Understanding the history of extreme wave events in the Tuamotu Archipelago of French Polynesia

17 Pages·2016·2.66 MB·English

MarineGeology380(2016)174–190
ContentslistsavailableatScienceDirect
Marine Geology
journal homepage: www.elsevier.com/locate/margo
Understanding the history of extreme wave events in the Tuamotu
Archipelago of French Polynesia from large carbonate boulders on
Makemo Atoll, with implications for future threats in the central
South Pacific
A.Y.AnnieLaua,b,⁎,JamesP.Terryc,AlanD.Zieglera,AdamD.Switzerd,YingsinLeed,SamuelEtiennee
aDepartmentofGeography,NationalUniversityofSingapore,1ArtsLink,Singapore117570,Singapore
bSchoolofGeography,PlanningandEnvironmentalManagement,TheUniversityofQueensland,StLuciaCampus,Brisbane,Qld,4072,Australia
cCollegeofSustainabilitySciencesandHumanities,ZayedUniversity,P.O.Box19282,Dubai,UnitedArabEmirates
dEarthObservatoryofSingapore,NanyangTechnologicalUniversity,50NanyangAvenue,639798,Singapore
eEPHE(ÉcolePratiquedesHautesÉtudes),PSLParisResearchUniversity,CNRSUMR6554LETGDinard,France
a r t i c l e i n f o a b s t r a c t
Articlehistory: Numerouslargecarbonatebouldersupto164tonnesinmasswereinvestigatedonthereefflatandbeachesof
Received22December2015 MakemoAtollintheTuamotuArchipelagoofFrenchPolynesiatorevealthepastoccurrenceandtoanticipate
Receivedinrevisedform14April2016 thefuturepotentialthreatofextremewaveevents,possiblygeneratedbytropicalcyclonesandtsunamis.The
Accepted21April2016 modernreefedgeandemergedmid-Holocenecoastallandformswereidentifiedassourcesofbouldersmobilized
Availableonline22April2016 duringextremewaveeventsinthepast.Theminimumflowvelocitiesproducedbyextremewaveeventswere
estimatedtoexceed5.4–15.7m/satthereefedgeondifferentpartsoftheatoll.Comparisonofuranium–thorium
Keywords:
agesofbouldercoralfabricwithwrittenhistoricalrecordsindicatesthattwolargeboulders(77and68tonnes)
Boulders
Waves werepossiblyemplacedonthereefflatbyapowerfulcycloneinFebruary1878.Althoughmostboulderdatesare
Tropicalcyclone olderthantheearliesthistoricalcycloneandtsunamirecordsinFrenchPolynesia,theiragesconcurwiththefol-
Tsunami lowing:(a)periodsof“storminess”(i.e.increasedcycloneactivitycomparedtotoday)inthecentralSouthPacific
Historicalrecords overthelastmillennium;and(b)periodsofhighsea-surfacetemperature(SST)attheGreatBarrierReef,possibly
SouthPacific associatedwithhigher-than-normalSSTsPacific-widethatfacilitatedthegenerationofcyclonesaffectingthe
centralSouthPacificOcean.NoneofthebouldersonMakemoweredatedyoungerthanCE1900,implyingthat
thelastcenturyhasnotexperiencedextremewavesofsimilarmagnitudeinthepast.Nevertheless,thefindings
suggestthatwavesofcomparablemagnitudestothosethathavetransportedlargebouldersonMakemomay
recurintheTuamotusandthreatenislandcoastsacrossthecentralSouthPacificinthefuture.
©2016ElsevierB.V.Allrightsreserved.
1.Introduction gravel.Consequently,largerdeposits,inparticulartall-standingcarbon-
atebouldersthatoriginatefromthereefframework,areimportantfea-
TheTuamotuArchipelago(Tuamotus)consistsof77atollsandone turesforinvestigatingtheageof,andenergyassociatedwith,extreme
emergedlimestoneisland,spreadover1500kminaWNW-ESEchain waveeventsinthearchipelago.Suchresearchhasproveneffectiveon
acrossthecentralSouthPacificOcean.Extremewavehazardassess- theGreatBarrierReefof Australia(Nott,1997; Yu et al.,2012; Liu
mentontheseinhabitedislandsishinderedbytheshortandpotentially etal.,2014),theRyukyuIslandsofJapan(Suzukietal.,2008;Goto
incompletehistoricalrecordoftropicalcyclonesandtsunamisinthere- etal.,2010;Araokaetal.,2013),TongatapuIslandinTonga(Frohlich
gion,aswellastheinfrequentoccurrenceofsucheventsinmodern etal.,2009);SumatrainIndonesia(Parisetal.,2010);TaveuniIsland
times.Proxyinformationisthereforeusefulforassessingthecurrent- inFiji(EtienneandTerry,2012);BonaireintheCaribbean(Engeland
day threat. However, fine-grained sedimentary evidence, such as May,2012)andLanyuIslandinTaiwan(Nakamuraetal.,2014).
tsunami-orcyclone-depositedsandsheets,isnotreadilypreservedin Theoverallaimofthisresearchistoimprovecurrentunderstanding
beach stratigraphies on low-lying coral atolls because of continual onthepotentialthreatsofextrememarinewavehazards,namelytsu-
reworkingbywaves(Parisetal.,2010).Narrowatollrimsarealsonot namisandtropicalcyclones,fortheatollsofthecentralSouthPacificre-
ideal locations for the preservation of storm ridges of reef-derived gion.Thisisachievedbyexaminingtheextremewaveeventhistoryfor
theTuamotusofFrenchPolynesia.Theobjectivesarethreefold.First,the
⁎ Correspondingauthor. physicalcharacteristicsoftheatollsintheTuamotusandtheirrelation-
E-mailaddress:[email protected](A.Y.A.Lau). shipwithHolocenesea-levelchangeinthecentralSouthPacificare
http://dx.doi.org/10.1016/j.margeo.2016.04.018
0025-3227/©2016ElsevierB.V.Allrightsreserved.
A.Y.A.Lauetal./MarineGeology380(2016)174–190 175
examined(Section2).Second,acomprehensiverecordofhistoricaltsu- windscanreach30knotsandgenerateswellsof3–5minamplitude.
namisandtropicalcyclonesiscompiledfortheTuamotuArchipelago Asaresult,islandsofcoralshinglehaveformedonthenorth-eastern
(Section3).Third,fieldinvestigationofnumerouslargecarbonateboul- atollrimsovertime(D'Hauteserre,1978;PirazzoliandMontaggioni,
dersonthereefflatofMakemoAtollinthearchipelagoisconductedto 1988).
interprethowandwhenbouldersweretransported(Sections4and5).
Finally,thesefindingsarecomparedwiththehistoricalrecordsoftsu- 2.2.Holocenesea-levelchangeandtectonicsintheTuamotuArchipelago:
namisandcyclonesinattempttoassessthethreatassociatedwithex- influenceoncoastalgeomorphology
tremewaveeventsinthegreaterTuamotus.
Relativesea-levelchangeduringthelastglacialmaximum(LGM)
2.Studyarea (23,000–17,000BP)intheTuamotuArchipelagowasmeasuredfrom
thecoralreefrecordatMururoaAtollatapproximately135–143m
2.1.TheTuamotuArchipelago belowpresentsealevel(p.s.l.)(Camoinetal.,2001).Followingthe
post-LGMmarinetransgression,thesealevelwasestimatedat17–
TheTuamotuArchipelago(Fig.1)isoneofthefiveislandarchipela- 23mbelowp.s.l.at9000BP,thenrosesharplyto2–3mbelowp.s.l.at
gosofFrenchPolynesia;theothersaretheSociety,Austral,Marquesas about8000BP(Camoinetal.,2001).SealevelintheTuamotusreached
andGambierarchipelagos.Situatedfarfromanytectonicboundary, aHolocenehighstandof0.8±0.2mabovep.s.l.fromabout4500BPto
theatollsintheTuamotuswerebuiltontopofvolcanicbasements after2000BP,buthassincegraduallydroppedtopresentsealevel
that formed from hot spot activity at least 37.5 million years ago (PirazzoliandMontaggioni,1988;Pirazzolietal.,1988a).Alternatively,
(ClagueandJarrard,1973;Pirazzolietal.,1988a).Thesubmarinevolca- recentresearchhassuggestedthesealevelhighstandintheTuamotus
nicedificessupportingthearchipelagoriseover4000mfromtheocean reachedabout1mabovep.s.l.andremainedstableuntilCE800,before
bottomatanangleexceeding45°.Submarineslopesbecomemoregrad- droppingsteadilyuntilCE1900,thensubsequentlyrosetopresentsea
ualatadepthof1000m(Vitousek,1963;Jordahletal.,2004).Atthesea levelduringthelastcentury(Dickinson,2003).
surface,low-lyingislandsformontopofatollreefs.Theyarecomposed Meanwhile,localtectonicactivityhasinducedverticalmovementof
ofcoralrubbledepositedbywavesontheoldcoralplatforms.Theeleva- atolls.InthetectonicallystablepartofthearchipelagotowardsVahitahi
tionofthesecoralislandsrarelyexceeds10mabovesealevel(Dupon, andMururoaattheeasternendoftheatollchain,thermalsubsidence
1986;Etienne,2012). occurs(Pirazzolietal.,1987a).TherateofsubsidenceofMururoaises-
AnnualweatherintheTuamotuArchipelagovarieslittle.Humidityis timatedat7–8mm/1000years,asdeterminedbytheK-Ardatingofthe
high(~80%)mostofthetime;thesummerseasonhasaslightlyhigher volcanicbasement(Trichetetal.,1984;Camoinetal.,2001).Incontrast,
temperaturefromNovembertoApril(25–30°C)thantheothersix- tectonicupliftisevidentatthesouthernboundaryonMakateaandAnaa
month period (24–28 °C) (Sachet, 1983). The Tuamotu coasts are (raisedcoralatolls),owingtolithosphericflexureassociatedwiththe
microtidal,withtidesrangingfrom0.2to0.7m.Mostwavesare1– hotspotwhereMehetiaiscurrentlylocatedandwhichcreatedTahiti
3mhighandbreakatperiodsof6–9s(IntesandCaillart,1994).The during the Pleistocene (Montaggioni et al., 1985; Pirazzoli et al.,
yearly average wave height measured in the western Tuamotus is 1988b;PirazzoliandMontaggioni,1988;MontaggioniandCamoin,
1.6m(Andréfouëtetal.,2012),althoughprevailingnorth-easttrade 1997;Dickinson,2001)(refertoFig.1).
Fig.1.LocationmapofMakemoAtollintheTuamotuArchipelagoofFrenchPolynesia.ThehotspotthatcreatedTahitiinthePleistoceneiscurrentlylocatedatMehetia.Thisactivevolcano
iscausingsubsidencearoundMehetia,andtheemergenceofatollsincludingAnaaandMakateaduetoflexuralupwarp(redcircle)outsidetheconeofsubsidence(orangecircle).Inferred
extentoftectonicverticalmovementsisadaptedfromDickinson(2001).Locationsofotheratollsmentionedinthetextarealsolabelled.A“+”indicatesthatlargeboulderdepositshave
beenreportedbyothers(seeSection3.3).+An=Anaa;+Hi=Hikueru;Ka=Kaukura;Ma=Makatea;Mu=Mururoa;+Nk=Nukutavake;+Np=Nukutepipi;Pu=Pukarua;Ra=
Raroia;Re=Reao;+Rg=Rangiroa;+Ta=Takapoto;+Tu=Tureia;Vh=Vahitahi;+Vt=Vairaatea.(InsetfromGoogleMap).
176 A.Y.A.Lauetal./MarineGeology380(2016)174–190
ThecombinedeffectsoflateHolocenesea-levelfallandlocaltec- consistsmainlyoftheshortbranchingcoralsAcroporaandPocillopora.A
tonic influences have resulted in the slight emergence of mid- mixedzoneofbranching,encrustingandmassivecoralsarefoundat
HolocenereefsabovemodernsealevelonatollsacrosstheTuamotuAr- 10–15mdepths.Adominanceofdomalandplatycorals,including
chipelago.Emergedmid-Holocenereefsaregenerallyfoundateleva- Porites,MontiporaandAstreopora,extendto15–20m.Encrustingcorals
tions ranging from 0.8–1.1 m. On the most uplifted atoll, Anaa, andelephantskincoralsPachyseriscanbefoundbelow20m,reaching
Holocenecoralreefs(14Cage2000–2600BP)occurat1.3–1.5mabove depthsover40minsomecases(Bruckner,2014).
p.s.l.(MontaggioniandCamoin,1997;Dickinson,2001).EmergedPleis-
tocenereefsanddepositsarealsopreservedasevidenceoftectonicup-
liftandlocalsea-levelfallonTuamotuatolls.Examplesincludeemerged 3.HistoricalextremewaveeventsintheTuamotus
reefs4–5mhighonAnaaandupto3.5monKaukura(Pirazzoliand
Montaggioni,1986;Pirazzolietal.,1988b).Elevateddome-shapedrem- 3.1.Tropicalcyclonesandstorms
nantsofHolocenealgalcrusts,indicatorsofpastsealevel,arealsopres-
entonsomeatolls.Exfoliationofemergedreefflats,coralconglomerate ThecycloneseasonintheSouthPacificcommonlyextendsfromNo-
platforms,andcorallinealgalcrustsbymechanicalerosion,mostlydur- vembertoApril.Onaverage,aboutninetropicalcyclonesoccureastof
ingcyclonicevents,havealsobeenobservedonseveralsouthernatolls: the 160°E meridian each year; however, the number per year was
Rangiroa,Reao,VahitahiandPukarua(Stoddart,1969;Ricard,1985; foundtovarygreatlyfrom3to17duringtheperiod1970to2006
Pirazzolietal.,1987a,1987b,1988a). (Terry,2007).ThisannualvariabilityislargelyrelatedtotheElNiño
SouthernOscillation(ENSO).Moretropicalcyclonesandtypicallystron-
2.3.MakemoAtoll gerstormsoccurinthecentralSouthPacificinElNiñoyears(Revelland
Goulter,1986;TerryandGienko,2010).Theeastwardshiftofwarmer
MakemoAtoll(16°37′S,143°40′W),lyinginthemiddleregionofthe SSTsduringElNiñoyearsalsoallowsmorestormstotravelfurther
Tuamotu Archipelago, is the fourth largest atoll in the group eastintothecentralSouthPacificwheretheTuamotusarelocated.
(~670km2).TheatollisorientedNW-SEandhasalengthof65km TropicalcyclonesarenotfrequentwithinFrenchPolynesia(Larrue
(Fig.2).Theleewardreefsinthesouthoftheatollaremostlysub- andChiron,2010).Cyclonehazardassessmentssuggesta50-yearre-
merged.Thewindwardreefsinthenorthoftheatollhavelowislands turnperiodforwavesexceeding12matboththeTuamotusandTahiti
with vegetation covering 56 km2. The islands support one village, intheSocietyIslands(Météo-France,1995;Damlamianetal.,2013;Ste-
Pouheva, having a population of less than 900 (L'Institut de la phensandRamsay,2014).DeAngelis(1983)statedthatonly20tropical
statistiquedelaPolynésiefrançaise,2012). cyclonesaffectedtheTuamotusinthe150yearsextendingfrom1825to
Onthenorthernshore,twodeeppasses(avainPolynesian)connect 1982(citedbyRuminski,1990).However,inthecycloneseasonof
thelagoonwiththeopenoceanandseparatethelow-lyingcoralislands 1982–83,aperiodunderthestronginfluenceofanElNiñoeventthat
(Fig.2).Carbonatebouldersarefoundmainlyonthe70–100mwide causedtheseasurfaceneartheTuamotustobesignificantlywarmer
reefflatthatisexpandingbyaccretioninanoceanwarddirectionto thannormalyears,fivetropicalcyclonesandonetropicaldepressionoc-
thenorth.AredalgalreefcrestcomprisingPorolithonorLithothamnion curredinthedurationofsixmonths.Fiveofthesestormstrackedpassed
(primary reef-building algae types) occurs at the reef edge where theTuamotus,causingseafloodingtoadepthofmorethan1m,soilero-
waveenergyishighest.Emergedfeatures,includingHolocenereefs, sion,anddisplacementofcoralrubbleon theatolls(Dupon,1986;
coralconglomerateplatforms,andbeachrockareexposedatheightsof Etienne,2012).
nomorethan0.6monandbehindthemodernreefflatonsomeparts Intotal,approximatelyonly24known(includingsomedoubtful)
oftheshore(Fig.3).Anarrowbeachupto20mwide,composedpri- tropicalcycloneshaveaffectedtheTuamotusin192yearsspanning
marilyofcoraldebrisofsandtocobblesize,islocatedfurtherbehind. 1822–2014(Table1).Therecordofstormhistoryisfragmented,asex-
Threetofourzonesofcoralscanbeidentifiedonthesubmarinereef emplifiedbythefactthatonlyonestormin1958wasdocumentedfor
slopeonTuamotuAtollreefs(Bruckner,2014).Theuppermost5–10m the72yearsbetween1907and1979.Sachet(1983)commentedthat
Fig.2.MakemoAtollinthecentralTuamotuArchipelagoconsistsofwindwardlowislandsformingthenorthcoast(darkgreyshading),submergedleewardreefsinthesouth(lightgrey),
acentrallagoon(whiteareabetweenthereefs).TherearetwodeeppassesconnectingthecentralatolllagoontotheopenPacific.Partsoftheshore(blueshading)werevisitedforboulder
datacollection,andtheseshoresweredividedintofivezones(AtoE)foranalyses.
A.Y.A.Lauetal./MarineGeology380(2016)174–190 177
(a) (b)
(c)
Fig.3.Emergedcoastalfeaturesincluding(a)emergedreefremnantsofdifferentformsand(b)calcareousbeachrockarepresentatpartsofthenorthernrim.(c)Ageneralizedprofileand
zonationsoftheMakemowindwardisland.Algalreefcrestisformedonthereefedgewherewaveenergyisthehighest,whichisthesourceofmostwave-transportedboulders.Boulders
aremostlydepositedonthe70–100mreefflat,whilesomeweretransportedfartherontoorbehindthebeach.Diagramisnotdrawntoscale.
theremusthavebeenotherstormsinpre-Europeantimes,butthereis onTarawaAtollinKiribati,morethan1000kmdistanttothenortheast
nowrittenrecordverifyingthis. (Stone,2015).ThisexampleindicateshowMakemomightbeaffected
Thecyclonesintheunusual1982–83season,andthedamagethey byswellsfromcyclonesthattrackanywherewithintheFrenchPolyne-
brought to the Tuamotus, have been reported in great detail (e.g. sianterritory.
Harmelin-VivienandStoddart,1985;Dupon,1986).Stormsurges3–
4minheightandfloodingofinhabitedareastomorethan1mwereex- 3.2.Tsunamis
periencedonsomeatolls(Harmelin-VivienandStoddart,1985;Dupon,
1986).VeryhighwaveswerereportedbyeyewitnessesonTureia(16– ThehistoricaltsunamirecordisverylimitedintheTuamotuArchi-
18m),Hao(15m)andAnaa(12m)(desGarets,2005;Damlamian pelago(Etienne,2012).Ithasbeensuggestedthatduetothesteepsub-
etal.,2013).Asaresultofpowerfulsurgesandwaves,largeboulders marineslopeandsmallsizeofatollislands,relativetolongtsunami
excavatedfrombeachrockandreefslopeswerethrownontothereef wavelengths,amplificationofwavesuponshoalingisminimal,resulting
flats,inboulderramparts,acrosstheinter-islandpassages(hoainPoly- in little destruction by tsunami waves on atolls (Vitousek, 1963;
nesian), or into atoll lagoons (Sachet, 1983; Harmelin-Vivien and StoddartandWalsh,1992;Reymondetal.,2012).However,theimpact
Stoddart,1985).Sheetsofcoralshingleandrubblewerealsodeposited ofthe2004IndianOceanTsunamiintheMaldiveshasshownthatatolls
onshorelines,whilebothaccumulationanderosionofsandonsmallis- arenotimmunetothedestructiveforcesoftsunamiwaves.In2004,
letsandinchannelswerenoticedondifferentpartsoftheaffectedatolls waveheightsof1.8mtogetherwithextensivefloodingcauseddamage
(Sachet,1983;Dupon,1986).Itismoredifficulttoassesstheimpactsof ofinfrastructureonMaléIslandatsouthernNorthMaléAtoll.Morethan
oldercyclonesbutinterestingperspectivesemergewithmultidisciplin- 80liveswerelostintheMaldivesduringthe2004IndianOceanTsu-
aryapproaches.Correlatingcyclonevelocityandatmosphericpressure nami(Kenchetal.,2006,2008).
derivedfromvariousarchives,Canavesio(2014)suggestedthatwaves Nevertheless,bothnumericalmodellingandrecentobservations
ashighas15.5–18.5mmighthavesweptAnaaAtollduringtheFebru- duringtsunamishavedemonstratedthattheTuamotuatolls,being
ary1906cyclonethatkilledbetween95and130inhabitants. protectedbyfringingreefs,receivesmallertsunamiwavesandsustain
Cyclone-generatedswellsareacommonhazardinFrenchPolynesia lessdamagethanvolcanicislandsinotherarchipelagosofFrenchPoly-
(Canavesio,2014).Theseswellsareusuallygeneratedbycyclonesys- nesia(Sladenetal.,2007).Accordingtowrittenrecords,changesin
temsoutsideoftheterritory,mostoftencomingfromthewesternPa- waterlevelwereobservedintheTuamotusfollowedthegenerationof
cific(Etienne,2012).IntheTuamotuArchipelago,swellamplitudesof tsunamisoccurringin1946(AleutianIslandearthquake),1960(Chile
upto10–15mwererecordedduringthe1982–1983cycloneseason. earthquake),2009(SouthPacificearthquake),2010(Chileearthquake)
Themeancyclonicswellheightnormallyrangesbetween4and12m and2011(Tohoku-okiearthquakeinJapan).Yet,Tuamotuatollssuf-
(Ministère du Développement Durable, 2006 as cited in Etienne, feredonlymoderatelyinthe1946and1960events(Reymondetal.,
2012).Ascyclonicswellhazardisnotdocumentedsystemicallyinthe 2012).Islandersobservedwaterlevelchangesoflessthan1monsev-
region,thereturnperiodforlargeswellscannotbedetermined.Yetit eralinhabitedatollsduringthe1960Chiletsunami(Vitousek,1963).
can be assumed that strong cyclones tracking through or near the WhereasintheMarquesasArchipelago,wherevolcanicislandsarenot
Tuamotuslikelygeneratedhighswells,affectingtheatolls.Cyclonic protectedbycoralreefs,run-upsexceeding10mfollowedthe1946
swells can also travel long distances. For instance, Cyclone Pam in and1960tsunamis(Schindeléetal.,2006;Etienne,2012).
March2015causedmuchdestructionasittrackedthroughthesouthern Anoraltraditionofaplausibletsunamidatingfromthe16thcentury
islandsofVanuatu.Butitspowerfulswellsalsodamagedinfrastructure wasreportedbyBourrouilh-LeJanandTalandier(1985)citingOttino
178 A.Y.A.Lauetal./MarineGeology380(2016)174–190
Table1
DetailsofhistoricalcyclonestrackedthroughandaffectedtheTuamotussince1822.
Date(Nameofstorm) Cycloneseason/El Damage/descriptions Ref.#
Niñointensity(ifany)
1822 Unknown Anaaandneighbouringatollsdevastated 6
1825 Unknown Anaahitbystorm 6
21Jan.1856(RecordedintheSociety 1855–56 HMSDidoofBritishRoyalNavywasdismastedinacycloneoffRaiateaoftheSocietyIslands. 4
Islands)
Jan1877 1876–77 “SeveralstormscauseddamagetovariousatollsincludingManihi”(Sachet,1983,p.3) 2,6
NoElNiño
Sept1877 1877–78 Hurricaneofparticularforce 1,2,5,6
StrongElNiño “SeveralstormscauseddamagetovariousatollsincludingManihi”(Sachet,1983,p.3)
6–7Feb.1878 Hurricaneofparticularforce 1,2,5,6,7
Extremelyviolent,117peoplekilledinKaukura.Damagedonebystormtidesratherthanwind.
14–15Jan.1903 1902–03 Hurricaneofparticularforce 2,4,6,7
ModerateElNiño Devastatedalargenumberofatolls,515peoplediedincluding377onHikueruAtoll.Damagedone
bystormtidesratherthanwind.
Bouldersof4–5tor/and4–6minlengthdepositedonRaroia
23–25Mar.1905 1904–05 ExtensivedamageonManihiandTakaroa 2,6,7
ModerateElNiño
6–8Feb.1906 1906–07 Hurricaneofparticularforce 2,5,6,7
ModerateElNiño ImmensedamageintheSocietyIslandsandTuamotus,includingonManihiandTakaroaatolls.
Northcoastsflooded,atleast121peoplekilled.Damagecausedbystormtidesratherthanwind.
Boulders4–6minlengthdepositedonRangiroa
Jan.1958(Doubtfulevent) 1957–58 “Teissier(1969)mentionsonlyonemorestormtohittheTuamotus,inJan.1958”(Sachet,1983,p.4) 3,6,12
StrongElNiño ButaccordingtoRef#(9),nocyclonetrackedthroughorneartotheTuamotusinthiscyclone
season.
5–9Dec.1977(Tessa) 1977–78 TrackedatnorthoftheTuamotuswithaminimumpressureof990hPa 3,11
WeakElNiño
27–29Nov.1980(Diola) 1980–81 OriginatedandtravelledintheTuamotus,maxwindspeed35knots 3,6,13
NoElNiño
22–26Jan.1983 1982–83 OriginatedandtravelledintheeasternTuamotus 3,6,7
(Nano) StrongElNiño
22–27Feb.1983 PassedclosetoTakapoto-Manihiarea,demolishingbuildingsandinstallations,breakingtress, 3,6,7
(Nisha/Orama) pilingupcoralshingle,tearinguplargeblocksofcoralfromreefandthrowingthemonreefflats
6–14Mar.1983 BoulderstransportedonNukutipipi(upto30m3),Takapoto(unrecordedsize),Anaa(45–145m3, 3,6,7
(Rewa) byCycloneOrama)
7–13Apr.1983 3,6,7
(Veena)
17–21Apr.1983 OriginatedandtravelledintheeasternTuamotus. 3,6,7
(Williams)
5–17Dec.1991 1991–92 Maximummeanwinds45m/s,lowestpressure976hPa.TrackedpassedthesouthernTuamotus 8
(Wasa/Arthur) StrongElNiño nearMururoa,butmostdamagewasintheSocietyIslandswherepropertiesweredestroyed.
5–9Feb.1992 OriginatedatnorthoftheTuamotusandtravelledthrougheasternTuamotus.Nodamagerecord 3,12
(Cliff) found.
29Jan.-2Feb.1998 1997–98 PassedthroughtheTuamotus,damagedonebypre-cycloneswellratherthanwinds. 3,8
(Ursula) StrongElNiño
30Jan.-3Feb.1998 PassedthroughthemiddleoftheTuamotus.TCsUrsulaandVelitogetherdamagedhouses,roads 3,9
(Veli) andbridgesonMataiva,andafewhousesonMakatea.AirstriponRangiroadisruptedbycoraland
sandwashedup.Nolossoflifeinthesetwoevents.
29Apr.-3May1998 Weakcyclone(tropicaldepression)passedovertheTuamotus.Tenpeoplekilledwhenwaves 3,9
(Bart) capsizedaboat.
23–29Feb.2000(Kim) 1999–2000 TrackedsouthoftheTuamotus.Swellsexceeded2mwererecordedattheGambierIslandsin 3,10,12
(Cyclonedidnottrackthroughthe NoElNiño southeastoftheTuamotus.
Tuamotus) (StrongLaNiña)
29Jan.-8Feb.2010(Oli)(Cyclonedid 2009–10 DidnottrackintheTuamotusbutasaseveretropicalcycloneofcategory4,muchdamagewas 3,12
nottrackthroughtheTuamotus) StrongElNiño doneontheSocietyIslandsandtheAustralArchipelagobythestormandrelatedlargeswells.
AccordingtoaresidentonMakemoAtoll,thelow-lyingbeachwasfloodedduringthestorm.
#References:
ForElNiñoconditionidentification
1:AllanandNicholls(1991).
2:WolterandTimlin(2011).
3:NOAAClimaticPredictionCenter(2015).
Forcyclonedescriptions
4:EditorofTheNauticalMagazine(1856)
5:Stoddart(1969),quoteddirectlyfromGeographicalhandbooksSeries(1943).
6:Sachet(1983),referredtotheworkofMorenhout(1937),Giovanelli(1940);Teissier(1969)andDeAngelis(1983).
7:Bourrouilh-LeJanandTalandier(1985),usingdatafromMeteorologicalBureauofPapeete,Tahiti,FrenchPolynesia.
8:Gill(1994).
9:ChappelandBate(2000).
10:Tahitipress(2010).
11:SouthernHemisphereTropicalCycloneDataPortal(2011).
12:JointTyphoonWarningCenterTropicalCycloneBestTrackDataSite(2014).
13:NOAAnationalclimaticdatacenter(2014).
A.Y.A.Lauetal./MarineGeology380(2016)174–190 179
(1965)fromRangiroaAtollintheeasternTuamotus.Thefolklorerefers 3.3.Wave-depositedbouldersonTuamotuatolls:publishedaccounts
to the conditions related to a sudden and brutal catastrophe that
destroyedthewesternandsouth-westernofRangiroa.Apassagebegins OwingtothelowreliefofatollsintheTuamotus,largeblocksand
“Souslesoleilmettantdeséclairséblouissantsdanslecreuxdesimmenses bouldersareprominentfeaturesthathavebeenrecordedhistorically.
vagues”(Bourrouilh-LeJanandTalandier,1985,p.314),whichtrans- IntheeasternTuamotus,agiantreefboulder(approx.150m3)on
latesto“Underthesettingsun,dazzlingflashesinthetroughsofhuge Vairaateawasindicatedon theFrenchchartno.5878 asa“Rocher
waves”.Sunnyweathermentionedsimultaneouslywithhugewaves Noir”orblackrock(Pirazzolietal.,1988a).Twootherreefbouldersof
impliestheeventwasatsunamiratherthanacyclone. upto15m3werefoundonNukutavakeAtoll;andseveralbouldersof
TsunamisbelievedtohaveaffectedotherpartsofFrenchPolynesia upto30m3werefoundonTureia.Alloftheseboulderswereconsidered
may have also struck the Tuamotu atolls. Bourrouilh-Le Jan and inearlierpublicationstoberemnantsofpastcyclones(Pirazzolietal.,
Talandier(1985),forexample,haveinferredthepossibleoccurrence 1988a). Carbon-14 dating of boulder coral fabric suggests that the
ofmultipletsunamisintheMarquesasArchipelagofromtheabandon- waveeventsthatbroughtthemonlandspannedtheperiod3995–
mentofcoastalhabitats.Similarly,examiningarchaeologicalandgeo- 1220BP(Pirazzolietal.,1988a).
morphologicalfeaturesinvariousPacificIslandcountries,Goffetal. Oneprominentreefboulderdatedat1470±60BPexistsonHikueru
(2011) presented the case for four plausible palaeo-tsunamis that inmiddleoftheTuamotuArchipelago(Pirazzolietal.,1988a).Asitis
couldhaveaffectedFrenchPolynesia.Threeabandonedoccupation situatedontopofapedestalthatis0.7mabovethepresentreefflat,
sitespreservedundersandlayersonUaHuka,NukaHivaandRurutu thisboulderisbelievedtohavebeenemplacedontothereefbycyclonic
islandsmighthaveresultedfromasingletsunamigeneratedbythe waveswhenthesealevelwashigher(Pirazzolietal.,1988a).Mean-
1450earthquake,whichGoffetal.(2012)proposedoccurredatthe whileonRaroia,bouldersweighing4–5tweretransportedontothe
Tonga-KermadecTrench(TKT),3000kmwestoftheTuamotus.Asum- coastbyacyclonein1903(Giovanelli,1940).BouldersonNukutipipi
maryofthese“equivocal”eventsispresentedtogetherwithotherhis- Atollupto30m3insizewereemplacedbyoneofthetropicalcyclones
toricaltsunamisreportedintheTuamotusinTable2. duringtheunusual1982–83cycloneseason(SalvatandSalvat,1992).
Table2
Detailsofconfirmedhistoricaltsunamisandpossible/inferredpalaeo-tsunamisintheTuamotusorFrenchPolynesiaingeneral.
Date/period Locationaffected Tsunamisource Evidence Impact/remarks Ref.#
ConfirmedhistoricaltsunamisintheTuamotuArchipelago
1Apr.1946 TheTuamotus Earthquake(Mw=8.1)intheAleutian Witnessesreport Waveheight1.9matHaoatoll 6
Islands,Alaska
22May1960 TheTuamotus Earthquake(Mw=9.5)atChile Witnessesreport Observationofwater-levelchangesofb1mreported 1
frommultipleatolls
29Sept.2009 TheTuamotus Earthquake(Mw=8.2)intheSamoa Tidegaugerecord Waveheight0.4matRangiroaatoll 7
Islands,SouthPacific
27Feb.2010 TheTuamotus Earthquake(Mw=8.8)atChile Tidegaugerecord Waveheight0.3matRikiteaIsland 8
11Mar.2011 TheTuamotus Earthquake(Mw=8.9)ateastJapan Tidegaugerecord Waveheight0.29matRangiroa 9
PossibletsunamiattheTuamotuArchipelago
16thcentury Rangiroa,the Oraltradition Suddenandbrutalwaves,destroyedsettlementsinSW 2
Tuamotus andSRangiroa
InferredtsunamiaffectingotherpartsofFrenchPolynesia
Latefirst HaneofUaHuka,the Suddenabandonment Inferredfromethnologystudies 3
millennium Marquesas ofcoastalhabitats
CE1430–1570 UaHuka,the PossiblyanearthquakeinSouth Mobilisedcoastaldune Dateinferredfromcarbon-14datingofsoiland 10
Marquesas America. coveringoccupation skeletons.
(orpossiblythe1450TKTTsunami)
CE1450–1500 NukaHiva,the Possiblyfromthe1450TKTTsunami Cleansandsheet Thesandsheetcouldbedepositedfromamajorstorm. 10
Marquesas betweenarchaeological Dateinferredfromcarbon-14datingofarchaeological
layers units.
CE1450–1600 Rurutu,theAustrals EarthquakeatTongaTrenchorSouth Cleansandsheet Dateinferredfromcarbon-14datingofculturalunits. 10
America(Possiblythe1450TKT betweenarchaeological
Tsunami) layers
16thcentury HaneofUaHuka,the Suddenabandonment Inferredfromethnologicalstudies 3
Marquesas ofcoastalhabitats
Possibly16th Huahine,Raiateaand Suddenabandonment Inferredfromethnologicalstudies 4,5
century Scilly,SocietyIslands ofcoastalhabitats
Post1650 Huahine,Society Cleansandlayerontop Dateinferredfromcarbon-14datingofboneunderneath 10
Islands ofabandoned sandlayer.
occupation
#References:
1:Vitousek(1963).
2:Ottino(1965)ascitedin#7.
3:KellumOttino(1971)ascitedin#7.
4:Sinoto(1978)ascitedin#7.
5:Semah(1979)ascitedin#7.
6:Bourrouilh-LeJanandTalandier(1985).
7:CEA(2009).
8:CEA(2010).
9:PacificTsunamiWarningCenter(2011).
10:Goffetal.(2011).
180 A.Y.A.Lauetal./MarineGeology380(2016)174–190
FurtherwestintheTuamotus,largecarbonateboulders(ofunre- andmedium-sizedboulderswerechosen:clastswithintermediate(b-)
cordedsize)onthenorthwestreefofTakapotosimilarlyoriginated axeslessthan1mweregenerallynotmeasured.However,sevenboul-
fromcyclonesin1982–83(Sachet,1983).OnthewesternrimofAnaa derswithb-axesb1mwereincluded,eitherbecausetheywereonly
Atoll,sevenboulders(25–145m3)movedbyCycloneOramaonthe slightlybelowthechosensizethreshold,orweredepositedfarinlandbe-
reef flat in 1983 were measured by Etienne et al. (2014). Another hindthebeach(135–161m)wheretheirpresencewasassumedtobe
eightlargerbouldersranginginvolume150to450m3wereencoun- representativeofthelimitofstrongwaveinundation.Forboulderorien-
tered,butcausesofdepositionarestillunknown(Etienneetal.,2014). tationandwavedirectionanalyses,thestudiedshorelinewasdivided
OnRangiroa,thelargestatollintheTuamotus,largecoralblockscan intofivezones(A–E)accordingtotheshoreorientation(refertoFig.2).
be found at multiple locations. Surveying three large blocks at the Foreachselectedboulder,thefollowinginformationwasrecorded:
southern,westernandnorthernrims,Stoddart(1969)foundthatthe (1)thedimensionsofthreeaxes(termeda-,b-,c-axesforlong,inter-
largestblockonthewesternrimmeasured5.5×5.5×4.5m.Research mediate,andshortaxisrespectively)measuredbytape;(2)theorienta-
byBourrouilh-LeJanandTalandier(1985)recordednumerousboulders tion of thea-axis in degrees; (3) shape(rectangular, triangular, or
onwesternatollrims,withthelargestblock(15×10×5m)weighting ellipsoidal);(4)GPSlocation(MobileMapper10;spatialaccuracy±
1500t.Recentlythedimensionandmassofthisboulderwereadjusted 0.5m);and(5)thedistancefromthealgalcrestreefedge(determined
to14×8×4.5m(approx.504m3)andabout1000t(Etienneetal., withaLaserTechnologyTruPulse360Blaserrangefinder;accuracy
2014).Whilethecauseofthewavesthatbroughtthelargestblock ±1m).Incaseswherethedistancetothereefedgecouldnotbemea-
ontothereefhasnotbeendetermined,boulders4–6minlengtharebe- sured,forexamplewhenobstructionswerepresent,thedistancewas
lieved to have been transported by cyclonic waves on Raroia and estimatedfromsatelliteimagesavailableonGoogleEarth(accuracyes-
Rangiroain1903and1906respectively(StoddartandWalsh,1992). timatedat2m).Forboulderorientationanalysis,onlyelongatedboul-
derswithasignificantlongaxis(a-axisN20%longerthanb-axis)were
4.Methods considered,becausesquarebouldersdonothavearepresentativelong
axisforthisanalysis.
4.1.Datacollection Carbonatesampleswerechiselledfrom24bouldersforage-dating
anddensitydetermination.Amongthem,22weredeadcoralsamples
OnMakemoAtoll,numerouslargebouldersexistonthenorthernatoll takenfromthesurfaceofboulders;theothertwosamplesweredead
rim.Thesehavenotpreviouslybeenstudied.Bouldersmeasuredinthis giantclamsthathadanchoredonseparateboulders.Samplesweregen-
investigationwerechosenbasedontheirsizeandaccessibility.Atthe erallycollectedfromboulderswiththelargestvolumeorthoselocated
westernsectionoftheisland,whereapproximately0.7kmofcoastlines farthestfromthereefedgewherewavesbreak.Theiragestherefore
hadabundantbouldersbutnoroadaccess,onlylarge(intermediate possiblyrepresentoccurrencesofthestrongestextremewaveeventsaf-
axesN2m)clastsweremeasured(zoneAinFig.2).Elsewhere,large fectingtheatoll.
Fig.4.Bouldervolumewascalculatedbasedongeneral3Dgeometricshapes.(a)Asthesimplegeometriccalculationofrectangularbouldersresultsinsignificantoverestimationof
volume,theirinitialvolumesweremultipliedbya0.7scalar(seetext).Nocorrectionwasappliedto(b)ellipsoidalor(c)triangularboulders.
A.Y.A.Lauetal./MarineGeology380(2016)174–190 181
4.2.Volumeanddensitydetermination estimatedvolumes.Spiskeetal.(2008)highlightedthattheporosityof
bouldersshouldnotbeneglectedindensitydeterminationofwave-
Thevolumeofeachboulderwascalculatedbasedonits3Dgeomet- emplacedcoastalboulders.TheyshowedthatArchimedeanbuoyancy
ricshape:e.g.,rectangularprism(‘rectangular’),triangularprism(‘tri- measurementsprovidethemostaccurateestimationofcoraldensity.
angular’),orellipsoidal.Thevolumes(V)wereestimatedthus: BulkdensitywasthereforecalculatedbasedontheArchimedesprinci-
Triangularboulders: ple:
(cid:4) (cid:5)
V¼ða(cid:2)b(cid:2)cÞ ð1Þ BD¼1:02(cid:3) Wa ð4Þ
2 ðW −W Þ
a f
Ellipsoidalboulders:
where1.02g/cm3isthedensityofseawater;W isthedryweight(g)of
(cid:1) (cid:3) a
sampleinair;andW istheweight(g)ofthesampleinseawater.Dry
4 a b c f
V¼ π (cid:2) (cid:2) ð2Þ weightwasdeterminedfrom22samples(17–83g)withadigitalbal-
3 2 2 2
ance(accuracyto0.01g).Tomeasurethecoralweightinseawater
withadensityof1.02g/cm3,seasaltwasaddedtoabeakerofdistilled
Rectangularboulders:
water of known volume. Salinity was monitored with a calibrated,
V¼0:7ða(cid:2)b(cid:2)cÞ ð3Þ handheldsalinitymeterwhilestirringuntiltheconcentrationreached
thatofnaturalseawater(35ppt).Theplastic-wrappedsamplewas
wherea,b,andcinallequationsarethelong,intermediate,andshort thentiedtoahangingscaleandloweredintothebeakerofseawater.
axes. For rectangular boulders, volumes are almost always TheW valuewasmeasuredimmediatelyaftersamplesubmergence.
f
overestimatedbythegeneralgeometriccalculationV=a×b×cbe- Bulkdensitywasalsobedeterminedbyasimplemethodusingdry
causebouldersarerarelyperfectlyrectangular(Terryetal.,2013).Com- mass(M)andbulkvolume(BV):
a
parison of accurate measurements of boulder volumes determined
usingterrestriallaserscanning,differential-GPS,andphotogrammetric BD¼Ma=BV ð5Þ
3Dmodelling(Scicchitanoetal.,2012;EngelandMay,2012;Gienko
andTerry,2014),hasdemonstratedthattheactualvolumesofboulders whereM wasdeterminedinthesamewayasW above,andbulkvol-
a a
areonlyafraction(0.41–0.76)ofthoseestimatedusingrectangularge- ume of the same sample was determined via water displacement.
ometry.Thus,inthisstudy,0.7wasusedtominimizeerrorintheesti- Briefly,theplastic-wrappedsamplewasplacedintoawater-filledbea-
mationofbouldervolumeandmass(Eq.3).Aconservativevalueof kerandthedifferenceinwaterlevelwasrecordedasthebulkvolumeof
0.7wasusedbecauseobservedboulderswerenothighlyweathered thecoralsample.
intorugged,irregularshapesthatwouldotherwisesupporttheuseof
smallerscalars.Nocorrectionwasneededforellipsoidalandtriangular 4.3.Wavevelocityestimation
bouldersbecausetheirvolumesarewellapproximatedbysimplegeo-
metricformulae(Fig.4). CarbonatebouldersontheMakemoreefflatoriginatedfromtwo
Thebulkdensity(BD)ofcollectedcoralsamples(n=22)wasused plausiblesources(Fig.5):(1)theactivereefedgeorreefslope(collec-
tocalculatethemass(M)ofallmeasuredboulders(M=BD×V)from tivelycalled“reef-edgeboulders”);or(2)frommid-Holoceneemerged
Fig.5.BouldersonthecoastofMakemoAtollwerederivedfromtwopossiblesources:someoriginatedfromtheemergedreefremnantsoremergedbeachrock(source1)andweremoved
byeitherrun-upwaves(landwarddirection)orbackwash(oceandirection).Theywerethendepositedeitherbehindthebeachontheislandas“beachtopboulders”,oronthereefsurface
as“reeftopboulders”.Thesurfaceappearanceofthesebouldersisconsistentwiththelocalreefremnant;andthethicknessofsuchbouldersiswithintherangeofreefemergencesincethe
mid-Holocene.Mostbouldersinthedatasetwerehoweverderivedfromtheactivereef(source2)andreferredtoas“reef-edgeboulders”.Atsomepartsofthereefedge,socketsofsimilar
sizeasbouldersdepositedonreefflatwereobservedatthereefedgeandcouldbeidentifiedasbouldersources.
182 A.Y.A.Lauetal./MarineGeology380(2016)174–190
reefremnantsorotheremergedfeatures(collectivelycalled“emerged 2ðρ=ρ −1Þgccosθ
u2≥ s w ðliftingÞ: ð8Þ
reefboulders”).Reef-edgebouldersweretransportedinthelandward C
l
direction by run-up flow; while emerged reef boulders were
transportedeitherbyrun-upontothebeachinthelandwarddirection, Forareef-edgeboulderinajoint-boundedsetting,transportcan
orbybackwashtothereefflatintheoceandirection(Fig.5).Thedirec- onlybeinitiatedbyliftingwhen:
tionofmovementofthelatterprocessisoftendebatable(discussed
belowinSection5.1).Whenaboulderexhibitedasurfaceappearance 2ðρ=ρ −1Þgcðcosθþμ sinθÞ
u2≥ s w s : ð9Þ
resemblingthatoftheremnantreefatthelocality,andhadatleast C
l
one axis that was shorter than the elevation of the emerged reef
(approx. 1.1 m), the boulder was considered to originate from the Inallequations,uisflowvelocity(m/s);bandcaretheb-andc-axes
emergedreef(i.e.,“emergedreefboulders”).Theemergedreefboulders (m),respectively;ρ,boulderdensity;ρ ,waterdensity(1.02g/ml);C ,
s w d
werefurtherdividedintotwogroupsbasedontheirpresentlocation: dragcoefficient(C =1.95);C,liftcoefficient(C =0.178);θ,angleof
d l l
“beachtop”or“reeftop”(Fig.5). thebedslopeatpre-transportlocation;μ,coefficientofstaticfriction
s
The hydrodynamic flow transport equations proposed by (μ =0.5);andg,accelerationofgravity(g=9.81m/s2).
s
Nandasenaetal.(2011)wereusedforestimatingplausibleflowveloc- Becausethestudiedreefflatisnotinclined,theangleofthebed
itiesthatmovedthecarbonatebouldersonMakemo.Assumingthat slopeθissetat0°.Whenθ=0°,thecalculatedflowvelocityrequired
clastswerealreadydetachedfromthereefpriortotransport,themini- to initiate boulder transport by lifting is the same for both non-
mumflowvelocities(MFVs)requiredtoinitiatebouldertransportfrom obstructed(SAorSM;Eq.8)andjoint-boundedpre-transportcondi-
twodifferentpre-transportpositionsbythreeinitialtransportmodes tions(Eq.9),becausesineθ°=0.
wereestimated(Nandasenaetal.,2011).Bouldersoriginallyfoundoc-
cupyinganon-obstructedsurface(eithersubaerial(SA)orsubmerged 4.4.Coralage-dating
(SM)setting)couldbetransportedbysliding,rolling(overturning)or
liftingbecausedrag,inertiaandliftforcescanallbeappliedtotheboul- Theagesofsurfacecoralsamplescollectedfrom20boulderswere
derbytheflowtoinitiatemovement.Incontrast,foraclastrestingina measuredbyuranium-thorium(U-Th)datingusinghigh-precisionther-
joint-bounded(JB)socket,liftistheonlyforcethatcanbeappliedini- malionizationmassspectrometry(TIMS)attheRadiogenicIsotopeFacil-
tially,becauseonlythetopsurfaceoftheboulderisexposed,andlateral ityoftheUniversityofQueensland(refertoZhaoetal.(2009))forthe
movementbyslidingandrollingisrestricted(Nandasenaetal.,2011) detailsofthisdatingmethod).DetailsoftheTIMSU-Thdatinglaboratory
(Fig.6). procedureareprovidedbyYuetal.(2012),whodatedover100coral
Emergedreefboulderslocatedonthesub-aerialreefflatpriorto samplesfrombouldersonthesouthernGreatBarrierReeftoexamine
movementcouldbetransportedbysliding,rollingorlifting(Fig.6). the decadal variations in storm activity in the region over the past
Reef-edgeboulderssituatedatelevationslowerthanthereefflatbefore century.
movement,similartotheinitiallocationforjoint-boundedboulders, Itisassumedthatlivecoralwasdislodgedfromtheupperlivingreef
couldonlybetransportedontothereefflatvialifting.Aftertheboulder surfaceatthetimeofpastextremewaveeventswhenaboulderwas
wasliftedontothereefflat,itcouldbemovedfurtherbyslidingor transported.Thus,theageofthecoralsamplesmaybetakentorepresent
rollingbylowerflowvelocities.AccordingtoNandasenaetal.(2011), theapproximatetimingoftheevent(Yuetal.,2009),oratleastthemin-
theinitialtransportofanon-obstructedemergedreefboulderinasub- imumtimethathaspassedsincetheeventoccurred.Timingofpre-
aerialorsubmergedsettingoccursforthefollowingconditionsviathree historicalextremewaveeventsinferredfromboulderagesyieldedby
processes: U-Thdatingofconstituentcoralsshouldbeinterpretedwithcaution,
however,aschallengesexistwiththismethod.Forexample,onTaveuni
2ðρ=ρ −1Þgcðμ cosθþ sinθÞ Island(Fiji),EtienneandTerry(2012)showedthatiftheoldestinstead
u2≥ s w s ðslidingÞ ðn:6Þ
C ðc=bÞþμ C oftheyoungestpartofalargeboulderissampledfordating,theresulting
d s l
‘boulderage’mightbe250yearsolderthantheactualbouldertransport
2ðρ=ρ −1Þgcðcosθþðc=bÞsinθÞ timing.OntheMakemoreefflat,bioerosionisnotintenseandboulders
u2≥ s w (cid:6) (cid:7) ðrollingÞ ð7Þ
C c2=b2 þC arenotseverelyweathered.Fossilcoralgrowthdirectionscanbeidenti-
d l fiedonmostboulders,thereforesuchageerrorishopefullyavoided.
FollowingtheapproachofYuetal.(2012),theageerrorduetosam-
plethicknesswasaddedtotheageuncertaintyrange,becausethedated
materialmaynotbetheyoungestpartofthecollectedcoralsample.In
samplepreparation,cubesofabout3cm3weresenttotheisotopefacil-
ityforU-Thdatinganalyses.GrowthratesofcoralsnearMakemoAtoll
are~10mm/yearonaverage:e.g.,onMooreaintheSocietyIslands,
theaveragegrowthrateofthemassivecoralPoriteswas10.7mm/
yearovertheperiodCE1800-1905(BessatandBuigues,2001);onTa-
hiti,themeanaggradationrateoftherobust-branchingAcroporacoral,
whichcommonlydominatestheupperforereef,was9.3–11mm/year
through the postglacial period from 14,000 BP (Montaggioni and
Camoin,1997). Each 3-cm thick sample thereforerepresents three
yearsofcoralgrowth,soanadditionalerrorof±3yearswasaddedto
eachdatingresult.
Fig.6.Illustrationoftwodifferentpre-transportsettings(SA/SMonleft;JBonright)and
therespectivepossibletransportmodes(sliding,rollingandlifting).Theboulder 5.Results
exampleontheleftisanemergedreefboulder;theoneontherightisareef-edge
boulder(seeFig.5).Thebouldersareassumedtoberectangular,anddetachedfromthe 5.1.Boulderdistribution
bedrockwitha-axisalignednormaltoflowinthepre-transportposition.Fdisdrag;Fm,
iannedrtcia-a;xFil,s,lirfte;sFpfe,cfrtiicvteiolyn.;TahnedtFerr,mreast’riasinthinegafxoirscoe.fTohveervtaurrinabinlegs(broatnadtiocnreafberoutottthheeba-- Atotalof286boulderswererecordedalongthe15-kmlongnorth-
axis).TheconceptisadaptedfromNandasenaetal.(2011). ernMakemoreefflat(Fig.7).Someofthelargestclastsreachedthe
A.Y.A.Lauetal./MarineGeology380(2016)174–190 183
‘meso-boulder’category(i.e.b-axisN4.1m),followingthescaleofTerry 5.2.Flowvelocityestimation
andGoff(2014).Atotalof45(16%)clastswereover20m3involume.
Thedensityofsamplesmeasuredbybothmass/volumeandArchimedean Theminimumflowvelocity(MFV)requiredtoinitiatethetransport
methodsweresimilar:1.67±0.2g/cm3and1.70±0.3g/cm3,respec- ofthe256reef-edgebouldersrangefrom5.4–15.7m/s.Velocitiesof
tively.Thus,aroundeddensityvalueof1.7g/cm3wasusedforthedeter- only1.5–4.4m/sweresufficienttoinitiatethemovementofthe30
minationofclastmass.Thelargestmeso-boulder(7.5×5.5×3.35m)had emergedreefbouldersbysliding.However,someemergedreefboul-
anestimatedmassof164.4t;thesmallest(0.77×0.68×0.66m)0.41t dersshowedevidenceofrollingorlifting,e.g.onebouldersittingon
(metrictonsortonnes,t).Themean(±standarddeviation)volumeand topofanother.Therefore,highervelocitiesassociatedwiththesetrans-
massofall286boulderswas11.8±11.3m3and20.0±19.2trespec- portmodesweretheassumedMFVs.
tively;themedian(±medianabsolutedeviation)volumewas8.9± TheMFVscalculatedfrombouldersrepresenttheflowgeneratedby
5.0m3andmedianmasswas15.2±8.4t. breakingwavesatthebouldersourcewheretransportisinitiated.Reef-
ThemajorityofthebouldersinallzonesexceptzoneBareorientated edgeboulderswereconsideredtocomefromthenearestreefedgeloca-
paralleltoslightlysub-paralleltotheshore(Table3).Theminimumdis- tion.Asnumerousbouldersweremeasuredinthisstudy,theMakemo
tancebetweenameasuredclastandthereefedgewas11m.Themax- reefedgewasdividedinto100-mintervalsonthemapforillustration
imum distance was 162 m for a small spherical boulder andanalyses(Fig.9).ThehighestMFVcalculatedfromallboulders
(1.17×1.16×1.14m;mass=0.82t)thatwasdepositedbehindthe withineach100-msectionthereforerepresentsaconservativeestimate
beachinthevegetationzone.Thedistributionofbouldersonthecoast ofthemaximumflowthathasimpactedthissectionofthecoastline.A
followsaclearlandward-finingtrend(Fig.8). flowvelocityexceedingthehighestMFVisassumedtobecapableofini-
Ofthe286bouldersinvestigated,30wereemergedreefboulders. tiatingtransportofallbouldersinthis100-msection.Foremergedreef
Sixteenoftheseweredepositedonthereeftop;theother14were boulders,itwasdifficulttoidentifytheoriginalsourcelocationandorig-
washedfartherlandwardonthebeachsurface.The2ndand7thlargest inaltransportdirection,i.e.eitherbyrun-uporbackwash.Evidenceof
boulders(84.5tand74.9t)werereeftopboulders,derivedfromthe run-uporbackwashcouldbeclearlyidentifiedforonlyfourboulder
emergedreef.Theotherreeftopboulderswereofvarioussizes,with sourcesontheemergedreef(labelledonFig.9).Colouredlinestoindi-
massesrangingfrom1.2to33.0t.Mostofthelargereeftopboulders cate MFVaredrawn atboulderlocationswhenabouldersourceis
weresituatedinzoneAinthewesternsectionofthestudiedshore.In unknown.
contrast,thebeachboulderswereallsmallbouldersweighingb4t. ThebouldersthatrequirehighestMFVsfortransportareclusteredin
Most(12of14)beachboulderswerefoundinzoneBwithmassesrang- zonesDandE,where19outof20measuredclastsrequiredN13m/sto
ingbetween0.4and2.2t.Thefarthestlandwardboulderwassituated betransported(Fig.9).Theexceptionisinthenorth-facingzoneC.De-
162mfromthereefedgeandwas67mfromtheemergedreef. spitethepresenceoflargebouldersinzoneA,thelargestofallwere
Fig.7.Distribution,size,andorientationofbouldersinZonesAtoEonMakemoAtoll.SeeFig.2forthelocationofeachzoneontheatoll.Rosediagramsofboulderorientationateachzone
aredisplayedincircles.Theboulderorientationsweregroupinto10°gridsforcalculation.AtallzonesexceptB,themajorityofthebouldersarealignedwitha-axesparalleltotheshore.
RefertoLauetal.(2014)formoreresultsanddiscussiononboulderorientationintheMakemoboulderfield.(SatelliteimagesfromGoogleEarth).