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NIWA Water & Atmosphere 11(2) 2003

The seas around New Zealand support a rich anddiverse range of corals. Some deep-sea species –found to depths of at least 2500 m – can beincredibly long-lived, with ages ranging fromdecades to hundreds of years. These include bothstony corals and the less well-known octocorals.Rapid changes in their environment (such asthose caused by fisheries, as well as changes inclimate and ocean patterns) can have significantimpacts on these animals. To identify and reducethese effects we must understand both thetaxonomy and biology of deep-sea corals. AtNIWA we are investigating ways to learn one ofthe basic pieces of information: the age ofindividual specimens of coral. The work so farshows that it is possible to get a good idea of theage of a coral, but it is not easy.

Problems in age measurementCorals come in all shapes and sizes. Thismeans that no single method can be used tomeasure the age or growth rate of all coraltypes. For example, the conventionaltechnique of ageing by counting concentricgrowth zones – as in fish otoliths (ear bones)– could be used for some coral species, but notfor colonial, tree-like corals.

MARINE BIODIVERSITY

Chronicles of the deep:ageing deep-sea corals in New Zealand watersDi Tracey

Helen Neil

Dennis Gordon

Steve O'Shea

How old is acoral? Finding theanswer requiressome rathercomplex steps.

Di Tracey, Helen Neiland Dennis Gordonare based at NIWA inWellington; SteveO'Shea is based atthe AucklandUniversity ofTechnology.

Because corals grow by depositing a calciumcarbonate (calcite) skeleton, radiocarbon datingcould provide estimates of ages. In corals themethod is complicated by two very differentsources of carbonate in the skeleton: from theadjacent water and from the coral’s diet. We needto explore the processes of growth and metabolismand determine the age of surrounding water andlevels of carbon before we can accept coral ageingmethods with confidence.

The calcium carbonate that forms on the newtips of corals is in balance with thesurrounding water. However, the mixing ofocean waters of different ages and the delayin exchange between the atmosphere and theocean cause the level of natural radiocarbon(14C) to vary. This mixing is known as themarine reservoir effect. The radiocarbon agesof the newly formed coral tips provide theknown radiocarbon signature or the“reservoir age”. If we can find out more aboutthe age of deep-sea waters in our region thenwe can also find out the age of the coral. (Theages of water masses are available fromtemperature records, including historical sea-surface temperature data.)

Di Tracey holding a branch of a large(broken) tree of bubblegum coral(Paragorgia arborea), radiocarbon-dated at300–500 years old. This species is amongthe most impressive octocorals and some ofthe largest specimens have been trawledfrom New Zealand waters. (Photo: AlanBlacklock)

Teachers: thisarticle is applicablefor NCEAAchievementStandards inBiology (1.2, 2.2,2.6), Physics (1.2,2.2, 2.5, 2.7),Chemistry (1.2,2.5). See othercurriculumconnections atwww.niwa.co.nz/pubs/wa/resources

NIWA Water & Atmosphere 11(2) 2003

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Colony of the stonycoral Madreporaoculata growing at adepth of 800 m.

Both coral growth and levels of 14C are affectedby climatic events, including El Niño cycles andchanges in sea-surface temperature. We can useenvironmental records and the known rate oftransfer of radiocarbon from the atmosphereinto deep water to interpret the age and growthof corals.

Ageing coral becomes even more difficult if thecoral's diet includes carbonate. However, littleis known currently about feeding in deep-watercorals.

Ageing methodsWe have searched the scientific literature todetermine what techniques are available toage corals and to validate these ages. Apartfrom radiocarbon dating (see above and panelnext page), methods include otherradioisotope techniques and transversesection counts. Validation can be achievedeither by bomb radiocarbon signatures or in-situ growth-rate studies. Our biologicalcollection has several samples of deep-seacoral species we can use for ageing studiesand further samples will be collected duringfuture marine surveys.

Stable isotopes and climatic recordsThe environmental conditions corals havebeen exposed to during their life are lockedin their carbonate matrix, and can beunravelled by using stable isotopetechniques. For example, changing stableisotope ratios of oxygen can indicate pastocean temperatures, while metabolicprocesses such as respiration influence thestable isotopes of carbon. We are using high-resolution historical temperature recordsderived from coral to provide insights intoseasonal and interannual variability andmultidecadal climate cycles. This informationcan help explain the variability of growthzones, and the bursts of growth (or slowgrowth), all of which help determine the ageof the coral.

Radiometric age validationRadiometric assessment measures theproportion of radioisotope pairs found incalcified structures such as fish otoliths andcorals, and gives a close estimate of age.Elements such as radium and thorium enter anorganism through normal metabolic processes,and reach their half-life after a known numberof years. The ratio of 210Pb/226Ra is used tocalculate age for potentially long-livedorganisms (that is, more than 60 years old).

Growth-zone countsThe age of some species of coral can beestimated by counting skeletal growth zonesalong the coral stem (a technique similar topreparing otoliths). X-rays of the slices cut fromthe growth axes can be fluoresced to make iteasier to count the zones.

Previous workIn preliminary work at NIWA in 1999, we agedsamples of bubblegum coral (the gorgonianParagorgia arborea – photo page 22). The colony

DefinitionsKinds of coralGorgonian: Half of all New Zealand's octocoral species (see below) aregorgonians (fan corals, Order Gorgonacea). Gorgonians have a central axialrod and a calcareous skeleton, which accounts for the yellow, orange, orlavender colours in some species.

Octocorals: Octocorals are recognised by their polyps, which typically haveeight tentacles, and virtually all form colonies. New Zealand's octocorals arevery diverse, but only 40 of at least 133 species have been identified withcertainty.

Stony corals: “True” corals (Order: Scleractinia) that build reef-likestructures. They are closely related to sea anemones but produce a calciumcarbonate skeleton. Some are solitary and comprise a single polyp (e.g.,cup corals) but most are large and are made up of branching colonies ofinterconnected polyps. Currently 127 species are known in New Zealand.

Ageing techniquesHalf-life: The time it takes for half of the original amount of a radioactiveelement to decay. 14C has a half-life of 5730 years.

Radiocarbon dating, bomb radiocarbon: See panel on page 24.

Stable isotopes: Stable isotopes are non-radioactive isotopes of elements,which occur naturally in the environment. Their ratios alter as they undergophysical, chemical and biological changes. For over 20 years coralskeletons from temperate regions have been used to reconstruct records ofisotopic and chemical ratios in seawater. (For more on stable isotopes, seeWater & Atmosphere 7(2): 22–28.)

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NIWA Water & Atmosphere 11(2) 2003

tip of this coral was dated at between 100 and 200years old and the base was between at least 300and 500 years old. Another New Zealandgorgonian, bamboo coral (family Isididae,Keratoisis sp., photo far left) was also dated atbetween 300 and 500 years. In Australia threeageing methods – growth zone counts,radiocarbon dating, and radium and thoriumradiometric ageing – were used to age two coloniesof Keratoisis. Although none of the methods agedthe corals precisely, the consensus was that theywere between 100 and 150 years old.

The Primnoidae is the least well known of NewZealand's gorgonian families. Although noNew Zealand species have been aged, branchesof small colonies overseas have been agedat about 112 years by using Lead-210(210Pb). Scientists sampled thin sections alongthe axis of the skeleton of the red tree coralPrimnoa resedaeformis in the Gulf of Alaska andfound the ages along the sections rangedbetween 8 and 105 years.

Other work on New Zealand deep-sea stonycorals has reported radiocarbon ages for severalspecies including Madrepora oculata (photo onpage 23) and Enallopsammia rostrata (above).Ages ranged from under 200 years to more than6000 years. For comparison, a study of the samespecies in the North Sea found ages rangingfrom 600 to 5000 years.

Complex steps to ageingStudying the age and growth of coral iscomplicated. We need to understand: thesource of carbonate (how much comes from theocean and how much from metabolism); theeffects of climatic events; how to interpretgrowth zones; the effect of 14C and biologicalprocesses such as feeding and reproduction;and how to overcome the lack of deep-seaenvironmental data records. We also need tofind out where on the coral we should besampling to get the best estimates of age. Atthe moment we know little about how deep-sea corals deposit their calcite, but we will beexploring this further so that we can havegreater confidence in our age estimates.

To confirm and validate age and growth, it willbe necessary to use a combination of some ofthe possible methods for ageing coral. Inaddition to ageing the corals, this work shouldyield a high-resolution record of oceantemperature during the past 100 years by usingstable-isotope signatures preserved in thecorals’ carbonate skeletons. �

A bamboo coral(Keratoisis sp.), aged at300–500 years old.(Photo: Alan Blacklock)

Cup coralsDesmophyllum

dianthus growing on acolony of another

stony coralEnallopsammia

rostrata, which hasbeen aged at between50 and 840 years old.

(Photo: AlanBlacklock)

Our work ageingcorals is part of theFRST-fundedprogrammes“Seamounts: theirImportance toFisheries andMarineEcosystems”(C01X0224) and“Consequences ofEarth–OceanChange”(C01X0203).

The 14C radiometric dating method is based on14C decay. It relies on the principle that aproportion of natural carbon in the atmosphere(as carbon dioxide) is the radioactive isotope14C. (Ordinary carbon has a molecular weight of12.) Natural atmospheric radiocarbon levelshave been modified by human activities (fossilfuel combustion, nuclear power plants andtesting). As a result the oceans have undergonea significant input of radiocarbon that is clearlyvisible in 14C time-series derived frommeasurements of seawater or by using proxiessuch as corals or bivalve shells.

The amount of radiocarbon incorporated in thehard parts of marine organisms such asbivalves, corals, and fish (for example, shellsand otoliths) can be measured. This is a well-known dating method and is used to age coralspecies both in New Zealand and overseas.

Atmospheric testing of nuclear devices duringthe 1950s and 1960s created a global signal of14C that can be traced in atmospheric,terrestrial, and marine environments. Bombradiocarbon quickly became incorporated inthe carbonate of hard tissues (coral skeletons,fish otoliths) in concentrations proportional tothose in the water column. The level of 14C inhard tissues can determine whether the coralswere growing before, during, or later than themain years of 14C increase (1958–65), usingappropriate reference 14C chronologies for theregion. Ages previously determined fromgrowth increment counts can be shown to beeither correct or incorrect and so this is apowerful validation procedure.

Radiocarbon dating andbomb radiocarbon