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ThegeologicalcycleofplasticsandtheiruseasastratigraphicindicatoroftheAnthropocene

JanZalasiewicza,*,ColinN.Watersb,JulianaIvardoSulc,PatriciaL.Corcoran,dAnthonyD.Barnoskye,AlejandroCearretaf,MattEdgeworthg,Agnieszka

Gałuszkah,CatherineJeandeli,ReinholdLeinfelderj,J.R.McNeillk,WillSteffenl,ColinSummerhayesm,MichaelWagreichn,MarkWilliamsa,AlexanderP.Wolfeo,

YasminYonana.Addresses:aDepartmentofGeology,UniversityofLeicester,UniversityRoad,LeicesterLE17RH,UKbBritishGeologicalSurvey,Keyworth,NottinghamNG125GG,UKcAssociationofPolarEarlyCareerScientists(APECS‐Brazil),RuaEdgarGerhke,Piratini,SapucaiadoSul‐RS.CEP93216180dDept.ofEarthSciences,UniversityofWesternOntario,London,ON,Canada,N6A5B7e.Dept.ofIntegrativeBiology,MuseumofPaleontology,MuseumofVertebrateZoology,UniversityofCalifornia,Berkeley,CA94720,USAfDepartamentodeEstratigrafíayPaleontología,FacultaddeCienciayTecnología,UniversidaddelPaísVascoUPV/EHU,Apartado644,48080Bilbao,Spain.gSchoolofArchaeologyandAncientHistory,UniversityofLeicester,UniversityRoad,LeicesterLE17RH,UKhGeochemistryandtheEnvironmentDivision,InstituteofChemistry,JanKochanowskiUniversity,15GŚwiętokrzyskaSt,25‐406Kielce,PolandiLEGOS(CNRS/CNES/IRD/UniversitéPaulSabatier),14avenueEdouardBelin,31400Toulouse,France.jDepartmentofGeologicalSciences,FreieUniversitätBerlin,Malteserstr.74‐100/D,12249Berlin,Germany.kGeorgetownUniversity,WashingtonDC,USAlTheAustralianNationalUniversity,CanberraACT0200,Australia.m.ScottPolarResearchInstitute,CambridgeUniversity,LensfieldRoad,CambridgeCB21ER,UK.

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nDepartmentofGeodynamicsandSedimentology,UniversityofVienna,A‐1090Vienna,AustriaoDepartmentofEarthandAtmosphericSciences,UniversityofAlberta,Edmonton,ABT6G2E3,Canada*Correspondingauthor.Tel.:+4401162523928.E‐mailaddress:jaz1@le.ac.uk(J.Zalasiewicz)AbstractTheriseofplasticssincethemid‐20thcentury,bothasamaterialelementofmodernlifeandasagrowingenvironmentalpollutant,hasbeenwidelydescribed.ItsdistributioninboththeterrestrialandmarinerealmssuggeststhatitcouldbeakeygeologicalindicatoroftheAnthropocene,withpotentialtobeacomponentoffuturegeologicalstrata.Mostimmediatelyevidentinterrestrialdeposits,itisclearlybecomingawidespreadcomponentofmarinesedimentarydepositsinbothshallow‐anddeep‐watersettings.Itisabundantandwidespreadasmacroscopicfragmentsandvirtuallyubiquitousasmicroplasticparticles;thesearedispersedbybothphysicalandbiologicalprocesses,notleastviathefoodchainandthe‘faecalexpress’routefromsurfacetoseafloor.Alreadyawidespreadanddistinctivelithologicalcomponentofstrata,theamountofplasticsseemslikelytogrowseveral‐foldoverthenextfewdecades,andtocontinuetobeinputintothesedimentarycycleovercomingmillenniaastemporarystores–landfillsites–areeroded.PlasticsalreadyenablefinetimeresolutionwithinAnthropocenedepositsviathedevelopmentofitsdifferenttypesandviatheartefacts(‘technofossils’)itismouldedinto,andmanyofthesemayhavelong‐termpreservationpotentialifburiedinstrata.KeywordsAnthropocene,Plastics,StratigraphyIntroductionTheconceptoftheAnthropocene,anepochoftimeinwhichhumanshavecometodominatemanysurfacegeologicalprocesses,hasbeenwidelydiscussedsinceitwasfirstproposedbyCrutzenandStoermer(2000)andCrutzen(2002).ThereisgeneralagreementthatsufficientevidenceisavailabletosuggestthattheAnthropoceneisarealgeologicalphenomenon,withthepotentialtobeformalizedwithintheGeologicalTimeScale(Zalasiewiczetal.,2008;Williamsetal.,2011;Watersetal.,2014).AlthoughmanysuggestionshavebeenputforthregardingthetimingoftheAnthropocene,thereisgrowingconsensusthatastartingtimearoundthemid‐twentiethcenturyandthepost‐WWII‘GreatAcceleration’ofpopulation,industryandresourceuse(Steffenetal.2007,2015)isoptimal.ThisispartlyaresultoftheincreaseinscaleofhumanimpactsontheEarthsystem,suchastheriseinCO2abovepre‐industriallevels.The“GreatAcceleration”periodisalsomarkedbykey,near‐synchronousstratigraphicmarkersthatenablethestrataofaputativeAnthropoceneEpochtobeidentified.Theseincludeartificialradionuclidesofthenuclearage(Hancocketal.,2014;Zalasiewiczetal.,2015;Watersetal.,2015),aluminiummetal(Zalasiewiczetal.,

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2014),flyashparticles(Rose,2015;Swindlesetal.,2015),persistentorganicpollutants(MuirandRose,2007)andavarietyofbiologicalindicators(Barnosky,2014;Wilkinsonetal.,2014).Onefurtherpotentialindicatorisplastic,asthisisamaterialthathasbeenmanufacturedinabundancesincethemid‐20thcentury.Currentlyindispensible,plasticsmakeanextraordinarycontributiontoourlives,andarekeytothemomentumofthetechnologicalrevolutionsincethestartofthe‘GreatAcceleration’.Thisismainlyduetotheirremarkablyversatileandusefulpropertiesandapplications.Forexample,plasticsarefundamentaltocontemporaryhygiene,whetheraswrappingforfoodstuffsandothermaterials,asdisposablegloves,coatsandmedicineencapsulationsusedinhospitals,andttheprovisionofinexpensivecleanwatersystems,suchaswaterbottlesorpipelines.Plasticsarealsocomponentsofmanyofourbuildings,toolsandmachines.Althoughnowindispensible,plasticsareeasilydisposable.Discardedinvariouswaysafteruse,weseethemwidelyaroundusaslitter.Thescopeandrangeofplasticcontaminationhasbecomeincreasinglyapparentoverthelastfewyearsanddecades,anditisnowregardedasamajor,andgrowing,environmentalhazard(seebelow).Acorollaryofthisdispersalisthatplasticsmightbeusedasmarkersoftime,andindicatorsoftheprocessesthatoccurredduringdeposition,muchinthewaythatgeologistsusefossilstocharacterizeanddatestrata.Itisthispotentialthatweexploreinthispaper.Plasticsarerelativelyeasilyrecognizable,withouttheneedforsophisticatedanalyticalequipment,asinthecaseforthedetectionofradionuclides.Theymay,therefore,beeffectiveandwidelyuseableprimarystratigraphicmarkersforAnthropocenestrata.However,togainanoverviewoftheirtrueutilityrequiressomeconsiderationabouttheirdistributionandbehaviorasageologicalmaterial,ratherthanasaproductofmaterialscience,orasanenvironmentalpollutant.Thisideaofplasticsasasignificantcomponentofthepresent‐daysedimentarycycleisgrowing,althoughstudiestoprovideclearanddetailedglobalcharacterizationhaveonlyjustbegun(e.g.Reed,2015;Corcoran2015).Thispaperthusplacescurrentknowledgeabouttheenvironmentalbehaviorofplasticsintoageneralgeologicalperspective.Inparticular,weconsidertheextenttowhichplasticsmayprovideapragmaticstratigraphicmarker,notjustinsoilsandotherterrestrialdeposits,butalsofarintothemarinerealm.Wedevelopthisanalysistoprovidethefirstpredictivemodelofthetransport,distributionandburialofplasticsassedimentaryparticlesinarepresentativearrayofglobalsedimentologicalsettings,bothterrestrialandmarine.Wealsoexaminethepossibilitythatplasticsmaybecomepreservedintothegeologicalfuture,toprovideevidenceofthishuman‐dominatedphaseofEarthhistory.Plastics,seenthroughthisprism,mayrangemorewidelythroughtimeandspacethancanbeseenbythecasualeye.Thenatureandproductionofplastics

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Plasticsaremalleablesolidsmadeofhighmolecularweightorganicpolymers.Mostareentirelysynthetic–primarilymadefrompetrochemicals–althoughsomearecellulose‐based.Thefirstplasticstobecomecommonlyusedwerepermanentlyhardandbrittle,suchasshellac,forgramophonerecordsfromthelatenineteenthcentury,andbakelite,producedwidelyfromthe1920stothe1940s(andstillinminorusetoday)(Albusetal.,2006).Viscosesilkandrayon,madefromacellulosebase,havebeenmanufacturedsincetheearly20thcentury,andremaininproduction.Nylon,polystyrene(PS),polyvinylchloride(PVC),polyethylene(PE)andpolytetrafluoroethylene(PTFE)begantobeproducedinthelate1930sand1940s,polypropylene(PP)andexpandedpolystyrenefoaminthe1950s,andpolyethyleneterephthalate(PET),fromwhichmostcontainersandbottlesarenowmade,waspatentedin1973(Fig.1).Developmentcontinuestothisday,withsome15‐20maingroupsofplastic.Manyofthesesubsequentplasticproductsretaintheirplasticityoncemoulded(Shahetal.,2008).

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Fig.1.Stratigraphicappearanceofsomemajortypesofplasticsandplasticartefacts.Gutta‐percha,hardenedsapofanyofeighttreespeciesfromsoutheastAsia,isnotstrictlyaplastic.Nevertheless,itfeaturesinsomeearlyhistoriesofthismaterial;between1850and1899,some27,000tonswerelaidontheseafloortoserveasinsulationfortelegraphcablesduetoitsresistancetosaltwatercorrosion(Tully,2009).Adaptedfrominformationmostlyinhttp://www.bpf.co.uk/Plastipedia/Plastics_History/Default.aspx.

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Theextraordinaryglobalexpansionofthisnowindispensiblematerial(AndradyandNeal,2009)canbeseeninthedramaticriseofproducedplastics,fromthelessthan2milliontonnesproducedin1950tothe300milliontonnesmadeannuallytoday(Fig.2).Thecumulativeamountproducedasof2015isontheorderof5billiontons,whichisenoughtowraptheEarthinalayerofclingfilm,orplasticwrap.Thecurrentglobalannualproductionrepresents~40kgofplasticsproducedannuallyforeachofthe7billionhumansontheplanet,approximatingthetotalhumanbiomass(Zettleretal.,2013).Theamountprojectedby2050,oncurrenttrends,isabout40billiontons(Rochmanetal.,2013),whichisenoughtowrap6layersofclingfilmaroundtheplanet.Itisanenormousindustry,currentlyusingapproximately8%ofglobaloilextractionforitsmanufacture(Thompsonetal.,2009).Approximately4%isusedasasourcematerialfortheplastics,and4%isusedtoprovidetheenergytoproducetheplastics:http://www.wastewatch.org.uk/data/files/resources/13/Plastics‐information‐sheet‐FINAL‐Oct‐08.pdf).

Fig.2.Growthofplasticsproduction:fromPlasticsEurope(2013,2015).Syntheticfibresproduction(metrictonsperyear)fromThompsonetal.(2004)andhistoricalstagesinplasticsdevelopment,ingreyboxes,fromThompsonetal.(2009).Mostoftheglobalplasticsthathavebeenproducedarestillpresentintheenvironment.OftheplasticsproducedinEurope,abouthalfisaccountedforby

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recycling,energyrecovery(i.e.incineration)andlandfill,withtheproportionsincineratedandputintolandfillvaryinggreatlyfromcountrytocountry(PlasticsEurope,2013,2015).Theproportionrecycled,withinthehalfthatisaccountedfor,istypically15‐25%inEurope(op.cit.),butfiguresprovidedbyBarnesetal.(2009)fortheUSAsuggestrecyclingratestherearebelow5%.Thehalfofplasticsproductionthatisnotaccountedfor(seealsoRochmanetal.,2013)presumablystaysintheenvironment,eitherascomponentsofsome‘permanent’objectorisdisposedofotherwise,includingcasuallyaslitter.PlasticsintheenvironmentPlasticsareusefultohumansbecausetheyarelight,strong,flexibleandrelativelyinert.Theyareinsolubleinwater,andresistanttobiologicaldecayandmuchchemicalattack,overatleastdecadestocenturies.Theyareeasilytransportedbywindandwaterthroughtheenvironment,wheretheymayaccumulate.Plasticsareprovingtobemuchmoremobilethananyotherhuman‐madematerial,suchasceramicsorglass.Ittookceramicsthousandsofyearstoachieveanythingresemblingaglobaldistribution,andtheyaredistributedmainlyinterrestrialdeposits,withverylittleincursionintomarineenvironments(Edgeworthetal.,2015).Frombeingalocal‘litter’problemafewdecadesago,plasticsareincreasinglyrecognizedasamajorenvironmentalcatastrophe,bothonlandandinthesea.Inresponse,therehasbeenarapidlyexpandingbodyofliteratureonthesubjectwithinthelastfewyears(e.g.seethereviewbyIvardoSulandCosta,2014).Plasticsintheenvironmentaredividedbroadlyintomacroplasticsandmicroplastics.Macroplasticsare>5mm,andincludeeverythingthatwewouldrecognizeaslitter,suchasplasticbagsandbottles,discardedfishingnets,plastictoys,andsectionsofplasticpiping(Fig.3).Insomesurveys,forinstancebycamerasonremotelyoperatedsubmarinevehicles,macroplasticsaretheonlyplasticsthatcanbeobserved(Wattersetal.,2010;RichardsandBeger,2011).

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Fig.3.PlasticdebrisonKamiloBeach,Hawaii(itemonrightofphotoisplastiglomerate);Fieldofviewis20cmacross(Photo:P.Corcoran).Incontrast,microplastics,whichare<5mm,arecommonlyinvisibletothenakedeye,particularlywhenmixedintosediment.Somemicroplasticsareoftheiroriginalsize,suchasthe10−1000ηmplasticmicrobeads(polyethylenemicrospheresthatareputintosomecosmetics,facialscrubsandtoothpaste,aswellaslentil‐sizedresinpellets(“nurdles”)thataretherawmaterialsforplasticproducts.Othermicroplasticshavebeenphysicallyorphysico‐chemicallydegraded.Amicroplasticcategoryrecentlyrecognizedasimportantisplasticfibres(~0.1mmacrossandusuallyupto2‐3mmlong),whicharelargelydetachedfromsyntheticfabricsduringwashing.Asinglesyntheticgarment,forinstance,canreleaseoverathousandfibresinasinglewashcycle(Fig.4).Toosmalltobefilteredouteitherbymachineorsewageplant,thesecantravelfarbyriverandseacurrent,andbecomedepositedinsedimentlayers(Browneetal.,2010,2011;Woodalletal.,2014).

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Fig.4.MicroplasticfibresfoundinbottomsedimentsofLakeOntario‐sampledbyglewcorer(Photo:AnikaBallent).Plasticscanbeconsideredsedimentarycomponentsinbothterrestrialandmarineenvironments,however,theirdistributiononlandappearstohavehadmuchlessstudythanthatinthesea(Thompsonetal.,2009;Rillig,2012).Thismaybearesultofthegreaterheterogeneityoflandscape,bothnaturalandanthropogenic,whichmakesanalysismoredifficult.Nevertheless,itisclearevenbycasualobservationthatmacroplasticdebrismaybefoundinmostinhabitedenvironments.Microplasticsarenoteasilyvisible,butmethodsfortheiranalysisintheenvironmenthavebeendeveloped.Theycanbeextractedfromwaterbyfiltering,andseparatedfromsedimentviasievingordensityseparationusingcentrifugeandsaltsolutions(Nuelleetal.,2014;Woodalletal.,2014;Corcoranetal.,2015).Nanoplasticsareparticlesthataretypicallytensofnanometersindiameter.Thesemaybeproducedintentionally,forexample,fordrugdelivery,detergentsorcosmeticuse,ortheymayresultfromfragmentationoflargerplasticparticles.Studiesofnanoplasticshaveindictedtheirlargesurfacetovolumeratio,which

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increasestheircapacitytoadsorborganiccompounds,potentiallyenablestheirabilitytopenetratecellwalls,andtheirempiricallydeterminedcapacitytoaffectgrowthandreproductionofatleastsomeaquaticinvertebrates(e.g.Besselingetal.,2014;DellaTorreetal.,2014;Velzboeretal.,2014).Thedistributionofnanoplasticparticlesinthenaturalenvironmentisverypoorlyknownbecauseofthetechnicaldifficultyofisolatingthemfromwaterorsediments,buttheyarealmostcertainlybecomingincreasinglycommonlydispersed.LandOnlandandawayfromshorelines,plasticlitteriswidelydistributedinthesurfaceenvironment,mostclearlyinandaroundurbanareasviacasuallittering.However,itsdistributionseemstohavehadlittledetailedstudy(Thompsonetal.,2009;Rillig,2012).Theuseofplasticsinagriculturehasgrownsincethe1960s,andHussainandHamid(2003)notedthatglobalagriculturalconsumptionofplasticsisat2.48milliontonsperyear.Theyareusedintransplantandbeddingplantproduction,asirrigationtape,traysandpots,tunnels,haybalewraps,andingreenhouseconstruction..Plasticsmaybecomeincorporatedintocultivationsoils,andthescaleofthisdistributionrequiresfurtherinvestigationandquantification.Anyplasticinclusionspresentinploughsoilsbecomethoroughlymixedwithothermaterialstothefulldepthofploughing.Thestratigraphicdistributionofplasticsbelowthesurfacecorrelatesstronglywiththedistributionoflandfillsites,whereplasticsinthelastfewdecadeshavecometomakeupapproximately10%byweightofthewasteburied(Thompsonetal.,2009).Wherelandfillsiteshavebeenmappedoutandtheiroperationdated,sedimentarydepositsuptoseveraltensofmetresthickwithconcentrationsofplasticsmaybedelineated(Figs.5,6).In1967,intheUK,plasticsformedabout3%ofmunicipallandfillwaste(Bridgewater,1986).However,increasingproductionofplasticsinthe1960scoincidedwithincreasedconsumerismandcasualdisposalofsingle‐usegoodsratherthanre‐useandrepair.Theproblemismorecriticalindevelopingcountrieswherethearrivalofpackagedgoodsisassociatedwithinefficientwastedisposal.Thiscontributedtotherapidincreaseintheproportionofplasticsinlandfillinthe1970s(Fordetal.,2014,fig.11).Subsequentlegislationacrossmanypartsoftheworldhasstimulatedincreasingreuseandrecyclingofplasticgoods,suchassupermarketplasticbags,butatbestthishasrestrictedratherthanoverturnedtherelentlessgrowthofplasticdisposal.Thedistributionoflandfillsitesalmostalwayscoincideswiththe(former)outcropofbulkminerals,suchasquarriesforaggregateandforbrickclay.Landfillsites,especiallymodernoneswithleak‐proofseals,tendtomummifymaterial‐evenpaperandfoodstuffs‐ratherthanencourageittodecay(RathjeandMurphy,1992).Inturn,plasticsmaybeexpectedtosurviveevenlongerinlandfillsthanatthesurface(cf.TanselandYildiz,2011),withthepotentialtobecomefossilizedorreworkedbyfutureerosion(seeDiscussionbelow).

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Fig.5.Distributionof3055wastedisposalsitesacrossEnglandandWalesactiveduringtheperiod1971−3(source:BritishGeologicalSurveydatabase,heldonbehalfofDepartmentofEnvironment).BoxshowslocationofFig.6.BGS©NERC2015.Allrightsreserved.

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Fig.6.LandfilllocationsinpartofeastLondon,showingoperationalhistory;sitesafter1960willincludesignificantplasticscontent(fromEnvironmentAgencydata).Roadnetworkshaveincreasinglybecomecorridorsofplasticdeposition,partlythroughsurfacedepositionofdiscardedmaterial,whereplasticsarelikelytodegradeorbedispersedrelativelyquicklyandnotaccumulateassubstantialdeposits.Notwithstandingthesearelikelytobezonesofmicroplasticsproductionthroughdegradationandfragmentation.Plasticsarealsousedinthelayingofcablesandpipesforservicesandcommunications,whicharedeliberatelyburiedinbackfilledtrenches,oftenunderoralongroads.Plasticsarealreadywidelyusedasstratigraphicmarkersinfieldarchaeologicalpractice‐asindicatorsofmodernorrecentlydisturbeddeposits(Fig.7).Evensmallamountsofplasticfoundasinclusionswithinalayercanbeusedasevidenceofdateofdeposition.Thiscanprovidepreciseconstraintsontheageofthespecificdepositwithinwhichitisfound,andalsoconfersrelativedatinginformationonlayersthatarestratigraphicallyabove(‘laterthan’)andbelow(‘earlierthan’)theplastics‐bearinglayer..

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Fig.7.1980splasticbagsintheupperfillofanornamentalmoatinTudorgardensfromevaluationatCedarsPark,Broxbourne,HertsbyMuseumofLondonArchaeology,2010(imagedreproducedcourtesyofMOLA).Theplasticinthiscasehasbeeninthegroundfor30years.Itiswellpreserved,presentingavisualandcolourfulmarkerintheprofileofanarchaeologicaldepositoranthrosol(acompletelyornearlycompletehuman‐madesoil).Asadateablehorizonwithinastratigraphicsequence,theplastic‐bearinglayerhereprovidesrelativedatingforalllayersabove(1980sorlater)andalllayersbelow(1980sorearlier).Itsutilityasastratigraphicmarkerextendstothewholesequence.LakesandriversPlasticshavebeenfoundinfreshwaterecosystems(Eerkes‐Medranoetal.,2015),suchaslakes(e.g.Eriksenetal.,2013;Imhofetal.,2013;Freeetal.,2014;Zbyszewskietal.,2014),andinrivers,suchastheThames(Morrittetal.,2014),Danube(Lechneretal.,2014)andYangtze(Zhaoetal.,2014).Plasticsareconsideredtobeatleastaswidelydistributedinlakesastheyareintheoceans(seebelow),butalthoughtheirdistributiononshorelinesandasfloatingdebrisonwaterhaslocallybeendetermined,asintheGreatLakesofNorthAmerica,theirdistributioninlakebottomsedimentshasonlyrecentlybeeninvestigated(Corcoranetal.,2015).Microplasticsareintroducedtoriversviawind,stormsewers,andwastewatertreatmentplants,andalsohostdistinctmicrobialcommunities(McCormicketal.,2014).However,thelowdensityofthemostcommonlyproducedplastics,

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polyethyleneandpolypropylene,meansthatasignificantproportionstayswithinoruponthewatercolumnandistransportedfartherdownstreamorouttolakesandseas(SadriandThompson,2014).Themajorityofplasticdebrisissourcedfromland,andthus,riversareconduitsforplasticstoentertheirfinalsink:themarineorlakerealms.Forexample,inSouthWalesabout80%oflitteronestuarinebeachescomesfromrivers(WilliamsandSimmons,1996),andnearToronto,Canada,plasticpelletswereobservedtravellingdowntheHumberRiverintoLakeOntario(Corcoranetal.,2015).Plasticsoftenactassedimentbafflesinrivers,asdoesvegetationandwooddebris.Alonglakeshorelinesandriverbanks,microplasticstendtobecometrappedinorganicdebrisbroughtinbywavesandcurrents(Zbyszewskietal.,2014;Corcoranetal.,2015).Inaddition,high‐densityplasticsmayrapidlyaccumulatewithinchannelbedloadwhere,mobileplasticelementsinthetractioncarpetmaybeabradedrapidly(WilliamsandSimmons,1996)andreducedtomicroplasticparticles.Betweenriversandthesea,mangrovescantrapplasticsaswellasmud(IvardoSuletal.,2014).NearshoremarineThatmacro‐andmicroplasticswereenteringtheseas,andwerelikelytocausesignificantenvironmentalimpact,wasobservedasearlyasthe1960sinseabirdpopulations(KenyonandKridler,1969;HarperandFowler,1987)andfromthe1970sontheseasurface(CarpenterandSmith,1972).Sincethen,boththephenomenonitself,andstudyintoit,havegrownmarkedly,particularlyinthelastdecade(seereviewofIvardoSulandCosta,2014formicroplastics;alsoLeinfelder,inpress).Attentionhasfocusedontheimpactofingestionandentanglementonbiota,ontheirdistributionwithinbothwaterandsediments,andonpossibletoxiceffects.Althoughplasticsaregenerallyinert,theycanaccumulatetoxins,suchasPCBs,ontheirsurfacesorreleaseharmfulconstituentssuchasbisphenolAastheyweather.Theseaisthefinalrestingplaceforarangeofdifferenttypesofhumanlitter,fromglasstometalstobuildingwaste,but.Plasticsformthemoststrikingcomponent.Makingupsome10%ofallhumanrefusebyweight,theselectivetransportofplasticsbywindandwatermeanthattheymakeup>50%ofmarinelitter,andlocallyconsiderablymore(Barnesetal.,2009).Asimilarselectiveconcentrationofcertainnaturalresistantrocktypes,suchasflintandveinquartz,occurswithinsedimentarydeposits.Therehavebeensomestudiesofphysicalsortingofplastics,particularlyincoastalareas.Forinstance,Browneetal.(2010)examinedthesortingofmicroplasticswithintheTamarestuarynearPlymouth,UK,andnotedsegregationoflighterandmoredensemicroplastics,althoughtheauthorsfoundnorelationshipbetweenmicroplasticsandclayparticledistribution.Isobeetal.(2014)notedselectivetransportofmesoplastics(~5mm)towardstheshoreandmicroplasticstowardsoffshoreintheSetoSeaofJapan.

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Plasticsentertheseaviarivers,frompointanddiffusesourcesalongtheshoreline,andfromships(thoughsuchdumpingisnowintheorybannedbyinternationalshippingregulation)(Ryanetal.,2009,Fig.1).Estimatesofplasticscurrentlyenteringtheseaeachyearrangefrom6milliontons(UNEP2009inPhametal.,2014)tobetween4.8and12.7millionmetrictons(Jambecketal.,2015),withtheamountpredictedtoincreasebyanorderofmagnitudeby2025(Jambecketal.,2015).Differencesinsourceareevident,forexamplearoundtheUKcoastline,withvariousproportionsderivedfromrivers,fly‐tipping,sewageoutfalls,shipdischarges(WilliamsandSimmons,1996)andcoastaltourismactivities.Arecentlypublishedstudy(Obbardetal.,2014)foundsignificantmicroplastics(38to234particlespercubicmeter),frozeninArcticseaice,havingseeminglybeenderivedfromthePacificOcean.TheArcticisthusamajorglobalsinkforthesetinyplasticparticles.However,meltingatcurrentratescouldunlockoveronetrillionpiecesofmicroplasticsoverthenextdecade.Rayonwasthemostprevalentmaterial,muchofitfromcigarettefilters(onecigarettefiltertipcomprises~10,000fibres)andhygieneproducts.Othermaterialsincludedpolyester,nylon,polypropylene(PP),polystyrene(PS),acrylicandpolyethylene(PE).Oncewithinthesea,lowdensityplasticsfloatinseawater,suchaspolyethylene(PE)andpolypropylene(PP),whichtogethermakeup~55%ofoutputinEurope(PlasticsEurope,2015).Theselowdensityplasticscanbemovedbywindstressandbysurfacecurrents,andbythismeanscanencircletheEarth,tobecomeconcentratedinmid‐oceangyres(e.g.the‘GreatPacificGarbagePatch’,somethousandkilometresindiameter:Mooreetal.,2001;Ryanetal.,2009;Lawetal.,2014).Therehavebeenwidelyreportedexamplesofspilledcargoesofsuchdistinctiveobjectsasplasticducks,trackedtorevealmarinepathways(e.g.EbbesmeyerandScigliano,2009;Hohn,2011).Plasticsmayultimatelyeitherbeloadedinsomefashiontosinktotheseafloor(seebelow)orbewashedupondistantbeaches.OnKoreanbeaches,Jangetal.(2014)foundthatmorethanhalfoftheplasticmaterialbeachedhadcomefromtheocean,vialong‐distancetravel,andnotfromnearbyland.Onceplasticfragmentswithdensities>1gm/cm3(e.g.PVC)areinthesea,theycanbemovedbytidalandstorm‐drivencurrentsinshallowwater,andbyvariousgravity‐drivencurrents(e.g.turbidityandcontourcurrents)indeepwaterbeforefinallybeingdeposited.However,low‐densityplasticshavealsobeenfoundinlake‐bottomsediments,havingbeendepositedasaresultofdensityincreasebymineralfillersduringproduction,ormineraladsorptionwhileinthewatercolumn(Corcoranetal.,2015;Corcoran,2015).Itisalsobeingincreasinglyrealizedthatthetransportofplasticsthroughthewatercolumnisoftenmediatedbiologically(seebelow)becausemicrobialfilmsrapidlydeveloponsubmergedmicroplastics,andchangetheirbuoyancy(LobelleandCunliffe,2011).Studiesofplasticsinsedimenttodatehavetypicallyfocusedontheamountandtypeofplasticspresentandontheirgeographicaldistribution.However,very

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fewinvestigationsincludedataontheverticaldistributionofplasticswithinthesediment(exceptionsincludeKusuiandNoda,2003,NgandObbard,2006,Turraetal.,2014;Corcoranetal.,2015).Inferencesonsuchdistributionmustbemadeusinggeneralsedimentaryfaciesconsiderations.Coastlinesandbeacheshaveunderstandablyattractedmuchattention,giventheirsensitivestatusinhumansocietyandthehighvisibilityofplasticlitterdepositedthere.Themonitoringofbeachlitter,mostlymacroplastic,istypicallydonebycountingitemsatthesurfaceperunitlength(e.g.,per100m)ofcoastline,andnotingsuchaspectsastype,composition,weightandvolume.ArecentstudyofKoreanbeaches(Hongetal.,2014)found300‐1000items/100m,includingpolystyrene(PS)fishingbuoys,andplasticbagsandbottles.Cigarettefiltertipsaregenerallythesinglemostcommonitemfoundinstudiesofsuchsortandinbeachcleanups.Ofthe~6trillioncigarettessmokedannually,thefilter‐bearingtipsofover4trillionendupaslittereachyear(Carlozo,2008).Plasticsarevirtuallyomnipresentinthecoastalzoneglobally,notonlyindenselypopulatedregions,butalsobecauseoflong‐distancetransporttoremoteareas.Barnes(2005)notedsubstantialamountsofmacroplasticsonremoteislands.OnsomeislandssuchasDiegoGarcia,hermitcrabshavetakentousingplasticbottletopsashomes(seealsoReed,2015,p.32).Barnes(2005)henotedadiminishingtrendofplasticsfromequatortopoleintheSouthernOcean,althoughnoticeableamountsstillreachAntarcticcoasts.InHawaii,accumulationsofplasticdebrishaveformedwhatCorcoranetal.(2014)referredtoas‘plastiglomerates’inwhichmeltedplasticassociatedwithcampfires(Fig.3)hasbondedbeachpebblesandsandtoformarock(theoreticallytheactivityofwildfiresandvolcanicactivitycouldalsocausemelting).Thesedensehybridplastic‐sedimentmaterialshavegoodpotentialforburialandlong‐termpreservation.Successivesurveyshaveshownthatamountsofplasticsincoastalsedimenthaveincreasedthroughtime,broadlymirroringtheriseinglobalproduction(Ryanetal.,2009;Fig.8herein;Claessensetal.,2011).ThistrendcontinuesinthatBritishbeachesin2009sawrecordlevelsoflitter,withanaverageof2195items/kminasurveyof374beachesnationwide,comparedwith1045items/kmin1994(Adam,2009).Thisresultisdespitestrenuousclean‐upeffortsbylocalauthoritiesandvolunteergroups,andtheactivitiesofbeachcombers.Peaklevelscanbemuchgreater,exceeding30000items/kmor‘muchhigher’inbeachesinEurope,AsiaandSouthAmerica(Phametal.,2014andreferencestherein).

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Fig.8.(Left)increaseinnumberofplasticbottles(leftbar)andlids(rightbar)onbeacheswithregularcleaningprogrammes(inblack)ornoformalcleaning(ingrey),redrawnfromRyanetal.(2009).(Right)MicroplastictimeseriesdatafromThompsonetal.(2004).Inthedynamicbeachenvironment,objectscanbeburiedandexhumedmanytimes(SmithandMarkic,2013).Overall,thefewstudies(e.g.Turraetal.,2014)examiningthethirddimensiononbeachessuggestthatplasticitemsmaylocallyextenddownwardsforasmuchas2m,withtherebeinganorderofmagnitudemoreburiedplasticthansurfaceplastic.Hence,thereisasedimentbodyforminginthecoastalzonethat,ifseenincrosssection,couldcontainsufficientmacroplasticmaterialtoberecognizabletothefieldgeologistasapost‐mid‐twentiethcenturydeposit(Fig.9).Insomeinstances,thesemacroplasticfragmentsarealreadyincludedinbeachrockdeposits,asintheBasquecoast(Irabienetal.,2015).

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Fig.9.Plasticfragmentincarbonate‐cementedbeachrockonGorrondatxe‐Azkorribeach,Basqueregion,Spain(Photo:H.Astibia).Suchdistribution,andparticularlyinremoteareas,maybesufficientlysporadictopreventconsistentidentificationofAnthropocenedeposits.However,microplasticparticlesaremoreabundant,andmorewidelyandevenlydistributed,thanaremacroplastics,andcanberecognizedeveninsamplesassmallas50gofcoastalsediment(Browneetal.,2010,2011).Thiscanincluderelativelylargeparticlessuchasresinpellets,thatarenear‐ubiquitousinsomebeachsediments.AroundSãoPauloinBrazil,pelletsarecommonlypresentatlevelsofupto10000/m3insediment,andlocallyofupto25000/m3(Turraetal.,2014).Smallmicroplasticsareparticularlyabundant.Largelycomposedofmicrofibres(Fig.4)detachedfrommachine‐washedartificialfabrics(Browneetal.,2011)andtransportedviasewageoutfallstoriversanddumpedsewagesludge,thesehavebecomeverywidelydispersed.Browneetal.(2011)suggestedthatfibreshavebecomeincorporatedin,androutinelyextractablefrom,shorelinesedimentsthroughouttheworld,inquantitiesthatrangefromtenstohundredsoffibresperlitreofsediment(Fig.10)(Browneetal.,2011;IvardoSulandCosta,2014,Fig.1).Forexample,Dekiffetal.(2014)reported~5‐25microplasticparticles(mostlymicrofibres)perkilogramofsedimentforNorderney(NorthSea),whereasReis(2014)foundanaverageof66/kgontheBalticislandofFehmarn.Thispotentiallyprovidesanear‐ubiquitoussignatureoftheAnthropoceneincoastalsettings.

Fig. 10 Reports on the amounts and distribution of microplastics in marine sediment samples. Stars, squares and circles represent the average number of items per cubic metre of sediment available and/or estimated. (A) Khordagui and Abu-Hilal, 1994; (B) Kusui and Noda, 2003; (C) Thompson et al., 2004; (D) McDermid and McMullen, 2004; (E) Ng and Obbard, 2006; (F) Ivar do Sul et al., 2009; (G) Costa et al., 2010; (H) Turner and Holmes, 2011; (I) Browne et al., 2011; (J) Hidalgo-Ruz and Thiel, 2013; (K) Woodall et al., 2014. Dashed squares represent deep-sea sediment core samples. Red crosses represent works that registered microplastics in sediments

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but did not allow estimation within the scale used here. Extracted and modified from Ivar do Sul and Costa (2014). OffshoremarineThisencompassesshelf,slopeandabyssalsediments,wheretheextentandstratigraphyofanthropogeniclitterhasbeenmadeclearerbyanarrayofrecentstudies.Mostattentionhasbeengainedbythevisibleplasticdebrisnowfloatinginthewater,followingthediscoverybyMooreetal.(2001)of‘theGreatPacificGarbagePatch’.PlasticsconcentrateintheslowlycirculatingwatersoftheNorthPacificgyre,withsimilarconcentrationsnowknowntobepresentintheothergreatgyresoftheworld(Lawetal.,2014;Fig.11).TheglobalassessmentbyEriksenetal.(2014)showed,intheirtitle,thescaleofthephenomenon:5trillionplasticpiecesweighingsome250000tonsarenowafloatatanyonetime.Theynotedoneunexpectedresult–macroplasticsmadeupthegreatmajorityofthisbyweight(ca85%).Theproportionofmicroplasticswasfarlessthanhadbeenexpected(seealsoCozaretal.,2014).Theoceangyresformthebasisofmodelledconcentrationsofsurfaceplasticdebriswithinthemid‐latitudesofalloceans(Fig.11).Thismimicsatmosphericcirculationpatterns,whichdeterminetheconcentrationofradiogenicfallout(e.g.Watersetal.,2015),thusprovidingapotentialdualsignatureinmarinesedimentsforrecognitionoftheAnthropocene.

Fig.11. ModelleddistributionofmicroplasticsinoceansurfacesshownbyEriksenetal.(2014,Fig.2)(1mm‐<4.75mm).Onshoreestimatedmassofmismanagedplasticwasteisinmillionsofmetrictons,generatedby2010within50kmofthecoast(Jambecketal.,2015).

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Zettleretal.(2013)foundthatmostfragmentscollectedfromthemarinewaterwerepolyethylene(PE)andpolypropylene(PP),tworesinscommonlyusedinpackagingandothersingle‐useplasticapplications.Thisplasticmarinedebrisiscolonizedbyacomplexmicrobialcommunitylivingonitandreferredtoasthe“Plastisphere”.Plastispherecommunitiesaredistinctfromthoseofsurroundingsurfacewater,implyingthatplasticsserveasnovelecologicalhabitatsintheopenocean.Microbesmaybetakingpartinthedegradationofplasticsviaphysicalormetabolicmeans.Bacteriaandfungiarewellknowntodegradehighlyrefractorycompounds,includingplastic,butthishasnotbeendemonstratedintheopenocean.Thelikelysinkforthe‘missingsurfacemicroplastics’notedaboveseemsbethedeepsea.Fischeretal.(2015)discoveredmicroplastics,mainlyfibres,atdepthsof4869−5766mintheKuril‐KamchatkaTrenchandadjacentabyssalplain.Evenatthesegreatdepths,concentrationswereashighas2000/m2.Woodalletal.(2014;seealsoGoldberg,1997,andVanCauwenbergheetal.2013forearlierrecords)examineddeep‐seasedimentcoresamplesfromthesubpolarNorthAtlanticandNorth‐eastAtlantic;theMediterranean;andseamountsontheSWIndianOcean.Allcontainedmicroplastics,mainlyasfibres,inabundancesrangingfrom1.4to40fibres(average13.4)per50mlofsediment(Fig.10).Thatwassome4ordersofmagnitudemoreabundantthaninthecontaminatedsurfacewatersabove.EventheIndianOceanseamounts,whichshowedthelowestabundances,wereconservativelycalculatedtohave4billionfibrespersquarekilometre,or4000/m2(Woodalletal.,2014).Howdidtheplasticsgettotheseoceanfloors,fardistantfromland?Thefibresweremostlycomposedofacrylicandpolyester,whicharedenserthanseawater.These,itwassuggested,mayhavebehavedlikefineclayparticles,slowlydriftinginstorm‐orturbiditycurrent‐generatednepheloidplumes,orcarriedbythermohalinecurrents.Therewerelow‐densityfloatingmicroplastics,too,thathadsunk.Thesecouldhavebeeningestedbyzooplanktonandejectedasfaecalpellets,orsankwiththemwhentheydied,orwithinthefaecesorbodiesoffishthatatethezooplankton(Boergeretal.,2010;Coleetal.,2013;Setäläetal.,2014).Themicroplasticscouldalsohavebeencaughtupingelatinousmarinesnow.Inthisrespect,microplasticsbehaveinasimilarwaytoothermicroplanktonictaxapreservedinthegeologicalrecord(e.g.coccolithoozes),andrepresentaprimarytoolofbiostratigraphicalcorrelationinthegeologicalrecordbecauseofawidespreaddistributionwithinstratathatarelikelypreservablelongintothefuture.Othersurveyshaveshownthespreadoflargerplasticfragments,bydredgingorbyremotelyoperatedunderwatervehicle(ROV)cameras.Bottles,plasticbagsandabandonedfishingnetsareabundant(Wattersetal.,2010;RichardsandBeger,2011;Tubauetal.,2015;Corcoran,2015andreferencestherein),andareoftenconcentratedbytopographyorcurrentsintosubmarinelows,suchasthebottomsofsubmarinecanyons(Schliningetal.,2013;Tubauetal.,2015).ThestudybyTubauetal.(2015)oftheseabedat24of26ROVdivesitesinthesubmarinecanyonsoftheNWMediterraneanatdepthsof140−1731m,showed

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thatplasticswerethedominantcomponentoflitter(72%).Mostofthelitterwasobservedoncanyonfloorsatdepthsover1000m,andmayhavebeencarriedtherebydown‐slopeflowsoriginatingnearshore.Litterdensityrangedupto11.8itemsper100msurveyline,andaveragedbetween8,000‐15,000items/km2,reachingamaximumof167,540litteritems/km2atonesite(Tubauetal.,2015).Phametal.(2014)consideredthattherelativescarcityofmacroplasticobjectsonshelveswasbecausetheywerebeingcurrent‐sweptintodeepwater,particularlyviasubmarinecanyons.Suchdeeperwaterandsubmarinecanyonenvironments,beinglessdisturbedbybottomtrawlingthanareshelfsediments,mayprovideagoodrecordofthehistoryofplasticsinfluxassociatedwiththeAnthropocene.Thisnewplastic‐dominateddebrislayeroverliesthedebrisofpreviouscenturies.Overall,thisearliermaterialissparser,butanotablecomponentisclinkerfromtheoldcoal‐firedsteamships,thrownoverboardenrouteandhenceforming‘pavements’belowthesailingroutes(Ramirez‐Llodraetal.,2011).Thus,inbothshorelineandoffshoresediments,thereisanear‐ubiquitousdistributionofmicroplasticfibres,invisibletothenakedeye,butsufficientlyabundanttobeextractablefrommostsedimentsamples,togetherwithscatteredmacroplastics.Thenumberofitemsvary,but,forexample,Phametal.(2014)usedsubmersiblecamerastoanalysevisibledebris(mostlyplastic)inthenorth‐eastAtlanticoffEurope.Debriswasfoundeverywhere,asfarastheMid‐AtlanticRidge,withdensitiesrangingfrom~100‐300objects/km2(incontinentalshelfareas)to200‐600objects/km2(oncontinentalslopesandoceanridges)to400‐700objects/km2(onsubmarinebanksandmounds)to600‐1200objects/km2insubmarinecanyons.Inaddition,theArgoprofilingfloatprogrammewasdevelopedtosowtheoceanwith3000floatstorecordthetemperatureandsalinityoftheoceandowntodepthsof2000m.Theprogrammeisintendedtooperateindefinitely,andwillprovidefurther‘scientificlitter’comprisingthemetre‐longplastichousingsofthefloatswhentheysinktotheoceanbedaftertheirbatteriesdieattheendofanapproximately4‐yearlife‐time(www.argo.ucsd.edu/;http://en.wikipedia.org/wiki/Argo_(oceanography).PreservationpotentialofplasticsinthegeologicalrecordThegeologicallongevityofplasticpolymersispoorlyknown,mainlyasthesearenovelmaterialsthathavebeenintheenvironmentforonlydecades.Willsuchplasticsstillberecognizableovergeologicaltimescales?Thedegradationofplasticscantakeplacechemically,bymodificationofthemolecularstructure,orphysicallyorbiologically(KayandBlond,2005;Shahetal.,2008).Chemicaldegradationcanresultfromalterationofmolecularbondsthroughheat(thermal)orsolarradiation(photo),chemicalreactions,andhydrolysisatveryhighorverylowpH.Physicaldegradationincludespartialortotalextractionofadditives(e.g.pigments,plasticizersandfillers),theactionofsolvents,internalchainreorganizationduetoheat,environmentalstress‐crackingandthermalstress.Biologicaldegradationbybacteriaandfungioccursfollowingdepolymerizationofplasticbyotherphysicalorchemicalprocesses.

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Plasticsareclearlylong‐livedonhumantime‐scales,especiallywhenburiedandbeyondthereachoftheultra‐violetlightpresentinsunlightthatcanbreakbondsintheirchemicalstructure,causingtheplasticstobecomebrittleandthenfragment(photodegradation)(Shahetal.,2008).Mostfragmentationoccursthroughphotodegradation,mainlyinbeachenvironments.Plasticsasawholeareresistanttomicrobialattack–whichunderliesagooddealoftheirpracticalutilityandoftheirlongevityintheenvironment.Nevertheless,someevidenceofdigestionbymicrobeshaslocallybeenobserved(HarshvardanandJha,2013;Yangetal.,2014;seealsoKasirajanandNgouajio, 2012), andplasticsmayhostmicrobialcommunitiesdifferenttothegenerallyambientones(McCormicketal.,2014).Thesuddenappearanceofplasticsasawidespreadnewadditiontothesurfaceenvironment,togetherwiththerapidevolutionaryratesobservedinmicrobessubjecttostrongselectivepressures,suggeststhatmicrobialdegradationmaybecomemorecommonovertime,notleastbecauseanymicrobesthatcanuseplasticsasafoodsourcewillbeselectivelyadvantaged.Nevertheless,thisiscurrentlyaminorfactor–anditmustbenotedthatmanyeminentlydigestibleanddecomposableorganictissues(shellbecauseofitsorganicmatrix;bone;wood)maybecommonlyfossilizedonceburied.However,incommonwithshells,plasticitemsmaybefossilizedin‘cast’and‘imprint’formevenifalltheoriginalmaterialislostthroughbiodegradation.Thustheformsofbiros,plasticbottlesorcompactdisks(CDs)maybefoundasfossilsinsedimentaryrockinthefutureeveniftheplasticitselfhasdegradedorbeenreplacedbyothermaterials.Coldertemperatureswithinthedeepocean,associatedwithalackofUVlight,makeplasticsonthesea‐bedmorelikelytobepreserved(GregoryandAndrady,2003).Intheseconditions,theyaresaidtolastfor‘centuriestomillennia’(GregoryandAndrady,2003),mostlyviainferencefromshort‐periodlaboratorystudies.Overlongertimescales,buriedwithinstrata,theirdiagenesisandfossilizationpotentialisatopicofconsiderableacademicinterest(althoughofnoanalyticalstudyasofyet,asfarasweareaware).Thenearestcomparisoniswiththelong‐chainpolymersinrecalcitrantorganicfossilssuchaswood,sporesandgraptolites.Thesefossilizebythelossofpartofthematerial,expelledashydrocarbonliquidorgas,toleaveacarbonizedhuskand,dependingonthesizeandrigidityofthefossilandthenatureoftheenclosingfossil,alsoanimpression(anexternalmould).Onpreliminaryconsideration,itseemsthatmanyplasticswillbehavesimilarlyovergeologicaltimescales.Thehydrocarbonsreleasedduringdiagenesismightcontributetofutureoilandgasdeposits.DiscussionPlasticdebrisofdifferentsizesarewidelydistributedonlandandinthesea.Onthelandsurface,thelocallyabundantbuthighlyheterogeneousdistributionofplasticsseemstoberatherpoorlydefinedbyscientificstudy.Thereisconsiderablepotentialforitsrecordinginarchaeologicalexcavations,byaminoradjustmentofexistingstandardmethodologies.Indevelopedcountrieswherelandfillsiteshavebeencategorized,mappedanddated,asintheUK,metresto

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tensofmetresthickconcentrationsofplastic‐rich(i.e.~10%)anthropogenicdeposits,,mayberecognized.Inthecoastalrealm,theaccessibilityandrelativeeaseofstudyofsomeenvironments(e.g.beaches),thelimitedvariabilityoftopographyandprocess,andtheenvironmentalsignificanceofthephenomenon,hasencouragedmoresystematicstudy.Itisclearthatplasticsarewidelydistributed,bothasmacroplasticsandasmicroplastics,totheextentthattheycanberetrievedfrommostseafloorsedimentsamplesinmostpartsoftheworld(Browneetal.,2011;Woodalletal.,2014;Corcoran,2015).Overall,therefore,plastics,andparticularlymicroplastics,maybeaneffectivemeanstorecognizeterrestrialandmarinesedimentsdepositedsincethemid‐twentiethcentury.Furtheranatomizationintimeandspaceisclearlysignificanttotheuseofplasticsasstratigraphicindicators.Withrespecttotime,anumberofissuesmayberaised.Theglobalsignalofartificialradionuclidesiseffectivelysharp‐based,startingin~1952(Hancocketal.,2014;Zalasiewiczetal.,2015;Watersetal.,2015),reachingpeaksinthemid‐1960s,thentailingoff.Incontrast,theplasticssignalhasamoregradualandcomplexpattern.Theremaybesporadicappearancesinthestratigraphicrecordofsomeearlyformsofplastic,notablybakeliteandrayon,fromtheearly20thcentury,mostlyconfinedtothevicinityofurbanareasinEuropeandNorthAmerica.Thisputative,localizedfore‐runnerplasticssignal(stilltobeconstrainedbystratigraphicstudies)thengiveswayinthemid‐20thcenturytoamorewidespreadsignalfromplasticsdispersal,increasingfromscarcelyperceptibletomarkedoverlittlemorethanhalfacentury(cf.Fig.2).Thesignificantpresenceofplasticsonland,withinlandfillsites,wasnotapparentuntiltheearly1970sindevelopedcountriesanddisplayedsomedegreeofdiachroneityoverthesubsequentdecadeorsoasplasticsbecameaglobalcommodity.Inthemarineenvironment,recognitionofplasticsasanenvironmentalproblemdidnotsurfaceuntilthelate1960s.Oversubsequentdecades,theevidencebasehasgrownwhilethevolumeofplasticsenteringthemarineenvironmenthasalsoincreasedexponentially.Littleresearchhasbeencarriedouttorecognizetheextentoftheplasticsignatureinthe1940sand1950sduringtheearlyyearsofitsusageandbeforeitsenvironmentalimpactwasrealized.Thatgradationalbeginningmakesdefining(orpreciselylocating)thebaseoftheAnthropoceneonthebasisofplasticmaterialssensulatoimpractical,althoughplasticsareclearlyaneffectiveidentifierofAnthropocenestrata.Therearemanyformsofplastic,whichhavebeendevelopedatdifferenttimes,andthesecouldbeusedastime‐specificspeciesindicators(Albusetal.,2006)(Fig.2).Forexample,acrylicfibreswerefirstcreatedbyDuPontin1941,butnotproducedinlargequantitiesuntilthe1950s.Thisissimilartothestratigraphicuseofartificialradionuclides,theonsetofsignaturesfordifferentisotopesbeingatdifferenttimes(Watersetal.,2015).However,theartificialradiogenicsignatureislargelyrestrictedtoEarth,butplasticscannowbefoundelsewhereinthesolarsystem,includingonthesurfaceoftheMoonandMarsandinorbitaroundtheEarth.

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Overgeologicaltimescales,theplasticsburiedinlandfillsitesmaybeinpartatime‐bombofplasticrelease.Somelandfills,inlowgroundintectonicallysubsidingareas,willsimplybeburiedbymorestrata,tobefossilizedaspalaeontologicalmiddens.Wherelandfillsareeroded,theywillbeginreleasingtheirdebris,includingplastic,intothesedimentarycycle(seebelow).Virtuallyallplasticsaremouldedintoartefactsofmanydifferentkinds,eachofwhichinthiscontextmayberegardedasatechnofossil(Zalasiewiczetal.,2014),whichisatracefossilproducedbyhumans.Technofossilsshowextremelyrapidevolution,entirelydetachedfromtheevolutionofthetrace‐makingorganism(i.e.ofhumans),andhencetheappearanceofthedifferentartefactscanmarkafinechronology.–eventotheday,asseeninthedate‐stampingofplasticfoodwrapping.ThischaracteroflitterhasbeenusedtodateextremefloodeventsaffectingtheOmancoast(HoffmannandReicherter,2014).Althoughitisimportanttorecognizethedistinctionbetweenproductiondateandthetimingofaccumulation,whichmaybemonthsoryearslater,thisisstillaresolutionofdatingrarelyavailabletogeologists.Spatially,plasticsneedtobeconsideredassedimentaryparticlescontainedwithin3‐dimensionalsedimentarybodies(sometimestermed‘lithosomes’)thathavebeenshapedbyavarietyofphysicalandchemicalprocesses.IntheAnthropocene,ofcourse,thesesedimentarybodiesarestillaccumulating.Suchfactorsassedimentationrate,transportpaths,sedimentarysortingandbiologicalinfluencebecomeimportant.Fociofanthropogenically‐influencedsedimentinput,suchaslarge‐riverdelta‐frontestuaries,alreadyidentifiedassensitiverecordersofotherkindsofhuman‐drivenperturbation(BianchiandAllison,2009)maymeritparticularstudy.Thetransportofplasticsbyorganisms(andviceversa,inconsideringfloatingplasticasvectorsforencrustingspecies)hashadconsiderablestudy,withingestion/entanglement(oftenfatal)byfishandlargervertebratesbeingmuchstudied(e.g.Gregory,2009).Inrecentyears,widerrelationsbetweenfilter‐feedingplankton,benthosandmicroplastics(Browneetal.,2008;Coleetal.,2013)havebeenconsidered(IvardoSulandCosta,2014).Indrawingupsomepreliminarypatternsofplasticdistributionvis‐à‐vissedimentology,onemightsuggestthefollowingascomponentsofapredictivemodeltobetestedandfurtherdeveloped(Fig.12):Thedynamiccoastalzonewilloftenhavearelativelythick(metres‐scale)plastic‐bearingsedimentbody.Plasticlevelscanbeveryhighinpopulatedareasandlower,butoftenstillmeasurable,inuninhabitedareasbecauseoflong‐distancetransport.Inzonesofwave/currentreworking,suchasbeaches,theplastics‐bearingsedimentbodymaybelocallysharp‐basedandshowinternalvariationsreflectingselectivetransportandsedimentarysorting,withattritionandenhancedphotodegradationofplasticparticlespriortoburial.Indepositionalareas,suchasdeltasandestuaries,wheresedimentbuildupdominates,plasticshavepreservationpotentialandmayshowastratigraphicpatternofupward

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increaseinrelativeabundance,reflectinghistoricincreaseinplasticproductionandrelease.Individualhighenergy‐events,suchasstorms(HoffmanandReicherter,2014)andtsunamis,maysweepdebrisfarinland.Incarbonate‐producingenvironments,plasticshavebeenobservedinbeachrock(CaraLauria,pers.comm.;Irabienetal.,2015)andmayprovidenucleationpointsformicrobialcarbonateprecipitation.

Fig.12.Conceptualmodelofplasticstransportthroughandaccumulationinthemarinerealm.Onshelves,theremaybecontinuouslycurrent‐sweptareassuchaspartsofthetidalNorthSea,wheresedimentissweptalongasshellysanddunes.Onlythedenserplasticfragmentsmightbeincorporatedhere,whilelighterorsmaller,butstillnegativelybuoyantparticles,suchasfibresmightbewinnowedouttotravelfurther.Onquieterormoredistalshelves,plasticsmaytravelwithdebrisinstormebbsurges(ortheebbcurrentsfromtsunamis)tobedepositedastempestitelayers.Alongcontinentalslopes,plasticswillbefunneledtogetherwithsedimentthroughsubmarinecanyons,asalreadyobserved(e.g.Phametal.,2014).Withinthese,thereislikelytobesize/shape/densitysortingoftheplasticdebris,asthereisoftheaccompanyingsediment.Muchoftheplastic,especiallythemicroplastic,willbetransportedthroughthecanyonstobedepositedontheturbiditefansthatextendfromthem,andwithinturbiditelayersthatcoverthefansurfaces.Theseturbiditelayerswillshowsize/shapedensitysortingofplasticfragments,comparabletothatseenindifferentfossilsinancientturbidites.Forexample,robustshellfragmentstypicallyendupinthebottom,BoumaA‐Bdivisionsofturbiditelayerswhilethelessdensegraptolitesaretypicallyconcentratedalittlehigher,intheripple‐laminatedBoumaCdivision(Daviesetal.,1997).Weexpectplasticfragmentstobehavesimilarly,andtobeconcentratedintheupper,C‐Edivisions,dependingontheirsize,shapeanddensity.OverthecourseoftheAnthropocene,theseturbidites,andthetempestitelayersnotedabove,arelikelytobeofminimalthickness(centimetrestodecimetres)butofwideextent.Plasticscontentwillreflectthedensityand

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behavior(vis‐à‐vislitter)ofpopulationalongtheterrestrialriversandcoastupstreamofthecanyon.Itislikelytoshowgoodpreservationpotentialinthesesettings.Beyondtheturbiditefanstherearethepelagicrealmsoftheoceanfloor,inpartanalysedbyWoodalletal.(2014).Here,sedimentationratesarelowandtheAnthropocenewillberepresentedbymillimetresinstratigraphicthickness,ifthat,andsotheplasticsmayrepresentasignificantpartoftheinput.Mostoftheseafloorisoxygenatedandburrowed(bioturbated)bybenthicorganisms,andtherefore,theplastic,overdepthsof(normally)afewcentimetreswill,liketherestofthesediment,bemixedinwitholderdeposits,andseparatedfromthembyadiffusegradationalboundary.Thisisoneofthepracticalproblemsofapplyingchronostratigraphyoververyshorttimeintervals(Zalasiewiczetal.,2007),butforpracticality’ssake,thisplastic‐bearingbioturbatedunitmightberegardedasAnthropocene.Thepreservationpotentialfortheplasticmaterial,likeforanyotherorganiccompound,willprobablyincreasestronglyunderdysaerobicoranaerobicconditions.These“deadzones”ofcoastalandopenmarinebottomwatersalsowillbecomemorefequentandmorewidespreadintheAnthropocene,owingtoland‐derivedanthropogenicnutrientrunoff,aswellasmorefrequentsurfacewaterstratificationcausedbywarmingseas(cf.Gruber,2011;Keelingetal.,2010).Theplasticmaterialitselfmightremainpreservedinthesesedimentsovergeologicallyrelevanttimeepisodes.Ontheotherhand,inthemoreaerated,carbonatesupersaturatedmarinesettingsoftropicallagoons,theplasticlasticfractionislikelytobecomeinitiallyincorporatedwithinearlycementedsedimentlayers.Iftheplasticfragmentsthendegradeorbecomefragmentedafterafewhundredyears,theplasticsshouldgiverisetoanewtypeofhighlyporous,vuggylimestonewithvoidsorpseudomorphsmirroringtheshapeofleachedplastictechnofossils.Somecontemporarysedimentaryunitsmaystillremaineffectivelyplastic‐free.WhereasbeachesinAntarcticahavebecomepollutedwithplastic,thefringingdeeper‐watersedimentsderivedfromthemeltingofrockdebris‐ladenglaciersshouldbepristine,asshouldremoteland‐basedice‐masses.Perhapssimilarly,thecontouritedriftsthatmantlethebaseoftheeasternNorthAmericancontinentalslope,derivedfromdeepsouth‐flowingcurrentsfromtheArcticCirclewillalsobeplastic‐free.Involcanicsettings,primarypyroclasticflowdepositsareunlikelytoincludeplastics,butthelow‐temperaturelahardepositsderivedfromthem,iftheyflowthroughpopulatedareas,willpickupplasticsontheway.Tsunamidepositsmayalsobeworthconsideringinthisrespect.Theywillgenerateanunsortedmassofmaterialsthat,ifsourcedfromurbanareas,willincludeasignificantproportionofplastics.Largeamountsofplasticsuddenlyentrainedinthiswaymayaccumulateincoastalzonesandsomewayinlandasraiseddeposits.Alternatively,thesematerialsmaybecarriedbackouttotheoceanasachaoticbackflowofdifferentmaterialsthatislesssortedthanforthoseintroducedthroughrivers.Onceidentified,tsunamidepositscouldalsobe

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usedastime‐specificstratigraphicsignals.InthecaseoftheBoxingDay2004tsunami,existingwastesinlandfillsiteswerealsotransportedouttosea,(e.g.inBandaAcehhttp://www.gdrc.org/uem/disasters/disenvi/tsunami.html).ConclusionsThereisagrowingabundanceofplasticsinthesurfaceenvironment,andthisphenomenonmaybeconsiderednotonlywithrespecttoenvironmentalpollution,butintermsofthemodificationandcharacterizationofrecent(generallypostmid‐20thcentury)andcontemporarystrata.Plasticsarenowwidelyenoughdistributedtocharacterizesuchstrataoverlargepartsoftheworld,eveninremoteenvironmentssuchasthatofthedeepseafloorandthepolarregions.Amongsuchenvironments,microplasticsareasuperficiallyinvisible,butpotentiallywidespreadmarker,directlyakintomicrofossilsinmoreconventionalpalaeontology.Thefewstudiescarriedouttodateindicatethatthepatternsofdistributionofplasticsasbothlargeandsmallparticles(‘clasts’)withinglobalsedimentarysystemsmaybehypothesized.Onceaccumulatedwithinsedimentarystrata,plasticparticlesarelikelytohaveavariable,butgenerallygoodpreservationpotential,comparabletothatofrecalcitrantorganicfossils.Theyarealreadypresentinsufficientnumberstobeconsideredasoneofthemostimportanttypesof‘technofossil’thatwillformapermanentrecordofhumanpresenceonEarth.Stratigraphically,plasticswithinsedimentscompriseagoodpracticalindicatorofAnthropocenestrata,usingamid‐20thcenturybeginningforthispostulatedepoch.However,theonsetofthismarkerislikelytohavebeengradualandnotperfectlyisochronousinstratigraphicsuccessions,withsignificantpresenceofplasticsinthemarineandterrestrialenvironmentsnotbeingrecordeduntilthelate1960stoearly1970s.Therefore,despitetheirutilityforpracticalstratigraphy,theyarenotacharacterthatmightbeexpectedtoactasprimarymarkersinanyfuturedefinitionofaformalreferencesectionfortheAnthropocene.ReferencesAdam,D.2009.Britishbeachlitterlevelshighestonrecord.TheGuardian,8April2009.http://www.theguardian.com/environment/2009/apr/08/beach‐litter‐record‐levels‐mcsAlbus,S.,Bonten,C.,Kebler,K.,RossiG.,WesselT.,2006.PlasticArt–APrecariousSuccessStory.Germany:TheAXAArtConservationProject.

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