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THE GREAT BARRIER REEF: THE CHRONOLOGICAL RECORD FROM A NEW BOREHOLE COLIN J.R. BRAITHWAITE,1 HE´LE`NE DALMASSO, 2 MABS. A. GILMOUR,3 DOUGLAS D. HARKNESS,4 GIDEON M. HENDERSON,5 R. LIN F. KAY,6 DICK KROON,7 LUCIEN F. MONTAGGIONI, 2 AND PAUL A. WILSON3 1

Division of Earth Sciences, The University of Glasgow, Glasgow G12 8QQ U.K. e-mail: [email protected] 2 Universite ´ de Provence, Centre de Se´dimentologie-Paleontologie, UPRESA 6019 CNRS, Place Victor Hugo, F-13331 Marseille ce´dex 3, France 3 Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, U.K. 4 Scottish Universities Environmental Research Centre, Rankine Avenue, East Kilbride G75 0QF U.K. 5 Department of Earth Sciences, The University of Oxford, Parks Road, Oxford OX1 3PR U.K. 6 Natural Environment Research Council, Polaris House, North Star Avenue, Swindon SN2 1EU, U.K. 7 Faculty of Earth Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands 8 School of Ocean and Earth Science, The University of Southampton, Southampton SO14 3ZH, U.K.

ABSTRACT: A new borehole, 210 mbsf (meters below sea floor) deep, drilled in Ribbon Reef 5 on the Great Barrier Reef off Cooktown, NE Australia, reveals a shallowing-upwards succession, the younger part of which is punctuated by a series of erosion surfaces. Nine depositional units have been defined by lithological changes and are numbered sequentially from the base of the hole upwards. Aminostratigraphy, magnetostratigraphy, radiocarbon dating, uranium series dating, and modeling together with strontium ratios have been applied in an attempt to establish a chronology of accumulation. Carbonate deposition began about 770 ka ago in a relatively deepwater slope environment and is represented by a series of debris flows. Lithoclasts within these rocks, indicate that older limestones already existed in the area. Subsequent accretion involved the downslope accumulation of grainstones and wackestones, sometimes cross-laminated, characterized by intervals with abundant rhodoliths and scattered, probably reworked, corals. Four units at the base of the hole reflect deposition that probably began during isotope stage 16 and continued through stage 15 from about 770 to about 564 ka. Unit 5 probably extended to stage 11 (about 400 ka), and unit 6 to stage 9 (; 330 ka). Typical reefal associations of corals and calcareous algae were established in this area only above depths of about 100 m in the borehole, units 5–4. The succession is apparently unbroken to an erosion surface at 36 mbsf indicating subaerial emergence. The lack of evidence of emergence below this surface reflects progressive accretion or progradation or both. Two younger erosion surfaces define further periods of lowered sea level. Unit 7 is attributed to deposition during isotope stage 7, but erosion during stage 8 resulted in the preservation of only 8 m of unit 7 limestones. Unit 8 is correlated with stage 5 (;125 ka), and unit 9 is interpreted as Holocene (post 7,700 ka). The limited thicknesses of units 7, 8, and 9 are considered to reflect erosion. The progressive shallowing brought the depositional surface within the zone exposed during lowstands, and there is no sedimentological evidence that aggradation was restricted by a lack of accommodation.

INTRODUCTION

Following its discovery by James Cook in 1770, and the visit of Charles Darwin in 1836, the Great Barrier Reef (GBR) has fascinated both science and the public at large, as demonstrated by the popularity of C.M. Yonge’s (1930) book and a succession of more general texts. The reef complex is almost 2,000 km long, covering an area of some 250,000 km 2 off the NE coast of Australia (Fig. 1), and size is undoubtedly part of the reason why it has captured popular imagination. Both public and scientific interests have focused on the biological characteristics of the GBR and its importance as a major ecosystem and center of biodiversity. Yonge (1930) carried forwards Darwin’s (1890) ideas concerning reef growth and structure and saw the edifice simply as a ‘‘barrier reef.’’ However, by the time of pubJOURNAL OF SEDIMENTARY RESEARCH, VOL. 74, NO. 2, MARCH, 2004, P. 298–310 Copyright q 2004, SEPM (Society for Sedimentary Geology) 1527-1404/04/074-298/$03.00

lication of Maxwell’s (1968) atlas, this view had been modified to take account of the likely effects of changing sea level and the position of the reef on a major block-faulted passive continental margin. We remained largely ignorant of the geology of the interior of ‘‘the reef’’ or of its detailed history. Davies et al. (1989) provided an important step forward in this, interpreting seismic stratigraphy to describe the development of the platform margin over the last 60 Ma (million years), but it was not clear from this when ‘‘the reef,’’ as it now is, was initiated, or how it subsequently developed. Many shallow exploratory holes have been drilled on the GBR, but only four (Fig. 1), at Anchor Cay, Michaelmas Cay, Heron Island, and Wreck Reef (Humber 1960), are regarded as ‘‘relatively deep.’’ Of these, only those at Heron Island (Davies 1974) and Michaelmas Cay (Richards and Hill 1942) sampled the Pleistocene, and neither shed much light on the structure or age of the rocks recovered. In 1990, during the Ocean Drilling Program (ODP) Leg 133, three holes were drilled on an upper slope terrace east of Cairns, in front of the outer reef barrier (Davies et al. 1993). Combined with seismic profiles across the margin, the cores from these were used by Davies (1991) and Davies and McKenzie (1993) to suggest that the shelf-edge reefs are less than 500 ka old. However, Feary et al. (1993) used the same data to conclude that the GBR might be as much as a million years old, while Montaggioni and Ve´nec-Peyre´ (1993) suggested, on the basis of diagnostic shallow-water benthic foraminifera, the likely existence of a succession of reefs throughout the Pleistocene as precursors to the modern reef system. In order to resolve these issues, in 1995 an International Consortium drilled two new holes (International Consortium 2001). These were sited at Boulder Reef, latitude 158 23.9449 S, longitude 1458 26.1829 E, about 5 km east of Cooktown on the Queensland coast, and at Ribbon Reef 5, latitude 158 22.409 S, longitude 1458 47.1499 E, approximately 49 km east of Cooktown (Fig. 1). The intention was to drill through the reef and to date the initiation of reef construction in the region by the onset of growth of the main tropical reefal framework. Preliminary results relating to the absolute age of the structure based on strontium ratios and magnetostratigraphy have appeared in International Consortium (2001). Tables of these results and stable-isotope data are to be found in GSA Repository item 2001051 Tables 1–3, available from GSA P.O. Box 9140, Boulder, CO 80301-9140 U.S.A. Here we are concerned with other dating methods applied, and with the resulting attempts to provide a cumulative chronology and correlation with offshore ODP boreholes. Sedimentological and diagenetic data will be limited to those necessary to illustrate the nature of the succession. The GBR is the largest structure of its kind on Earth. Knowledge of its history is therefore important for its own sake but also provides new data on current issues of sea level fluctuation and climatic change. METHODS

A ‘‘Mobil’’ drill installed aboard a 50 ton jack-up platform was towed to the drilling sites on a 40 m barge. Positions were established using

GREAT BARRIER REEF CHRONOLOGY satellite-based Global Positioning (GPS) and, after jack-up at high tide, the barge floated free to anchor. Drilling was based on a diamond-tipped NQ core barrel (75 mm core) and a wireline rod system. The power limit of the equipment was approximately 400 m, but drilling was terminated at 210 m at an apparently thick siliciclastic unit, on the assumption that the entire ‘‘reef’’ section had been penetrated. After completion, the hole was logged using geophysical methods and cased with slotted PVC tubing, to allow for future geochemical monitoring. Recovery is shown in Figure 2, and depths are reported as meters below sea floor (mbsf). The lithology of the core has been described to millimeter intervals, with each core run recovery hung from the top of the recorded interval. The accuracy of depth references therefore depend on the proportion of core recovered. Some 202 thin sections have been examined for evidence of diagenetic change. These will be described elsewhere. XRD analyses on 57 splits from approximately 234 samples used for stable-isotope analyses provide a guide to mineral composition. Reference cores are currently stored in the Department of Geology at the University of Sydney, NSW, Australia. Age constraints are based on five independent chronostratigraphic methods: 1) aminostratigraphy; 2) magnetostratigraphy; 3) radiocarbon dating; 4) strontium isotope stratigraphy, and 5) uranium-series dating. With one exception, these methods have all been limited to some degree by diagenetic alteration of the corals analyzed. Samples for amino acid analyses (n 5 22) were prepared using the method of Sykes et al. (1995) and analyzed on a Perkin-Elmer Applied Biosystems 402 Amino Acid Analyzer using phenylisothiocyanate (PITC) to derivatize. The amino acid analyzer chemistry was then changed to enable D/L isomers to be identified using Murphy’s reagent on a chiral column. Regrettably, although amino acids were present, concentrations declined with supposed age and were ultimately so low that samples were too viscous to transfer. No D/L isomers could be determined, and therefore no ages suggested. While these results are disappointing they confirm comments by Hearty (1998) on three counts: 1) alteration occurs in samples that are shallowly buried and (importantly) leached, 2) the epimerization reaction responds to warmer interglacial periods with faster rates, and 3) the near absence of amino acids is in itself an indicator of relatively older ages. The overall results of Paleomagnetic determination (n 5 42 samples) were initially reported in International Consortium (2001). Although carbonates have only weak magnetic intensities, these are easily measured on modern cryogenic magnetometers and magnetization is stable. The samples from Ribbon Reef 5 displayed almost ideal paleomagnetic behavior, with simple predominantly single-component remanences close to that expected of the geocentric axial dipole field. Magnetization intensities (typically of 0.5 mA/m) have median destructive fields of ; 20 mT. Partial demagnetization, using alternating fields, reveals a small soft component that was typically removed at the first demagnetization step of 5 mT. Thereafter, samples showed only single polarity components characteristic of a stable primary remanence (McNeill and Kirschvink 1993; Lu et al. 1996). Approximately 10% by weight of each sample for radiocarbon analysis was removed by acid hydrolysis with 0.5M HCl. Etched samples were then hydrolyzed to CO2 using 4M HCl and converted to benzene using conventional methods (Harkness and Wilson 1972). Analyses were by liquid scintillation counting of benzene/scintillant cocktail (5 radiometric analyses). Results have been normalized to d13C 5 225‰ on the PDB scale. Stable carbon isotope ratios were measured on sample benzene combusted to CO2 using a dual-inlet mass spectrometer with a multiple ion beam collection facility (VG OPTIMA), calibrated with international reference materials to a precision of 6 0.1‰. The dates quoted are in conventional 14C years BP and are expressed at 6 1 s (68%) level for overall confidence. The appropriate reservoir correction (; 450 6 50 yr.) should be subtracted for direct comparison with the terrestrial-based timescale. Of the 11 ages obtained for Ribbon Reef 5 materials, a number suggest ‘‘infinite’’ values— that is, a situation in which no significant 14C signal could be detected

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above the analytical background set when the identical treatment and monitoring conditions were applied to a sample of geologically old carbon. The 87Sr/86Sr ratios of eight samples from three units in the Ribbon Reef 5 borehole have been reported previously (International Consortium 2001). Samples from corals and micritic limestone were obtained by microdrilling (Wilson et al. 1998). Measurements were made with a VG Sector 54 multicollector instrument using a three-collector multidynamic peak-switching program, and ratios were normalized to 86Sr/88Sr 5 0.0194. Conversion of Sr-isotope ratios to an inferred age was achieved by reference to the seawater curve (linear fit from 2.5 ma to present, of Hodell et al. 1990). During this work, results obtained on laboratory standards were: SRM 987 5 0.710256 6 14 3 106 (2sn21, n 5 46), with modern seawater inferred from a coral from the Philippines (‘A2’) 5 0.709182 6 15 3 106 (2sn21, n 5 6). The systematics of 87Sr/86Sr ratios are less prone to diagenetic alteration of primary values than U-series dating, and Sr stratigraphy is therefore a powerful tool in offshore carbonate sequences because of the relative separation from sources of radiogenic Sr. Metastable carbonate minerals are normally altered to calcite in pore fluids having an 87Sr/86Sr composition identical to that of seawater at the time of deposition (Ludwig et al. 1998; Quinn et al. 1991; Ohde and Elderfield 1992; Aharon et al. 1993; Wilson et al. 1998). Fortunately, this condition usually holds even during alteration in meteoric waters because, in these, sedimentary carbonates are the only source of Sr. Samples for uranium series analyses (n 5 5) were washed three times with ultrapure water to remove surface contaminants before total dissolution with nitric acid, and spiked with a mixed 229Th/ 236U spike. One sample containing some silicate detritus was subjected to total dissolution using HF. Uranium and thorium fractions were separated on anion exchange columns using standard techniques (Edwards et al. 1986). Both uranium and thorium were loaded onto graphite-coated Re filaments and analyzed using a Finnigan MAT262 mass spectrometer equipped with a retarding potential quadrapole and secondary electron multiplier (described by van Calsteren and Schwieters 1995). A dynamic peak-switching routine was employed measuring 234U/ 236U and 235U/ 236U (a proxy for 238U, assuming a 238U/ 235U natural ratio of 137.88) and 230Th/ 229Th and 232Th/ 229Th. Although 232Th abundance is not required for the age calculation it was always measured to determine any detrital thorium input. The standard method of obtaining a U-series age relies mainly on the ingrowth of 230Th towards secular equilibrium. However, it is also theoretically possible to use the 234U/ 238U ratio to estimate an age without depending on 230Th. Although the 234U/ 238U ratio is notoriously susceptible to diagenetic alteration, simple modeling of U-series systematics, allows some age constraints to be derived from 234U/ 238U data. A second argument suggests that a sample assumed to be from isotope stage 5e (see text) has a d 234U value that is too high, and has therefore suffered the addition of 234U. It is possible to use the deviation of d 234U from that expected for this sample to assess the rate of addition. Results of the various methods are given in Table 1. Oxygen and carbon isotope values (d18O%o PDB) were reported in International Consortium (2001) and are deposited as ICGBRD (Supplementary Table 2). SEDIMENTOLOGICAL BACKGROUND

Three sets of factors control biological and lithological variation within the cored interval. (1) Variation in the depositional environment, including changes in ecology, due to changes in depth, local hydrodynamic energy, sea-surface temperature and nutrients, and rates of deposition. (2) Positive and negative changes in relative sea level, as recorded in unbroken sedimentary sequences and in erosion surfaces, respectively. 3) Diagenetic changes, including those in isotope geochemistry controlled by groundwater flow and the relative positions of the water table with respect to the contemporary sediment surface. Sedimentological evidence in the Ribbon Reef 5 core indicates that sea level has moved several times with respect

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FIG. 1.—General location of the Ribbon Reef 5 borehole and other localities referred to relative to the Great Barrier Reef.

to the sequence represented, and therefore, to varying degrees, the isotope geochemistry of the rocks reflects the overprinting of these successive events. Nine units, summarized in Figure 2 are defined. These are a significant refinement of the three generalized depositional sequences described by the International Consortium (2001). Unit 1: 210.85–184 mbsf The borehole on Ribbon Reef 5 was terminated at a depth of 210.85 mbsf, considered to be the base of the carbonate succession. Low recovery in the last few meters was due in part to relatively large volumes of apparently unconsolidated sand-grade sediment. Two lithologies are present in core at intervals up to 184 mbsf and are probably interbedded: 1) distinctive, locally micropeloidal, carbonate mudstones and 2) grainstones. From 209.3 to 184 mbsf most of the rocks recovered (Fig. 2) are dense white or gray grainstones. Within these, bioclasts 2–3 cm in diameter in-

clude neomorphosed corals, Millepora, Tubipora, a faviid and an unidentified mussid, bivalves, and gastropods including trochids and smaller forms, echinoderm plates, and thin phylloid and encrusting coralline algae, together with fragments that resemble lithoclasts (208.2 mbsf). The sandsize clasts include in addition the dasycladacean Halimeda (commonly as molds), and foraminifera, Marginopora, Heterostegina, textularids, miliolids, and encrusting forms, together with a few quartz grains (at 208.5 mbsf). Although many corals are neomorphosed, a few are well-preserved and retain traces of aragonite cement (184.2 mbsf.) Beds ; 10 cm thick at 199.3 and 200.3 mbsf are faintly laminated. The grainstones are commonly friable, but from 208.5 to 208 mbsf and from 185.8 to 185 mbsf the recovery consists of loose and apparently uncemented carbonate sand. Wackestones–packstones 11 and 20 cm thick appear at 199.3 and 181.4 mbsf, respectively. Dark silty mudstones with scattered silt-size quartz grains are present at 205.2, 193 to 193.1, 190.6 to 190, 188.1 to 188.4, and 185 to 184.3 mbsf. Planar-laminated (2–4 mm) and structureless intervals within these contain molds of bivalves and Halimeda, burrows (190.2 mbsf), and

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FIG. 2.—Schematic lithological log and summary recovery log (black bars) for the Ribbon Reef 5 core.

scattered corals (4 cm at 184.8 mbsf with a thick coralline crust). However, at 190.6, 190.4, and 184.5 mbsf lamination is folded and locally draped around areas of grainstone. These sections lack macrofossils but contain large numbers of a planktonic foraminifer resembling Orbulina universa. At 184.5 mbsf they also include en echelon tension fractures. At 184.2 mbsf a sharp contact in one core section indicates that siltstones extend into borings in a grainstone surface that truncates bioclasts, and the two lithologies are probably interbedded. Larger fragments of corals (Millepora and ?Echinopora) are associated with the core but seem not to be related to the siltstones. Stable oxygen isotope values show a marked break in this unit, with those below ; 196 mbsf around zero and those above generally between 24 and 25‰ (Fig. 3). There is a parallel shift in stable carbon isotope values from approximately 12‰ to values close to zero. Interpretation.—The folded mudstones and lithoclasts are interpreted as representing slumps and the grainstones reflect downslope incursions of shallow-water sediment, perhaps as turbidites. Although they are not common, the lithoclasts are important because they imply the presence nearby of some older limestone edifice. Other boreholes (Davies 1974; Marshall 1983) have revealed Pleistocene rocks that may be of greater age than those recovered in Ribbon Reef 5 but also limestones considered to be Miocene. Thus the sequence sampled in Ribbon Reef 5 is only the base of the preserved carbonate accumulation in this area, and older carbonates may be present elsewhere in the GBR. The sedimentological assembly seems to reflect deposition on a deep slope or ramp. Both the d18O and d13C values are higher than in any section of the core except the Holocene, and these and the presence of aragonite may indicate the preservation of a deeper,

FIG. 3.—Plots of d18O and d13C (‰ PDB) for the Ribbon Reef 5 core.

and probably cooler, marine isotopic signal untouched by freshwater influence. Unit 2: 184–155 mbsf The boundary between units 1 and 2 is based on the last appearance of folded mudstones. The rocks recovered between 184 and 155 mbsf are generally fine-grained grainstones–packstones, with minor intervals of mudstones–wackestones and only scattered larger bioclasts. There are, however, long intervals of poor or no recovery (Fig. 2). The lower part of this unit is gray, but at ; 170 mbsf the color changes to a conspicuous

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C.J.R. BRAITHWAITE ET AL. TABLE 1.—Methods and results of radiometric and other dating. Method

Sample (mbsf)

Unit

Age (ka)

Magnetic susceptability Radiocarbon

210 2.4 3.1 4.2 6.2 9.1 10.6 14 16.1 16.5 21.3 17.75 16.1 56.4 94.9 118 184.8 5 samples 206–210 3 samples 122, 113, and 88

1 9 9 9 9 9 9 9 8 8 8 8 8 6 6 6 6 1 4&5

770 4.46 4.95 6.03 6.72 6.43 7.20 7.72 29.9 .45 32.47 .45 125.7 322 564 616 414.92 Mean 770 Mean 600

U-Series (TIMS)

87

Sr/86Sr

Error 60.05 60.05 60.05 60.05 60.05 60.05 60.05 60.02 60.02 60.6 619 678 651 619 6280 6280

pink. Most of the grainstones are structureless, but faint, gently inclined lamination is present in a 26 cm interval below 162.25 mbsf. At ; 162.3 mbsf dilated en echelon shear fractures imply post-cementation (?downslope) movement. Scattered larger bioclasts include corals (a 4.5 cm Millepora with a Magilus boring at 181 mbsf), solitary corals, fragments of coralline algal crusts, Halimeda, bivalve and gastropod shell fragments, with bryozoans and serpulids on some shell surfaces. Smaller components comprise fragments of these together with foraminifera including Heterostegina and Marginopora. Many grains show a chalky alteration or appear as molds. Thin (10 cm) rudstones consisting of platy grain-supported clasts 7–10 mm diameter are present at 174.6 and 176.6 mbsf. These include fragments of coral, coralline algae, and mollusc shells. Small amounts of grainstone and wackestone have filtered between these to locally form geopetal floors. Pink wackestones–mudstones are present at 166 mbsf (13 cm), 164 mbsf (8 cm), and 163.4 to 163 mbsf. These contain a 7 cm Galaxaea (at 163.1 mbsf) but typically smaller fragments of coral (some extensively bored), coralline algae (rhodoliths at 164 mbsf), bivalve and gastropod shell fragments, echinoderm plates, bryozoa, barnacles, and foraminifera, including Heterostegina and encrusting forms associated with vermetids. Irregular grainstone areas 3–4 cm diameter at 163 and 162.8 mbsf resemble lithoclasts. A thin (10 cm) pink mudstone at 162.7 mbsf has a steeply inclined upper boundary with overlying grainstones. Coralline algae throughout this unit are principally Mesophyllum and Lithophyllum. Foraminifera include Heterostegina, Amphistegina, Marginopora, miliolids, and others. Values for d18O vary around 24‰ to 25‰, and there is a similar fluctuation in d13C around zero, although this is superimposed on a general decline to 22‰, with a spike close to 26‰ just above the upper boundary of the unit (Fig. 3). Interpretation.—The upwards change in color in the core, from gray to pink, probably reflects a diagenetic oxidation–reduction boundary, within the groundwater, rather than any sedimentological variation. Although corals are common, they do not represent a growth framework, and the algal flora, dominated by melobesiids, implies cool (by tropical standards) and relatively deep water. The diverse fauna of bivalves, gastropods, bryozoans and echinoderms suggests that although deposition may have occurred in relatively deep water, perhaps low on a slope, much of the sediment was derived from shallower areas. The carbon values in particular imply a phreatic rather than a vadose diagenetic influence (Morse and Mackenzie 1990). Unit 3: 155–131 mbsf Recovery is poor to 155 mbsf, and includes broken friable core and uncemented sand (142.3 to 141.85 mbsf), but there is a marked change in the character of deposition. The rocks in the interval are dominantly pink

grainstones (they coarsen to granule grade, 2 to 3 mm, for ; 10 cm at 149.5 mbsf). At 137.3 mbsf they show cross-lamination (, 10o), and at 131 mbsf form several 5 to 10 mm graded laminae. There is a sharp, irregular boundary at 131.6 mbsf between dense mudstone surrounding a large branching coral with a thick coralline crust and a faintly laminated grainstone. Grain-scale porosity is common. Thin and locally laminated packstones–wackestones are present at 147.4 (6 cm) and 139.2 (; 10 cm) mbsf. Within all of these, neomorphosed coral fragments (some . 6 cm) include: extensively bored faviids at 155, 154.6, 154.4, 153, 152.3 to 152.0, 143, 143.2, 139, 138.7, 136.8, and 136.5 mbsf; Porites at 152.5 mbsf; Millepora at 154.3, 153.2, and 134.4 mbsf, and branching Acropora at 154.2, 153.1, and 131.5 mbsf together with smaller fragments of corals and coralline algal crusts, Halimeda molds, mollusc shells including Trochus, echinoid spines, and foraminifera (Heterostegina, Marginopora, miliolids, and textularids, as well as encrusting forms). Recovery is patchy, but larger corals appear at intervals of ; 10 cm in the material retained. They are significantly smaller from 152 to 143 mbsf and nowhere form a framework. Many carry centimeter-thick (5 cm at 133.5 mbsf) coralline algal crusts (Lithophyllum and Dermatolithon) and are extensively bored. Rhodoliths up to 4 cm in diameter are common, at 154.4, 154.2, 152.4, 152.2, 151.3, 150.1, 144, 143.8, 142.9, 142.4, 133.5, and 131.8 (7 cm) mbsf, and form a distinctive layer at 154.5 mbsf. There is a progression from an assemblage characterized by melobesiids to one dominated by lithophyllids. At the base of the unit, there is a marked change in d18O from variation between 24‰ to 22‰ in unit 2 to more uniform values around 26‰ from 155 to 132 mbsf. In the same interval carbon values decrease dramatically from about 22‰ to (nearly) 26‰ (Fig. 3). Interpretation.—The gradual up-hole transition from an algal flora dominated by melobesiids to one consisting mainly of lithophyllids is taken to imply a warming and shallowing of waters. Rhodoliths are common, and corals increase both in size and numbers up sequence. However, grainstones dominate the succession and corals are not sufficiently closely spaced to imply the presence of a framework. The cross-lamination and graded beds are interpreted as indicating downslope transport rather than in situ accumulation, and graded layers suggest that this may have been by turbidity currents or in response to storms. The progressive decrease in both d18O and d13C values implies an increasing vadose diagenetic influence, but gamma logs (International Consortium 2001) provide no indication of any break in deposition. The irregular boundary between mudstones–wackestones and grainstones at 131.6 mbsf may reflect sea-floor cementation or represent a marine erosion surface. Unit 4: 131–99 mbsf At 131 mbsf well-cemented laminated grainstones are overlain by pink mudstones–wackestones. These are the dominant lithologies between 132 and 99 mbsf (Fig. 2), and this transition is taken as the unit boundary. However, grainstones follow from 129.7 to 121.4 mbsf with a mudstone interval of ; 15 cm at 129.5 mbsf. Local patches of grainstone fill burrows within mudstones from 130 to 122 mbsf. Gently inclined graded laminae are present from 123 to 122.5 mbsf (where there are no macrofossils) and in ; 10 cm intervals at 111.2 and 111.4 mbsf. Variable recovery between 122 and 118 mbsf (Fig. 2) includes pink wackestones–packstones with intervals of a few centimeters of grainstones. Recovery above is dominated by wackestones–packstones with a few centimeters of grainstones at 116.2, 112.2, 111.4, 111.1, 110.7, 110.4, and 110.2 mbsf, and in smaller less regular patches. Grainstone is present from 106 to 105 mbsf and further centimeter-scale layers between 104 and 103 mbsf. From 103 to 99 mbsf the sequence is almost entirely of grainstone, although thin mudstones are intercalated at 102.8 and 102.6 mbsf. Coral fragments are relatively sparse, and although some are as large as 5 to 8 cm these are limited to 1 to 2 per meter. The general distribution of Acropora, Millepora, faviids, and Porites is shown in Figure 4. Less common genera include: Pocillopora at

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FIG. 4.—Distribution of the principal corals and lithologies in unit 4. Note here and in subsequent diagrams that the corals identified are by no means the total present. It is considered that distributions shown are nevertheless a reflection of the real distribution of fragments attributed to the genera named. Mudstones and wackestones are shown as a gray tone; grainstones appear white.

129.0, 120.3, 122.3, and 106.3 mbsf; Stylophora at 121.2 and 108.3 mbsf; Tubipora at 108.6 and 107.6 mbsf; and Fungia at 127.4, 111.3, and 108.4 mbsf. Many corals are intensely bored with thick coralline algal crusts seen at 130.7, 130.5, 130.2, 129.5, and 127.5 mbsf. Independent branching coralline algae appear at 130.8, 130.5, 130.3, 129.5, 127.2, and 122.4 mbsf. Rhodoliths are present between 116.5, and 112.6 and also occur 103.6 mbsf (Fig. 4). At the base of the unit at 131 mbsf coralline algae are predominantly mastophoroids, but from ; 110 mbsf upwards these are replaced by lithophylloids. There appear to be relatively few other fossils, but small bivalves, gastropods (including a small cowrie), echinoid spines, bryozoans, and serpulids have all been noted. There is commonly extensive vuggy porosity. Sand-size bioclasts in all grainstones within this unit include Halimeda, Amphiroa, coralline algae, molluscan shell fragments, and echinoderm plates, together with foraminifera, commonly Amphistegina, Calcarina, Marginopora, and encrusting forms, and alcyonarian and sponge spicules. A few quartz grains are present, and a peak on the gamma log at ; 103 mbsf (Fig. 3) is thought to reflect a relatively clay-rich incursion. However, it is uncertain which, if any, of several carbonate mudstones between 103 and 105 mbsf it represents. In contrast with adjacent units the behavior of carbon and oxygen isotopes is antithetic. Whereas the d18O values show a progressive rise from values close to 26‰ to about 25%o at 95 mbsf, carbon values rise to a peak of 22‰ at about 120 mbsf before falling towards 26‰ (Fig. 3). Interpretation.—Calcareous algae at the base of the unit are predomi-

nantly mastophorids, indicating a transition to shallower and possibly warmer waters (Adey 1986; Borowitzka and Larkum 1986). However, lithophyllids again dominate from about 110 mbsf upwards. Rhodoliths form a number of distinctive beds associated with laminated grainstones that imply sediment transport. It is not clear if both grainstones and rhodoliths were transported downslope, or if the rhodoliths represent in situ accumulations. Corals are sometimes relatively large, and are more diverse than those below, but are not sufficiently common to imply the establishment of an in situ growth framework and show a nearly symmetrical distribution within the unit (Fig. 4) that argues for a cycle rather than any consistent shallowing or deepening. Stable-isotope values are typical of the vadose diagenetic zone but the contrasting behavior of the carbon and oxygen ratios is anomalous. Unit 5: 99–73 mbsf The boundary at 99 mbsf is taken at the beginning of a sequence to 73 mbsf dominated by grainstones, although wackestones become more common above ; 85 mbsf. Centimeter clasts of coral, coralline algal crusts, gastropods, and bivalves locally form coarse rubble with a wackestone matrix. Recovery in this interval is generally good, although the rocks become increasingly vuggy, but it is poor from 96 to 93 and from 76 to 73 mbsf (Fig. 2), where some sections consist only of loose sand containing scattered centimeter clasts of coral. Small (2 to 2.5 cm) coral fragments

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FIG. 5.—Distribution of the principal corals and lithologies in unit 5 (as in Fig. 4.)

with thin or no coralline crusts are present at the base together with fragments of thick coralline crust, gastropods, and bivalves. Corals increase in both size (3 to 4 cm) and numbers to 96.5 mbsf. From 97.25 mbsf they are accompanied by other centimeter-size bioclasts, forming a rubbly mass. However, there are intervals below 93, 89, and 84 to 80 mbsf where larger fragments are sparse or absent. Other bioclasts include spines of the echinoid Heterocentrotus, and a few molds of bivalves, but these also seem to be relatively poorly represented. From 95.5 mbsf corals are relatively large (to 15 cm) and include faviids, Acropora and Millepora (Fig. 5). Less common genera include Fungia at 93.2, Galaxaea at 81.8 and 80.6,? Pocillopora at 76.2, 73.5, 73.2, and 73.0, Leptoria at 73.7, Porites at 73.0, and Tubipora (11 cm) at 82.7 mbsf. Many corals are extensively bored, and most carry crusts of coralline algae. These are typically thin, but isolated examples are up to 3 cm thick, and some incorporate other encrusting species including corals. Throughout this unit coralline algae are dominated by lithophyllids, but these are replaced at intervals by mastophorids and Hydrolithon reinboldi occurs sporadically at 94.0, 90 to 88, 82.0, and 74.0 mbsf. The lower part of the unit is characterized by an increase in the foraminiferan Calcarina calcar together with Amphistegina, Heterostegina, Planorbulina, and Marginopora, but planktic forms are also present between 86 and 80 mbsf. There is an abrupt decrease in d18O values at the base of the unit to around 26‰, but values peak at about 25‰ around 78 mbsf (Fig. 3). Carbon values fluctuate erratically in this interval, varying from about 26‰ to 24‰ with no overall trend. There is a marked gamma peak at 92 mbsf, but this does not correspond to any lithological change. Interpretation.—Although corals continue to increase in numbers upwards, they are not sufficiently abundant or contiguous to suggest the establishment of any extensive framework. The initial dominance of lithophyllid algae indicates relatively shallow waters, but their replacement upsection by mastophorids suggests that the area had effectively become a reef environment (Adey 1986; Borowitzka and Larkum 1986). However, the coral fauna is dominated alternately by branching and massive forms, and these, with the algae, suggest fluctuations in water depth.

Unit 6: 73–36 mbsf The position of the lower boundary of unit 6 is based largely on the change in the core in both density and diversity of corals, which from this

FIG. 6.—Distribution of the principal corals in unit 6 (as in Fig. 4.)

point upward are commonly in contact. From 73 to 71 mbsf the material recovered consists largely of fragments of faviids, together with Acropora,? Pocillopora, and Porites or of unconsolidated sand. From 70 to 57 mbsf recovery is good, consisting predominantly of grainstones with less common wackestones and mudstones. Rudstones are present at 47.2 and 46.6 mbsf, and the section to 36 mbsf is best described as ‘‘rubble facies.’’ Some corals are relatively large, and in comparison with units below are generally closer together, potentially forming a framework. However, given the diameter of the borehole (75 mm) it is not practical to discriminate between material that is in situ and larger fragments. The variations in distribution of the principal coral genera are shown in Figure 6. Other corals include: Leptoria, at 73.6 (. 30 cm), 69.2, 61.6, and 42.4 mbsf; Goniopora at 66.1 and 65.8 mbsf; Millepora at 49.3, 48.1, 47.8, 44.1, 37.9, and 37.4 mbsf; Galaxaea at 37.8; and Tubipora at 55.0 mbsf. Many corals carry thick coralline algal crusts incorporating bryozoa and encrusting foraminifera, and the same algae locally form isolated rhodoliths (66.6, 60.6, 39.4, and 38.2 mbsf). Hydrolithon occurs sporadically at 73, 72, 70, 60, and 58 mbsf. Smaller bioclasts include increasing numbers of bivalves and gastropods (cowries), together with Halimeda. Recovery is poor from 57 to 51 mbsf. Rudstone intervals include rhodoliths, fragments of coral (Acropora), molluscs, and coralline algae. Coral-fragment rudstones dominate the interval from 38 to 36 mbsf, locally with very thick (1 to 3 cm) coralline algal crusts, forming a rubble framework. At 39 mbsf pink mudstones and grainstones contain encrusting Porites, and Galaxaea, faviids, Millepora, and Acropora cf. A. palifera. These are relatively fresh and carry fewer borings than others. Numbers of Halimeda and bryozoans increase towards the top of the unit in parallel with encrusting foraminifera, including acervulinids and victorinellids, as well as benthic forms, Amphistegina, Planorbulina, Heterostegina, miliolids, and textularids. An irregular surface at 36 mbsf forms a clear upper boundary to unit 6

GREAT BARRIER REEF CHRONOLOGY (Fig. 2). The surface and extensive vugs in the rocks below (44 to 36 mbsf) are coated with a pale brown material. This varies from dense silty mudstone with vermiform pores to peloidal grainstone containing small lithoclasts and scattered bioclasts. Larger ovoid, sometimes concentrically laminated, structures resemble pedotubules. Apparently isolated areas of grainstone 2 to 3 cm diameter appear in the core at 36 mbsf. The rocks in the entire interval, from 73 to 71 mbsf, are generally vuggy with conspicuous calcite linings. Crystals forming these may carry scalenohedral or rhombohedral terminations, but in some vugs surfaces are smooth and crystal terminations concordant. The decline in d18O values that began in unit 5 continues at the base of the unit (Fig. 3), with some values approaching 27‰. There is a narrow interval from about 60 to 50 mbsf with values around 26‰, but from this depth to 36 mbsf the signal is marked by noise, with values varying from 28‰ (one 210.3‰) to 26‰. In the lower part of the unit carbon values increase from 26‰ to 24‰ but fall dramatically to almost 210‰, in parallel with the fall in oxygen, and continue to behave in parallel, with values between 24‰ and 28‰, to the top of the unit. A broad gamma excursion at 55 mbsf (International Consortium 2001) has no lithological correlative. Interpretation.—There is no abrupt lithological change at the lower boundary of this unit, but the transition from unit 5 is marked by an increase in the proportions of encrusting foraminifera and bryozoans, together with the appearance of increasing numbers of echinoderms and molluscs. At the base, the dominant corals are massive forms, and calcareous algae are lithophyllids that differ in no important respect from those in unit 5. However, with some variation, interpreted as reflecting changes in water depth, there is an upward transition, from about 115 mbsf, to predominantly branching corals and mastophorid algae including Hydrolithon onkodes that imply a progressive shallowing. The surface at 36 mbsf is interpreted as a karst surface, and the brown sediment mantling it and occupying the vuggy porosity below is interpreted as a paleosol (Braithwaite 1975, 1983). Areas of grainstone within this may be sections through irregular pinnacles on the karst surface or lithoclasts. These features provide the oldest evidence of subaerial emergence within the core. However, the surface does not correspond with any specific gamma signal, and the diagenetic boundary, reflected in variations in stable isotopes, occurs within the unit rather than at the margin (Fig. 3). The cement linings to vugs are significant. Whereas scalenohedral terminations imply that pores were flooded at the time of crystal growth, smooth concordant surfaces indicate air-filled spaces. Thus, together these suggest that the most recent growth was at least at the boundary between the phreatic and vadose zones, although it may not have been contemporary with deposition of the paleosol capping the unit. This interpretation is borne out by the stable isotopes, which imply an increased 12C influence from the paleosol in the upper half of the unit. Unit 7: 36–28 mbsf Larger corals are common in this interval and include both robust branching and massive forms. The sparse matrix is of grainstones or wackestones containing Halimeda (locally prominent) together with fragments of coral, coralline algae, and mollusc shells, including Turbo. In some areas the grainstones are burrowed, and graded bedding is seen at 33.15 mbsf. However, some intervals, at 33.7, 30.7, and 30.2 mbsf, consist predominantly of smaller rubbly coral fragments. Among the larger corals: Acropora is seen at 35.8, 35.6, 35.4, 35.1, 32.8, 32.7, 32.6, and 28.3 mbsf; faviids at 35.4, 33.2, 32.1, 31.9, 31.5, 30.7, 30.1, 29.3, 29.1, and 28.3 mbsf; Leptoria at 29.3 mbsf; Porites at 35.4, 30.7, 30.1, and 28.2 mbsf; and Pocillopora at 35.0, 34.1, 31.4, 31.1, and 30.1 mbsf. However, it is important to emphasize that there are many more, smaller unidentified fragments. Most are neomorphosed, but some contain local remnants of acicular marine cement. Thick crusts of coralline algae (predominantly mastophoroids) are seen at 34.5, 33.2, 31.4, and 29.2 mbsf and small rhodoliths at 35.3, 33.5, 33.1, and 31.9 mbsf. There is an irregular contact at 28 mbsf

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below which the rocks are conspicuously vuggy with extensive calcite cement linings. Some irregular cavities, apparently extending downwards from the contact surface, are filled with pale brown sediment with a crudely laminated texture. The laminae wrap around cement-filled tubules containing areas of yellowish blocky calcite resembling calcitized plant cells and forming well-organized groups. There is no indication in d18O values (ranging irregularly between 26‰ to a little below 28‰) of the presence of this surface. Carbon values, in dramatic contrast to those below, range from approximately 27‰ at either boundary to a spike in which they approach zero (Fig. 3). There is a general increase in the gamma response within the unit but no specific signal reflecting either boundary. Interpretation.—The karst surface and paleosol interpreted as capping unit 6 represents an important break in the sequence. The domination of much of the section from 35 to 28 mbsf by grainstones with conspicuous Halimeda, and the diverse coral and algal assemblages that they contain, suggest derivation from a shallow-water lagoonal environment. The irregular surface at 28 mbsf, and the related vuggy porosity, is interpreted as a karst surface, and the brown sediment containing preserved rootlets is regarded as a paleosol (Braithwaite 1975, 1983). As indicated, there is no gamma excursion associated with this and it (apparently) does not include potassium-bearing clays. The d18O signal for this interval is characterized by an erratic fluctuation interpreted to reflect diagenetic overprinting (International Consortium 2001). The ‘‘noisy’’ signal and lower minimum values first evident in the upper part of unit 6 may reflect the lower limits of penetration of vadose conditions. However, a single carbon ratio, approaching zero, implies a more normal marine influence. Unit 8: 28–15.85 mbsf Recovery is poor between 28 and 25 mbsf. The boundary between units 7 and 8 is placed at the unconformity identified at the top of unit 7. Between 28 and 15.85 mbsf the rocks recovered vary from grainstones to rudstones or locally mudstones, the latter including scattered angular quartz grains. Halimeda is commonly prominent in rocks containing larger corals that include: massive faviids at 16 and 20.5 mbsf; Porites, at 25.0, 24.4, 24.3, 21.4, 20.6, 20.3, 19.6, and 19.3 mbsf; robust branching Acropora, at 27.1, 26.1, 25.1, 21.9, 21.2, 20.1, 19.6, 19.2, 19.0, 18.0, 17.8, 17.7, 17.5, 17.4, 17.0, 16.7, 16.3, and 16.1 mbsf; Pocillopora, at 17.0 and 18.0 mbsf; and Galaxaea at 21.2 and 21.0 mbsf. Many are extensively bored, with their surfaces encrusted by coralline algae (up to 3 cm thick). Rhodoliths were noted only at 24.8 mbsf. Scattered mollusc shells are present, including pectenids at 24.3 mbsf. At the top of this sequence (15.85 mbsf) there is a prominent surface thinly covered with pale brown porous sediment containing yellow-brown cellular areas of blocky calcite resembling calcitized plant cells. Similar sediment extends in cavities to at least 21 mbsf. The rocks are generally vuggy, and there is extensive dissolution of bioclasts. Multiple calcite cements are present locally, and cement surfaces include both well-defined crystal terminations and smooth concordant boundaries. There is no specific response in d18O values that resemble those in the upper part of unit 6 and in unit 7. However, they generally increase from 28‰, with a slightly more negative spike, to values of about 26‰. By contrast carbon values peak close to zero at the base of the unit before a progressive but irregular upward decrease to values approaching 28‰. A marked gamma peak at ; 25 mbsf, is apparently unrelated to sediment characteristics but a smaller peak at ; 16 mbsf may correlate with the material capping this unit. Interpretation.—The numbers and diversity of the corals, and the widespread presence of Halimeda, suggest a well-developed shallow reef or lagoonal environment in this interval. However, the paleokarst surface at 15.85 mbsf and the associated brown cavity-filling sediment, interpreted as a paleosol (Braithwaite 1975, 1983), are once more indicators of sub-aerial emergence. The calcite cements include the concordant surfaces characteristic of the vadose zone.

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C.J.R. BRAITHWAITE ET AL. Unit 9: 15.85–0 mbsf

From 15.85 mbsf to the surface recovered samples vary from grainstones to packstones. There is almost no recovery between 11 mbsf and the top of unit 8. Well-preserved corals include: faviids (at 15.2 and 5.5 mbsf), Leptastrea, at 9.7, 6.2, and 3 mbsf; Goniopora, at 14.2 and 2.7 mbsf; Porites, at 11.0, 10.1, and 9.1 mbsf; Pocillopora, at 15, 10.1, 9.3, 7.0, 5.1, and 3.1 mbsf; Acropora, from 11.1 to 1.0 mbsf; and Millepora at 3.3 mbsf. Coral surfaces are commonly infested with endolithic algae and fungi, with larger clionid and sipunculid borings, and crypts that may be attributed to Lithophaga. Many carry thick crusts of coralline algae that incorporate vermetids, the encrusting foraminifera Carpentaria and Homotrema, bryozoans, serpulids, barnacles, and small oysters. Within the grainstones, smaller bioclasts include molluscan shell fragments (Patella), echinoderm plates and spines (Heterocentrotus), and a variety of benthic foraminifera (Marginopora), Halimeda, and alcyonarian and sponge spicules. The rocks remain cavernous and open pores are locally lined with thin gray laminated micritic crusts that form millimeter-scale columnar stromatolites. There is no widespread calcite cement in this interval, but spherular and acicular aragonite cements are present locally. With one prominent excursion (to 24‰), d18O values within the unit increase progressively from 26‰, typical of most of the lower sequence, to values closer to 22‰. Carbon values peak at nearly 12‰ but reach a low of 23‰ that corresponds with part of the oxygen low (Fig. 3) and appears to correlate with an exceptionally low response in the gamma signal. Interpretation.—The numbers and diversity of corals within this unit imply deposition in a shallow reef environment, although probably behind or below any active reef edge (Done 1982; Roberts et al. 1992; Cabioch et al. 1999). The taphonomy of the corals, commonly with extensive boring and encrustation of their surfaces, suggests that accumulation rates were relatively slow, and the distribution of aragonite cements indicates that sea-floor cementation occurred simultaneously within a meter of the sediment surface. RESULTS OF AGE DATING AND CORRELATION

The reliability of the ages provided by the various dating methods (Table 1) must be considered before any attempt can be made to provide a chronology for the core succession. Palaeomagnetic analyses indicate a normal polarity throughout the core, implying that the rocks were deposited entirely within the Brunhes polarity interval and are therefore younger than 770 ka. Plotting 87Sr/86Sr ratios against a regression line derived from Hodell et al. 1990 (Fig. 7) implies that all samples are younger than 1 Ma. Five samples from unit 1 (; 206–210 mbsf) yield Sr isotope compositions within a narrow range (0.709134–0.709144) and the mean of these values (0.709139) provides an apparent age of 770 6 280 ka, which is consistent with deposition close to the Brunhes–Matuyama boundary (International Consortium 2001). Three samples from units 4 and 5 (122, 113, and 88 mbsf) yield mean Sr isotope ratios of 0.709148, suggesting an apparent age of approximately 600 6 280 ka. However, there are prominent 87Sr/ 86Sr omissions in units 4 and 5 relative to the regression line (accepting error bars), probably reflecting a break in the depositional record. Some of the radiocarbon dates provide equivalent uncertainties. Samples from unit 9 (, 14 mbsf, Table 1) provide dates of 4.46, 4.95, 6.03, 6.72, 6.43, 7.20, and 7.72 ka, respectively, (all 6 0.05 ka, uncorrected for marine reservoir effects and variations in 14C content). These are consistent with a Holocene age for the youngest part of the succession and agree with dates for the GBR provided by Davies and Hopley (1983). However, unit 8 samples from 16 mbsf and 21.3 mbsf suggest ages of 29.9 6 0.2 and 32.5 6 0.5 ka, respectively, whereas those from 16.5 mbsf and 17.7 mbsf suggest ages of . 45 ka. These ages contradict one another, and all four reflect a period when world sea level was significantly lower than their positions in the core imply. They lie below a surface interpreted as resulting from subaerial erosion, and both petrography and stable isotopes indicate that

FIG. 7.—Plot of 87Sr /86Sr ratios for Ribbon Reef 5 (large circles) relative to the regression line through data from Hodell et al. (1990).

samples from this level are diagenetically altered. The unit 8 radiocarbon dates are therefore regarded as reflecting the time of alteration and of cement emplacement, that is, of exposure and the addition of younger carbon. Five samples, from 16.1 (unit 8), 56.4 (unit 6), 94.9 (unit 5), 118 (unit 4), and 184.8 (unit 1) mbsf, were subjected to Uranium-series (TIMS) analysis. The upper four samples were of coral and the lowermost of carbonate sand. The latter returned a very high 232Th content of 0.5 ppm. This suggests, as might be expected, that the sediment is contaminated with detrital material and is not likely to return a meaningful U/Th age. Petrography and stable isotopes point to modification of the system by freshwater flux (on at least three occasions), and the coral samples must therefore also be viewed with suspicion. However, despite this a highly plausible date of 125.7 6 0.6 ka from 16 mbsf implies that the top of unit 8 was deposited during the last interglacial. This conflicts with the ages suggested for the same interval by radiocarbon dates but is consistent with deposition when sealevel was relatively high (isotope Stage 5e). The remaining coral samples, from 56.4 (unit 6), 94.9 (unit 5), and 188 (unit 1) mbsf, have 230Th/ 234U values that, for the observed 234U/ 238U ratios, are not possible within a closed system and cannot therefore provide 230Th ages. However, it is theoretically possible to use 234U/ 238U to derive an age without the use of 230Th. This approach relies on the assumption that seawater 234U/ 238U has been constant through time and that samples have not gained uranium. Ages calculated from 234U/ 238U for samples from 56.4, 94.9, and 118 mbsf are 322 6 19 ka, 564 6 78, and 616 6 51 ka, equated respectively with isotope stages 9, 14 and 15. In terms of the supposed sedimentary history these might be considered reasonable. However, because the 234U/ 238U ratio is notoriously susceptible to diagenetic alteration they most probably represent minimum ages. Nearly all corals described in the literature with ages . 40 ka give initial 234U/ 238U values above those of modern seawater (Bard et al. 1991) as a result of diagenetic alteration (Henderson et al. 1993). If 234U has been added to the corals analyzed here then 234U/ 238U will be higher than for a closed system and calculated ages are therefore too young. The one coral from this study returning a reasonable age (from unit 8,

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16 mbsf) also has an initial 234U/ 238U ratio higher than that of seawater, supporting the suggested addition of 243U and the fact that the 234U/ 238U ages obtained are minimal values. If the rate of addition of 234U is a constant this sample could be used to better constrain the ages of the deeper samples. Unfortunately, the implied rate of addition is higher than that possible for the deeper samples, and cannot have been constant. All that can be concluded is that the three 234U/ 238U ages quoted above are probably minimum values. As a further guide to chronology it is helpful to compare the sedimentological changes recorded in Figure 2 with those indicated in the ODP borehole at site 823A (Alexander 1996). The latter have been dated using both stableisotope stratigraphy and biostratigraphy. Within the interval from 0 to ; 60 mbsf, a series of well-defined cycles are indicated by changes in d18O (from the tests of foraminifera) and Al and Si content (Fig. 8), with the last two used as proxies for the input of terrigenous clays and quartz (Alexander 1996). Spikes in these correlate with high sealevel stands. A biostratigraphically wellconstrained age of 930 ka has been determined for samples from about 80 mbsf (Alexander 1996). Below this, the amount of terrigenous material introduced to the system was higher than at any time since. This is interpreted as reflecting a relatively open nearshore system with land-derived material washed directly to the offshore zone, in as much as it implies that there was no well developed barrier (?reef) at this time. Although generally reducing in abundance, terrigenous material continues to provide a ‘‘noisy’’ signal from the base of the hole through to isotope stage 13 (?) at about 50 mbsf depth. Above this, relatively well-defined excursions are indicated towards the top of the succession (Fig. 8) that are interpreted as reflecting the high stands in Ribbon Reef 5, on the basis of the principle that, during low stands, terrigenous sediments would be blocked behind the emergent reef (except perhaps in bypass channels). By contrast, during transgressions, and particularly in the early stages where ‘‘catch-up’’ growth could be anticipated, they could be flushed seawards so that peaks correspond closely with glacial sealevel maxima recorded in d18O excursions. However, it appears from the succession in Ribbon Reef 5 that in this area there was no general supply of terrigenous material to the sediments accumulating along the platform margin and that these were therefore dominated by carbonates. This begs the question of whether the terrigenous material in the 823A borehole was provided by a bypass system or by shore-parallel transport along the margin. Finally, we should refer also to the results from earlier boreholes. On Heron Island (Marshall 1983) reef limestones extend from the surface to 154 m, interleaved below with quartz sands that extend to the base of the hole in what are considered to be Miocene deposits. Davies (1974) identified five dissolution unconformities within this borehole at depths of 20, 35, 75, 95, and 140 m. Only the shallowest of these appear to have any correlatives in the Ribbon Reef 5 core. On Michaelmas Cay, limestones were 110 m thick. The deeper Wreck Island bore reached 420 m and again included rocks thought to be of middle Miocene age. Further shallow drilling (Marshall 1983) revealed the base of the Holocene at depths ranging from 7 to 14.3 m, although seismic refraction had previously suggested an upper limit closer to 32 m. TOWARDS A CHRONOLOGY OF DEPOSITION

Notwithstanding the wide variety of dating methods applied, the age data from Ribbon Reef 5 (Table 1) leave many uncertainties. It is uncertain when the carbonate succession represented in the Ribbon Reef 5 borehole began to be deposited. The Sr/Sr age of 770 6 280 ka (Fig. 7) has a large potential error and may reflect the age of reworked material, although more than one sample is represented. The lithoclasts present in the rocks in unit 1 near the base of the hole indicate that there were remnants of an older structure nearby and therefore that the oldest in situ rocks recovered do not reflect the initiation of carbonate deposition in the area. More of this older carbonate edifice, which might be Miocene, Pliocene, or early Pleistocene, might be found elsewhere in the GBR beneath late Pleistocene successions as represented in Ribbon Reef

FIG. 8.—Plot of d18O (‰ PDB) and Al and Si (%) for ODP borehole 823A; arrows indicate suggested cooling cycles (data from Alexander 1996).

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FIG. 9.—Comparative d18O (‰ PDB) plots for ODP sites 607 and 823A (after Alexander 1996), together with Feary et al. (1993) correlation, and suggested equivalents in Ribbon Reef 5 chronology.

5. Although unit 1 seems to have been deposited in relatively deep water it probably indicates a low sealevel stand, with emergent surfaces upslope providing lithoclasts and the slope and energy for the transport of slumps (recorded in sediment disturbance) and density currents. The age of approximately 414 ka attributed to the sample from 184.8 mbsf (Table 1) is at variance with others and probably reflects cement growth and neomorphism of older material by freshwater flushing, during the emergence following stage 6. There is no direct or indirect sedimentological evidence of older subaerial exposure, and stable isotopes suggest that there has been little freshwater influence within unit 1. The base of unit 1 in Ribbon Reef 5 may correlate with the lower boundary of Feary et al.’s (1993) Sequence 3 in their description of ODP Leg 133 and represents, as they suggest, a major disconformity (see Fig. 9). Feary et al. (1993) regard this, on the basis of biostratigraphical correlation, as dating to approximately 760 ka, in broad agreement with suggestions for the rocks in Ribbon Reef 5 provided by magnetostratigraphy and 87Sr/ 86Sr ratios. Unfortunately the Leg 133 boundary is not tightly constrained because it lies between the biostratigraphic datum represented by the acme of small Gephyrocapsa at 930 ka and that corresponding to the first appearance of Pseudoemiliania lacunosa at 465 ka (Wei and Gartner 1993). The age of 760 ka selected for the boundary (Feary et al. 1993) is based solely on estimated sedimentation rates, and the associated error is unknown. The boundary is interpreted on seismic lines through sites 820 and 821 as an offlap surface with downlap of Sequence 3 reflectors. This part of the succession comprises clayey wackestones with local coarse shelfderived wackestones that include fragments of Halimeda, corals, bryozo-

ans, and molluscs, and is interpreted (Feary et al. 1993) as reflecting deposition from an aggrading shelf. Small ‘‘reefal’’ mounds are recorded on a number of seismic lines but, given the conclusions drawn here, may represent residuals of older limestones. No dates are available for unit 2 but the 234U/ 238U age from unit 4 (616 ka), the 87Sr/86Sr age from unit 4 (600 ka), and the 234U/ 238U age from the base of unit 5 (564 ka), all seem to point to deposition in mid-Brunhes time and imply that unit 2 was deposited in the Brunhes interval. Thus, units 1 to 4 together indicate deposition of about 110 m in a relatively short period with sedimentological data reflecting progressive slope progradation. Note, however, that Feary et al. (1993) described their Sequence 3 as marking the beginning of an aggradational phase. Faunal evidence in Ribbon Reef 5 suggests a gradual shallowing, with transport downslope of scattered corals and rhodoliths. Carbonate sands show low-angle lamination that also suggests downslope reworking. No subaerial exposure surface has been identified in this interval so deposition must have occurred at sufficient depth to escape the effects of any lowered sea levels. The 234U/ 238U ratio suggests an age of 322 ka or greater (isotope stage 9) for the middle of unit 6 (56.4 mbsf). During the period of deposition of unit 5, reef growth was established and followed a ‘‘catch-up’’ mode as waters became progressively shallower. Although there are both faunal and floral indications of changes in depth across the boundary between units 4 and 5 there is no sedimentological evidence that these reflect a major lowstand event or events. As in the succession below there is no inference of subaerial emergence and no indication of the very shallow water that might

GREAT BARRIER REEF CHRONOLOGY have been expected during such a period. Sealevel lows in the mid-Brunhes interval may have been insufficient to result in exposure. Curiously this interval is represented by a gap in 87Sr/86Sr data (Fig. 7) that implies an omission in the depositional record between units 4 and 5. The subaerial erosion surface at the top of unit 6 represents the first of a series of significant marine regressions (Fig. 9). The erosional unconformity at the top of Feary et al.’s (1993) Sequence 3, characterized by them as ‘‘continuous and discontinuous high and low-amplitude subparallel and divergent reflectors’’ in the shelf area may correlate with this. It was attributed by Feary et al. (1993), on the somewhat insecure basis of sedimentation rates, to approximately 365 ka, lying between the Pseudoemiliania lacunosa datum at 465 ka and that marked by the first appearance of Emiliana huxleyi at 275 ka. An age closer to 350 ka would correlate better with the low stand of isotope stage 10. Such an age is in accord with the imprecise 234U/ 238U ages and 87Sr/86Sr projections for the rocks below. There is no reliable radiometric age for unit 7 in Ribbon Reef 5, the interval from 36 to 28 mbsf. It is bounded at the top by an emergent surface that may correlate with the upper boundary of Feary et al.’s (1993) Sequence 2. Feary et al. (1993) described this surface as an unconformity and placed it above the Emiliana huxleyi datum of 275 ka (see Fig. 9). Sequence 2 itself is predominantly a clay-rich calcareous mud with three interbedded bioclast wackestones. Carbonate content increases upwards in each muddy bed. A relatively large ‘‘reefal’’ mound at the top of Sequence 2 has been identified on a seismic line adjacent to the present barrier (Feary et al. 1993). However, unit 7 preserves only 8 m of limestones and in a scenario correlating it with Sequence 2 should represent at least 100 ka. Unit 8 in Ribbon Reef 5 reflects deposition during Isotope stage 5 (Fig. 9). A sample from 16 mbsf at the upper margin of the unit has provided a 230Th/ 234U age of 125.7 ka. The karst surface bounding the unit may then logically be correlated with the upper limit of Feary et al.’s (1993) Sequence 1, described as ‘‘deeply dissected,’’ and dated at approximately 80 ka. The overlying succession, unit 9, is confidently referred to the Holocene on the basis of a series of radiocarbon dates and a lack of alteration. Sequence 1 of the Feary et al. (1993) succession comprises two similar subsequences of approximately equal thickness, separated by a continuous high-amplitude reflector. Each subsequence contains reflectors that onlap the underlying surface and are assumed to indicate a relative rise in depositional base level. However, because Sequence 1 can be recognized only on the outer shelf and thins towards the present reef, the truncation of the succession probably indicates erosion. There seems to be no equivalent to this subdivision in the depositional record of Ribbon Reef 5, but there may be a speculative correlation with the stable isotope signal, indicated by decreases in both the carbon and oxygen ratios (Fig. 3). Holocene deposition was established about 8 ka ago in Ribbon Reef 5 and reef growth reached sea level by about 4 ka. At ; 16 m thickness the Holocene accumulation is double that of unit 7 and 25% thicker than unit 8. If these various correlations and ages are accepted, the differences in thickness of the units preserved in Ribbon Reef 5 imply either that rates of production and accumulation changed dramatically, or that much of the succession is missing. The lack of sedimentological evidence of ‘‘condensed sequences’’ and the presence of three subaerial erosion surfaces implies the latter. Comparison with well-exposed Pleistocene successions on Aldabra (Braithwaite et al. 1973) and in Kenya (Braithwaite 1984) shows that stage 5 limestones commonly form a thin veneer on much older rocks. In these successions both older and younger units are also thin and locally any or all of them may be absent. This might in part reflect a lack of accommodation space but also, and more importantly, the volume of rock removed by karst erosion. At the level reached by deposition at the end of unit 6, isotope Stage 9, there was already little space for more sediment to be preserved as subsidence was not sufficiently rapid. Comparable reductions in sediment thickness and omissions in preservation are recorded by Ve´zina et al. (1999) from data on Grand Cayman, and by Hearty (1998) on Eleuthera. Every highstand results in added deposition, but the erosion that follows in every lowstand

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truncates or may entirely remove older deposits. Only where boreholes recover deeper successions can we expect to find low-stand sequences represented (Camoin et al. 2001). CONCLUSIONS

The lower part of the Ribbon Reef 5 succession suggests that deposition of unit 1 commenced around 770 ka, perhaps in isotope Stage 16, with the formation of debris flows on a relatively deep slope. The presence of lithoclasts indicates that older areas of limestone were exposed nearby at that time and may therefore be present below this succession elsewhere in the GBR. However, the delivery of terrigenous material offshore suggests that this was a relatively open system and that there was no well-defined reef (platform) edge. The overlying sequence reflects a progressive but probably not continuous relative shallowing to about 36 m below the present sealevel (isotope stage 9). This may have been driven as much by progradation as by other factors. Faunal and lithological variations in units 5 and 6 (isotope stages 11 and 9) suggest fluctuations in sea level, and one well-defined boundary might be interpreted as a marine omission surface, but no evidence of subaerial emergence has been identified over a period that may have been of the order of 400 ka. It seems unlikely that reef framework environments were well established nearby (forming a significant barrier) until late in the deposition of unit 5, during isotope Stage 11. A paleosol at 36 m lies above a zone of extensive leaching of bioclasts and indicates an interval of emergence and erosion, but it is not known how much of the succession was removed. The truncating surface is thought to represent isotope Stage 8. Later submergence, during what we assume to be isotope Stage 7, resulted in the deposition of at least 10 m of marine limestone in a reefal environment, before a further fall in sea level resulted in the formation of the erosion surface marked by a paleosol at 28 mbsf. The paleosol bounding unit 7 (isotope stage 7) therefore represents stage 6. The severe truncation of stage 7 seems to correlate with observations elsewhere (Hearty 1998; Ve´zina et al. 1999). When sea level rose again, during what was probably isotope Stage 5, it is thought to have reached a level well above the present datum. However, there is no evidence of the total thickness of limestone deposited, and no evidence in the sequence preserved of a progressive shallowing. The subsequent fall in sealevel (isotope stage 4) must have resulted in substantial erosion and in the formation of a karst surface, now at 15.85 mbsf, with a thin paleosol contained within surface cavities. The subsequent Holocene rise in sea level, first evident 7.7 ka ago, resulted in the accumulation of reef framework and reef-derived sediments to the level of the present sea floor, which may have been reached 3 to 4 ka ago. With the exception of the addition of aragonite cements these sediments have been essentially unaffected by diagenetic change. ACKNOWLEDGMENTS

This work has been based on the collaboration of a large number of individuals and organizations, which we are happy to acknowledge and thank for their support. The drilling program was initiated by Peter Davies of the University of Sydney, Australia. It was funded jointly by The University of Sydney, the Natural Environment Research Council in the UK, The ETH Zurich Research Council, Switzerland, the French National Coral Reef Committee, the CEA (The French Nuclear Agency), and the University of Granada in Spain. Drilling was with the permission and support of the Great Barrier Reef Marine Park Authority and the Queensland Parks and Wildlife Authority, and we thank in particular Craig Sambal and Jenny Le Cusion. Drilling operations were under the guidance of Jack Pheasant and Alistair Skinner of the British Geological Survey, and the Ribbon Reef 5 borehole was completed thanks to the tireless work of Phil Manning of the University of Sydney. Thanks are also due to Maggie Cusack at the Division of Earth Sciences at the University of Glasgow (UK) for amino acid analyses; to Roy Thompson at the Department of Geology and Geophysics in the University of Edinburgh (UK) for paleomagnetic determinations; to Harry Elderfield at the Marine Geochemistry Laboratory, University of Cambridge (UK), for strontium ratios; to Judith McKenzie at the Geological Institute ETH Zurich, Switzerland, for oxygen isotope analyses; and to Juan

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Braga of the University of Granada, Spain, for comments on coralline algae. The 14C dates were determined at the NERC Radiocarbon Laboratory at the Scottish Universities Environmental Research Centre at East Kilbride, (UK); uranium-series analyses at the NERC uranium series facility in the Department of Earth Sciences, The Open University, Milton Keynes (UK); and modeling of U-series systematics, at the Lamont Doherty Geological Observatory of Columbia University (USA). Christian Betzler and Chip Fletcher provided constructive comments on an earlier version of the manuscript; and David Budd and John Southard are thanked for their meticulous editing. REFERENCES ADEY, W.H., 1986, Coralline algae as indicators of sea-level, in Van de Plassche, O., ed., Sealevel Research: A Manual for the Collection and Evaluation of Data: Norwich, U.K., Geo Books, p. 229–280. AHARON, P., GOLDSTEIN, S.L., WHEELER, C.W., AND JACOBSON, G., 1993, Sea-level events in the South Pacific linked with the Messinian salinity crisis: Geology, v. 21, p. 771–775. ALEXANDER, I.A., 1996, Late Quaternary sedimentation off the Queensland continental margin (northeast Australia) in response to sea level fluctuations [unpublished Ph.D. Thesis]: University of Edinburgh, 489 p. BARD, E., FAIRBANKS, R.G., HAMELIN, B., ZINDLER, A., AND HUANG, C.T., 1991, Uranium 234 anomalies in corals older than 150,000 years: Geochimica et Cosmochimica Acta, v. 55, p. 2385–2390. BOROWITZKA, M.A., AND LARKUM, A.W.D., 1986, Reef algae: Oceanus, v. 29, p. 49–54. BRAITHWAITE, C.J.R., 1975, Petrology of palaeosols and other terrestrial sediments on Aldabra, western Indian Ocean: Royal Society (London), Philosophical Transactions, Series B, v. 273, p. 1–32. BRAITHWAITE, C.J.R., 1983, Calcrete and other soils in Quaternary Limestones: structures, processes and applications: Geological Society of London, Journal, v. 141, p. 685–699. BRAITHWAITE, C.J.R., 1984, Depositional history of the late Pleistocene limestones of the Kenya coast: Geological Society of London, Journal, v. 141, p. 685–699. BRAITHWAITE, C.J.R., TAYLOR, J.D., AND KENNEDY, W.J., 1973, The evolution of an atoll, the depositional and erosional history of Aldabra: Royal Society (London), Philosophical Transactions, Series B, v. 266, p. 307–340. CABIOCH, G., MONTAGGIONI, L.F., FAURE, G., AND RIBAUD-LAURENTI, A., 1999, Reef coralgal assemblages as recorders of paleobathymetry and sea level changes in the Indo-Pacific province: Quaternary Science Reviews, v. 18, p. 1681–1695. CAMOIN, G.F., EBREN, PH., EISENHAUER, A., BARD, E., AND FAURE, G., 2001, A 300,000 yr coral reef record of sea level changes, Mururoa atoll (Tuamotu archipelago, French Polynesia): Palaeogeography, Palaeoclimatology, Palaeoecology, v. 175, p. 325–341. DARWIN, C., 1890, On the structure and distribution of coral reefs: London, Ward Lock & Co., 154 p. DAVIES, P.J., 1974, Subsurface solution unconformities at Heron Island, Great Barrier Reef: 2nd International Symposium on Coral Reefs, Brisbane, Proceedings, v. 2, p. 273–278. DAVIES, P. J., 1991, Origins of the Great Barrier Reef: Search, v. 23, p. 193–196. DAVIES, P.J., AND HOPLEY, D., 1983, Growth fabrics and growth rates of Holocene reefs in the Great Barrier Reef: Australian Bureau of Mineral Resources, Journal of Australian Geology and Geophysics, v. 8, p. 237–252. DAVIES, P.J., AND MCKENZIE, J.A., 1993, Controls on the Pliocene–Pleistocene evolution of the northeastern Australian continental margin, in McKenzie, J.A., Davies, P.J., Palmer-Julson, A.A., Betzler, C.G., Brachert, T.C., Chen, M-P.P., Crumie`re, J-P., Dix, G.R., Droxler, A.W., Feary, D.A., Gartner, S., Glenn, C.R., Isern, A., Jackson, P.D., Jarrard, R.D., Katz, M.E., Konishi, K., Kroon, D., Ladd, J.W., Martı´n, J.M., McNeill, D.F., Montaggioni, L.F., Mu¨ller, D.W., Omarzi, S.K., Pigram, C.J., Swart, P.K., Symonds, P.A., Watts, K.F., and Wei, W., eds., Proceedings of the Ocean Drilling Program, Scientific Results, College Station, Texas, v. 133–132, p. 755–762. DAVIES, P.J., MCKENZIE, J.A., PALMER-JULSON, A.A., BETZLER, C.G., BRACHERT, T.C., CHEN, MP.P., CRUMIE`RE, J-P., DIX, G.R., DROXLER, A.W., FEARY, D.A., GARTNER, S., GLENN, C.R., ISERN, A., JACKSON, P.D., JARRARD, R.D., KATZ, M.E., KONISHI, K., KROON, D., LADD, J.W., MART´ıN, J.M., MCNEILL, D.F., MONTAGGIONI, L.F., MU¨LLER, D.W., OMARZAI, S.K., PIGRAM, C.J., SWART, P.K., SYMONDS, P.A., WATTS, K.F., AND WEI, W., 1993, Proceedings of the Ocean Drilling Program, Scientific Results, Northeast Australian Margin, College Station, Texas, v. 133–132, 903. p. DAVIES, P.J., SYMONDS, P.A., FEARY, D.A., AND PIGRAM, C.J., 1989, The evolution of the carbonate platforms of Northeast Australia, in Crevello, P.D., Sarg, J.F., and Read, J.F., eds., Controls on Carbonate Platform and Basin Development: SEPM, Special Publication 44, p. 233–258. DONE, T.J., 1982, Patterns in the distribution of coral communities across the central Great Barrier Reef: Coral Reefs, v. 1, p. 95–107. EDWARDS, R.L., CHEN, J.H., AND WASSERBURG, G.J., 1986, 238U– 234U– 230Th– 232Th systematics and the precise measurement of time over the past 500,000 years: Earth and Planetary Science Letters, v. 81, p. 175–192. FEARY, D.A., SYMONDS, P.A., DAVIES, P.J., PIGRAM, C.J., AND JARRARD, R.D., 1993, Geometry of Pleistocene facies on the Great Barrier Reef outer shelf and upper slope, seismic stratigraphy of sites 819, 820 and 821, in McKenzie, J.A., Davies, P.J., Palmer-Julson, A.A., Betzler, C.G., Brachert, T.C., Chen, M-P.P., Crumie`re, J-P., Dix, G.R., Droxler, A.W., Feary, D.A.,

Gartner, S., Glenn, C.R., Isern, A., Jackson, P.D., Jarrard, R.D., Katz, M.E., Konishi, K., Kroon, D., Ladd, J.W., Martı´n, J.M., McNeill, D.F., Montaggioni, L.F., Mu¨ller, D.W., Omarzi, S.K., Pigram, C.J., Swart, P.K., Symonds, P.A., Watts, K.F., and Wei, W., eds., College Station, Texas, Proceedings of the Ocean Drilling Program, Scientific Results, v. 133–132, p. 327–351. HARKNESS, D.D., AND WILSON, H.W., 1972, Some applications in radiocarbon measurement at the SURRC: Eighth International Conference on Radiocarbon Dating, Proceedings, Royal Society of New Zealand, v. 1B, p. 102. HEARTY, P.J., 1998, The geology of Eleuthera Island, Bahamas: a Rosetta Stone of Quaternary stratigraphy and sea-level history: Quaternary Science Reviews, v. 17, p 333–355. HENDERSON, G.M., COHEN, A.S., AND O’NIONS, R.K., 1993, 234U– 238U ratios and the 230Th ages for Hateruma Atoll corals: implications for coral diagenesis and seawater 234U/ 238U ratios: Earth and Planetary Science Letters, v. 115, p. 65–73. HODELL, D.A., MEAD, G.A., AND MUELLER, P.A., 1990, Variations in the strontium isotopic composition of seawater during the past 8 million years: Chemical Geology, v. 80, p. 291–307. HUMBER (Barrier Reef Oils Pty Ltd.), 1960, H.B.R. Wreck Island No. 1. Qld, Well Completion Report: Australian Bureau of Mineral Resources, Petroleum Search Subsidiary Acts Report, no. 62/1021 (unpublished). INTERNATIONAL CONSORTIUM, 2001, Alexander, I.A., Andres, M., Braga, J., Braithwaite, C.J.R., Davies, P.J., Elderfield, H. Gilmour, M.A., Kay, R.L.F., Kroon, D., McKenzie, J.A., Montaggioni, L.F., Thompson, R., and Wilson, P.A., New constraints on the origins of the Great Barrier Reef, Results from an international project of deep boreholes: Geology, v. 29, p. 483–487. LU, G., AHARON, P., WHEELER, C.W., AND MCCABE, C., 1996, Magnetostratigraphy of the uplifted former atoll of Niue, South Pacific: implications for accretion history and carbonate diagenesis: Sedimentary Geology, v. 105, p. 259–274. LUDWIG, K.R., HALLEY, R.B., SIMMONS, K.R., AND PETERMANN, Z.E., 1988, Sr isotope stratigraphy of Enewetak Atoll: Geology, v. 16, p. 173–177. MARSHALL, J.F., 1983, Lithology and diagenesis of the carbonate foundations of modern reefs in the southern Great Barrier Reef: Journal of Australian Geology and Geophysics, v. 8, p. 253–265. MAXWELL, W.G.H., 1968, Atlas of the Great Barrier Reef: Amsterdam, Elsevier, 258 p. MCNEILL, D.F., AND KIRSCHVINK, J.L., 1993, Early dolomitization of platform carbonates and the preservation of magnetic polarity: Journal of Geophysical Research, v. 98, B5, p. 7977–7986. MONTAGGIONI, L.F. AND VE´NEC-PEYRE´, M-T., 1993, Shallow water foraminiferal taphocoenoses at site 821: implications for the evolution of the central Great Barrier Reef, northeast Australia, in Davies, P.J., McKenzie, J.A., Palmer-Julson, A.A., Betzler, C.G., Brachert, T.C., Chen, M-P.P., Crumie`re, J-P., Dix, G.R., Droxler, A.W., Feary, D.A., Gartner, S., Glenn, C.R., Isern, A., Jackson, P.D., Jarrard, R.D., Katz, M.E., Konishi, K., Kroon, D., Ladd, J.W., Martı´n, J.M., McNeill, D.F., Montaggioni, L.F., Mu¨ller, D.W., Omarzi, S.K., Pigram, C.J., Swart, P.K., Symonds, P.A., Watts, K.F., and Wei, W., eds., Proceedings of the Ocean Drilling Program, Scientific Results, Northeast Australian Margin, College Station, Texas, v. 133–132, p. 365–378. MORSE, J.W. AND MACKENZIE, F.T., 1990, Geochemistry of Sedimentary Carbonates: Amsterdam, Elsevier, Developments in Sedimentology, 48, 707 p. OHDE, S., AND ELDERFIELD, H., 1992, Strontium isotope stratigraphy of Kita-daito-jima Atoll, North Philippine Sea: implication for Neogene sea-level change and tectonic history: Earth and Planetary Science Letters, v. 113, p. 473–486. QUINN, T.M., LOHMANN, K.C, AND HALLIDAY, A.N., 1991, Sr isotopic variations in shallow water carbonate sequences: Stratigraphic, chronostratigraphic, and eustatic implications of the record at Enewetak Atoll: Palaeoceanography, v. 6, p. 371–385. RICHARDS, H.C., AND HILL, D., 1942, Great Barrier Reef bores, 1926 and 1937. Descriptions, analyses and interpretations: Report of the Great Barrier Reef Committee, v. 5, p. 1–122. ROBERTS, H.H., WILSON, P.A., AND LUGO-FERNANDEZ, A., 1992, Biologic and geologic responses to physical processes: Examples from modern reef systems of the Caribbean–Atlantic region: Continental Shelf Research, v. 12, p. 809–834. SYKES, G.M., COLLINS, M.J., AND WALTON, D.I., 1995, The significance of a geochemically isolated intracrystalline organic fraction within biominerals: Organic Geochemistry, v. 23, p. 1059–1065. VAN CALSTEREN, P., AND SCHWIETERS, J.B., 1995, Performance of a thermal ionization mass spectrometer with a deceleration lens system and post-deceleration detector selection: International Journal of Mass Spectrometry and Ion Processes, v. 146–147, p. 119–129. VE´ZINA, J., JONES, B., AND FORD, D., 1999, Sea-level highstands over the last 500,000 years: evidence from the Ironshore Formation on Grand Cayman, British West Indies: Journal of Sedimentary Research, v. 69, p. 317–327. WEI, W., AND GARTNER, S., 1993, Neogene calcareous nannofossils for sites 811 and 819 through 825, offshore northeastern Australia, in McKenzie, J.A., Davies, P.J., Palmer-Julson, A.A., Betzler, C.G., Brachert, T.C., Chen, M-P.P., Crumie`re, J-P., Dix, G.R., Droxler, A.W., Feary, D.A., Gartner, S., Glenn, C.R., Isern, A., Jackson, P.D., Jarrard, R.D., Katz, M.E., Konishi, K., Kroon, D., Ladd, J.W., Martı´n, J.M., McNeill, D.F., Montaggioni, L.F., Mu¨ller, D.W., Omarzi, S.K., Pigram, C.J., Swart, P.K., Symonds, P.A., Watts, K.F., and Wei, W., eds., Proceedings of the Ocean Drilling Program, Scientific Results, Northeast Australian Margin, College Station, Texas, v. 133–132, p. 19–37. WILSON, P.A., JENKYNS, H.C., ELDERFIELD, H., AND LARSON, R.L., 1998, The paradox of drowned carbonate platforms and the origin of Cretaceous Pacific guyots: Nature, v. 392, p. 889–894. YONGE, C.M., 1930, A Year on the Great Barrier Reef: London, Putnam, 246 p. Received 3 January 2003; accepted 16 September 2003.