Beach Stability on a Tropical Uplifted Coral Atoll: Niue Island

<p>Fundamental knowledge about the change and dynamics, and what thresholds drive sediment accumulation in tropical reef settings are poor. Little is also known about how they may respond to the higher and stormier seas that are predicted in an enhanced greenhouse world. Niue's rocky shore setting and the regular occurrence of small isolated pocket-beaches provides an ideal environment to investigate key factors that drive beaches to accumulate or erode within a tropical reef setting. Niue is the largest uplifted coral atoll in the world, covering an area of 200 km^2 and rising to 70 m above sea level. The island is characterised by a series of Pleistocene reef terraces with distinct platforms forming at the base at approximate mean sea level. Lateral reef growth at sea level is juxtaposed with landward retreat of the limestone cliffs leading to the formation of shore platforms. Geomorphological surveys of 9 sites revealed a combined reef platform width of up to 150 m with the widest section found on the leeward side of the island on the north western coast and the narrowest (<30 m) being located on the more exposed south eastern coast. Therefore, their distribution is likely related to the energy environment around the island. Beaches up to 12 m wide and 50 m long are only found in protected coves along the shoreline. Their development is determined by platform width, with beaches only occurring in areas where platform width is more than 60 m. While distance from the reef crest played a role in dissipating wave energy across the platform therefore reducing beach erosion, beach stability is reliant the morphology of the underlying ramp on the landward edge of the platform. Beaches increased in width at higher elevations therefore implying that a higher ramp can effectively reduce the amount of wave energy reaching the landward edge of the beach resulting in the accumulation of sediment. Composition analysis of 51 samples reveal that the Niuean beaches are largely composed of unconsolidated bioclastic sand and gravels derived from the surrounding reef platform. They are characterised by an assemblage of chlorozoan carbonates typical of tropical areas, in which coral and coralline algae are prominent (>50%) except on the north western platforms (Hio and Tuapa) where foraminifera is the key component. Radiocarbon dating further indicates the youth of these beaches returning modern ages for reef flat microatolls as well as the beach sand itself. These sedimentary environments on Niue are therefore intrinsically linked to the platform biota and their preservation also dependent on the frequency of cyclones. The fast recovery of the foraminifera-rich north western beaches following Tropical Cyclone Heta (2004) is an indication that the foraminifera community can re-establish quicker after cyclones. This therefore confirms that the beaches are highly dynamic, and build out or erode during alternated calm and stormy conditions. The close links between beach accumulation and their biotic communities will be strongly affected by human-induced climate change, likely leading to the beaches becoming more ephemeral in the future.</p>

Beach Stability on a Tropical Uplifted Coral Atoll: Niue Island

<p>Fundamental knowledge about the change and dynamics, and what thresholds drive sediment accumulation in tropical reef settings are poor. Little is also known about how they may respond to the higher and stormier seas that are predicted in an enhanced greenhouse world. Niue's rocky shore setting and the regular occurrence of small isolated pocket-beaches provides an ideal environment to investigate key factors that drive beaches to accumulate or erode within a tropical reef setting. Niue is the largest uplifted coral atoll in the world, covering an area of 200 km^2 and rising to 70 m above sea level. The island is characterised by a series of Pleistocene reef terraces with distinct platforms forming at the base at approximate mean sea level. Lateral reef growth at sea level is juxtaposed with landward retreat of the limestone cliffs leading to the formation of shore platforms. Geomorphological surveys of 9 sites revealed a combined reef platform width of up to 150 m with the widest section found on the leeward side of the island on the north western coast and the narrowest (<30 m) being located on the more exposed south eastern coast. Therefore, their distribution is likely related to the energy environment around the island. Beaches up to 12 m wide and 50 m long are only found in protected coves along the shoreline. Their development is determined by platform width, with beaches only occurring in areas where platform width is more than 60 m. While distance from the reef crest played a role in dissipating wave energy across the platform therefore reducing beach erosion, beach stability is reliant the morphology of the underlying ramp on the landward edge of the platform. Beaches increased in width at higher elevations therefore implying that a higher ramp can effectively reduce the amount of wave energy reaching the landward edge of the beach resulting in the accumulation of sediment. Composition analysis of 51 samples reveal that the Niuean beaches are largely composed of unconsolidated bioclastic sand and gravels derived from the surrounding reef platform. They are characterised by an assemblage of chlorozoan carbonates typical of tropical areas, in which coral and coralline algae are prominent (>50%) except on the north western platforms (Hio and Tuapa) where foraminifera is the key component. Radiocarbon dating further indicates the youth of these beaches returning modern ages for reef flat microatolls as well as the beach sand itself. These sedimentary environments on Niue are therefore intrinsically linked to the platform biota and their preservation also dependent on the frequency of cyclones. The fast recovery of the foraminifera-rich north western beaches following Tropical Cyclone Heta (2004) is an indication that the foraminifera community can re-establish quicker after cyclones. This therefore confirms that the beaches are highly dynamic, and build out or erode during alternated calm and stormy conditions. The close links between beach accumulation and their biotic communities will be strongly affected by human-induced climate change, likely leading to the beaches becoming more ephemeral in the future.</p>

Inter-Habitat Variability in Parrotfish Bioerosion Rates and Grazing Pressure on an Indian Ocean Reef Platform

Parrotfish perform a variety of vital ecological functions on coral reefs, but we have little understanding of how these vary spatially as a result of inter-habitat variability in species assemblages. Here, we examine how two key ecological functions that result from parrotfish feeding, bioerosion and substrate grazing, vary between habitats over a reef scale in the central Maldives. Eight distinct habitats were delineated in early 2015, prior to the 2016 bleaching event, each supporting a unique parrotfish assemblage. Bioerosion rates varied from 0 to 0.84 ± 0.12 kg m−2 yr−1 but were highest in the coral rubble- and Pocillopora spp.-dominated habitat. Grazing pressure also varied markedly between habitats but followed a different inter-habitat pattern from that of bioerosion, with different contributing species. Total parrotfish grazing pressure ranged from 0 to ~264 ± 16% available substrate grazed yr-1 in the branching Acropora spp.-dominated habitat. Despite the importance of these functions in influencing reef-scale physical structure and ecological health, the highest rates occurred over less than 30% of the platform area. The results presented here provide new insights into within-reef variability in parrotfish ecological functions and demonstrate the importance of considering how these interact to influence reef geo-ecology.

Analysis of the relationship between porosity and permeability in reservoir modeling using the petrophysical rock type approach

The carbonate reservoir is one of the reservoir characters found in hydrocarbon fields in Indonesia. Carbonate reservoirs have complex porosity and permeability relationships. So it is necessary to do a special reservoir character that is different from the siliciclastic reservoir. Efforts that can be made to assist the development of this hydrocarbon field are to analyze the reservoir character in more detail using the petrophysical rock type (PRT) approach. This approach is used by combining geological elements such as the depositional environment, the petrophysical properties of the rock, as well as the fluid flow in it which is reflected by capillary pressure and water saturation. Modeling with this method is expected to be a method that can increase hydrocarbon production optimally in Xena Field. The object of research from Xena Field is Zone A2 which is included in the Parigi Formation. The Parigi Formation is one of the main hydrocarbon-producing reservoirs. The data used in this study are routine core analysis (RCAL) rock data on JLB-07, JLB-08, JLB-02, JLB-23 wells, wire log data (gamma-ray log, resistivity log, density log, neutron log) of 30 wells, and 2D seismic data. The depositional facies are divided into 2 facies, namely the margin reef platform facies and the interior platform facies. Identification of rock type (RT) using the flow zone indicator (FZI) method. The rock type in this field can be divided into 4 rock types, namely RT 1, RT 2, RT 3, RT 4 with RT 1 being able to drain the best fluid and RT 4 to drain the worst fluid. Reservoir property modeling is controlled by facies and rock type (RT) models. The margin reef platform facies are associated with RT 1 and RT 2. The interior platform facies are associated with RT 2 and RT 3.

Assessing Reef Island Sensitivity Based on LiDAR-Derived Morphometric Indicators

Reef islands are some of the most highly sensitive landforms to the impacts of future environmental change. Previous assessments of island morphodynamics primarily relied on historical aerial and satellite imagery. These approaches limit analysis to two-dimensional parameters, with no ability to assess long-term changes to island volume or elevation. Here, we use high-resolution airborne LiDAR data to assess three-dimensional reef island features for 22 islands along the north-western coast of Australia. Our primary objective was to utilize two regional LiDAR datasets to identify characteristics indicative of island sensitivity and future vulnerability. Results show reef platform area to be an accurate predictor of island area and volume suggesting larger island volumes may reflect (1) increased carbonate production and supply from the reef platform and/or (2) enhanced shoreline protection by larger reef platforms. Locations of foredune scarping (an erosional signature) and island orientations were aligned to the regional wind and wave climate. Reef island characteristics (island area, volume, elevation, scarping, and platform area) were used to rank islands according to sensitivity, using a new Island Sensitivity Characteristics Index (ISCi) where low ISCi indicates stable islands (large areas and volumes, high elevations, and fewer scarped areas) and high ISCi indicates unstable islands (small areas and volumes, low elevations, and more scarped areas). Comparison of two LiDAR surveys from 2016 and 2018 validates the use of 3D morphometrics as important (direct) measurements of island landform change, and can complement the use of 2D parameters (e.g., area) moving forward. Results demonstrate that ongoing use of airborne LiDAR and other 3D technology for monitoring coral reef islands at regional scales will enable more accurate quantification of their sensitivity to future impacts of global environmental change.

Satellite Derived Bathymetry on Shallow Reef Platform: A Preliminary Result from Semak Daun, Seribu Islands, Java Sea, Indonesia

Derivation of the bathymetric model from satellite imaging for non-navigable coastal waters has been developed. It is the purpose of this presented paper to assess the depth accuracy of the bathymetric model derived from such optical satellite imagery. The study domain is situated in the Semak Daun reef platform, Java Sea, Indonesia. The area represents shallow sub- and inter-tidal water with various benthic covers. Satellite imagery used here is retrieved from the European Space Agency Sentinel-2 satellite observation system. Two methods in deriving bathymetry from optical imagery are used. The first one is the empirical band ratio transform algorithm and the second one is the analytical approach. Coefficients involved in both models are obtained from means of calibration against sounding data from a single-beam echo-sounding survey. About 9% of sounding data are used for the calibration, while the rests are used to validate the resulting bathymetric models. It is found that both methods can successfully be applied at depth of up to 10 m. The root mean square errors indicated by both models are comparable. Accuracy measures in the order of 1.9 m are obtained with a coefficient of determination of 0.7. The results presented here confirm the applicability of satellite-derived bathymetry for mapping shallow seabed complying to the category zone of confidence C as of the International Hydrographic Organization standard. It should be bear in mind that such an assessment is typical for the environmental condition considered in this study.