The Great Barrier Reef - Wiley Online Library

Sedimentology (2009) 56, 1591–1622

doi: 10.1111/j.1365-3091.2008.00982.x

The Great Barrier Reef: a 700 000 year diagenetic history COLIN J. R. BRAITHWAITE* and LUCIEN F. MONTAGGIONI *Department of Geographical and Earth Sciences, The University of Glasgow, Glasgow G12 8QQ, UK (E-mail: [email protected]) FRE 2761 CNRS, Centre de Se´dimentologie et de Paleóntologie, Universite´ de Provence, 3 Place Victor Hugo, 13331 Marseille cedex 3, France ABSTRACT

A deep borehole through Ribbon Reef 5 in the Great Barrier Reef off northeastern Australia has identified a variety of cements, including epitaxial, radial prismatic and spherular aragonite, together with blocky, prismatic and fibrous calcite. These cements are discontinuously arranged within the sequence that consists predominantly of grainstones but locally includes clotted muddy and filamentous textures that may be of microbial origin. Calcite cements vary in morphology with groups of crystals that include acute scalenohedral, rhombohedral and flattened concordant terminations; these show varying densities of inclusions that locally define growth zones and in some terminations divide in the manner of ‘split crystals’ to form fibrous fringes. Morphological changes in calcite are inferred to reflect changes in water chemistry and crystal growth rates at the time of growth, allied to their relationship to the palaeo-water table, and linked in turn to changes in sealevel. Neomorphism and dissolution are widespread and variations in the severity of both imply response to the degree of undersaturation of pore waters that at times were probably balanced within very narrow limits. A total of 10 depositional units are identified. Those units at the base of the borehole reflect deposition and diagenesis within a marine environment. The influence of meteoric waters, indicated by stable isotopes, is first apparent at the top of Unit 1 and in Unit 2 (184 to 155 m below sea floor). Petrographic evidence of vadose conditions appears at the tops of Unit 3 (131 to 99 m below sea floor). Units 4 to 8, all deposited under marine conditions, provide isotopic evidence of meteoric or mixing-zone waters and petrographic indicators of vadose conditions, typically at the top of the units. Evidence indicates that in Unit 5 the water table was mobile and Units 6a, 6b, 7 and 8, all characterized by ultraviolet fluorescent cements, are capped by sub-aerial erosion surfaces. Unit 9 (the Holocene) reflects the recent re-establishment of marine conditions. The extent of alteration of the entire sequence reflects the substantial and pervasive influence of meteoric waters. This effect is interpreted as a result of a greater rainfall and river flow from the mainland during early and late stages of interstadial periods. The study reflects progress in the ability to recognize the diagenetic signal generated by sea-level change. However, whereas the isotopic results reflect the changing relationships between vadose and phreatic zones in groundwater systems beneath successive emergent surfaces, their correspondence with petrographic features is expressed only weakly and commonly lacks the systematic sequential overprinting implied by the distribution of cathodoluminescent zones of cements in many ancient limestones. Keywords Climate, diagenesis, Great Barrier Reef, Pleistocene, sea-level.

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C. J. R. Braithwaite and L. F. Montaggioni

INTRODUCTION Much of the extensive work on Holocene and Quaternary reef sequences has focused on their ecology and depositional history, related to changing sea-level and thus to climate (Webster & Davies, 2003; Camoin et al., 2004; Fournier et al., 2004; Montaggioni, 2005). A wide variety of diagenetic features has been described in Holocene successions (Shinn, 1969; Land & Goreau, 1970; Schroeder & Purser, 1972; Sibley & Murray, 1972; Macintyre, 1977; James & Ginsburg, 1979; Marshall, 1983; Lighty, 1985; Macintyre & Marshall, 1988; Budd & Land, 1990; Reid & Macintyre, 1998 and others). Descriptions of changes in Quaternary sequences are less common and have tended to emphasize the variety of cements present (Cullis, 1904; Pierson & Shinn, 1985; Dullo, 1986; Saller & Moore, 1991; Quinn, 1991) and their relationship to fresh or saline waters or to vadose and phreatic zones (Schroeder, 1973; Steinen & Matthews, 1973; Steinen et al., 1978; Buchbinder & Friedman, 1980; Longman, 1980; Aı¨ssaoui et al., 1986; Beach, 1995; Melim et al., 2001) or flow rates of active waters (Marshall, 1986). These are important observations, bearing as they do on understanding of the creation or preservation of porosity and the progressive stabilization of the mineral assemblages. However, with few exceptions (e.g. Strasser & Strohmenger, 1997; Melim, 1996; Sherman et al., 1999), they commonly neglect the relationship between diagenesis and the sequential fluctuations of both sea-level and the boundaries between the various water bodies. Matthews & Frohlich (1987) set out to provide a predictive diagenetic model, testing the premise that a static one-dimensional model sufficiently describes a sequence of distinctive diagenetic facies assemblages related to positions relative to the water table and mixing zone. Changes in sea-level might be assumed to result in a paragenesis in which successive events simply overprint cement sequences. However, the model shows that, as a result of varying exposure to the influence of widely diverse water chemistries, the relationship of features to particular surfaces is longer systematic (Matthews & Frohlich, 1987). Nevertheless, coupled twodimensional models (Whitaker et al., 1997) suggest a pattern that has elsewhere been shown to include intervals of pervasive change related to exposure separated by intervals in which a single generation of cement reflects relative stability. The deep borehole through the outer margin of the Great Barrier Reef (GBR) provides

an opportunity to investigate further the relationship between sea-level change and diagenesis over a longer timescale.

METHODS This work is derived from the petrological examination of the core from a deep borehole through the outer margin of the GBR drilled in 1995. The broad features of the core were described by Alexander et al. (2001) and Braithwaite et al. (2004). The present study is based on the examination of 370 thin sections and 234 analyses for both d18O and d13C. Gaps in recovery mean that all depths quoted are approximate, to within about 25 cm. Dating results and stable isotope data are available as GSA Repository item 2001051, and tables listing results of analyses can be obtained from GSA, PO Box 9140, Boulder, CO 80301-9140, USA, or http://www.geosociety. org/pubs/ft2001.htm (entry 051). Stable isotope analyses were conducted. Approximately 50 mg of drilled sample [mostly coral above ca 100 m below sea floor (mbsf) but including coralline algae and limestone below] was placed in an achat bowl and crushed in methanol using a mortar. Crushed samples were reacted at 90 C in a 100% phosphoric acid (Al-Assam et al., 1990; Sharma & Clayton, 1965) common acid bath using a VG Isogas Autocarbonate preparation system (VG Limited, Cheshire, UK). The gas evolved was distilled cryogenically to remove water produced in the reaction. Samples were run on a VG Precision Isotope Ratio Mass Spectrometer (PRISM). Instrument accuracy, as determined from an internal carbonate standard, is 0Æ08 ± 0Æ03& (1r) for d18O and 0Æ06 ± 0Æ03& (1r) for d13C. For X-ray diffraction a few milligrams of bulk carbonate were placed in a small mortar and ground to a fine powder in ethanol, sufficient for the preparation of smear slides. The samples to 24Æ23 mbsf were analysed using a Scintag XDS 2000 X-ray diffractometer (Scintag Inc., Sunnyvale, CA, USA). Selected samples from the remaining core were analysed using a Debeyeflex 1001 (Seifert, Ahrensberg, Germany) with a position-sensitive detector (Braun; Ar, metal wire).

BACKGROUND The GBR is an iconic structure almost 2000 km long, covering an area of some 25 000 km2 off

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The Great Barrier Reef the north-eastern coast of Australia. The borehole through Ribbon Reef 5 (latitude 1522Æ40¢ S, longitude 14547Æ149¢ E), approximately 49 km east of Cooktown (Fig. 1), penetrated some 210 mbsf and revealed a broadly shallowing-up carbonate succession punctuated by a series of erosion surfaces (Fig. 2). Ten depositional units have been identified (Braithwaite et al., 2004) the oldest of which was dated at ca 770 ka (marine isotope stage, MIS, 19). For the most part the boundaries of the units recognized in Braithwaite et al. (2004), based on changes in deposition, also correspond to shifts in diagenetic and geochemical characteristics. The ages assigned to the lower part of the succession are at best tentative and diagenetic characteristics shed some light on the nature of the events recorded.

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DIAGENETIC FEATURES

Aragonite cements Aragonite appears in three forms, all of which are common in Holocene reefs worldwide (Shinn, 1969; Schroeder, 1973; Lighty, 1985; Macintyre & Marshall, 1988). The most widespread aragonite in the GBR consists of acicular crystals grown epitaxially on the surfaces of corals or other aragonite bioclasts and as extensions of crystals forming trabeculae or other structural elements (Fig. 3A). By contrast, prismatic crystals of aragonite, that may be blunt ended, form crudely radial or mesh-like clusters unrelated to substrate characteristics. These crystals are relatively rare here and in Fig. 3B are overgrown by spherular aragonite. Some (e.g. at ca 5Æ5 mbsf) show divided

Fig. 1. Map illustrating the northeastern coast of Australia and the position of Ribbon Reef 5 relative to the principal topographic features. 2008 The Authors. Journal compilation 2008 International Association of Sedimentologists, Sedimentology, 56, 1591–1622

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Fig. 2. Summary lithology of the Ribbon Reef 5 borehole. 2008 The Authors. Journal compilation 2008 International Association of Sedimentologists, Sedimentology, 56, 1591–1622

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Fig. 3. (A) Acicular epitaxial aragonite crystal on coral, core run 2/12, depth ca 2 mbsf, scale bar 240 lm. (B) Radial prismatic and spherular crystals of aragonite (arrows R and S), core run 6/2, depth 10Æ21 mbsf, scale bar 240 lm. (C) Spherular (botryoidal) growth of aragonite, note division of outer margin to form a split (fibrous) fringe (arrow), core run 2/2, depth 3Æ41 mbsf, scale bar 110 lm. (D) Epitaxial aragonite overgrown by extension of neomorphic calcite as cement, core run 9/7, depth 17 mbsf, scale bar 240 lm.

terminations that suggest a transition to split crystal growth (Grigor’ev, 1965; Maleev, 1972; Grańa´sy et al., 2005; discussed below). Spheroidal (botryoidal) aragonite cements are characterized by sweeping radial extinction with poorly defined crystal elements (Fig. 3C) and sometimes also by concentric growth rings. A few examples (e.g. 3Æ4 mbsf) locally divide along their outer margins to assume a radially fibrous form (Fig. 3C). Botryoids occur locally in the youngest unit, typically within or between coralline algal crusts but either were not formed or are not preserved at deeper levels. Epitaxial cements may be overlain and preserved locally by internal sediment or overgrown by calcite cement that may itself be epitaxial on neomorphic crystals (Fig. 3D).

Interpretation Epitaxial, prismatic and spherular aragonite cements are all reported to form at or within a few metres beneath the sea bed. It has long been

accepted that they may form rapidly. Shinn (1969), for example, recorded growth of aragonite in the Persian Gulf within about 20 years and bulk cementation within ca 400 to 1000 years. Friedman (1998) described aragonitic ooids in the Bahamas cemented within a single year, although he unfortunately omitted the mineralogy of the cement. Grammer et al. (1999) reported the results of experiments on the Bahamas Banks which suggest that initial cementation by fibrous aragonite occurs within ca 8 months in water depths of up to 60 m.

Calcite cements Calcite cements are ubiquitous. Low-magnesium calcite is the common form. Although acicular high-magnesium calcite (Fig. 4A) was detected (X-ray diffraction) in the younger intervals, such crystals are difficult to separate petrographically from aragonite. Acicular crystals are non-fluorescent (Fig. 4B). The morphology of


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Fig. 4. (A) Prismatic crystals of high-magnesium calcite, core run 6/2, depth 10Æ2 mbsf, scale bar 240 lm. (B) Acicular non-fluorescent crystals of (?) high-magnesium calcite, core run 29/6, depth 61Æ9 mbsf, scale bar 240 lm. (C) Prismatic crystals of low-magnesium calcite, core run 20/7, depth 44Æ3 mbsf, scale bar 1Æ8 mm. (D) Calcite crystals with scalenohedral ‘dog-tooth’ terminations, core run 15/7, depth 32Æ3 mbsf, scale bar 1 mm. (E) Calcite crystals with rhombohedral ‘nail-head’ terminations, core run 17/3, depth 36Æ8 mbsf, scale bar 1 mm. (F) Calcite crystals with flattened concordant terminations overlain by scalenohedral surface, core run 17/3, depth 36Æ8 mbsf, scale bar 1 mm.

low-magnesium calcite varies widely. Blocky, roughly equant grains are widespread but in some sections occur alongside radial groups of crystals of prismatic (Fig. 4C) or blade-like form, with a greater length-to-width ratio. Where they form linings to open pores, terminations may be of three kinds: (i) acute scalenohedral, ‘dog-tooth’

(Fig. 4D): (ii) obtuse-angled rhombohedral, ‘nailhead’ (Fig. 4E); or (iii) flat and concordant between adjacent crystals (Fig. 4F). Some cement crusts, including overgrowths on echinoid spines, comprise a number of discrete growth increments, separated by thin sediment intercalations or stained surfaces (Fig. 5A). Many


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Fig. 5. (A) Overgrowth cement on echinoid spine with stained surfaces (arrows) defining distinct growth increments. Note variation in crystal terminations. Core run 20/7, depth 44Æ3 mbsf, scale bar 1 mm. (B) Meniscate bridge with contrasting terminations on crystals, arrows indicate surfaces reflecting adjacent water-filled and air-filled cavities, core run 9/7, depth 17 mbsf, scale bar 240 lm. (C) Coarse calcite crystals packed with inclusions defining bundles of subsidiary crystals, core run 27/9, depth 59 mbsf, scale bar 240 lm. (D) Coarse blocky calcite overgrowing radially fibrous precursor, core run 27/2, depth 60Æ7 mbsf, scale bar 240 lm.

crystals show alternations of dense inclusioncharged and limpid inclusion-free zones. Towards the tops of sedimentary cycles, cements may drape from the margins of pores or form meniscate bridges between grains with adjacent pores showing contrasting terminations (Fig. 5B). Successive surfaces may show transitions from scalenohedral or rhombohedral terminations to concordant surfaces and vice versa (Fig. 4F). Locally, coarse calcite crystals contain cores that consist of bundles of elongate sub-crystals (Fig. 5C) separated by linear inclusions and, in relatively rare examples, limpid crystals have overgrown a radially fibrous precursor (Fig. 5D). Clusters of triangular (in section) crystals may be trigonal or represent the terminations of acute rhombohedra but resemble the dendritic crystals described by Jones et al. (2000) from hot springs in New Zealand. However, hexagonal and nearcircular cross-sections are also recorded (Fig. 6A) with stained surfaces defining successive growth

increments. Some prismatic crystals and overgrowths on some echinoid spines are characterized by divided outer margins, again forming ‘split crystals’ (Grigor’ev, 1965) as in Fig. 6B. However, multilayered radial fibrous growths, widespread in the Moruroa core (Aı¨ssaoui et al., 1986) and common in speleothems, are rare here, although in a few samples coarse ragged interpenetrant crystals resemble flowstone (Fig. 6C). At intervals within the core, calcite cements are UV fluorescent, dimly in some areas but brightly in others; these include relatively isolated prismatic crystal fringes (Fig. 6D), successive generations of cement with varying characteristics (Fig. 6E) and well-defined concentric oscillatory zones (Fig. 6F).

Interpretation High-magnesium calcite cements are known to form rapidly in the marine environment, at or close to the sea floor. However, isotopic analyses


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Fig. 6. (A) Blocky calcite crystals with hexagonal and near-circular cross-sections, core run 104/5, depth 161Æ5 mbsf, scale bar 240 lm. (B) Prismatic calcite crystals with divided margins (arrows) reflecting split crystals, core run 4/7, depth 5Æ5 mbsf, scale bar 120 lm. (C) Ragged interpenetrant calcite crystals (partly neomorphosed?), core run 13/3, depth 28Æ2 mbsf, scale bar 1Æ8 mm. (D) Prismatic calcite crystals with UV fluorescent growth zones, core run 17/3, depth 36Æ84 mbsf, scale bar 240 lm. (E) Calcite cements with contrasting UV fluorescent growth increments, core run 27/2, depth 60Æ7 mbsf, scale bar 240 lm. (F) Prismatic calcite crystals with well-defined UV fluorescent oscillatory growth zones, core run 21/4, depth 48Æ2 mbsf, scale bar 240 lm.

(see below) indicate that most of the calcite described here is the low-magnesium form precipitated from meteoric waters. Crystallization can occur rapidly under such conditions and Dravis (1996) recorded growth of calcite cements in less than 10 years.

Split crystals, forming bow-tie, wheatsheaf and spherular forms have been recorded widely in carbonates (Folk et al., 1985; Buczynski & Chafetz, 1991; Gonza´lez et al., 1992; Verrecchia et al., 1995; Fernańdez-Dıáz et al., 1996; Morse et al., 1997) and form a consanguineous series. These


The Great Barrier Reef crystals may result from a number of different physico-chemical and biochemical driving mechanisms but broadly reflect rapid growth. The growing face of a crystal divides, forming a series of contiguous divergent projections initially separated by no more than dislocations. In the early stages such growth may result in a single crystal with radiaxial extinction. However, as divergence increases bow-ties, opposed bundles of divergent sub-crystals, are formed and with continued growth and progressive lateral expansion of the bow these form first wheatsheafs and then spherules. This transition was illustrated by Grigor’ev (1965) and examples are figured by FernańdezDıáz et al. (1996). Crystals characterized by acute scalenohedral, or obtuse-angled rhombohedral terminations are interpreted as the products of pores that were flooded. Contrasting forms can occur in adjacent pores and changes in morphology have been shown to reflect local variations in water chemistry and kinetics (Given & Wilkinson, 1985; Fernańdez-Dıáz et al., 1996; Morse et al., 1997). Where terminations are flat and concordant between adjacent crystals (cf. Binkley et al., 1980) growth is interpreted as having been within the vadose zone and limited to water films. Centimetre pores may have concordant surfaces across their roofs, formed above the saturated zone, with conventional pinnacle terminations on crystals lining the flooded lower margins. Transitions between pinnacled and concordant growth in successive growth surfaces in crystals are interpreted as reflecting a falling water table with reversed sequences generated where the water table was rising. The contrasts between inclusion-charged and limpid inclusion-free zones in blocky calcite are interpreted as reflecting changes in growth rates. The ease of incorporation of foreign bodies (or liquids) into crystals as inclusions can be related to growth rate, increasing with more rapid crystal growth, and is recorded in a range of natural crystals (Roedder, 1984; Budd, 1988; Landi et al., 2004). The sporadic appearance of fluorescent cements within the succession is significant. It is well-known that low molecular weight fluorescent compounds are concentrated in the vicinity of coral reefs (Gentien, 1981). The presence of fluorescent organic acids can be related to runoff from adjacent rivers, correlating with more negative values of d18O and d13C (Ayliffe et al., 2004). Fluorescent compounds are taken up by coral skeletons during growth (Isdale, 1984; Boto

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& Isdale, 1985; Scoffin et al., 1989, 1992) and may also be implicated in calcite nucleation (Neuweiler et al., 2003). If present in ambient waters, these compounds are also likely to be incorporated into abiotic cement crystals during growth. The timescales for the formation of fluorescent cement zones are, however, unlikely to mirror those of the annual growth bands in corals. Ragged interpenetrant crystals may reflect neomorphic replacement of a fibrous phase. Comparable textures attributed to calcitization of botryoidal aragonite have been figured by Sandberg (1985). However, in this instance, there seems no inherent reason why the precursor should not have been calcite. Similar calcite crystals were described by Folk and Assereto (1976) and Chafetz et al. (1985) in speleothem crusts.

Neomorphism As in many Quaternary sequences, grain alteration ranges from patchy neomorphism of corals that preserves details of the original trabecular structure (Fig. 7A) to coarse crystal growths that obliterate structures and extend into adjacent intraskeletal pores as cement (Fig. 7B). Similar changes have been reported widely by Cullis (1904), Richards & Hill (1942), Folk (1965), Bathurst (1983), Dullo (1986), Zhu et al. (1988) and others. In addition to formerly aragonitic bioclasts, including corals, some mollusc shells (Fig. 7C) and Halimeda (Fig. 7D), neomorphism locally extends to the high-magnesium calcite of both coralline algae (Fig. 7E) and echinoid plates. Within corals it is common to find small areas of acicular epitaxial crystals preserved beneath cement overgrowths on neomorphic calcite crystals (Figs 3D and 7B). These crystals retain distinct optical properties and are probably still aragonite. Coarsening of muddy or silty matrices to microspar is recorded locally (Fig. 7F).

Interpretation Neomorphism is essentially a process of ‘calcitization’. Ostensibly a solid-state process it reflects the migration of a microscale or nanoscale dissolution front through the grain or crystal, preserving elements of structural features. While neomorphism most obviously affects aragonite structures, it may also apply to calcite. The mineralogy and structure of the bioclasts affected and the severity of neomorphic effects reflect the levels of saturation and rates of flow of the pore fluids (Banner & Wood, 1964; James, 1972,


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Fig. 7. (A) Neomorphosed coral preserving traces of trabecular structure, core run 21/12, depth 46Æ4 mbsf, scale bar 120 lm. (B) Coarse neomorphic calcite replacing coral and extending as cement into intraskeletal pores, core run 9/7, depth 17 mbsf, scale bar 240 lm. (C) Neomorphosed mollusc shell, core run 9/7, depth 17 mbsf, scale bar 240 lm. (D) Neomorphosed Halimeda, core run 10/11, depth 20 mbsf, scale bar 240 lm. (E) Neomorphosed high-magnesium calcite in coralline alga, core run 100/3, depth 154Æ5 mbsf, scale bar 1Æ8 mm, XP. (F) Aggrading neomorphism of calcitic muddy/silty matrix, core run 121/2, depth 201 mbsf, scale bar 240 lm.

1974; Pingitore, 1976, Pingitore, 1982). However, it is arguable whether the areas of microspar described result from cementing of a micritic precursor (cf. Munnecke et al., 1997), or are a reflection of ‘Ostwald ripening’, demonstrated experimentally by Adelsek et al. (1973), in which smaller grains dissolve preferentially, driven by

boundary free energy, with the carbonate released crystallizing on the surfaces of larger grains.

Dissolution Dissolution ranges from the selective removal of compositional cores (Fig. 8A) or zones within


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Fig. 8. (A) Selective dissolution of zones within calcite crystals, core run 29/6, depth 61Æ9 mbsf, scale bar 120 lm. (B) Indiscriminate dissolution of calcitic bioclasts and neomorphosed muddy calcitic sediment, core run 113/6, depth 178 mbsf, scale bar 1 mm. (C) Collapse following selective dissolution of compositional zones within calcite crystals, core run 29/6, depth 61Æ9 mbsf, scale bar 120 lm. (D) Coral extensively dissolved but preserving areas of epitaxial aragonite, core run 100/2, depth 154Æ8 mbsf, scale bar 1Æ8 mm. (E) Patchy dissolution of magnesium calcite in echinoid spine with later cement, core run 14/3, depth 31 mbsf, scale bar 240 lm. (F) Patchy dissolution of magnesium calcite in coralline alga core run 59/4, depth 113Æ2 mbsf, scale bar 1 mm.

calcite cements or bioclasts, to the indiscriminate dissolution of both calcitic bioclasts (Fig. 8B) and muddy calcitic sediment. Selective dissolution of cement crystals may be so pervasive as to have allowed the remaining zones to collapse (Fig. 8C). Nevertheless, acicular epitaxial crystals may be

preserved within corals even where the original coral structure has suffered extensive dissolution (Fig. 8D). Patchy loss of high-magnesium calcite is seen in both coralline algae (Fig. 8E) and echinoderm plates (Fig 8F). In one example, the stereome of an echinoid spine has dissolved,


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leaving the cement to form a porous framework. Locally, dissolution has delicately etched exposed crystal faces, generating dentate fringes (cf. Jones & Pemberton, 1987) that resemble both ‘split crystals’ and the ‘spiky calcite’ figured by Folk et al. (1985). Dissolution surfaces are overgrown locally by cement.

Interpretation In the face of the contrast between indiscriminate dissolution that may remove large areas of bioclasts or cross-cut bioclasts, matrix and cement, and the precision of removal of specific compositional zones of crystals, there can be little doubt that the rate of delivery and the level of undersaturation of fluid at the reactive face varied widely and at times were delicately balanced within very narrow limits. For abiotic cements, grain composition and the level of saturation and rates of flow of the surrounding fluid are the dominant factors. However, where bioclasts are also involved, Walter (1985) showed experimen-

tally that rates of dissolution are controlled in part by grain microstructure, reflecting the sizes and architecture of component crystallites (cf. Banner & Wood, 1964). Examples of cement overgrowing dissolution surfaces of metastable grains attest to the temporal variability of groundwaters.

Features of cycle surfaces Younger sedimentary cycles within the succession are capped by sediments that include brown crumb-like muddy aggregates, some of which contain poorly preserved roots (Fig. 9A). Denser areas and larger peloids show tapering shrinkage fractures (Fig. 9B). A variety of structured and structureless micritic micropeloids (Fig. 9C) are present, sometimes in relatively large volumes. Calcified filaments occur locally towards the top of the succession (Fig 9D) but the matted filaments and associated matted needle crystals described elsewhere are either absent or not preserved here.

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Fig. 9. (A) Preserved roots in muddy aggregate, core run 18/10, depth 37Æ9 mbsf, scale bar 1 mm. (B) Muddy peloids showing tapering shrinkage fractures, core run 13/6, depth 29Æ4 mbsf, scale bar 1 mm. (C) Structureless micritic micropeloids, core run 29/4, depth 62 mbsf, scale bar 1 mm. (D) Calcified microbial filaments (arrow), core run 21/4, depth 48Æ2 mbsf, scale bar 1 mm. 2008 The Authors. Journal compilation 2008 International Association of Sedimentologists, Sedimentology, 56, 1591–1622


Interpretation The crumb-like aggregates capping sequences are similar to features recorded in palaeosols from Aldabra and Kenya (Braithwaite, 1975, fig. 5; Braithwaite, 1983, fig. 5C & D) described as micropeds or crumb aggregates in pedological terminology (FitzPatrick, 1993, figs 6Æ11, 6Æ12 and 13Æ8). Calcified filaments seem to reflect precipitation within the sheaths of microbial filaments and appear monocrystalline. Filaments have been described from a range of environments that commonly are sub-aerial (James, 1972; Steinen, 1974; Klappa, 1979; Jones & Kahle, 1993). However, here the close association with corals and encrusting algae suggests that they form part of the marine biota. Some of the micropeloids present resemble those described by Macintyre (1985) and later attributed to bacterial precipitation (Chafetz, 1986) but the relatively large size and traces of structure within others suggests that they may

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represent muddy clasts or micritized bioclasts. With the exception of the preserved roots, few of these features can be described as uniquely characteristic of sub-aerial emergence. However, their association with features of cement distribution (Fig. 4F) and indicators of significant dissolution and erosion (described below) places them securely in this context.

LITHOLOGY AND DIAGENESIS OF UNITS Detailed lithological descriptions and justifications of boundaries are provided in Braithwaite et al. (2004), summarized in Fig. 2, and are only outlined here. The Units described correspond to Milankovich cycles of sea-level change (Braithwaite et al., 2004). Cross-plots of stable isotope compositions in individual units are illustrated in Fig. 10, and comparison of isotopic variations with depth and diagenetic variations

Fig. 10. Cross-plot of stable isotope compositions illustrating spatial distributions and overlaps of units. 2008 The Authors. Journal compilation 2008 International Association of Sedimentologists, Sedimentology, 56, 1591–1622

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Fig. 11. The left hand graph illustrates the down-hole variation of d18O and d13C relative to the boundaries of depositional units. The graph on the right illustrates schematically the parallel variation in diagenetic features. Note that this graph records variations in numbers of features and thus reflects both diversity and history of diagenesis.

are shown in Fig. 11. The general distribution of diagenetic features with depth is shown in Fig. 12.

Unit 1: 211 to 184 mbsf The borehole was terminated at 210Æ85 mbsf in the belief that the material penetrated represented the beginning of carbonate accumulation (Braithwaite et al., 2004). Recovery of the lowest 34 m commonly was poor. The core, with conspicuous grey staining, is either friable or comprises abraded fragments; a large proportion consists of disaggregated carbonate sand. The sequence comprises

grainstones to wackestones, interbedded with micropeloidal laminated carbonate mudstones. The latter locally show synsedimentary folding with tapering en echelon spar-filled fractures. Bioclasts in the coarser lithologies include corals, bivalves and gastropods, barnacles, echinoderm plates, articulate and encrusting coralline algae, and larger foraminifera, together with varied lithoclasts that include grainstone fragments with surface borings resembling those of clionids. Scattered sharply angular grains of quartz, feldspar and mafic minerals, approximately 0Æ15 mm in diameter are present at intervals. Mudstones at the top of the sequence lack macrofossils but



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Fig. 12. Simplified core-log showing variations in characteristics of cements and distribution of other diagenetic features relative to depth compared with the assumed chronology based on marine oxygen isotope stages (MIS) and interpreted diagenetic environments.

contain large numbers of a planktonic foraminifer resembling Orbulina, while those at the base include scattered Globigerina. Although some corals are neomorphosed extensively others appear fresh, retaining their structure and traces of aragonite cement. Still others, together with fragments of aragonitic mollusc shells, have dissolved, leaving pores that are lined with a single generation of scalenohedral calcite. The crystals lie at varied angles to the

pore walls and some are doubly terminated, suggesting a sparse nucleation. Some peloidal areas are relatively porous as a result of dissolution of the peloids, leaving the granular cement as a supporting framework. Traces of early fibrous cement are preserved in some bioclasts where the bulk of the skeleton has dissolved. At intervals packstones and wackestones have a patchy crystalline matrix that also reflects aggrading neomorphism.


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The distribution of cements is quite variable and they are not fluorescent. The lower part of the unit consists largely of disaggregated grains, relatively large proportions of which are fractured, probably by drilling. There is no obvious gradation within the interval and the upper boundary is taken at the last appearance of folded mudstones. Although petrographic characteristics remain relatively uniform, the interval can be divided isotopically into two sub-units. The lower portion, below ca 200 mbsf, is characterized by relatively high d18O and d13C values, varying from )1Æ87& to +0Æ35& (average )0Æ25&) and from +1Æ22& to +2Æ83& (average +2Æ37&), respectively, values that are only approached in the sequence above in the Holocene. Above ca 200 mbsf values for both carbon and oxygen fall, d18O ranges from )5Æ82& to )0Æ08& (average )3Æ75&) and d13C from )1Æ83& to +2Æ55& (average +0Æ31&) (Fig. 11).

Interpretation The base of Unit 1 has been dated at ca 760 ka bp (Braithwaite et al., 2004), placing the onset of deposition at approximately MIS 19 or 18. The folded mudstones are interpreted as slumps of deep-water pelagic sediment, whereas the grainstones with their shallow-water biota imply downslope incursions of shallow-water deposits as debris flows. Together these are interpreted as reflecting deposition on a deep slope or ramp; it is arguable whether they imply highstand or lowstand conditions. Models of platform margin development typically show an upward transition from ramp to platform and similar transitions are seen in the early stages of development of the north-eastern Australian Platform (Davies et al., 1989). In a ramp context lowstands would imply a shift downslope of a relatively narrow production zone. However, it has been accepted widely that, because of the greater area of production, platforms are able to shed much larger volumes of sediment downslope in highstand intervals (Schlager et al., 1994). The relief of the area exposed may also drive larger volumes of meteoric water deeper into the sediment apron. The preservation of aragonitic bioclasts implies that the rocks have not been exposed since deposition, although in a relatively arid climate aragonite may survive protracted exposure (Brachert et al., 2006). In support of exposure, calcite cements are ubiquitous and both dissolution and neomorphism are common. However, Schlager & James (1978) found sea water generated effects that resembled

those of freshwater diagenesis in rocks of the deep slope of Tongue of the Ocean in the Bahamas and Melim et al. (2001) made similar observations on material from deep boreholes beneath the western flank of the Great Bahama Bank. Cross-plots of d18O against d13C fall into two groups (Fig. 10). Those at the deepest level lie within fields defined by Hudson (1977) and Nelson & Smith (1996) and subsequent workers as reflecting the general characteristics of marine sediments and cements. Above ca 196 mbsf, however, there is an abrupt shift to strongly negative oxygen values and to carbon values around zero. Although these values still correspond to ‘average marine limestones’ the change in character may reflect re-equilibration. This change is unlikely to be in response to an increasing temperature (Nelson & Smith, 1996). Average d18O values of )3Æ75& more typically reflect alteration by meteoric waters. The abrupt change in both curves at 184Æ2 mbsf reflects an outlier, a single sample consisting of 37% aragonite, but provides a convenient marker for the boundary.

Unit 2: 184 to 155 mbsf The rocks in this interval are predominantly grainstones to packstones, with minor intervals of wackestones or mudstones and only scattered larger bioclasts. The latter include corals, coralline algae (forming rhodoliths at 164 mbsf), phylloid calcareous algae, mollusc shells, echinoderm plates and spines, alcyonarian spicules, bryozoans and benthic foraminifera, lithoclasts, and a few small grains of quartz and feldspar. Rocks at 168 mbsf also contain Globigerina. Perched areas of muddy sediment are common between grains, forming geopetal floors in shelter porosity and within bioclasts such as foraminifera. The lower part of the unit is grey but, at 171 mbsf, this changes to a conspicuous pink. Vuggy porosity (at ca 163 mbsf) is associated with fractures reflecting lateral dilation. Neomorphism is widespread and affects corals, Halimeda and mollusc shells. Locally, the matrix is also neomorphosed (178, 168 and 161 mbsf). Many bioclasts have been dissolved, with pores lined or filled with a single generation of granular calcite cement but locally including scattered doubly terminated scalenohedral crystals (178 mbsf). At 166 mbsf coral with intraskeletal pores plugged with dense muddy sediment has been extensively dissolved, leaving an intricate system of pores with a thin, discontinuous fringe of cement. An


The Great Barrier Reef adjacent coral retains neomorphic areas that provide nucleation points for, and merge imperceptibly with, equant granular cement. Prismatic crystals with hexagonal cross-sections and both scalenohedral and low-angle rhombohedral terminations are present towards the top of this interval (163 and 161 mbsf) where prismatic palisade-like crystals have grown epitaxially on a foraminifer. At the top of the unit (161 mbsf) prismatic crystals include faintly fluorescent trails of inclusions that define two distinct but optically continuous growth phases. Similar trails are present in epitaxial cements on echinoderm plates (173 mbsf). Micropeloidal sediments form geopetal floors in some pores (161 mbsf). Dissolution of aragonite is common, but echinoid spines also show partial loss with overgrowth cement filling the resulting pores (162 mbsf). Locally, selective dissolution has formed narrow pores beneath blocky cement (173 mbsf) and within zones in epitaxial calcite overgrowths. At least two phases of dissolution are separated by the formation of overgrowth cement. Further evidence of this is seen where echinoid stereome has dissolved leaving the cement as an open framework and where foraminifera have dissolved beneath cement (161 mbsf). Dissolution is more widespread towards the top of the unit and the cement itself is distributed irregularly. Isotope values show a slight upwards decline within the unit (Fig. 11). Those for d18O vary from )5Æ02& to )3Æ23& (average )4Æ28&), while those for d13C continue the slight declining upwards trend established in Unit 1, ranging between )2Æ09& and +0Æ75& (average )0Æ41&).

Interpretation No samples have been dated within this interval but it is correlated tentatively with MIS 17. The corals do not represent a growth framework and the algal flora, dominated by melobesiids, implies cool relatively deep water, probably again on a slope. However, the diversity of bioclasts implies that many were derived from shallower areas. The fluorescent growth phases recognized locally may reflect the inclusion of land-derived organic compounds (Gentien, 1981; Neuweiler et al., 2003). In larger pores crystals show well-defined terminations, indicating that pores were flooded and that diagenesis occurred in the phreatic zone. Neomorphism and dissolution are widespread, the latter extending to dissolution of calcite bioclasts following cement deposition. Together with the preservation of traces of aragonite, these

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features indicate the highly localized variation in water chemistry. Carbon and oxygen isotope values are essentially co-variant at levels that overlap those established in the upper part of Unit 1 with a more marked decline in d13C. These values correspond with those established for meteoric diagenesis by Hudson (1977) but it seems most likely that this was a result of headdriven flow of meteoric waters, reflected in the sporadic occurrence of fluorescent zones, rather than sub-aerial exposure.

Unit 3: 155 to 131 mbsf There is no significant diagenetic boundary at the base of this unit that is marked by a change in the character of deposition. The rocks are dominantly grainstones, locally coarsening to rudstones. Some intervals are graded or show low-angle cross-lamination. Scattered larger corals include both branching and massive forms and fragments increase in size and numbers up sequence. These corals commonly carry thick coralline algal crusts and at some levels also form independent rhodoliths; they are accompanied by phylloid Halimeda, mollusc shells, echinoderm plates and benthic and encrusting foraminifera. Only one lithoclast has been identified (143 mbsf) and there are few silicate grains. Locally, long grain contacts indicate significant compaction (148, 145Æ6 mbsf), although there is little dissolution at common boundaries. Dissolution has resulted in widespread grain-scale porosity. Corals typically are neomorphosed but vary in their preservation. Some retain traces of trabecular structures in a brownish colour and trails of inclusions, but elsewhere contiguous neomorphic and epitaxial cement crystals resemble sparry cement (154 mbsf). Locally, granular calcite overgrows acicular epitaxial crystals that preserve a separate birefringence, suggesting that they are still aragonite (154Æ5, 151Æ5 mbsf). However, coralline algae show a striking replacement by coarse calcite not seen elsewhere. The cement is typically a single generation of limpid blocky calcite that overgrows epitaxial cements in corals but two growth episodes can be recognized locally. Larger rhombic or prismatic crystals are seen within pores near the top of the section and, at the base of the unit, it tends towards a prismatic, palisade-like habit lining open pores. Terminations of crystals commonly are acute scalenohedra but locally appear rounded. At ca 148Æ5 mbsf crystals are triangular in cross-section. Some crystals lining pores have a


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distinct thin limpid outer zone and in larger pores (135 mbsf) trails of dark inclusions separate two distinct growth generations. Locally, the earlier of these is bounded by a concordant surface while the later is terminated by flattened ‘nail-head’ pinnacles. Echinoderm plates commonly carry thick epitaxial overgrowths and appear as single crystals but locally (149 mbsf) inclusion trails imply at least two phases of growth. Scattered relatively large rhombic crystals are present at 152Æ5 mbsf. A narrow fracture at 143 mbsf has been bridged by later cement growth. Partially dissolved calcite bioclasts include echinoderm plates, with the resulting pores filled by overgrowth cement; in some only the stereome has dissolved, leaving the cement as a supporting framework. Dissolution of corals has left intraskeletal sediments and cement as a frame together with patches of coarse neomorphic crystals that provide nucleation points for later calcite cement. Pervasive dissolution within and beneath overgrowth and pore-lining cements is seen at ca 150 and 151Æ5 mbsf with selective dissolution of zones forming narrow ‘letter-box’ slots (150 mbsf). At 149 mbsf pores along boundaries between cement crystals and between bioclasts and cement appear to have collapsed. Ultimately this process generates loose grains, although these may retain cement in their interiors or attached to their margins. There is a marked change in d18O values at the base of the unit, that now range between )6Æ67& and )4Æ32& (average )5Æ67&). Carbon values continue the decline established in Unit 2, varying from )0Æ28& to )5Æ5& (average )2Æ98&).

Interpretation The upwards transition in the core to an algal flora dominated by lithophyllids is taken as an indication of warming and shallowing of waters during MIS 17. Cross-lamination and graded beds suggest that these are slope deposits with transport probably dominated by storms (Braithwaite et al., 2004). Synsedimentary fracturing (142 mbsf) indicates reworking of the slope. At ca 135 mbsf the earlier of two cement generations is bounded by concordant terminations that imply air-filled pores and thus emergence. However, there is no evidence here of the fluorescence that elsewhere reflects the incursion of landderived waters. Neomorphism and dissolution, including that of calcite bioclasts, selective dissolution of calcite cements, and collapse of the resulting pores, vary locally, implying the

restricted distribution of waters of differing compositions. Geochemically, the unit is characterized by abrupt changes in stable isotopes. Oxygen values drop sharply, fluctuating above or around )6&. With a few outliers, carbon values show a progressive decrease from about )1& to values approaching )5&. These values suggest an increasing influence of meteoric processes. Similar steps in carbon values have been attributed to the change from a phreatic to a vadose environment (Allan & Matthews, 1977).

Unit 4: 131 to 99 mbsf Laminated grainstones occur throughout the unit but at ca 131 mbsf are replaced largely by pink mudstones and wackestones. Coral fragments are relatively sparse, although some are centimetres in diameter, and reflect a relatively diverse fauna. Coralline algae are commonly the dominant bioclasts; at the base of the unit they are predominantly mastophoroids, but these are replaced upwards by lithophylloids. At some intervals, interfaces between contiguous algal crusts are filled by brownish calcified filaments. Smaller bioclasts include fragments of mollusc shells, echinoid spines, bryozoans and serpulids, phylloid calcareous algae, benthic foraminifera, alcyonarian spicules and possible sponge spicules. Unusually, fragments include barnacle remains. Small angular quartz grains are present at ca 119 mbsf and, remarkably, a single ooid is present near the base of the sequence. Corals in the interval commonly are neomorphosed and, although some epitaxial aragonite is preserved beneath blocky calcite, preservation is poor. As in other units, it is difficult to separate coarse neomorphic growths from sparry cement. However, fungal borings in coral surfaces that remain in place attest to an in situ replacement origin (121Æ5 mbsf). Nevertheless, coral at 120 mbsf includes acicular epitaxial crystals with distinct optical characteristics; these may be aragonite but have been engulfed by calcite grown epitaxially on neomorphic spar. Some mollusc shells are also neomorphosed but others are partly dissolved and replaced by sparry calcite. These shells are also difficult to differentiate as patches of colour are commonly the only traces of primary structure in neomorphic crystals. At the top of the unit many bioclasts have dissolved. Some algal clasts show selective dissolution (112Æ6 mbsf), and partially dissolved echinoid spines are surrounded by coarse epitaxial calcite


The Great Barrier Reef overgrowths. However, at 128 mbsf cement surfaces have been corroded. Indiscriminate dissolution, including patchy dissolution of areas of the muddy matrix, becomes more common upwards. Sinuous cavities at 113Æ25 mbsf are coated with a rusty brown stain associated with small opaque granules. At 111Æ4 mbsf tapering fractures are lined or filled with blocky calcite. Borings into mudstone contain two generations of geopetal fills. At 123 mbsf inclusion trails differentiate four discrete growth stages in blocky calcite cement and at 102Æ3 mbsf two generations are separated by a dusty coating of internal sediment. In the uppermost 6 to 7 m the interface between generations commonly defines a concordant surface. Extensive secondary cavities are lined with a single generation of coarse blocky calcite that locally also occludes primary pores. However, as some bioclasts dissolved after cement growth there must have been at least two growth episodes. Crystals lining open pores typically have acute scalenohedral terminations but flattened rhombohedral forms are also present throughout the section with no systematic distribution. Cross-sections vary from hexagonal to trigonal, with local examples appearing almost circular. None of the cement is fluorescent. Micropeloidal sediment is present within some dissolution cavities and borings (115Æ5 mbsf). Unlike adjacent units (Fig. 11), the behaviour of carbon and oxygen isotopes is antithetic. Whereas d18O values vary from )7Æ76& to )4Æ41& (average )5Æ1&), but showing a gradual progressive increase, carbon values, range from )6Æ06& to )1Æ2& (average )4Æ39&), and, after rising to )1Æ2& at ca 120 mbsf fall towards )6& at the upper boundary.

Interpretation Unit 4 reflects a transition from a relatively deep slope to shallower and warmer waters, perhaps during MIS 15. Corals are larger and more diverse than in the units below but do not form a growth framework. Laminated grainstones locally suggest current transport, but rhodoliths are increasingly common and, dominated by lithophyllids, may reflect in situ deposition. These again appear to be slope deposits but with an increasing proportion of sediment derived from shallow water. Corals typically are neomorphosed, although some epitaxial aragonite is preserved beneath blocky calcite. The limited distribution of multiple cement generations implies an equally limited and highly localized flow of formation waters. Some cement

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surfaces within the uppermost 6 to 7 m are concordant, indicating air-filled pores and a relatively extensive vadose zone. Cement-filled fractures and borings in mudstone surfaces reflect early lithification. Cements are not fluorescent but many surfaces carry a rusty ferruginous stain that may indicate the flow of oxidized waters. Oxygen isotope values show a progressive drift upwards from around )6& to about )5&. Carbon values increase sharply upwards from the base of the unit with spikes approaching mixing-zone values before falling to around )6&, confirming meteoric influence at the top of the unit.

Unit 5: 99 to 73 mbsf The unit is dominated by grainstones, although wackestones are more common above ca 85 mbsf. Centimetre clasts of coral, coralline algae and bivalves locally form coarse rubble with a wackestone matrix. The rocks become increasingly vuggy upwards and sections from 96 to 93 mbsf and 76 to 73 mbsf consist essentially of loose sand with scattered bioclasts, quartz grains and lithoclasts. Corals increase in numbers up sequence, many at the base of the unit are extensively bored and most carry thick crusts of coralline algae. Typically the latter are lithophyllids but they are replaced at intervals by mastophorids, locally including Hydrolithon reinboldi. There is a progressive increase in the foraminifer Calcarina calcar along with a variety of other benthonic and planktonic forms. Corals commonly are neomorphosed with trails of inclusions locally preserving structures, but many show patchy dissolution with pores filled with blocky calcite. There is commonly a high secondary porosity. The patchy cement is commonly a single generation of blocky calcite. However, two generations are present in cavities between coralline algal crusts (99Æ4 mbsf) and forming the epitaxial cement of an echinoid spine. Inclusions at ca 92 mbsf indicate two growth phases separated by a concordant surface. At 88Æ2 mbsf similar epitaxial cement suggests more phases. Growth here is strongly asymmetric and on the lower surface three principle zones are separated by a rusty stained surface. The inner and outer zones comprise couplets in which the inner margin is charged with inclusions and the outer margin is limpid and inclusion free. Only the outermost zone is limited by coarse growth pinnacles. In corals the typical cement forms as epitaxial extensions of neomorphic crystals. Prismatic cements are rare and either were absent or are not


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preserved. Mollusc shells are affected by both neomorphism and dissolution with secondary pores remaining open or lined with blocky calcite. Within muddy lithologies some areas are coarser, reflecting aggrading neomorphism. At 92 mbsf pore surfaces carry a rusty brown stain associated with scattered opaque granules and at 88Æ2 mbsf some cavities contain two (perhaps three) generations of orange-brown muddy sediment, each overlain by granular calcite. Irregularly shaped opaque red-brown granules form part of a distinctive granular sediment at ca 87 mbsf. One associated grain may be glaucony. At ca 82 mbsf irregular vugs in muddy areas are filled with blocky calcite. Mostly this consists of a single growth generation but inclusion trails, including those within the overgrowth on an echinoid spine, locally suggest three major increments with two additional minor growths. Three cement relationships have been identified at 80Æ73 mbsf. Granular crystals with acute-angled (scalenohedral) terminations appear to have nucleated directly on the surfaces of corals and pore-lining sediment. More extensive growths form on neomorphic crystals but include both scalenohedral pinnacles and a blunt castellation. Faint inclusions in cements within neomorphosed corals define four growth stages with pinnacle terminations reflecting growth in flooded pores, although crystals lining the roofs of cavities commonly are larger than those on the floors. However, at 79Æ2 mbsf, trails of inclusions in granular calcite indicate two growth increments, the earlier with a concordant surface, the later with scalenohedral terminations. Some cement near the top of the section shows a series of fluorescent zones. Micropores parallel to growth zones (80Æ73 mbsf) reflect selective dissolution of cements. However, the outer margins of crystals may also be divided longitudinally to resemble bundles of isolated sub-crystals. These crystals maintain a common optical orientation and are unlikely to be ‘split crystals’. Dissolution to form similarly oriented spikes has been described by Jones & Pemberton (1987). Oxygen values range from )5Æ98& to )4Æ55& (average )5Æ41&), but the carbon signal is particularly noisy with wide variations from )7Æ12& to )2Æ19& (average )4Æ62&) and no consistent pattern.

Interpretation Unit 5 may represent the deposits of MIS 13. The lower part is dominated by grainstones and there seems to be little evidence of any extensive

growth framework. Nevertheless, the progressive replacement of lithophyllid algae by mastophorids suggests that the area effectively had become a relatively shallow reef (Braithwaite et al., 2004). Crystal terminations vary throughout the unit; and concordant surfaces are present at ca 99, 88, 84 and 79 mbsf, sometimes accompanied by scalenohedral or flattened ‘nail-head’ forms that are fluorescent locally, indicating incursions of waters containing dissolved organic compounds. In some cases the concordant surfaces are the oldest, indicating that the water table had risen, flooding part of the vadose zone; these point to local vadose conditions but, although the core becomes increasingly vuggy upwards, no welldefined erosion surface has been identified. Stable isotope values confirm the influence of meteoric waters.

Unit 6: 73 to 37 mbsf The diversity of cements, the occurrence of a putative erosion surface and distinct suites of stable isotope values suggest that Unit 6, as originally defined by Braithwaite et al. (2004), should be divided. However, in order that correspondence should remain clear units have not been renumbered.

Unit 6a: 73 to 57 mbsf The sedimentological boundary at the base of this unit is based largely on changes in the density and diversity of corals, an increase in the numbers of encrusting foraminifera and bryozoans, and increasing numbers of echinoderm and molluscan clasts. Although there is a change in average isotope values, there is no corresponding diagenetic change. Rocks include grainstones and rudstones with lesser intervals of wackestones and mudstones. The lowest interval is dominated largely by fragments of faviids together with branching forms while younger rocks include more massive genera; many of these carry thick coralline algal crusts that locally also form rhodoliths. Hydrolithon occurs at intervals. Smaller bioclasts include increasing numbers of mollusc shells, Halimeda, bryozoans and encrusting and benthic foraminifera with a single grain of glaucony at ca 72Æ5 mbsf. Irregular areas at ca 68Æ5 mbsf may represent lithoclasts or pinnacles on an erosion surface. Within this interval some bioclasts are wellpreserved, but corals, thick-shelled molluscs and echinoid spines all show patchy calcitization


The Great Barrier Reef (neomorphism) and dissolution, locally including selective dissolution of growth zones. Nevertheless, primary structures of aragonitic bioclasts and acicular epitaxial crystals in corals (ca 71 mbsf) sometimes are preserved. Secondary pores within some echinoid spines carry a thin lining of scalenohedral crystals in which inclusion trails imply at least two growth phases. At ca 72Æ5 mbsf irregularly distributed granular cement includes four or five growth phases, separated by inclusion trails defining concordant surfaces, although terminations of some later increments are scalenohedral. Crystals generally show a dull orange-red UV fluorescence contrasting with the more characteristic blues elsewhere. At 65Æ5 mbsf dense mudstones contain an extensive system of secondary cavities lined with coarse limpid calcite terminating in scalenohedral crystals. From 65Æ5 to 64Æ3 mbsf crystals have a pale blue UV fluorescence with distinct oscillatory zones. Older zones are generally dull and the younger ones bright, but a third dull zone is present locally. In some areas laminar bright zones imply rapid changes in growth conditions. The terminations of these crystals generally are not concordant but at ca 63 mbsf two generations are separated by a sculpted dissolution surface. At ca 62 mbsf muddy peloidal grainstones contain numerous mouldic pores. Narrow fissures are occupied by two mudstone increments. At ca 61Æ5 mbsf, corals retain traces of acicular epitaxial crystals overlain by blocky calcite or muddy internal sediment but retaining an independent birefringence; these have a faint yellow fluorescence, contrasting with the blue fluorescence of the internal sediment. Similar acicular crystals locally form a fringe where coral trabeculae have dissolved. Peloidal internal sediment is overlain by granular cement. Trails of inclusions define four growth increments but surfaces generally do not follow crystal faces and presumably reflect dissolution. In one area selective dissolution has resulted in the collapse of the remaining zones to form a mass of flat plates. Fluorescent blocky calcite at 61Æ55 mbsf includes several growth increments. The earliest, with a faint pale blue fluorescence, is cut off by an irregular erosion surface. This surface is overlain by cement with alternate light and dark blue oscillatory zones that is itself abruptly truncated along a smooth dissolution surface. This surface is overlain by generally dull crystals in which only the outermost margins show a faint yellow fluorescence and trails of dusty inclusions locally

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define three or four concordant growth boundaries. The final increment is non-fluorescent but there is no consistent pattern. Granular cement lining open pores at 61Æ19 and 60Æ7 mbsf is characterized by obtuse-angled ‘nailhead’ terminations. Some crystals contain inclusions which suggest that they may have had a fibrous precursor. At 60Æ7 mbsf two ‘scalenohedral’ cements are separated by thin internal sediment. The older of these is non-fluorescent but both the internal sediment and the younger cement fluoresce pale blue, the latter with a thick non-fluorescent outer rim. Similar but relatively elongate crystals at 59Æ7 mbsf grow from a smooth surface of blocky calcite that may be either a concordant growth surface or reflect corrosion. However, these features occur within areas of grainstone that are themselves bounded by surfaces characterized by iron oxide (?) granules and rusty surface staining. At ca 59 mbsf coarse prismatic calcite crystals grow on the geopetal fill of a large gastropod. These crystals are truncated and overlain by cement that extends as the principal (final) fill of smaller pores. Within the major cavity the bounding surface is a smooth and concordant surface and is overlain by four additional zones in which local faint inclusion trails suggest that there may have been as many as 10 subsidiary growth phases. The surface of the last growth increment shows a transition within a few millimetres from a smooth concordant surface to one defined by scalenohedral pinnacles and a similar transition is present in an earlier growth stage, with thicker growths on the floor of the chamber. The thick final zone of the cement contains swarms of minute fibrous crystals at its base, arranged roughly normal to the scalenohedral surfaces of the crystals beneath; although they have been overgrown they retain a distinct birefringence. Elsewhere, sparse elongate squareended crystals may be aragonite. Discontinuities in the boundaries between cement sequences imply that they have been broken. The major increments show a faint pale blue fluorescence, but later crystals have dull cores and brighter rims. Small distinctive areas of cement at 59 mbsf rest on a truncated surface of prismatic crystals and bioclasts. These crystals are relatively isolated, rising from the substrate at a variety of angles, and are elongate with tapering or rounded terminations. The crystals include large numbers of brownish linear inclusions parallel to their axes and narrow limpid zones on their outer margins.


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Samples from ca 57 mbsf appear to form part of an irregular lace-like erosion surface overlain by poorly sorted muddy sediment containing lithoclasts. Irregular cavities are filled with one or two generations of blocky calcite with a faint pale blue fluorescence. Crystals may extend as trigonal scalenohedral surface growths but are overlain by dark silty mud containing Heterostegina and other foraminifera as well as neomorphosed or dissolved and re-cemented echinoid spines. Between 59 and 57 mbsf cements show an abrupt change in complexity. At ca 59 mbsf inclusions locally define at least 10 growth stages, whereas rocks immediately above show only one or two. This observation suggests a change in diagenetic environment that is supported by rapid variation in d13C. Values for d18O are relatively stable, varying from )6Æ57& to )5Æ36& (average )6Æ07&) but with a central broad excursion. Values for d13C are more variable and range between )7Æ25& and )3Æ78& with an average of )5Æ73&.

Interpretation The base of Unit 6a was provisionally assigned an age of approximately 465 ka BP (Braithwaite et al., 2004), approximately MIS 11. The transition from generally massive corals to branching forms indicates a progressive shallowing of the reef system and supports the suggestion of Lea et al. (2000) that the significant sea-level and seasurface temperature (SST) changes of MIS 11 were responsible for the initial formation of the GBR per se. There is evidence of sporadic establishment of vadose conditions throughout the interval but these do not necessarily reflect a single event. Fluorescent cements, present at a number of levels and more common towards the top of the unit, may indicate pulsed incursions of meteoric waters containing dissolved organic matter. Stable isotope compositions dominantly reflect meteoric waters but, unlike some other intervals, the oxygen and carbon signals are closely parallel. The irregular surface at 57 m is interpreted as an erosion surface, based on the increasing alteration and complexity of cement sequences beneath it (Figs 11 and 12) but no palaeosol has been identified. Unit 6b: 57 to 36 mbsf Recovery was poor to 51 mbsf but rocks above this sub-unit include pink mudstones and grainstones with encrusting Porites, faviids and

branching Acropora. Coralline algae are dominated by mastophorids with Hydrolithon onkodes. There is a progressive increase in green algae, benthic foraminifera and bryozoans. Rudstones with rhodoliths appear at 47 mbsf and coral fragments with thick coralline algal crusts dominate the interval from 38 to 35 mbsf. Areas of muddy sediment commonly include scattered small quartz grains. Although some bioclasts are well-preserved, most coral and molluscan fragments, and many echinoid spines, show patchy calcitization. The interval from 44 to 36 mbsf is characterized by extensive vugs occupied by pale brown peloidal sediment with vermiform pores. Scattered lithoclasts are present, both near the base and towards the top of the unit, and areas of grainstone at 36 mbsf may represent either lithoclasts or isolated pinnacles on an irregular surface. Quartz grains are present at 41Æ9 and 42 mbsf. Above 56 mbsf, relatively large coral fragments are neomorphosed extensively, although they preserve traces of trabecular structure. The primary porosity is filled locally by a single generation of blocky calcite epitaxial on neomorphic crystals. At ca 52 mbsf extensive secondary cavities are filled with similar calcite but two zones are present locally. Some grainstone areas appear to represent lithoclasts. Here, and at ca 50 mbsf, patches of acicular epitaxial crystals in corals retain a distinct birefringence but are overgrown by calcite crystals several millimetres in diameter. Coralline algae and encrusting foraminifera have been extensively corroded. Opaque brown granules on some grain surfaces are associated with discontinuous rusty staining that in larger pores becomes a dark ferro-manganese coating. Numerous mouldic pores contain only a thin lining of blocky calcite but in one larger vug with a rusty muddy lining two increments of coarse calcite are present. At ca 49Æ5 mbsf the earlier of two cements includes at least four sub-zones. A large pore at ca 49 mbsf, adjacent to an area of packstone, is lined with coarse calcite with dark–bright–dark oscillatory fluorescent zoning and partly concordant terminations. In smaller cavities coarse ‘nailhead’ crystals have grown from the roofs, while smaller crystals lining the floor have their outer margins charged with inclusions. As in other sequences neither the cements nor the fluorescence patterns are consistent laterally. At ca 48Æ2 mbsf the outer tips of calcite crystals ca 2 mm long are dark with inclusions. Under UV these have dark cores overlain by multiple bright


The Great Barrier Reef oscillatory zones. Corals and mollusc shells have dissolved and are represented by coarse calcite cement. In one pore, faint fluorescent zoning in blocky calcite records transitions from pinnacles to concordant terminations, to pinnacles. Similar morphological changes are present at intervals to ca 47Æ8 mbsf but are absent in smaller pores. At 46Æ4 mbsf coral and molluscan fragments are replaced by coarse neomorphic calcite in which structures are preserved locally. Calcified filaments coat and invade pores on the outer surfaces of corals, some of which have completely dissolved. At 46Æ4 mbsf eight or nine growth phases of coarse blocky calcite are preserved. Locally the cement forms meniscate bridges and in the largest pores inclusion trails record alternations between concordant and pinnacle surfaces. The first concordant phase was followed by the introduction of small amounts of sediment but there were at least five concordant increments, followed by inclusion-charged pinnacles, three to four additional concordant phases and new large prismatic crystals with either concordant or scalenohedral terminations. The larger crystals are fluorescent, the inner zones are pale blue and the outer zones dull red, with inclusions of pale yellow. At 45Æ4 mbsf parallel fluorescent banding implies > 6 growth phases characterized by concordant surfaces, before the final growth of ‘nail-head’ crystal faces. Similar cements are overlain locally by internal sediment and a thin additional cement crust. Irregular cavities within coralline algae and mudstone carry only thin cement linings, implying that the dissolution forming them was a relatively late event. Cements in grainstones and packstones at 44Æ3 mbsf are varied. Echinoid spines carry extensive epitaxial growths with inclusions marking at least three growth stages. The first terminates in a pinnacled surface but stages two and three are smoothly rounded, lacking structurally controlled terminations. An adjacent spine carries five growth stages that are all smoothly rounded. Residual pores are lined predominantly by crystals with scalenohedral terminations but in the largest pores prismatic crystals (> 3 mm long), with a distinctive dark–bright–dark concentric zoning, overlie a concordant surface limiting non-fluorescent cement that locally includes two growth zones. A prominent fracture at 44 mbsf includes four major cement growth zones and two to three thin minor zones. The earlier zones all appear to terminate in concordant surfaces. At 42Æ2 mbsf inclusion trails in coarse

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blocky calcite, with well-defined scalenohedral terminations, define three earlier concordant surfaces. Four growth zones with either scalenohedral or concordant terminations in adjacent pores are seen at 41Æ9 mbsf. Corals with sparse coatings of calcified microbial filaments contain inclusions outlining the remains of aragonitic epitaxial cement. Traces of both high-magnesium calcite and aragonite have been recognized in X-ray analyses. The rocks at and above 41Æ5 mbsf are of particular interest. Bioclasts have been dissolved extensively, although some corals and mollusc shells preserve traces of their structure. However, at 41 and 39Æ2 mbsf small areas within neomorphic corals contain relicts of acicular epitaxial cement that retain a distinct birefringence. Pores are filled typically with blocky calcite and in some areas only micritic envelopes define original grain surfaces. Terminations in open pores may be either scalenohedral or concordant. At 41Æ5 mbsf a small area of crumb-like aggregates resembles the texture of some soils and at 39 mbsf patches of muddy sediment with a quasi-peloidal texture are associated with tubules resembling rootlets. Two granular calcite cements are separated by trails of dusty inclusions that locally define smooth concordant surfaces but, in some areas, the outer surface appears to have been eroded. The outer margin of the earlier cement has a thin blue fluorescent zone. At 38Æ6 mbsf neomorphic crystals in corals are locally several millimetres in length. Small areas within coralline algae appear to represent neomorphosed spherular cement overlain by acicular cement. Concordant and scalenohedral terminations are present locally at 38 mbsf where there are two phases of cement growth. In some areas both phases are non-fluorescent but elsewhere the later phase comprises dark inner and bright blue fluorescent outer zones. At 37Æ9 mbsf wackestone is cut by a fracture ca 0Æ75 mm wide filled with muddy peloidal sediment in which rounded and sinuous areas resemble rootlets but do not preserve detail of internal structures. A second fracture includes peloids resembling Favreina pellets. Lithoclasts are present at 39, 37Æ2 and 36Æ8 mbsf. These lithoclasts are encrusted with algae at 37Æ2 mbsf where they are associated with sediment that again includes structures resembling rootlets. Two calcite cements are associated locally with the surrounding sediment; both have dark cores and thin pale blue fluorescent rims. Where present, the earlier surface carries a rusty ferruginous stain. In larger pores crystals have


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concordant terminations. The common progression in sequences is from concordant to pinnacle terminations but at 36Æ8 mbsf this order is reversed. In some areas crystals of the later cement include numerous near-parallel inclusions. The isotopic signal initially is similar to that in Unit 6a with d18O values ranging from )9Æ28& to )5Æ72& (average )6Æ32&) and d13C values fluctuating between )10Æ28& and )3Æ47& (average )5Æ02&). However, extreme values of both oxygen and carbon, reaching )9Æ28& and )10Æ28&, respectively, form prominent spikes above ca 50 mbsf (Fig. 11).

Interpretation A tentative date of 322 ka implies deposition of the unit during MIS 9. The continuing upwards transition from massive to branching corals and the appearance of mastophorid algae including H. onkodes reflects a progressive shallowing but there is no abrupt lithological break in the succession. Inclusion trails marking four to five growth phases of blocky calcite cement near the top of the succession define smooth concordant surfaces that imply vadose conditions. Later growths commonly carry scalenohedral pinnacles indicating that pores were subsequently flooded. However, in at least one example a reverse order suggests that the water table was falling. There is a striking increase in cement complexity near the middle of the units for which there is no obvious explanation. Fluorescent cements are common throughout the uppermost two-thirds of the sequence, with a conspicuous oscillatory zoning indicating greater incorporation of organic compounds which, as in other intervals, is interpreted as reflecting incursions of terrestrially derived waters. There is a parallel increase in vuggy porosity towards the top of the sequence implying proximity to an exposed karst surface. The brown sediment mantling this surface at 36 mbsf is interpreted as a palaeosol and must have penetrated several metres into the rock beneath. As in the units below, the carbon signature is characterized by significant noise but closely parallels that of oxygen above ca 40 mbsf at around )6& with fewer negative excursions. Values for d18O remain at around )6& until small groups form spikes of )9Æ28& at ca 49 mbsf and )8Æ03& at 46 mbsf. Thus, although there are deeper intervals in which vadose conditions seem to have been established, the surface at 36 mbsf is the first evidence in the core of sub-aerial exposure resulting in the development of a palaeosol.

Unit 7: 36 to 28 mbsf Unit 7 is characterized by the presence of relatively large corals, including both massive and branching forms. A number of intervals consist of rubbly fragments with a matrix of grainstones or wackestones with conspicuous Halimeda and fragments of coral, coralline algae, mollusc shells (including Turbo), echinoderm plates and foraminifera including Marginopora. At 35Æ8 mbsf corals have been extensively neomorphosed but retain traces of their original structure in trails of inclusions and locally include relicts of aragonitic cement (confirmed by X-ray diffraction of bulk samples) in the lower half of the sequence. Muddy areas appear relatively coarse and may reflect aggrading growth (cf. Adelsek et al., 1973; Munnecke et al., 1997). Patches of sediment at 32Æ3 mbsf include a rusty brown soil-like material consisting of crumb-like aggregates. Many bioclasts have dissolved and both primary and secondary cavities are lined or filled with blocky calcite in which both concordant and ‘nail-head’ terminations are represented but scalenohedral terminations are relatively rare. The transition from draped concordant surfaces to angular crystal terminations commonly occurs within a few millimetres. At 32Æ3 mbsf neomorphic crystals in coral are extended as scalenohedral cement. Commonly there is only a single growth phase. Most of the cement and neomorphic calcite shows a dull reddish UV fluorescence and small areas of later cement fluoresce pale blue. By contrast, at 34Æ4 mbsf, this order is reversed, and there are areas in which bright and dull zones alternate. There commonly is evidence of an early fibrous cement (e.g. at 31Æ9 mbsf) overgrown and locally replaced by blocky calcite but with trails of inclusions defining the original structure. At higher levels in the core, cements become more complex with two to four distinct increments separated by dust lines at ca 31 mbsf; none of these show any fluorescence. Traces of aragonite are still found in rocks that have experienced both extensive neomorphism and dissolution of corals, coralline algae and echinoid spines. Crystal terminations commonly form smooth concordant surfaces and at 31 mbsf the cement rests on a dissolution surface truncating both cement and bioclasts. At 30Æ8 and 29Æ4 mbsf muddy and peloidal areas with swirling crudely laminated textures are associated with areas of crumb-like aggregates, irregular peloids and


The Great Barrier Reef tapering fractures, the bulk fabrics resembling those of palaeosols. Rootlet tubules are also present but only rarely preserve remnants of calcitized cells. The walls of an extensive secondary cavity system locally form a series of concave sculpted surfaces. These surfaces are overlain by patches of muddy sediment with residual pores lined with coarse blocky calcite. At 29Æ4 mbsf large areas of granular cement fluoresce pale blue and at 29Æ1 mbsf similar crystals show faint concentric zones. Neomorphic areas and younger cements are typically non-fluorescent but with an orange-red tint to crystal boundaries. Locally, cement with oscillatory zones overlies non-fluorescent crystals. At the top of this sequence rocks are conspicuously vuggy. At 28Æ2 mbsf large areas comprise coarse calcite crystals, in which closely spaced lamination is crossed by radially divergent inclusions that appear to represent crystal boundaries; these are associated with brown-stained areas in which structures resembling roots are surrounded by a crude concentric lamination. Stable isotope values are broadly similar to those below, with d18O ranging between )7Æ6& and )6Æ01& (average )6Æ46&) and d13C from )7Æ08& to a peak of )0Æ18& (average )5Æ18&).

Interpretation The base of Unit 7 rests on an erosion surface that has been assigned an age of approximately 275 ka (MIS 8Æ4). Deposition probably took place during MIS 7. The diverse coral and algal assemblage suggests derivation from a shallow lagoonal environment. The irregular karst surface and vuggy porosity at ca 28 mbsf is mantled by brown sediment containing preserved roots and interpreted as a palaeosol that penetrates several metres into the rocks below. Typically the cement is of blocky calcite that, at higher levels within the unit, is characterized by two to four discrete growth increments and fluorescent oscillatory zones. Linings to larger pores include both lowangle ‘nail-head’ and concordant terminations, indicating that air-filled and flooded pores coexisted, probably along the lower margins of the vadose zone. Areas of cement associated with some larger cavities resemble the fibrous ragged crystals characteristic of speleothems. The d18O signal is relatively noisy and, from a base of ca )7&, the carbon signal essentially forms a single spike with a peak of )0Æ18& at ca 32 mbsf that may reflect mixing zone conditions. However, cross-plots generally are consistent with meteoric-derived waters.

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Unit 8: 28 to 15Æ85 mbsf A sample from the upper margin of the unit has been reliably dated at 125Æ7 ka (230Th/234U, Braithwaite et al., 2004), indicating deposition during MIS 5. The rocks of this unit predominantly are grainstones and rudstones with a few intervals of mudstone. Scattered lithoclasts and quartz grains appear towards the top (17Æ5 and 16Æ7 mbsf). Corals include massive faviids, Porites, and robust branching forms that commonly are bored extensively. Phylloid calcareous algae are common and many corals carry thick surface crusts of coralline algae, that locally also form rhodoliths. Benthic foraminifera are common, together with molluscan fragments and other smaller bioclasts. Corals typically are neomorphosed, with traces of their structure preserved in trails of inclusions but replacive crystals commonly are finer-grained than in many examples below. Patches of linear inclusions locally represent remnants of epitaxial aragonite cement but elsewhere coarse neomorphic grains locally extend into adjacent pores as epitaxial calcite cement. Many bioclasts have dissolved and extensive secondary cavities are filled by blocky calcite. At 20 mbsf echinoid spines have been replaced by calcite, with large areas retaining no trace of the original stereome. There is typically only a single growth generation of cement, locally fluorescent, but in larger pores at 24Æ8 mbsf trails of inclusions suggest two to four growth increments and at ca 24 mbsf more than six growth increments with additional minor phases. The early generations commonly are not fluorescent but sporadically throughout the interval later crystals show faint unzoned fluorescence. Cements include concordant, rhombohedral ‘nail-head’ and acute scalenohedral terminations, but also concave scalloped surfaces reflecting dissolution. Scattered occurrences at other depths show faint stained surfaces that define two to four successive crystal faces but cannot be traced laterally. Locally, the outer margins of crystals are finely divided forming congruent fibrous terminations. At ca 25 mbsf successive concordant layers form asymmetrical meniscate drapes against pore margins, implying growth in vadose water films. At 23Æ9 mbsf and more commonly upwards in the succession, traces of epitaxial cement in corals retain an independent birefringence and aragonite has been identified by X-ray diffraction. These traces may be preserved even where the structure of the coral has been lost. Other


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cements include calcified filaments and peloidal micrites. At 21 mbsf grainstone is overlain by a soil-like interval that includes numerous laminae wrapped around sinuous tubular structures; these locally contain areas of brownish blocky calcite and resemble cellular structures described from Aldabra (Braithwaite, 1975) and interpreted as roots. These roots commonly are packed tightly with little interstitial material but are associated locally with crumb-like aggregates that are remarkably porous and, in contrast to the calcitized roots, generally lack cement. At 18Æ3 mbsf a large cavity within a coral contains clusters of muddy peloids of varying size with a soil-like (glaebular) texture. At the same depth a muddy coating on limpid cement is contained within radiating ragged crystals that have no consistent extinction. Areas of reddish-brown muddy sediment at 17Æ5 mbsf contain tapering fractures. The unit is capped by a prominent karst surface but clearly surface deposits extend several metres into the rocks below. Values of d18O range from )8Æ55& to )5Æ64& (average )6Æ44&) and those for d13C from )7Æ76& to )0Æ41& (average )5Æ0&).

Interpretation Recovery was poor for several metres above the unconformity marking the top of Unit 7. Corals with thick coralline algal crusts are common and diverse above this. The biota suggests deposition in a well-developed shallow reef or lagoonal environment (Braithwaite et al., 2004). The upper limit of the unit is a karst erosion surface bearing a palaeosol that extends at least 5 m into the cavities below, with scattered lithoclasts and quartz grains towards the top. Blocky calcite cement crystals are only weakly or non-fluorescent and are unzoned, perhaps suggesting less terrestrial organic input than in some other intervals. Although neomorphism and dissolution are widespread both aragonite and highmagnesium calcite occur throughout the unit, attesting to the incomplete mineralogical stabilization and the inefficiency of meteoric alteration, the latter reflecting local compositional variability of the waters responsible. Concordant surfaces indicate the periodic establishment of vadose conditions. In general, variations in crystal morphology in larger pores reflect changes between air-filled and flooded pores, depending on the connectivity of the pore system and its evolution. At 21 mbsf a concordant surface is characterized by a series of concave scallops that reflect dissolution in response to a change to undersaturated

water chemistry. There is a general increase in d18O values from ca )8& at the base of the unit to ca )6&, contrasting with carbon values of ca )7Æ6& at the base of the unit spiking close to zero before falling to meet those of oxygen towards the top. Cross-plots suggest a scatter between limestones with cements derived from the mixing zone, and those reflecting meteoric waters.

Unit 9: 15Æ85 to 0 mbsf The final unit from 15Æ85 mbsf to the surface is regarded as Holocene and includes rocks that vary from grainstones to packstones and contain well-preserved massive and branching corals. Many are bored extensively by Cliona, sipunculids and Lithophaga, and carry thick crusts of coralline algae, vermetids, bryozoans and foraminifera, together with varied serpulids, barnacles and small oysters. Coralline algae form scattered rhodoliths. Molluscan shell fragments are common among smaller bioclasts with benthic foraminifera and Halimeda. Open pores are lined locally with greyish millimetre-scale microbialites similar to those described by Camoin et al. (1999). Lithoclasts are present at ca 3Æ15 mbsf and scattered quartz grains are present at 5Æ5 and 4 mbsf. Among a variety of cements present, the most common is acicular epitaxial cement in corals; this is irregularly distributed and is absent in some sections. It does not generally occlude pores. Prismatic aragonite cement occurs sporadically, sometimes occluding pores. Both cements are present to within ca 2 mbsf. Spherular aragonite and blocky calcite are less common. In one pore (at 5Æ5 mbsf) prismatic crystals show ragged divided terminations; these may reflect dissolution (cf. Jones & Pemberton, 1987) but more probably imply growth of parallel sub-crystals (split crystals). Similar fibre crystals at 3Æ4 mbsf form a distinctive fringe on concentrically banded spherular cement. Spherular (botryoidal) and prismatic aragonite crystals are present only locally within the encrusting framework, with spherular growths in particular appearing to favour spaces within coralline algal aggregates. Both morphologies include examples in which the outer margins are divided, with terminations interpreted as ‘split-crystals’ (prismatic crystals at ca 5Æ5 mbsf and spherular growths at 3Æ4 mbsf). Micropeloidal cements and calcified microbial filaments occur locally and brown calcified filaments form an integral part of many coral and algal growths. Corals appear fresh and retain


The Great Barrier Reef aragonite but locally the structure shows a sweeping ‘spherular’ extinction (at 3Æ3 mbsf) and at ca 4 mbsf scattered larger crystals are present within trabeculae. At 5Æ5 mbsf a number of brownish areas appear to lie within coralline algae. These areas have a yellow-grey birefringence (masked by body colour) with a patchy extinction resembling that of ‘microflamboyant’ chalcedony. It has not been possible to identify these further. Scattered irregular dissolution (?) cavities are filled with dark silty mudstone containing smaller bioclasts. Values of d18O range from )4Æ18& to )0Æ91& (average )2Æ7&), with d13C from )2Æ9& to +1Æ69& (average +0Æ7&).

Interpretation Unit 9 reflects Holocene deposition and rests on the eroded surface of rocks believed to have been deposited during MIS 5. The number and diversity of corals implies deposition in a shallow reef environment, although this may have been behind or below any active reef edge. Extensive borings in corals suggest that accumulation was slow, particularly in the upper part of the unit. Boundstones of coral and coralline algae are common but, between these, a variety of sediments ranging from mudstones to grainstones have been emplaced into irregular cavities that reflect significant reworking of surface deposits. There may be two or three such sediments within the same thin section and some include lithoclasts and silt-sized quartz grains. Deposits commonly appear uncemented but the fact that they locally exclude impregnating stained resin suggests the presence of cryptic grain-surface cement. The ‘spherular’ extinction in corals may be a primary feature but more probably reflects incipient neomorphism. Stable isotopes show a marked departure from those in older units. There are increases in d18O to values only exceeded in the lower half of Unit 1. In parallel, d13C values typically are positive. A single value of )2Æ9& at ca 8Æ3 mbsf is mirrored by a fall in a group of d18O values to ca )4& (Fig. 11). It is not clear what event triggered this excursion. Both sets of data show a steep rise from the boundary and cross-plots are consistent with marine sediments and cements. DISCUSSION Although the upper part of the core carries clear indications of repeated intervals of sub-aerial exposure the diagenetic signal is muted. This

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observation contrasts with results in other areas, where distinctive diagenetic sequences are associated with sea-level changes. Each of the most recent exposure events on Moruroa resulted in the generation of a characteristic diagenetic couplet. In the lower parts of couplets pores are filled by blocky calcite deposited within the phreatic zone and commonly formed as a result of a single growth event. By contrast, the upper parts are typified by high secondary porosity filled by extensive multilayered growths of radial-fibrous calcite. These layers imply multiple growth episodes but their separation by intervals in which there is no repetition suggests that they formed within relatively brief periods and do not reflect overprinting by successive sea-level changes. The layers carry evidence of cementation within the vadose zone and are capped by palaeosols. Each couplet therefore reflects a single sea-level cycle. This finding seems counterintuitive; surely sealevel variations are of greater magnitude than the thicknesses of these units? However, much depends on the hydraulic conductivity of the rocks and the nature of the pore fluids. When sealevel falls, meteoric water invades pores, dissolution begins and cements begin to form and progressively occlude pores. Much of the rock through which waters must pass is eroded and it is this process that provides the carbonate for cementation. It is for this reason that dissolution decreases and saturation increases with depth (McClain et al., 1992), with a sharp increase at the fresh/saltwater boundary (Anthony et al., 1989). Controls on solubility include CO2 concentrations and changes in alkalinity that may result from bacterial decay of organic matter (Melim et al., 2002). The depth to which circulation of meteoric waters occurs has traditionally been seen as governed by the Ghyben–Herzberg model, based on the relationship between hydraulic conductivity and recharge. This has translated as a ‘rule of thumb’ ratio of 1:40 for the height of the water table above sea-level and the depth of the marine water below. However, Budd & Vacher (1991) suggest that this is unrealistic in older rocks, both because of the difficulty in estimating the height of the palaeo water table above contemporary sealevel and because it assumes static conditions that are inherently impossible. Budd and Vacher argue that the thickness of the freshwater lens can be approximated better as 1% of island width. Buddemeier & Oberdorfer (1986), describing the situation on Enewetak, indicated that the freshwater body is probably only a few metres thick, whereas the mixing zone beneath is likely to be


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tens of metres. The Great Barrier Reef is some 60 km from the present Australian coast and locally more than 100 km from the nearest significant relief. Thus, the freshwater phreatic zone may not have been thick even when sea-level was at its lowest. The models of cyclic cement distribution that have been described elsewhere are not replicated in the GBR. The common radial-fibrous calcite cements so characteristic of the Moruroa core are not represented here and, although there are changes in the complexity of diagenetic features in the GBR, these vary erratically and do not form well-defined cycles. Figures 11 and 12 include schematic illustrations of GBR cement variability based on cement generations and the presence of features such as neomorphism or dissolution. Although this is a crude measure and the sense of change is not always the same, the pattern of variability corresponds closely with that of the stable isotopes. A similar multicycle diagenetic evolution was described by Fournier et al. (2004) from the Oligo-Miocene carbonate succession of Malampaya (Philippines). In shallow-water carbonate systems that have undergone repeated phases of sub-aerial exposure the vertical distribution of cements and vugs is commonly quite erratic and it is not possible to locate accurately the vadose and phreatic zones related to a given exposure event. This view was borne out by the forward modelling of Matthews & Frohlich (1987). However, Whitaker et al. (1997), using similar methods, generated a clear stacked sequence of diagenetic zones but found, nevertheless, that spatial associations between unconformity surfaces and diagenetic zones that suggested causality were misleading. In the GBR, the greatest diagenetic diversity occurs in Units 6 to 8 towards the top of the core. Variations become more pronounced and can be related to emergent surfaces and the domination of the stable isotope record by a meteoric water signature. Fluorescence is more common in cements in this part of the core and is attributed to the greater incorporation of organic compounds within the growing crystals. This is thought to reflect a greater flow of freshwaters passing from coastal vegetation during wetter periods that may also explain the sporadic appearance of detrital quartz during earlier deposition. In the Moruroa core fluorescence is limited and the contrasting signals within diagenetic cycles are expressed more clearly. The interpretation placed on these differences is that the climate on Moruroa generally was drier and the

contrasts between Moruroa and the GBR data reflect regional climatic variations over a substantial period. The current rainfall in northeastern Australia is >3000 mm year)1 and, in contrast to some other published views, Kershaw & Nanson (1993) showed that the region was generally substantially wetter during both early and late stages of interstadials. Kershaw et al. (2003), based on data from Ocean Drilling Programme (ODP) Site 820, were able to show that on the adjacent continent glacial periods were generally more arid than interglacials. Wetter intervals allowed the development of a complex angiosperm rainforest on the coast that may have been responsible for the fluorescent compounds present in the cements. Because river flow was substantially reduced during lowstands, cement generation may have been limited to periods of around 10 ka in interstadials when run-off was highest. The least effect that this would have is to generate two diagenetic cycles within a single sea-level cycle; however, this neglects the known sea-level variations within the major Milankovich cycles. In areas with a wetter climate and greater throughput of meteoric waters, the ability to differentiate such cycles may always be severely limited.

CONCLUSIONS Detailed examination of the diagenetic characteristics of a 210 m core through Ribbon Reef 5 in the Great Barrier Reef reveals a variety of cements, epitaxial, radial prismatic and spherular aragonite and blocky, prismatic and fibrous calcite, together with local calcified microbial filaments and peloidal sediments. Calcite cements vary in morphology and crystals include those showing acute scalenohedral, low-angle rhombohedral and concordant terminations. These crystals are associated with others in which finely divided margins represent a transition to ‘split crystals’. The variations in morphology are interpreted as reflecting changes in water chemistry during crystal growth. Neomorphism and dissolution are widespread and the severity of both varied, probably in response to the degree of undersaturation of the pore waters with respect to both aragonite and calcite. The diversity of diagenetic features varies in parallel with changes in stable isotopes even when there is no overt evidence of emergence. However, diversity increases in the upper part of the core where emergent surfaces are capped by palaeosols. Whereas on Moruroa


The Great Barrier Reef couplets representing a division between cements deposited in phreatic and vadose zones can be attributed to specific emergent surfaces, here the distinction is blurred. Unit 1 deepest in the core carries a wholly marine isotopic signature. In Unit 2 there is evidence of freshwater and mixing-zone but in neither of these is there petrographic evidence of vadose conditions. These conditions first appear towards the top of Unit 3. In Unit 4 mixing-zone waters near the base gave way to freshwater and vadose conditions at the top where petrography identifies a thicker vadose zone but no emergent surface has been recognized. Unit 5 is generally similar but here transitions within cement from flattened concordant to pinnacle crystal terminations reflect a rising water table during diagenesis. Diagenetic features are more diverse in Unit 6a. Deposition reflects a shallowing upwards, culminating in an erosion surface. No palaeosol has been recognized here but fluorescent cements are relatively common. Stable isotopes confirm the influence of meteoric and mixing zone waters. Unit 6b is also capped by an erosion surface that here carries a palaeosol. Fluorescence is common in cements in which sequences of crystal terminations reflect both rising and falling sea-levels. The diagenetic assembly here is the most diverse in the succession. Unit 7 is relatively thin and is capped by an erosion surface bearing a palaeosol; vadose features are common. Cements include coarse ragged crystal growths that may represent speleothems. Unit 8 is characterized by the influence of both mixingzone and wholly meteoric waters. Petrographic evidence indicates a water table that at times was either rising or falling. Cements are less fluorescent than those below but the interval is capped by a karst surface bearing a palaeosol. Finally, Unit 9 reflects the re-establishment of wholly marine conditions. Although it is possible to recognize diagenetic signals generated by sea-level change, the results suggest that in the Great Barrier Reef such changes are not reflected in either diagenetic sequences simply related to emergent surfaces, or well-defined sequential overprinting. The petrographic and geochemical variations recorded are products of the relationships between marine and meteoric, and vadose and phreatic, zones in groundwater systems. Because these variations were both spatially limited and dynamic, their effects occupy limited foci and discordant time frames and few can be attributed to specific emergent surfaces. Comparison with data from

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Moruroa suggests that diagenesis in the Great Barrier Reef may reflect relatively high rainfall.

ACKNOWLEDGEMENTS The drilling programme on the GBR was initiated by Peter Davies of the University of Sydney. 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, and Commissariat a` l’Energie Atomique, and the University of Granada in Spain. Drilling was with the support of the Great Barrier Reef Marine Park Authority and the Queensland Parks and Wildlife Authority, and we thank Craig Sambal and Jenny Le Cusion. Drilling operations were under the guidance of Jack Pheasant and Alistair Skinner of the British Geological Survey and Phil Manning of the University of Sydney. We are particularly indebted to Miriam Andres, formerly of ETH Zurich, now with Cheveron, San Ramon, CA, for providing the analyses and details of the analytical methods. CJRB gratefully acknowledges the generous support of the UK Natural Environment Research Council and an Emeritus Fellowship from the Leverhulme Trust. Finally we would also like to acknowledge the constructive criticism of referees.

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Manuscript received 10 July 2007; revision accepted 30 April 2008
