Palaeogeography, Palaeoclimatology, Palaeoecology 268 (2008) 91–105
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Palaeogeography, Palaeoclimatology, Palaeoecology
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The chronology and structure of the western New Caledonian barrier reef tracts
Guy Cabioch a,⁎, Lucien Montaggioni b, Nicolas Thouveny c, Norbert Frank d, Tokiyuki Sato e,Véronique Chazottes b, Hélène Dalamasso b, Claude Payri f, Michel Pichon g, Anne-Marie Sémah h
a Institut de Recherche pour le Développement, Unité de Recherche “Paléotropique”, BP A5, 98.848 Nouméa CEDEX, New Caledoniab Université de Provence, Laboratoire des Systèmes et Réservoirs Carbonatés, EA 4992, Place Victor Hugo, 13331 Marseille CEDEX 3, Francec UMR 663, CEREGE, BP 80, 13.545 Aix-en-Provence CEDEX 4, Franced Laboratoire des Sciences du Climat et de l'Environnement, Laboratoire des Sciences du Climat et de l'Environnement, UMR 1572 CEA, CNRS, UVSQ, Avenue de la Terrasse, 91198 Gif-sur-Yvette, Francee Institute of Applied Earth Sciences, Faculty of Engineering and Resource Science, Akita University, Tegata-Gakuencho, Akita 010-8502, Japanf Institut de Recherche pour le Développement, UMR 7138 «Systématique, Adaptation, Evolution», BP A5, Nouméa CEDEX, New Caledoniag Laboratoire des Ecosystèmes Aquatiques Tropicaux and Méditerranéens, Ecole Pratique des Hautes Etudes, Université de Perpignan, 66.860 Perpignan CEDEX, Franceh Institut de Recherche pour le Développement, Unité de Recherche “Paléotropique”, 32 avenue Henri Varagnat, 93143 Bondy CEDEX, France
⁎ Corresponding author. Tel.: +687 26 07 39; fax: +68E-mail address: [email protected] (G. Cabioch)
0031-0182/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.palaeo.2008.07.014
a b s t r a c t
a r t i c l e i n f oArticle history:
The mainland of New Caled Received 20 January 2008Received in revised form 10 July 2008Accepted 22 July 2008Keywords:Barrier reefNew CaledoniaQuaternary reef development and climateChronologyCoral Sea
onia (“Grande Terre”) is surrounded by one of the largest continuous barrier reefsystem in the world. In order to study the development history and internal structure of this system, drillingoperations have been carried out at two sites (Amédée and Kendec islets), located on the barrier reef tractextending from the south-west 33 to the north-west of New Caledonia. The extracted cores are 128.50 and148.75m in length respectively. Lithological and paleoecological descriptions, combinedwithUranium / Thoriumdating, magnetostratigraphy and nannofossil-based biostratigraphy allowed a reef evolutionary scheme to bedrawn. As a result of the interplay between margin subsidence and sea-level changes, 11 reef units formedsuccessively during interglacial episodes at high sea levels in both sites. Depending on the subsidence rate of thewestern shelf-margin, major building in New Caledonia appears to have started during the MIS 11 (400,000 yr)from shallow-water carbonate platform deposits older than 780,000 yr. Comparing development patternsbetween the New Caledonian barrier reef and the Australian Great Barrier Reef clearly indicates that both globalclimate and regional tectonic history have been themajor controls on reef initiation and growth along both sidesof the Coral Sea. Climatic conditions are likely to have not been optimal before the late Quaternary, probablyresulting in luxuriant reef expansion only during the last 400,000 yr.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
In 1842, Darwin was the first to try explaining the formation ofbarrier reefs as the result of progressive subsidence of fringing reefsrimming volcanic land masses (Darwin, 1842). As an alternative tosubsidence, Daly (1910) emphasized the importance of changes in sealevel during the Pleistocene as a major control of reef development.Afterwards, although our knowledge on the role of glacio-eustatic sea-level changes was greatly improved owing to increasing deep reefdrilling, the subsidence-control concept was still widely accepted asthe best scenario to explain the physiographical and structuralattributes of present-day barrier reefs (Steers and Stoddart, 1977;Guilcher, 1988). For barrier reefs associated with continental coastsrather than volcanic islands, the origin appears to be more complexbecause the reefs overcap antecedent tectonic structures (Holpey,
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1982; Cabioch et al., 1999). Nevertheless, Purdy (1974), Purdy andWinterer (2006) pointed out that, in some cases, one of the principalfactors that have controlled the morphology and structure of barriersand atolls is dissolution by (meteoric) fresh water during Pleistocenelow sea-level stands.
While up to 100 m-long, carbonate sequences were extracted froma number of atolls or shelf-margin platforms (Funafuti: Bonney, 1904;Cullis, 1904; Ohde et al., 2002; Kita-Daito-Jima Atoll: Hanzawa, 1941;Ohde and Elderfield,1992; Bikini: Johnson et al., 1954; Enewetak: Laddand Schlanger, 1960; Tracey and Ladd, 1974; Ludwig et al., 1988; Quinnand Matthews, 1990; Quinn et al., 1991-a; Mururoa: Trichet et al.,1984; Buigues, 1985; Aissaoui et al., 1990; Buigues,1996; Camoin et al.,2001; lagoons of Pukapuka and Rakahanga atolls, northern CookIslands: Gray et al., 1992; Bougainville guyot in Vanuatu: Quinn et al.,1991-b; Montaggioni et al., 1991; Taylor et al., 1994; Florida Keys:Multer et al., 2002; northern Belize: Mazzullo, 2006) and sometimeswere chronologically constrained, deeply cored series recovered fromtypical barrier reefs remain still rare. Until now, data are only availablefrom New Caledonia (Coudray, 1976; Cabioch et al., 1999), Belize
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(Gischler et al., 2000; Purdy et al., 2003; Gischler and Hudson, 2004;Mazzullo, 2006) and the Australian Great Barrier Reef (Richards andHill, 1942; Alexander et al., 2001; Webster and Davies, 2003;Braithwaite et al., 2004; Braithwaite and Montaggioni, in press). Thebarrier reef system in New Caledonia (South-West Pacific) is one of thelongest and the most continuous in the world. Located at the easternborder of the Coral Sea, it surrounds the main New Caledonian island(so-called “Grande Terre”) that stretches out between 19° and 23°South and 163° and 168° East (Fig. 1-A and B). This barrier locallydevelops as double reef tracts, interrupted by passes usually incontinuation with the main rivers. The distance from the coastline tothe barrier reef front varies from place to place, ranging from severalhundred meters to several tens of km. Accordingly, lagoonal, back-reefzones range from several hundred meters to several tens of km inwidth. Two vast lagoon systems occur at both island extremes, e.g. theso-called “South-West Lagoon of Nouméa” and “ Great North Lagoon”.
The first comprehensive study devoted to the internal structureand growth history of the New Caledonian barrier reef system wascarried out by Coudray (1976). From the analysis of a 226 m-long corepenetrating Ténia islet (western barrier area), he claimed that reefbuilding initiated during the Plio-Pleistocene transition and produced4 superimposed units in relation to successive changes in sea level:
Fig. 1. A: Location of New Caledonia in the South-West Pacific relative to main physiography fsurrounding barrier reef system. Positions of drilling sites along the south-west to north-w
Unit 1 from 226 to 105 m characterized by layers rich in packstones,wackestones and sands with coralline algal debris, foraminifera,mollusks and scarce corals and interpreted as back-reef and / orlagoonal deposits; Unit 2 from 105 to 40 m composed of grainstonesand packstones with foraminifera, molluscs, coralline algal and coraldebris interpreted as lagoonal and subreefal deposits; Unit 3 from 40to 11 mmainly characterized by coral and coralline algal boundstonesmixed with packstone rich in foraminifera, molluscs and algal debrisinterpreted as subreefal to reefal deposits; and Unit 4 from 11m to topcomposed of biodetrital sands (Coudray, 1976).
In order to clarify the origin and development patterns of barrierreef tracts associated with continental land masses, a drillingprogramme was conducted during the last decade. Cores wererecovered from a variety of sites around the mainland, especiallyfrom the western side: Amédée islet, situated about 3 km back fromthe south-west outer reef line and Kendec islet located in the middleof the Koumac pass which cuts the barrier reef (Fig. 1-B). Morerecently, additional drilling operations were performed in Ténia isletbut the analyses of cores are still in progress. The principal aim of thisstudy is to provide new data on the anatomy, growth history andchronology of the western New Caledonian barrier reef tract. Ourresults will be compared with those recently obtained from the Great
eatures; B: General physiography of the New Caledonian mainland (“Grande Terre”) andest barrier tract are marked: Amédée, Ténia and Kendec islets.
Fig. 2. Thin-section photomicrographies from core Amédée 4: A: sample 333 AM4 (100 m below core top, bct, Unit 3): foraminiferal–coralline algal packstone / wackestone withAmphistegina sp. (Amp), Amphiroa sp. (Ar art). Low-magnesian calcitic sparite (Cal) replace the micritic matrix; B: sample 330 AM4 (99.40 m bct, Unit 3): coralline algal (Ar)wackestone rich in debris of Amphiroa sp. (Ar art). The Halimeda segment (Hal) is calcitized. Calcitic sparite (Cal) replaces the micritic matrix; C: sample 269 AM4 (86.30 m bct, Unit4): calcitized Halimeda (Hal); D: sample 204 AM4 (68.90m bct, Unit 5): packstone / wackestonewith debris of coralline algae (Ar), echinids (Ech) and foraminifera (For). The micrite ispartly replaced by calcitic sparite (Cal); E: sample 191 AM4 (66.80m bct, Unit 5): wackestone / packstonewith various skeletal debris including gastropods (Gastr) and Halimeda (Hal)which are dissolved (moldic cavities); F: sample 105 AM4 (53 m bct, Unit 6): paleosoil characterized by numerous pisoids (arrows); G: sample 105 AM4 (53 m bct, Unit 6): detailedview of pisoids (arrows); H: sample 95 AM4 (50.80 m bct, Unit 7): moldic cavity of gastropod (Gastr).
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Fig. 3. Thin-section photomicrographies from core Kendec: A: sample 423 K (119.40 m below core top, bct, Unit 2): coralline algal (Ar) packstone / wackestone. The micritic matrix isreplaced by calcitic sparite (Cal); B: sample 395 K (116.60m bct, Unit 2): foraminiferal–coralline algal packstone / wackestone rich (coralline algae = Ar; foraminifera soritid = Sor). Themicritic matrix is dissolved or replaced by calcitic sparite (Cal); C: sample 213 K (48.15 m bct, Unit 8): wackestone / packstone enclosing partly dissolved coralline algal debris (Dis Ar).The micritic matrix is dissolved or replaced by calcitic sparite (Cal); D: sample 119 K (35.50 m bct, Unit 8): algal crust (Ar) with calcitized vermetid gastropod (V). Numerous cavities(Cav) due to dissolution; E: sample 102 K (32.80 m bct, Unit 8): moldic cavity of gastropod (Gastr) and cementation by calcite (Cal); F: sample 73 K (28.10 m bct, Unit 9): wackestonewith coralline algal debris (Ar) and echinoidal debris (Ech); G: sample 12 K (8.70 m bct, Unit 11): layer of coralline algae (Ar) associated with vermetid gastropod (V); H: sample 12 K(8.70 m bct, Unit 11): coral colony (Cor) encrusted by layers of coralline algae (Ar) associated associated with vermetid gastropod (V).
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Table 1Results of U-series open system models for corals from core Amédée 4 (modified afterFrank et al. (2006)); (1) Conventional age; (2) U-series open system ages calculatedusing the model of Thompson et al. (2003); (3) U-series open system ages calculatedusing the age equation of Villemant and Feuillet (2003)
Sample Depth Age (1) Age (2) Age (3)
(m) (yr) (yr) (yr)
AM4-14-1 19.0 132,200±1900 124,300 ±2800 123,550 ±3850AM4-14-2 19.0 124,700 ±1600 112,000 ±4600 110,900 ±5200AM4-14-3 19.0 134,700 ±2700 120,600 ±4400 119,600 ±5800AM4-14-4 19.0 127,400 ±3600 102,600 ±4400 100,550 ±5450AM4-14-5 19.0 145,300 ±4800 102,600 ±4400 104,500 ±6000AM4-15P 21.5 144,800 ±2500 122,200 ±4400 120,500 ±6200AM4-23G 30.5 127,600 ±2200 122,100 ±5100 121,850 ±6450AM4-30Fs 33.45 129,300 ±2400 122,300 ±4300 121,800 ±5700AM4-34G 35.1 117,100 ±1800 127,500 ±5400 128,850 ±7150AM4-52-1Fa 40.2 328,000 ±10,000 253,000⁎ ±11,000 128,850 ±10,900AM4-52-2Fa 40.2 328,000 ±11,000 241,000⁎ ±11,000 236,000 ±10,900AM4-52-3Fa 40.2 412,000 ±19,000 237,000⁎ ±11,000 245,100 ±8000AM4-52-4Fa 40.2 309,000 ±7800 235,000⁎ ±11,000 222,150 ±8550AM4-58Fu 42.5 356,000⁎ ±47,000 278,000 ±12,000AM4-77G 45.45 479,000⁎ ±100,000AM4-82P 48.75 N440,000⁎
AM4-145A 59.3 N440,000⁎
⁎ Ages calculated using the Equation defined by Frank et al. (2006).
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Barrier Reef of Australia, extending along the opposite side of the CoralSea.
2. Materials and methods
The barrier reef from New Caledonia is more than 900 km long andwell developed along thewesternmargins of the island (Fig. 1-B). Twodrilling sites therefore were selected there, from south to north. Thesouthernmost site is located at Amédée islet, close to Aboré Reef andBoulari Pass, through which a 128.50-m long core (core Amédée 4)was extracted. The northernmost site relates to Kendec islet, risingthrough Koumac pass; only one core, 148.75-m long, was drilled. It isnoteworthy that Coudray (1976) studied and described a 226 m-longcore at Ténia islet, a sandy cay overlying the barrier reef flat, justbehind the reef front line. Unfortunately, only few chronological datawere gained from the Ténia core. More recently, new cores, still understudy, were recovered at Ténia site.
The 46 to 35 mm-diameter cores were extracted using a sedidrill-500 coring engine able to penetrate more than 100 m deep. Becauseaccessibility to barrier reefs is particularly difficult for heavy drillingengines and it is necessary to keep drilling engines water-safe, theislets located close to or at the top of barrier reef flats have served as“natural” platforms for drilling operations. During coring, the core bitwas advanced every 1.5 m, using a double-tube casing. A depthaccuracy of ±0.1 to 0.5 m has been estimated. Recovery, depending onthe nature of penetrated sediments and rocks, ranges from 0 to 100%.Unconsolidated, fine-grained material is usually difficult to recover.Moreover, there were large empty cavities. The core depths are givenaccording to the core top (below core top, bct).
Chronology is based mainly on several dating methods, includingU-series measurements, biostratigraphy (based on nannofossils) andmagnetostratigraphy. Chronostratigraphical correlations between thereef sequences were also attempted, using the occurrence ofunconformities (i.e. emergence surfaces and diagenetic features).
Petrographic analysis of thin sections was performed usingstandard petrographic microscopy to analyze the texture, composition(coral, coralline algae, Halimeda, foraminifera, molluscs and echino-derms and lithoclasts) and diagenetic features (Figs. 2 and 3).
Unconformities and associated diagenetic features were used asindicators of sub-aerial exposure during low sea-level stands.Recognition of exposure surfaces was based on both petrologicalobservations of hand specimens and thin sections, and shifts inprofiles of oxygen isotopic ratios from top downcore. The occurrenceof typical paleosoils, containing residual flora (pollens), were regardedas reliable markers of periods of emergence. Isotope analyses wereprovided by Michael Joachimski at the University of Erlangen-Nürnberg (Germany), restricted to bulk carbonate samples, in orderto prevent δ18O variations related to the lithology. Results are reportedin standard delta notation (per mil versus Pee Dee Belemnite,‰ PDB).Reproducibility relative to an internal standard was b0.1‰.
In this study, radiometric dating of corals from core Kendec wasperformed by thermal ionization mass spectrometry (TIMS U/Th) atLSCE (Gif-sur-Yvette, France). The dated samples were pieces of pristinecoral colonies containing generally more than 95% aragonite. Dates aregiven in years B.P. or ka (1 ka=1000 yr B.P.). In core Amédée 4, the U/Thcoral dates were previously obtained by Frank et al. (2006). These datespreviously published by Frank et al. (2006) are reported in Table 1.
Magnetostratigraphy can be used because the reef carbonates cancontain variable concentration of ferrimagnetic particles such astitano-magnetite inherited from the Earth's crust or biogenicmagnetite. The deposition of these magnetic particles in the coralreef environment occurs under the influence of the geomagnetic field;after compaction and chemical precipitation of carbonate cements,the porosity decreases, favouring a final lockin-in of the (post-)detritalmagnetization. Sensitive rock magnetometers allow measuring veryweak remanent magnetizations and construct magnetostratigraphic
scales from boreholes of from outcrops. The correlation with thegeomagnetic polarity time-scale thus provides series of precise ageswhich help to constrain the chronology of the carbonate reefconstruction (e.g. Aissaoui et al., 1990). Cores Amédée 4 and Kendecwere subsampled for rock magnetic and paleomagnetic studies. Hardlithological facies (packstone and grainstone) were selected forextraction of hemi-cylindrical blocks oriented in the vertical plane(bottom/up) fromwhich two or three specimens of 3 cm×2 cm×2 cm(i.e. ~18 cm3) corresponding to a mass of ~20 to 40 g of carbonatematerial. The magnetic and paleomagnetic measurements wereperformed at the Rock and paleomagnetic laboratory of CEREGE atPlateau de l'Arbois, A. Volume magnetic susceptibilities weremeasured using a Kly-2 kappabridge (sensitivity=10−8 SI unit) andwere normalized by the mass of each specimen, and expressed asmass specific susceptibility [m3 kg−1]. Natural and anhystereticremanent magnetizations (NRM and ARM) were measured using athree axis rockmagnetometer 2Gmodel 760-R (sensitivityof 10−11A·m2)installed in a shielded room. The natural remanent magnetizations(NRM) were demagnetized i) by alternating field (AF) using the «in line»degaussing system coupledwith themagnetometer allowing a completedemagnetization to be performed step by step (from 5 to 60 mT); ii) bystepwise thermal treatment from80 °C to 220 °C in a shielded furnace. Incore Amédée, 39 layers were collected between 30 and 128.50 m, with areasonably good resolution between 50 and 70m (1 site each 2m) and arather poor resolution between 80 and 128.50 m (1 site each 5 m). Incore Kendec, 29 layers were collected between 10 and 148.75 m, with aresolution of 1 layer each 6 m down to 83 m and 1 layer each 5 mbetween 110 and148.75m.No sampleswere recovered from the interval85 to 110 m.
Taxonomic identification of calcareous nannofossils at the specieslevel was carried out in the two cores, using smear slides, in order todefine major Quaternary biozones diagnostic in terms of biostrati-graphy. Estimates of total species abundance are labelled as follows:A: abundant, C: common, R: rare, +: present, but not counted.
3. Results
3.1. Lithology
Although lithology varies greatly from core to core, several distinctbiofacies can be recognized, including 2 major coral growth frame-work and 7 detrital facies, in the sense of Montaggioni (2005).
Fig. 4. A: Hand-sample 52 bis AM4 (39.80 m below core top, bct, Unit 9): colony of Favia gr. palida showing a bioerosion cavity (B) (scale bar=7 cm); B: sample 81 bis AM4 (48.60 mbct, Unit 7): colony of Plesiastrea cf. versipora in growth position (arrow) (scale bar=7 cm); C: sample 164 AM4 (60.85 m bct, Unit 5): pedogenetic features (oxydation, dissolution)(scale bar=7 cm); D: sample 459 AM 4 (119.30 m bct, Unit 1): rhodolith-bearing facies (arrows) (scale bar=7 cm); E: sample 12 K (8.60 m below core top, bct, Unit 11): successionof coral debris (Co) and crusts of coralline algae with vermetid gastropods (Ar / V); presence of several bioerosional cavities (arrows) (scale bar=7 cm); F: sample 18 K (11.40 m bct,Unit 10): skeletal detritus embedding coralline algal debris (scale bar=7 cm); G: sample 102 K (32.80 m bct, Unit 8): calcitized corals (arrows) below an unconformity (scalebar=5 cm); H: sample 247 K (53.70 m bct, Unit 7): mudstone to wackestone with rare skeletal debris (scale bar=7 cm).
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The coral growth framework facies comprise branching andmassive coral forms in growth position according to the orientationof their growth axis, usually encrusted by coralline algal veneers. Thealgal crusts in this framework facies are mainly composed: 1/ in coreAmédée 4, of Hydrolithon onkodes (at various depths as for exampleat 35.65 m and 41 m) and Lithophyllum spp. (including L. pustulatum at32 m, 35 m and L. tamiense at 41 m); in core Kendec, of Hydrolithononkodes,Neogoniolithon spp. and Lithophyllum spp. including L. pustulatumfrom top to around 50 m. The corals in this framework facies arerepresented: 1/ in core Amédée 4, by branching and tabular acroporids(including Acropora spp.), domal faviids (including Leptoria phrygiaat 21.50 m, Favia gr. palida at 39.80 m, Fig. 4-A, Favia sp. at 44 m,montastreinae at 44.80 m, Plesiastrea versipora at 48.60 m, Fig. 4-B) andporitids (including Porites spp. at 17m, cfGoniopora sp. at 44.60m, Poriteslutea at 48.75 m); 2/ in core Kendec, by branching and tabular acroporids(including Acropora spp.), branching pocilloporids (including Pocilloporasp. at 6.60 m), domal faviids (including Platygyra pini at 5.20 m) andporitids (including Porites spp.).
Detrital facies include skeletal rubble and carbonate sand facies. Therubble facies is composed mostly of a mixture of unsorted, angular torounded, gravelly fragments of corals, bivalves, foraminifera andcalcareous algae. The carbonate sand facies are subdivided into 6subfacies, on the basis of major skeletal contributors. The mostsignificant are the following: coral–molluscan–foraminiferal grain-stones to packstones; foraminiferal–bryozoan–molluscan–echinidgrainstones / packstones to packstones / wackestones; coralline algal–foraminiferal grainstones / packstones to packstones / wackestones;alcyonarian spiculite grainstones; coralline algal–planktonic foraminif-eral grainstones / packstones to packstones / wackestones; rhodolitic–foraminiferal rudstones / grainstones to rudstones / wackestones.
Based on lithological attributes, and occurrence of unconformities,several carbonate units can be defined from the different sequences.
Fig. 5. Core Amédée 4: Lithology (A), oxygen isotopic profile (B), magnetostratigraphical daunconformities and subunit boundaries respectively.
In core Amédée 4 (Fig. 5-A), 11 units were recognized. The substrate,at around 126.50 m core depth, is composed of basalts more or lessaltered. Unit 1, extending from 126.50 to 115 m, consists of rudstones /grainstones rich in rhodoliths (Fig. 4-D), foraminifera andmollusks,withrare coral debris. This unit was previously specifically studied andinterpreted as a platform developing in relatively shallow-waterenvironments, at around 30 water depth (Payri and Cabioch, 2004).Unit 2, from 115 to 105 m, is typified by a so-called “foramol”(foraminifera / mollusks) association, with fragments of the greenalgaeHalimeda, coralline algae and a few recrystallized coral clasts. Unit3, between 105 and 85 m, refers to grainstones / packstones andpackstones / wackestones rich in benthic foraminifera, algal debris andmollusks (Fig. 2-A and B). The calcareous clasts are generallyrecrystallized (Fig. 2-C). Unit 4, from 85 to 70 m, mostly contains piecesof recrystallized corals, coralline algae and vermetid gastropods. From116 to 69.50 m, the sequence is composed of recrystallized coral andmolluscan debris, interbedded with laminated carbonate micrites andpackstones / wackestones rich in debris of coralline algae andforaminifera (Fig. 2-D). Dissolution features are particularly abundant,indicating severe sub-aerial alteration. At 70 to 69.50 m, the rock isseverely karstified, but reveals the presence of calcareous organismspartly preserved. Unit 5, from 70 to 60 m, consists of molluscangrainstones and packstones, with occasional corals and various skeletaldebris (Fig. 2-E). This unit is capped by soil features including calcareouscrusts and oxydation (Fig. 4-C). Unit 6, from 60 to 52 m, is composedmainly of mollusks, foraminifers, and a variety of bioclasts; coralelements occur in the formof scatteredmolds. From52 to 46m,Unit 7 ismade up of coral framework, interbedded with layers of grainstones/packstones rich in molluscs, echinids, foraminifers, Halimeda andcoralline algae. Most ofHalimeda andmolluscs are intensively dissolved(Fig. 2-H). Coral framework is volumetrically dominant at the base of theunit, mainly characterized by domal faviids including Plesiastrea
ta (C) and age–depth relationship. The waved lines and dashed indicate the position of
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versipora at 48.60 m and poritids including Porites lutea at 48.75 m. InUnit 8, from 46 to 40m, the deposits consist of thin coralgal framework,associated with layers of molluscan–foraminiferal grainstones. From69.50 to 47 m depth, the reef sequence is punctuated by several minordiscontinuities; diagenetic alteration increases downwards. At 52m and60 m, there are several sedimentary and diagenetic features, indicatingepisodic emergence, such as terrigeneous soils, solution and moldiccavities (Fig. 2-E and F). Unit 9, from 40 to 37 m, consists of coral,coralline algal and foraminiferal grainstones. Unit 10, from 37 to 14m, ismostly composed of coral framework, encrusted by coralline algal andforaminiferal veneers, and mixed with grainstones / packstones rich inarticulated and branched coralline algae, Halimeda, mollusks, echinidsand foraminifera. A marked change in texture and color occurs at 26 mdeep. The 14-m boundary exhibits fragments derived from a mm-thickcalcareous crust (calcrete), and therefore is interpreted as an emresionunconformity. Unit 11, from 14 m to core top, is composed of skeletalgrainstones, with scarce coral colonies characteristic of shallow-watersettings.Moreover, this unit is onlycharacterizedbyaragonitic andhigh-magnesian cements related to marine diagenesis. It is noteworthy thatUnits 2, 3, 5, 6 and 10 can be delineated in several subunits, from sharpchanges in facies and/or in diagenesis.
In core Kendec (Fig. 6-A), 11 units can be observed. Unit 1, from148.75 to 131.60 m core depth, is composed of grainstones dominatedby benthic and planktonic foraminifers, bivalves, Halimeda, spongespicules and quartz debris. Unit 2, from 131.60 to 112.75 m, iscomposed of grainstones or packstones rich in corals, encrustingcoralline algae, foraminifers and molluscs. The subordinate compo-nents include echinids, articulated coralline algae, gastropods andalcyonarian spicules (Fig. 3-A and B). Unit 3 exhibits intervals of fine-
Fig. 6. Core Kendec: Lithology (A), oxygen isotopic profile (B), magnetostratigraphical dataunconformities and subunit boundaries respectively.
grained sands rich in coralline algae, echinids and foraminifers. Unit 4,from 75.70 to 69.60 m is made up of coral framework accompaniedwith encrusting coralline algae, benthic foraminifers, bivalves, Hali-meda and alcyonarian spicules. Unit 5, from 69.60 to 63.60 m, consistsof grainstones rich in mollusks, benthic foraminifers, encrustingcoralline algae, coral, and, to a lesser extent, in echinids, Halimeda andalcyonarians. Unit 6, from 63.60 to 56.10 m is composed predomi-nantly of mollusks and coral fragments, associated with echinids,encrusting coralline algae, Halimeda and benthic foraminifers. Unit 7,from 56.10 to 50.10 m, is compositionally quite similar to theunderlying unit (Fig. 4-H). At 50.10 m, the top of Unit 7 is marked byan unconformity with changes in color; coral clasts are severelydissolved. In Unit 8, from 50.10 to 30.60 m, biofacies are dominated bymollusks and coral colonies, generally calcitized (Fig. 4-G), associatedwith coralline algae, foraminifers, echinids and alcyonarians (Fig. 3-Cand D). From 50.10 m upwards, the units are enriched in in-situ coralcolonies. At 21.60 m and 30.60 m respectively, occur iron-rich crusts.An unconformity surface is particularly well marked between 32.7 and33.6 m and typified by firmly lithified, iron-rich grainstones, withextensively neomorphized coral debris. At 30.60 m, sub-aerialdiagenetic features can be observed (Fig. 3-E). Unit 9, from 30.60 to21.60 m, contains numerous in-situ coral colonies. The subordinatecomponents are coralline algae, foraminifera, Halimeda, echinids,bryozoans and alcyonarian spicules (Fig. 3-F). At 21.60 m, an iron-richlayer can be observed. Unit 10, from 21.60 to 9.60 m, is enriched incoral colonies, interbedded with sands composed of coralline algae,foraminifers, echinids, and Halimeda (Fig. 4-F). Millimetre-thickcalcareous crusts occur at 12.7 and 13.1 m respectively, and a paleosoilis present at 12.8 m. The upper surface of Unit 10, at 9.60 m, appears in
(C) and age–depth relationship. The waved lines and dashed indicate the position of
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the form of a thin calcareous, associated with slightly calcitized coralelements. Unit 11, from 9.60 m to core top, is composed of sands andgravels derived from corals, and encrusted by coralline algae and coraldebris encrusted by coralline algae (Figs. 3-G, H and 4-E). Thediagenetic features are only represented by marine aragonitic andhigh-magnesian cements.
3.2. Oxygen isotopic data
As a whole, the lithological boundaries identified in the corescoincide with significant changes in δ18O ratios measured in bulkcarbonate samples.
In core Amédée 4 (Fig. 5-B), the values of δ18O range from −4.84 to−1.70‰ PDB. It is noteworthy that the values decrease from −0.10 to−4.30‰ , from core top to 70 m deep. Between 70 m and the base ofthe core, the ratio values range from −3 to −4.85‰. These variationsare in good agreement with changes in petrology. The unconformitysurface observed at 70 m deep, is particularly well marked in theisotopic signal. This surface clearly separates an upper, weaklydiagenetically altered reef from a lower, extensively calcitized unit.
In core Kendec (Fig. 6-B), shifts in δ18O curve are positivelycorrelated with those observed in core Ténia 3. The δ18O ratios rangefrom −6.43 to −1.01‰. Between 131.60 and 106.75 m deep, the δ18Oprogressively decreases up to −5.81‰. Then, from 106.75 to around70 m, the δ18O range from −5 to −6‰. In the 70–50 m interval, theδ18O values average −4‰. Between 50 and 30 m, the δ18O valuesdecrease from −4 to −6‰. From 30 m to core top, the values increasesignificantly from −6 to −1‰. The changes in δ18O values correlatewell with changes in lithology.
3.3. U/Th dating
U/Th dating by TIMS were previously performed on coral samplesextracted from Amédée site, including core Amédée 4 and a fewsections from core Amédée 1, 2 and 5 (Frank et al. 2006). The relevantresults revealed that both distribution of U and Th isotopes wereaffected by sub-aerial diagenesis during emergence of the reef units.By contrast, the Holocene corals seem to have been well preserved.Therefore, various correction models were tested by Frank et al.(2006) to try to get reliable ages from the late Pleistocene sequences. Itappeared that the model from Thompson and Goldstein (2005) fittedbest with the data. We report in Table 1 the data previously publishedby Frank et al. (2006). The dates obtained in core Amédée 4 and also inthe other short cores from this site (core Amédée 1, 2 and 5) show thatthe Holocene unit is clearly identified from 14 to 0 m core depth.Stages MIS 5 (100 to 130 ka) andMIS 7 (221 to 248 ka) relate to Unit 10(37–14m) andUnit 9 (40–37m) respectively. Unfortunately, the oldestMIS stages cannot be accurately dated because coral material fromthese time ranges were severely affected by late meteoric diagenesis.
The new U/Th dates obtained from core Kendec are 4.48±0.10 and6.44±0.12 ka from samples 2 K1 (3.20 m deep) and 3 K1 (5.15 m deep)respectively (Table 2). Although the coral at 3.2 m contains only 93.5%of aragonite, we consider that the age of 4.48 ka is valid because theremaining calcite is a high-magnesian calcite deposited in the marineenvironment. Although the coral analyzed at 17.35 m contains anexcess of δ234U (572.2) and that an age cannot be given, we canassume that this coral lived during the last interglacial period based
Table 2TIMS U/Th results from corals sampled in core Kendec
Samples Depth Age Δ (abs) Initial δ234U Δ (abs) % aragonite
(m) (yr)
2K1—3.2 m 3.2 4.48 ± 0.10 149.8 ± 5,0 93.53K1—5.15 m 5.15 6.44 ± 0.12 145.6 ± 4,0 99.836K1—17.35 m 17.35 312.00 ± 12.67 572.2 ± 8,4 96.0
on it depth in core i.e. on its stratigraphical position below theuppermost uncorformity. Below 17.35 m, several corals were sampledbut some of them contain less than 65% of aragonite (63.8% aragoniteat 28.5 m, 100% low magnesian calcite at 113 m) and were not dated.
3.4. Magnetostratigraphy
The susceptibility values range from −46 · 10− 10 m3 kg− 1
to 455 · 10−10 m3 kg−1 along core Amédée and from −15·10−10 m3 kg−1
to 434·10−10 m3 kg−1 along core Kendec comprising negative and positivevalues. Negative susceptibilities imply the large dominance of thediamagnetism of carbonates. Nevertheless, a part of these carbonatesmay have Fe-magnetic properties as revealed by the occurrence ofremanentmagnetization.Weak positive susceptibility values denote thedominance of a paramagnetism (silicates, clays) or a weak ferrimagnet-ism (highly diluted iron oxides). The measurements made at a periodicinterval reveal polarity reversals, basedon relative up–downdirection inthe unoriented core. This can be correlated to the geomagnetic polaritytime-scale of the Quaternary.
The mass specific NRM values range from 5 ·10−9 A m2 kg−1 to1 ·10−6 Am2 kg−1 in core Amédée and from10−8 Am2 kg−1 to 2 ·10−6 Am2 kg−1 in core Kendec. These values are thus one to three orders ofmagnitude higher than the sensitivity of the magnetometer. Theanhysteretic remanent magnetization intensity values are 10 to 100times higher than the NRM intensity value. The values of theseremanent magnetizations demonstrate that despite their weakmagnetic susceptibility, these coral reef carbonate materials containa sufficient amount of ferrimagnetic particles.
The medium destructive field of the NRM lies for most studiedlayers between 20 and 40mT. In few examplesMDFweaker than 5mTor higher than 60 mT were found. Thermal demagnetization at 220 °Cis enough to remove 1/2 to 4/5 of the NRM intensity. AF as wellas thermal demagnetization diagrams (orthogonal plots) show that aweak viscous overprint is removed at 5 or 10 mT, or at 100 °C,isolating a stable single component remanent magnetization probablycarried — considering the coring site position versus the main NewCaledonia island — by titano-magnetite grains deposited as detritalmaterial in the reef environment.
The stable NRM isolated at 40 mT AF (or at 220 °C) alloweddistinguishing three populations of inclination values (since the corewas collected using a rotary-drill system, the declination could not beused:
1) inclinations between −80 and −20° indicating a NRM acquired innormal polarity field (the inclination of the field created by thedipole field at −21° latitude equals −37.5°);
2) inclination between 20 and 60° indicating a NRM acquired in areverse polarity field.
3) inclination between −20 and +20° indicating either transitionalfield directions of an imperfect removal of a spurious (viscous)remanence.
The interpreted polarity and the correlation to polarity chronsof the Brunhes and late Matuyama are shown in Figs. 5-C and 6-C.Unfortunately, some intervals are represented by inappropriatematerials in both cores, however, the inclination record allowsidentifying normal and reverse magnetozones; a major transitionfrom the reverse polarity to the normal polarity is attributed to theMatuyama/Brunhes transition dated at 778 ka BP. It is clearlyestablished at ca 85 m in core Amédée, and at 70 m in core Kendec.
The return to a normal polarity at about 120 m in core Amédée,and beneath 120 m in core Kendec possibly corresponds to therecord of the Jaramillo subchron dated between 990 ka BP and 1070 kaBP. However the record of these two normal magnetozones shouldbetter be confirmed by further paleomagnetic analyses on newsamples and/or by independent datings.
Table 3Nannofossil occurrence in cores Amédée 4 and Kendec. A = abundant; C = common;R= rare; + = present, but not counted
Core Amédée 4
Age (Ma) 0.25–0.41 0.41–0.85
Sample depth (m) 38.5 50.1 56.05 124.2
Calcidiscus leptoporus R RGephyrocapsa caribbeanicaGephyrocapsa oceanica C C +Gephyrocapsa parallela A C + CGephyrocapsa spp. (small) C A + AHelicosphaera hyalina RPseudoemiliania lacunose CReticulofenestra spp. (small) R AUmbilicosphaera sibogae RCore Kendec
Age (Ma) notchronostratigraphicallysignificant
notchronostratigraphicallysignificant
Sample depth (m) 66.9 144.30
Calcidiscus leptoporusGephyrocapsa caribbeanicaGephyrocapsa oceanicaGephyrocapsa oceanica (large)Gephyrocapsa parallela +Gephyrocapsa spp. (small) + +Helicosphaera hyalinaPseudoemiliania lacunosa +Reticulofenestra spp. (small) +Umbilicosphaera sibogae +
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3.5. Nannofossils
In core Amédée 4 (Table 3), only 4 intervals contain nannofossils,respectively at 38.50 m, 50.10 m, 56.05 m and 124.20 m deep. In the 3uppermost layers, Gephyrocapsa oceanica, Gephyrocapsa parallela andGephyrocapsa spp. (small) are abundant to present while Calcidiscusleptoporus and Reticulofenestra spp. (small) are rare. In the lowermostinterval, at 124.20 m, Calcidiscus leptoporus, Gephyrocapsa spp.(small), Helicosphaera hyalina, Pseudoemiliania lacunosa, Reticulofe-nestra spp. (small) and Umbilicosphaera sibogae are abundant to rare.According to the calcareous nannofossil biostratigraphy, the 3 young-est beds can be correlated to the 0.41–0.85 million yr (Ma) interval,while the oldest one has an age ranging from 0.41 to 0.85 Ma.
In core Kendec (Table 3), only 2 layers yielded nannofossils. At66.90 m, the occurrence of Gephyrocapsa spp. (small) and Umbilico-sphaera sibogae is not chronostratigraphically significant. At 144.30 m,the record of Gephyrocapsa parallela, Gephyrocapsa spp. (small),Pseudoemiliania lacunosa and Reticulofenestra spp. (small) is also notchronostratigraphically significant.
4. Discussion
4.1. Age and structure of the western New Caledonian barrier reefsystems
On the basis of sedimentological, paleoecological and diageneticanalyses from a single drillhole, Coudray (1976) estimated that initiationof New Caledonian barrier reef systems occurred at around 2 Ma.
The present study has generated a significant body of newinformation about the growth history of New Caledonian barriersystems was gained based on detailed lithological, paleoecological andchronostratigraphical analyses of additional reef cores. Especially, itreveals that carbonates may have deposited earlier than previouslyexpected. In core Amédée 4, given the Brunhes–Matuyama reversal wasidentified at 80 m deep, the carbonate basement initiation is shown tohave occurred long before 0.78 Ma. (Fig. 7). In core Kendec, this
boundary has been recorded at 60 m. This age pattern has beenconfirmed by nannofossil biostratigraphy in both cores. Moreover, theBrunhes–Matuyama boundary is materialized by changes in lithology,i.e. betweenUnits 3 and4at85m incoreAmédée4, andbetweenUnits 5and 6 at 60 m in core Kendec. Based on the composition of sediments,especially the scarcity of coral elements, the lower units in cores (in coreAmédée 4 from 126.50 to 52m and in core Kendec from148.75 to 50m)may relate to a ramp or a slope depositional system rather than a typicalreef system, dominated by in-situ coral framework. By contrast,considering the abundance of coral colonies, with some still resting ingrowth position, the upper units (in core Amédée 4 from 52 to 11m andin core Kendec from 50 to 9.60 m) are regarded as part of typicalframework reefs, presumably representing inner reef flat zones.
In addition in cores Amédée 4 and Kendec, the field inversion,respectively recognized at around 122 and 117.30 m, could beattributed to the Matuyama–Jaramillo reversal (Fig. 7). However,these results must be used with caution. The nannofossil-basedbiostratigraphy cannot provide reliable information regarding thestratigraphical location of this reversal in the cores, although theMatuyama–Jaramillo boundary appeared to be marked by unconfor-mities between Units 1 and 2 in core Amédée 4, and between Units 2and 3 in core Kendec.
Due to a good preservation of coral material, the upper carbonatesequences gave reliable U/Th and radiocarbon ages. The base of theHolocene section ranges in depth as follows: in core Amédée 4, at 14m(Unit 11) and in core Kendec, at 9.60 m (Unit 11). Generally, theunconformity between the Holocene and the 125 ka-old section ismarked by significant diagenetic features generated by a long-termsub-aerial exposure. The Holocene–Pleistocene transition in coresAmédée 4 and Kendec exhibits typical calcrete crusts. The lastinterglacial episode (MIS 5) is represented by Unit 10 (14 to 37 mdeep) in core Amédée 4. This unit can be subdivided into 2 subunitsfrom 37 to 26 m and 26 to 14 m on the basis of an abrupt change inlithology at 26 m. From correlation with core Amédée 4, the 125 kareef unit can be identified as Units 9 and 10 in Core Kendec. In bothreef sequences, an unconformity surface was clearly observed at16.30 m and 21 m respectively, and marked by sharp shifts in δ18O(Figs. 5-B and 6-B). The development of 2 distinct units during the MIS5 interval could be related to the two high sea-level stands separatedby an about 10-m drop in sea level that occurred between 125 and115 ka (Thompson and Goldstein, 2005).
The reef units deposited during the interglacial stages older thanMIS5 are less easy to identify. In core Amédée 4, the MIS 7 unit has beendated, from 40 to 37 m and identified as Unit 9 (Frank et al. (2006)). Incore Kendec, the section related to MIS 7 is not clearly identifiable. AtAmédée site, Unit 7 can be correlated with MIS 11 (400 ka).
According to the chronostratigraphy of the two sequences, theevolutionof the carbonate barrier system is believed tohave beendrivenby 100,000-year orbital eccentricity cycles, at least in the relatively age-constrained upper sections. Furthermore, the existence of subunits,which have resulted principally from changes in deposition style,strongly support the influence of environmental disruptions operatingwithin the time period of a 100,000-year cycle. As pointed out by Perkset al. (2002), tropical climate variability and marine productivity werepredominantly controlled by the 23,000-year orbital precession cyclesduring the past 500 ka. In this respect, the occurrence of subunits fromthe upper parts of the sequences may reflect environmental changes inresponse to 23,000-year periods. Similarly, Liu and Herbert (2004)demonstrated that changes in climate and marine productivity in thetropics were predominantly controlled by the 41,000-year obliquitycycle from 1.8 to 1.2 Ma. The subunits described from the lower coresectionsmayhave deposited during41,000-year periods. Due to a lack inage resolution, this still remains highly speculative although suchanalyseswere performed in the Florida Keys (Multer et al., 2002) and theGreat BahamaBank (Kievman,1998). In addition, numericalmodelingofthe reef growth history at Hawaii revealed that for the past 250 ka,
Fig. 7. Time correlations between units from the cored reef sequences (Amédée and Kendec) in New Caledonia.
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repeated drowning and emergence of reefs were governed dominantlyby precessional and high-frequency, suborbital variations in sea level(Webster et al., 2007).
The tectonic behavior of the “Grande Terre” and, especiallysubsidence rates, can be inferred from the stratigraphic attributes ofeach cored carbonate sequence. In 1976, at Ténia, Coudray estimated arate of subsidence at 0.12 mm yr−1 and it was thought to be constant.This rate was calculated by Coudray who assumed that the of 226 m-thick reefal deposits at Ténia encompassed the whole 1.8 MaQuaternary. The coastal area from Ténia to Bourail is known to havebeen colonized by a barrier reef rimming a shore line that wassubjected to a low subsidence rate during the Quaternary. This schemeis consistent with the Quaternary tectonic history of the western NewCaledonian margin (Cabioch et al., 1996).
Assuming the sea level was 6 m higher than today, at about 125 ka(Broecker et al., 1968; Bloom et al., 1974), the subsidence rate isestimated at 0.16 mm yr−1 at the Amédée site, between 125 ka and theearly Holocene (Frank et al. (2006)). If this assumption is correct, the780 ka-old reef should lie at 128m. This disagrees with stratigraphy andthe present-day position of the Brunhes–Matuyama boundary found at80 m. Such discrepancies are likely to result from differential tectonicmovements affecting the New Caledonian mainland, during the lateQuaternary. Analyses of seismic lines and bathymetric records from theNouméa lagoon revealed that lagoonal deposition operated throughonly two glacial–interglacial cycles, probably in response to an increasein the rate of subsidence (Chevillotte et al., 2005; Lagabrielle et al., 2005).This hypothesis is consistent with our chronological results.
At theKendec site, the subsidence rate canbeestimatedat0.1mmyr−1,for the past 125 ka. If we use this rate, the Brunhes–Matuyama boundary
should be at 78 m deep. But, the magnetostratigraphic data showed thatthis limit was at around 60 m. This may be due to an increase in thesubsidence rateduring the lateQuaternaryasobservedat theAmédéesite.
4.2. Stratigraphical correlations between barrier reefs at the regional towest Pacific scales
Comparison between the investigated sites in New Caledoniaindicates that each carbonate series of Pleistocene age can besubdivided into 2 main sequences. The upper sequence is character-ized by the dominance of coral and coralline algal frameworks asfound in core Amédée 4 from 52 to 11m and in core Kendec from 50 to9.60 m. The lower sequence is mainly composed of grainstones,packstones or wackestones rich in debris of mollusks, calcareousalgae, foraminifers echinids and, occasionally, corals.
Two interpretations can be proposed to explain such a cleardifferentiation between the two sequences. The first hypothesis isthat, during the Quaternary, climatic conditions were not optimal forluxuriant reef growth fromabout 0.5Ma. Thiswas claimed previously toexplain the anatomy of the Australian Great Barrier Reef (Davies andPeederman, 1998; Alexander et al., 2001; Webster and Davies, 2003;Braithwaite et al., 2004). Inparticular, Davies and Peederman (1998) andWebster and Davies (2003) pointed out that, on the north-eastAustralian margin, at least in its central part, the settlement of typicaltropical reefsdidnotoccur earlier than452 to365ka (close toMIS11 and9). Indeed, atRibbonReef 5,well developed reef tractswere encounteredfrom core top to 96m.However, it is noteworthy that the Sr stratigraphyused by Alexander et al. (2001) and Braithwaite et al. (2004) can bequestioned since the Pleistocene 87/86Sr variation is not easily resolvable
102 G. Cabioch et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 268 (2008) 91–105
(Hodell et al., 1990). Our data seem to be consistent with Davies,Peederman and Webster assumption. Droxler and Farrell (2000) andDroxler et al. (2003) pointed out the MIS 11 is characterized by theoccurrence of highest values in δ18O in the late Quaternary and anincrease in carbonate production. Conversely, it is noteworthy thatYamamoto et al. (2006) did not observe a change at this period butdescribed 2 superimposed sequences in the emerged Pleistocene reeftracts in the Ryukyu islands (Japan): an upper carbonate sequence,deposited during the past 800 ka, and a lower one, consisting ofsiliciclastics andmixed carbonate-siliciclastics. These authors concludedthat changes in the mode of sedimentation were probably the result ofincreasing amplitude in sea-level fluctuations at around 0.8 Ma, i.e. atthe onset of themid-Pleistocene Climate Transition. In our study, such achange at around800 ka in themodeof sedimentationwasnot observedcontrary to the Great Barrier Reef. But more data are required to clarify
Fig. 8. Comparison between reef sequences from core Amédée 4 (New Caledonia) and Core Rib
this point in New Caledonia. Both observations in New Caledonia andnorth-east Australia (Fig. 8) indicate that some delay in reef initiationmay have taken place in different areas of the Coral Sea, probablydependingon their latitudinal location. Furthermore, at the scale of NewCaledonia, changes in latitude may have had differential effects on thediagenetic evolution of barrier reef bodies. For example, the occurrenceof paleosoils, calcretes and neomorphized features can vary from site tosite according to rainfall intensity during emergence phases.
The second hypothesis to explain the occurrence of 2 superimposedsequences is that the inundation of the outer New Caledonian shelfmay have started earlier than MIS 11, but the reef sequences older thanMIS 11 have deposited off the modern barrier reef system due to tectoniccollapses. An alternative hypothesis to account for the occurrence of non-reefal sequences at the base of the cores, may be proposed. These mayhave deposited along the slope, during interstadial / stadial episodes
bon Reef 5 (Australian Great Barrier Reef) (Alexander et al., 2001; Braithwaite et al., 2004).
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rather than during high sea-level stands. Solving this problem wouldrequire drilling along the fore reef slopes characterized by the presence ofseveral groups of terraces up to 120 m even more (Flamand, 2006).
The marked differences in the thickness of reef sequencesobserved between the Great Barrier Reef and New Caledonia can bedue to differences in subsidence rates and geodynamical evolution oftheir margins. Interference of regional tectonics with global toregional climatic conditions has driven the settlement and develop-ment of barrier reefs on both sides of the Coral sea.
4.3. Stratigraphical correlations between barrier reefs at the global scale
The origin and development patterns of barrier reef systems fromdeep reef coring series are poorly documented in most tropical areas.In the western Atlantic Ocean, significant data were obtained from theFlorida and Belize.
Some 60 m-long and shallow cores were recovered in the KeyLargo limestones in the Florida Keys covering the last 420 ka (Multeret al., 2002). In the Florida Keys, the analysis of 69 deep and shallowcores drilled into the Key Largo limestone shows the occurrence of 5Quaternary units (Multer et al., 2002). Unit Q1 is characterized by theabundance of quartz grains and grainstones rich in mollusks andbenthic foraminifera, with some coralline algae and scarce branchingcorals. Unit Q2 is very similar to the previous unit but poorer in quartzcomponents, while the coralline algae and corals increase inabundance. Multer et al. (2002) assumed that the Units Q1 and Q2may have been deposited during the MIS 11 event, expressing theinitiation of the Florida platform. Unit Q3 is rich in massive coral andcoralline algal boundstones mixed with wackestones and packstonesdominated by mollusks and benthic foraminifera. This unit wasassumed to have formed during the MIS 9 event (Multer et al., 2002).Unit Q4 is very similar in composition to Unit Q3 but the volume of
Fig. 9. Development model for the New Caledonian barrier reef tract over the pas
coralgal boundstones tends to decrease. This unit refers to MIS 7highstand (Multer et al., 2002). Unit Q5 is mainly composed of coralgalboundstones and also contains high amount of Halimeda; it depositedduringMIS 5e. It appears that the Key Largo limestone formed throughseveral phases. During the MIS 11 highstand, a shallow-waterplatform developed. Then, during MIS 9 to 5, typical reef tracts settledand flourished. Although this scenario seems to be partly differentfrom the evolution of the New Caledonian barrier reef tract, there issome similarity in the timing and the process of reef settlement. Inboth sites, coral reef framework appear not to have initiated prior toMIS 9, from non-reefal, carbonate foundations.
Some relatively short cores were also extracted from the Belizianbarrier reef. These cores have provided data on the Holocene and lastinterglacial (MIS 5e) reef growth (Gischler et al., 2000; Purdy et al.,2003). Unfortunately, there is no available information on thechronology of depositional events prior to the last interglacial episode(Purdy et al., 2003). This lack of dating was also pointed out byMazzullo (2006) who provided new data on the overall evolution ofthe Quaternary Belize platform. Based on analyses of lithofaciescoupled with seismic profiles, the development of the Belizian barrierreef appears to have been controlled by tectonics and sea-levelchanges, throughout the Quaternary. To date, except the 125 ka reef,the older Pleistocene units are not yet chronologically constrained.
5. Conclusion
Coring investigations carried out through the south-west to north-west parts of the New Caledonian barrier system indicate that thelatter results from the superimposition of 11 sedimentary units thathave deposited during successive transgressive to high sea stands ininterglacial periods. These units are separated from each other byunconformities that are interpreted as formed during low sea stands
t 400 ka according to eustatic and tectonic controls. 150°E 160°E 170°E 18°S.
104 G. Cabioch et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 268 (2008) 91–105
in glacial periods (Fig. 9). Accordingly, several glacial–interglacialcycles can be identified. But, only the upper sections of the twosequences that refer to 100 ka eccentricity cycles can be accuratelydated or linked to a given isotopic stage. Furthermore, the occurrenceof subunits within some of the depositional units may expressenvironmental changes within the time range of interglacials. Thesesubunits may be tied to 23 ka precessional orbital cycles. Thesuccession of the carbonate sequences into units and subunits appearsto reflect the control of orbital cycles operating at different time scales.
The age of reef initiation differs slightly from site to site, accordingto the subsidence rate of the shelf-margin at each site. Based on avariety of dating methods (Uranium-series, mainly open system agesfor all ages older than the Holocene and radiocarbon, magnetostrati-graphy, nannofossil biostratigraphy), major building in New Caledoniaappears to have started during the MIS 11 from shallow-watercarbonate platform deposits older than 780 ka at Amédée and Kendecsites. This is confirmed by recent reflection seismic records obtainedfrom the outer shelf of the north-eastern New Caledonian margin.These records revealed that during the Pliocene and the Quaternary, a250 to 450-m-thick carbonate platform capped by the barrier reef hasformed (Chardon et al., 2008). The study of additional cores extractedfrom Ténia is still in progress. Preliminary results based on nannofossilbiostratigraphy indicate that the carbonate system has probablyinitiated as early as 1.4 Ma.
Although marked differences can be observed in the structure andage of the barrier reefs in New Caledonia and Australia, the deve-lopment patterns appear similar. On both sides of the Coral Sea,barrier reef building has been controlled by climatic changesinterplaying with tectonic motions. Coral reefs flourished from MIS11 until now. The MIS 11 highstand can be considered as a period ofluxuriant reef expansion in this part of the Pacific.
Acknowledgements
We thank all the successive Directors of IRD Centre, at Nouméa:F. 2Jarrige, C. Colin and F. Colin for their assistance and help duringcoring operations and core studies. Yvan Join, Jean-Louis Laurent,Claude Ihilly, Denis Utramadra, Julien Perrier and John Butscher arethanked for their experience during the drilling operations. We alsothank the “Phares et Balises” Service (MM. Trigalo, Babin and Mary),Province Sud of New Caledonia (M. Farman) and Province Nord of NewCaledonia for their assistance. Our thanks are extended to J. Récy andB. Pelletier for their assistance, UMR Geosciences Azur 6526 for itsfinancial support during the field trip, the French Navy (MarineNationale Française) for assistance in carrying equipment and to thepeople from the different areas for their hospitality and help duringdrilling operations. Similarly, we thank M. Joachimski (ErlangenUniversity, Germany) for isotopic analyses, T. Pilorge for preparingthin sections, H. Boucher and S. Caquineau for DRX analyses, B.Cahuzac (University of Bordeaux I) and K. Fujita (University ofRyukyus) for identification of foraminifera. We are particularlygrateful for the constructive comments from Reviewers E. Gischlerand J. Webster and Editor T. Corrège who significantly improved themanuscript. This work is jointly supported by the French NationalProgramme ECLIPSE (“Environnement et CLImat du Passé: hiStoire etEvolution”) sponsored by INSU-CNRS and by IRD (Institut deRecherche pour le Développement).
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