Central Great Barrier Reef Shelf Research Articles

Impact of Typhoon Chan-hom on sediment dynamics and morphological changes on the East China Sea inner shelf

The East China Sea (ECS) inner shelf is highly impacted by tropical cyclones. Due to the limited availability of observations, the sediment dynamics and associated sedimentary process under the impact of typhoons are still poorly understood. In this study, a high-resolution unstructured-grid FVCOM model was employed to investigate the rapid responses of sediment dynamics on the ECS inner shelf to the passage of Typhoon Chan-hom in July 2015. The results indicated that there was an evident asymmetry in sediment dynamics around the typhoon track. During the typhoon, sediments were resuspended in nearshore area and transported downwind with increasing southwestward and seaward sediment flux. The gear-like sediment delivery presented a ratchet effect that induced an evident spatial asymmetry in responses of sediment dynamics and shelf sedimentation to the short-lived typhoon event. The coastal upwelling normally existing in summer was intermittently blocked by the passage of typhoon, and rapidly transited to downwelling, which substantially enhanced the cross-shelf sediment transport. As a result, severe erosions were mainly occurred near the typhoon track within 20-m isobaths, in contrast to newly formed sedimentary layers (~10 cm in thickness) in areas far south of the typhoon track and around the 50-m isobaths. The spatial asymmetry of sedimentary response to the typhoon passage is therefore of importance to locating sediment records that well preserved the typhoon signals. The ratchet effect we proposed provided reasonable interpretations of the spatially asymmetric responses of shelf sedimentations in the northern Gulf of Mexico and the central Great Barrier Reef shelf to tropical cyclones, respectively.

Internal structure and accretionary history of a nearshore, turbid-zone coral reef: Paluma Shoals, central Great Barrier Reef, Australia

Nearshore environments directly influenced by fluvial sediments and dominated by terrigenoclastic sedimentation are generally considered marginal for coral reef growth. There are, however, a growing number of examples from the modern and fossil record that clearly show that reef-building and terrigenous sedimentation are not mutually exclusive. Although a number of descriptive accounts of nearshore turbid-zone reefs have emerged in recent years, our understanding of the accretionary architecture and history of such reefs remains extremely limited. Here we detail the depositional facies and accretionary history of an inshore, turbid-zone coral reef complex, Paluma Shoals, on the central Great Barrier Reef Shelf of Australia. High turbidity (>40 NTU for over 30% of the time) and terrigenous sedimentation — both at levels close to the perceived limits for coral growth — are episodically important influences at Paluma Shoals. Despite these potentially limiting conditions Paluma Shoals exhibits high live coral cover (~ 50–60%, but locally up to 80%), with the intertidal reef flats compositionally dominated by G. aspera microatolls, and Galaxea sp. and Porites sp. colonies. Seventeen cores from 5 cross-reef transects reveal that the internal reef structure comprises an unconsolidated detrital coral rubble framework, 2.0–2.7 m thick, dominated by fragments of Acropora pulchra, T. frondens and M. mollis. These occur within a mixed terrigenoclastic–carbonate sediment matrix. Basal facies, in particular, contain a high (~ 60%) proportion of terrigenoclastic sediment. Radiocarbon dating indicates that the reef initiated above Pleistocene clays ~ 1600 cal yr BP. The present reef flat was emplaced within the last ~ 150–100 cal yr BP. Within the last ~ 60 years, areas of the North Shoal have become partially smothered as a result of intertidal sandflat progradation. The depositional sequence at Paluma Shoals shows; 1) that long-term (millennial timescale) reef accretion has occurred under conditions dominated by terrigenoclastic sediment accumulation; 2) that relatively rapid vertical reef growth and progradation can occur in these ‘marginal’ settings; and 3) that these reefs are susceptible to natural episodic mortality events associated with the remobilisation of intertidal and shallow subtidal sediments. This study provides important insights into coral reef development where environmental conditions are perceived to be marginal for hermatypic coral growth.

Long-shelf sediment transport and storm-bed formation by Cyclone Winifred, central Great Barrier Reef, Australia

Three major sediment facies belts occur on the Great Barrier Reef shelf: an inner shelf prism of Holocene terrigenous sand and mud, a patchy middle shelf veneer of palimpsest muddy shelly calcsand (< 1 m thick) above a weathered Pleistocene erosion surface, and an apron of modern carbonate mud and sand around reefs of the main reef tract. These sediments were mobilized and strongly affected by the passage of severe tropical Cyclone Winifred (central pressure 958 hPa), which traversed the central Great Barrier Reef shelf on February 1, 1986. Re-surveys after the passage of Cyclone Winifred showed that (i) a storm-induced shell lag and/or a normally graded bed of terrigenous fine sand-mud had been deposited at depths down to 20 m; (ii) middle shelf longitudinal bedforms were widespread at depths of 28–35 m, and comprised a furrowed substrate of palimpsest muddy shell gravel surmounted by 40–150 m wide ribbons of quartzose and bioclastic sand up to 15 cm thick; (iii) fields of northward-facing, 1–2 m wavelength megaripples were present adjacent to zones of ribbons, and in the alleys between ribbons; (iv) five days after the cyclone, mud was still settling from suspension, and large parts of the inner shelf were bathed in muddy hypopycnal river plumes, some of which reached 30 km seaward to the inner edge of the reef tract; and (v) suspended mud, derived in part by unmixing of the seabed, was present throughout the shelf water column, and a seaward-thinning mud drape up to 40 cm thick had accumulated on and seawards of the inshore sediment prism, tapering seawards to a few mm thick only over most of the middle shelf. It is inferred that storm-waves and currents associated with the passage of Cyclone Winifred caused widespread unmixing of bottom sediments. Offshore, long-shelf transport of bedload sand ribbons and megaripples was affected by powerful shelf-parallel currents of velocity 1–3 m/s. As the storm passed, the graded, seaward-thinning sand–mud bed was deposited over the inner-middle shelf. On the middle shelf, bioturbation and downward mixing of the mud drape started immediately after the passage of the cyclone, and was well advanced after 3 months. In this way, the ephemeral seabed features produced by passage of a cyclone are gradually degraded, and the storm-layer stratigraphy becomes incorporated within a thin middle shelf veneer of muddy shelly calcsand.

RECONSTRUCTING HOLOCENE SEA-LEVEL CHANGE FOR THE CENTRAL GREAT BARRIER REEF (AUSTRALIA) USING SUBTIDAL FORAMINIFERA

We assessed the utility of subtidal foraminifera to reconstruct Holocene relative sea levels from the central Great Barrier Reef shelf, Australia. We collected contemporary foraminiferal samples from Cleveland Bay and Bowling Green Bay, with water depths from 24.2 m to 248.0 m Australian Height Datum (AHD). The subtidal foraminiferal assemblages were divided into two distinct foraminiferal zones: an inner shelf zone occupied by Elphidium hispidulum, Pararotalia venusta, Planispirinella exigua, Quinqueloculina venusta and Triloculina oblonga; and a middle shelf zone dominated by Amphistegina lessonii, Dendritina striata and Operculina complanata. The zonations of the study areas and relative abundances of individual species indicate that the distributions of subtidal foraminifera are related to water depth. We used the subtidal data to develop a transfer function capable of inferring past water depths of sediment samples from their foraminiferal content. The results indicated a robust performance of the transfer function (r 2 jack 5 0.90). We produced ten sea-level index points, which revealed an upward trend of Holocene relative sea level from 28.86 6 4.5 m AHD at 9.3–8.6 cal kyr BP to a mid-Holocene high stand of +1.72 6 3.9 m AHD at 6.9–6.4 cal kyr BP. Sea level subsequently fell from the highstand to the present-day. The sea-level reconstructions are consistent with geophysical models and previous published data.

INTERTIDAL MANGROVE FORAMINIFERA FROM THE CENTRAL GREAT BARRIER REEF SHELF, AUSTRALIA: IMPLICATIONS FOR SEA-LEVEL RECONSTRUCTION

Research Article| July 01, 2005 INTERTIDAL MANGROVE FORAMINIFERA FROM THE CENTRAL GREAT BARRIER REEF SHELF, AUSTRALIA: IMPLICATIONS FOR SEA-LEVEL RECONSTRUCTION Sarah A. Woodroffe; Sarah A. Woodroffe 1Department of Geography, University of Durham, South Road, Durham, DH1 3LE, UK. E-mail: S.A.Woodroffe@durham.ac.uk Search for other works by this author on: GSW Google Scholar Ben P. Horton; Ben P. Horton 2Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, PA, 19104-6316, USA. Search for other works by this author on: GSW Google Scholar Piers Larcombe; Piers Larcombe 3Centre for Environment, Fisheries and Aquaculture Science, Pakefield Road, Lowestoft, Suffolk, NR33 0HT, UK. Search for other works by this author on: GSW Google Scholar John E. Whittaker John E. Whittaker 4Micropalaeontology Research, Department of Palaeontology, The Natural History Museum, London SW7 5BD, UK. Search for other works by this author on: GSW Google Scholar Journal of Foraminiferal Research (2005) 35 (3): 259–270. https://doi.org/10.2113/35.3.259 Article history received: 12 Aug 2003 accepted: 01 Dec 2004 first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Sarah A. Woodroffe, Ben P. Horton, Piers Larcombe, John E. Whittaker; INTERTIDAL MANGROVE FORAMINIFERA FROM THE CENTRAL GREAT BARRIER REEF SHELF, AUSTRALIA: IMPLICATIONS FOR SEA-LEVEL RECONSTRUCTION. Journal of Foraminiferal Research 2005;; 35 (3): 259–270. doi: https://doi.org/10.2113/35.3.259 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyJournal of Foraminiferal Research Search Advanced Search Abstract Contemporary foraminiferal samples and environmental information were collected from three fringing mangrove environments (Sandfly Creek Transect 1 and 2, and Cocoa Creek) in Cleveland Bay, and an estuarine mangrove environment (Saunders Creek) in Halifax Bay, on the central Great Barrier Reef (GBR) coastline, Australia, to elucidate the relationship of the foraminiferal assemblages with the environment. The data support the vertical zonation concept, which suggests that the distribution of foraminifera in the intertidal zone is usually a direct function of elevation, with the duration and frequency of subaerial exposure as the most important factor. An agglutinated foraminiferal assemblage dominated by Miliammina fusca, Trochammina inflata, Ammotium directum and Haplophragmoides sp. exists at the landward edge of the field sites, in a zone between just above Mean Low Water of Neap Tides to Highest Astronomical Tide level (a vertical range of 1.8 m). In addition, a foraminiferal assemblage dominated by Ammonia aoteana is found at all sites, existing between just below Mean Low Water of Neap Tides and Mean High Water of Neap Tides (a vertical range of 0.8 m). These assemblages may be used to reconstruct sea level from fossil cores from the area. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Patterns of mixed siliciclastic‐carbonate sedimentation adjacent to a large dry‐tropics river on the central Great Barrier Reef shelf, Australia

The Great Barrier Reef represents the largest modern example of a mixed siliciclastic‐carbonate system. The Burdekin River is the largest source of terrigenous sediment to the lagoon and is therefore an ideal location to investigate regional patterns of mixed sedimentation. Sediments become coarser grained and more poorly sorted away from the protection of eastern headlands, with mud accumulation focused in localised ‘hot spots‘ in the eastern portion of embayments protected from southeast trade winds. The middle shelf has a variable facies distribution but is dominated by coarse carbonate sand. North of Bowling Green Bay, modern coarse carbonate sand and relict quartzose sand occur. Shore‐normal compositional changes show Ca‐enrichment and Al‐dilution seawards towards the reef, and shore‐parallel trends show Al‐dilution westwards (across bays) along a Ca‐depleted mixing line. Intermediate siliciclastic‐carbonate sediment compositions occur on the middle shelf due to the abundance of relict terrigenous sand, a pattern that is less developed on the narrow northern Great Barrier Reef shelf. Rates of sediment deposition from seismic evidence and radiochemical tracers suggest that despite the magnitude of riverine input, 80–90% of the Burdekin‐derived sediment is effectively captured in Bowling Green Bay. Over millennial time‐scales, stratigraphic controls suggest that sediment is being preferentially accreted back to the coast.

The hydrodynamic and sedimentary setting of nearshore coral reefs, central Great Barrier Reef shelf, Australia: Paluma Shoals, a case study

The Great Barrier Reef (GBR) shelf contains a range of coral reefs on the highly turbid shallow inner shelf, where interaction occurs with terrigenous sediments. The modern hydrodynamic and sedimentation regimes at Paluma Shoals, a shore‐attached ‘turbid‐zone’ coral reef, and at Phillips Reef, a fringing reef located 20 km offshore, have been studied to document the mechanisms controlling turbidity. At each reef, waves, currents and near‐bed turbidity were measured for a period of ≈1 month. Bed sediments were sampled at 135 sites. On the inner shelf, muddy sands are widespread, with admixed terrigenous and carbonate gravel components close to the reefs and islands, except on their relatively sheltered SW side, where sandy silty clays occur. At Paluma Shoals, the coral assemblage is characteristic of inner‐shelf or sheltered habitats on the GBR shelf (dominated by Galaxea fascicularis, up to &gt;50% coral cover) and is broadly similar to that at Phillips Reef, further offshore and in deeper water. The sediments of the Paluma Shoals reef flats consist of mixed terrigenous and calcareous gravels and sands, with intermixed silts and clays, whereas the reef slope is dominated by gravelly quartz sands. The main turbidity‐generating process is wave‐driven resuspension, and turbidity ranges up to 175 nephelometric turbidity units (NTU). In contrast, at Phillips Reef, turbidity is &lt;15 NTU and varies little. At Paluma Shoals, turbidity of &gt;40 NTU probably occurs for a total of &gt;40 days each year, and relatively little time is spent at intermediate turbidities (15–50 NTU). The extended time spent at either low or high turbidities is consistent with the biological response of some species of corals to adopt two alternative mechanisms of functioning (autotrophy and heterotrophy) in response to different levels of turbidity. Sedimentation rates over periods of hours may reach the equivalent of 10 000 times the mean global background terrigenous flux (BTF) of sediment to the sea floor, i.e. 10 000 BTF, over three orders of magnitude greater than the Holocene average for Halifax Bay of &lt;3 BTF. As elsewhere along the nearshore zone of the central GBR, dry‐season hydrodynamic conditions form a primary control upon turbidity and the distribution of bed sediments. The location of modern nearshore coral reefs is controlled by the presence of suitable substrates, which in Halifax Bay are Pleistocene and early Holocene coarse‐grained (and relatively stable) alluvial deposits.

Variability of nutrient regeneration rates and nutrient concentrations in surface sediments of the northern Great Barrier Reef shelf

Sediment nutrient concentrations and fluxes were examined on the northern Great Barrier Reef (GBR) shelf to determine the extent of temporal and spatial variability. Despite a clear sediment gradient of increasing carbonate content seaward and differences in river discharge along-shelf, no significant differences in nutrient concentrations or fluxes across the sediment–water interface were observed for most nutrients along- or across-shelf over time. Only interstitial P and DOC concentrations displayed significant differences along-shelf and only in the wet season. Rates of nutrient regeneration were highly variable, but contributed, on average, 11% of N and 22% of P requirements for phytoplankton production, similar to the benthic contribution for primary production on the central GBR shelf. The lack of clear patterns in the size of the sediment nutrient pools and fluxes are contrary to patterns on most other shelves, and may be partly the result of intense dilution, mixing and transport processes operating at small to large spatial scales over time.

Stratigraphic control of the geochemistry of Holocene inner-shelf facies, Great Barrier Reef

Cleveland Bay and Halifax Bay are adjacent embayments, situated on the inner-shelf region (0–20 m) of the central Great Barrier Reef shelf off Townsville, northeast Australia. These bays contain Holocene sediments up to 5 m in thickness, deposited during the last stages of the post-glacial sea-level rise. X-ray diffraction and X-ray fluorescence techniques were used to examine the mineralogy and geochemistry of the main Holocene facies, sampled in 40 vibrocores. The sediments of southern Halifax Bay and Cleveland Bay consist mainly of quartz (ca. 50%), alkali feldspars (ca. 20%), clay minerals (ca. 20%) consisting of mixed-layer clays, smectite, kaolinite and illite, and carbonate (ca. 10%) including aragonite and calcite. The Holocene facies sequence comprises, in order of increasing age, modern bay, shoreline and mangrove sediments. The oxides of the major components (SiO 2, Al 2O 3, Fe 2O 3, MgO, CaO, K 2O, Na 2O, P 2O 5) in Halifax Bay show the predicted linear correlations with geochemically related minor elements (Pb, Rb, Sr, Y, Zr, Ga, Zn, Ni, Co, Mn, Cr, Ti, Sc, V, Ba), and can be used to discriminate Holocene facies. However, in Cleveland Bay these elements are poorly correlated within each facies and provide very poor facies discrimination. These findings probably result from the presence of sandy shoreline sediments up to 2 m in thickness in the Holocene sequence of Cleveland Bay, with the consequent development of a weak oxic zone and migration of elements between adjacent facies. A similar oxic zone is poorly developed in Halifax Bay because the shoreline facies there is either thin or absent. That the chemical signatures of Holocene inner-shelf facies are spatially-variable, and are strongly influenced by the stratigraphy, has wide implications for geochemical studies of marine sediments.

Ecology of free-living nematodes from the continental shelf of the central great barrier reef province

Nematode assemblages from five sites on the continental shelf of the central Great Barrier Reef (GBR) along a transect ranging from inshore muddy quartz to offshore carbonate sands were examined in austral spring (October 1987) and summer (January 1988). Assemblages at all sites were characterized by very even distributions (low dominance) of families, genera and species. Average per cent abundances of the dominant family (Chromadoridae), genus ( Theristus) and species ( Spilophorella paradoxa), respectively, were 16, 10 and 8% of the total number of specimens identified. The diversity of species ( H′ = 3·35–4·08) from the GBR was higher than that normally calculated for shelf nematode assemblages; species richness (SR = 10·64–16·67) and evenness (J′ = 0·82–0·91) values approximate those characteristic of deep-sea nematode assemblages, suggesting equitable exploitation of the interstitial environment on the GBR shelf by many species. This was also indicated by a nodal analysis of the 22 dominant species, which exhibited no strong preference for any particular sediment or site. Non-selective deposit and epistrate feeders were the most abundant trophic types among the nematodes. The abundance of non-selective deposit feeders was significantly greater in the summer than spring assemblages, accompanying differences also observed in the per cent composition of several species. Shortterm fluctuations in species composition and high species diversity of nematodes on the central Great Barrier Reef shelf may be the result of high sediment heterogeneity (caused by cyclones and frequent prawn trawling), a rich bacterial flora, continuous warm temperatures and low macroinfaunal abundance.

Shelf-wide erosion, deposition, and suspended sediment tranport during Cyclone Winifred, central Great Barrier Reef, Australia

ABSTRACT Observation of shelf sediments collected immediately before and after Cyclone Winifred crossed the central Great Barrier Reef shelf (1 February 1986) confirmed that the storm produced a normally graded, mixed terrigenous-carbonate storm layer extending 30 km offshore in water up to 43 m deep. Distinct post-Winifred changes in the cross-shelf distribution of organic carbon and carbonate in the mud-fraction of the sediment suggest that suspended sediment transport was extensive and that the storm layer had multiple sediment sources. On a shelf-wide scale, the storm layer is composed almost entirely of reworked shelf sediment. Erosion depths were greater on the mid-shelf (20-40 m water depth) than on the inner shelf( 6.9 cm and 5.1 cm, respectively. Particles finer than medium sand were eroded and transported out of the mid-shelf. The inner-shelf portion of the storm layer formed by the combination of three sediment sources, including 1) seaward transport of terrigenous sediment in buoyant freshwater flood plumes, 2) resuspension and settling of inner-shelf sediment, and 3) resuspension and shoreward transport of mid-shelf sediment. Mass-balance calculations predict that at least 10-30% of the inner-shelf storm layer is composed of mid-shelf mud. Combined wave and wind-forced currents probably resuspended mid-shelf material and drove the suspended fraction alongshelf and shoreward a minimum distance of 15 km. The results suggest that tropical cyclones are capable of sporadic but efficient cross-shelf transport of suspended sediment. On shallow cyclone-prone shelves, suspended sediment may easily be exchanged between adjacent sedimentary facies. In ancient shelf sequences, the transport history of the mud will be complex, and stratigraphically equivalent facies may have similar mud types but completely different sands.

The Cyclone Winifred Storm Bed, Central Great Barrier Reef Shelf, Australia

ABSTRACT Shelf sediments were collected immediately before (8-20 January 1986), immediatly after (9-10 February 1986), and one year after (27 February 1987) Cyclone Winifred crossed the central Great Barrier Reef shelf on 1 February 1986. The storm produced a normally graded, mixed terrigenous-carbonate storm bed more than 11 cm thick, covering an area at least 1,200 km2, and extending more than 30 km offshore to water depths greater than 40 m. The storm layer encompasses two lithologically distinct, shelf-parallel facies belts. The nearshore (0-25 m depth) storm bed is 311 cm thick, thins offshore, and grades upward from medium sand to silty clay. Offshore (25-43 m depth) the storm bed thickens to more than 11 cm and consists of normally graded, well sorted, relict quartz and skeletal, gravelly sands capped by a thin mud veneer. The cross-shelf distributions of grain size, mineralogy, carbonate content, and carbon isotope ratios were very similar before and after Winifred, suggesting the storm bed did not result from offshore-directed storm currents but, rather, in situ resuspension and settling of the shelf sand with shoreward transport of mud. In contrast to storm sedimentation models which predict thinning and fining offshore, Winifred created a layer that becomes thicker and coarser grained in deeper water in response to cross-shelf substrate changes. After one year the Winifred storm layer was well preserved nearshore ( 30 m water depth) below fairweather wave base. Preservation may not be a matter of insulating the layer from physical reworking below fairweather wave base but of burying it where rates of sediment accumulation outpace bioturbation. The effects of storms similar in size to Winifred may be well concealed in ancient shelf sequences subject to rapid biological remixing of storm layers.