Sedimentation on three caribbean atolls: Glovers reef,lighthouse reef and turneffe Islands,belize

Summary The chief mode of carbonate sedimentation on the Belizean atolls Glovers Reef, Lighthouse Reef and Turneffe Islands is the accumulation of organically-derived particles. Variations in the distribution of the composition and grain-sizes of surface sediments, collected along transects across the atolls, are environmentally controlled. Two major sediment types may be distinguished. (1) Reef and fore reef sediments are dominated by fragments of coral, coralline algae andHalimeda. Mean grain-sizes range from 1–2 mm. (2) Back reef sediments contain more mollusk fragments, more fine-grained sediment (<125 μm) and appear to have fewerHalimeda fragments. In addition, sediments from inner platforms and shallow lagoonal parts of Glovers and Lighthouse Reefs comprise non-skeletal grains, namely fecal pellets. Sediments from lagoonal patch reefs may contain up to 20% coral fragments. Mean grain-sizes range from 0.1–1 mm and are finest on the inner platform and lagoon floor of the back reef environment. Within the reef and fore reef environments, it is not possible to distinguish sub-environments on the basis of textural and compositional differences of the sediments. Sediments from patch reefs contrast with those from back reef lagoons and inner platforms and are similar in terms of grain-sizes and compositions to reef and fore reef surface sediments. Non-skeletal grains forming in shallow parts of the back reef in Glovers and Lighthouse Reefs are interpreted to be indurated by interstitial precipitation of calcium carbonate from warm, supersaturated water flushing the sediment. The lack of hardened non-skeletal particles in the back reef sediments of Turneffe Islands is most probably due to the abundance of muddy, organic-rich sediment in the well-protected lagoon. Fine sediment is less permeable and organic films prevent cement overgrowth on particles.     

Pleistocene marine depositionalenvironments from Mauritius island,Indian Ocean

The Pleistocene elevated marine carbonate complexes on Mauritius island correspond to three types of depositional zones: (1) reef-crest zone, (2) backreef zone, (3) littoral zone.On reef crests, the primary frame-builders are scleractinian corals (Acroporidae, Faviidae, Poritidae) and crustose coralline algae. The secondary builders are encrusting Foraminifers (Homotrematidae) chiefly, Molluscs (Vermetidae), Bryozoans (Cheilostomata) and Serpulids. Associated internal sediments are coarse to fine sand-sized grainstones or packstones and wakestones. In decreasing order of abundance, skeletal elements include fragments of corals, branched and crustose red algae, Pelecypods, Gastropods, benthonic Foraminifera (Amphisteginidae, Peneroplidae, Calcarinidae, Nummulitidae, Alveolinidae and Homotrematidae), Echinoderms and Alcyonarians; the most abundant mud-sized grains are identified as tunicate spicules. Primary marine cements are needle, palisade, micrite and pelletal ones.The backreef zone is characterized by coarse tomedium grainstones, packstones and wakestones. In addition to pellets the most common components are skeletal in nature; they broadly show the same mode of occurrence as the reef-crest unit. Primary cements locally occur in packed fibrous or micrite form.In littoral areas, well-sorted grainstones to poorlysorted mudstones can be described. The distribution of the various categories of biogenic particles is dependent upon the extent of reef tracts; ancient beaches from narrow reefs are characterized by coralgal facies while consolidated muddy banks in large complexes display high amount of molluscan detritus. Primary cementation appears mainly to be the result of the precipitation of fibrous or micrite calcite.The morphology, biological associations andsediments of these Pleistocene limestones are homologous with those of the adjacent modern environments.     

The modern reef complex,Jeddah area,Red Sea: a facies model for carbonate sedimentation on embryonic passive margins

The modern reef complex north of Jeddah comprises an offshore knoll platform and a fringing reef, subdivised into several depositional zones: tops and upper flanks of offshore reefs; lower flanks of offshore reefs and nearby inter-reef areas; fringing forereef, reef flat and backreef zones, and beach. Sixty-seven sediment samples were collected from the different zones and have been analysed in order to define relationships between the distribution of sedimentary facies and the depositional environments, and to furnish a reliable facies model by using multivariate analysis. Six types and subtypes have been objectively differentiated on the basis of total biogenic component and foraminiferal associations. Grain size data allowed us to discriminate three textural types, whereas five chemotypes have been recognized according to trace element concentration. Regarding the offshore reef platform, poorly sorted, medium sands of molluscan-coralline algal-Amphistegina and Cd types are restricted to the lower flanks of buildups and to the adjacent inter-reef deposits, whereas the tops and upper flanks of theses buildups are characterized by moderately sorted, coarse sands of coralline algal-Tubipora-Amphistegina-encrusting foraminiferal-bryozoan types, with a Mn chemotype. Concerning the fringing reef system, backreef areas exhibit poorly sorted, fine sands of molluscan-Ammonia-Peneroplis and Fe-Cu types. Moderately sorted, coarse sands of coralgal-Calcarina-Spiroloculina and Fe-Zn types are found on the reef flat. The forereef zone is characterized by poorly sorted, fine sand of Triloculina-encrusting foraminiferal-bryozoan and Zn-Mn types. The lateral limits of the various biotypes roughly coincide with the distribution of the relevant living organic communities. Trace elements appear to be either bound to the reef-associated silicate fractions or incorporated into the carbonate skeletons. On the basis of prevailing water conditions, physiography, biological and sedimentological attributes, the fringing reef can be regarded as an asymmetrical structure, with bidimensional (lateral and vertical) facies zonation; in contrast, the offshore platform is a symmetrical structure, with one dimensional (depth-dependent) facies zonation. This system is believed to represent a modern example of a laterally undifferentiated, offshore reef tract in a relatively enclosed basin, at an embryonic passive continental margin.     

Variability of Symbiodinium Communities in Waters,Sediments, and Corals of Thermally Distinct Reef Pools in American Samoa

Reef-building corals host assemblages of symbiotic algae (Symbiodinium spp.) whose diversity and abundance may fluctuate under different conditions, potentially facilitating acclimatization to environmental change. The composition of free-living Symbiodinium in reef waters and sediments may also be environmentally labile and may influence symbiotic assemblages by mediating supply and dispersal. The magnitude and spatial scales of environmental influence over Symbiodinium composition in different reef habitat compartments are, however, not well understood. We used pyrosequencing to compare Symbiodinium in sediments, water, and ten coral species between two backreef pools in American Samoa with contrasting thermal environments. We found distinct compartmental assemblages of clades A, C, D, F, and/or G Symbiodinium types, with strong differences between pools in water, sediments, and two coral species. In the pool with higher and more variable temperatures, abundance of various clade A and C types differed compared to the other pool, while abundance of D types was lower in sediments but higher in water and in Pavona venosa, revealing an altered habitat distribution and potential linkages among compartments. The lack of between-pool effects in other coral species was due to either low overall variability (in the case of Porites) or high within-pool variability. Symbiodinium communities in water and sediment also showed within-pool structure, indicating that environmental influences may operate over multiple, small spatial scales. This work suggests that Symbiodinium composition is highly labile in reef waters, sediments, and some corals, but the underlying drivers and functional consequences of this plasticity require further testing with high spatial resolution biological and environmental sampling.     

Taphonomy of coral reefs: a review

This review summarises the major factors that affect the post-mortem history of skeletons in a coral reef environment. Skeletal material is traced from life, through death, breakdown, transport, burial and diagenesis to its final fossil form. The fact that most reef sediments are of skeletal composition poses problems of concentration or dilution of individual grain types in taphonomic analysis of reefs. Rates of supply of grains vary, not only with organism abundance and skeletal growth rates, but also with rates of physical and biological breakdown to transportable sediment. Physical and organic processes affect sedimentary structures and textures by mixing or segregating skeletal grains, though biogenic processes normally dominate in the protected setting of reef lagoons. The soft and hard substrates associated with reefs present different media for calcium carbonate accumulation and post-depositional disturbance, for example, loose sediments suffer bioturbation and rocks surfaces suffer bioerosion. The wide range of durability of skeletons and their susceptibility to diagenesis contribute further to the complexities of the preservation of coral reefs.     

Reef to basin sediment transport using Halimeda as a sediment tracer,Grand Cayman Island,West Indies

Fragments of the calcareous green alga Halimeda form a large part of the sediment in the fringing reef system and adjacent deep marine environments of Grand Cayman Island, West Indies. Nine species combine to form three depth-related assemblages that are characteristic of the major reef-related environments (lagoonpatch reef, reef terraces, and deep reef). These modern plant assemblages form the basis of the use of Halimeda as a sediment tracer. Halimeda-based tracer studies of Holocene sediments indicate that only sediments containing deep reef species of Halimeda are presently being transported through the reef system by sediment creep and being deposited at the juncture of the upper and lower island slope. Sediments containing shallow reef Halimeda are retained within the reef and lithified by marine carbonate cements. Tracer studies of Pleistocene sediment indicate large amounts of reef-derived carbonate sand containing deep water Halimeda were produced during interglacial high stands of sea level. Much of this material was removed by turbidity currents moving out of the reef system to the island slope down submarine channels perpendicular to the reef trend. These channels may still be identified on bathymetric profiles, but are no longer receiving coarse reef debris and are veneered with a blanket of pelagic carbonate mud.     

Offshore sedimentary facies of a modern carbonate ramp, Kuwait, northwestern Arabian-Persian Gulf

The Kuwait example studied here may serve as a model for ancient carbonate ramp systems just as the classical—but markedly different—southern Arabian-Persian Gulf ramp of the Trucial Coast (United Arab Emirates). Five sedimentary facies may be distinguished on the modern southern Kuwait carbonate ramp based on quantitative sedimentological, mineralogical, and geochemical analyses of 130 surface sediment samples and by using multivariate statistics. These facies include (1) inner ramp ooid-skeletal grainstone with common aggregate grains, peloids, and molluscs, (2) limited occurrences of nearshore quartz-ooid sand, (3) mid ramp mollusk packstone to grainstone, (4) outer ramp mollusk-marl wackestone with abundant siliciclastic fines, and (5) coralgal grainstone that is found on small nearshore patch reefs and outer ramp pinnacle and platform reefs. In addition to facies (1), an aggregate grain packstone to grainstone sub-facies is mapped out where abundances of this grain type exceed 20%. Ooid-skeletal grainstone, mollusk packstone to grainstone, and coralgal grainstone are predominantly aragonitic with 5–10% insoluble residue on average. Mollusk-marl wackestone has 55% insoluble residue on average with aragonite and low-magnesium calcite predominating in the carbonate fraction. Dolomite in this facies is interpreted to be of eolian origin derived from the upwind deserts of Syria and Iraq. Facies distribution is correlated with water depth, and hence controlled by depositional energy, primarily wavebase. This correlation is seen in the results of statistical analyses and in the fact that facies boundaries are more or less parallel to depth contours. Ooid-skeletal grainstones are found in depths from 0 to <10 m. The boundary between the mollusk packstone to grainstone and the mollusk-marl wackestone, which also marks the transition from grain-supported to mud-supported textures, is situated between 15–20 m depth and is much sharper than the boundary between the ooid-skeletal and the mollusk packstone to grainstone facies. Carbonate-dominated facies may also be distinguished geochemically as indicated by significantly different carbon and oxygen isotope compositions. The latter should be kept in mind when using bulk isotope values for chemostratigraphy or for paleo-environmental reconstructions in fossil carbonate ramps and platforms.An erratum to this article can be found at .     

Influence of setting,sieve size,and sediment depth on multivariate and univariate assemblage attributes of coral reef-associated mollusc death assemblages from the Gulf of Aqaba

Measures of diversity and ecology of marine invertebrate assemblages depend on a variety of factors including environmental conditions and methodological decisions. In this study, the influence of such factors on multi- and univariate assemblage parameters of molluscan death assemblages from the Gulf of Aqaba (Red Sea, Jordan) was evaluated. Sediment samples were collected at two coral reef types, a patch reef at 13 m of water depth characterized by fine-grained sediments and a Millepora-fringing reef with coarse-grained sediments at 5 m of water depth. The upper and lower 10 cm of the sediment column were separately removed and sieved with mesh sizes of 1 and 2 mm. A large dataset of 6400 bivalve and gastropod shells was compiled to evaluate how setting, sediment depth, and sieve size influenced taxonomic composition and species richness, species-abundance patterns and the Shannon–Wiener index, the number of drilled shells per species and drilling frequency (DF) of the assemblage. Setting had the strongest impact on all aspects, followed by sieve size, but sediment depth was insignificant, probably due to complete homogenization of the sediments by reworking and bioturbation. Multivariate assemblage parameters distinguished much better between categories (setting, sieve size) than univariate measures. Sieve size-related disagreements recognized between the two higher taxa are mostly due to the underlying difference in body-size distribution of bivalve and gastropod assemblages. We conclude that species richness and other ecological characteristics of molluscan death assemblages in coral reef-associated sediments will most strongly reflect habitat complexity of the sites chosen, are significantly influenced by methodological decisions (i.e., sieve size), will only poorly preserve temporal patterns, and the results will differ between bivalves and gastropods.     

Particulate matter filtration and seasonal nutrient dynamics in permeable carbonate and silicate sands of the Gulf of Aqaba,Red Sea

This study compares mineralization in permeable silicate and carbonate sands in the shallow shelf of the Gulf of Aqaba. From July 1999 to March 2000, we monitored concentrations of inorganic nutrients in water and pore water at two neighboring sites, one dominated by silicate, the other by carbonate sand. Although the carbonate was coarser than the quartz sand, organic matter, dissolved inorganic nitrogen (DIN), and ortho-phosphate concentrations in the biogenic carbonate sediment always exceeded those in the terrigenic silicate sands (factor 1.5–2.0 for organic matter, 1.7–14.0 for nutrients). Higher nutrient concentrations in the water column during winter months caused increases in pore-water nutrient concentrations in both sediments down to 10 cm depth with no significant delay, emphasizing the effect of advective transport of solutes and particles into permeable sands. An experiment was conducted where sieved clean quartz and carbonate sands of same grain size (250–500 µm) were incubated in-situ. Although exposed to the same water and boundary current conditions, the sieved carbonate sand accumulated more organic matter and developed higher nutrient concentrations than the incubated silicate sediment. We conclude that the mineralogical characteristics of the carbonate sand (higher porosity, sorption capacity and pH buffer capacity) enhance the filtration capacity, and the biocatalytic conversion efficiency relative to the smooth crystalline quartz grains.     

Sedimentary facies from the modern coral reefs,Jordan Gulf of Aqaba,Red Sea

Summary Sixty-three sediment samples collected from the modern fringing reefs off the Jordan coast (Gulf of Aqaba, Red Sea) have been analysed in order to determine variations of composition and texture by using correspondence factor analysis. From the shore seawards, the following physiographic zones are recognized: beach; shallower backreef zone; reef flat zone; forereef zone including sandy or coral-built slopes. Eight sediment facies and subfacies can be recognized on the basis of total component composition and foraminiferal assemblages and four sediment facies can be recognized using grain-size data. Wellsorted, fine to medium, quartzofeldspathic sands (terrigenous facies) occur on beaches and outer sandy slopes close to wadi mouths. Backreef areas exhibit relatively well-sorted fine sands of terrigenous-coral and Miliolidae-Soritidae facies. Poorly sorted, coarse sands of coral-coral-line algal and Homotremid facies characterize reef flats and the upper parts of coral-built forereef areas, which respecitively display an Amphistegina-Spirolina subfacies and an Acervulina one. Poorly sorted, medium sands of coralmolluscan-foraminiferal (Amphistegina-Acervulina) facies are restricted to the lower parts of the forereef zone. Latcral limits of the various biofacies coincide with the distribution of the related organic communities.     

The influence of water quality and sediment geochemistry on the horizontal and vertical distribution of phosphorus and nitrogen in sediments of a large,shallow lake

Distinct horizontal water column concentration gradients of nutrients and chlorophyll a (Chl a) occur within large, shallow, eutrophic Lake Taihu, China. Concentrations are high in the north, where some of the major polluted tributaries enter the lake, and relatively low in the south, where macrophytes generally are abundant. It is not clear, however, whether these water column concentration gradients are similarly reflected in spatial heterogeneity of nutrient concentrations within the bottom sediments. The main objective of this study was therefore to test if horizontal and vertical variations in the phosphorus and nitrogen content in bottom sediments of Lake Taihu are significantly related to (1) horizontal variations in overlying water column nutrient concentrations and (2) other sediment geochemical constituents. We measured the concentration of total phosphorus (TP) and total nitrogen (TN) in surficial sediments (0–2 cm) and TP, TN and Chl a concentrations in water column samples, collected from 32 sites in 2005. In 2006 sediment, TP, TN, carbon, iron and manganese concentrations were measured vertically at 2 cm intervals, extending to a depth of approximately 20 cm, at an additional eight sites. Linear correlation analysis revealed that surficial sediment TP concentrations across the 32 stations were related significantly, though weakly, to annual mean water column concentrations of TP, TN as well as Chl a. Correlations of surficial sediment TN with water column variables were, however, not significant (P > 0.05). Amongst the geochemical variables tested, the vertical variability of sediment TP concentrations was most strongly related to sediment manganese and carbon concentrations. A multiple stepwise linear regression revealed that the combination of sediment manganese and carbon concentrations explained 91% of the horizontal variability in sediment TP concentrations and 65% of the vertical variability. Handling editor: Luigi Naselli-Flores     

The geological effects of hurricanes on coral reefs and the interpretation of storm deposits

Hurricanes occur in belts 7° to 25° north and south of the equator. Reefs growing in these belts suffer periodic damage from hurricane-generated waves and storm surge. Corals down to 20m depth may be broken and removed, branching colonies being much more susceptible to breakage than upright massive forms. Sand cays may be washed away and former storm ridges may migrate to leeward across reef flats to link with islands. Reef crest and reef front coral debris accumulate as talus at the foot of the fore-reef slope, on submarine terraces and grooves, on the intertidal reef flat as storm ridges of shingle or boulders and isolated blocks of reef framework, as accreting beach ridges of leeward migrating shingle, as lobes and wedges of debris in back-reef lagoons, as drapes of carbonate sand and mud in deep off-reef locations in the fore-reef and lagoonal areas. In addition to the coarse debris deposited, other features may aid the recognition of former hurricane events, including the assemblage of reef biota, its species composition and the structure of the skeletons; graded internal sediments in framework cavities; characteristic sequences of encrusting organisms; characteristic shapes of reef flat microatoll corals; and submarine cement crusts over truncated reef surfaces. The abundance of reef flat storm deposits whose ages cluster around 3000–4000 y BP in certain parts of the world most likely relate to a slight fall in relative sea level rather than an increase in storminess during that period. A higher frequency of storms need not result in more reef flat storm deposits. The violence of the storm relative to normal fair-weather conditions influences the extent of damage; the length of time since the previous major storm influences the amount of coral debris created; the length of time after the hurricane, and before a subsequent storm influences the degree of stabilization of reef-top storm deposits and hence their chances of preservation.     

Meiofauna sediment relations in leeward slope turf algae of Heron Island reef

As part of studies investigating the influence of grazers on reef meiofauna, we assessed the density, composition and richness of meiofauna (retained on a 100-μm sieve) on the leeward reef slope of Heron Reef, GBR, Australia using an airlift vacuum sampling device. Estimates of meiofauna densities ranged between 40 individuals 10 cm⁻² and 290 individuals 10 cm⁻², which is considerably lower than many estimates from carbonate sediments and hard substrates from other reefs and marine habitats. The 17 taxa of meiofauna were dominated by harpacticoid copepods (40%) and nematodes (32%). Varying sediment load within algal turfs explained 37% of variation of meiofauna density. A model is proposed in which increased shelter afforded by high living coral cover reduces meiofaunal losses from grazing and increases sediment loads, balanced by areas of low coral cover in which sedimentation rates are lower and grazing rates higher. At none of the four sites did major differences in abundance occur between November and March sampling events. Together these observations suggest that epilithic meiofaunal communities are generally spatially and temporally predictable at small scales in this reef system, indicating that their ecological services are similarly conservative. Handling editor: I. Nagelkerken     

Wave-current interactions on a shallow reef (Nicaragua,Central America)

Measurements of wave height and currents associated with normal trade-wind conditions have been made on a linear reef that parallels the northern and northeastern coast of Great Corn Island, eastern shelf of Nicaragua, Central America. Analyses indicate that waves breaking over the reef crest generate lagoonward flow normal to the reef. Average reef-normal flow was in the range of 10 to 20 cm/s; however, individual wave surges reached values of up to 180 cm/s. The strength of the over-the-reef flow is modulated by the tide. Lagoon currents are weak (2–5 cm/s) and change direction with the tide as the lagoon fills and drains. Long-period oscillations in water level (30 s to 20 min) and in the current were observed, and may be important in transporting fine-grained sediments out of the reef-lagoon system. Strong, short-duration surge currents ( <5 s) transport coarse sediment from the breaker zone to the seaward margin of the backreef lagoon.     

An abrupt backreef infilling in a Holocene reef, Paraoir, Northwestern Luzon, Philippines

We describe a sudden backreef infilling at the west coast of Luzon, Philippines, which occurred after 324 ± 12 yr ago (year BP, before 1950 AD). Results of 30 ²³⁰Th-dated fossil corals from the surface and 5 cores, 17–29.1 m in length, recovered from a Holocene reef at Paraoir show that the reef flat developed in two stages. The reef margin is dated at 10,256 ± 50 (2σ) yr BP at 23.9 m below mean sea level (MSL) and about 6,654 ± 29 yr BP at 3.7 m below MSL with ages increasing with depth. The reef flat was formed with sediments of 818–324 yr BP old, which do not follow an age–depth correlation. The evidence suggests that a backreef moat remained empty throughout the buildup of the reef for about 6 kyr and was filled abruptly with a 26-m-thick succession of rubble and bioclastics by an extreme wave event (EWE) after 324 ± 12 yr BP. Field evidence, historical records, and tsunami simulation suggest the EWE sedimentation was likely caused by a single severe tropical cyclone, although the possibility of tsunami is not ruled out. The Paraoir reef flat was built up in a mode different from previously reported cases of Holocene reefs.     

Untersuchungen zum Energiefluß in Sedimenten der Kieler Bucht

Over a course of one year total oxygen consumption of sediment cores was determined for the period of one week in a laboratory flow-through system. The cores were collected during different months of the year and at two localities. One of the areas of investigation, at about 1 m water depth near the beach, had a carbon demand of 160 g C/m²/year. The other one, at a water depth of 11 m–a very heterogeneous sediment with a large gravel fraction — had a carbon demand of 100 g C/m²/year. As additional parameters for characterization of the sediments and interpretation of the determined metabolic rates particulate organic carbon and nitrogen, ATP, chlorophyll and pheopigments, bacterial counts and fauna composition were used. Both areas can be described as non detritus-accumulating sediments.     

Effects of nourishment on the form and function of an estuarine beach

Beach nourishment programs in estuaries can enhance shore protection, but they decrease habitat suitability by creating higher berms and wider backshores than would occur under natural conditions. Use of sediment sources from outside the area can result in sedimentary characteristics that differ from native sediments on the surface and at depth, altering conditions for both aeolian transport to dunes and interstitial fauna. Field data were gathered on an estuarine beach to determine differences in beach profile change, depth of sediment reworking, and potential for aeolian transport due to nourishment. Data were gathered over a 20-month period 6 months prior to nourishment, 3 days after nourishment, 6 months after nourishment, and 14 months after nourishment when the beach was mechanically graded to eliminate a vertical scarp in the foreshore. The nourishment consisted of 87,900 m³ of sediment emplaced to create a 1.34-km-long, 30-m-wide berm 2.3 m above mean tide level. Seven percent of the fill was removed from the profile within 6 months after nourishment, accompanied by 7 m in horizontal retreat of the artificial berm. The fill on the backshore remained above the zone of wave influence over a winter storm season and was separated from the active foreshore by the scarp. Nourished sediments on the intertidal foreshore were significantly different from native sediments to a depth of 0.20 m below the surface. A lag surface of coarse sediment formed by deflation on the backshore, resulting in a rate of aeolian transport <2% of the rate on the wave-reworked foreshore.Nourishing a beach to a level higher than would be created by natural processes can create a profile that compartmentalizes and restricts transport of sediment and movement of fauna between the foreshore and backshore. Mechanical grading can eliminate the scarp, allow for faunal interaction, and reestablish wave reworking of the backshore that will facilitate aeolian transport. Using an initial design to nourish the backshore at a lower elevation and allowing a dune to provide protection against flooding during major storms could prevent a scarp from forming and eliminate the need for follow-up grading.     

Distribution of gravel-sized empty tests of large benthic foraminifers as practical depositional indicators in tropical reef and shelf carbonate environments

To explore the utility of gravel-sized tests of large benthic foraminifers (LBFs) as practical paleoenvironmental indicators of tropical reef and shelf carbonate environments, depth and spatial distributions of gravel-sized empty tests of LBFs were examined using 39 surface sediment samples collected from depths shallower than 200 m off the west coast of Miyako Island (Ryukyu Islands, northwest Pacific). Distributions of the LBF tests were mainly related to water depth, topography, and substrate type. Q-mode cluster analysis based on the binary (presence/absence) data of LBF associations (4–2-mm size fraction) clearly delineates four depositional environments: bay, back reef to fore reef, flat shelf, and shelf slope. Application of this modern dataset to fossil LBF data from larger foraminiferal limestones of the Pleistocene Ryukyu Group indicate that a test section was deposited in an outer flat shelf at depths between 54 and 99 m. Comparisons of these results with previous reports suggest that our foraminiferal analysis using gravel-sized tests is methodologically easier than conventional analyses including smaller sized tests to distinguish similar levels of depositional environments. However, taxonomic and environmental similarities make the applicability of this dataset to fossil LBF data from Quaternary tropical carbonate environments in the northwest Pacific.     

Deposition and cycling of carbon and nitrogen in carbonate mud of the lagoons of Arlington and Sudbury Reefs, Great Barrier Reef

The dynamics of carbon and nitrogen in carbonate mud were examined in the lagoons of Arlington and Sudbury Reefs, Great Barrier Reef. Most (89–93%) of the organic carbon and total nitrogen depositing to the carbonate mud zones was mineralized over a sediment depth of 1 m, with ∼50% of CO₂ produced during microbial decomposition involved in carbonate precipitation/dissolution reactions. There was proportionally little burial of organic carbon (10–11%) or nitrogen (7–10%). Nitrogen budgets suggest rapid turnover of porewater inorganic N pools on the order of hours to a few days. Incubation experiments indicate carbonate dissolution in surface deposits (≤20 cm depth) and carbonate precipitation in deeper sediments. Depth-integrated reaction rates indicate net carbonate precipitation of 7–10 mol CaCO₃ m² year⁻¹ over a depth of 1 m. Budget calculations at the whole-reef scale imply that deposition of CaCO₃ in the mud zones of both lagoons may equate to 50–90% of total reef carbonate production, with organic carbon fluxes equating to nearly all net primary production on each reef. These biogeochemical estimates point to the functional importance of carbonate mud zones in the lagoons of the shelf reefs of the Great Barrier Reef.