Mechanisms of spur and groove development and implications for reef platform evolution

Quaternary Science Reviews 231 (2020) 106155

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Quaternary Science Reviews journal homepage: www.elsevier.com/locate/quascirev

Mechanisms of spur and groove development and implications for reef platform evolution Stephanie Duce a, c, *, Belinda Dechnik b, c, Jody M. Webster c, Quan Hua d, James Sadler e, Gregory E. Webb e, Luke Nothdurft f, Marcos Salas-Saavedra e, Ana Vila-Concejo c a

Centre for Tropical Environmental and Sustainability Studies, James Cook University, Townsville, QLD, 4811, Australia Department of Oceanography, Federal University of Espirito Santo, Vitoria, ES-CEP, 29075-910, Brazil Geocoastal Research Group, School of Geosciences, University of Sydney, Sydney, NSW, 2006, Australia d Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW, 2234, Australia e School of Earth and Environmental Sciences, The University of Queensland, Brisbane, QLD, 4072, Australia f School of Earth, Environmental and Biological Sciences, Queensland University of Technology, Brisbane, QLD, 2000, Australia b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 April 2019 Received in revised form 18 December 2019 Accepted 23 December 2019 Available online xxx

Contemporary understanding of Holocene coral reef development is based primarily on sub-surface investigations of reef flat, back reef and lagoon zones. Few studies of Holocene fore reef development exist, constituting a significant gap in our understanding of reef evolution. The spur and groove (SaG) zone is a distinct, understudied, feature of fore reefs worldwide. We review SaG development from previous studies and present 29 new SaG short cores with 52 14C and UeTh ages from six fore reef regions of Heron and One Tree reefs, the first such data from the Great Barrier Reef. Remarkably, we found that SaGs do not necessarily accrete in the same direction as their adjacent reef flat. We identified three modes of reef flat and SaG lateral accretion: Mode 1 e lagoonward accretion of both the reef flat and SaGs; Mode 2 elagoonward accretion of the reef flat but seaward accretion of the SaGs; Mode 3 seaward accretion of both the reef flat and SaGs. Most SaG zones (five of the six studied) accreted in a seaward direction (Modes 2 or 3). Hydrodynamic conditions and local topography appear to be the dominant factors determining which mode occurs. Episodic high-energy events are also likely to play an important role in SaG formation. Our findings suggest that traditionally held models of reef evolution whereby lagoonal, mature reefs fill, developing into senile platform reefs, may not hold. Rather, reef flats may continue to expand seaward on their leeward, and semi-exposed fronts to increase in size while maintaining their lagoons. © 2019 Published by Elsevier Ltd.

Keywords: Reef growth Coral reef geomorphology Spur and groove Accretion Holocene Great barrier reef

1. Introduction How coral reefs grow and evolve over time in response to environmental conditions has long fascinated scientists (e.g. Darwin, 1842; Maxwell, 1968; Hopley et al., 2007) and has important applications for reconstructing past sea level and environmental conditions and for providing insights to potential future changes in reef environments (Woodroffe and Webster, 2014; Webster et al., 2018). Extensive coring, seismic and SCUBA studies have improved our understanding of reef development and

Abbreviations: SaGs, Spurs and grooves. * Corresponding author. College of Science and Engineering, James Cook University, Townsville, QLD, 4811, Australia. E-mail address: [email protected] (S. Duce). https://doi.org/10.1016/j.quascirev.2019.106155 0277-3791/© 2019 Published by Elsevier Ltd.

controlling factors throughout the Holocene (Gischler, 2011). For the majority of reefs in the Indo-Pacific, modern reef growth was initiated on pre-Holocene limestone foundations between 9500 and 7000 yr BP (Montaggioni, 2005). Reefs then adopted one of three vertical growth responses to sea-level rise: “keep-up”, “catchup” or “give-up” (Neumann and Macintyre, 1985). After reaching sea-level “keep-up” and “catch-up” reefs then accreted laterally once sea level stabilized. Variations in reef growth, morphology and internal architecture of Holocene reefs reflect differing sea-level histories, wave energy regimes and water quality combined with neotectonics and antecedent topography (Montaggioni, 2005; Dechnik et al., 2017). A comprehensive review of 684 reef cores collected throughout the Indo-Pacific revealed that the majority of samples, on which our contemporary understanding is based, were collected from reef top zones between the reef crest and back reef

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(Montaggioni, 2005). Few studies have made sub-surface observations of reef-front and fore reef zones due to the difficulty of coring in these environments (Montaggioni, 2005; Roff et al., 2015). The fore reef zone thus constitutes a significant gap in our understanding of geological and geomorphic coral reef evolution and development. In the Great Barrier Reef (GBR) it was thought that after reefs reached sea-level, lateral accretion occurred in a predominantly lagoonward direction, driven by hydrodynamic energy and antecedent substrate morphology, particularly slope angle (e.g. Davies et al., 1977; Marshall and Davies, 1982; Hopley, 1982; Marshall and Davies, 1985). This was based primarily on cores from reef flats and lagoonal patch reefs throughout the GBR and limited samples from the near vertical southern fore reef at One Tree Reef in the southern GBR (~20 m water depth) which revealed Pleistocene-aged substrate at/near the surface implying little, if any, lateral accretion had occurred on this margin during the Holocene (Davies and Marshall, 1979). Those concepts informed the evolutionary classification of Holocene shelf reefs by Hopley (1982, refer to his Fig. 9.2 p.253). This classification recognizes a progression in reef morphology and development from “juvenile” reefs, characterized by initial colonization and growth on submerged antecedent topography, to “mature” reefs that have reached modern sea level and developed reef flats commonly enclosing relatively shallow lagoons. Mature reefs were then suggested to accrete laterally in a lagoonward direction until complete infilling of the lagoon resulted in a “senile” platform reef. During the senile phase lateral seaward accretion may occur on their leeward flanks (Hopley, 1982). Recently, closely spaced core transects across the reef flat at Heron and One Tree reefs in the southern GBR found the direction of lateral reef flat accretion varies according to exposure to hydrodynamic energy with reef flats exposed to high wave energy tending to accrete lagoonward while protected reef flats accret in a seaward direction (Dechnik et al., 2016; Webb et al., 2016). However, given the aforementioned lack of cores from the fore reef zone the mechanisms by which seaward lateral reef flat accretion occur remain poorly understood. Spurs and grooves (SaG) are a distinctive morphology of fore reefs worldwide (Roberts et al., 1992). They are composed of parallel, linear ridges (spurs) separated by troughs or depressions (grooves) that form finger-like shapes extending down the reef slope into deeper water (Guilcher, 1988). Given their location at the seaward fringe of modern reefs, the direction, rates and mechanisms of SaG formation and growth likely play an important role in the development of reef platforms. Indeed, whether the formation of these distinctive features is dictated primarily by pre-existing gullies eroded into the antecedent substrate during sea-level low stands (e.g. Newell, 1954; Purdy, 1974), or is produced actively by wave and current induced abrasion of the grooves (e.g. Cloud, 1959, Blanchon and Jones, 1997), or coral growth aligned to the direction of dominant wave energy (e.g. Shinn, 1963; Maxwell, 1968; Shinn et al., 1981; Kan et al., 1997a), has been the subject of much debate and speculation (Sneh and Friedman, 1980; Gischler, 2010). Few studies have dated sub-surface material from SaG features to provide a chronology for their development and determine their implications for reef platform evolution. Shinn et al. (1981) collected six cores along the top of a spur and into the adjacent grooves at Looe Key reef in Florida. They found that the middle of the spur was located on bedrock but the shallower end of the spur initiated on a layer of carbonate sand with no existing topography of the underlying basement that could have controlled the location of the SaG features. This spur showed lateral accretion from the middle in both lagoonward and seaward directions (Shinn et al., 1981). Kan et al. (1997a) found a similar growth pattern based on

exposures from a reef cutting excavated at Miyako Island in southern Japan. Both sites appear to show a spur initiating as a patch reef and then accreting both seaward and lagoonward. At Ishigaki Island, Japan Yamano et al. (2003) examined a trench created through the reef and fore reef and documented the seaward accretion of the reef flat after it reached sea level by “engulfing spurs and filling grooves” on the fore reef. Kan et al. (1997b) also found that lateral seaward accretion of the reef flat at Tonaki Island, Japan was facilitated by SaG development. This suggests that SaGs could provide a mechanism for reef flats to expand seaward. However, with few, and localised studies, the modes and mechanisms of fore reef SaG development globally, and their influence on reef flat growth and evolution, are variable and remain far from certain. The fore reefs and SaGs in the GBR have never been cored. Thus, many fundamental questions remain about the features including their composition, age, accretion patterns and the environmental forcing mechanisms responsible for their formation. To address these questions we present 37 radiocarbon dates, 15 UraniumeThorium (UeTh) dates and bio-lithofacies descriptions from a suite of 29 short cores from SaG zones at six sites across Heron and One Tree reefs in the southern GBR. By combining these fore reef cores with extensive existing reef flat core data we provide a better understanding of the role that SaGs play in reef platform development and evolution within the previously understudied fore reef zone.

2. Study sites We present 29 short cores from six sites, three at Heron (14 cores) and three at One Tree (15 cores) reefs, in the southern GBR, Australia (Fig. 1, Table 1). Heron (23
270 S, 152
570 E) and One Tree (23
300 S, 152
060 E) reefs are mature lagoonal reefs (Hopley et al., 2007) on the mid-continental shelf of Australia in the CapricornBunker groups (CBG) of reefs. The region receives oceanic trade wind-driven swells with an average annual significant wave height of 1.15 m from an east southeasterly direction (Harris et al., 2015a). Heron Reef is mostly sheltered from these swells by surrounding reefs whereas One Tree Reef is fully exposed (Duce et al., 2016). The meso-tidal area has semidiurnal tides with a mean spring tidal range of 3 m (Vila-Concejo et al., 2014). The antecedent Pleistocene basement at One Tree Reef is located approximately 13 m below present mean sea level (MSL) (Barrett and Webster, 2012) and Holocene reef growth is believed to have initiated at ~8370 cal yr BP (Dechnik et al., 2015). At Heron Reef the Pleistocene basement lies between 12 and > 30 m below modern sea level and the timing of Holocene reef growth initiation was before 8400 cal yr BP (SalasSaavedra et al., 2018). Heron Reef SaG cores were collected from the northwestern (site abbreviation: H-NW), northern (HeN), and southwestern (HSW) fore reefs (Fig. 1). The reef margin at H-NW is characterized by a secondary rim, approximately 25 m seaward of the reef platform rim (Jell and Webb, 2012). One Tree Reef SaG cores were collected from the eastern (OT-E), southeastern (OT-SE) and northern (OT-N) fore reefs. The SaGs at H-NW and OT-SE were classified as “short and protected” under the SaG classification system presented in Duce et al. (2016), while those at HeN, H-SW and OT-N were classified as “long and protected”. Spurs and grooves at OT-E were classified as “exposed to wave energy” (see Duce et al. (2016) for a detailed description of each class). The general morphology and substrate characteristics of each core site at Heron and One Tree reefs are shown in Figs. S1 and S2, and the fore reef slope angles and wave energy conditions are summarised in Table 1.

S. Duce et al. / Quaternary Science Reviews 231 (2020) 106155

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Fig. 1. Study sites a) overview of the Great Barrier Reef, Australia with the study region offshore from Gladstone and b) satellite image showing the location of coring sites at Heron and One Tree reefs.

3. Methods 3.1. Core collection Cores were recovered using a hand-held, pneumatic drill (Shinano SI-5501, 10 mm air drill) powered by standard SCUBA tanks with a commercial grade water swivel, diamond drill bit and barrel with a diameter of 48 mm and a length of 700 mm (Fig. S3). Core bores commonly had to be abandoned if a layer of sediment or rubble was encountered. This introduced some bias towards well consolidated framework into our sampling. Where possible, vertical cores were taken in transects along the length of a spur top allowing the direction of lateral spur accretion to be determined. Cores were also taken horizontally into the spur wall. It was not possible to core the groove floor due to the presence of unconsolidated sand and/or rubble. Core location was measured using a Global Positioning System on a buoy at the surface. Hereafter the terms “outer” and “inner” are used to refer to the seaward (deeper) and lagoonward (shallower) ends of the SaG zone, respectively. Water depth was measured at the top of the hole using a standard diving pressure gauge. The time was recorded simultaneously and depths were corrected to MSL. When a piece of core was extracted the down bore depth was measured before drilling continued to provide accurate recovery data. Recovery was high (average 96%, Table 1), although this partly reflects the necessary abandonment of holes when unconsolidated material was encountered. 3.2. Core logging and dating Cores were split, photographed and logged to identify downcore bio-lithofacies based on those described by Montaggioni (2005), and Dechnik et al. (2015) (Fig. 2 a-d). Corals were identified to genus level where possible following the taxonomic descriptions of Veron (2000), and Humblet et al. (2015). Corals were assessed to determine whether they were in-situ (i.e., in growth position and not deposited as rubble or debris) based on the criteria detailed in Blanchon and Perry (2004), and Webb et al. (2016). Preference was to date well-preserved in-situ samples. The most pristine samples were rigorously vetted by examination under Scanning Electron Microscope (SEM) to assess for microboring, diagenesis and secondary carbonate material, e.g., muds, cements (Webb et al., 2016) (Fig. 2 e-h). Samples assessed under SEM were not coated or treated and the material examined

was used for dating. A Dremel drill with diamond wheel was used to extract very fine sub-samples (e.g., removing any cements from within the corallite if revealed by the SEM analysis leaving only the pristine carbonate material for dating). Sub-samples were ultrasonically bathed in Milli-Q water to remove any unlithified fill. 3.2.1. Radiocarbon dating Accelerator Mass Spectrometry (AMS) radiocarbon dating of 37 samples was carried out using the STAR facility at ANSTO (Fink et al., 2004; Hua et al., 2001). Radiocarbon results are reported as percent modern carbon (pMC) and conventional radiocarbon ages in years before present (yr BP) where 0 yr BP is AD 1950 (Stuiver and Polach, 1977) and are shown in Table 2. All replicates show good agreement, within 1 s uncertainties of their pMC values or conventional 14C ages, giving high confidence in the quality of the sample material and the dates obtained. 3.2.2. UeTh dating Fifteen UeTh dates were obtained (Table 3) following the methods previously described by Clark et al. (2014a), Clark et al. (2014b) and Salas-Saavedra et al. (2018). Samples were measured for U and Th isotopes on a Nu Plasma multi-collector-inductively coupled plasma-mass spectrometer at the Radiogenic Isotope Facility, the University of Queensland. All samples plot within 4% of ocean d234U (Gallup et al., 1994) (Fig. S4) and are considered reliable. UeTh ages were presented as cal yr BP (relative to 1950) for ease of comparison with calibrated radiocarbon ages (Fig. S4). Ten of the UeTh samples were replicates of samples dated using AMS 14 C. The ages obtained by the two methods show excellent agreement (R2 ¼ 0.99, Fig. S4). 3.2.3. Reef accretion rates Fig. 3 shows the axes along which rates of vertical and lateral accretion and wall accretion were calculated. Rates were calculated as difference in age divided by distance between the two samples where there were two available dates and the ages were outside the error range of one another. Spur lateral accretion rates were calculated based on the uppermost (surface) ages from cores. When considering these lateral accretion rates the depth of the spur surface at each core should be taken into account as spurs can slope down the fore reef and thus water depth can vary along the length of the spur, unlike reef flats for which lateral accretion rates have previously been reported which are predominantly of equal depth.

4 Table 1 Summary of cores collected from each site. Relative wave exposure (Duce et al., 2016), slope angle of the fore reef and approximate spur dimensions are given along with site water depth in meters below MSL. For cores collected into the spur wall the distance up the wall from the groove bottom is given in parenthesis. Asterisks denote cores where the depth down bore was not measured and, thus, percent recovery could not be calculated. For those two cores the depth down bore values are the lengths of material recovered. Site (Lat. Long.) “class” from Duce et al. (2016)

Relative wave exposure (Slope
) Length (L) Width (W) Height (H)

Core ID

Environment drilled

Site depth (distance up wall from groove bottom, m)

Depth down bore (cm)

Percent recovery (%)

Heron

H-NW Northwestern fore reef (“Blue Pools”) (23.4345 S, 151.9235 E) “SaP” HeN Northern fore reef (“Libby’s Lair”) (23.4351 S, 151.9453 E) “LaP” H-SW Southwestern fore reef (“Harry’s Bommie”) (23.4565 S, 151.9262 E) “LaP”

Low (8) L e 60 m W e 15 m H e 3 m

BP1 BP2 BPSW1

Spur top (inner) Spur top (mid) Spur wall (mid)

1.06 1.46 6.76 (1.30)

12.5 49 57

40 100 91

Low (2) L e 50 m W e 8 m H e 2 m

OT-E Eastern fore reef (23.5074 S, 152.0981 E) “EWE” OT-SE Southeastern fore reef (23.5125 S, 152.0900 E) “SaP” OT-N Northern fore reef (“Long Bank”) (23.4874 S, 152.0664 E) “LaP”

High (2) L e 145 m W e 13 m H e 2.5 m

LL1 LL2 LLSW1 LLSW2 LL4 HBSW1 HB2 HB3 HB4 HB5 HBSW2 ES1 ES2 ESW1

Spur Spur Spur Spur Spur Spur Spur Spur Spur Spur Spur Spur Spur Spur

top (mid) top (outer) wall (outer) wall (outer) top (outer) wall (mid) top (outer) top (inner) top (inner) top (inner) wall (mid) top (inner) top (mid) wall (mid)

1.66 1.26 2.36 2.86 1.76 3.46 2.01 1.36 0.86 0.86 3.26 2.82 4.82 6.52

70 20 13 32 54 17 40.5 15 26.5 28.5 29 10.1 18 18.5

100 85 100 99 100 65 86 63 75 81 100 100 100 95

SS1 SSW1 Sp_1 Sp_2 LB_SaG_SC_1 LBS1 LBS2 LBSW1 LBSW1a LBS3 LBS4 LBSW2

Spur top (outer) Spur wall (outer) Spur top (inner) Spur top (mid) Reef edge Spur top (inner) Spur top (inner) Spur wall (inner) Spur wall (inner) Spur top (outer) Spur top (outer) Spur wall (outer)

1.72 2.92 1.4 1.72 ~0 1.12 0.92 2.37 2.37 1.97 1.72 2.87

19 38 18 11 27 52 18 3 11.5 4.5 23.2* 20.5*

100 93 100 91 100 84 97 100 100 78 ? ?

One Tree

Moderate/Low (4.5) L e 53 m W e 13 m H e 4 m

Moderate/High (5) L e 36 m W e 8 m H e 2 m Low/Moderate (3) L e 120 m W e 5 m H e 1.5 m

(1.10) (0.4) (0.7)

(0.9)

(0.3)

(0.6)

(50 cm) (44 cm)

(110 cm)

S. Duce et al. / Quaternary Science Reviews 231 (2020) 106155

Reef

S. Duce et al. / Quaternary Science Reviews 231 (2020) 106155

4. Results 4.1. SaG composition and chronology Ten coral genera were identified in the 29 short cores (Fig. 4a). The most common genus was Acropora, present in 52% of cores. Cores from Heron Reef contained more rubble (present in 50% of cores) than One Tree Reef (13%). Robust coral genera including Isopora and Porites, as well as degraded unidentified corals, were more abundant in cores from One Tree than Heron reef. No clear trends in bio-lithofacies were apparent between cores from the inner, outer or wall locations at each site. Figs. 5 and 6 show planview and cross-sectional views of core transects with down core bio-lithofacies and dates for two representative sites (OT-E and HNW). Figures and dates for all sites are provided in Figs. S5eS8 and described in detail in the supplementary materials. Calibrated 14C and UeTh dates (Tables 2 and 3) show that near-surface ages of SaG features ranged from >6400 cal yr BP to modern times. The oldest spurs from all sites were at the exposed eastern fore reef of One Tree (OT-E) with a surface age of 6412 cal yr BP at the outer end of the spur (ES2) (Fig. 5). The other SaG cores at One Tree Reef all showed surface ages <3000 cal yr BP (Tables 2 and 3; Figs. S5 and S6) except the inner-most spur core at the moderate/high relative wave exposure southeastern site (OT-SE), which had a surface age of 5151 cal yr BP (Fig. S5). At Heron Reef SaGs on the northern side (H-NW and HeN) were older than those on the southwestern margin (H-SW). At H-NW the age of the top of the spur on the “secondary rim” (BP2 ¼ 4304 cal yr BP) was within error of the outermost age of the reef flat dated by Webb et al. (2016) at 4156 cal yr BP (their Fig. 7). At H-NW the short

5

section of core at the edge of the reef flat (BP1) returned a modern age (50 cal yr BP) and is most likely representative of the thin veneer of modern coral that can be seen growing at the site today (Figs. 6 and S1a). In-situ dates from cores at HeN both returned upper ages around 4000 cal yr BP (LL1 ¼ 4041 and LL4 ¼ 4028 cal yr BP) (Fig. S7). The oldest surface age at H-SW was 2561 cal yr BP at the inner-most spur top (HB5) and the youngest (HB2: 846 cal yr BP) was from the seaward end of the more “consolidated” spur (Fig. S8). Seaward of HB2 the spur continued but was predominately composed of un-filled Acropora thicket with very high live coral cover, which was impossible to drill (Fig. S1l). This suggests the spur is actively prograding and will continue to consolidate as it grows closer to sea-level. Cores taken horizontally into the spur walls at each site mostly returned relatively modern outer ages (Fig. 4b). Only three of the seven wall cores returned surface ages greater than 1000 cal yr BP and these cores were taken from less than 50 cm up the wall (as measured from the floor of the groove). 4.2. SaG accretion rates Rates of vertical accretion range from 0.26 m/ka at high energy OT-E to 6.12 m/ka at relatively protected OT-N (Table 4). By combining the surface ages from the SaGs with previously published surface ages (see Table 5 for references) from the Heron and One Tree reef flats we were able to calculate rates of lateral accretion at both fore reef and reef flat sites. The lateral accretion rates of spurs were variable between reefs and at different locations on the same reef ranging from 10 to 141 m/ka in a seaward direction and as much as 392 m/ka in a lagoonward direction (Table 5, Figs. 7 and 8).

Fig. 2. Principle sedimentary facies identified in the cores (aed) and scanning electron microscope images of sample vetting (eeh). a) framestone and bindstone, b) rudstone, c) grainstone, and d) rubble. Labels show an area of oxidation (Ox.), branching Acropora colony (Acr.), vermetid gastropod (V), encrusting foraminifera (EF), crustose coralline algae (CCA), and Acropora branch rubble (Acr. Br). e) Aragonite cements (Arg. C) infilling a coral pore and tiny micro-borer holes (Mc. B). f) large grained peloidal, high magnesium calcite cement infilling a coral pore. g) a high magnesium calcite tube surrounded. by aragonite cement. h) an example of well-preserved coral wall with fibrous microstructure suitable for dating.

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Table 2 AMS radiocarbon age results from Heron and One Tree SaG cores. The material dated and growth position are also indicated (in-situ, NI ¼ not in-situ and NEI ¼ not enough information). Samples shaded in grey are duplicates. Lab ID

Material

Growth position

Reef (Site ID)

Sample ID

d13C (o/oo)

OZT340 OZT341 OZT342 OZU154 OZU155 OZT343 OZU156 OZT344 OZT345 OZU162 OZT346 OZT348 OZT349 OZU173 OZT350 OZT351 OZT352 OZU157 OZU170 OZU158 OZU159 OZU160 OZU161 OZU163 OZU164 OZU165 OZU166 OZU167 OZU168 OZU169 OZU171 OZU172 OZU177 OZU178 OZU179 OZU180 OZU181

Acropora Platygyra Montastrea Montastrea Montipora Favia CCA CCA Acropora Acropora Goniopora Platygyra Acropora Acropora Leptoria Isopora CCA Favites Favites Acropora Acropora Acropora Acropora Isopora Acropora CCA Goniopora Favid Isopora CCA CCA Goniopora Isopora Isopora Isopora Isopora CCA

in-situ in-situ in-situ in-situ in-situ in-situ

Heron (H-NW) Heron (H-NW) Heron (H-NW) Heron (H-NW) Heron (H-NW) Heron (H-NW) Heron (H-NW) Heron (H-SW) Heron (H-SW) Heron (H-SW) Heron (H-SW) Heron (HeN) Heron (HeN) Heron (HeN) Heron (HeN) Heron (HeN) Heron (HeN) One Tree (OT-E) One Tree (OT-E) One Tree (OT-E) One Tree (OT-E) One Tree (OT-E) One Tree (OT-E) One Tree (OT-N) One Tree (OT-N) One Tree (OT-N) One Tree (OT-N) One Tree (OT-N) One Tree (OT-N) One Tree (OT-N) One Tree (OT-N) One Tree (OT-N) One Tree (OT-SE) One Tree (OT-SE) One Tree (OT-SE) One Tree (OT-SE) One Tree (OT-SE)

BP1.1 BP2.1 BP_SW1_1.2 BP_SW1_1.2 BP_SW1_1.5 BP_SW1.6 BP_SW1_1.8 HB2.3 HB4.1 HB5.5 HBSW2.1 LL1.1 LL1.4 LL2.1 LL4.1 LLSW2.1 LLSW2.4 ES1a ES1b ES2.2 ES2a ES2b ESW1.1Ac_lo LB_SaG_SC1.1 LB_SaG_SC1.3 LB_SW1.4 LB_SW2.1 LB_SW2.3 LBS2 LBS4.4 LBS4.5 LBS5 SP1.1a SP1.1b SS1.2 SSW1.1 SSW1.4

1.2 0.4 2.1 0.6 0.0 0.9 1.3 2.9 0.6 2.4 5.6 2.0 2.1 0.8 1.4 1.4 3.0 0.6 1.9 0.4 1.7 1.6 0.3 0.0 0.6 2.1 1.1 1.2 0.8 2.4 1.5 4.1 0.0 3.2 1.3 0.2 3.5

in-situ NI in-situ in-situ in-situ NEI in-situ NEI in-situ in-situ in-situ in-situ in-situ NEI in-situ NEI NEI NEI in-situ in-situ NEI in-situ in-situ in-situ in-situ in-situ NEI NEI

14

pMC

C Age (yr. BP)

Mean

1s

Mean

108.32 59.18 112.83 112.87 57.41 56.74 65.53 84.97 71.50 70.64 93.61 60.55 62.06 66.26 60.63 61.82 70.06 79.36 80.21 47.15 78.66 78.22 45.28 73.48 73.76 81.19 92.72 91.61 77.27 92.07 91.87 77.96 54.63 54.70 81.03 96.10 80.17

0.42 0.24 0.43 0.27 0.19 0.20 0.20 0.31 0.27 0.21 0.27 0.23 0.19 0.20 0.16 0.17 0.27 0.24 0.23 0.17 0.22 0.23 0.17 0.21 0.22 0.22 0.28 0.25 0.23 0.24 0.25 0.22 0.19 0.19 0.27 0.28 0.25

Modern 4215 Modern Modern 4460 4550 3395 1310 2695 2790 530 4030 3835 3305 4020 3865 2860 1855 1770 6040 1930 1975 6365 2475 2445 1675 605 705 2070 665 680 2000 4855 4845 1690 320 1775

1s 35

30 30 30 30 35 25 25 30 25 25 25 25 30 25 25 30 25 25 30 25 25 25 25 25 25 25 25 25 30 30 30 25 25

Cal ages (cal yr BP) 2s range 16 4445 21 21 4801 4854 3381 948 2612 2680 263 4206 3915 3296 4176 3961 2737 1511 1418 6726 1592 1662 7140 2281 2255 1322 371 449 1772 437 444 1686 5290 5283 1342 102 1428

Median 57 4134 40 40 4497 4580 3115 730 2275 2362 0 3891 3638 2989 3877 3681 2454 1295 1220 6230 1352 1395 6566 1993 1950 1122 91 265 1522 149 243 1422 4987 4974 1135 0 1225

50 4304 33 34 4641 4745 3257 846 2398 2531 155 4041 3780 3139 4028 3822 2617 1396 1314 6465 1472 1527 6838 2126 2083 1230 239 351 1638 313 330 1558 5151 5140 1243 38 1318

(a) (b) (a) (a) (b) (b) (b) (b) (b) (b) (b) (b) (b) (b) (b) (b) (b) (b) (b) (b) (b) (b) (b) (b) (b) (b) (b) (b) (b) (b) (b) (b) (b) (b) (b) (b) (b)

(a) - Calibration was performed using OxCal 4.2 program and combined bomb coral 14C data from Heron Reef., Abraham Reef and Holmes Reef (Dawson et al., 2014). (b) - Age calibration was performed using Marine13 data (Reimer et al., 2013) with OxCal program 4.2. Respective DR values of 4 ± 40 yr (Druffel and Griffin, 2004) and 2 ± 112 yr (Hua et al., 2015) for the period 0e5.4 cal kyr BP and 5.4e8 cal kyr BP were employed for age calibration.

Most of the SaG zones (five of the six studied) showed lateral accretion in a seaward direction, the exception being OT-E (transect O9, Fig. 8). Lateral accretion rates of the reef flats were predominantly in a seaward direction (six of nine sites) and varied between 16 and 281 m/ka. The three sites that showed lagoonward lateral reef flat accretion (between 42 and 174 m/ka) were on the relatively exposed eastern or southern reef flat at One Tree (Fig. 8). 5. Discussion 5.1. Bio-lithofacies The 29 short cores presented here represent the first subsurface material collected from SaGs in the GBR and provide new insight into the internal anatomy and composition of SaGs. The range of coral genera identified was similar between sites with Acropora, usually exhibiting robust branching morphology, the dominant genus overall (Fig. 4a). Similarly, spurs in Florida were found to be composed primarily of coalesced in-situ Acropora palmata colonies “oriented away from the sea” (Shinn, 1963). In the Yucatan Peninsula spurs also were dominated by Acropora palmata but largely in the form of unconsolidated cobble gravel (Blanchon et al., 2017b). Spurs in the Ryukyu Islands, Japan are

predominantly built by tabular Acropora (Kan et al., 1995, 1997b; Yamano et al., 2001) whereas a spur in Belize was found to be dominated by foliaceous Agaricia sp. (Shinn et al., 1982). Bio-lithofacies observed in SaG cores from different sites seem to reflect differences in the hydrodynamic energy regime in accordance with the trends identified by Montaggioni (2005) review of Indo-Pacific reef cores. Montaggioni (2005) recognized that reefs that are subject to permanent wave energy predominantly consist of in-situ interlocking framework corals. Accordingly, the cores at the highest energy sites in this study, OT-E and OT-SE, were dominated by framework corals (robust Acropora and Isopora) (Figs. 5 and S5). Reefs in the lowest energy settings tend to be predominantly detritus with only scattered in-situ corals (Montaggioni, 2005) as is the case at lower energy SaGs at Heron Reef (Fig. 6, S7 and S8) with 50% of cores containing unconsolidated rubble. In contrast <15% of cores from the higher energy environment of One Tree Reef contain rubble (Fig. 4a).

5.2. Vertical accretion rates Vertical accretion rates of the SaG cores range from 0.26 to 6.12 m/ka (Table 4). This is low in comparison to vertical accretion rates for framework-dominated reefs reported elsewhere in the

Table 3 UraniumeThorium isotopic data from Heron and One Tree fore reef cores. Material

Growth Reef

U (ppm)

BP_SW_1.5 HB5.2 HB5.5 LL1.2 LL1.4 ESW1.1P_up

3.1791 3.3477 2.7128 2.5734 2.8887 2.9005

232

Th (ppb)

(230Th/232Th) (230Th/238U)

(234U/238U)

337 ± 15 395 ± 2 46 ± 0 8671 ± 84 101 ± 1 9862 ± 85

1.1474 1.1462 1.1449 1.1477 1.1476 1.1432

position Montipora Acropora Acropora Platygyra Acropora Porites

in-situ in-situ NI in-situ in-situ in-situ

Acropora

in-situ

Acropora

in-situ

Acropora

in-situ

Isopora

in-situ

Acropora

NEI

Goniopora NEI Favid

in-situ

Isopora

in-situ

Goniopora in-situ

Heron (H-NW) Heron (H-SW) Heron (H-SW) Heron (HeN) Heron (HeN) One Tree (OTE) One Tree (OTE) One Tree (OTE) One Tree (OTE) One Tree (OTN) One Tree (OTN) One Tree (OTN) One Tree (OTN) One Tree (OTN) One Tree (OTN)

± ± ± ± ± ±

0.0019 0.0014 0.0013 0.0008 0.0015 0.0013

1.366 0.704 5.431 0.038 3.896 0.061

± ± ± ± ± ±

0.058 0.003 0.018 0 0.008 0.001

ESW1.1Ac_lo 4.0057 ± 0.0019 0.098 ± 0.001 8516 ± 52

0.04773 ± 0.0008 0.02735 ± 0.00014 0.0303 ± 0.00011 0.04194 ± 0.00012 0.04474 ± 0.00026 0.06873 ± 0.0003

± ± ± ± ± ±

0.0017 0.002 0.0014 0.0011 0.0022 0.0013

Uncorrected age (ka) 4.629 2.633 2.923 4.056 4.333 6.754

± ± ± ± ± ±

0.079 0.014 0.012 0.013 0.027 0.032

Corrected age (ka) 4.618 2.627 2.871 4.056 4.298 6.753

± ± ± ± ± ±

0.08 0.015 0.028 0.013 0.032 0.032

Corrected age (cal yr BP) 4552 2561 2805 3990 4232 6687

± ± ± ± ± ±

80 15 28 13 32 32

Corrected initial (234U/238U) 1.1494 1.1473 1.1462 1.1494 1.1494 1.1459

± ± ± ± ± ±

0.0017 0.002 0.0014 0.0011 0.0022 0.0014

0.06856 ± 0.00026 1.1457 ± 0.0014 6.721 ± 0.027

6.72 ± 0.027

6654 ± 27

1.1484 ± 0.0014

6.478 ± 0.021

6412 ± 21

1.1468 ± 0.0013

6.929 ± 0.029

6.898 ± 0.033

6832 ± 33

1.1449 ± 0.0017

LB_SaG_SC1.1 3.8983 ± 0.0014 0.474 ± 0.002 547 ± 5

0.02191 ± 0.00016 1.1495 ± 0.0015 2.097 ± 0.015

2.094 ± 0.016

2028 ± 16

1.1504 ± 0.0015

LB_SaG_SC1.3 3.544 ± 0.001

0.02268 ± 0.00011 1.1476 ± 0.001

2.176 ± 0.011

2.167 ± 0.012

2101 ± 12

1.1485 ± 0.001

ES2.2

4.2761 ± 0.0015 0.177 ± 0.001 4858 ± 22

0.06607 ± 0.0002

ES2.3

3.03 ± 0.0015

0.07039 ± 0.00027 1.142 ± 0.0016

3.613 ± 0.013 179 ± 1

1.178 ± 0.002 207 ± 1

1.1441 ± 0.0013 6.479 ± 0.021

LBSW2.1

5.2441 ± 0.0025 0.03 ± 0.001

1904 ± 55

0.00358 ± 0.00005 1.1467 ± 0.0013 0.341 ± 0.005

0.341 ± 0.005

275 ± 5

1.1469 ± 0.0013

LBSW2.3

2.4482 ± 0.0012 0.014 ± 0

2290 ± 55

0.00417 ± 0.00004 1.1453 ± 0.0021 0.398 ± 0.003

0.398 ± 0.003

332 ± 3

1.1454 ± 0.0021

LBS2

3.193 ± 0.0012

0.01766 ± 0.0001

1.69 ± 0.01

1624 ± 10

1.1475 ± 0.0014

LBS5

3.0815 ± 0.0007 0.256 ± 0.001 663 ± 4

1.739 ± 0.009

1673 ± 9

1.149 ± 0.0011

0.213 ± 0.001 802 ± 6

1.1468 ± 0.0014 1.692 ± 0.01

0.01819 ± 0.00009 1.1483 ± 0.0011 1.741 ± 0.009

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Sample ID

7

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Fig. 3. Schematic diagram of SaGs defining the orientation axes of vertical, lateral, and wall accretion. Note that lateral accretion can be either seaward or lagoonward.

Indo-Pacific which range from 1 to 30 m/ka with a modal rate of 6e7 m/ka, or detritus-dominated facies, which range from 0.2 to 40 m/ka (Montaggioni, 2005). However, the only vertical accretion rates for spurs reported in the literature are similar to our results with 1.6 m/ka at Looe Key Reef, Florida (Shinn et al., 1981) and between 1.2 and 3.2 m/ka (mean 2.0 m/ka) at various reefs in the Ryukyus, Japan (Kan and Hori, 1993; Kan et al., 1997b; Yamano et al., 2001). Fore reef percussion cores from turbid inner shelf fringing reefs (without SaGs) in the central GBR show vertical accretion rates between 1.1 m/ka (Ryan et al., 2016) and 35.2 m/ka (Roff et al., 2015) and 1.46 and 9.88 m/ka in Western Australia (Collins et al., 2003; Twiggs and Collins, 2010), while fringing reefs in Hawaii have fore reef vertical accretion rates ranging between 0.17 and 23 m/ka (Grigg, 1998; Engels et al., 2004). Slow vertical accretion in fore reef environments has been attributed to wave energy causing breakage and abrasion of living corals (Grigg, 1998) and the dominance of slower growing robust coral morphotypes (Montaggioni, 2005). Accordingly, the cores from the highest energy site in this study show the slowest rates of vertical accretion (OT-E ¼ 0.26 m/ka), while the less exposed sites have higher rates (e.g. HeN ¼ 1.74 and OT-N ¼ 6.12 m/ka). Additionally, our cores are short (<70 cm) and thus capture only the uppermost layer of spurtop growth. This layer is closest to present, stable sea level, thus restricted vertical accommodation may also be slowing vertical accretion (e.g., Kan et al., 1995; Cabioch et al., 1999; Gischler, 2008; Woodroffe and Webster, 2014). Spur vertical accretion rates may have been greater earlier in the Holocene when there was greater vertical accommodation; however, longer cores are needed to determine this. 5.3. Wall accretion rates A lateral wall accretion rate could be calculated only at OT- SE (0.13 m/ka). To our knowledge no other cores have ever been recovered from spur walls, therefore no comparisons can be made. However, Kan et al. (1997a) reported no evidence of lateral expansion parallel to the long axis of a spur at Miyako Island, southern Japan and it has been speculated that sediment movement through the grooves scours the spur wall and inhibits live coral growth (e.g. Shinn, 1963; Hopley, 1982; Kan et al., 1995). Indeed, the importance of erosional processes on spur walls is demonstrated by the relatively smooth walls of spurs and the presence of highly abraded, rounded boulders and rubble pieces in grooves classified as “Exposed to Wave Energy” by Duce et al. (2016) (e.g. Fig. S2c top of frame). Clasts in these “Exposed to

Wave Energy” grooves would have undergone transport processes to produce their characteristic rounded shape. Extreme movement of groove sediment during storm events also has been reported (e.g., Hubbard et al., 1991, Blanchon and Jones, 1997; Kan, 1995). All spur wall samples older than 1000 cal yr BP in this study were located less than 50 cm from the base of the wall suggesting that coral growth is prohibited near the groove floor by sediment accumulation, movement and/or that active scouring and abrasion is causing erosion of the wall. Nevertheless, the majority of spur wall cores collected (4 of 7) had outer ages younger than 1000 cal yr BP (Fig. 4b). Spur walls, particularly in lower energy environments, commonly were covered by luxuriant coral growth (e.g. Figs. S1 and S2). Hence, lateral wall accretion is occurring under present conditions. It is possible that lower light availability farther down on the spur wall or beneath an overhang plays a role in prohibiting coral growth. However, this is unlikely as all cores were recovered from less than 10 m depth in clear waters where low light would likely favour different species, morphologies or cause slower growth rates rather than an absence of live coral (Anthony and Hoegh-Guldberg, 2003). 5.4. Spur lateral accretion rates Lateral accretion rates of spurs were variable between reefs and at different locations on the same reef ranging from 10 to 141 m/ka in a seaward direction and as much as 392 m/ka in a lagoonward direction (Table 5). These rates are within the range of previously published reef flat lateral accretion rates, which vary from 17 to 333 m/ka (Yamano et al., 2003). At most sites in this study spur top age was negatively correlated with distance seaward of the reef crest (i.e., the top of the spur gets younger further seaward of the reef crest) suggesting most of the spurs are accreting laterally in a seaward direction (Fig. 9). Only two other studies have dated material in a transect along the top of a spur allowing comparison of lateral accretion rates and direction. In Florida, Shinn et al. (1981) showed a lagoonward lateral accretion rate of 14 m/ka from the middle of a spur to the inner end, and a seaward lateral accretion from the middle to outer end at 13 m/ka. Kan et al. (1997a) found a similar growth pattern at Miyako Island, southern Japan, with lagoonward lateral accretion of 8 m/ka and seaward lateral accretion of 11 m/ka. Both sites suggest a spur forming from a patch reef and accreting in both seaward and lagoonward directions. None of our sites exhibited this mode of lateral accretion although the rates of lateral seaward accretion at H-SW (17 m/ka) and OT-N (24 m/ka) (Table 5) are similar to those reported by Shinn et al. (1981), and

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Fig. 4. a) Frequency percentage of cores from each reef that contained each of the bio-lithofacies. b) Surface ages of spur wall samples, plotted against the distance up the wall from the groove floor. The dotted line shows the height above which it appears coral growth dominates over erosion processes based on the presence of modern wall ages/live coral. Note, error bars are typically smaller than the symbols.

Kan et al. (1997a). 5.5. Relationship between reef flat and SaG accretion patterns Reef flats in this study showed lagoonward lateral accretion at OT-E and OT-SE, the most exposed sites (Fig. 9 a and b), and seaward lateral accretion at the remaining sites (Fig. 9 c). A metaanalysis of reef flat cores by Dechnik et al. (2016) found 80% of exposed mid-outer platform reef flats in the GBR showed lagoonward lateral accretion while protected mid-outer platforms exhibited seaward lateral accretion. They concluded that hydrodynamic energy is the dominant driver of lateral reef flat accretion direction. However, their dataset contains no cores from the fore reef zone. The SaG cores from different sites presented here showed a mix of lagoonward and seaward lateral accretion of the fore reef

and the adjacent reef flat (refer to trend lines in Fig. 9 a-c). Hence, our data show for the first time that fore reefs do not necessarily accrete in the same direction as the adjacent reef flat. Their direction of accretion also appears to be influenced by hydrodynamic energy with “Exposed to Wave Energy” SaGs accreting lagoonward while those classified as “Short and Protected” or “Long and Protected” accreted seaward. Kennedy and Woodroffe (2002) noted that fringing reefs, which may appear similar in surface morphology, can have differing and complex modes of threedimensional growth. The same may be true of SaGs. Based on the SaG cores presented here and existing reef flat cores from Heron and One Tree reefs we identified three modes of combined fore reef and reef flat growth (Fig. 9).

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Fig. 5. a) Overview of the site on the eastern fore reef of One Tree Reef (OT-E). b) Aerial image showing a plan view of core locations. Ages of the top material dated are labelled in white boxes with an asterisk denoting dates from material which may not be in-situ. c) Cross-sectional view of core locations and logs with calibrated 14C ages shown in black and UeTh ages shown underlined in blue. All ages are presented with 2s uncertainties shown in cal yr BP. Note that ES1 and ES2 are from different spurs. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 6. a) Overview of the site on the northwestern fore reef of Heron Reef (H-NW). b) Satellite image showing a plan view of core locations. Blue points are cores into the top of the spur, red arrows show cores into the spur wall and grey squares are long cores on the reef flat presented by Webb et al. (2016). Calibrated ages of the top material dated (or xintercept ages for the long cores) are labelled in white boxes. c) Cross-sectional view of core locations and logs with calibrated 14C ages shown in black and corrected UeTh ages shown underlined in blue. All ages are presented with 2s uncertainties shown in cal yr BP. Grey cores (WH1-5) are from Webb et al. (2016) and the x-intercept ages at the tops of these cores are shown in red. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

5.5.1. Mode 1: lagoonward lateral accretion of the fore reef and reef flat Mode 1 is represented by the exposed eastern side of One Tree Reef (OT-E, “Exposed to Wave Energy” SaGs (Duce et al., 2016)). Cores suggest spurs at OT-E reached their present elevation (~ -5 m MSL) by ~6400 cal yr BP and the adjacent reef flat reached its present elevation (0.35 m MSL) by ~6000 cal yr BP (Dechnik et al., 2016) giving a lagoonward lateral accretion rate from mid spur (ES2) to reef crest of 392 m/ka (Transect O9, Fig. 8c and Table 5). The eastern reef flat also was found to have accreted lagoonward at a rate of 42 m/ka (Dechnik et al., 2016). We note that the relatively young age (1314 cal yr BP, Fig. 5) from the inner spur top (ES1) is not entirely consistent with this interpretation (it suggests lagoonward lateral accretion of the SaG zone, but at a slower rate, ~8 m/ka).

However, given the similarity in surface ages of the mid spur top and spur wall (ES2 ¼ 6412 and ESW1 ¼ 6687 cal yr BP respectively), it is likely that ES1 is superficial modern growth atop an older feature. Harris et al. (2015b) dated reef flat micro-atolls and sediment cores from lagoonal sand aprons at One Tree Reef and found evidence of a “turn-off” of reef flat carbonate production and sand apron accretion around 2000 cal yr BP, which they attributed to a relative sea-level fall of 1e1.3 m. Dechnik et al. (2017) suggested that the hiatus in reef flat growth began earlier at ~3900 cal yr BP, continued until ~1500 cal yr BP, and was likely driven by a relative fall in sea level of ~0.5 m based on short cores from the reef flats of One Tree and Heron reefs. Either way, the reef flat and fore reef at OT-E had apparently “turned off” earlier than the rest of the reef

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Fig. 7. a), c) and d) show surface ages in cal yr BP from published studies of Heron Reef (Dechnik et al., 2016; Webb et al., 2016) in black and surface ages from the fore reef obtained by this study underlined in red. Asterisks indicate dates from material that may not be in-situ. Arrows labelled H1eH5 show the core transects for which lateral accretion rates were calculated and correspond to Table 5 b) shows calculated rates and inferred direction of lateral accretion for all transects. Dashed arrows show rates for the fore reef SaG zone and full arrows show rates for the reef flat. White arrows and text show rates calculated in this study between two dates only while black arrows and text show the mean lateral accretion rates calculated on x-intercepts of lines of best fit for the top-most in-situ ages published by Dechnik et al. (2016). Relative wave exposure from Duce et al. (2016) is shown in b). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

during the sea-level highstand. At ~6500 cal yr BP, when vertical accretion of the spur appears to have ceased, it would have been ~5.5e6.3 m below mean sea level. This early “turn-off”, preceded by a slow vertical accretion rate (0.26 m/ka), despite accommodation

being present, may suggest that vertical growth was limited by hydrodynamic energy. This phenomenon has been reported for high energy reef flats in Hawaii, which exhibited virtually no accretion to depths of 12 m owing to high wave energy (Grigg, 1998;

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Table 4 Vertical accretion rates calculated from cores in the fore reef SaG zone at Heron and One Tree reefs. Reef

Environment

Facies

Core ID

Depth at top of core (MSL)

Upper age Lower age

Distance between ages (cm)

Vertical accretion rate (m/ka)

HeN

Spur top (mid)

In-situ domal coral and in-situ branching coral

LL1

1.66 m

42

1.74

H-SW

Reef edge

In-situ robust branching coral and rubble

HB5

0.86 m

17

0.70

OT-E

Spur top (mid)

In-situ robust branching corals

ES2

4.82 m

11

0.26

OT-N

Reef edge

In-situ robust branching coral and rubble

LB_SaG_SC1

0

16

2.19

OT-N

Spur top (inner)

In-situ robust branching coral and domal coral

LB_S1

1.12 m

30

6.12

OT-N

Vertically into ridge on spur wall (outer)

NEI domal corals

LB_SW2

2.87 m

4056 4298 2627 2871 6478 6898 2028 2101 1624 1673 341 398

8

1.40

Table 5 Lateral accretion rates calculated at Heron and One Tree reef flat and fore reef zones based on ages from this and other studies. The core transects over which accretion rates were calculated are displayed in Figs. 7 and 8. Negative lateral accretion rates (underlined) imply lagoonward accretion while positive rates imply seaward accretion. Accretion rates for spurs are in bold, all others are reef flat lateral accretion rates. Asterisks indicate average accretion rates published by Dechnik et al. (2016) or Webb et al. (2016) and calculated based on all available surface ages. Reef

Transect Relative wave exposure

Heron

H1 H2 H3 H4 H5 One O1 Tree O2 O3 O4 O5 O6 O7 O8 O9 O10

Low

Slope (
)

Cores source

Distance (m)

Inner age (cal yr BP)

Outer age (cal yr BP)

Lateral accretion rate (m/ ka)

<1

Webb et al. (2016) Dechnik et al. (2016) Dechnik et al. (2016) This study: HB4 to HB2 Salas-Saavedra et al. (2018) This study: LBS1 to LBS4 This study: LB_SaG_SC1 to LBS1 Marshall and Davies (1982) to LB_SaG_SC1 Dechnik et al. (2016) Dechnik et al. (2016) Dechnik et al. (2016) Dechnik et al. (2016) This study SP1 to SS1 Dechnik et al. (2016) to ES2 Dechnik et al. (2016)

20 101 50 29 48 32 57 625

5120 5200 5070 2584 3235 1624 2094 4250

4100 4050 1080 846 1837 313 1624 2094

20* 68* 16* 17 34 24 141 281

206 99 302 90 40 134 122

6750 6180 1250 4780 5151 6070 5420

5860 6750 6150 6230 1243 6412 6070

231 174 55* 86* 10 ¡392 42*

Low/Moderate

Low/Moderate

Moderate Moderate/High High

4.5 <1 3 2 <1

<1 <1 5 2 <1

Grossman and Fletcher, 2004). OT-E already may have filled much of its accommodation at the higher sea-level and when relative sealevel fell the higher hydrodynamic energy may have limited further growth and likely caused erosion of the surface. Indeed, Davies (1977) postulated that under the present sea-level and hydrodynamic regime the windward fore reef of One Tree Reef would be incapable of producing sufficient calcium carbonate to equal the amount likely to be transported away. Thus, Davies (1977) suggested that SaGs on the windward sides of One Tree Reef and other exposed reefs in the Capricorn-Bunker groups represent the “original front of an eroding reef” or may be inherited from solution channels in the antecedent substrate (Davies, 1977). Our cores at OT-E are consistent with this hypothesis, but modern live coral cover at the site (Fig. S2) shows that accretion is still occurring and suggests an equilibrium between erosion and accretion currently exists. A number of authors have documented Indo-Pacific reefs where SaG features are hypothesized to be predominantly erosional and “cut” into the reef slope (e.g., Cloud, 1954; Munk and Sargent, 1954; Cloud, 1959; Battistini et al., 1975). Gischler (2010) suggested that such erosion could be due to the transgressive-regressive nature of Holocene sea-level in the Indo-Pacific. This study is the first report of sub-surface material from SaGs that is consistent with that hypothesis.

5.5.2. Mode 2: lagoonward lateral accretion of the reef flat and seaward lateral accretion of the fore reef Mode 2 occurs at the moderately exposed southeastern side of One Tree Reef (OT-SE) where SaGs were classified as “Short and Protected” (Duce et al., 2016). Similar to OT-E the reef flat at OT-SE reached MSL by ~6200 cal yr BP and showed lagoonward lateral accretion at a rate of 86 m/ka (Dechnik et al., 2016) (Transect O7, Fig. 8c). However, unlike OT-E the outer spur at OT-SE (SS1, Fig. S5) did not reach its present elevation (1.72 m MSL) until relatively recently (1243 cal yr BP) resulting in a seaward lateral accretion rate of 10 m/ka (Transect O8, Table 5 and Fig. 8c). The possibility that this core penetrated superficial modern growth (as believed to be the case at ES1) cannot be ruled out. However, modern ages of the spur wall at the site (SSW1: 38 and 1318 cal yr BP) provide evidence that this entire feature is relatively young (Fig. S5). Dechnik et al. (2017) found that after the hiatus beginning around 3900 cal yr BP, reef growth re-initiated on the inner and outer edges of the Heron and One Tree reef flats around 1500 cal yr BP. They speculated that continuous progradation was possibly limited to outer reef margin environments or to protected valleys or lows in the original Pleistocene platform based on the core transect of SalasSaavedra et al. (2018). Our fore reef dates support this hypothesis at OT-SE. A seaward shift in the area of active reef growth also was documented in the fringing reefs of the Ryukyu Islands, Japan

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Fig. 8. a), b), d) and e) show surface ages in cal yr BP from published studies of One Tree Reef (Dechnik et al., 2016; Harris et al., 2015b; Marshall and Davies, 1982) in black and surface ages from the fore reef obtained by this study underlined in red. Asterisks indicate dates from material that may not be in-situ. Arrows labelled O1eO10 show the core transects for which lateral accretion rates were calculated and correspond to Table 5 c) shows calculated rates and inferred direction of lateral accretion for all transects. Dashed arrows show rates for the fore reef SaG zone and full arrows show rates for the reef flat. White arrows and text show rates calculated in this study between two dates only while black arrows and text show the mean lateral accretion rates calculated on x-intercepts of lines of best fit for the top-most in-situ ages published by Dechnik et al. (2016). Relative wave exposure from Duce et al. (2016) is shown in c). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

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Fig. 9. a-c) show upper-most ages plotted against distance lagoonward () or seaward (þ) from the reef crest for sites showing each mode of accretion. Linear trends between cores at each site are plotted. The colour of the symbols denote different sites. Symbol shape denotes the source of the age data with circles for this study, squares for Dechnik et al. (2016), triangles for Webb et al. (2016) and diamonds for Salas-Saavedra et al. (2018). Open symbols show ages we argue are from superficial modern material (ES1 at OT-E (a) and BP1 at HNW (c)). Alongside each scatter plot schematic diagrams show the three apparent modes of lateral accretion at the reef flat (blue arrows) and SaG zones (black arrows) of the study sites. The relative importance of erosion, accretion and relative wave energy is shown. Sites with question marks are those for which data are in-conclusive. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

where the reef crest reached sea level about 5500 cal yr BP causing back reef accumulation to slow/stop (Kan, 2007) and active reef growth and expansion of the fore reef SaG zone then occurred (Kan and Hori, 1993). A similar seaward shift in the area of active reef growth also was documented at Kikai Island, Japan driven by episodic tectonic uplift events causing relative sea level fall (Webster et al., 1998).

5.5.3. Mode 3: seaward lateral accretion of both the reef flat and the fore reef Mode 3 occurs on the northern side of One Tree Reef (OT-N) and southern side of Heron Reef (H-SW). Both sites are exposed to moderate to low wave energy and were classified as having “Long and Protected” SaGs (Duce et al., 2016). The reef flat at OT-N likely reached its present elevation at a similar time to the southern and eastern flats (~6000 cal yr BP) (Dechnik et al., 2016) (Fig. 8a). However, fewer reliable dates make the pattern of lateral accretion difficult to determine (Dechnik et al., 2016). Our short core towards the seaward edge of the reef flat returned a surface age of 2094 cal yr BP while the adjacent inner spur reached its present elevation (~-1 m MSL) at 1690 cal yr BP giving the reef flat in this area a seaward lateral accretion rate of 141 m/ka (Transect O2, Table 5, Fig. 8c). The spur appears to be continuing its seaward lateral accretion at a rate of 24 m/ka from 1689 to 313 cal yr BP (Transect O1).

At the southwestern side of Heron Reef (H-SW) reef flat core transects have been dated either side of the SaG cores presented here. Approximately 1.3 km west of the SaG cores the reef flat was found to have accreted seaward at a rate of 16 m/ka from 5070 cal yr BP until 1080 cal yr BP (Dechnik et al., 2016; Transect H3, Fig. 7b) while 260 m east of the SaG cores the reef flat had a seaward accretion rate of 34 m/ka between 3235 cal yr BP and 1837 cal yr BP (Salas-Saavedra et al., 2018) (Transect H5, Fig. 7b). We found the surface age at the inner edge of the SaG zone was 2561 cal yr BP suggesting that this part of the reef flat reached its present extent between 724 and 1481 years earlier than the reef flat to the east and west. This shows considerable variation in the timing when the reef flat reached its present extent over a relatively small area and is in keeping with the evidence presented by Salas-Saavedra et al. (2018) that large variations exist in the underlying Pleistocene topography beneath this smooth modern margin. Spurs at H-SW prograded seaward with a lateral accretion rate of 17 m/ka and may provide the mechanism by which the seaward accretion of the reef flat occurred (Transect H4, Fig. 7b; refer to sections 5.6 and 5.7 below, for further discussion). Examples of reef margins fitting mode 3 also have been described in the Ryukyu Islands (Kan and Hori, 1993; Kan et al., 1997b; Yamano et al., 2001, 2003). Kennedy and Woodroffe (2002) described a similar model of fringing reef development (their Fig. 15 “model B”) showing seaward lateral accretion of a fringing reef with little or no remaining vertical accommodation.

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We tentatively ascribe H-NW and HeN on the northern protected side of Heron to mode 3 although the behavior of these margins is complex and difficult to interpret from available data (Figs. 7 and 9). In contrast to the southwestern side of Heron Reef, the northern reef flat prograded seaward at rates between 20 and 68 m/ka until lateral accretion ended ~4000 cal yr BP (Dechnik et al., 2016; Webb et al., 2016). Fore reef spurs on this side of the reef (H-NW and HeN) also stopped significant vertical and lateral accretion around the same time (Figs. 6 and S7). Dechnik et al. (2016), and Webb et al. (2016) suggested that the halt in reef flat lateral accretion may have been due to the steepness of the fore reef slope. However, analysis of Laser Airborne Depth Sounder data (resolution 25 m) shows the slope of the fore reef at HeN is 2
. This is gentler than most of the SaGs in this study, which exhibited seaward lateral accretion. However, the slope of the fore reef at HNW is relatively steep (~8
) and thus may be a factor in limiting the available substrate for recruitment and requiring considerably more vertical accretion for any lateral accretion to occur. Webb et al. (2016) found that reef flat cores at H-NW had high rubble and sand content perhaps implying lower resistance to wave energy and that progradation may be hampered by destruction during episodic high energy events. We suggest that low wave energy (providing less optimal coral growth conditions, slower rates of cementation and higher susceptibility to destruction in high energy events), as well as an abundance of surrounding sediment (limiting hard substrate available for coral settlement) may be the main causes of the apparent reef growth hiatus at these sites. Kan and Hori (1993) found that lateral accretion of SaGs halted ~2500 cal yr BP at leeward Minna Reef in Japan. They suggested that it may relate to the accumulation of sandy sediments on the reef slope or climatic change. Blanchon et al. (2017a) also hypothesized that high sediment flux inhibiting coral recruitment was responsible for the halt

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in growth of an incipient reef crest in north east Yucatan, Caribbean. The geometry at H-NW is further complicated as these SaGs occur on a “secondary rim” feature (Fig. 6). Similar ages on the secondary rim spur and the outer reef flat suggest the features grew simultaneously. Only one other secondary rim feature has previously been cored, at Gable Reef in the southcentral GBR (Hopley et al., 2007). There, the secondary rim was found to have developed atop a detrital spit and reached sea level around 6500 yr BP, approximately 1000 years before the inner reef rim (Hopley et al., 2007). A longer core through the secondary rim at H-NW is required to determine the substrate on which this secondary rim developed and to verify whether the initial implication of our analysis, i.e., that it grew simultaneously with the reef flat, is correct. 5.6. Possible mechanisms of SaG formation and evolution Based on the geomorphic, chronostratigraphic and hydrodynamic evidence we discuss several possible mechanisms of SaG formation and evolution for the three modes, and identify the key environmental drivers (Fig. 10). We consider that these modes and mechanisms likely represent a continuum, with erosion and accretion processes of varying degrees of importance to each mode. 5.6.1. Mode 1 Mode 1 SaGs are likely to form on gently sloping fore reefs (~2
) continuously exposed to relatively high wave energy (e.g., OT-E and other “Exposed to Wave Energy” SaGs (Duce et al., 2016)). We speculate that coral growth on the Pleistocene reef basement began sometime before 8000 cal yr BP (Dechnik et al., 2015). The spurs then continued to grow upwards and lagoonward with accretion

Fig. 10. Conceptual plan view and cross-sectional diagrams showing the possible evolution and formation mechanisms of the three modes of SaG development identified in this study. The typical characteristics of the physical environment in which they are likely to be found are also presented.

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determined by accommodation and wave energy, slowing as they got closer to sea level. Infilling and cementation of the coral framework would be accelerated by hydrodynamic pumping (Storlazzi et al., 2003) causing the features to be more consolidated than those that formed under lower energy conditions (mode 3). The rhythmic spacing of the mode 1 grooves could then be explained by a process analogous to swash rip current formation (Dalrymple et al., 2011) such that onrushing wave energy in the swash zone splits and preferentially flows back at a point of lower topographic relief (the groove) thus inducing more turbulence and scouring within the groove (Werner and Fink, 1993) (Fig. 10). We suggest two possible formation and evolution scenarios consistent with the core data recovered at OT-E. Under scenario 1a) following the relative sea level fall, by ~2000 cal yr BP at the latest (Harris et al., 2015b; Dechnik et al., 2017), fore reef areas that were previously too deep, would come under the direct effect of waves, thereby increasing hydrodynamic energy adequately to cause erosion of the fore reef, particularly “cutting” into the grooves. Although it would require extreme force to cut into well consolidated reef framework, Newell et al. (1951) speculated that grooves at Andros Island in the Bahamas were likely incised by erosion during high energy storm events. Formation scenario 1b) presumes that grooves existed in the antecedent substrate and when Holocene reefs began to grow, vertical accretion within the grooves would have been considerably suppressed by sediment build-up and abrasion while the spurs would provide enhanced growth conditions for corals (Fig. 10). The antecedent SaG morphology would be enhanced by this growth differential. Rogers et al. (2015) measured higher alongshore flows and turbulence on the spur than the groove and suggested that this would increase the nutrient uptake of corals on the spur and prevent sediment build-up; thus enhancing coral growth on the spur in comparison to the groove. In the absence of longer cores and/or seismic investigation we cannot determine whether the antecedent substrate on which the modern mode 1 SaGs formed was already characterized by rhythmically spaced grooves formed by karstification during sea level low-stand as proposed by Newell (1954), and Purdy (1974). However, it seems implausible that such grooves would be as closely spaced as the grooves at OT-E today, which have a mean spacing of 21 m (Duce et al., 2016) because such a gentle gradient slope (2
) would require far fewer channels to drain the surface water (Orange et al., 1994). Given the presence of SaGs in the fossil record dating back to the Ordovician (Barnes, 1965) it is probable that the Last Interglacial reef at this site would have been characterized by SaGs and the Last Interglacial grooves would have been enhanced by karst erosion during Last Glacial Maximum exposure providing a template for the modern SaG features. Dechnik (2016) calculated that approximately 1 m of erosion due to karstification was likely to have occurred at the mid-outer platform reefs of the GBR during sub-aerial exposure in the Last Glacial Maximum. Thus, scenario 1b seems more likely than 1a. 5.6.2. Mode 2 Mode 2 SaGs are likely to form on relatively steep fore reef slopes (5
) subject to moderate or episodic wave energy (Fig. 10). These spurs are better consolidated than those in lower energy conditions (mode 3) suggesting that wave pumping enhances the infilling and cementation of the substrate, similar to “Exposed to Wave Energy” mode 1 SaGs. In the Florida Keys, cores penetrating to Pleistocene limestone at adjacent spurs and grooves showed a 4e5 m thick layer of carbonate sand between the Pleistocene basement and the SaG system demonstrating that antecedent topography did not play a role in SaG formation (Shinn, 1963). Similar conclusions were reached at Carrie Bow Cay in Belize, although the basement substrate was not penetrated (Shinn et al.,

1982). We suggest that, similar to the spurs in Florida and Belize, mode 2 spurs probably develop atop sandy deposits at the edge of the fore reef rather than directly on Pleistocene limestone. Material eroded from the SaG zone may be transported onto the reef flat, particularly during high energy events, facilitating lagoonward lateral accretion of the reef flat. However, some material must also build up at the outer ends of these spurs, allowing them to creep seaward over these talus substrates (as long as the fore reef is not too steep) as suggested by Blanchon and Jones, 1997 at Grand Cayman Island. Storlazzi et al. (2003) suggested that spurs at a shallow, high energy Hawaiian fringing reef may be constructed by the binding of coral rubble. It is likely that steepness of the fore reef slope limits continued seaward accretion of these features as was the case at China Reef in Japan (Kan et al., 1995). This is particularly likely at OT-SE where the fore reef drops sharply to ~ 40 m MSL just seaward of the current SaG zone (Davies, 1977; Davies and Marshall, 1979). It is possible that wave energy drives rhythmic, rip-like water currents to open and maintain the rhythmic spacing of the grooves. In addition, at meso-tidal reefs, which have relatively high tidal ranges and are ponded over low tide, it is possible that during the ebbing tides water draining from the lagoon over the reef crest could produce grooves as the water channelizes into the paths of least resistance (e.g., at OT-SE). This tidal influence may be particularly important for SaG formation in meso-tidal environments and sets them apart from SaGs in microtidal areas such as the Caribbean. 5.6.3. Mode 3 Mode 3 SaGs form in moderate wave energy and moderate gradients (1e5
) and the examples presented here all fit within the “Long and Protected” class of SaG (Duce et al., 2016). We suggest the cross-sectional profile of these spurs is similar to mode 2 and not controlled by SaG features in the antecedent topography. Indeed, at Heron Reef Salas-Saavedra et al. (2018) showed large variations in the underlying Pleistocene topography near SaG cores H-SW. Mode 3 SaGs are associated with seaward accreting reef flats. We suggest this may be because wave energy is not sufficient, and/or the elevation of the reef flat is too high (disconnecting the fore reef and reef flat during lower tidal phases) for waves to transport debris lagoonward of the reef crest and facilitate lagoonward lateral accretion. In the Ryukyu Islands SaG formations have been found to facilitate seaward lateral accretion of reef flats by trapping detritus in the inner ends of the grooves during storms and allowing the reef flat to build outwards over them (Kan and Hori, 1993; Kan et al., 1997b; Yamano et al., 2001). Our spur wall cores from higher than 50 cm above the bed all showed relatively modern ages, which supports the hypothesis by Roberts et al. (1975) that spurs grow wider and eventually coalesce with neighboring spurs. This is another mechanism by which the reef flat can expand seaward facilitated by spur development. A recent study also found SaG features in the Florida Keys had been filled with sediment leading to net accretion in seafloor elevation (Yates et al., 2016). Detrital deposits could also accumulate at the ends of spurs allowing them to extend further seaward. Live coral growth at the spur ends could be shaped along the long axis by wave energy while coral growth in the groove is deterred by sediment as found to be the case at spurs in the Florida Keys (Shinn, 1963) and supported by our findings of spur wall ages (Fig. 4b). As suggested in mode 2, above, ebbing tidal flows off the reef platform may contribute to groove formation. This mechanism could be particularly important on the leeward side of meso/macrotidal reefs, which are less influenced by waves and become disconnected from the reef flat at low tidal phases (e.g., OT-N). Another possible mechanism of SaG initiation is suggested

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Fig. 11. Proposed “proto-spur” features at northern One Tree Reef. a) two “proto-spurs” covered in soft corals with some massive corals and rippled carbonate sand in the groove. b) view along the top of a “proto-spur” with soft corals and tabular Acropora. c) close-up of the encrusted, largely unconsolidated carbonate detritus which these features seem to be composed of. d) an example of rhythmically spaced rubble horns of beach cusps. White scale bars in each image represent approximate 1 m.

based on visual inspection of outer SaGs at OT-N (Fig. S6). Single beam bathymetry and SCUBA inspection revealed water depth in the grooves to be 12 m below mean sea level and spurs as high as 4 m. The spurs were primarily composed of unconsolidated rubble/ unfilled branching coral framework colonised by soft corals, encrusting coralline algae and some live plate, encrusting and massive corals (Fig. 11); and therefore could not be cored. No change to the 2D morphology (i.e. length and width) of these submerged rubble ridges/spurs can be detected between aerial imagery from 1978 to 2013. We propose that these ridges of rubble may represent “proto-spurs” potentially deposited by a storm or cyclone event prior to 1978. A modern example of a potentially analogous process is shown in Fig. 11d with rhythmically spaced rubble ridges deposited on a beach fronted by a fringing reef. These features are, however, sub-aerially, exposed, whereas proto-spurs would remain submerged on the reef front. Once deposited these rubble ridges, which are well below wave base under modal wave conditions and thus would not experience re-working, would undergo cementation and consolidation by coralline red algae and corals and infilling by debris and eventually grow upwards until reaching sea level. At reefs in southern Japan that experience frequent typhoons, overlapping spurs with different orientations have been reported (pers. comm. Kan, 2016). These could potentially represent “protospurs” formed by typhoon events that approached from differing directions. In the Yucatan, Blanchon et al. (2017b) presented evidence of hurricane waves producing linear ridges of coral rubble that developed into spurs and grooves. This suggests that occasional high-energy events are critical to the initiation of at least some SaG features, particularly on relatively low energy, leeward reef fronts. Equally, there have been reports of storms completely

removing SaG features (not just breaking and removing corals from the spurs) from fore reefs in Belize (Stoddart, 1962) and Barbados (Lewis, 2002). This suggests that these SaG features must have been quite poorly consolidated and probably not rooted directly on antecedent limestone substrate. They may be examples of “protospurs” that had not yet been adequately consolidated to withstand the hydrodynamic conditions of a powerful storm. 5.7. Implications for reef platform growth and evolution The traditional model that platform reefs in the GBR developed through juvenile, mature and senile phases via progressive windward to leeward lateral accretion of the reef flat and infilling of the lagoon (Hopley, 1982; Hopley et al., 2007) has recently been questioned. Dechnik et al. (2016) found that wave energy likely determines whether a reef flat accretes laterally in a seaward or lagoonward direction and Harris et al. (2015b) suggested that sand aprons in the southern GBR no longer appear to be accreting under present sea-level conditions. SaGs from modes 2 and 3 provide potential mechanisms for seaward lateral accretion of reef flats. Harris et al. (2015b) suggested that no significant accretion of the sand apron or reef flat has occurred at One Tree Reef over the past 2000 years; in contrast, our SaG cores show lateral and vertical accretion of the fore reef has continued over this interval. This may be because reef crests and flats have reached their maximum elevation and hydrodynamic conditions at present are incapable of transporting sufficient sediment from their seaward growth fronts to continue leeward reef flat expansion and lagoon filling. This suggests that under present sea level conditions and energy regimes lagoonal, mature reefs, like One Tree Reef, may not fill their lagoons to eventually develop into senile platform reefs. Rather,

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where slope and substrate permit, the reef flats may continue to expand seaward on their leeward, and semi-exposed fronts to increase in size while maintaining their present lagoons (assuming, probably optimistically, that changes in climatic and environmental conditions do not severely hinder reef growth into the future). Longer cores penetrating the antecedent substrate are required to assess this. Regardless, the relationship between SaGs and reef flats is more complex and site specific than previously thought.

Sadler: Methodology, Investigation, Writing - review & editing. Gregory E. Webb: Resources, Writing - review & editing, Funding acquisition. Luke Nothdurft: Resources, Investigation, Writing review & editing. Marcos Salas-Saavedra: Investigation, Writing review & editing. Ana Vila-Concejo: Conceptualization, Methodology, Writing - review & editing, Supervision, Funding acquisition.

6. Conclusion

Radiocarbon dating was funded by an Australian Institute of Nuclear Science and Engineering (AINSE) Postgraduate Award (ALNSTU11922) and the University of Sydney, Australia (SD). Scanning Electron Microscope time at the Australian Centre for Microscopy and Microanalysis and the Queensland University of Technology Central Analytical Research Facility, CT scanning and UeTh dating were funded by Australian Research Council Discovery Project (DP120101793). Field work was supported by a One Tree Island Research Station Student Fellowship, Australia (SD), an Australian Coral Reef Society Danielle Simmons Student Research Award (SD), a Heron Island Research Scholarship, Australia (JS) an Australian Research Council Future Fellowship (AVC, FT100100215) and return to work grant for Women in Science at the University of Sydney, Australia (AVC). We sincerely thank staff at Heron Island and One Tree Island Research Stations. The help of field volunteers Russell Graham, Kent Holmes, Bevan Yiu also is appreciated greatly. Thanks to Matthew Kosnik, Jennie Mallela, Stewart Fallon and Andrew Wilson for advice regarding coring equipment and techniques.

This is the first study to examine multiple (29) SaG short cores from two reefs and different environments of the same reef. Our sites at Heron and One Tree reefs in the southern GBR, represent reefs with differing hydrodynamic regimes and differing broad scale reef slope morphology. We show that spurs are primarily composed of Acropora corals as well as crustose coralline algae making bindstones and rudstones. The bio-lithofacies tend to reflect differences in hydrodynamic energy. Spurs show comparatively low rates of vertical accretion (0.26 and 2.19 m/ka), which likely reflects the relationship between wave energy, accommodation and slope. Most spurs show active coral growth and lateral accretion of the spur wall, although this is inhibited within 50 cm of the groove floor most likely by sediment accumulation and scouring. Lateral accretion rates of spurs are variable between reefs and at different locations on the same reef ranging from 10 to 141 m/ka in a seaward direction and as much as 392 m/ka in a lagoonward direction. Combining our SaG cores with a comprehensive existing suite of reef flat cores revealed that fore reef lateral accretion does not invariably occur in the same direction as the lateral accretion of the adjacent reef flat. We identified three modes of reef flat and SaG development characterized by: 1) leeward accretion of both the fore reef SaGs and reef flat; 2) leeward accretion of the reef flat and seaward accretion of the fore reef SaGs; 3) seaward accretion of both the reef flat and fore reef facilitated by SaG growth. Of the six study sites, evidence of mode 1 was found at “Exposed to Wave Energy” SaGs at one site (OT-E); evidence of mode 2 was found at “Short and Protected” SaGs at one site (OT-SE) and the remaining four sites, characterized by “Long and Protected” SaGs, exhibited mode 3. We conclude that hydrodynamic energy, particularly the occurrence of high energy events, and fore reef slope angle are the primary environmental factors controlling SaG formation and development. We suggest antecedent topography, may be important, particularly at mode 1 SaGs, but is unlikely to be the main factor driving SaG morphology. Our findings suggest that under present sea level conditions lagoonal, mature reefs may not fill their lagoons and eventually develop into senile platform reefs. Rather, if environmental conditions allow it, reef flats may continue to expand seaward on their leeward and semi-exposed fronts to increase in size while maintaining their lagoon. Additional cores of greater length are required to test the SaG formation mechanisms proposed for each mode and to better understand the implications of fore reef development to the evolution of reef platforms. CRediT authorship contribution statement Stephanie Duce: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft, Visualization, Project administration, Funding acquisition. Belinda Dechnik: Methodology, Formal analysis, Investigation, Writing - review & editing. Jody M. Webster: Conceptualization, Methodology, Writing - review & editing, Supervision, Funding acquisition. Quan Hua: Formal analysis, Investigation, Writing - review & editing. James

Acknowledgements

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