Great Barrier Reef Marine Park sedimentology revealed �

issue 84 Dec 2006

GreatBarrier Reef Marine Park sedimentology revealedNew research into inter-reefal environments will assist reef managersEmma Mathews and Andrew Heap

GeoscienceAustraliahascompletedadetailedspatialanalysisofseabedenvironmentsintheGreatBarrierReef(GBR)MarinePark.Theanalysiswasbasedonanewdatasetofmorethan3000samples,whichisaccessibleonlinefromGeoscienceAustralia’smarinesamplesdatabase.

Theresults,tobepublishedlaterthisyear(Mathews&Heap2006),representthefirstregionalassessmentofpostglacialsedimentsacrosstheentireGBRsinceMaxwell’spioneeringworkin1968,andprovidethefirstquantifiedcomparisonsinaspatialframework.TheinformationcanbeusedbymanagersandplannerstomakebetterdecisionsinAustralia’shighestprofileWorldHeritageArea.

Spatial studyAquantitativespatialstudyoftheseabedsedimentscomprisinginter-reefalenvironmentsgivesimportantcluesaboutthesourcesofsediment,theirrelativeinfluenceonGBRecosystems,andsedimentdistributionbyhydrologicalprocesses.

Thestudyalsocontributestoourunderstandingoftheevolutionduringthepast18000yearsofthenortheastAustralianmargin,thelargesttropicalmixedsiliciclastic–carbonatemarginonEarthandthemoderntypecaseusedtocompareancientrocksfromsimilarenvironmentselsewhereontheEarth.

Inter-reefalenvironmentscover327950squarekilometresor95%ofthetotalareaoftheGBRMarineParkbutaremuchlessstudiedthanthereefs.Theyformaconnectednetworkofhabitatsthatsupportawiderangeofbiologicalcommunitiesinadditiontothereefs(Chin2003).SpatialchangesinthecompositionandtextureofseabedsedimentshelptocharacterisethebenthicenvironmentscoveredbytheexistingGBRMarineParkplanningscheme.

Figure 1. LocationofsamplescollectedwithintheGreatBarrierReefMarinePark.

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Largest sediment databaseThestudyisbasedonmorethan3000surfacesedimentsamplescollectedintheGBRMarineParksince1984andrepresentsthelargestsedimentdatabaseassembledforanypartoftheshallowtropicalAustralianshelf(figure1).Nearlyhalfofthesampleswerecollectedbetween2003and2005bysixregionalsurveysconductedbytheAustralianInstituteofMarineScienceandCSIROfortheGBRSeabedBiodiversityProject,aprogramrunbytheReefCRCinTownsville.GeoscienceAustralia’scontributiontothisprogramwastoproduceaquantitativeregionalsynthesisoftheseabedsedimentologyfromasystematicanalysisofthetextureandcompositionofthesamples.ThenewdataarepartofafundamentalnationalmarinedatasetmaintainedbyGeoscienceAustralia.

Regional sedimentologyPreviousworkfromtheGBRshowsthatpostglacialsedimentsareessentiallymadeupofamixtureofcalciumcarbonatefromtheskeletalremainsofmarineorganisms(molluscs,foraminifers,coralsandalgae)andsiliciclasticsedimentsmainlyderivedfromsourcesonland(Maxwell1968,1969;Maxwell&Swinchatt1970;Flood&Orme1988).

Table 1 Calculatedareaof%sedimentconcentrationsintheGreatBarrierReefMarinePark

Sediment attribute km2 Per centGravel %

0–20 196,050 56.820–40 23,100 6.740–60 8,400 2.460–80 3,000 0.980–100 2,500 0.7Sum: 233,000 67.5No data: 112,150 32.5

Sand %0–20 10,140 2.920–40 24,940 7.240–60 57,010 16.560–80 81,370 23.680–100 59,560 17.3Sum: 233,010 67.5No data: 112,170 32.5

Mud %0–20 120,190 34.820–40 59,050 17.140–60 33,640 9.860–80 14,400 4.1780–100 5,730 1.7Sum: 233,020 67.5No data: 112,160 32.5

Bulk carbonate %0–20 14,580 4.220–40 28,680 8.340–60 43,310 12.660–80 66,170 19.280–100 86,560 25.9Sum: 239,300 69.3No data: 105,880 30.7

Carbonate sand %0–20 15,720 4.620–40 20,960 6.140–60 29,160 8.560–80 41,230 11.980–100 99,880 29.0Sum: 206,960 60.0No data: 138,220 40.0

Carbonate mud %0–20 16,800 4.920–40 34,870 10.140–60 36,130 10.560–80 39,400 11.480–100 50,660 14.7Sum: 177,870 51.5No data: 167,310 48.5

Marine Park total 345,180 100.0

*Calculations weremadeusingAlbersEqualAreaProjection.

Great Barrier Reef Marine Park sedimentology revealed �

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Thedistributionofthesetwomajorcomponentsshowsacross-shelfvariationwhereinthesiliciclasticsedimentsarerestrictedtotheinnershelfregionsclosetothecoast,andthecarbonatesedimentsaredominantonthemiddleandoutershelves(Belperio,1983).

This‘model’providedausefulregionalframeworkforthecurrentstudy,whichaddsessentialdetailbycharacterisingthegrainsizeofthebulksedimentandquantifyingtheareasofshelfcomprisingdifferentproportionsofgravel(>2mm),sand(63µm–2mm),andmud(<63µm)inboththebulkandcarbonatefractions.Theresultisourmostup-to-daterepresentationofpostglacialsedimentsforthismargin.

Key findingsTheGBRMarineParkisgenerallygravel-poor,withsedimentscomprisinglessthan20%gravelcovering196050squarekilometresormorethan56%ofthetotalmarineparkarea(figure2;table1).Relativelyhighgravelconcentrations(morethan40%)coveranareaof13900squarekilometres(4%oftotalarea),andarerestrictedtoafewareasnexttotheouter-shelfreefsandinBroadSoundontheinnershelf.

MostofthesedimentintheGBRMarineParkiscomposedofsand,withconcentrationsofmorethan60%covering140900squarekilometresofthemarinepark,ormorethan40%ofthetotalarea(figure3;table1).Sandconcentrationsshowveryhighspatialvariabilityacrossthemarinepark(muchhigherthanpreviouslyreported)andaregenerallyhigherontheoutershelfandinthesouth.Thishighdegreeofspatialvariabilityisalsoafeatureofthecarbonateconcentrationsandprincipallyreflectstheinsituproductionofcarbonatebymarineorganisms.

AcrosstheentireGBRMarinePark,mudconcentrationsabove60%cover20150squarekilometres(6%)ofthemarinepark(figure4;table1).NorthofTownsville,mudcomprisesarelativelysmallamountofthemiddleandoutershelfsediments,withhighestconcentrationsreaching40%ontheinnershelf.SouthofTownsville,similarmudconcentrationsoccuronlyonthemiddleshelf,andextendintotheCapricornChannel.

Carbonateisthedominantsedimenttype,withconcentrationsabove60%covering152700squarekilometres(45%)ofthemarinepark(figure5;table1).Concentrationsincreasefrommorethan20%nearthecoasttomorethan80%onthemiddleandoutershelves.Concentrationsoflessthan20%occurinembaymentsnorthof

Figure 2. Mapofthe%gravelweightconcentrationinsediment.

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theBurdekin,Pioneer,andFitzroyrivers,whichcurrentlydeliverthehighestquantitiesofsiliciclasticsedimenttotheinnershelf,andonthesouthernborderofthemarineparkwheresiliciclasticsandsaretransportednorthintotheparkfromFraserIslandandHerveyBay.

AclassificationofsedimenttypeshasrevealedthattheMarineParkisdominatedbygravelly,muddysand,whichcovers53350squarekilometresor16%ofthemarinepark.Othercommonsedimenttypesincludegravellysandandslightlygravelly,muddysands,whichcover48830squarekilometres(15%)and46650squarekilometres(14%),respectively.Themostscarcesedimenttypesincludethegravelsandmuds,withmuddygravelcovering600squarekilometres

andslightlygravellymudcovering1100squarekilometres;bothmakeuplessthan1%ofthetotalmarineparkarea.

ThisnewstudyaddsconsiderablymoredetailtothemodelofpostglacialsedimentologyforthenortheastAustralianmargin,specificallyaboutspatialvariabilityinthedistributionsofthesedimentfractions.Whilethereisageneralincreaseinthecarbonatecontentandadecreaseinsiliciclasticcontentacrosstheshelf,asshownbypreviousstudies,ourresultsshowthattheseacross-shelfdistributionsarecomplexanddonotholdforallpartsofthemargin.Overall,thetextureandcompositionofpostglacialsedimentsshowstrongcorrelationswiththedominantsedimentsources,withoverprintingbyhydrodynamicprocesses.

Applications for managementThespatialanalysisofseabedsedimentintheGBRrevealsinformationaboutthetextureandcompositionofinter-reefalseabedhabitatsandtheirvariability.TheGBRMarineParkisauniquenaturalenvironmentofnationalandinternationalsignificance,anditsmanagersrequiredetailedmappingtofullyunderstandthenatureoftheseabedanditshabitats.Ourquantitativespatialanalysisoftheseabedsedimenttextureandcomposition,andtheassociateddistribution

“ThisnewstudyaddsconsiderablymoredetailtothemodelofpostglacialsedimentologyforthenortheastAustralianmargin”

Figure 3. Mapofthe%sandconcentrationinsediment.

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patterns,givesasystematicseabedclassification.ThesedataprovidethebestpossiblespatialinformationformarineparkmanagerstomakeinformeddecisionsandquantitativecomparisonsbetweenareasofdifferenthabitattypesinAustralia’shighestprofileWorldHeritageArea.

AssembledseabedsedimentinformationprovidesaconsistentandrobustdatasetthatcanbeusedtohelpcharacterisethedifferentmanagementzonesmakinguptheGBRMarinePark.Overall,seabedsedimentsrangefromgravel-poorwithlocalisedhighgravel

concentrations,tosedimentswithhighsandandcarbonateconcentrations,andhighmudwithlowcarbonateconcentrations.Together,thisvariabilityinsurfacesedimentsshowsthepotentialrangeofinter-reefalseabedhabitatsandtheirphysicalcharacteristics.Seagrassmeadows,mudandsandflats,andgravelandshoalbottomsarejustsomeoftheinter-reefhabitatsalreadyidentified.

ThecollectionandmappingofseabedsedimentsampleshasbeenundertakenaspartoftheGBRSeabedBiodiversityProject,whichaimstoformaninventoryoftheseabed’scharacteristics,suchassedimenttypesandfishandinvertebrateassemblages(Pitcheretal2002).SeabedsedimentdatawillbeaddedtoexistingregionalmapsoftheGBRMarineParkandwillprovidemoredetailtohelpmanagersconservesignificanthabitatsandbiologicalcommunities.Inareaswherebiologicalinformationisnotavailable,‘proxies’suchasseabedsedimentscanhelpdeterminetherelationshipbetweenphysicalenvironmentsandbiologytopredictbiodiversity.

Themainadvantageofusingsedimentdataisthattheycanbedeterminedacrossbroadregions,eveninareasthatlackbiologicaldata.OurquantitativespatialinformationaboutseabedsedimentsprovidesaplanningandmanagementframeworkfromwhichtoassessnewplanningproposalsintheGBRMarinePark,andtomonitor

“OurquantitativespatialinformationaboutseabedsedimentsprovidesaplanningandmanagementframeworkfromwhichtoassessnewplanningproposalsintheGBRMarinePark,andtomonitorhabitats”

Figure 4. Mapofthe%mudconcentrationinsediment.

Great Barrier Reef Marine Park sedimentology revealed �

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habitats.Thisapproachwillimprovemarineparkmanagementanddecisionmaking.Furtherwork,investigatingthelinksbetweensediments,habitatsandmarinebiotaingeneral,isnecessaryfortheconservationandappropriatefuturemanagementofinter-reefalareasinthepark.

ThenextstageofGeoscienceAustralia’sworkwillbetocharacteriseandquantifythespatialvariabilityofsedimentswithintheframeworkofthecurrentGBRplanningscheme.Byrevealingthetextureandcompositionofdifferentmanagementzones,weprovidevitalbaselineinformationthatcanbeusedtomonitorpotentialchangestoseabedhabitatsandbiologicalcommunities.

Related websiteGeoscienceAustralia’smarinesamplesdatabasewww.ga.gov.au/oracle/mars

link

For more information

phone EmmaMathewson+61262499295

email emma.mathews@ga.gov.au

ReferencesBelperioAP.1983.TerrigenoussedimentationinthecentralGreatBarrierReeflagoon:amodelfromtheBurdekinregion.BMRJournalofAustralianGeologyandGeophysics8:179–190.ChinA.2003.Inter-reefalandlagoonalbenthos.StateoftheGreatBarrierReefOn-line,GreatBarrierReefMarineParkAuthority,www.gbrmpa.gov.au/corp_site/info_services/publications/sotr/benthos/index.html(viewed28June2006).FloodPG&OrmeGR.1988.Mixedsiliciclastic/carbonatesedimentsofthenorthernGreatBarrierReefprovince,Australia.InDoyleLJ&RobertsHH(eds),Carbonate–clastictransitions,Elsevier,Amsterdam.MathewsEJ&HeapAD.2006.Sedimentologyandgeomorphologyofinter-reefalseabedenvironmentsoftheGreatBarrierReef,northeastAustralia.GeoscienceAustraliaRecord.MaxwellWGH.1968.AtlasoftheGreatBarrierReef,Elsevier,Amsterdam.MaxwellWGH.1969.ThestructureanddevelopmentoftheGreatBarrierReef.In:StratigraphyandPalaeontology:EssaysinHonourofDorothyHill,AustralianNationalUniversityPress.MaxwellWGH&SwinchattJP.1970.GreatBarrierReef:regionalvariationinaterrigenous–carbonateprovince.GeologicalSocietyofAmericaBulletin81:691–724.PitcherCR,VenablesW,EllisN,McLeodI,PantusF,AustinM,CappoM,DohertyP&GribbleN.2002.GreatBarrierReefSeabedBiodiversityMappingProject:Phase1,ReporttoCRCReef,CSIROMarineResearch.

Figure 5. Mapofthe%bulkcarbonateconcentrationinsediment.