Skeletal Porosity Research Articles

Quantifying attributes of boring bivalve populations in corals using micro-computed tomography

Bioerosion plays a crucial factor in shaping the structure and function of coral reef ecosystems, with bioeroders actively altering both the physical and ecological dynamics of coral substrates. Despite their importance, studying internal bioeroders in corals presents significant challenges owing to their cryptic nature within the skeletal structures. Additionally, invasive methods are often required to reveal the subtle and microscopic bioerosive alterations they induce in calcium carbonate substrates. Here, we demonstrate the effectiveness of high-resolution micro-computed tomography (μCT) in quantifying the abundance, size, distribution, and growth directions of coral bioeroders such as cryptic calcareous bivalves in the northern Red Sea. We scanned three coral species inhabited by bioeroders, followed by the utilization of three-dimensional image analysis software to identify, count, and measure each bivalve within the coral skeleton, along with quantifying boring cavity volumes. We revealed that μCT captures small boring cavities (< 1mm), providing more accurate abundance estimates of live and dead boring bivalves than the skeleton decalcification technique, with the added benefits of being rapid and non-destructive in contrast to traditional methods. Furthermore, measurements of empty cavity volumes enabled the estimations of the contribution of bioeroders to the overall coral skeletal porosity. Overall, our study highlights μCT as a practical and effective tool for studying cryptic coral bioeroders, providing novel ecological insights into bioeroder population ecology and coral-bioeroder interactions.

EFFECT OF GRAIN SHAPE ON BEDLOAD TRANSPORT OF CORAL GRAVELS UNDER TURBULENT FLOW

Coral islands with low-lying accumulations of wave-deposited carbonate bioclasts are suffering significant erosion and flooding threats due to sea level rise and wave climate changes. Properties of coral gravels are distinct from natural quartz sands in its larger skeletal porosity and more irregular shape (de Kruijf et al., 2021). This study aims to investigate how the particle shape affects bedload transport process of coral gravels under turbulent flow by means of laboratory experiments, which can provide insights for better predictive skills of morphology change of coral islands.

Impacts of Particle Shape, Skeletal Porosity and Density on the Settling Velocity of Gravel‐Size Coral Debris

AbstractKnowledge of the hydrodynamic behavior of carbonate bioclasts is essential for solving coastal morphodynamic problems in marine carbonate depositional environments. Marine carbonate grains are produced within the sedimentary system itself and are deformed by subsequent hydrodynamics, resulting in particle densities, sizes and shapes that are distinct from those of terrigenous sediments. In this study, we investigated the shapes and settling characteristics of gravel‐size natural coral debris collected from the coasts of Okinawa Prefecture, Japan. Based on the shape parameters and settling velocities obtained using image analysis techniques and settling tube experiments, respectively, a correlation model for predicting the settling velocity of coral gravel particles was developed by extending the previous formula of Dietrich (1982, https://doi.org/10.1029/wr018i006p01615) over particle Reynolds number values ranging from 384.23 to 4439.76. The model determines the settling velocity as a function of particle nominal diameter, form, roundness, and surface texture parameters that were selected after examining several other shape descriptors. Compared with other models, the proposed model showed better settling velocity predictive skills for coral gravel particles classified into four shape types. The proposed model also determines how the drag coefficient of coral gravel particles should be modified from that for spherical particles, by accounting for the abovementioned coral gravel particle shape parameters. The obtained data set of settling velocity and modification of the drag coefficient can be used for quantitative interpretation of suspension, transport, and sedimentation processes in marine carbonate depositional environments.

Coral micro- and macro-morphological skeletal properties in response to life-long acclimatization at CO2 vents in Papua New Guinea

This study investigates the effects of long-term exposure to OA on skeletal parameters of four tropical zooxanthellate corals naturally living at CO2 seeps and adjacent control sites from two locations (Dobu and Upa Upasina) in the Papua New Guinea underwater volcanic vent system. The seeps are characterized by seawater pH values ranging from 8.0 to about 7.7. The skeletal porosity of Galaxea fascicularis, Acropora millepora, massive Porites, and Pocillopora damicornis was higher (up to ~ 40%, depending on the species) at the seep sites compared to the control sites. Pocillopora damicornis also showed a decrease of micro-density (up to ~ 7%). Thus, further investigations conducted on this species showed an increase of the volume fraction of the larger pores (up to ~ 7%), a decrease of the intraskeletal organic matrix content (up to ~ 15%), and an increase of the intraskeletal water content (up to ~ 59%) at the seep sites. The organic matrix related strain and crystallite size did not vary between seep and control sites. This multi-species study showed a common phenotypic response among different zooxanthellate corals subjected to the same environmental pressures, leading to the development of a more porous skeletal phenotype under OA.

Linking Internal Carbonate Chemistry Regulation and Calcification in Corals Growing at a Mediterranean CO2 Vent

Corals exert a strong biological control over their calcification processes, but there is a lack of knowledge on their capability of long-term acclimatization to ocean acidification. We used a dual geochemical proxy approach to estimate the calcifying fluid pH (pHcf) and carbonate chemistry of a Mediterranean coral (Balanophyllia europaea) naturally growing along a pH gradient (range: pHTS 8.07-7.74). The pHcf derived from skeletal boron isotopic composition (11B) was 0.3-0.6 units above seawater values and homogeneous along the gradient (mean  sem: Site 1 = 8.39  0.03, Site 2 = 8.34  0.03, Site 3 = 8.34  0.02). Also carbonate ion concentration derived from B/Ca was homogeneous (mean  sem (µmol kg-1): Site 1 = 579  34, Site 2 = 541  27, Site 3 = 568  30) regardless of seawater pH. Furthermore, gross calcification rate (GCR, mass of CaCO3 deposited on the skeletal unit area per unit of time), estimated by a “bio-inorganic model” (IpHRAC), was homogeneous with decreasing pH. The homogeneous GCR, internal pH and carbonate chemistry confirm that the features of the ‘building blocks’ – the fundamental structural components - produced by the biomineralization process were substantially unaffected by increased acidification. Furthermore, the pH up-regulation observed in this study could potentially explain the previous hypothesis that less ‘building blocks’ are produced with increasing acidification ultimately leading to increased skeletal porosity and to reduced net calcification rate obtained by including the total volume of the pore space. In fact, assuming that the available energy at the three sites is the same, this energy at the low pH sites could be partitioned among fewer calicoblastic cells that consume more energy given the larger difference between external and internal pH compared to the control, leading to the production of less building blocks (i.e., formation of pores inside the skeleton structure, determining increased porosity). However, we cannot exclude that also dissolution may play a role in increasing porosity. Thus, the ability of scleractinian corals to maintain elevated pHcf relative to ambient seawater might not always be sufficient to counteract declines in net calcification under ocean acidification scenarios.

The problem of melting ice in a porous medium saturated with ice and gas

The problem of ice formation in a dry, cold, porous medium saturated with ice and gas (air) when pumping warm water is considered in a flat one-dimensional self-similar formulation. The task was considered in volume area. During the injection of warm water from the beginning deep into the reservoir, it spread in a volume region that will divide the reservoir into 3 zones. The first zone was filled with water, the second zone was filled with ice and water, and the third zone was filled with ice and gas. To describe the process of heat and mass transfer, the following hypotheses were used: the temperature of the saturated substance (water, ice or gas) is equal to the temperature of the porous medium; ice and skeleton still; water, ice and skeleton of the reservoir are incompressible; skeletal porosity is constant. On the basis of constructed self-similar solutions, a numerical analysis was performed illustrating the effect of the initial parameters of a dry porous medium saturated with ice and gas, as well as the temperature of the injected water on the temperature and pressure distribution in the porous medium. It has been established that an increase in the temperature of the injected water does not lead to a significant increase in the area of ice decomposition. It is also established that if the pressure of the injected water is increased, this will not lead to a large increase in the area of ice decomposition. However, based on the results obtained, it can be seen that the speed of movement of the melting boundary increases, in particular, as the pressure increases by <i>p<sub>e</sub></i>=0.05 MPa, the intermediate region increases by one and a half times. It was found that it is economically more profitable to pump water with a lower temperature, because water with a higher temperature slightly increases the freezing area of the porous soil.

Annual response of two Mediterranean azooxanthellate temperate corals to low-pH and high-temperature conditions

Ocean acidification (OA) and warming related to the anthropogenic increase in atmospheric CO2 have been shown to have detrimental effects on several marine organisms, especially those with calcium carbonate structures such as corals. In this study, we evaluate the response of two Mediterranean shallow-water azooxanthellate corals to the projected pH and seawater temperature (ST) scenarios for the end of this century. The colonial coral Astroides calycularis and the solitary Leptopsammia pruvoti were grown in aquaria over a year under two fixed pH conditions, control (8.05 pHT units) and low (7.72 pHT units), and simulating two annual ST cycles, natural and high (+3 °C). The organic matter (OM), lipid and protein content of the tissue and the skeletal microdensity of A. calycularis were not affected by the stress conditions (low pH, high ST), but the species exhibited a mean 25 % decrease in calcification rate at high-ST conditions at the end of the warm period and a mean 10 % increase in skeletal porosity under the acidified treatment after a full year cycle. Conversely, an absence of effects on calcification and skeletal microdensity of L. pruvoti exposed to low-pH and high-ST treatments contrasted with a significant decrease in the OM, lipid and protein content of the tissue at high-ST conditions and a 13 % mean increase in the skeletal porosity under low-pH conditions following a full year of exposure. This species-specific response suggests that different internal self-regulation strategies for energy reallocation may allow certain shallow-water azooxanthellate corals to cope more successfully than others with global environmental changes.

Gains and losses of coral skeletal porosity changes with ocean acidification acclimation.

Ocean acidification is predicted to impact ecosystems reliant on calcifying organisms, potentially reducing the socioeconomic benefits these habitats provide. Here we investigate the acclimation potential of stony corals living along a pH gradient caused by a Mediterranean CO2 vent that serves as a natural long-term experimental setting. We show that in response to reduced skeletal mineralization at lower pH, corals increase their skeletal macroporosity (features >10 μm) in order to maintain constant linear extension rate, an important criterion for reproductive output. At the nanoscale, the coral skeleton's structural features are not altered. However, higher skeletal porosity, and reduced bulk density and stiffness may contribute to reduce population density and increase damage susceptibility under low pH conditions. Based on these observations, the almost universally employed measure of coral biomineralization, the rate of linear extension, might not be a reliable metric for assessing coral health and resilience in a warming and acidifying ocean.

Morphological plasticity of the coral skeleton under CO2-driven seawater acidification.

Ocean acidification causes corals to calcify at reduced rates, but current understanding of the underlying processes is limited. Here, we conduct a mechanistic study into how seawater acidification alters skeletal growth of the coral Stylophora pistillata. Reductions in colony calcification rates are manifested as increases in skeletal porosity at lower pH, while linear extension of skeletons remains unchanged. Inspection of the microstructure of skeletons and measurements of pH at the site of calcification indicate that dissolution is not responsible for changes in skeletal porosity. Instead, changes occur by enlargement of corallite-calyxes and thinning of associated skeletal elements, constituting a modification in skeleton architecture. We also detect increases in the organic matrix protein content of skeletons formed under lower pH. Overall, our study reveals that seawater acidification not only causes decreases in calcification, but can also cause morphological change of the coral skeleton to a more porous and potentially fragile phenotype.

PETROPHYSICS OF LOWER CRETACEOUS PLATFORM CARBONATE OUTCROPS IN PROVENCE (SE FRANCE): IMPLICATIONS FOR CARBONATE RESERVOIR CHARACTERISATION

High resolution petrophysical analyses were carried out on Urgonian (Lower Cretaceous) carbonates from outcrops in Provence, SE France. Porosity and permeability were measured on 541 plug samples selected from grain‐supported carbonates analogous to those in the age‐equivalent Shu'aiba and Kharaib Formation reservoirs in the eastern Arabian Plate. The sampling strategy allowed property heterogeneities from centimetre to kilometre scales to be investigated, as well as correlations between porosity and permeability in several different reservoir rock types. Property spatial modelling sensitivity analyses were also undertaken.The relative abundance of microporosity, grain size and sedimentary‐diagenetic anisotropy were the main geological parameters which controlled the petrophysical characteristics of the grainstones studied. Increasing microporosity decreased permeability but resulted in an increase in the homogeneity of the reservoir rocks and therefore in their predictability. An increase in grain size, from fine sand to gravel, and in the amount of intergranular pores, enhanced permeability significantly but resulted in a decrease in the homogeneity (and therefore predictability) of the reservoir rock.At a plug scale, poro‐perm relationships are very good and can be used predictively for fine grainstones dominated by microporosity; but relationships are moderate to weak for coarse rudstones with mixed pore types, including intraskeletal pores. In grainstone units, weak sedimentary anisotropy, such as decametre‐scale cross‐bedding, did not prevent the prediction of the horizontal property distribution from vertical data over a few hundreds of metres. In these units, the lateral correlation of rock properties follows periodic variograms with a 7 m wavelength. The lateral distribution of properties in coarse‐grained and heterogeneous rudstones with complex pore types and intense sedimentary heterogeneities, such as channel structures, was however more difficult to predict from a vertical data set.Upscaling poroperm data from plug scale to reservoir scale is linear in the case of grainstones with intergranular microporosity, but is non‐linear in the case of skeletal rudstones with coarser pore types including skeletal porosity.

Development and Validation of Computational Fluid Dynamics Models for Prediction of Heat Transfer and Thermal Microenvironments of Corals

We present Computational Fluid Dynamics (CFD) models of the coupled dynamics of water flow, heat transfer and irradiance in and around corals to predict temperatures experienced by corals. These models were validated against controlled laboratory experiments, under constant and transient irradiance, for hemispherical and branching corals. Our CFD models agree very well with experimental studies. A linear relationship between irradiance and coral surface warming was evident in both the simulation and experimental result agreeing with heat transfer theory. However, CFD models for the steady state simulation produced a better fit to the linear relationship than the experimental data, likely due to experimental error in the empirical measurements. The consistency of our modelling results with experimental observations demonstrates the applicability of CFD simulations, such as the models developed here, to coral bleaching studies. A study of the influence of coral skeletal porosity and skeletal bulk density on surface warming was also undertaken, demonstrating boundary layer behaviour, and interstitial flow magnitude and temperature profiles in coral cross sections. Our models compliment recent studies showing systematic changes in these parameters in some coral colonies and have utility in the prediction of coral bleaching.

LA-ICP-MS analyses on coral growth increments reveal heavy winter rain in the Eastern Mediterranean at 9 Ma

Sediment particles incorporated into coral skeletons reflect variation in composition and amount of suspended material in ambient water during coral growth. They can be used to identify periods of enhanced storm frequency and associated freshwater discharge. Tortonian (Late Miocene) Porites corals from Crete (Aegean Sea, Eastern Mediterranean) show pronounced annual density bands in X-ray photographs. δ 18O compositional variability reflects the annual banding equivalent with a ~ 7 °C annual sea surface temperature (SST) cycle over a seven-year period. Fine sediment particles are concentrated in layers with skeletal porosity parallel to growth increments. Variations in the chemical composition of coral skeletons can result from changes in the environment. Therefore, variations in element concentrations with a spatial resolution of ~ 500 µm along a transect perpendicular to growth increments were measured using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Complementary X-ray diffraction, X-ray fluorescence and scanning electron microscope analytical techniques were applied in order to characterize the sediment particles. Besides Sr/Ca and U/Ca variability indicating SST seasonality, the alternation of layers of pure aragonite and layers of aragonite coated with sediment particles (e.g., kaolinite, montmorillonite, quartz) is reflected in systematic variations in major and trace element concentrations. This element pattern results from a seasonal environmental mechanism where periodically enhanced input of sediment particles into the coral reef environment causes high concentrations of non-lattice bound elements (e.g., Al, Mn, Mg). Potential sources for the non-lattice bound elements (i.e., elements related to the clay minerals) such as airborne Saharan dust, Nile-derived sediment suspension as well as volcanic ash fall can be excluded on the basis of chemical, mineralogical or geological evidence. In contrast, normalized element patterns indicate that the clay minerals represent weathering products of local rocks, e.g., ophiolites, which at present crop out on Crete in mountainous areas, and formed islands exposed to erosion during the Late Miocene. High concentrations of the non-lattice bound elements correlate with low SSTs of the winter months. The discrepancy in SST estimations between δ 18O and Sr/Ca seasonality suggests that fresh water originating from intense winter rain caused a reduction of the annual δ 18O amplitudes and transported suspended material to the near-shore reef. Thus, we relate the seasonality in non-lattice bound element concentrations to climate variations in the Eastern Mediterranean region during the Late Miocene. Our results indicate that the present-day Mediterranean-type climate with a strong seasonality of precipitation and heavy winter rainfall events may have occurred at least temporarily during the Late Miocene at ~ 9 Ma in the Eastern Mediterranean.

Taphonomic signatures of turbid-zone reef development: Examples from Paluma Shoals and Lugger Shoal, inshore central Great Barrier Reef, Australia

Numerous coral reefs and coral communities occur along the nearshore regions of the central Great Barrier Reef (Australia) coastline. These develop under conditions of episodically high, wave-induced sediment resuspension and turbidity, and terrigenoclastic sediment accumulation. Cores recovered from two such reefs at Paluma Shoals and Lugger Bay are used to assess the taphonomic features and processes associated with framework development in such settings. Reef framework development is dominated by the accumulation of mainly rubble coral and isolated in situ coral colonies. The reef fabric varies from clast- to matrix-supported in character and the matrix comprises poorly sorted mixed terrigenoclastic/carbonate sediments. Taphonomic signatures from the different reef sites and facies show little variability and thus a distinct suite of taphonomic criteria, indicative of nearshore, turbid-zone reef development, are delineated. The degree of both calcareous encrustation and internal bioerosion exhibits a high degree of between-clast variability and many clasts are preserved in near pristine condition; where present calcareous encrustation is restricted to thin, isolated crusts comprising mainly of the coralline algae Lithoporella or Lithothamnion, and isolated serpulid worms and cemented bivalves. No biological binding of framework constituents occurs. Bioerosion is dominated by traces of Entobia sp. (sponge borings) and, locally, Gastrochaenolites sp. (bivalve traces), although only a limited suite of bioerosion trace morphologies is present. There is little or no cementation of the reef framework and only isolated acicular aragonite or peloidal micrite cements occur within intra-skeletal cavities. Significant occlusion of primary and secondary skeletal porosity by uncemented, fine-grained, mainly terrigenous sediment is, however, common throughout the reef sequence. The taphonomic criteria identified all have good preservation potential and should be readily identifiable within core sections. Such criteria thus have potential for aiding the recognition of reefs that either developed under similar environmental conditions of high turbidity and terrigenous-sediment accumulation, or which were subject to similar conditions during more short-lived phases of reef accretion.

Hydrodynamic behaviour of Nummulites: implications for depositional models

Large benthic foraminifers are considered to be good indicators of shallow marine carbonate environments in fossil series. Over the last 50 years, the palaeoenvironment of Tertiary Nummulites accumulations has been a matter of debate, particularly because of difficulties in interpreting these deposits, and in this way, the absence of analogues in present-day seas does not help. The aim of this paper is to insight the different ways Nummulites tests and clasts may accumulate according to their hydrodynamic behaviour. Based on experimental measurements and on SEM observations, it appears that the high primary skeletal porosity of Nummulites made them easily transportable. The calculated threshold shear velocities confirm that large-sized Nummulites can be moved by weak wave-driven currents. This peculiar hydrodynamic behaviour of Nummulites could explain the diversity of depositional models. Depending on local hydrodynamic conditions, autochthonous Nummulites deposits can be preserved as in situ winnowed bioaccumulations or be accumulated offshore, onshore or alongshore, away from the original biotope.

Skeletal micro-density, porosity and bulk density of acroporid corals

Users of the buoyant weighing technique to measure growth in scleractinian corals have often assumed a specific gravity of coral skeletal material equivalent to that of pure aragonite (2.94 g cm −3). Previous studies have demonstrated that this assumption is not valid for some taxa, although data are very limited. In this study, density of the carbonate skeleton, here termed micro-density, is determined for eight species of Acropora corals from three sites in Australia, using Archimedean principles. Micro-density ranged from averages per species of 2.781 to 2.873 g cm −3. Significant differences were recorded between some species, and between some colonies of the same species, but micro-density was relatively constant within a colony and for a single species between sites. Micro-density was below the value for pure aragonite by 2–6%, indicating that variation in this parameter must be accounted for when comparing growth and porosity measurements of coral skeletons obtained from buoyant weight methods. Possible explanations for the deviation from the specific gravity of pure aragonite are the variable inclusion of low-density organic material into the skeletons, and/or variation in the chemistry and micro-architecture of the skeleton. Skeletal bulk density and porosity were calculated for branch tips from a range of Acropora species. Precision of the porosity measurement for dried specimens was reduced in the initial trials by the potential for coral skeletons to retain air in skeletal voids, introducing errors into the measurement of skeletal matrix volume. A modification of existing techniques to calculate porosity on dried samples is recommended, including soaking in acetone to displace air from skeletal voids and to remove waxy residues, and vacuum infiltration by water. Skeletal bulk density of Acropora branch tips ranged from 0.86 to 1.68 g cm −3 and porosity ranged from 33 to 70%. There was a good correlation between skeletal porosity calculated using displacement techniques and the same parameter measured from image analysis of branch sections. The use of Archimedean principles to determine density is not new to coral biology, and yet there has been no published standard methodology. The methods described permit precise and accurate quantification of bulk density, micro-density and porosity, using basic laboratory equipment, which may provide a means of investigating the effects of environmental alterations such as elevated dissolved nutrient levels.

The Role of Skeletal Porosity in Aragonite Neomorphism--Strombus and Montastrea from the Pleistocene Key Largo Limestone, Florida

ABSTRACT The neomorphism of aragonite to calcite involves complex textural and chemical alterations that are in part controlled by the degree of communication between diagenetic and ambient formation fluids. Two taxa, Montastrea annularis and Strombus gigas, from the vadose zone of the Pleistocene Key Largo Formation in southern Florida, were examined in order to evaluate the effect of initial skeletal porosity on system closure during neomorphism. Fabrics of replacement calcite in both taxa are similar, consisting of blocky crystals largely restricted to areas of former skeletal aragonite. Oriented aragonite and organic inclusions result in the preservation of relic fabrics in both taxa. The chemical signatures of coralline and molluscan aragonite and calcite are strongly dependent on initial skeletal porosity. Compared to original aragonite, Montastrea calcite is enriched in magnesium (1,151 ± 64 ppm versus 2,114 ± 619 ppm) and depleted in strontium (7,397 ± 376 ppm versus 2,510 ± 43 ppm). These data, in conjunction with large calcite oxygen depletions (-3.8 ± 0.2% versus -5.7 ± 0.3%) and variable calcite carbon depletions (-0.6 ± 0.6% versus -1.3% to -7.3%), demonstrate significant exchange between neomorphic front and formational fluids. In contrast, Strombus calcite composition is largely inherited from original aragonite and is only slightly enriched in strontium and magnesium (2,299 ± 316 ppm versus 2,519 ± 759 ppm and 96 ± 23 ppm versus 173 ± 79 ppm), while carbon exhibits a small depletion (+5.3 ± 0.5% versus +4.4 ± 0.8%). Similarity of Str mbus aragonite and calcite compositions reflects high rock/water ratios in relatively closed diagenetic systems. Calcite oxygen depletion (-0.5 ± 0.5% versus -3.6 ± 0.4%) observed in Strombus, however, requires some exchange between formation and neomorphic front fluids. Differences in the thickness of replacement calcite rims between taxa reflect variable skeletal porosities and rates of migration of neomorphic fronts. Neomorphism was six times faster (by volume) in porous Montastrea than in dense Strombus skeletal structures. Thus, skeletal porosity influences degrees of system openness during aragonite neomorphism.

Osteoporosis in postmenopausal women

Eleven postmenopausal complete denture patients participated in a study to evaluate some possible predictors of osteoporosis. Most participants in the study reported a low caloric intake and consumed considerably less than the recommended daily allowances of sodium, cholesterol, calcium, fluoride, magnesium, zinc, and folic acid. Many participants in the study were taking additional daily vitamin and mineral supplements. The CCT as measured on radiographs of the second phalynx of the fifth digit of the right hand correlated linearly with the CBD corrected for soft tissue. Panoramic radiographs revealed that all individuals had severe residual ridge resorption. All serum calcium and phosphorus means were within the normal range, while more than 60% of the patients had below normal plasma levels of 25-hydroxyvitamin D. In conclusion, although based on a small sample, it appears that the diet of elderly women in New York is somewhat deficient for adequate skeletal homeostasis. Ideally, the vitamin D status of each patient should be determined and proper supplements prescribed. However, the high cost of analysis suggests that dietary analysis be used on a selected but more frequent basis. Radiation techniques for measuring skeletal porosity are also too complex to perform on a routine basis and should, like dietary analysis, be reserved for patients in whom other clinical signs and symptoms indicate metabolic bone disease.

Selective solution of macroinvertebrate calcareous hard parts: a laboratory study

Ten suites of 16 common types of invertebrate hard parts were placed in acid baths for 24 hours to determine relative rates and common styles of dissolution. Skeletal mineralogies included aragonite and both high-magnesium and low-magnesium calcite. Hard parts included barnacle cxoskelctons, cchinoid tests, gastropod opercula and gastropod and bivalve shells. Calcitic barnacle plates dissolved most rapidly, aragonitic and high magnesium calcitic hard parts showed intermediate rates, and the calcitic shells of the oyster dissolved at the lowest rate. The surface area to weight ratio of the hard parts correlated (r=0.650) significantly with the hard part's rate of dissolution. Skeletal remains with a high surface area to weight ratio dissolved faster than those with a low surface area to weight ratio. Skeletal porosity and mineralogy appeared to be responsible for additional variation in the rate of dissolution. The effect of the surface area to weight ratio is sufficient to overcome the effect of mineralogy. Dense, compact aragonitic hard parts can persist longer than porous, thin calcitic remains. Typical features associated with skeletal degradation include development of chalky textures, thinning of distal margins, surface etching and formation of holes in bivalve muscle scars. Such features may aid in the recognition of partial dissolution of skeletal remains in the rock record. □Taphonomy, paleoecology, fossil-diagenesis.

Mineralogy and chemistry of the lower Pennsylvanian Kendrick fauna, eastern Kentucky, U.S.A.: 3. Diagenetic and paleoenvironmental analysis

The preservation of metastable calcite and aragonite of the Kendrick Shale Member fauna is related to the impervious nature of the coal and possibly the shale units of the lower Pennsylvanian Breathitt Formation. These units prevent the circulation of diagenetic meteoric water through the formation, and interaction with the metastable carbonate components of the Kendrick marine beds. Compaction and cementation are the only major diagenetic processes determined for the Kendrick shale and its fauna. The interstitial marine water expelled during compaction of the Breathitt Formation is responsible for the minor post-compaction cementation of the shale and occlusion of primary pore space in fossil components. This cement has little effect on the chemistry and mineralogy of the low-porosity brachiopod and mollusc shells. However, the high-Mg calcite crinoids and intermediate-Mg calcite rugose corals are affected by the cement due to their high skeletal porosity. The incorporated calcite cement has altered the original elemental chemistry, increased the δ 18O and δ 13C and slightly altered the mineralogy. The average water temperature calculated for Kendrick seawater is 26°C, with a probable yearly range of ∼ 17–35°C. These temperatures are complemented by normal salinity conditions with, at times, low salinity. In addition, Kendrick seawater was slightly euxinic during deposition of the marine shale and its fauna.

Mechanical properties of coral skeleton: compressive strength and its adaptive significance

Measurement of the compressive strength and elastic modulus of the skeletal material of three common Caribbean corals suggests that the mechanical properties of coral skeleton are an important factor in the adaptive repertoire of these animals. The strength (stress at fracture) of the specimens tested is 12–81 meganewtons/meter2, with material from branched colonies being generally stronger than material from massive colonies. These values are lower than the strength of most other carbonate skeletal materials, but higher than that of carbonate engineering materials like concrete and limestone. The comparatively low strength of coral skeleton may be the result of architectural properties produced by the requirements of competing adaptive factors, such as polyp phototropism, or it may reflect the low probability that a colony will be broken, and therefore need to be stronger, before it achieves reproductive parity. The skeleton of the three species tested here is strongest when stress is applied parallel to the growth direction of the polyps. Strength varies inversely with skeletal porosity. Decreasing porosity in highly stressed colonies represents a potentially valuable adaptation for enhancing strength. The adaptive value of porosity modification may explain differences in porosity and strength between highly stressed branched growth forms and more moderately stressed massive growth forms. Boring organisms reduce the strength of coral skeleton by increasing its porosity. Only minor amounts of boring can produce strength reductions of up to 50%. Specialized, stress-minimizing branch arrangements help maximize resistance of coral structures to mechanical degradation in situations where colony size is unusually large or hydraulic energy dangerously high.