The Antarctic Centennial Oscillation: A Natural Paleoclimate Cycle in the Southern Hemisphere

30 Pages·2017·8.03 MB·English

climate
Article
The Antarctic Centennial Oscillation: A Natural
Paleoclimate Cycle in the Southern Hemisphere
That Influences Global Temperature
W.JacksonDavis1,2,*,PeterJ.Taylor1andW.BartonDavis1
1 EnvironmentalStudiesInstitute,SantaCruz,CA95062,USA;[email protected](P.J.T.);
[email protected](W.B.D.)
2 DivisionofPhysicalandBiologicalSciences,UniversityofCalifornia,SantaCruz,CA95064,USA
* Correspondence:[email protected]@earthlink.net
Received:5October2017;Accepted:3January2018;Published:8January2018
Abstract: Wereportapreviously-unexplorednaturaltemperaturecyclerecordedinicecoresfrom
Antarctica—the Antarctic Centennial Oscillation (ACO)—that has oscillated for at least the last
226 millennia. Here we document the properties of the ACO and provide an initial assessment
ofitsroleinglobalclimate. Weanalyzedopen-sourcedatabasesofstableisotopesofoxygenand
hydrogen as proxies for paleo-temperatures. We find that centennial-scale spectral peaks from
temperature-proxyrecordsatVostokoverthelast10,000yearsoccuratthesamefrequencies(±2.4%)
inthreeotherpaleoclimaterecordsfromdrillsitesdistributedwidelyacrosstheEastAntarcticPlateau
(EAP),and>98%ofindividualACOsevaluatedatVostokmatch1:1withhomologouscyclesatthe
otherthreeEAPdrillsitesandconversely. IdentifiedACOssummatewithmillennialperiodicityto
formtheAntarcticIsotopeMaxima(AIMs)knowntoprecedeDansgaard-Oeschger(D-O)oscillations
recordedinGreenlandicecores. HomologousACOsrecordedatthefourEAPdrillsitesduringthe
lastglacialmaximumappearedfirstatlowerelevationsnearesttheoceanandcenturieslateronthe
highEAP,withlatenciesthatexceeddatinguncertainty>30-fold.ACOhomologsatdifferentdrillsites
becamesynchronous,however,duringthewarmerHolocene. Comparativespectralanalysissuggests
thatthemillennial-scaleAIMcycledeclinedinperiodfrom1500to800yearsoverthelast70millennia.
Similarly,overthelast226millenniaACOrepetitionperiod(mean352years)declinedbyhalfwhile
amplitude (mean 0.67 ◦C) approximately doubled. The period and amplitude of ACOs oscillate
inphasewithglacialcyclesandrelatedsurfaceinsolationassociatedwithplanetaryorbitalforces.
WeconcludethattheACO:encompassesatleasttheEAP;istheproximatesourceofD-Ooscillations
intheNorthernHemisphere;thereforeaffectsglobaltemperature;propagateswithincreasedvelocity
astemperatureincreases;doubledinintensityovergeologictime;ismodulatedbyglobaltemperature
variationsassociatedwithplanetaryorbitalcycles;andistheprobablepaleoclimateprecursorofthe
contemporaryAntarcticOscillation(AAO).PropertiesoftheACO/AAOarecapableofexplaining
thecurrentglobalwarmingsignal.
Keywords: Antarctic Oscillation; Southern Annular Mode; Antarctic Isotope Maxima;
Dansgaard-Oeschgeroscillations;icecores;paleoclimate;Holocene;AICC2012;Vostok;anthropogenic
globalwarming
1. Introduction
GlobaltemperaturehasoscillatedatleastsincethebeginningofthePhanerozoicEon542million
years(My)ago,manifestaslongclimatecyclesthatrepeatwithaperiodof135–150My[1–3].Fasterand
smallertemperatureoscillationsaresuperimposedonthecoolingphaseofthemostrecentlongcycle
beginningatleast~55millionyearsbeforepresent(Mybp)[4],formingtheearliestMarineIsotope
Climate 2018,6,3;doi:10.3390/cli6010003 www.mdpi.com/journal/climate
Climate 2018,6,3 2of30
Stages(MISs;[5]), orglobalglacialcycles(“iceages”). MISsrecurredinitiallyevery~41thousand
years (Ky) [5], the same repetition period as the integrated Obliquity/Precessional Orbital Cycle
(OPOC; [6,7]), and grew larger by more than an order of magnitude over millions of years as
the Earth cooled an additional ~1.5 ◦C [5]. MISs shifted to a quasi-regular period of ~80–120 Ky
and doubled in amplitude at the Mid-Pleistocene Climate Transition 1.25–0.75 My ago [5,8–14].
Thistemperature-oscillationpatterncontinuestodominatemulti-millennialclimatevariabilityup
to the present, inducing quasi-regular global glaciations separated by shorter warm interstadials.
Thecontemporaryclimateconstitutesaprolongedwarminterstadialcomparabletothosethathave
alternatedinthepastwithcoldstadialscorrespondingtoglobalglacialmaxima.
Superimposed upon these multi-millennial climate cycles are numerous shorter global and
regional climate cycles ranging in period from several millennia down to a few weeks. Included
among these faster oscillations are millennial-scale cycles, particularly the Bond cycle [15–23],
and centennial-scale cycles, notably the Antarctic Oscillation (AAO) known also as the Southern
Annular Mode (SAM) and tracked quantitatively by means of the Southern Oscillation Index
(SOI)[24–31]. TheseinterdependentSouthernHemisphere(SH)temperature-proxyoscillationsexhibit
bothcentennial[24–27]anddecadal[28–31]frequencycomponents. Similarperiodicityappearsin
independentreconstructionsofmorecontemporarytemperatureproxiesfromJamesRossIsland[27]
andsnowaccumulationinstackedrecordsfromsnowpitsatVostok[32].
Theseandothercentennial-scaletemperatureoscillationsrecurredwithregularityinthepast
and are, therefore, likely to continue to oscillate at similar periods to affect future climate [33].
Understandingthesecyclescanthereforeimprovetheempiricalbasisforclimateprojectionacross
timescalesmostimmediatelypertinenttohumanaffairs,decadestocenturies[34,35]. Analysisof
oscillatoryclimateregimensandtheirinteractionsondiversetimescalesalsopromisestoimprove
ourunderstandingofregionalandglobalclimatedynamics,includingespeciallythebalancebetween
human and natural forcing of present and future climate change, which are among today’s most
compellingenvironmentalandpolicyissues.
Towardtheseendswereporthereapreviously-unexploredcentennial-scaletemperature-proxy
oscillation,theAntarcticCentennialOscillation(ACO),recordedinstableisotopesfrozeninicecores
atVostok,Antarctica[36,37]andthreeadditionalAntarcticdrillsitesdistributedwidelyontheEast
Antarctic Plateau (EAP), namely, EPICA Dronning Maud Land, EPICA Dome C and Talos Dome.
Paleoclimaterecordsfromthesefourdrillsiteshavebeensynchronizedonthemostpreciseice-core
chronologyavailable,theAntarcticIceCoreChronologyof2012,orAICC2012[38,39],enablingthe
mostaccuratepossibleestimatesoftherelativetimingofACOoccurrenceatdifferentdrillsites.
Centennial-scale temperature oscillations in the Vostok paleoclimate record were recognized
qualitatively nearly two decades ago [40], but their nature and spatial extent have not been
characterizedpreviously,“precludingadefinitiveconclusionabouttheirmeaning”[40](p. 111). Herewe
characterize the ACO quantitatively, document its occurrence at the aforementioned four ice-core
drillsitesontheEAP,exploretheACOcontributiontoDansgaard-Oeschger(D-O)oscillationsinthe
NorthernHemisphere(NH),drawparallelswithcontemporaryclimatecyclesincludingparticularly
theAntarcticOscillation(AAO),andevaluatepossibleimplicationsforthecurrentandfutureclimate.
In particular, the contemporary global warming increase of ~0.8 ◦C recorded since 1850 has
been attributed widely to anthropogenic emissions of carbon dioxide (CO ) into the atmosphere.
2
Recent research has shown, however, that the concentration of CO in the atmosphere has been
2
decoupled from global temperature for the last 425 million years [3] owing to well-established
diminishingreturnsinmarginalradiativeforcing(∆RF)asatmosphericCO concentrationincreases.
2
MarginalforcingoftemperaturefromincreasingCO emissionsdeclinedbyhalffrom1850to1980,
2
andbynearlytwo-thirdsfrom1850to1999[3]. ChangesinatmosphericCO thereforeaffectglobal
2
temperatureweaklyatmost.Theanthropogenicglobalwarming(AGW)hypothesishasbeenembraced
partly because “...there is no convincing alternative explanation ... ” [41] (p. 12). The ACO provides
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a possible alternative explanation in the form of a natural climate cycle that arises in Antarctica,
propagatesnorthwardtoinfluenceglobaltemperature,andpeaksonapredictablecentennialtimetable.
2. Methods
Open-accessdatabasescontainingstableisotoperecords[36–38]andcomputedplanetaryorbital
(Milankovitch)parametersandconsequentsurfaceinsolation[6,7]wereanalyzedusingconventional
methods detailed in the Supplementary Materials (SM). The cycle nomenclature proposed by
Wunsch[42]wasmodifiedforthecentennial-scaleAntarcticcyclesidentifiedhere. Thestarttimefor
thisanalysis,226,408yearsbefore1950(yb1950),istheoldestforwhichthesamplingresolutionofthe
Vostoktemperature-proxyrecordissufficienttodetectcentennial-scaleeventsincompliancewiththe
Nyquist-Shannonsampling-frequencycriterionoftwosamplespercycle(SM).Oxygenandhydrogen
isotope ratios are used widely and interchangeably as paleoclimate temperature proxies [43–47],
althoughtheformercontainsaproxycomponentoficevolumeandotherenvironmentalsignalsnot
presentinthelatter[43,48]. Weusebothproxiesinthisstudyasisconventional.
Three definitions were used to describe temperature-proxy oscillations comprising ACOs:
oscillations larger than 0.25 ◦C in amplitude (~90% of 546 cycles identified over 226 millennia),
oscillationsfrom0.045to0.25◦Cinamplitude(~10%of546cycles),andsmalloscillationscharacterized
byapositiveinflectionofthetemperature-proxyrecordfollowedbyanegativeinflectiononarisingor
fallingbackgroundtemperature-proxycurve(anadditional104cyclescreatingamaximumsample
sizen=650). ThesesmallestACOsareaccompaniedbylargerhomologsinclimaterecordsfromother
drill sites, showing that they are genuine temperature-proxy oscillations and not noise. The third
definitionwasusedonlytocomputecycleperiod;excludingitincreasedmeancomputedperiodby
~15%. AllotherpropertiesofACOcycleswerecomputedusingonlythefirstandsecondACOcycle
definitions. ACOpropertiesevaluatedincludeperiod(trough-to-troughandpeak-to-peak),amplitude
(halftheexcursionofthewarmingphaseplushalftheexcursionofthecoolingphase),andwarming
andcoolingdurationandrate,asdetailedintheSM.Reductionoftemperature-proxyandatmospheric
carbondioxide(CO )concentrationdatatoanalyticformatandcolor-codingofcentennial-scaleclimate
2
cyclesdepictingtheabovethreedefinitionsappearinSMTableS1,enablingreplicationofthisstudy
andsupportingfurtherindependentanalysis.
ThisstudyislimitedtoEastAntarctictemperature-proxyrecordsfromicecoresextractedfrom
fourdrillstationsontheEAPsynchronizedontheAICC2012chronology: Vostok,EPICADomeC
(EDC),TalosDome(TD)andEPICADronningMaudLand(EDML)[38]. Meaningfulcomparison
of ACO cycles across these diverse climate records required equalizing corresponding sampling
resolutions as closely as possible to Vostok by averaging (filtering). The averaging protocols for
non-Vostok records were established using a simple arithmetic protocol (SM) and are required to
replicate the results of our study. Averaging protocols are therefore reported in figure legends as
thebinwidthoverwhichaverageswerecomputedfromtheoriginalrawtemperature-proxydata
followed in parentheses by the start time of the averaging. Quantitative analysis of the timing of
homologouscyclesindifferentclimaterecordswasrestrictedtothemostrecognizablecyclesassociated
withwell-establishedclimateevents. Theseunambiguous“signpost”ACOcyclescanbematched
objectively based on their position relative to known climate events, following the methodology
establishedbynumerouspreviousclimateinvestigators[38,40,49].
Weusedstandardquantitativemethodologiesinthisstudy,includingFourier(spectral)analysis,
auto-andcross-correlation,andconventionalt-tests. Ourresultsshow,however,thatclimatedata
includingthoseevaluatedheredonotnecessarilycompriseastationarytimeseries,implyingthat
modernadvancedmethodsthatassumeneitherlinearitynorstationaritymaybemoreappropriate.
Suchmethodologiesinclude,tovariousdegrees,WaveletAnalysis(whichisnonethelessbasedonthe
Fouriertransform),EmpiricalModeDecomposition,Hilbert-Huangtransform,etc. Weconsiderthese
moreadvancedanalyticapproachesunnecessaryinthisinitialanalysisoftheACO,butourfindings
nonethelessimplythattheirabsencehereconstitutesamethodologicallimitation. Asdevelopedin
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theDiscussion,weconsiderthislimitationinconsequentialbecausewemainlyevaluatedshorttime
segmentsoftheclimaterecordthatcompriserelativelystationarytimeseriesasconfirmedbyauto-
andcross-correlationresults.
Weevaluatedthemajorpotentialsourcesofnoiseinthereconstructedclimaterecords,including
frequencyaliasing,datinguncertainty,varianceincycleamplitude,cycleperiodicity,sampleresolution
andstratigraphicnoisefromnon-climatesignalsintheoriginalice-corerecords,andconcludedthat
theydidnotaffectthepresentinterpretationssignificantly(SM).TheSASJMPsoftwareusedherefor
spectralpoweranalysisemploysFisher’sKappaTestStatistic(κ)asawhite-noisetest(SM).Theκ
statisticreturnstheprobabilitythatthedistributionanalyzedisgeneratedbyGaussian(random)white
noiseagainstthealternativethatthedistributionisnonrandom(SM)andisreportedinthefigure
legends. Tocomputeconfidenceintervalsofspectraldensitypeaks,Fouriercoefficientswereestimated
usingforwardstepwiseregression(SM).Suchstepwiseselectioncanexaggeratetheprecisionofthe
alphaestimatesbyasmuchasanorderofmagnitude[50],however,andwethereforeconservatively
settheconfidencelimitthresholdforspectralpeaksatanorderofmagnitudehigher(0.005,99.5%)
thanusual(0.05,95%).
3. Results
3.1. CycleNomenclature
We used the cycle nomenclature developed by Wunsch [42] as modified here (SM). By this
nomenclature, the ACO documented in the present study is designated as the TO cycle
C350V
(Temperature-proxyOscillationCentennial−scale350yearmeanperiodVariable)basedonitsmeanrepetition
period of 352 years over the time period evaluated, 226.4 to 0.149 Kyb1950, and based on the
observedsystematicvariationofACOperiodandamplitudeinthetimeseriesasdocumentedbelow.
The acronyms “ACO” and “TO ” are used interchangeably throughout this paper. A second,
C350V
millennial-scale oscillation, termed the TO cycle using the modified Wunsch nomenclature,
K1500V
appearshomologouswiththepreviously-identifiedBondcycleintheNH[17–21]andisalsoevaluated
hereusingtemperature-proxydatafromtheSH.
3.2. CycleForm
The TO cycles analyzed here are evident in raw temperature-proxy records as separate,
C350V
individually-identifiable oscillations, each composed of several sampling data points (Figure 1).
Thesecentennialtemperature-proxycyclesaresuperimposedonascendingtemperaturephasesofMISs
suchastheLastGlacialTermination(LGT;Figure1a)andrecoveryfromtheAntarcticColdReversal
(ACR,Figure1b),ondescendingphasesofthetemperature-proxyrecordsuchasthecoolingphaseof
theACR(Figure1b),andonrelativelyflatregionsoftheVostoktemperature-proxyrecordincluding
the Last Glacial Maximum (LGM) and the Holocene (Figure 1a,d, respectively). All 650 TO
C350V
cyclesidentifiedinthisstudyoverthe226.4Kyevaluatedusingthethreedefinitionsdescribedinthe
Methodsarelabeledstartingwiththemostrecentcycleat149yb1950ontheVostokGT4glaciological
chronologyusingalphanumericsymbols,followingtheconventionadoptedforMISs[5](Figure1).
Thesetimeseriesshowqualitativelyanapparentprogressivedeclineincycleperiod(anincreasein
frequency)combinedwithaproportionalincreaseincycleamplitude,bothofwhichareconfirmed
quantitativelybelow.
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Climate 2018, 6, 3 5 of 30

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MThaexcimaluibmra; tLioGnTs,c Lalaesot nGtlhaceialel fTtearbmovinea(tai)onp.e rTthaien csatloibarlaltrieocno rsdcaslien o(na– tdh)e. left above (a) pertains to all
records in (a–d).
3.3. CyclePeriodicity
3.3. Cycle Periodicity
SpectralanalysisofAntarcticpaleoclimaterecordswasdonepreviouslyfortheHoloceneforthe
timeSppeericotrdalf raonmaly11si,5s 0o0f yAbnptatrocttihce pparleesoecnlitm[5a1te]. rWeceoerdxtse wndast hdiosnaen palryesviisohuesrlye ufosrin thget hHeomloocreenaec fcourr tahtee
tdimatea psyernicohdr ofrnoimze 1d1o,5n00th yebpA ItoC Cth2e0 p12recshenrot n[5o1lo].g Wyea nexdteinncdl uthdiisn agncalliymsiast eherreec oursdinsgf rtohme mVoorset oakc,cuErDatCe,
dEaDtaM sLynanchdroTnDiz;e[3d8 ,o3n9 ]t.hWe eAeIvCaClu20a1te2 tchhersoencolliomgayt eanredc oinrdclsuodvinegr bcrliomadateer rtiemcoerdsps afnrosmth aVnosptroekv, ioEuDsCly,
EdDonMe,Li nacnludd TinDg; t[h38e,H39o]l. oWceen ee,vaanludaotev ethreoslde ecrliamnadtelo rnegceorrdtism oevpeerr biordosa,dteore tnimabel espdaentesc ttihoanno pfraebvrioouadsleyr
dcoomnep, leinmcleundtionfgc ythcele Hpeorloiocdenicei,t ieasn.dT hoveerre soulldtienr gasnpde cltornaglepro wtimered peenrsiiotydsp, etroi oednoagbrlaem dsestehcotwionp eoafk as
bartodaedceard caol,mcepnletmenennita lo,fm cyilclelen pneiarlioadnidcimtieusl. tTi-hmei lrleesnunltiianlgp seprieocdtricailt pieosw(Feirg duernes2i)ty. Sppeerciotrdaolgpreaamkss oshccouwr
peaks at decadal, centennial, millennial and multi-millennial periodicities (Figure 2). Spectral peaks
occur at similar frequencies in ice-core records from different drill sites (cf. Figure 2a,b), consistent
Climate 2018,6,3 6of30
Climate 2018, 6, 3 6 of 30
atsimilarfrequenciesinice-corerecordsfromdifferentdrillsites(cf. Figure2a,b), consistentwith
with the hypothesis that the same temperature cycles manifest at different Antarctic drill sites located
thehypothesisthatthesametemperaturecyclesmanifestatdifferentAntarcticdrillsiteslocatedon
on the EAP.
theEAP.
Broadband periodograms covering several hundred millennia exhibit the primary MIS
BroadbandperiodogramscoveringseveralhundredmillenniaexhibittheprimaryMISfrequency
frequency peak at a period of 106–108 Ky, as well as secondary and tertiary peaks of Milankovitch
peak at a period of 106–108 Ky, as well as secondary and tertiary peaks of Milankovitch cycles at
cycles at the orbital precession (41 Ky) and obliquity (21 Ky) cycle periods (Figure 2a,b), replicating
theorbitalprecession(41Ky)andobliquity(21Ky)cycleperiods(Figure2a,b),replicatingprevious
previous results [36] (cf. our Figure 2a with Figure 4a in [36] (p. 432)). Spectral analysis of shorter
results [36] (cf. our Figure 2a with Figure 4a in [36] (p. 432)). Spectral analysis of shorter records
records shows possible representations of the precession orbital cycles in the Talos Dome climate
showspossiblerepresentationsoftheprecessionorbitalcyclesintheTalosDomeclimaterecordat
record at 38.3 Ky (Figure 2c), but corresponding peaks at EDML differ in period by up to 25% and
38.3Ky(Figure2c),butcorrespondingpeaksatEDMLdifferinperiodbyupto25%andhencetheir
hence their etiology is correspondingly less certain. The spectral periodograms constructed over
etiologyiscorrespondinglylesscertain. Thespectralperiodogramsconstructedovershortertime
shorter time periods (Figure 2c,d) also lack the longer MIS cycle (Figure 2c,d) as expected, since the
periods(Figure2c,d)alsolackthelongerMIScycle(Figure2c,d)asexpected,sincethetimeperiod
time period evaluated is too short to detect it.
evaluatedistooshorttodetectit.

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ddrriillll ssiitteess.. AArrrroowwss aanndd aassssoocciiaatteedd nnuummbbeerrss ddeessiiggnnaattee ppeeaakkss aanndd ccoorrrreessppoonnddiinngg nnuummeerriiccaall vvaalluueess iinn
tthhoouussaannddss ooff yyeeaarrss ((KKyy)).. PPaarrttss ((aa,,bb)) ssppaann 442200 aanndd 443344 KKyy ooff tthhee cclliimmaattee rreeccoorrdd ffrroomm VVoossttookk aanndd EEPPIICCAA
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PPaarrttss ((cc,,dd)) ssppaann sshhoorrtteerr ttiimmee ppeerriiooddss aavvaaiillaabbllee ffoorr tthhee TTaallooss DDoommee ((TTDD)) ((115533 KKyy)) aanndd EEPPIICCAA DDrroonnnniinngg
MMaauudd LLaanndd ((EEDDMMLL) )(1(14444 KKy)y c)licmlimataet reecreocrodrsd, rse,srpeespcteivcteilvye. lTy.heT ahbesaenbcsee nocfe a opfeaakp neeaakr n1e0a0r K1y0 0inK pyaritns
p(ca,drt)s r(ecs,udl)tsre fsruolmts tfhroe mreltahteivreellya tsihvoelryt tsihmoer tpteirmioedp eevraioludaetevda,l uwahteicdh, wprheicclhudperse cidluednetisfiicdaetniotnifi ocfa tlioonngoerf
mlonugltei-rmmiluleltni-nmiaill lceyncnleiasl (csyecel etesx(ts)e. eTtheex st)i.teTsh, etimsitee ps,etriimodesp ceorvioedresdc oavnedr e(idna pnadre(ninthpeasreesn) tFhiessheesr)’sF iKshaperp’as
(KMapetphaod(Ms)e atnhdod ths)ea pnrdobthabeiplirtyo bthabaitl tithye tpheartiothdeogprearmiosd oregsrualmt fsroremsu wlthfirtoem nowishei ateren: o(ais)e Vaorset:o(ka:) 4V2o0s ttoo k0:
K42y0bt1o9500K (y4b41.9985,0 p( <4 45.9 ×8 ,1p0−<24)5; (×b)1 E0−D2C4):; 4(3b4) EtoD 0C K:4y3b419to500 (K5y9.b11, 9p5 <0 3(5 ×9 .110,−p28<); 3(c×) T1D0−: 1285)3; (tco) 0T DK:y1b51395to0
(02K4.y7b31, 9p5 <0 1( 2×4 1.703−1,1p); <(d1) ×ED1M0−L1:1 )1;4(4d t)oE 0D KMyLb:1194540 t(o210.3K6y, bp1 <9 510 ×( 2110.−3116),. p <1×10−11).
Spectral analysis of temperature-proxy records over shorter time periods (narrower frequency
Spectralanalysisoftemperature-proxyrecordsovershortertimeperiods(narrowerfrequency
bandwidths) were done for the period from ~12,500 to 0 Kyb1950, confirming the results of previous
bandwidths)weredonefortheperiodfrom~12,500to0Kyb1950,confirmingtheresultsofprevious
studies [51]. The Vostok periodogram (Figure 3) shows six discernible (p < 0.005) centennial-scale
studies [51]. The Vostok periodogram (Figure 3) shows six discernible (p < 0.005) centennial-scale
spectral power density peaks ranging from 193 years, near the average period of the ACO across the
spectral power density peaks ranging from 193 years, near the average period of the ACO across
Holocene, to 1096 years, near the reported millennial period of the Bond cycle in the NH [17–21].
Relaxation of confidence limits to 95% (p < 0.05) adds three discernible spectral peaks at 282, 448 and
580 years which however we do not consider further here owing to the possible exaggeration of
Climate 2018,6,3 7of30
theHolocene,to1096years,nearthereportedmillennialperiodoftheBondcycleintheNH[17–21].
Relaxationofconfidencelimitsto95%(p<0.05)addsthreediscerniblespectralpeaksat282,448and
5C8li0maytee 2a0r1s8, w6, h3 i ch however we do not consider further here owing to the possible exaggeratio7 nof o30f
precisioninthemathematicalmethodusedtocomputetheconfidencelimits(Methods).Thesefindings
precision in the mathematical method used to compute the confidence limits (Methods). These
confirmpreviousevidenceofmultiplecentennialpeaksovertheHolocene[51](cf. Figure3herewith
findings confirm previous evidence of multiple centennial peaks over the Holocene [51] (cf. Figure 3
their Figure 6, p. 356) and they are reminiscent of multiple discernible spectral peaks recorded in
here with their Figure 6, p. 356) and they are reminiscent of multiple discernible spectral peaks
climaterecordsfromtheNH,e.g.,[23].
recorded in climate records from the NH, e.g., [23].

Figure 3. Spectral power density periodogram of temperature-proxy records from Vostok over the
Figure3. Spectralpowerdensityperiodogramoftemperature-proxyrecordsfromVostokoverthe
Holocene. Arrows and associated numerals designate spectral peaks at the indicated periods in years
Holocene.Arrowsandassociatednumeralsdesignatespectralpeaksattheindicatedperiodsinyears(y)
t(hya) tthaaret adriesc deirsnciebrlneiwbliet hwiniththine itnhde iicnadteicdacteodn ficodnenficdeelnimcei tlsim.Ditiss.c Derinsciberlenipbelaek pseaatkps <at0 p.0 <0 05.0a0re5 laarbee llaebde1le–d6
f1o–r6r effoerr ernecfeerteontchee tsoa mtheep seaamksep poertarkasy epdoirntrsauybesde qiune snutbfisgeuqrueesn.tT hfiegcuornefis.d Tenhcee cliomniftisdeanrecere lpimresitesn taerde
breypbreesset-nfittedex bpyo bneesntt-ifaitl ecxuprvoensenfittitaeld cutorvsetse pfiwttiesde tfoo rswteaprwdirseeg froerswsiaornd dreagtareosvsieornt hdeatwa hoovleer ftrheeq wuehnoclye
sfrpeeqcutreunmcyr esppreecsterunmted rienprtheseepneterido dino gtrhaem p(eMrieotdhoogdrsaman d(MSMeth).oFdissh aenr’ds KSMap)p. aFiasnhdert’hs eKcaoprrpeas paonndd itnhge
pcororrbeasbpiolintyditnhgat ptrhoebpaebriiloitdyo tghraatm three spuelrtisofdroogmrawmh riteesunlotiss efraorme 1w7h.3i4tea nnodispe< ar8e.7 1×7.3140 −an7,dr eps <p e8c.7ti v×e 1ly0.−7,
respectively.
PeriodogramsoftheremainingthreeAICC2012climaterecordsduringtheHolocenearesimilar
Periodograms of the remaining three AICC2012 climate records during the Holocene are similar
to the periodogram of the Vostok record (Figure 4). All are bounded near the low end by a peak
to the periodogram of the Vostok record (Figure 4). All are bounded near the low end by a peak
corresponding approximately to the mean period of the TO cycles and near the high end by
corresponding approximately to the mean period of the TOCC35305V0 Vcycles and near the high end by a
apeakcorrespondingtotheBondcycleintheNHandrangingfrom825to1027years. Betweenthese
peak corresponding to the Bond cycle in the NH and ranging from 825 to 1027 years. Between these
extremeslieatleastfouradditionalcentennial-scalepeaksinallAICC2012climaterecordsevaluated.
extremes lie at least four additional centennial-scale peaks in all AICC2012 climate records evaluated.
Theperiodsofthesepeakscoincidecloselywiththesixdiscernible(p<0.005)spectralpeaks
The periods of these peaks coincide closely with the six discernible (p < 0.005) spectral peaks
identifiedintheVostokclimaterecord(adjacentarrowsinFigure4demarcatedbyovals).Thedifference
identified in the Vostok climate record (adjacent arrows in Figure 4 demarcated by ovals). The
between the closest corresponding peaks in other records was computed in both relative terms
difference between the closest corresponding peaks in other records was computed in both relative
(positiveandnegativesignsincluded)and,moreconservatively,inabsoluteterms(signsomitted).
terms (positive and negative signs included) and, more conservatively, in absolute terms (signs
Themean(n=6)relativeandabsolutedifferences,respectively,betweenthefrequencyofVostokand
omitted). The mean (n = 6) relative and absolute differences, respectively, between the frequency of
non-Vostokspectralpeaks(Figure4)are1.0%and5.8%(VostokversusEDC;Figure4a),−1.5%and3.8%
Vostok and non-Vostok spectral peaks (Figure 4) are 1.0% and 5.8% (Vostok versus EDC; Figure 4a),
(VostokversusTD;Figure4b),and−0.7%and4.7%(VostokversusEDML;Figure4c). Homologous
−1.5% and 3.8% (Vostok versus TD; Figure 4b), and −0.7% and 4.7% (Vostok versus EDML; Figure 4c).
spectralpeaksatdifferentdrillsitesthereforematcheachotheronaveragewithin±0.2%(relative
Homologous spectral peaks at different drill sites therefore match each other on average within ±0.2%
difference)or±2.4%(absolutedifference).
(relative difference) or ±2.4% (absolute difference).
Most but not all matching spectral peaks at EDC, TD and EDML are as distinct as the
Most but not all matching spectral peaks at EDC, TD and EDML are as distinct as the
correspondingpeaksintheVostokrecord(Figure4). Forexample,peak6atVostokwithaperiodof
corresponding peaks in the Vostok record (Figure 4). For example, peak 6 at Vostok with a period of
1096years,whichmaybeanalogouswiththeBondcycleintheNH(theTO cycle),ismatched
1096 years, which may be analogous with the Bond cycle in the NH (the TOK1K5150000VV cycle), is matched
withacomparablepeakat1050yearsintheTDrecord. Wedidnotassessthestatisticaldiscernibility
with a comparable peak at 1050 years in the TD record. We did not assess the statistical discernibility
of non-Vostok spectral peaks and have no explanation for the occasional apparent differences in
relative spectral density energy of peaks that are otherwise matched in period. The findings
nonetheless support overall the hypothesis that the same temperature-proxy cycles manifest at
geographically-distributed EAP drill sites.
Climate 2018,6,3 8of30
of non-Vostok spectral peaks and have no explanation for the occasional apparent differences in
relative spectral density energy of peaks that are otherwise matched in period. The findings
nonetheless support overall the hypothesis that the same temperature-proxy cycles manifest at
geographically-distributedEAPdrillsites.
Climate 2018, 6, 3 8 of 30

FigureFi4g.urSe p4e. cStpraelctpraol wpoewredre dnesnitsyityp epreiroioddooggrraammss ooff AAnntatracrtcict ictemtepmerpaeturaret-uprreo-xpyr orexcyorrdesc oorvders tohve er the
Holocene (~12,000—0 yb1950). Each graph compares the spectral periodogram of Vostok (dotted
Holocene(~12,000—0yb1950). EachgraphcomparesthespectralperiodogramofVostok(dotted
curves; from Figure 3) with three other drill sites on the East Antarctic Plateau (EAP; solid curves): (a)
curves;fromFigure3)withthreeotherdrillsitesontheEastAntarcticPlateau(EAP;solidcurves):
EDC; (b) TD; and (c) EDML. Ovals encompass arrows designating homologous pairs of spectral
(a)EDC;(b)TD;and(c)EDML.Ovalsencompassarrowsdesignatinghomologouspairsofspectral
peaks, which match on average within ±0.2−2.4% (see text). This comparison highlights similar
peaks,spwechtricahl pmeaaktsc hin otnhea dvieffreargenet wcliimthaitne ±re0co.2rd−s2, .s4h%ow(isnege ttheex st)a.mTe hcyisclceos mreppraersiesnotnedh aigt hdlriigllh stistess imilar
spectrdailsppeerasekds winidtehlye odni ftfheer eEnAtPc. lOimrdainteater elcaoberlds so,ns thhoew leifnt gof tehaechs agrmapehc cyocrlreessproenpdr etos eVnotsetdoka, twdhirliel l sites
disperosreddinwateid lealbyelos nonth teheE rAigPh.tO cordrriensaptoendla btoe ltsheo nsitteh ceolmefptaorefde awcihthg Vraopsthokc.o Trhree sspitoens,d titmoeV poesrtioodks, while
ordinactoevlearbeedl saonndt h(einr igphatrecnotrhreessepso) nFdisthoetrh’se sKitaepcpoam panadre dthwe itchoVrroesstpookn.dTinhge spitreosb,atbimiliteyp ethriaotd sthceo vered
and(inpepriaordeongtrhaemsse sr)esFuilsth ferorm’s Kwahpitpe anoainsed atrhee: (cao) rErDesCp, o1n2d,0i5n0g top r~o0 bKaybbi1li9t5y0t (h2a6t.5t5h, ep p<e 1r0io−1d1);o (gbr)a TmDs, result
fromw11h,1it6e0 ntoo ~is0e Kayrbe1:9(5a0) (E45D.1C, p, 1<2 2,.0756 0× t1o0−~230); K(c)y EbD19M5L0, (1226,0.5005 ,top ~<0 1K0y−b1119)5;0( b(3)5T.9D4, ,p1 <1 3,1.86 0× 1to0−1~7)0. Kyb1950
(45.1,p<2.76×10−23);(c)EDML,12,000to~0Kyb1950(35.94,p<3.8×10−17).
Spectral power periodograms of non-stationary time series can be ambiguous in the frequency
domain inasmuch as the changing frequency of any variable climate cycle can broaden (“smear”) a
Ssppeeccttrraall ppoowweer rppeaekr,i orednodgerrianmg sit olefsns odnis-tsintactt.i oAndadrityiotnimalley,s ienr itehse cevaennbt eofa dmisbcriegtue ofruesquinentchye sfhrieftqs uency
domaiinn tihnea sumnduecrlhyiansg trhefeercehnacne gsiignngalf,r peqowueern cpyeroiofdaongryamvas rciaanb lmeacnliifmesat tdeisctyincclte scpaenctrbarl opaedakesn e(v“esnm ear”)
when they originate from the same, variable (non-stationary) climate cycle. We therefore
a spectral power peak, rendering it less distinct. Additionally, in the event of discrete frequency
conservatively sought additional confirmatory evidence for nonrandom periodicity in the time
shiftsintheunderlyingreferencesignal,powerperiodogramscanmanifestdistinctspectralpeaks
domain of Antarctic climate records in the form of lagged or serial autocorrelation correlelograms.
even when they originate from the same, variable (non-stationary) climate cycle. We therefore
We generated correlelograms over relatively short time frames of the Vostok record dated on the GT4
conservativelysoughtadditionalconfirmatoryevidencefornonrandomperiodicityinthetimedomain
chronology, including from 75 to 55 Kyb1950, 20 to ten Kyb1950, and ten to 0.149 Kyb1950. The GT4
ofAntcahrrcotincocloligmy aftoerr eVcoosrtdosk inditfhfeersf oornmlyo fslliagghgtleyd forormse rtihael aAuItCoCco2r0r1e2l acthioronncoolorrgeyl eolovgerr atmimse. Wpeerigoednse rated
correlelogramsoverrelativelyshorttimeframesoftheVostokrecorddatedontheGT4chronology,
Climate 2018,6,3 9of30
includingfrom75to55Kyb1950,20totenKyb1950,andtento0.149Kyb1950. TheGT4chronology
for Vostok differs only slightly from the AICC2012 chronology over time periods analyzed here,
asdemColnimsattre a20t1e8d, 6b, 3y thenear-identityoftherespectiveagemodelsoverthetimeperiodstu9d oife 3d0 here
(SMFigureS3).
analyzed here, as demonstrated by the near-identity of the respective age models over the time period
ThenullhypothesisthatTO cyclescompriserandomvariationincyclestructurewastested
studied here (SM Figure 3). C350V
by means of cyclic autocorrelation coefficients. We find that autocorrelation coefficients alternate
The null hypothesis that TOC350V cycles comprise random variation in cycle structure was tested
betweebnyp moseiatnivs eoaf ncydcnlice gaauttiovceorarteltahtieonsa cmoeeffpiceierinotsd. icWitey fainsdt htheacto aruretoscpoorrnedlaitniognT cOoeCf3f5ic0iVenctys calletefrrneaqtue ency
(Figureb5e)tw. eNene aprospietiavke sanadn dnetgraotuivgeh ast ,thnee saarmlye aplelroiofdtihcietys easa tuhteo ccoorrrreeslpaotniodningc oTeOffiC3c50iVe ncytcslea rfreeqduisecnecryn ibly
differen(Ftifgruorme 5z)e. rNoeaatr lpoewakasl panhda tlreovueglhss(,a ntelaeralsyt aaltl pof< th0e.s0e5 )a.uTtohceosrerealauttioonc ocroreeflfaictiieonntsr earseu ldtisscseurpnipblleym ent
and extdeinffderesnpt efcrotrma lzepreor aiot ldoowg arlapmhas letoveclso (nafit lremastt ahta pt <T 0O.05). Thecsye caluetsoccoormrelpatriiosne rnesounltrsa snudpopmlempeenrti odic
C350V
sequenacneds. eSxutecnhdp sopseicttirvael apuertioocdoorgrrealmatsi oton croensufirltms wthoaut lTdOnC3o50tV bceycpleoss sciobmlepruisnel ensosntrhaendsohmor tpetirmiodeics eries
sequences. Such positive autocorrelation results would not be possible unless the short time series
evaluatedrepresentrelativelystationarytimeseriesoverthetimeperiodsevaluated.
evaluated represent relatively stationary time series over the time periods evaluated.

Figure 5. Serial (lagged) autocorrelelograms of the Vostok temperature-proxy record over three
Figure 5. Serial (lagged) autocorrelelograms of the Vostok temperature-proxy record over three
representative time periods. These graphs demonstrate nonrandom periodicity in the repetition
representativetimeperiods.Thesegraphsdemonstratenonrandomperiodicityintherepetitionperiod
period of the ACO in the Vostok record and relative stationarity across the designated time periods.
oftheACOintheVostokrecordandrelativestationarityacrossthedesignatedtimeperiods.Theserial
The serial autocorrelation coefficient between high-pass filtered and normalized Vostok temperature-
autocorprreolxayti roencocrdose fisfi icni eenacthb ceatswe peleonttehdi gagha-ipnasts sthefi oltredreerd ofa tnhed anutoorcmorarelilzateidon Vcoonsvtoekrtetde mto ptheer atitmuer ela-gp roxy
recordsoifs tihne eVaocshtocka rseecoprldo tstheidfteadg satienpswtitshee reolardtiveer toof itthseelfa (uKtyo)c. oTrimreel apteiorinodcso rnevperersteendtetdo athree: (taim) 7e5 ltaog 55o fthe
VostokrKeycbo1r9d5s0h; (ibft)e 2d1 sttoe tpewn iKsyebr1e9la50ti;v (ce) ttoenit tsoe l0f.1(K49y K).yTbi1m95e0p. eriodsrepresentedare:(a)75to55Kyb1950;
(b)21totenKyb1950;(c)tento0.149Kyb1950.
Climate 2018,6,3 10of30
3.4. GeographicDistributionofACOs
ThedemonstrationofsimilarspectraldensityperiodogramsatdifferentdrillsitesontheEast
Antarctic Plateau suggests that TO cycles are common to all four ice-core records evaluated
C350V
here. We tested this hypothesis more directly by examining individual TO cycles using the
C350V
identicalapproachpioneeredbypreviousinvestigators,namelyevaluationofcyclewaveformand
sequence(Methods). ThreeofthefourAICC2012-synchronizedAntarcticclimaterecordsoverthe
period70to63Kyb1950weresampledoriginallyatahighenoughfrequencytodetectcentennial-scale
temperature-proxyoscillationsincompliancewiththeNyquist-Shannonsampling-frequencycriterion:
Vostok, the nearby (~550 km) and slightly lower-elevation (~248 m) EDC, and the more distant
(~2300km)andstilllower-elevation(~596m)EDML(Figure6)(distancesbetweendrillsitesand
elevationsarefrom[51]).
The time period from 70 to 63 Kyb1950 encompasses two well-studied climate events,
AntarcticIsotopeMaximum(AIM)#19andAIM#18[38],whichwereformerlydesignatedasAIM#5
andAIM#4[40],respectively.TheseAIMsprecedeD-Oevents#19and#18,respectively,inGreenland
icecoresfromtheNH,withsouth-to-northpropagationlatenciesof1.5to3.0millennia[38,40,49,52].
Examination of individual TO cycles shows that AIM # 19 is composed of multiple TO
C350V C350V
cycles,identifiedasACO195a,195and194,whileAIM#18iscomposedofTO cycles186–181.
C350V
These identified TO cycles are recognizable across the three climate records compared (grey
C350V
dashedconnectorlinesinFigure6),consistentwiththehypothesisthatTO cyclesaredistributed
C350V
geographicallyacrosstheEAPtoatleastthesethreedrillsites.
TheidentifiedTO cyclesthatcomposeAIM#19andAIM#18donotoccuratthesametime
C350V
indifferentclimaterecords,i.e.,thehomologouscyclesatonedrillsiteareshiftedforwardorbackward
intimerelativetootherdrillsiteswhendisplayedonacommonchronology(Figure6). Forexample,
meanpeak-to-peaklatenciesfromVostoktoEDCovertenhomologouscyclesthatencompassAIM
#19(cycles195a-190)wascomputedas−158.5years. ThenegativesignimpliesthatVostokpeaks
laghomologouspeaksatEDC.Similarly,peaksofthetenTO cyclesassociatedwithAIM#18,
C350V
186a-178areasynchronous, occurringatameantimelatencyfromVostoktoEDCof −211.2years
(Vostoklags). Inbothoftheseexamples,however,themeanlatencyissmallerthanthereporteddating
uncertaintyforthisportionoftherespectiveclimaterecords[38]andthereforeintheseexamplesthe
observeddifferenceinlatenciescannotbedistinguishedfromchronologicalnoiseordatinguncertainty.
In contrast to these examples, the mean latency between the peaks of homologous TO
C350V
cyclesfromVostoktoEDMLforcycles195a-190is−335.1years(VostoklagsEDML),whilethemean
latencybetweenhomologouscycles186a-178is−557.7years(Figure6a). Thesemeansarediscernibly
greaterthanthecorrespondingmeanslatenciesbetweenVostokandEDC(t-test,p<0.0006,n=10).
The Vostok-to-EDML mean latency also exceeds the sum of the decadal variance that arises from
averagingandtheestimateddatinguncertaintyforthisglacialperiod,whichis500years[38](p. 1737).
TheselatencydifferencesfromVostoktoEDMLcannot,therefore,beattributedtodatinguncertainty.
Instead, the differences in latency between homologous cycles at different drill sites are genuine
(non-artifactual)climatesignals.
Thecorrespondingmeanpeak-to-peaklatencyfromEDCtoEDMLbetweenhomologouscycles
195a-190is−176.6years(EDClags),discerniblysmallerthanthemeanlatencyfromVostoktoEDML
of −335.1 years (t-test, p < 0.0009, n = 9). For cycles 186a-178, the comparable mean homologous
peak-to-peaklatenciesare−322.6years(EDCtoEML)and−557.6years(VostoktoEDML),discernibly
differentfromeachother(t-test,p<0.0006,n=10). Chronologicaluncertaintyreportedforthisperiod
oftheAICC2012chronologyisfrom20to100years[38](p. 1737). Themeasureddifferencesinmean
latenciesarethereforenotonlydiscerniblewithhighprobabilitybutagainexceedthedatinguncertainty
of the AICC2012 chronology, in these examples by a factor up to >30. Comparing Vostok-to-EDC
latencywithEDC-to-EDMLlatencyyieldsnon-discernibledifferencesforcycles195a-190(p>0.66)
andcycles186a-178(p>0.13). Inbothcasesthecorrespondingmeanlatenciesnonethelessexceed
reportedchronologicaluncertainty.