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Geomorphology304(2018)64–73 ContentslistsavailableatScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph Implications of sea-level rise in a modern carbonate ramp setting StephenW.Lokiera,⁎,WesleyM.Courta,TakumiOnumab,AndreasPaula aGeosciencesDepartment,KhalifaUniversityofScienceandTechnology,PetroleumInstitute,P.O.Box2533,AbuDhabi,UnitedArabEmirates bAppliedGeoscienceDepartment,JGI,Inc.,Tokyo,Japan a r t i c l e i n f o a b s t r a c t Articlehistory: Thisstudyaddressesagapinourunderstandingoftheeffectsofsea-levelriseonthesedimentarysystemsand Received10April2017 morphologicaldevelopmentofrecentandancientcarbonaterampsettings.Manyancientcarbonatesequences Receivedinrevisedform13December2017 areinterpretedashavingbeendepositedincarbonaterampsettings.Thesesettingsarepoorly-representedin Accepted16December2017 theRecent.Thestudydocumentsthepresent-daytransgressivefloodingoftheAbuDhabicoastlineatthesouthern Availableonline28December2017 shorelineoftheArabian/PersianGulf,acarbonaterampdepositionalsystemthatiswidelyemployedasaRecent analoguefornumerousancientcarbonatesystems.Fourteenyearsoffield-basedobservationsareintegrated Keywords: withhistoricalandrecenthigh-resolutionsatelliteimageryinordertodocumentandassesstheonsetofflooding. Sealevel Transgression Predictedratesoftransgression(i.e.landwardmovementoftheshoreline)of2.5myr−1(±0.2myr−1)basedon Carbonateramp globalsea-levelrisealonewerefarexceededbythefloodingratecalculatedfromtheback-steppingofcoastalfea- Coastalerosion tures(10–29myr−1).Thisdiscrepancyresultsfromthedynamicnatureofthefloodingwithincreasedwater PersianGulf depthexposingthecoastlinetoincreasederosionand,thereby,enhancingback-stepping.Anon-accretionary transgressiveshorelinetrajectoryresultsfromrelativelyrapidsea-levelrisecoupledwithalow-anglerampgeom- etryandapaucityofsediments.Thefloodingisrepresentedbythelandwardmigrationoffaciesbelts,arangeof erosivefeaturesandtheonsetofbioturbation.EmployingIntergovernmentalPanelonClimateChange(Church etal.,2013)predictionsfor21stcenturysea-levelrise,andallowingforthepost-floodinglagtimethatistypical forthestart-upofcarbonatefactories,itiscalculatedthatthecoastlinewillcontinuetoretrogradefortheforesee- ablefuture.Totalpassiveflooding(withoutconsideringfeedbackinthemodificationoftheshoreline)bytheyear 2100iscalculatedtolikelybebetween340and571mwithafloodingrateof3.40–8.64myr−1.However,adopting theobservationthatglobalsea-levelriseonlyaccountsfor15%oftherecordedshorelineretreat,thisfigurerises dramaticallytoatotallikelydynamicflooding(consideringmodificationstotheshoreline)ofbetween2.3and 3.8km.Lossofmicrobialandmangalhabitatswillsubjecttheexposedshorelinetoincreasingerosion.Shoreline retreatwillthreatenexistingcoastalinfrastructure. ©2017ElsevierB.V.Allrightsreserved. 1.Introduction fortheAbuDhabicoastlineduringtheLateHoloceneregressionand stillstandiswell-documented(Evansetal.,1969;KinsmanandPark, Thepasttwocenturieshavewitnessedariseinglobalsea-levelat 1976; Lokier and Steuber, 2008; Stewart et al., 2011; Lokier et al., anacceleratingrate,with3.2mmyr−1observedfrominstrumentalre- 2015).Yetonlyasmallnumberofpreviousstudieshaveconjectured cordsoverthepasttwodecades(ChurchandWhite,2011;Meyssignac theeffectsofaeustaticsea-levelrise,withthecreationofnewaccom- andCazenave,2012;Churchetal.,2013).Thisindustrial-erasignature modationthatonlyrecentlyoverwhelmedsedimentproductionand of global sea-level rise has been documented in a range of coastal thusinitiatedtheonsetofatransgressivephase(EvansandKirkham, systems(Singh,1997;Leathermanetal.,2000;RankeyandMorgan, 2002;Lokieretal.,2015). 2002;Woodroffe,2008;FordandKench,2015;Romineetal.,2016; Theprimaryobjectiveofthisstudyistoutilisefourteenyearsof Testutetal.,2016).However,todate,therehavenotbeenanystudies field-basedobservationsalongwithhistoricalandrecentsatelliteimag- oftheeffectsofthistransgressiononsedimentarysystemsandshore- erytodocumentandassesstheprocessoftransgressioninanaridcarbon- linemorphology in a carbonate ramp setting. The coastline of Abu ateandevaporitecoastaldepositionalsystemthatismorphologically Dhabi,lyingatthesouthernshoreofthePersian/ArabianGulf,hereafter analogous to numerous ancient carbonate ramp systems. The study referredtoastheGulf,isanidealsettingforstudyingtheprocessof alsoaimstoestablishthediagnosticsedimentaryfeaturesthatcanbe transgressioninsuchasetting.Theoverallprogradationalgeometry employedtoassistintheidentificationoftransgressivesurfacesinancient sedimentary sequences throughout the stratigraphic record. These ⁎ Correspondingauthor. outcomesprovidearesourcewithwhichtointerpretthesequencestrat- E-mailaddresses:[email protected](S.W.Lokier),[email protected](T.Onuma). igraphiccontextinancientcarbonaterampsettingsand,conversely,to https://doi.org/10.1016/j.geomorph.2017.12.023 0169-555X/©2017ElsevierB.V.Allrightsreserved. S.W.Lokieretal./Geomorphology304(2018)64–73 65 considerthelikelyimplicationsofglobalsea-levelriseonfutureshoreline Temperatures measured in the study area regularly exceed 50 °C morphodynamics.Furthermore,predictionsofprobableratesofgeomor- onsummerdaysyetmaydropaslowas7°Cduringwinternights phicchangearefundamentaltoplanningfuturecoastlinemanagement (LokierandFiorini,2016).Theprevailingwindisthenorthwesterly (Pethick,2001;Romineetal.,2016). Shamal.Stormsarerareandtypicallyofrelativelyshortdurationand lowenergy. 2.Setting 3.Methods TheGulfisashallow(average35m)microtidal(1–2m)epeiricsea lyingatthenorthernmarginoftheArabianPlate(Fig.1).Thefloorofthe This study utilised and integrated multiple datasets including almost completely land-locked basin dips gently north-eastward satellite imagery, shoreline transect surveys and long-term repeat towardsthecoastofIran.Thesedimentsoftheseaflooraredominated field observations along a 16 km section of coastline containing a by muddy to fine-grained carbonates (Evans, 1966; Uchupi et al., rangeofsettingsrepresentativeoftheGulfcoastalsystemoftheUAE 1996)andassociatedhardgrounds(Shinn,1969).Thereisageneral (Fig.1).ThestudyemployedQuickbirddatawithaspatialresolution increaseinsiliciclasticcontenttowardstheIranianshoreline(Emery, of 0.6 m/pixel for the Panchromatic Band, acquired in 2003, 2004, 1956). 2005and2007,andWorldView-2datawitha0.5m/pixelresolution TheAbuDhabicoastlineoftheGulfcomprisesanorthward-dipping, for the same band acquired in 2010. Multi-band data covering the low-angle, carbonate ramp characterised by the transition from a visible-nearinfraredregionwithaspatialresolutionof2.4m/pixels supratidalsabkha,throughabroadcarbonate-dominatedintertidal and2.0m/pixel,forQuickbirdandWorldView-2respectively,were environment,intoasubtidalcarbonatesetting(Evansetal.,1964). alsousedalongwithPAN-sharpenedcolourimageswith0.6m/pixel Thecoastislocallyisolatedfromopen-marineconditionsbyacomplex forQuickbirdand0.5m/pixelforWorldView-2data.Multi-banddata ofpeninsulas,shoalsandislandsthatformedasHolocenesedimentsac- wereconvertedtospacereflectanceusingparametersprovidedbythe cretedaroundPleistocenelimestoneassociatedwiththeunderlyingGreat TechnicalNoteofDigitalGlobe,andNormalizedVegetationIndexim- PearlBank(PurserandEvans,1973;Harris,1994).ThesouthernGulf ageswereproducedinordertotracethedistributionchangeofonshore shorelinehasbeentectonicallystablethroughoutthelateQuaternary vegetationaswellasmicrobialmatsintheintertidalzone.Theresultant (Stevensetal.,2014). images were examined and compared in order to trace temporal TheemirateofAbuDhabiexperiencesanaridclimatewithamean morphologicalchangesalongthecoastline. annualrainfallof72mm,concentratedduringFebruaryandMarchas Field-basedobservationshavebeenregularlymadeintheAbuDhabi localised,brief,torrentialrainstorms(Raafat,2007).Meanannualevap- sabkhasincetheyear2000.Thoughnotinitiallyfocusedonthisspecific orationof2.75mexceedsrainfallbyalmost4000%(Bottomley,1996). study, the earlier field observations (prior to 2007) provided an Iraq Kuwait Persian/Arabian Iran Gulf Abu Dhabi Island Bahrain Persian/Arabian Qatar Gulf GOumlfa onf Saudi Arabia United Arab Oman Emirates Khawr Jazirat Abu Qirqishan Kushayshah Al Dabb’iya Abu Al Abyad K h a wr Q a ntur N 10 km Fig.1.SatelliteimageoftheAbuDhabicoastlinebetweenAbuDhabiIslandintheeastandAbualAbyadIslandinthewest,seeinsetmapforregionalcontext.Thelocationofthe16km-long studyareaishighlightedbythewhitebox,theblackarrowindicatesthepositionoftheislandshowninFig.2,thewhitearrowindicatesthelocationofFigs.4and6.LandsatETMimage acquired3rdJuly2002. 66 S.W.Lokieretal./Geomorphology304(2018)64–73 invaluableresourceforselectinglocationsformorerobust,long-term 4.Evidenceofshorelinechange monitoring.Observationswerealsoregularlyundertakenoutsideof thefocusareasinordertoensurethatthewidestpossiblegamutof Shorelinesarehighlydynamicfeaturesinanear-continuousstateof datawasincorporatedintothestudy.Fieldobservationsalsoincluded morphologicalfluxatarangeofspatialandtemporalscales(Woodroffe, surveyingtwotransectsnormaltotheshorelineinordertomeasure 2008;Rankey,2011).ThedGPSsurveyinthisstudyestablishedthat theheightandslopeofthecarbonaterampcoastalsystem.Thesurvey the upper intertidal to supratidal portion of the ramp system dips wasundertakenusinganAltusAPS-3differentialglobalpositioningsys- very gently seawards at an angle of between 0.07° and 0.08°. By tem(dGPS).Ateachmeasurementposition,atleast10measurements employing the Intergovernmental Panel on Climate Change (IPCC) were made and the average of these was used to geolocated the globalmeansealevelriserateof3.2mmyr−1fortheperiodbetween position. 1993and2010(Churchetal.,2013),anaveragetransgressionrateof Aparticularchallengeresultingfromthelow-anglegeometryof 2.5±0.2myr−1canbepredicted.Itcanbeanticipatedthatsucha theAbuDhabicoastalsystemisdemarkingadefinitive‘coastline’. significantrateoftransgressionwouldresultinsubstantialcoastline In this carbonate ramp system the horizontal distance between change.Fieldandremotesensingobservationsofarangeoferosional mean high water and mean low water may locally exceed 4 km, andconstructionalfeaturesfromtheAbuDhabicoastlinearedocumented withahighdegreeofvariabilityresultingfromeventhemildest herein. windsoratmosphericpressurevariations.Intheabsenceofadefin- Anumberofmorphologicalfeaturesweremonitoredinthevicinity itivecoastline,weemploythemicrobialmatbeltasadatum.The ofalow-lyingvegetatedislandinthewestofthestudyarea(Fig.2). landwardandseawardlimitsofmicrobialmatbeltsarestronglycon- Unfortunately,dredgingandlandreclamationactivitydestroyedthis trolledbytheirpositionintheintertidalzone(LokierandSteuber, areain2011therebyabruptlycurtailingmonitoring.Priortothisloss,a 2008).Theseawardextentofthebeltisconstrainedbyabalancebe- seriesofspitsdevelopedatthewesternshoreoftheislandandmigrated tweenmicrobialmatgrowthrateandthepresenceofgrazingorgan- landwards over a monitoring period ofseven years. The spits were isms-particularlygastropods.Themat'slandwardlimitisgoverned composedofcoarsesandtograveldominatedbywholeandfragmented bythelimitsofflooding;microbialmatsmustberegularlyinundated gastropods with subordinate bivalves and other skeletal allochems. toavoiddesiccation. Thesebodiesmigratedtowardsthesoutheastoverapustularmicrobial 5.4.2003 18.6.2004 4.7.2005 A B C B C2 B M S S C S N 100 m 13.2.2007 8.10.2010 D E B B Be S2 S S Fig.2.Seriesofsatelliteimagesovera7-yearperiodshowinggeomorphologicalchangesassociatedwithaspitinthewestofthestudyarea(refertoFig.1forthelocationoftheimages). A)Satelliteimageshowingaseriesofspitslandward(south)ofalow-reliefisland.B)Spitsretrogradeoverthemicrobialmat(S).Inthenorthoftheimagesandbars(B)migratesouth towardsastandofmangroves(M).Acrevassesplaydevelopsinthesouthoftheimage(C).C)Spitsandsandbarscontinuetoretrogradetowardsthesouth.Thespitisbreachedwithanew crevassesplaydevelopingwithaneasterlyorientation(C2).D)Continuedsouthernspitmigration.Sandbarsbegintoburymangrovesandisolatethemfrommarinecirculation.Significant buildingofthebeachonthewesternmarginofthespit(Be).E)Spitscontinuetoretrogradetowardsthesouth-totalmigrationof218min7yearsand7months.Mangrovestandtotally destroyedbymigratingsandbars.Newspitsystemdeveloping(S2).RefertoFig.1forthelocationwithinthestudyarea.AllimagesfromQuickbird2(0.6m/pixelspatialresolutionforthe PanchromaticBand)exceptthoseof10/10/10whichareWorldView-2(0.5m/pixelresolution).Infraredisdisplayedasred,redasgreenandgreenasblue. S.W.Lokieretal./Geomorphology304(2018)64–73 67 mat(Courtetal.,2017)underlainbyananoxic,organic-richmudto Stormeventsresultinlocalisedbreachingofbeachridgestoproduce fine-sandgradesediment.Themigrationwasmonitoredbetween crevassesplayscomprisingcoarse-grainedcarbonatesands(Figs.2C, April2003andAugust2010usingacombinationoffieldobserva- 3C). Onceemplaced, thesesands are not susceptible to reworking, tions(Fig.3A)andremotesensingimagery(Fig.2).Overthisseven thereby contributing to the aggradation of the island surface. andahalfyearperiod,thespitmigrated218mlandwardoverthe Retrogradingsandbodiesalsohaveasignificantimpactonthedevelop- microbialmats,equatingtoanaveragerateof29myr−1.Similar mentofmangals(Fig.3D–F).Wheretheaerialroots(pneumatophores) back-steppinghasbeenobservedinbeachridgeselsewherealong ofmangrovesareburiedbymigratingsands,themangrovetreesdie thecoast(Fig.3B). andbecomevulnerabletosubsequenterosion(Ellison,1999). A Landwards 8/10/2007 27/12/2007 28/2/2008 15/5/2008 30/12/2008 2 m B C 30 cm D E F 13/2/2007 50 m 8/10/2008 50 m 40 cm G H 20 cm Fig.3.FieldphotographsofarangeoferosionalandmigrationalfeaturesobservedattheAbuDhabicoastline.A)Observationsofaspitmigrating17.5mlandwards(southeast)overthe microbialmatsoveraperiodof15months.Arrowsdenotetheobservedlocationofthetipofthespitatthespecifieddates.B)Beachridgeback-stepping(directionofwhitearrow)towards thesouthoveramicrobialsurfacewithfootprints(blackarrows).C)Breachingofabeach-ridgewithassociatedcrevassesplay.Blackarrowsindicatethelimitsofthebreach,blackdashed linedenoteslimitofwashover(personforscaleis1.75mtall).D&E)PairedQuickbird2(left)andWorldview2(right)satelliteimagesshowingtheinundationandlossofmangroves (whitearrow)byretrogradingsandbodiesoveraperiodof20months.F)FieldphotographofthemangroveskilledthroughburialbyretrogradingsandbodiesasindicatedinpanelsD&E. Viewtowardsthenorth.G)HardgrounderosionontheleewardsideofthepeninsulashowninFig.1(bendingpersonforscaleis1.15mhigh,viewtowardsthenorth).H)Erosionof microbially-boundlayersofmuddysandsintheupperintertidalzone,whitearrowsindicatedistinctmicrobialsurfaces,landwards(south)totopofimage. 68 S.W.Lokieretal./Geomorphology304(2018)64–73 Intheintertidalzonetotheseawardandeastoftheislandthereis changesintheamountanddurationoftidalfloodingandanychanges significantexposureanderosionofthehardgrounds(Fig.3G).During inenergywithinthedepositionalsystem. rarehigherenergystormevents,clastsfromthehardgroundarecarried Thearealextentofthemicrobialmatbeltcanbeclearlydefinedfrom landwardontothesurfaceofthesabkhawheretheyareincorporatedas satelliteimagery(Fig.4).Inthefield,itispossibletosub-dividethemi- aminorcomponentintobeachridgesormoremuddyfaciesasangular crobialmatbeltintoanumberofmorphologicalsub-zones(Courtetal., extraclasts.Erosionisalsoobservedatthelandwardlimitoftheinter- 2017).Nobioturbationisobservedwithinthemicrobialmatbeltorin tidalzone.Firmgrounds,formedthroughthebindingofsedimentsby facieslandwardofthemicrobialmats.Duringtheperiodofobservation, interstitialmicrobialgrowth,arebeingeroded(Fig.3H).Fragmentsof avarietyofchangeshavebeenobservedinrelationtothemicrobialmat thefirmgroundareeasilyabradedtoformsub-roundedintraclastsor, belt. iftransportedmoredistally,potentiallyextraclasts. Satelliteandfieldobservationsshowthatboththeseawardand Within the middle to lower intertidal zone, tidal creeks are landward margins of the microbial mats are migrating landwards retrogradingthroughheadwarderosionoftheunderlyinghardground (Fig.4).Theseawardmarginofthematsisbeingburiedbeneaththe facies.Retrogradinglobesoftheintertidalbioclasticandpeloidalsands back-stepping bioclastic and peloidal sands of the intertidal zone locallydivertthemost-landwardportionoftheintertidalcreeks;this (Fig.5A–E).BetweenFebruary2007andSeptember2010theseaward processsometimesdivertsebbtidalflowtoadifferentcreekcatchment marginofthemicrobialmatslocallyretrogradedbybetween60and resultinginabandonmentoftheoldcreeksystem(Fig.4). 100m,equivalenttoarateof16–27myr−1.Theretrogradingbioclastic andpeloidalsandsinitiallycoverthematsurfacewithathinsediment veneer(Fig.5A)thatresultsinamorphologicalchangeinthegrowth 4.1.Migrationoffaciesbelts ofthemats.Low-lyingareasofthematandgapsbetweenpolygons (Fig.5B)arepreferentiallycoveredbysedimenttherebyreducingthe Asmentionedpreviously,theextentofthemicrobialmatbeltis small-scalereliefofthematsurface.Withinweeks,thissedimentis stronglycontrolledbyitspositionwithintheupperpartoftheintertidal colonised and stabilised by interstitial microbial growth (Fig. 5C). zone. The community composition, morphological characteristics Topographicallyhigherareasofthemicrobialmats,suchasmatmargins andresilienceofthemicrobialmatsareparticularlysusceptibletoany continuetogrowbutareincreasinglysusceptibletograzingbybenthic A B C IT IT MM MM IT MM MM P ST P ST ST Road 555...444...222000000333 200 m N 18.6.2004 4.4.2005 D E b a 13.2.2007 8.10.2010 Fig.4.Seriesofsatelliteimagesovera7-yearperiodshowinggeomorphologicalchangesassociatedwiththemicrobialmatbelt,refertoFig.1forthelocationoftheimages.A)Satellite imageshowingthesupratidal(ST),microbialmatbelt(MM)andintertidal(IT)environmentsoftheAbuDhabiSabkha.B)Unusuallyhightidesresultinfloodingandpondingofwaterin thesupratidalzone(P).C)Increasedmicrobialmatdevelopmentinthenorth-westernfloodedarea.Retrogradationofthelandwardsmarginofthemicrobialmatbeltinmuchofthearea (arrow).D)Imagewithfloodingoftidalcreeks,intertidalbioclasticandpeloidalsandfaciesretrogradeoverthemicrobialmatbelt(arrows).E)Intertidalsandshaveback-steppedovera distanceof60mat‘a’and100mat‘b’.Thelandwardmarginofthemicrobialmatshasmigratedlandwardsbyonly35moverthesameperiod.AllimagesfromQuickbird2(0.6m/pixel spatialresolutionforthePanchromaticBand)exceptthoseof8/10/10whichareWorldView-2(0.5m/pixelresolution).Infraredisdisplayedasred,redasgreenandgreenasblue. S.W.Lokieretal./Geomorphology304(2018)64–73 69 A B 1 m 50 cm C D M B F G P 10 cm 10 cm E F R 10 cm 50 cm Fig.5.Fieldphotographsoftransgressivefeaturesassociatedwiththemicrobialmatbelt.A)Viewseawardsshowingthebacksteppingofthelowerintertidalbioclasticandpeloidalsand faciesovermicrobialmatswithapoorly-definedpolygonalmorphology.Theretrogradationalfrontisindicatedbythewhitearrow.Smallstandsofmangrovescanbeseeninthedistance (blackarrow).B)Well-developedleatherypolygonalmicrobialmats(foreground)beingburiedbythebacksteppingbioclasticandpeloidalsandfacies(blackarrow)–viewtowardsthe north(seawards).Notehowthesandsareinfillingareasbetweentheupliftedpolygonmargins(whitearrow),therebyeffectively‘smoothing’thetopography.C)Bioclasticandpeloidal sandfacies(leftofimage,seawards)retrogradingoverspongymicrobialmat(rightofimage,landwards).Thesandsarebeingstabilisedbythedevelopmentofathininterstitialmicrobial growthwithinthesedimentsurface(blackarrow).D)Followingburialbeneaththebioclasticandpeloidalsandfacies,themicrobialmats(M)aresusceptibletodegradation.Surface feedingbygastropods(G)andcrabsproducespeloids(P)andfeedingpellets(F)respectively.InfaunalbioturbationislimitedtodwellingburrowsofScopimeracrabicauda(B).Note thatalloftheburrowingandfeedingisconcentratedinthetopographicallylowareaswherethesemicrobialmatsareabsent.E)Crosssectionshowingthelaminatedbioclasticand peloidalsandfacies(arrow)retrogradingoverthespongymicrobialmat(blackarrow)-thedashedwhitelinedemarksthebaseandfrontofthetransgressingunit.Thesurfaceofthe sandsiscolonisedbyaninterstitialpalegreenmicrobialcommunity.Thedarkgreycolouroftheunderlyingsedimentsreflectstheanoxicconditionsinthesub-surfacewhere microbialmatshavedecayedwithonlyisolatedrelicsremaining(whitearrow).F)Areasofrippeduppustularmat–viewtowardsthesouth(landwards).Areasofmataredislodged duringinundationbyhigher-energyevents,suchasstorms,orthroughsimplefloatingwhengasistrappedbelowtheimpermeablemat.Thedisplacedmaterialmaybeoverturned (blackarrow),piledup(whitearrow)orformelongateroles(R). faunathatresidewithinthenow-adjacentsands.Withincreasingburial, Thelandwardmarginofthemicrobialmatsisalsoback-stepping andovertime,themicrobialmatsbecomesusceptibletodegradation. throughoutthestudyarea,though,atarateofonly10myr−1thisis Increasedfloodingofthemicrobialmats,andthepresenceofhostsed- significantly slower than the retrogradation of the seaward margin iment,facilitatesthepresenceofincreasednumbersofgrazingfauna overthesameobservationperiod(Fig.4).Thedisparityintheretrogra- dominatedbygastropods,ostracodsandcrabs(Fig.5D).Thepresence dationratesbetweentheseawardandlandwardmarginsofthemicrobi- ofasignificantorganiccomponentwithinthesedimentresultsinthe almatbelthasresultedinthebeltbecomingsignificantlynarroweras onsetofanoxicconditionsimmediatelybelowthesedimentsurface canbeclearlyvisualisedfromNormalizedDifferenceVegetationIndex (Fig.5E).Burrowingisabsentfromthemicrobialmatsandsupratidal images(Fig.6).Throughoutthestudyareathelandwardmarginofthe facies.islimitedtosimpleshallowdwellingburrowsconstructedby microbialmatbeltischaracterisedbyathinpustularmatthatisdevelop- crabs(Scopimeracrabicauda).Burrowsareabsent. ingaboveamixedcarbonate-evaporitesequencewithabundantsandto 70 S.W.Lokieretal./Geomorphology304(2018)64–73 4.7.2005 50 m 8.10.2010 50 m Fig.6.PairedNormalizedDifferenceVegetationIndeximagesfrom4/7/2005(Quickbird2)and8/10/2010(Worldview2)showingthattheseawardlimitofthemicrobialmatbelthas movedsignificantlylandwardwhilethelandwardlimithasretrogradedtoalesserextentoverthe5yearsofobservation.Thedashedwhitelineshowstheformerpositionofthe seawardmarginofthemicrobialmat.Distributionofthevegetation,includingmicrobialmats,ishighlightedbythegreentoredcolours.Thetrackwaysseenintheimages(white arrow)arearesultofseismictruckssurveyingtheareaduringthe20thCentury.ThelocationoftheimageisindicatedbythewhitearrowinFig.1. gravelgradegypsum.Thegypsumwasformedthroughtheevaporation retreat,afigurethatisverysimilartothe12%notedfromtheopen ofgroundwaterintheshallowsub-surfaceofthesupratidalsabkha beachesoftheMediterraneancoastofFrance(BrunelandSabatier, (LokierandSteuber,2008). 2009).Thislargediscrepancyarisesfromtheassumptionthatma- Duringfieldwork,largeareasofthemicrobialmatwereobservedto rinetransgressionisapassivesubmersionprocesswithoutmodifica- episodicallyberemovedeitherduringrarehigher-energystormevents tionoftheshoreline.Suchamodelisover-simplifiedbecauseduring ormoregentlythroughraftingresultingwhentrappedgasbelowthe transgressioncoastallandformsareforcedtomigrateinorderto impermeablematsurfacebuoys-upthematduringperiodsofexcessive maintaintheirspecificpositioninrelationtothecoastalenergygra- flooding.Suchdenudationexposestheunderlyingsediment(Fig.5F) dient(Pethick,2001).Itisinferredthatthismodelofdynamicactive either to renewed microbial colonisation or, likely in the case of submersion(BrunelandSabatier,2009)willexposethecoastlineto storm-events,toerosiontoformlocaliseddepressionsthatmaydevelop increasederosion(Leathermanetal.,2000;Woodroffe,2008)and intointertidalponds(Pauletal.,inprep.). resultinshorelinesretreatingatarategreaterthanthatpredicted forsea-levelrisealone.Further,thecarbonatefactoryintheseshal- 5.Discussion lowtointertidalenvironmentsdoesnotproducesufficientsediment tofilltheaccommodationspacenewlycreatedbyrisingsealevel;a Theremotesensingandfieldobservationsdocumentedinthisstudy situationthatiscompoundedbythelackoffluvialsystemsandthe recordashorelinethathasenteredaphaseoferosionandretrograda- prevailingonshore,Shamal,windbothofwhichpreventtheinflux tionconsistentwiththeonsetofmarinetransgression.Thisprovides ofdetritalsedimentfromtheland. anexceptionalopportunitytoestablishthediagnosticsedimentary Itiswidelyacceptedthatthemuch-employedBrunn(1962)model featuresthatwouldprovideevidenceoftransgressionofacarbonate forshorelineretreatisover-simplified,neglectingsite-specificgeomor- rampsystemwithinthegeologicalrecord.Employingtheaxiomthat phologicalandsedimentologicalfactors(CooperandPilkey,2004).The improvedknowledgeofthepastandpresentarekeytomoreaccurate modelisparticularlyinadequateinthevicinityofbarrierislands,capes predictionsofthefuture,weconjecturethelikelyfutureeffectsofshore- and spits (Leatherman et al., 2000). Local-scale variations in the lineretreatandimplicationsthereof. geomorphologicalresponseofthecoastlinetosea-levelchangeare common (Webb and Kench, 2010; Ford and Kench, 2015; Romine 5.1.Calculatedtransgressioncomparedwithmeasuredretrogradation etal.,2016),withadjacentportionsofthecoastlinevariouslydisplaying erosion,accretionornosignificantchange.Thenatureoftheresponseis UnderthetectonicallystableconditionsofthesouthernGulfshore- controlledbytheantecedenttopography(Romineetal.,2016),changes line(Stevensetal.,2014;Lokieretal.,2015;PfefferandAllemand, inboundaryconditions(WebbandKench,2010)andsedimentsupply 2016),relativesealevelcanbeconsideredasequivalenttoeustatic (Carrascoetal.,2016;Testutetal.,2016). sealeveloverthedurationofthestudy.Thepredictedaveragerateof flooding for the Abu Dhabi coastline of 2.5 ± 0.2 m yr−1 is much 5.2.Recognisingtransgressioninthestratigraphicrecord lowerthananyoftheobservedratesofcoastalfeatureretrograda- tion,i.e.10–29myr−1.Consequently,eustaticsea-levelriseaccounts Canthepresent-dayfloodingattheAbuDhabicoastlinebeconsidered for only an average of 15% (8.6–25%) of the observed shoreline as representative of a transgressive event in the true sequence S.W.Lokieretal./Geomorphology304(2018)64–73 71 stratigraphiccontext?Transgressionmaybedefinedasalandwardshift infaunal detrital sedimentfeeders. Burrows are limited to shallow, ofthemarinesysteminitiatedwherebase-levelrisesataratehigher open,dwellingburrowsandbioturbationisunlikelytobesointensive thantheshorelinesedimentationrate(Catuneanuetal.,2009).Thetrans- astodestroyprimarysedimentarystructures.Undertheseconditions, gressivepartofthesequenceliesimmediatelyonthemaximumregres- theonsetofbioturbationlocallyrecordstheonsetoftransgression. sivesurface(transgressivesurface)ofthepreviousregressivephase Locally,microbialmatsarerapidly(hourstoweeks)buriedbeneath (Catuneanuetal.,2011).Presentedwiththesedefinitions,weconclude relatively-thickcoarsegrainedcarbonatesands,associatedwiththeret- thattheobservationsmadeduringthisstudysupporttheinterpretation rogradationofspitsorwashoverdeposits.Underthesecircumstances thattheAbuDhabicoastlinehasenteredaphaseoftransgressionrecord- theburiedmicrobialmatwillbeisolatedfrombioturbationresulting ingthetransitionfromaprogradationaltoaretrogradationalgeometryof inasedimentarysequencecharacterisedbycoarse-grainedsediments thesedimentarybodies. directlyoverlyingfinergrainedfaciesdisplayingamicrobiallamination Thearchitectureoftransgressivedepositsisaproductoftherateof andlackingfaunalbioturbation. sedimentsupply,therateofsea-levelchange,thegeomorphologyof theshorelinesystemandtheenergyregime–asingletransgressive transectcomprisessectorsrecordingdepositionandsectorsoferosion 5.3.Projectionsof21stcenturytransgression (Cattaneo and Steel, 2003). The relatively high rate of relative sea-levelrise,low-anglegeometryandlimitedsedimentsupplyatthe BasedondifferentRepresentativeConcentrationPathways(RCP) AbuDhabicoastlinesupportanon-accretionarytransgressiveshoreline scenarios, the IPCC 5th Assessment Report predicts a likely global trajectory(sensuHelland-HansenandGjelberg,1994).Underthese meansea-levelriseofbetween0.44(0.28–0.61)m(RCP2.6)and0.74 conditions,arapidlandwardshiftintheshorelinewouldbeexpected (0.52–0.98)m(RCP8.5)bytheyear2100(Churchetal.,2013).Given toinitiallyproduceonlythintransgressivedepositswithinsitudrown- thatthereiscurrentlynosignificantverticallandmotion(subsidence ingoftheshoreface(CattaneoandSteel,2003).Thisisparticularly or tectonic uplift) at the southern shore of the Gulf (Pfeffer and thecaseincarbonate-dominatedsystemswherealagtimeofafew Allemand, 2016), these global mean sea-level rise figures can be hundredtoafewthousandyearsiscommonbeforethecarbonatefactory employedtopredicttherangeofthedegreeoflikelyfloodingofthe isfullyfunctioningagain(Tipper,1997;KempandSadler,2014). naturalAbuDhabicoastline. Thepresenceofcohesive,microbially-bound,fine-grainedsediment Basedontheangleofslopeandtheamountofprojectedsealevel furtherinhibitstheformationofatidalravinementsurface,except riseitiscalculatedthatby2100theamountoftotalpassivefloodingis where headward erosion occurs at the thalweg of tidal channels. tobelikelybetween340(216–471)m(RCP2.6)and571(401–756)m Headwarderosionoftidalcreekshasbeendocumentedfromotherset- (RCP8.5)(Fig.7).TherateoffloodingattheAbuDhabicoastlineiscal- tings(Knightonetal.,1991;RankeyandMorgan,2002).Duringhigh culatedtolieintherangebetween3.40(1.54–5.25)myr−1(RCP2.6) tides,seawaterincursion is concentratedwithin slighttopographic and8.64(5.79–12.12)myr−1(RCP8.5)bytheyear2100.Thiswould depressions,thecentreofwhichbecomesincreasinglysusceptibleto resultinasignificantreductioninlandareainnaturalcoastalsystems. tidalscouringandsubsequentdeepening(Knightonetal.,1991).This Theabovefiguresassumepassivefloodingofthecoastlinewithout processisproducinghighlylocalisedtidalravinementsurfaceswhere anydynamicresponsetosea-levelrise,asdiscussedearlier.Adopting tidalcreekserodetheunderlyinghardground. anassumptionthatsea-levelrisewillcontinuetoaccountforonly15% Transgressivelags,occurringaseitherconglomeratic,glauconitic, ofshorelineretreat,wecansignificantlyrevisetheabovefigures.The fossiliferous or geochemical deposits (Kidwell, 1989; Cattaneo and amountoftotaldynamicfloodingby2100wouldbelikelybetween Steel,2003),aretypicallyexpectedwherethereissedimentstarvation 2.26 (1.44–3.14) km (RCP2.6) and 3.81 (2.68–5.04) km (RCP8.5). of the shelf (Swift, 1976) or condensation resulting from dynamic SincemostoftheAbuDhabicoastlineisnotembankedbyacliffor bypass(Kidwell,1989).Lagdepositsarenotobservedinthecurrent scarp,thereisnotheoreticallimittotheamountoflandwardmigration study as there is a paucity of bioclasts, intraclasts or extraclasts. oftheshoreline. Althougherodedfirmgroundandhardgroundclastswereobserved, Ofcourse,theassumptionthatsea-levelrisewillcontinuetocontrib- thesewererelativelyrareandonlylocallyconcentratedin,forexample, uteonly15%ofshorelineregressionfortheremainderofthe21stcentury beachridgesorspits.Carbonatesedimentationrates,thoughlowat ishighlyconjecturalandopentosignificanterror.Localisedvariationsin 13–50cm/ky(Kinsman,1969;LokierandSteuber,2008),aresufficient sedimentavailabilityandtransportpathwaysmakepredictionshighly topreventsedimentstarvation. challenging(Romineetal.,2016).Thecreationofnewaccommodation Ithaspreviouslybeeninferredthat,withintidalflatsystems,new and onset of improved marine circulation may promote a positive accommodationisinitiallyrapidlyfilledbymicrobialmats(Strasser, responsefromthecarbonatefactory(Eberli,2013),withanincreasein 2015)untiltheyaresufficientlyinundatedtopermitgrazingorganisms carbonateproductionthatmaygosomewaytooffsettingthetransgres- todestroythem(Farmer,1992).Duringthepresentstudy,weobservea siveprocess.Conversely,assurfacewatertemperaturesriseandpH distinctnarrowingofthemicrobialmatbeltatthecoastline.Thesuper- decreasesthestressonmarineorganismswillinhibitcarbonateproduc- position of the back-stepping upper intertidal microbial mats over tion.Ashasbeendocumentedelsewhere,thereisnormallyasignificant supratidal-depositedevaporitefaciesformsadistinctstratigraphicrela- lagtimebeforetheorganiccarbonatefactoryisfullyestablished(Kemp tionshipthatisindicativeoftransgression(Lokieretal.,2013).Should andSadler,2014).Inthelow-anglecarbonaterampsetting,thetrans- themicrobialmatsberapidlyburied,increasingthelikelihoodofpreser- portofsedimentswill becontrolledbywavesandtides.Increased vation,thentheymayactasanaquitardpreventingdissolutionofthe water depth will result in greater waveenergy reaching the shore underlyingevaporitefaciesandallowingtheirretentionintotheburial therebyincreasingshorelineerosion(Woodroffe,2008)andsediment regime.Wherethesurfacemicrobialmatbeltisdestroyed,eitherduring reworking; likewise, tidal energy may locally increase or decrease higherenergyeventssuchasstormsorthroughtheheadwarderosion (CattaneoandSteel,2003). oftidalchannels,theunderlyingunlithifiedsedimentisexposedand The sedimentary column in the shallow sub-surface of the Abu issusceptibletoerosion. Dhabicoastlinecontainsasignificantproportionofevaporiteminerals Bioturbationislimitedtotheintertidalandsubtidalfaciesthatlie (LokierandSteuber,2008)thatwillbehighlysusceptibletoinsitudis- seawardofthemicrobialmatbelt.Duringflooding,thesurfacemicrobi- solutionduringmarineflooding.Wherethesesedimentsaremechani- almatbecomesaccessibletoarangeofgrazingandburrowingfauna callyeroded,theirdissolutionwillbeevenmorerapidandtheywill (LokierandSteuber,2008).However,theretrogradingfaciesrapidly notcontributetothesedimentsupply.Thelossoftheseevaporitemin- burytheorganic-richmicrobialmats,resultingintheonsetofanoxic eralswillfurtherlimitthesupplyofreworkedsedimentduringthe conditionsintheshallowsub-surfaceandaconsequentinhibitionof floodingphase. 72 S.W.Lokieretal./Geomorphology304(2018)64–73 800 700 600 m) on (500 P8.5 ssi400 RC e r g ns300 Tra R C P 2.6 200 100 0 2000 2020 2040 2060 2080 2100 Year Fig.7.CalculatedrangesoftransgressionofthenaturalAbuDhabicoastlineforthe21stcenturyforIPCCRCP2.6andRCP8.5scenarios. 5.4.Environmentalandsocietalimplications andenhancingshorelineretreat.Thelow-anglecarbonaterampsetting supportsanon-accretionarytransgressiveshorelinetrajectorywithlit- ThecalculatedfiguresfortransgressionoftheAbuDhabicoastline, tlelikelihoodofawell-developedravinementsurfaceoroftransgres- eventhoseassumingpassiveflooding,havesignificantenvironmental sivelagdeposits.Theonsetoftransgressioninsuchacontextmaybe andsocietalimplications.Naturalecosystemsareunlikelytorespond difficulttorecogniseinthestratigraphicrecord.Identificationwould positivelytosuchrapidinundation,itislikelythatmangalsandmicro- beaidedbytherecognitionofback-steppingofmarinefacies,theburial bialmatswillbemostaffected.Increasingenergyattheshorelinewill ofmicrobialmatsandtheonsetofbioturbationbymarineorganisms. furthererodemicrobialmatswhileincreasedinundationwillinhibit EmployingIPCCRCPscenariosforfuturesea-levelriseandtheangle recolonisation.Floodingwilldrownmangroves,withsedimentsmoth- ofslopeofthecoastalsystem,calculatedlikelytotalpassivefloodingof ering of their aerial pneumatophores resulting in the death of the theAbuDhabicoastlinerangesbetween340mand571mbytheyear trees(Ellison,1999)andthelossofmangals.Thelossofthemangal 2100,atarateofbetween3.40myr−1and8.64myr−1.Thesevalues andmicrobialmatcommunitieswillhavesignificantimpactsonthe increasesignificantlyifitisassumedthatfuturefloodingwillcontinue protectionandstabilisationofcoastalsediments,enhancingerosion tobedynamic,atarateakintothatobservedduringthedurationof andfurtherpromotingshorelineretreat. thecurrentstudy.Underthisscenario,calculatedlikelytotaldynamic Thesocietalimplicationsofsea-levelriseareenormous.Globally,in floodingby2100wouldbebetween2.26kmand3.81km.Allofthese excessof2billionpeople(37%ofglobalpopulation)livewithin100km figures,both passive anddynamic, imply a significantreduction in ofacoastline(Cohenetal.,1997)withover145millionpeopleliving landareawithsubstantialenvironmentalandsocietalconsequences. within1mofpresentdaymeansealevel(Anthoffetal.,2006).Inthe UnitedArabEmiratesc.5millionpeople(85.5%ofthepopulation) Acknowledgments resideinthelow-lyingcitiesoftheGulfcoastline(DESA,2016).Man- agementofthecoastalsystemrequirespredictiveratesoftransgression TheauthorsaregratefultoJapanOilDevelopmentCompanyLtd. andgeomorphicchange(Pethick,2001;BrunelandSabatier,2009). 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Implications of sea-level rise in a modern carbonate ramp setting PDF

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by  Stephen W. Lokier
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Preview Implications of sea-level rise in a modern carbonate ramp setting

Geomorphology304(2018)64–73 ContentslistsavailableatScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph Implications of sea-level rise in a modern carbonate ramp setting StephenW.Lokiera,⁎,WesleyM.Courta,TakumiOnumab,AndreasPaula aGeosciencesDepartment,KhalifaUniversityofScienceandTechnology,PetroleumInstitute,P.O.Box2533,AbuDhabi,UnitedArabEmirates bAppliedGeoscienceDepartment,JGI,Inc.,Tokyo,Japan a r t i c l e i n f o a b s t r a c t Articlehistory: Thisstudyaddressesagapinourunderstandingoftheeffectsofsea-levelriseonthesedimentarysystemsand Received10April2017 morphologicaldevelopmentofrecentandancientcarbonaterampsettings.Manyancientcarbonatesequences Receivedinrevisedform13December2017 areinterpretedashavingbeendepositedincarbonaterampsettings.Thesesettingsarepoorly-representedin Accepted16December2017 theRecent.Thestudydocumentsthepresent-daytransgressivefloodingoftheAbuDhabicoastlineatthesouthern Availableonline28December2017 shorelineoftheArabian/PersianGulf,acarbonaterampdepositionalsystemthatiswidelyemployedasaRecent analoguefornumerousancientcarbonatesystems.Fourteenyearsoffield-basedobservationsareintegrated Keywords: withhistoricalandrecenthigh-resolutionsatelliteimageryinordertodocumentandassesstheonsetofflooding. Sealevel Transgression Predictedratesoftransgression(i.e.landwardmovementoftheshoreline)of2.5myr−1(±0.2myr−1)basedon Carbonateramp globalsea-levelrisealonewerefarexceededbythefloodingratecalculatedfromtheback-steppingofcoastalfea- Coastalerosion tures(10–29myr−1).Thisdiscrepancyresultsfromthedynamicnatureofthefloodingwithincreasedwater PersianGulf depthexposingthecoastlinetoincreasederosionand,thereby,enhancingback-stepping.Anon-accretionary transgressiveshorelinetrajectoryresultsfromrelativelyrapidsea-levelrisecoupledwithalow-anglerampgeom- etryandapaucityofsediments.Thefloodingisrepresentedbythelandwardmigrationoffaciesbelts,arangeof erosivefeaturesandtheonsetofbioturbation.EmployingIntergovernmentalPanelonClimateChange(Church etal.,2013)predictionsfor21stcenturysea-levelrise,andallowingforthepost-floodinglagtimethatistypical forthestart-upofcarbonatefactories,itiscalculatedthatthecoastlinewillcontinuetoretrogradefortheforesee- ablefuture.Totalpassiveflooding(withoutconsideringfeedbackinthemodificationoftheshoreline)bytheyear 2100iscalculatedtolikelybebetween340and571mwithafloodingrateof3.40–8.64myr−1.However,adopting theobservationthatglobalsea-levelriseonlyaccountsfor15%oftherecordedshorelineretreat,thisfigurerises dramaticallytoatotallikelydynamicflooding(consideringmodificationstotheshoreline)ofbetween2.3and 3.8km.Lossofmicrobialandmangalhabitatswillsubjecttheexposedshorelinetoincreasingerosion.Shoreline retreatwillthreatenexistingcoastalinfrastructure. ©2017ElsevierB.V.Allrightsreserved. 1.Introduction fortheAbuDhabicoastlineduringtheLateHoloceneregressionand stillstandiswell-documented(Evansetal.,1969;KinsmanandPark, Thepasttwocenturieshavewitnessedariseinglobalsea-levelat 1976; Lokier and Steuber, 2008; Stewart et al., 2011; Lokier et al., anacceleratingrate,with3.2mmyr−1observedfrominstrumentalre- 2015).Yetonlyasmallnumberofpreviousstudieshaveconjectured cordsoverthepasttwodecades(ChurchandWhite,2011;Meyssignac theeffectsofaeustaticsea-levelrise,withthecreationofnewaccom- andCazenave,2012;Churchetal.,2013).Thisindustrial-erasignature modationthatonlyrecentlyoverwhelmedsedimentproductionand of global sea-level rise has been documented in a range of coastal thusinitiatedtheonsetofatransgressivephase(EvansandKirkham, systems(Singh,1997;Leathermanetal.,2000;RankeyandMorgan, 2002;Lokieretal.,2015). 2002;Woodroffe,2008;FordandKench,2015;Romineetal.,2016; Theprimaryobjectiveofthisstudyistoutilisefourteenyearsof Testutetal.,2016).However,todate,therehavenotbeenanystudies field-basedobservationsalongwithhistoricalandrecentsatelliteimag- oftheeffectsofthistransgressiononsedimentarysystemsandshore- erytodocumentandassesstheprocessoftransgressioninanaridcarbon- linemorphology in a carbonate ramp setting. The coastline of Abu ateandevaporitecoastaldepositionalsystemthatismorphologically Dhabi,lyingatthesouthernshoreofthePersian/ArabianGulf,hereafter analogous to numerous ancient carbonate ramp systems. The study referredtoastheGulf,isanidealsettingforstudyingtheprocessof alsoaimstoestablishthediagnosticsedimentaryfeaturesthatcanbe transgressioninsuchasetting.Theoverallprogradationalgeometry employedtoassistintheidentificationoftransgressivesurfacesinancient sedimentary sequences throughout the stratigraphic record. These ⁎ Correspondingauthor. outcomesprovidearesourcewithwhichtointerpretthesequencestrat- E-mailaddresses:[email protected](S.W.Lokier),[email protected](T.Onuma). igraphiccontextinancientcarbonaterampsettingsand,conversely,to https://doi.org/10.1016/j.geomorph.2017.12.023 0169-555X/©2017ElsevierB.V.Allrightsreserved. S.W.Lokieretal./Geomorphology304(2018)64–73 65 considerthelikelyimplicationsofglobalsea-levelriseonfutureshoreline Temperatures measured in the study area regularly exceed 50 °C morphodynamics.Furthermore,predictionsofprobableratesofgeomor- onsummerdaysyetmaydropaslowas7°Cduringwinternights phicchangearefundamentaltoplanningfuturecoastlinemanagement (LokierandFiorini,2016).Theprevailingwindisthenorthwesterly (Pethick,2001;Romineetal.,2016). Shamal.Stormsarerareandtypicallyofrelativelyshortdurationand lowenergy. 2.Setting 3.Methods TheGulfisashallow(average35m)microtidal(1–2m)epeiricsea lyingatthenorthernmarginoftheArabianPlate(Fig.1).Thefloorofthe This study utilised and integrated multiple datasets including almost completely land-locked basin dips gently north-eastward satellite imagery, shoreline transect surveys and long-term repeat towardsthecoastofIran.Thesedimentsoftheseaflooraredominated field observations along a 16 km section of coastline containing a by muddy to fine-grained carbonates (Evans, 1966; Uchupi et al., rangeofsettingsrepresentativeoftheGulfcoastalsystemoftheUAE 1996)andassociatedhardgrounds(Shinn,1969).Thereisageneral (Fig.1).ThestudyemployedQuickbirddatawithaspatialresolution increaseinsiliciclasticcontenttowardstheIranianshoreline(Emery, of 0.6 m/pixel for the Panchromatic Band, acquired in 2003, 2004, 1956). 2005and2007,andWorldView-2datawitha0.5m/pixelresolution TheAbuDhabicoastlineoftheGulfcomprisesanorthward-dipping, for the same band acquired in 2010. Multi-band data covering the low-angle, carbonate ramp characterised by the transition from a visible-nearinfraredregionwithaspatialresolutionof2.4m/pixels supratidalsabkha,throughabroadcarbonate-dominatedintertidal and2.0m/pixel,forQuickbirdandWorldView-2respectively,were environment,intoasubtidalcarbonatesetting(Evansetal.,1964). alsousedalongwithPAN-sharpenedcolourimageswith0.6m/pixel Thecoastislocallyisolatedfromopen-marineconditionsbyacomplex forQuickbirdand0.5m/pixelforWorldView-2data.Multi-banddata ofpeninsulas,shoalsandislandsthatformedasHolocenesedimentsac- wereconvertedtospacereflectanceusingparametersprovidedbythe cretedaroundPleistocenelimestoneassociatedwiththeunderlyingGreat TechnicalNoteofDigitalGlobe,andNormalizedVegetationIndexim- PearlBank(PurserandEvans,1973;Harris,1994).ThesouthernGulf ageswereproducedinordertotracethedistributionchangeofonshore shorelinehasbeentectonicallystablethroughoutthelateQuaternary vegetationaswellasmicrobialmatsintheintertidalzone.Theresultant (Stevensetal.,2014). images were examined and compared in order to trace temporal TheemirateofAbuDhabiexperiencesanaridclimatewithamean morphologicalchangesalongthecoastline. annualrainfallof72mm,concentratedduringFebruaryandMarchas Field-basedobservationshavebeenregularlymadeintheAbuDhabi localised,brief,torrentialrainstorms(Raafat,2007).Meanannualevap- sabkhasincetheyear2000.Thoughnotinitiallyfocusedonthisspecific orationof2.75mexceedsrainfallbyalmost4000%(Bottomley,1996). study, the earlier field observations (prior to 2007) provided an Iraq Kuwait Persian/Arabian Iran Gulf Abu Dhabi Island Bahrain Persian/Arabian Qatar Gulf GOumlfa onf Saudi Arabia United Arab Oman Emirates Khawr Jazirat Abu Qirqishan Kushayshah Al Dabb’iya Abu Al Abyad K h a wr Q a ntur N 10 km Fig.1.SatelliteimageoftheAbuDhabicoastlinebetweenAbuDhabiIslandintheeastandAbualAbyadIslandinthewest,seeinsetmapforregionalcontext.Thelocationofthe16km-long studyareaishighlightedbythewhitebox,theblackarrowindicatesthepositionoftheislandshowninFig.2,thewhitearrowindicatesthelocationofFigs.4and6.LandsatETMimage acquired3rdJuly2002. 66 S.W.Lokieretal./Geomorphology304(2018)64–73 invaluableresourceforselectinglocationsformorerobust,long-term 4.Evidenceofshorelinechange monitoring.Observationswerealsoregularlyundertakenoutsideof thefocusareasinordertoensurethatthewidestpossiblegamutof Shorelinesarehighlydynamicfeaturesinanear-continuousstateof datawasincorporatedintothestudy.Fieldobservationsalsoincluded morphologicalfluxatarangeofspatialandtemporalscales(Woodroffe, surveyingtwotransectsnormaltotheshorelineinordertomeasure 2008;Rankey,2011).ThedGPSsurveyinthisstudyestablishedthat theheightandslopeofthecarbonaterampcoastalsystem.Thesurvey the upper intertidal to supratidal portion of the ramp system dips wasundertakenusinganAltusAPS-3differentialglobalpositioningsys- very gently seawards at an angle of between 0.07° and 0.08°. By tem(dGPS).Ateachmeasurementposition,atleast10measurements employing the Intergovernmental Panel on Climate Change (IPCC) were made and the average of these was used to geolocated the globalmeansealevelriserateof3.2mmyr−1fortheperiodbetween position. 1993and2010(Churchetal.,2013),anaveragetransgressionrateof Aparticularchallengeresultingfromthelow-anglegeometryof 2.5±0.2myr−1canbepredicted.Itcanbeanticipatedthatsucha theAbuDhabicoastalsystemisdemarkingadefinitive‘coastline’. significantrateoftransgressionwouldresultinsubstantialcoastline In this carbonate ramp system the horizontal distance between change.Fieldandremotesensingobservationsofarangeoferosional mean high water and mean low water may locally exceed 4 km, andconstructionalfeaturesfromtheAbuDhabicoastlinearedocumented withahighdegreeofvariabilityresultingfromeventhemildest herein. windsoratmosphericpressurevariations.Intheabsenceofadefin- Anumberofmorphologicalfeaturesweremonitoredinthevicinity itivecoastline,weemploythemicrobialmatbeltasadatum.The ofalow-lyingvegetatedislandinthewestofthestudyarea(Fig.2). landwardandseawardlimitsofmicrobialmatbeltsarestronglycon- Unfortunately,dredgingandlandreclamationactivitydestroyedthis trolledbytheirpositionintheintertidalzone(LokierandSteuber, areain2011therebyabruptlycurtailingmonitoring.Priortothisloss,a 2008).Theseawardextentofthebeltisconstrainedbyabalancebe- seriesofspitsdevelopedatthewesternshoreoftheislandandmigrated tweenmicrobialmatgrowthrateandthepresenceofgrazingorgan- landwards over a monitoring period ofseven years. The spits were isms-particularlygastropods.Themat'slandwardlimitisgoverned composedofcoarsesandtograveldominatedbywholeandfragmented bythelimitsofflooding;microbialmatsmustberegularlyinundated gastropods with subordinate bivalves and other skeletal allochems. toavoiddesiccation. Thesebodiesmigratedtowardsthesoutheastoverapustularmicrobial 5.4.2003 18.6.2004 4.7.2005 A B C B C2 B M S S C S N 100 m 13.2.2007 8.10.2010 D E B B Be S2 S S Fig.2.Seriesofsatelliteimagesovera7-yearperiodshowinggeomorphologicalchangesassociatedwithaspitinthewestofthestudyarea(refertoFig.1forthelocationoftheimages). A)Satelliteimageshowingaseriesofspitslandward(south)ofalow-reliefisland.B)Spitsretrogradeoverthemicrobialmat(S).Inthenorthoftheimagesandbars(B)migratesouth towardsastandofmangroves(M).Acrevassesplaydevelopsinthesouthoftheimage(C).C)Spitsandsandbarscontinuetoretrogradetowardsthesouth.Thespitisbreachedwithanew crevassesplaydevelopingwithaneasterlyorientation(C2).D)Continuedsouthernspitmigration.Sandbarsbegintoburymangrovesandisolatethemfrommarinecirculation.Significant buildingofthebeachonthewesternmarginofthespit(Be).E)Spitscontinuetoretrogradetowardsthesouth-totalmigrationof218min7yearsand7months.Mangrovestandtotally destroyedbymigratingsandbars.Newspitsystemdeveloping(S2).RefertoFig.1forthelocationwithinthestudyarea.AllimagesfromQuickbird2(0.6m/pixelspatialresolutionforthe PanchromaticBand)exceptthoseof10/10/10whichareWorldView-2(0.5m/pixelresolution).Infraredisdisplayedasred,redasgreenandgreenasblue. S.W.Lokieretal./Geomorphology304(2018)64–73 67 mat(Courtetal.,2017)underlainbyananoxic,organic-richmudto Stormeventsresultinlocalisedbreachingofbeachridgestoproduce fine-sandgradesediment.Themigrationwasmonitoredbetween crevassesplayscomprisingcoarse-grainedcarbonatesands(Figs.2C, April2003andAugust2010usingacombinationoffieldobserva- 3C). Onceemplaced, thesesands are not susceptible to reworking, tions(Fig.3A)andremotesensingimagery(Fig.2).Overthisseven thereby contributing to the aggradation of the island surface. andahalfyearperiod,thespitmigrated218mlandwardoverthe Retrogradingsandbodiesalsohaveasignificantimpactonthedevelop- microbialmats,equatingtoanaveragerateof29myr−1.Similar mentofmangals(Fig.3D–F).Wheretheaerialroots(pneumatophores) back-steppinghasbeenobservedinbeachridgeselsewherealong ofmangrovesareburiedbymigratingsands,themangrovetreesdie thecoast(Fig.3B). andbecomevulnerabletosubsequenterosion(Ellison,1999). A Landwards 8/10/2007 27/12/2007 28/2/2008 15/5/2008 30/12/2008 2 m B C 30 cm D E F 13/2/2007 50 m 8/10/2008 50 m 40 cm G H 20 cm Fig.3.FieldphotographsofarangeoferosionalandmigrationalfeaturesobservedattheAbuDhabicoastline.A)Observationsofaspitmigrating17.5mlandwards(southeast)overthe microbialmatsoveraperiodof15months.Arrowsdenotetheobservedlocationofthetipofthespitatthespecifieddates.B)Beachridgeback-stepping(directionofwhitearrow)towards thesouthoveramicrobialsurfacewithfootprints(blackarrows).C)Breachingofabeach-ridgewithassociatedcrevassesplay.Blackarrowsindicatethelimitsofthebreach,blackdashed linedenoteslimitofwashover(personforscaleis1.75mtall).D&E)PairedQuickbird2(left)andWorldview2(right)satelliteimagesshowingtheinundationandlossofmangroves (whitearrow)byretrogradingsandbodiesoveraperiodof20months.F)FieldphotographofthemangroveskilledthroughburialbyretrogradingsandbodiesasindicatedinpanelsD&E. Viewtowardsthenorth.G)HardgrounderosionontheleewardsideofthepeninsulashowninFig.1(bendingpersonforscaleis1.15mhigh,viewtowardsthenorth).H)Erosionof microbially-boundlayersofmuddysandsintheupperintertidalzone,whitearrowsindicatedistinctmicrobialsurfaces,landwards(south)totopofimage. 68 S.W.Lokieretal./Geomorphology304(2018)64–73 Intheintertidalzonetotheseawardandeastoftheislandthereis changesintheamountanddurationoftidalfloodingandanychanges significantexposureanderosionofthehardgrounds(Fig.3G).During inenergywithinthedepositionalsystem. rarehigherenergystormevents,clastsfromthehardgroundarecarried Thearealextentofthemicrobialmatbeltcanbeclearlydefinedfrom landwardontothesurfaceofthesabkhawheretheyareincorporatedas satelliteimagery(Fig.4).Inthefield,itispossibletosub-dividethemi- aminorcomponentintobeachridgesormoremuddyfaciesasangular crobialmatbeltintoanumberofmorphologicalsub-zones(Courtetal., extraclasts.Erosionisalsoobservedatthelandwardlimitoftheinter- 2017).Nobioturbationisobservedwithinthemicrobialmatbeltorin tidalzone.Firmgrounds,formedthroughthebindingofsedimentsby facieslandwardofthemicrobialmats.Duringtheperiodofobservation, interstitialmicrobialgrowth,arebeingeroded(Fig.3H).Fragmentsof avarietyofchangeshavebeenobservedinrelationtothemicrobialmat thefirmgroundareeasilyabradedtoformsub-roundedintraclastsor, belt. iftransportedmoredistally,potentiallyextraclasts. Satelliteandfieldobservationsshowthatboththeseawardand Within the middle to lower intertidal zone, tidal creeks are landward margins of the microbial mats are migrating landwards retrogradingthroughheadwarderosionoftheunderlyinghardground (Fig.4).Theseawardmarginofthematsisbeingburiedbeneaththe facies.Retrogradinglobesoftheintertidalbioclasticandpeloidalsands back-stepping bioclastic and peloidal sands of the intertidal zone locallydivertthemost-landwardportionoftheintertidalcreeks;this (Fig.5A–E).BetweenFebruary2007andSeptember2010theseaward processsometimesdivertsebbtidalflowtoadifferentcreekcatchment marginofthemicrobialmatslocallyretrogradedbybetween60and resultinginabandonmentoftheoldcreeksystem(Fig.4). 100m,equivalenttoarateof16–27myr−1.Theretrogradingbioclastic andpeloidalsandsinitiallycoverthematsurfacewithathinsediment veneer(Fig.5A)thatresultsinamorphologicalchangeinthegrowth 4.1.Migrationoffaciesbelts ofthemats.Low-lyingareasofthematandgapsbetweenpolygons (Fig.5B)arepreferentiallycoveredbysedimenttherebyreducingthe Asmentionedpreviously,theextentofthemicrobialmatbeltis small-scalereliefofthematsurface.Withinweeks,thissedimentis stronglycontrolledbyitspositionwithintheupperpartoftheintertidal colonised and stabilised by interstitial microbial growth (Fig. 5C). zone. The community composition, morphological characteristics Topographicallyhigherareasofthemicrobialmats,suchasmatmargins andresilienceofthemicrobialmatsareparticularlysusceptibletoany continuetogrowbutareincreasinglysusceptibletograzingbybenthic A B C IT IT MM MM IT MM MM P ST P ST ST Road 555...444...222000000333 200 m N 18.6.2004 4.4.2005 D E b a 13.2.2007 8.10.2010 Fig.4.Seriesofsatelliteimagesovera7-yearperiodshowinggeomorphologicalchangesassociatedwiththemicrobialmatbelt,refertoFig.1forthelocationoftheimages.A)Satellite imageshowingthesupratidal(ST),microbialmatbelt(MM)andintertidal(IT)environmentsoftheAbuDhabiSabkha.B)Unusuallyhightidesresultinfloodingandpondingofwaterin thesupratidalzone(P).C)Increasedmicrobialmatdevelopmentinthenorth-westernfloodedarea.Retrogradationofthelandwardsmarginofthemicrobialmatbeltinmuchofthearea (arrow).D)Imagewithfloodingoftidalcreeks,intertidalbioclasticandpeloidalsandfaciesretrogradeoverthemicrobialmatbelt(arrows).E)Intertidalsandshaveback-steppedovera distanceof60mat‘a’and100mat‘b’.Thelandwardmarginofthemicrobialmatshasmigratedlandwardsbyonly35moverthesameperiod.AllimagesfromQuickbird2(0.6m/pixel spatialresolutionforthePanchromaticBand)exceptthoseof8/10/10whichareWorldView-2(0.5m/pixelresolution).Infraredisdisplayedasred,redasgreenandgreenasblue. S.W.Lokieretal./Geomorphology304(2018)64–73 69 A B 1 m 50 cm C D M B F G P 10 cm 10 cm E F R 10 cm 50 cm Fig.5.Fieldphotographsoftransgressivefeaturesassociatedwiththemicrobialmatbelt.A)Viewseawardsshowingthebacksteppingofthelowerintertidalbioclasticandpeloidalsand faciesovermicrobialmatswithapoorly-definedpolygonalmorphology.Theretrogradationalfrontisindicatedbythewhitearrow.Smallstandsofmangrovescanbeseeninthedistance (blackarrow).B)Well-developedleatherypolygonalmicrobialmats(foreground)beingburiedbythebacksteppingbioclasticandpeloidalsandfacies(blackarrow)–viewtowardsthe north(seawards).Notehowthesandsareinfillingareasbetweentheupliftedpolygonmargins(whitearrow),therebyeffectively‘smoothing’thetopography.C)Bioclasticandpeloidal sandfacies(leftofimage,seawards)retrogradingoverspongymicrobialmat(rightofimage,landwards).Thesandsarebeingstabilisedbythedevelopmentofathininterstitialmicrobial growthwithinthesedimentsurface(blackarrow).D)Followingburialbeneaththebioclasticandpeloidalsandfacies,themicrobialmats(M)aresusceptibletodegradation.Surface feedingbygastropods(G)andcrabsproducespeloids(P)andfeedingpellets(F)respectively.InfaunalbioturbationislimitedtodwellingburrowsofScopimeracrabicauda(B).Note thatalloftheburrowingandfeedingisconcentratedinthetopographicallylowareaswherethesemicrobialmatsareabsent.E)Crosssectionshowingthelaminatedbioclasticand peloidalsandfacies(arrow)retrogradingoverthespongymicrobialmat(blackarrow)-thedashedwhitelinedemarksthebaseandfrontofthetransgressingunit.Thesurfaceofthe sandsiscolonisedbyaninterstitialpalegreenmicrobialcommunity.Thedarkgreycolouroftheunderlyingsedimentsreflectstheanoxicconditionsinthesub-surfacewhere microbialmatshavedecayedwithonlyisolatedrelicsremaining(whitearrow).F)Areasofrippeduppustularmat–viewtowardsthesouth(landwards).Areasofmataredislodged duringinundationbyhigher-energyevents,suchasstorms,orthroughsimplefloatingwhengasistrappedbelowtheimpermeablemat.Thedisplacedmaterialmaybeoverturned (blackarrow),piledup(whitearrow)orformelongateroles(R). faunathatresidewithinthenow-adjacentsands.Withincreasingburial, Thelandwardmarginofthemicrobialmatsisalsoback-stepping andovertime,themicrobialmatsbecomesusceptibletodegradation. throughoutthestudyarea,though,atarateofonly10myr−1thisis Increasedfloodingofthemicrobialmats,andthepresenceofhostsed- significantly slower than the retrogradation of the seaward margin iment,facilitatesthepresenceofincreasednumbersofgrazingfauna overthesameobservationperiod(Fig.4).Thedisparityintheretrogra- dominatedbygastropods,ostracodsandcrabs(Fig.5D).Thepresence dationratesbetweentheseawardandlandwardmarginsofthemicrobi- ofasignificantorganiccomponentwithinthesedimentresultsinthe almatbelthasresultedinthebeltbecomingsignificantlynarroweras onsetofanoxicconditionsimmediatelybelowthesedimentsurface canbeclearlyvisualisedfromNormalizedDifferenceVegetationIndex (Fig.5E).Burrowingisabsentfromthemicrobialmatsandsupratidal images(Fig.6).Throughoutthestudyareathelandwardmarginofthe facies.islimitedtosimpleshallowdwellingburrowsconstructedby microbialmatbeltischaracterisedbyathinpustularmatthatisdevelop- crabs(Scopimeracrabicauda).Burrowsareabsent. ingaboveamixedcarbonate-evaporitesequencewithabundantsandto 70 S.W.Lokieretal./Geomorphology304(2018)64–73 4.7.2005 50 m 8.10.2010 50 m Fig.6.PairedNormalizedDifferenceVegetationIndeximagesfrom4/7/2005(Quickbird2)and8/10/2010(Worldview2)showingthattheseawardlimitofthemicrobialmatbelthas movedsignificantlylandwardwhilethelandwardlimithasretrogradedtoalesserextentoverthe5yearsofobservation.Thedashedwhitelineshowstheformerpositionofthe seawardmarginofthemicrobialmat.Distributionofthevegetation,includingmicrobialmats,ishighlightedbythegreentoredcolours.Thetrackwaysseenintheimages(white arrow)arearesultofseismictruckssurveyingtheareaduringthe20thCentury.ThelocationoftheimageisindicatedbythewhitearrowinFig.1. gravelgradegypsum.Thegypsumwasformedthroughtheevaporation retreat,afigurethatisverysimilartothe12%notedfromtheopen ofgroundwaterintheshallowsub-surfaceofthesupratidalsabkha beachesoftheMediterraneancoastofFrance(BrunelandSabatier, (LokierandSteuber,2008). 2009).Thislargediscrepancyarisesfromtheassumptionthatma- Duringfieldwork,largeareasofthemicrobialmatwereobservedto rinetransgressionisapassivesubmersionprocesswithoutmodifica- episodicallyberemovedeitherduringrarehigher-energystormevents tionoftheshoreline.Suchamodelisover-simplifiedbecauseduring ormoregentlythroughraftingresultingwhentrappedgasbelowthe transgressioncoastallandformsareforcedtomigrateinorderto impermeablematsurfacebuoys-upthematduringperiodsofexcessive maintaintheirspecificpositioninrelationtothecoastalenergygra- flooding.Suchdenudationexposestheunderlyingsediment(Fig.5F) dient(Pethick,2001).Itisinferredthatthismodelofdynamicactive either to renewed microbial colonisation or, likely in the case of submersion(BrunelandSabatier,2009)willexposethecoastlineto storm-events,toerosiontoformlocaliseddepressionsthatmaydevelop increasederosion(Leathermanetal.,2000;Woodroffe,2008)and intointertidalponds(Pauletal.,inprep.). resultinshorelinesretreatingatarategreaterthanthatpredicted forsea-levelrisealone.Further,thecarbonatefactoryintheseshal- 5.Discussion lowtointertidalenvironmentsdoesnotproducesufficientsediment tofilltheaccommodationspacenewlycreatedbyrisingsealevel;a Theremotesensingandfieldobservationsdocumentedinthisstudy situationthatiscompoundedbythelackoffluvialsystemsandthe recordashorelinethathasenteredaphaseoferosionandretrograda- prevailingonshore,Shamal,windbothofwhichpreventtheinflux tionconsistentwiththeonsetofmarinetransgression.Thisprovides ofdetritalsedimentfromtheland. anexceptionalopportunitytoestablishthediagnosticsedimentary Itiswidelyacceptedthatthemuch-employedBrunn(1962)model featuresthatwouldprovideevidenceoftransgressionofacarbonate forshorelineretreatisover-simplified,neglectingsite-specificgeomor- rampsystemwithinthegeologicalrecord.Employingtheaxiomthat phologicalandsedimentologicalfactors(CooperandPilkey,2004).The improvedknowledgeofthepastandpresentarekeytomoreaccurate modelisparticularlyinadequateinthevicinityofbarrierislands,capes predictionsofthefuture,weconjecturethelikelyfutureeffectsofshore- and spits (Leatherman et al., 2000). Local-scale variations in the lineretreatandimplicationsthereof. geomorphologicalresponseofthecoastlinetosea-levelchangeare common (Webb and Kench, 2010; Ford and Kench, 2015; Romine 5.1.Calculatedtransgressioncomparedwithmeasuredretrogradation etal.,2016),withadjacentportionsofthecoastlinevariouslydisplaying erosion,accretionornosignificantchange.Thenatureoftheresponseis UnderthetectonicallystableconditionsofthesouthernGulfshore- controlledbytheantecedenttopography(Romineetal.,2016),changes line(Stevensetal.,2014;Lokieretal.,2015;PfefferandAllemand, inboundaryconditions(WebbandKench,2010)andsedimentsupply 2016),relativesealevelcanbeconsideredasequivalenttoeustatic (Carrascoetal.,2016;Testutetal.,2016). sealeveloverthedurationofthestudy.Thepredictedaveragerateof flooding for the Abu Dhabi coastline of 2.5 ± 0.2 m yr−1 is much 5.2.Recognisingtransgressioninthestratigraphicrecord lowerthananyoftheobservedratesofcoastalfeatureretrograda- tion,i.e.10–29myr−1.Consequently,eustaticsea-levelriseaccounts Canthepresent-dayfloodingattheAbuDhabicoastlinebeconsidered for only an average of 15% (8.6–25%) of the observed shoreline as representative of a transgressive event in the true sequence S.W.Lokieretal./Geomorphology304(2018)64–73 71 stratigraphiccontext?Transgressionmaybedefinedasalandwardshift infaunal detrital sedimentfeeders. Burrows are limited to shallow, ofthemarinesysteminitiatedwherebase-levelrisesataratehigher open,dwellingburrowsandbioturbationisunlikelytobesointensive thantheshorelinesedimentationrate(Catuneanuetal.,2009).Thetrans- astodestroyprimarysedimentarystructures.Undertheseconditions, gressivepartofthesequenceliesimmediatelyonthemaximumregres- theonsetofbioturbationlocallyrecordstheonsetoftransgression. sivesurface(transgressivesurface)ofthepreviousregressivephase Locally,microbialmatsarerapidly(hourstoweeks)buriedbeneath (Catuneanuetal.,2011).Presentedwiththesedefinitions,weconclude relatively-thickcoarsegrainedcarbonatesands,associatedwiththeret- thattheobservationsmadeduringthisstudysupporttheinterpretation rogradationofspitsorwashoverdeposits.Underthesecircumstances thattheAbuDhabicoastlinehasenteredaphaseoftransgressionrecord- theburiedmicrobialmatwillbeisolatedfrombioturbationresulting ingthetransitionfromaprogradationaltoaretrogradationalgeometryof inasedimentarysequencecharacterisedbycoarse-grainedsediments thesedimentarybodies. directlyoverlyingfinergrainedfaciesdisplayingamicrobiallamination Thearchitectureoftransgressivedepositsisaproductoftherateof andlackingfaunalbioturbation. sedimentsupply,therateofsea-levelchange,thegeomorphologyof theshorelinesystemandtheenergyregime–asingletransgressive transectcomprisessectorsrecordingdepositionandsectorsoferosion 5.3.Projectionsof21stcenturytransgression (Cattaneo and Steel, 2003). The relatively high rate of relative sea-levelrise,low-anglegeometryandlimitedsedimentsupplyatthe BasedondifferentRepresentativeConcentrationPathways(RCP) AbuDhabicoastlinesupportanon-accretionarytransgressiveshoreline scenarios, the IPCC 5th Assessment Report predicts a likely global trajectory(sensuHelland-HansenandGjelberg,1994).Underthese meansea-levelriseofbetween0.44(0.28–0.61)m(RCP2.6)and0.74 conditions,arapidlandwardshiftintheshorelinewouldbeexpected (0.52–0.98)m(RCP8.5)bytheyear2100(Churchetal.,2013).Given toinitiallyproduceonlythintransgressivedepositswithinsitudrown- thatthereiscurrentlynosignificantverticallandmotion(subsidence ingoftheshoreface(CattaneoandSteel,2003).Thisisparticularly or tectonic uplift) at the southern shore of the Gulf (Pfeffer and thecaseincarbonate-dominatedsystemswherealagtimeofafew Allemand, 2016), these global mean sea-level rise figures can be hundredtoafewthousandyearsiscommonbeforethecarbonatefactory employedtopredicttherangeofthedegreeoflikelyfloodingofthe isfullyfunctioningagain(Tipper,1997;KempandSadler,2014). naturalAbuDhabicoastline. Thepresenceofcohesive,microbially-bound,fine-grainedsediment Basedontheangleofslopeandtheamountofprojectedsealevel furtherinhibitstheformationofatidalravinementsurface,except riseitiscalculatedthatby2100theamountoftotalpassivefloodingis where headward erosion occurs at the thalweg of tidal channels. tobelikelybetween340(216–471)m(RCP2.6)and571(401–756)m Headwarderosionoftidalcreekshasbeendocumentedfromotherset- (RCP8.5)(Fig.7).TherateoffloodingattheAbuDhabicoastlineiscal- tings(Knightonetal.,1991;RankeyandMorgan,2002).Duringhigh culatedtolieintherangebetween3.40(1.54–5.25)myr−1(RCP2.6) tides,seawaterincursion is concentratedwithin slighttopographic and8.64(5.79–12.12)myr−1(RCP8.5)bytheyear2100.Thiswould depressions,thecentreofwhichbecomesincreasinglysusceptibleto resultinasignificantreductioninlandareainnaturalcoastalsystems. tidalscouringandsubsequentdeepening(Knightonetal.,1991).This Theabovefiguresassumepassivefloodingofthecoastlinewithout processisproducinghighlylocalisedtidalravinementsurfaceswhere anydynamicresponsetosea-levelrise,asdiscussedearlier.Adopting tidalcreekserodetheunderlyinghardground. anassumptionthatsea-levelrisewillcontinuetoaccountforonly15% Transgressivelags,occurringaseitherconglomeratic,glauconitic, ofshorelineretreat,wecansignificantlyrevisetheabovefigures.The fossiliferous or geochemical deposits (Kidwell, 1989; Cattaneo and amountoftotaldynamicfloodingby2100wouldbelikelybetween Steel,2003),aretypicallyexpectedwherethereissedimentstarvation 2.26 (1.44–3.14) km (RCP2.6) and 3.81 (2.68–5.04) km (RCP8.5). of the shelf (Swift, 1976) or condensation resulting from dynamic SincemostoftheAbuDhabicoastlineisnotembankedbyacliffor bypass(Kidwell,1989).Lagdepositsarenotobservedinthecurrent scarp,thereisnotheoreticallimittotheamountoflandwardmigration study as there is a paucity of bioclasts, intraclasts or extraclasts. oftheshoreline. Althougherodedfirmgroundandhardgroundclastswereobserved, Ofcourse,theassumptionthatsea-levelrisewillcontinuetocontrib- thesewererelativelyrareandonlylocallyconcentratedin,forexample, uteonly15%ofshorelineregressionfortheremainderofthe21stcentury beachridgesorspits.Carbonatesedimentationrates,thoughlowat ishighlyconjecturalandopentosignificanterror.Localisedvariationsin 13–50cm/ky(Kinsman,1969;LokierandSteuber,2008),aresufficient sedimentavailabilityandtransportpathwaysmakepredictionshighly topreventsedimentstarvation. challenging(Romineetal.,2016).Thecreationofnewaccommodation Ithaspreviouslybeeninferredthat,withintidalflatsystems,new and onset of improved marine circulation may promote a positive accommodationisinitiallyrapidlyfilledbymicrobialmats(Strasser, responsefromthecarbonatefactory(Eberli,2013),withanincreasein 2015)untiltheyaresufficientlyinundatedtopermitgrazingorganisms carbonateproductionthatmaygosomewaytooffsettingthetransgres- todestroythem(Farmer,1992).Duringthepresentstudy,weobservea siveprocess.Conversely,assurfacewatertemperaturesriseandpH distinctnarrowingofthemicrobialmatbeltatthecoastline.Thesuper- decreasesthestressonmarineorganismswillinhibitcarbonateproduc- position of the back-stepping upper intertidal microbial mats over tion.Ashasbeendocumentedelsewhere,thereisnormallyasignificant supratidal-depositedevaporitefaciesformsadistinctstratigraphicrela- lagtimebeforetheorganiccarbonatefactoryisfullyestablished(Kemp tionshipthatisindicativeoftransgression(Lokieretal.,2013).Should andSadler,2014).Inthelow-anglecarbonaterampsetting,thetrans- themicrobialmatsberapidlyburied,increasingthelikelihoodofpreser- portofsedimentswill becontrolledbywavesandtides.Increased vation,thentheymayactasanaquitardpreventingdissolutionofthe water depth will result in greater waveenergy reaching the shore underlyingevaporitefaciesandallowingtheirretentionintotheburial therebyincreasingshorelineerosion(Woodroffe,2008)andsediment regime.Wherethesurfacemicrobialmatbeltisdestroyed,eitherduring reworking; likewise, tidal energy may locally increase or decrease higherenergyeventssuchasstormsorthroughtheheadwarderosion (CattaneoandSteel,2003). oftidalchannels,theunderlyingunlithifiedsedimentisexposedand The sedimentary column in the shallow sub-surface of the Abu issusceptibletoerosion. Dhabicoastlinecontainsasignificantproportionofevaporiteminerals Bioturbationislimitedtotheintertidalandsubtidalfaciesthatlie (LokierandSteuber,2008)thatwillbehighlysusceptibletoinsitudis- seawardofthemicrobialmatbelt.Duringflooding,thesurfacemicrobi- solutionduringmarineflooding.Wherethesesedimentsaremechani- almatbecomesaccessibletoarangeofgrazingandburrowingfauna callyeroded,theirdissolutionwillbeevenmorerapidandtheywill (LokierandSteuber,2008).However,theretrogradingfaciesrapidly notcontributetothesedimentsupply.Thelossoftheseevaporitemin- burytheorganic-richmicrobialmats,resultingintheonsetofanoxic eralswillfurtherlimitthesupplyofreworkedsedimentduringthe conditionsintheshallowsub-surfaceandaconsequentinhibitionof floodingphase. 72 S.W.Lokieretal./Geomorphology304(2018)64–73 800 700 600 m) on (500 P8.5 ssi400 RC e r g ns300 Tra R C P 2.6 200 100 0 2000 2020 2040 2060 2080 2100 Year Fig.7.CalculatedrangesoftransgressionofthenaturalAbuDhabicoastlineforthe21stcenturyforIPCCRCP2.6andRCP8.5scenarios. 5.4.Environmentalandsocietalimplications andenhancingshorelineretreat.Thelow-anglecarbonaterampsetting supportsanon-accretionarytransgressiveshorelinetrajectorywithlit- ThecalculatedfiguresfortransgressionoftheAbuDhabicoastline, tlelikelihoodofawell-developedravinementsurfaceoroftransgres- eventhoseassumingpassiveflooding,havesignificantenvironmental sivelagdeposits.Theonsetoftransgressioninsuchacontextmaybe andsocietalimplications.Naturalecosystemsareunlikelytorespond difficulttorecogniseinthestratigraphicrecord.Identificationwould positivelytosuchrapidinundation,itislikelythatmangalsandmicro- beaidedbytherecognitionofback-steppingofmarinefacies,theburial bialmatswillbemostaffected.Increasingenergyattheshorelinewill ofmicrobialmatsandtheonsetofbioturbationbymarineorganisms. furthererodemicrobialmatswhileincreasedinundationwillinhibit EmployingIPCCRCPscenariosforfuturesea-levelriseandtheangle recolonisation.Floodingwilldrownmangroves,withsedimentsmoth- ofslopeofthecoastalsystem,calculatedlikelytotalpassivefloodingof ering of their aerial pneumatophores resulting in the death of the theAbuDhabicoastlinerangesbetween340mand571mbytheyear trees(Ellison,1999)andthelossofmangals.Thelossofthemangal 2100,atarateofbetween3.40myr−1and8.64myr−1.Thesevalues andmicrobialmatcommunitieswillhavesignificantimpactsonthe increasesignificantlyifitisassumedthatfuturefloodingwillcontinue protectionandstabilisationofcoastalsediments,enhancingerosion tobedynamic,atarateakintothatobservedduringthedurationof andfurtherpromotingshorelineretreat. thecurrentstudy.Underthisscenario,calculatedlikelytotaldynamic Thesocietalimplicationsofsea-levelriseareenormous.Globally,in floodingby2100wouldbebetween2.26kmand3.81km.Allofthese excessof2billionpeople(37%ofglobalpopulation)livewithin100km figures,both passive anddynamic, imply a significantreduction in ofacoastline(Cohenetal.,1997)withover145millionpeopleliving landareawithsubstantialenvironmentalandsocietalconsequences. within1mofpresentdaymeansealevel(Anthoffetal.,2006).Inthe UnitedArabEmiratesc.5millionpeople(85.5%ofthepopulation) Acknowledgments resideinthelow-lyingcitiesoftheGulfcoastline(DESA,2016).Man- agementofthecoastalsystemrequirespredictiveratesoftransgression TheauthorsaregratefultoJapanOilDevelopmentCompanyLtd. andgeomorphicchange(Pethick,2001;BrunelandSabatier,2009). 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