,each transitionalinterval representsthe thicknessofsediment comprisedbetween30◦ and 150◦ angulardeviation,andthesetwolimitscaneasilybedefined graphically.Thedatawere rescaledtothe samelowresolutionof coreMD90-949,whichisequivalenttodatingeachlevelusingthe samedepositionrate.There waspersistingdisagreement between theapparentdurationsobtainedafterthistreatment.However, re-ducingtheresolutionrequiresalsototakeintoaccountadditional smearingofdirectionsovereachsampledepth.Weintegratedthe directionswithintheMD90-949 equivalentsample thickness. The pattern of changes recorded at ODP site 664D is now closer to

those from cores MD95-2183 andMD90-961 (Fig. 9c), but there arepersistingdifferencesthatpointout thelargeuncertaintieson thelowerandupperreversallimits.

After calculating the respective durations of the sedimentary records for the four more recent reversals, Clement (2004) re-ported that the apparent duration of transitional intervals (de-finedfromthethicknessofsedimentwithintermediatedirections) varieswithsitelatitude.Shorterdurationsareobservedatlow lat-itude sites and longer durations at equatorial and mid-latitudes. As there is no obvious reason that all reversals have the same duration, it is more reasonable to focus on a specific reversal to investigate this dependence. The last reversal is better doc-umented than any other, and we thus took the opportunity of the present four new B–M records to test this relationship fur-ther. We usedthe B–M databaseofClement (2004) after remov-ingtheremagnetizedrecordfromLakeTecopa(Valet etal., 1989; Larson andPatterson,1993) andincludedthepresentrecords ex-cept thosefromMD90-949thathasnotransitional direction.The distribution ofsitelatitudesis notuniformwitha largeramount ofsiteslowerthan20◦,agroupingaround35–45◦andafewcores athigherlatitudesbutbelow60◦.Depositionratesvaryfromless than1cm/kyrupto13 cm/kyr. Giventherapidityofthereversal we considerthat lower resolutionthan 700 yrcannot reasonably accountfordifferencesoftheorderof1or2 kyr.

In Fig. 10a we plotthe deposition rate asa function of tran-sition thickness for all records (the three southern hemisphere sitesweregivenequivalentpositivelatitudes).Reversalswith sim-ilar duration should thus be distributedalong a unique line. The three groups with equivalent sitelatitudes mentioned above are distributed along three lines withdifferent slopes (Fig. 10a), but thelowestslopeconcernsthemediumandnotthehighlatitudes. Asafurthercheckonthestabilityoftheresults,weeliminatedthe recordswithdepositionrateslowerthan3 cm/kyrandplottedthe reversaldurations asa functionof sitelatitude(Fig. 10b). Unlike

Clement (2004),we foundthat thedistribution doesnotincrease withsitelatitudebutratherresemblesabellpatternthatpeaksat 30–40◦oflatitude.

7. Conclusion

We investigatedwhatconfidence canbe giventosedimentary recordsofgeomagneticreversalswithdeposition rateslowerthan 5 cm/ka that representabout 95% of the data published during thepast40years.Theapproach reliedon acomparisonof

multi-ple recordsof thelast reversal fromone highlatitude andthree equatorialsites.Allmeasurementswereperformedonsingle sam-ples after stepwise a.f. demagnetization. After removal of a soft overprint, three of the four transitional intervals were character-izedbyasuccessionofscattereddirectionsthatimpededaproper determinationof their characteristiccomponent. We did notfind evidenceforsignificantdowncorechangesinmagneticmineralogy, coercivityandmagneticgrainsizesbetweenfullpolarityand tran-sitional intervals. The magnetization of the transitional samples differedbyuptothreeordersofmagnitudebetweencoresbutthe percentageoforientedmagneticgrainswassimilarforhighly and weaklymagnetizedtransitionalsamples.Therefore,thenumberof oriented grains cannot be fully responsible for the poor defini-tionofthecharacteristiccomponent.Itisalsonotstraightforward to envisage theprocess by whicha low transitional geomagnetic fieldwouldgenerateerraticmagneticbehavior forstronglyaswell asfor weaklymagnetized samples.We favor the hypothesis that therapidly changingnon-dipolefield during thereversal was in-consistent with the slow timing of magnetization lock-in. These resultshavedirect implicationsforstudies ofgeomagnetic excur-sionsinadditiontopotentialproblemslinkedtopost-depositional reorientations(RobertsandWinklhofer,2004) thatwerenicely ex-emplifiedby the record of the fourthcore MD98-2183 fromthe westernequatorialPacific. Inthiscasetheprogressiverotation of the vector is easily simulated by the smearing of the geomag-netic signal generatedby post-depositionalprogressive reorienta-tions. To summarize, we identified two kinds of recordings, but bothyield erroneousfeatures ofthe reversingfield. Thefirst one isbyfarthemostfrequentandshouldhopefullybeaddressedby veryhighresolutionsampling,whilethesecondoneislikely hope-less.

Thepresentanalysiswas restrainedtolow-medium accumula-tion rate records. Very few reversal records have been obtained from sediments with rapid accumulation rates (10 cm/kyr and above) from deep-sea sediments (Channell and Lehman, 1997; Channell et al., 2004; Mazaud et al., 2009) and we should ex-cludetransitions measured usingU-channels (Valet andFournier, 2016). We must add interesting data from other environments (Liddicoat, 1982; Glenetal., 1999; Sagnottietal., 2014), particu-larlyfromlacustrinesedimentsbutwithmorecomplexchronology. Assumingnosmearingofthesignal,samplingusing8ccstandard paleomagnetic cubes with a 10 cm/kyr accumulation rate yields a 200 yr resolution for a 1 kyr long transition which is similar tocharacteristictimesofnon-dipolar components(Lhuillieretal., 2011) that would be integrated over a 2 cm sample thickness. Weare inclined toconsider thatmeasurements ofvery thin sed-imentlayers remainthe mostpromisingfutureforreversal stud-ies.

Acknowledgements

ThisstudyissupportedbytheERCadvancedgrantGA 339899-EDIFICEundertheERC’s7thFrameworkProgram(FP7-IDEAS-ERC). Dataareavailableuponrequestatvalet@ipgp.fr.ThisisIPGP(CNRS UMR7154)contributionnumber3777.

Appendix A. Supplementarymaterial

Supplementarymaterialrelatedtothisarticlecanbefound on-lineathttp://dx.doi.org/10.1016/j.epsl.2016.07.055.

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