Coral Sediments Research Articles

A taphonomic clock based on the coral grain size of beach sediments from the Yongle Atoll, South China Sea

The grain size characteristics and aging of beach sediments from six islands of the Yongle Atoll, South China Sea, were measured to examine the taphonomic degradation of coral clasts in sediment reservoirs of reef environments. Seventy-one beach sediment samples were collected from 22 transects on 6 islands for sediment characteristic analyses, and 16 bulk sand samples and 28 coral samples were selected for 230Th dating. The mean grain size and ages of coral clasts showed a significant negative correlation (F25 = −0.721, p = 0.01). A grain size age model was established to explain the taphonomic degradation mechanism of coral clasts over time, and an empirical relationship between the mean grain size and 230Th ages was obtained from the Yongle Atoll. The mismatch of the taphonomic clock by grain size data was analyzed, and it was mainly caused by the influence of sorting processes and the intrinsic morphology of coral skeletons that occurred at the early stage of the breakdown process. The application of the model to a nearby atoll island showed that for coral sediments with homogeneous taphonomic conditions, the predicted age was well matched with the measured age. This work presents a general framework for understanding coral clast degradation trajectories with time in taphonomic environments, and it provides a feasible method to predict the coral age independent of other dating methods.

Experimental study of the incipient motion threshold of coral sediment for oscillatory flow

In this study, the incipient motion thresholds of coral sediment with the action of oscillatory currents were experimentally recorded by conducting a total of 405 laboratory experiments, including an analysis of nine sieve median diameters, five water depths, and four wave periods. The coral sediment Shields parameters for oscillatory flow were compared to those for unidirectional flow and to those of shell Shields parameters for oscillatory flow. The results showed that there was an extensive scatter range in the Shields parameter for oscillatory flow of coral sediments. Additionally, the coral sediment Shields parameter for oscillatory flow was slightly different from that for unidirectional flow. The use of the Shields curve form to represent the coral sediment threshold for oscillatory flow was not appropriate. Using the critical near-bottom peak velocity to represent the incipient motion threshold of coral sediment for oscillatory flow was a better option. The use of experimental particle data analyses and the equation for the critical near-bottom peak velocity of quartz sand for oscillatory flow was modified to obtain a critical near-bottom peak velocity equation for coral sediment. This equation adds the influence of shape through the median Corey shape factor.

New records and distribution of marine free-living nematodes in the Maldivian Archipelago

Biodiversity estimations, in particular in tropical regions, are a useful tool in research focused on the knowledge and conservation of ecosystems. Inferences on the biogeography of marine nematodes are often difficult due to the poor knowledge of the species distribution. In particular, limited information on the composition of the free-living marine nematodes is available from the Indian Ocean and Maldivian Archipelago. This study aims at providing a checklist of the free-living marine nematodes associated with coral sediments of the Maldives. Forty-six species, in 28 genera and 10 families, are newly recorded for the Archipelago, increasing the number of known nematode species to 111, in 79 genera, and 25 families. The distribution range of the species is updated, and a noticeable faunistic richness is recorded for the southern archipelago. The presence of six species described first in Maldives by Gerlach and still known only for this region is confirmed. The present synthesis of the faunistic and taxonomic data available so far for the Maldives Islands integrates the global information on the distribution of this phylum and provides information for future interesting comparisons from a zoogeographical perspective.

Meiofauna associated with coral sediments in the Maldivian subtidal habitats (Indian Ocean)

Coral reefs are important hot spots of biodiversity, but despite the problems related to their conservation, studies on meiobenthic biodiversity are scarce. This paper provides a preliminary overview of the meiofauna and nematodes inhabiting coral sediments in the outer reef and lagoon habitats of the Maldives, and investigates their relationships with micro-habitat type. The abundance and community structure of the meiofauna and nematodes reveal significant differences between stations. Coral fragments and rubble from the outer reefs seem to act as a trap for the finest sediment, which might contribute to creating a heterogeneous micro-habitat suitable for both epifaunal (Epsilonematidae and Draconematidae genera) and sediment-dwelling (Tricoma, Richtersia, Ptycholaimellus and Molgolaimus) taxa. On the other side, the lagoon’s sediments are mainly colonized by dwelling taxa, probably due to the low-flow regime that enhances the deposition of organic matter and pelite fractions. A high level of diversity was recorded in both the habitats studied. Among the factors that probably determine the high diversity in the lagoon is the presence of small biogenic structures. These are characterized by a high degree of angularity, which might add more complexity to the habitat. The nematodes reveal an overlap in the taxonomic composition between the Maldives and other geographically distant areas, possibly supporting the existence of iso-communities that are typical of the coral degradation zone. Nematode trophic composition reflects differences in availability of food resources; epigrowth and non-selective deposit feeders are dominant in the lagoon, while selective deposit and epistrate feeders are dominant in the outer reefs.

New Amphicocconeis species off Mascarenes (Western Indian Ocean) and related taxa

Two new species were found on coral sediments of Mascarene archipelago: Amphicocconeis mascarenica sp. nov., relatively frequent in Réunion and rare in Rodrigues Islands, and A. rodriguensis sp. nov., relatively rare in Rodrigues and scarce in Réunion. Comparison was made with A. disculoides and A. debesii (Hustedt) De Stefano, and with species pertaining to Psammococconeis Garcia (P. brasiliensis and P. disculoides sensu Garcia). The Mascarenes taxa do not have simple raphe endings but terminal raphe fissures bent to the same side, which is a characteristic feature shared by Amphicocconeis taxa. Amphicocconeis rodriguensis has a raphe valve relatively similar to that of A. debesii (Hustedt) De Stefano, but with a denser striation; furthermore, the valvocopula of the sternum valve is narrow with simple to chiselled fimbriae. Amphicocconeis rodriguensis sternum valve is close to that of A. debesii (Hustedt) De Stefano. Amphicocconeis mascarenica shows a number of striae relatively low on both valves and both valvocopulae are generally extended and present rows of alternated small holes, which seems a unique feature for Amphicocconeis.

Distribution of radionuclides in the guano sediments of Xisha Islands, South China Sea and its implication

Several natural and anthropogenic radionuclides ( 210Pb, 226Ra and 137Cs) in guano-phosphatic coral sediments and pure guano particles collected from Ganquan, Guangjin, Jinqing and Jinyin Islands of the Xisha archipelago, South China Sea, were analyzed. The Constant Initial Concentration (CIC) model and the Constant Rate of Supply (CRS) model were applied for age calculation. The average supply rate of 210Pb was 126 Bq m −2 a −1, very close to the flux of northern hemisphere average (125 Bq m −2 a −1). The activities of anthropogenic radionuclides in the sediments were very low, indicating that human nuclear tests did not notably impact this region. The main source of radionuclides in the sediments was from atmospheric precipitation, and the organic matter derived from plant and produced by nutrient-rich guano could further enhance them.

DMS and DMSP in mucus ropes, coral mucus, surface films and sediment pore waters from coral reefs in the Great Barrier Reef

Concentrations of dimethylsulphide (DMS) and its precursor compound dimethylsulphoniopropionate (DMSP), two sulphur compounds that are involved in the formation of clouds, were measured for mucus ropes, coral mucus, surface films and sediment pore waters collected from three coral reefs in the Great Barrier Reef, Australia. The concentrations of DMS (61–18 665 nm) and DMSP (1978–54 381 nm) measured in mucus rope samples are the highest yet reported in the marine environment. The values exceed concentrations of DMS and DMSP reported from highly productive polar waters and sea ice algal communities. Concentrations of DMSP in coral mucus ranged from 1226 to 25 443 nm, with mucus from Acropora formosa containing the highest levels of DMSP. Dimethylsulphide and DMSP in surface microlayer samples from three coral reefs were two to four times subsurface (0.5 m) concentrations. In coral-reef sediment pore waters, concentrations of DMS and DMSP were substantially higher than water-column concentrations, suggesting that coral sediments may be a significant source of these two compounds to reef waters. Overall, the results strongly suggest that coral reefs in the Great Barrier Reef are significant sources of these two sulphur substances.

Three-dimensional simulation of the Home Island freshwater lens: preliminary results

Home Island is one of the two main inhabited islands of the Cocos (Keeling) Islands, situated about 2800 km north-west of Perth. The island has an average elevation of approximately 2 m above MSL and supports a population of about 450. The geology of the island consists of coral sediments overlying a volcanic seamount. The upper sediments are of primary importance from a hydrogeological viewpoint. These sediments consist of unconsolidated coral sands and other fragments and coral limestone. An unconformity between the younger (Holocene) unconsolidated sediments and older (Pleistocene) coral limestone was found at depths between 10 m to more than 17 m. The island has a long term average annual rainfall of 1950 mm. Water supply is based primarily on groundwater extraction from a local freshwater lens and supplementary rainwater is collected from roofs. Groundwater is currently extracted via seven galleries. A recent study based on a two dimensional model (using the SUTRA package), simulating impacts of various level of pumping up to 200 m 3 day −1 determined the sustainable yield as 140 m 3 day −1. This paper describes progress regarding the development of a three-dimensional model of the freshwater lens using the SALTFLOW package, by discretising the aquifer to about 90 000 nodes and 83 000 elements. This is followed by the steady-state calibration of the model, estimation of the parameters affecting the system and assessing the sustainable yield of the lens. Preliminary results of the three dimensional modelling suggest that the sustainable extraction rate is less than 200 m 3 day −1, which is consistent with the results of the two dimensional modelling. This research project is continuing towards simulation of tidal forces, transient calibration of the model over a 10 year period from January 1988 to December 1997, estimation of sustainable yield of freshwater lens and simulation of various management options.

Designing for Climate Variability

Hurricanes, blizzards, and floods. The atmosphere has visited extreme weather in the United States over the last five years. Is this climate change or climatic variability? Is the distinction important in the context of infrastructure design and planning? To address these questions, the meaning and relevance of the notions of climate change and climate variability must be clearly established in the context of natural hazards, and infrastructure engineering. Multidecadal time scales (whether natural or anthropogenically induced) have been firmly identified in the climate record. Compelling evidence of this variability has been found in data sources as diverse as tree rings, and cores taken from coral reefs, sediments, and glaciers across the globe. It is therefore submitted that the criteria used in engineering design of lifeline infrastructures, land-use planning, and water resources management among others must incorporate this knowledge.

Nitrogenous excretion by meiofauna from coral reef sediments: Mecor 5

Nitrogenous excretion rates of meiofauna from coral sediments of the Great Barrier Reef, Australia were studied at winter temperatures. Data on single taxa fitted a geometric regression: Log N excretion=-0.206+0.865 log x where log x=dry weight in μg. Abundance and biomass data on meiofaunal taxa were obtained from Lizard Island, Orpheus Island and Davies Reef. Using excretion rates and biomass data, meiofauna were calculated as contributing 10.18, 12.32, and 1.60 mg N m-2 h-1 at the above areas respectively.

Elutriation: Theoretical Considerations and Methodological Improvements

The conventional elutriation method used for separating meiofauna and sediment yields insufficiently correct quantitative results. A critical evaluation of the physical background of this method leads to the construction of a differently shaped separation funnel and to improvements allowing better controlled, gradual variations in waterflow velocity and sieve mesh sizes. Comparative tests of the conventional and the new method, using similarly composed coral sediments and organism mixtures demonstrated a significantly higher efficiency of the new method: only 5 /r~ of the organisms contained in the test samples disappeared with the new method, compared with 16 /c, using the conventional one. Moreover, the new method elutriates with a much higher degree of purity: an average of 23 mg sediment accumulated in the sieves, as opposed to 2380 mg with the old method.