Browsing by Author "Webster, JM"

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    Deglacial mesophotic reef demise on the Great Barrier Reef
    (Elsevier Science BV, 2013-12-15) Abbey, E; Webster, JM; Braga, JC; Jacobsen, GE; Thorogood, GJ; Thomas, AL; Camoin, G; Reimer, PJ; Potts, DC
    Submerged reefs are important recorders of palaeo-environments and sea-level change, and provide a substrate for modem mesophotic (deep-water, light-dependent) coral communities. Mesophotic reefs are rarely, if ever, described from the fossil record and nothing is known of their long-term record on Great Barrier Reef (GBR). Sedimentological and palaeo-ecological analyses coupled with 67 C-14 AMS and U-Th radiometric dates from dredged coral, algae and btyozoan specimens, recovered from depths of 45 to 130 m, reveal two distinct generations of fossil mesophotic coral community development on the submerged shelf edge reefs of the GBR. They occurred from 13 to 10 ka and 8 ka to present. We identified eleven sedimentary fades representing both autochthonous (in situ) and allochthonous (detrital) genesis, and their palaeo-environmental settings have been interpreted based on their sedimentological characteristics, biological assemblages, and the distribution of similar modern biota within the dredges. Facies on the shelf edge represent deep sedimentary environments, primarily forereef slope and open platform settings in palaeo-water depths of 45-95 m. Two coral-algal assemblages and one non-coral encruster assemblage were identified: 1) Massive and tabular corals including Porites, Montipora and faviids associated with Lithophylloids and minor Mastophoroids, 2) platy and encrusting corals including Porites, Montipora and Pachyseris associated with melobesioids and Sporolithon, and 3) Melobesiods and Sporolithon with acervulinids (foraminifera) and bryozoans. Based on their modem occurrence on the GBR and Coral Sea and modem specimens collected in dredges, these are interpreted as representing palaeo-water depths of <60 m, <80-100 m and >100 m respectively. The first mesophotic generation developed at modern depths of 85-130 m from 13 to 10.2 ka and exhibit a deepening succession of <60 to >100 m palaeo-water depth through time. The second generation developed at depths of 45-70 m on the shelf edge from 7.8 ka to present and exhibit stable environmental conditions through time. The apparent hiatus that interrupted the mesophotic coral communities coincided with the timing of modem reef initiation on the GBR as well as a wide-spread flux of siliciclastic sediments from the shelf to the basin. For the first time we have observed the response of mesophotic reef communities to millennial scale environmental perturbations, within the context of global sea-level rise and environmental changes. © 2013, Elsevier Ltd.
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    Development of an inshore fringing coral reef using textural, compositional and stratigraphic data from Magnetic Island, Great Barrier Reef, Australia
    (Elsevier, 2012-03-01) Lewis, SE; Wüst, RAJ; Webster, JM; Shields, GA; Renema, W; Lough, JM; Jacobsen, GE
    The stratigraphy of fringing coral reef environments and platforms provides unique insights into reef development and evolution. This study used twelve sediment cores from three transects across a reef flat in Nelly Bay, Magnetic Island (NE Australia), to examine its development over the Holocene. The maximum thickness of the Holocene reefal material based on seismic and bore log data was around 5 m and comparable to other fringing reefs in the Great Barrier Reef (GBR). Six different sedimentary facies were identified in the cores and two coincided with reef accretion: the lower muddy sand with coral rubble and the upper siliciclastic sand with coral rubble. Radiocarbon and U-series dates show that the Nelly Bay fringing reef initiated around 6300 yr BP on a gently sloping, unconsolidated or weakly cemented Pleistocene alluvial sedimentary facies. The ages of four massive coral heads (range between 5790 and 6290 calibrated C-14 yr BP) were almost contemporaneous with reefal initiation indicating that conditions were favourable for reef growth around this time. Vertical accretion rates for the Nelly Bay fringing reef were, on average about 0.5 mm/yr over the last 6000 yr, although rates were as high as 5 mm/yr during initiation. Average lateral accretion rates varied from 98 to 120 mm/yr, which are comparable to rates of other fringing reefs in the region. The age structure of the lower muddy sedimentary facies was consistent with the classic seaward prograding model of fringing reef development. In contrast, the upper siliciclastic sand with coral rubble facies was much younger in age (< 1100 yr BP) than the models and appears to represent an erosional unconformity between the upper and lower reefal facies possibly linked to late Holocene sea-level fall. This younger facies extends laterally across the reef flat and has no obvious prograding symmetry. Our integrated approach, using multi-proxy analyses of sediment cores from the Nelly Bay reef flat, highlights the complex development of this reef which include changes in coral composition and rubble preservation, reef evolution in response to sea-level change and changes in reef accretion rates. (C) 2012 Elsevier Ltd.
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    Development of the Great Barrier Reef from the last glacial maximum to present
    (Australasian Quaternary Association Inc., 2022-12-06) Husdell, M; Dunbar, GB; Bostock, HC; Webster, JM; Hua, Q
    The Great Barrier Reef (GBR) is located on the north-eastern continental margin of the Australian continent and is the largest tropical coral reef system in the world. Reefs are known to form on the continental shelf during interglacial sea level highstands and are exposed and die during glacial sea level lowstands. However, sonar and drilling studies have shown a series of backstepping fossilreefs migrated across the shelf edge during sea level transgression prior to the onset of the modern GBR at approximately 9 ka. While cores drilled through reefs provide direct evidence of reef growth and composition at a fixed point, such cores are often temporally discontinuous. By contrast, distal marine sediment cores can provide a more continuous time series of the development of both reefs and sedimentary processes along the margin. This study focussed on the analysis of piston cores (FR4/92-11 and FR4/92-12) collected from the Queensland Trough. The cores were subsampled at regular intervals for elemental (XRF) and mineralogical (XRD) analyses, and planktic foraminifera (Globigerinoides ruber) for oxygen isotope measurements and radiocarbon dating. A principal component analysis revealed four phases (‘clusters’) of development of the GBR margin from the Last Glacial Maximum to present. These represent 1) primarily hemipelagic carbonate sediments deposited during the glacial sea level lowstand, 2) mid-deglaciation sedimentation dominated by High Mg Calcite as sea level transgression and slope fossil-reefs initiated, 3) late deglaciation (11.8-8 ka) sedimentation dominated by a pronounced spike in terrigenous accumulation as sea level transgression reached the shelf break and 4) reef sedimentation dominated by Sr-rich aragonite in the early to late Holocene. Changes in carbonate mineralogy likely reflect differences in the terrigenous sediment flux and reef assemblages (dominance of corals versus crustose coralline algae and microbialite) between the fossil-reefs and the modern reef system.
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    Evolution of the Great Barrier Reef Halimeda carbonate factory in response to Holocene environmental changes, determined from radiocarbon and nitrogen isotope measurements
    (Australian Nuclear Science and Technology Organisation, 2021-11-17) McNeil, M; Hua, Q; Erler, DV; Nothdurft, LD; Webster, JM; Moss, P
    The inter-reef Halimeda algal bioherms of the northern Great Barrier Reef (GBR) have accumulated up to 25 m of positive relief and up to four times greater volume of calcium carbonate than the nearby coral reefs during the Holocene. Covering > 6000 km² on the continental shelf, the Halimeda bioherms represent a significant contribution to the development of the northeast Australian neritic carbonate factory and sedimentary archive of post-glacial environmental changes. However, the geochronological record of initiation and development of the Halimeda bioherm carbonate factory throughout the Holocene was poorly constrained and based on very few radiocarbon dates, considering their vast area. Additionally, the main source of nitrogen (N) fuelling the productivity of these biogenic structures had not been traced geochemically, and there was no understanding of any temporal variation. A total of 63 samples of calcium carbonate Halimeda and foraminifera grains, mollusc shells, and bulk soil were dated by ¹⁴ C AMS using the VEGA facility at the ANSTO Centre for Accelerator Science. Changes in the terrestrial vegetation community were determined from palynological analysis at the University of Queensland. Nitrogen isotopes were measured on a subset of 27 Halimeda samples using isotope ratio mass spectrometry (IRMS) at the Centre for Coastal Biogeochemistry at Southern Cross University. Using the persulfate-denitrifier method previously developed for foraminifera and coral skeletal material, nitrogen bound within the Halimeda skeletal organic material (SOM) was targeted. This allowed for fossil samples down-core to be analysed, thus providing a 5000-year record of Halimeda δ¹⁵ N-SOM. A temporal sequence of facies transitions that record terrestrial and marine environmental changes over a full sea level cycle from the Last Interglacial, to Last Glacial Maximum (LGM) and deglacial, to transgressive estuarine sediments and shallow marine carbonates was reconstructed. Halimeda growth and carbonate deposition had established by 11,143 (+237/-277) cal. yr BP, just ~450 years after the marine transgression. The Halimeda carbonate factory was productive at least 2100 years earlier than Holocene coral reefs in the study area. Our results provide the first direct in-situ measurements of Halimeda bioherm ages from at or near the Holocene/Pleistocene boundary, recording the timing of bioherm initiation in response to post-glacial sea level rise. The average Halimeda skeletal δ¹⁵ N-SOM was 6.28 ± 0.26 ‰, consistent with δ¹⁵ N-NO₃ - from western tropical South Pacific (WTSP) thermocline waters, geochemically validating shelf-break upwelling of an oceanic N source. From 5000 to 2000 cal. yr BP, Halimeda δ¹⁵ N-SOM decreased by 1-2 ‰, reaching a minima of 5.5 ‰ that persisted for almost 1000 years. The Halimeda δ¹⁵ N-SOM variation reflects changes in regional climate and intensified El Niño activity and is consistent with other regional climate proxy records at this time. Thus, the inter-reef Halimeda algal bioherms of the GBR are an important carbonate ecosystem that record a >10,000-year near-continuous record of northeast Australian Holocene oceanographic and environmental changes in response to sea level and climatic drivers, potentially filling spatial and temporal gaps not covered by coral and other proxies. © The Authors
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    Gloria Knolls Slide: a prominent submarine landslide complex on the Great Barrier Reef margin of north-eastern Australia
    (Elsevier B.V., 2017-03-01) Puga-Bernabéu, A; Beaman, RJ; Webster, JM; Thomas, AL; Jacobsen, GE
    We investigate the Gloria Knolls Slide (GKS) complex on the Great Barrier Reef margin of north-eastern Australia, the largest extant mixed carbonate-siliciclastic province in the world. Based on the most complete bathymetric and sub-bottom profile datasets available for the region, we describe the main surface and subsurface geomorphologic characteristics of this landslide complex. The GKS forms a 20 km along-slope and 8 km across-slope indentation in the margin, extending from 250 to 1350 m depth, and involves a volume of 32 km3 of sediment remobilized during three events. Three main seafloor terrains can be distinguished based on seafloor morphology: a source area, a proximal depositional area and a distal depositional area. The source area includes a main headwall scarp with a maximum height of 830 m and a secondary scarp at 670 m depth. The proximal depositional area is flat and smooth, and lacks debris exposed on the seafloor. The distal depositional area has a hummocky surface showing a distinctive cluster of eight knolls and over 70 small debris blocks. A dredge sample from the top of the largest knoll at a depth of 1170 m reveals the presence of a cold-water coral community. In the sub-bottom profiles, the mass-transport deposits in the GKS are identified below the background sediment drape as partially confined, wedge-shaped bodies of mostly weak amplitude, transparent reflectors in the proximal depositional area; and more discontinuous and chaotic in the distal depositional area. The failed sediment slabs of the GKS were evacuated, transported and disintegrated downslope in three events following a sequential failure process spreading successively from the lower slope to the upper slope. The first event initiated at the lower slope at the depth of the secondary scarp, moved downslope and disintegrated over the basin floor leaving coherent blocks. The subsequent second and third events were responsible for the formation upslope of the main scarp in the GKS. The timing of emplacement of the first GKS event, constrained by radiometric age of fossil biota from the surface of the largest slide block, was at least before 302 ± 19 ka. The presence of alternating mixed carbonate and siliciclastic lithologies that build the slope might have played an important role as a preconditioning factor in this region. Preliminary estimations suggest that unusually large seismic events were the most likely triggering mechanism for the GKS. This work contributes to the understanding of large mass-movement deposits in mixed carbonate-siliciclastic margins and provides a useful morphologic characterization and evolutionary model for assessing its tsunamigenic potential with further numerical simulations. In addition, the discovery of a cold-water coral community on top of the largest knoll has implications for identifying similar landslide-origin cold-water coral communities on the GBR margin. © 2016 Elsevier B.V.
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    The implications of a relative sea-level fall along eastern Australia between 1200 and 800 Years BP.
    (Asia Oceania Geosciences Society Brisbane, 2013-06-24) Lewis, SE; Wüst, RAJ; Webster, JM; Collins, J; Jacobsen, GE
    AOGS (Asia Oceania Geosciences Society) will hold their 10th Anniversary meeting for the first time in the Southern Hemisphere at the Brisbane Convention and Exhibition Centre from the 24th to 28th of June 2013. AOGS was established in 2003 to promote geosciences and its application for the benefit of humanity, specifically in Asia and Oceania and with an overarching approach to global issues. AOGS holds annual conventions providing a unique opportunity of exchanging scientific knowledge and discussion to address important geo-scientific issues among academia, research institution and public.Asia Oceania Geosciences Society (AOGS) was established in 2003 to promote geosciences and its application for the benefit of humanity, specifically in Asia and Oceania and with an overarching approach to global issues. Asia- Oceania region is particularly vulnerable to natural hazards, accounting for almost 80% human lives lost globally. AOGS is deeply involved in addressing hazard related issues through improving our understanding of the genesis of hazards through scientific, social and technical approaches. AOGS holds annual conventions providing a unique opportunity of exchanging scientific knowledge and discussion to address important geo-scientific issues among academia, research institution and public. Recognizing the need of global collaboration, AOGS has developed good co-operation with other international geo-science societies and unions such as the European Geosciences Union (EGU), American Geophysical Union (AGU), International Union of Geodesy and Geophysics (IUGG), Japan Geo-science Union (JpGU), and Science Council of Asia (SCA). © 2013, AOGS.
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    IODP Expedition 325 to the Great Barrier Reef: unlocking the history of reef growth and demise since the Last Glacial Maximum
    (Australian Geosciences Council, 2012-08-05) Webster, JM; Braga, JC; Humblet, M; Potts, DC; Iryu, Y; Hinestrosa, G; Bourillot, R; Seard, C; Camoin, G; Yokayama, Y; Thomas, AL; Thompson, B; Esat, TM; Fallon, SJ; Dutton, A
    Predicting how the Great Barrier Reef (GBR) will respond to future global climate changes and over what time frame is crucial. Fossil reefs record critical data on geomorphic and ecological consequences of both long-term and abrupt centennial-millennial scale environmental changes. The Integrated Ocean Drilling Program (IODP) Exp. 325 investigated a succession of submerged fossil reefs on the shelf edge of the GBR to establish the course of sea-level change, define sea-surface temperature variations but also analyse the impact of these environmental changes on reef growth since the Last Glacial Maximum (LGM). Thirty-four boreholes were cored from 17 sites along four transects at three locations (Hydrographers Passage, Noggin Pass and Ribbon Reef) in water depths between 42 to 167 m. These cores record responses of the GBR to past environmental stresses similar to current scenarios of future climate change (i.e. changing sea-levels, SST’s, water quality). Initial lithologic, biologic and chronologic data document an active coral reef system that grew, drowned and backstepped up-slope as sea level rose since the LGM. We present an overview of the main Exp. 325 results, including a synthesis of the dating, paleoclimate and reef response team’s findings, in the context of the available site survey data (bathymetry, seismic, seabed imagery). Finally, we discuss the broader implications of these data for understanding how the geometry, composition and development of the GBR responded to repeated and major environmental disturbances since the LGM.
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    Late holocene sea-level fall and turn-off of reef flat carbonate production: rethinking bucket fill and coral reef growth models
    (GeoScienceWorld, 2015-02-01) Harris, DL; Webster, JM; Vila-Concejo, A; Hua, Q; Yokoyama, Y; Reimer, PJ
    Relative sea-level rise has been a major factor driving the evolution of reef systems during the Holocene. Most models of reef evolution suggest that reefs preferentially grow vertically during rising sea level then laterally from windward to leeward, once the reef flat reaches sea level. Continuous lagoonal sedimentation (“bucket fill”) and sand apron progradation eventually lead to reef systems with totally filled lagoons. Lagoonal infilling of One Tree Reef (southern Great Barrier Reef) through sand apron accretion was examined in the context of late Holocene relative sea-level change. This analysis was conducted using sedimentological and digital terrain data supported by 50 radiocarbon ages from fossil microatolls, buried patch reefs, foraminifera and shells in sediment cores, and recalibrated previously published radiocarbon ages. This data set challenges the conceptual model of geologically continuous sediment infill during the Holocene through sand apron accretion. Rapid sand apron accretion occurred between 6000 and 3000 calibrated yr before present B.P. (cal. yr B.P.); followed by only small amounts of sedimentation between 3000 cal. yr B.P. and present, with no significant sand apron accretion in the past 2 k.y. This hiatus in sediment infill coincides with a sea-level fall of ∼1–1.3 m during the late Holocene (ca. 2000 cal. yr B.P.), which would have caused the turn-off of highly productive live coral growth on the reef flats currently dominated by less productive rubble and algal flats, resulting in a reduced sediment input to back-reef environments and the cessation in sand apron accretion. Given that relative sea-level variations of ∼1 m were common throughout the Holocene, we suggest that this mode of sand apron development and carbonate production is applicable to most reef systems. Copyright © 2020 Geological Society of America
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    Morphological variation, composition and age of submerged reefs on the Great Barrier Reef
    (Geological Society of Australia, 2010-07-04) Abbey, E; Webster, JM; Jacobsen, GE; Thomas, AL; Henderson, G; Reimer, PJ; Braga, JC; Tudhope, AW; Beaman, RJ; Bridge, T; George, N
    Coral reefs are powerful indicators of environmental changes, such as sea level, salinity and sea surface temperature fluctuations. Many sites within the Indo‐Pacific and the Caribbean have been investigated, yet the Great Barrier Reef (GBR) remains largely under‐represented in early deglacial, Late Pleistocene records. IODP Expedition 325 Site Survey (Proposal 519) returned with 4200 km2 of high‐resolution multibeam bathymetry of submerged reef features on the shelf edge, revealing extensive terraces, barrier reefs, lagoons, pinnacles and palaeo‐channels. Fossil coral reef specimens were collected from these features (in situ and loose) at depths ranging from 45–160 m and dated using 14C AMS and U‐Th. Preliminary results suggest the morphology of the features and the timing of drowning are influenced by a number of complex factors, possibly including, but not limited to variations in sea level, latitude, shelf width, local weather patterns and reef community composition.
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    Morphology and evolution of drowned carbonate terraces during the last two interglacial cycles, off Hilo, NE Hawaii
    (Elsevier, 2016-01-01) Puga-Bernabéu, Á; Webster, JM; Braga, JC; Clague, DA; Dutton, A; Eggins, SM; Fallon, SJ; Jacobsen, GE; Paduan, JB; Potts, DC
    The eastern side of Hawaii Island is a rapidly subsiding margin dominated by drowned carbonate platforms. We present detailed bathymetric and backscatter data, remotely operated vehicle and submersible observations, sedimentological and 14C accelerator mass spectrometry and U/Th age data from seven submerged terraces (H7, H2a–d, H1a–b) in water depths between 1100 and 25 m off Hilo, north-eastern Hawaii. The main carbonate deposits on these terraces are coral deposits, rhodolith beds, coralline algal mounds, crusts, pavements and tabular sheets. We identified five previously described sedimentary shallow- to deep-water facies and one new facies type that are consistent with reef drowning on a rapidly subsiding margin. We used palaeobathymetric data derived from the sedimentary facies, age versus depth relationships, and published sea-level curves, to estimate a uniform long-term subsidence rate of 2.80 ± 0.36 m/ky for the eastern side of Hawaii over the last 150 ky. Terrace H7 developed about 380 ka based on data from the western side of the island. Active coral growth on terrace H2d occurred during the Marine Isotope Stage (MIS) 6 to 5 transition, and the terrace drowned during the peak of MIS 5e when sea level rose faster than reefs could grow. Favoured by the gentle platform gradient, reefs established progressively landwards with a backstepping pattern during MIS 5e to form the terraces H2c and H2b 122 ka. Final turn-off of shallow water carbonate production on terraces H2b–d coincided with the relative sea-level rise of the interstadial MIS 5a. Bathymetry and submersible data suggest that carbonate sediments on terraces H2a and H1b were deposited over an antecedent topography of local lava deltas emplaced during rising sea levels at ca. 85 and 65 ka, while terrace H1a established on lava delta substrates of the Mauna Loa volcano ca. 11 ka. We conclude that the initiation, growth and drowning of coral-reef terraces off Hilo differ in some ways from the pattern observed in the submerged terraces in the western side of Hawaii and that the platform evolution off Hilo is more strongly influenced by emplacement of offshore lava flows. © 2015, Elsevier B.V
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    Rapid relative sea-level fall along north-eastern Australia between 1200 and 800 cal. yr BP: an appraisal of the oyster evidence
    (Elsevier, 2015-12-01) Lewis, SE; Wüst, RAJ; Webster, JM; Collins, J; Wright, SA; Jacobsen, GE
    A fast-paced post-glacial sea-level rise and subsequent mid-Holocene sea-level highstand are well documented at several far field locations away from the presence of former ice sheets but sea-level development during the late Holocene remains ambiguous. In this study, we present new data from modern and fossil oysters attached to shoreline rocks along the north-eastern Australian coastline that reveal new constraints on the nature and timing of relative sea-level change over the past 2500 yr. Surveyed elevations of various contemporary oyster zones contextualize modern oyster growth forms in relation to sea-level datum and build the reference for our fossil oyster data. Based on survey data and field observations we developed a robust set of criteria for measuring fossil oysters to determine their relative sea-level position and constrain the uncertainties associated with these reconstructions. Thick (> 10 cm) fossil oyster visors above the equivalent modern growth suggest higher relative sea-levels in the past (i.e. > 1200 cal. yr BP). Radiocarbon analyses of the modern oyster visors suggest continuous lateral accumulation over the past ~ 800 yr which implies relatively stable sea-level over this period. The modern and fossil dataset defines a distinct and rapid relative ~ 1 m sea-level fall between 1200 and 800 cal. yr BP. Whether the sea-level fall was stepped or followed a broader smooth/monotonic pattern is unclear. The timing coincides with the initiation of some inshore fringing coral reefs in the Great Barrier Reef region and other major geomorphological changes along the coastal zone. A combination of various factors may have been the driving mechanism behind this relative sea-level fall with rates between 1.0 to 5.2 mm yr− 1. © 2015, Elsevier B.V.
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    Sea level change over the past 2500 years from northeastern Australia
    (Australian Geosciences Council, 2012-08-05) Lewis, SE; Wust, RAJ; Webster, JM; Collins, J; Wright, SA; Jacobsen, GE
    This study C-14 dated living and fossil oyster bed deposits (>50 cm thick) to examine when sea level fell to its present position. These thick oyster bed deposits are formed in the inter-tidal zone and are relatively long-lived features with one bed dated in this study growing for ∼1000 years. Consistent growth rates at each sampling location indicated that oyster bed accumulation over time has been continuous, although growth rates varied markedly between locations from 0.30 mm/year to 6.4 mm/year suggesting the influence of localised environmental variability in water circulation, wave exposure and boulder lithology. We will show that a rapid sea-level fall occurred between 800 and 1200 years BP where sea level fell from ∼ + 1 m to present position with rates up to 7 mm/year but no less than 2.5 mm/year. The rapid fall supports the stepped sea-level model. This rapid fall influenced the movements of indigenous people as well as coincided with the development of inshore fringing coral reefs and geomorphological changes along the coastal zone.