Post on 31-May-2020
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.
Great Barrier Reef Marine Park sedimentology revealed �
issue 84 Dec 2006
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 �
issue 84 Dec 2006
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.
Great Barrier Reef Marine Park sedimentology revealed �
issue 84 Dec 2006
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.
Great Barrier Reef Marine Park sedimentology revealed �
issue 84 Dec 2006
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 �
issue 84 Dec 2006
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.