Aragonitic Skeletons Research Articles

XANES SPECTROSCOPY OF SULFUR IN EARTH MATERIALS

X-ray absorption near-edge structure (XANES) spectroscopy is the ideal non-destructive technique for characterizing and quantifying S species in compositionally complex natural materials such as silicate glasses and minerals, coals, asphalts and asphaltenes, kerogens and humic substances. Sulfur absorption edges represent the transition of S 1 s and 2 p core electrons to unoccupied antibonding orbitals at the bottom of the conduction band. Shifts in the position of the absorption-edge feature of S K - and L -edge XANES spectra constitute a chemical ruler for oxidation state of both inorganic and organic species of S; the S K edge shifts from 2469.5 eV for chalcopyrite (2− oxidation state) to 2482 eV for gypsum (6+). However, chemical state of S in Earth materials is most readily assigned by comparing the overall XANES profile with spectra for reference compounds. Sulfur XANES spectra are reviewed for pyrite, troilite, pyrrhotite and NiAs-type Co 0.923 S and Ni 0.923 S, niningerite (MgS), oldhamite (CaS), alabandite (MnS) and cubic FeS, and sphalerite and related phases, as well as for selected solid-solutions of the monosulfides. Sulfur XANES spectra for FeS, CoS, NiS, MgS, CaS, MnS and ZnS have been simulated by multiple scattering calculations. The S K -edge XANES of transition-metal-bearing monosulfides generally show anomalous absorption consistent with hybridization of the final S 3 p σ* antibonding states with empty 3 d orbitals on the metal atoms. Various applications of S K - and L -edge XANES fingerprinting are discussed, including speciation of inorganic S in basaltic glasses, lazurite and hauyne, identification of organic functional groups of S in coals, kerogens and humic substances extracted from subtropical soils and marine sediments, and the association of sulfated sugars with calcification of coral aragonite skeletons.

Carbonate mineralogy of free-living bryozoans (Bryozoa: Otionellidae), Otago shelf, southern New Zealand

On the open temperate shelf off Otago Peninsula, eastern South Island, New Zealand, in water depths ranging from 90 to 140 m, free-living motile bryozoans of the genus Otionellina survive in shifting quartzofeldspathic sands. Unlike most bryozoans, these free-living colonies characteristically precipitate skeletons of aragonite. Here, we report on skeletal carbonate mineralogy of 104 specimens of Otionellina from four different species, across the shelf, which are also in different stages of development. All were dominantly composed of aragonite, with only five specimens containing 0–15 wt.% calcite (but with an overall mean of <1 wt.%), with low to intermediate MgCO 3 content in the calcite (<5 wt.% MgCO 3). Mineralogy in Otionellina varies, but is not related to species, colony size, or environmental factors such as water temperature, salinity, water depth, and substratum. Otionellinids may lack the environmentally influenced biomineralisation pathways found in other carbonate-producing taxa. It appears that most free-living (or “vagrant”) bryozoans are aragonitic, although some rare examples contain up to 5% calcite; there may be no free-living genera which are always entirely aragonite. While preservation of aragonitic skeletons is by no means certain, the presence of free-living bryozoans indicates a sandy outer-shelf environment and a diagenetic history that allows for preservation of aragonite.

Distribution of magnesium in coral skeleton

Ion micro‐probe imaging of the aragonite skeleton of Pavona clavus, a massive reef‐building coral, shows that magnesium and strontium are distributed very differently. In contrast to strontium, the distribution of magnesium is strongly correlated with the fine‐scale structure of the skeleton and corresponds to the layered organization of aragonite fibers surrounding the centers of calcification, which have up to ten times higher magnesium concentration. This indicates a strong biological control over the magnesium composition of all structural components within the skeleton. Magnesium may be used by the coral to actively control the growth of the different skeletal crystal components.

Perspectives in carbonate paleoclimatology

Sedimentary carbonate is the most successful material in paleoclimatological researches. Researches in this field are now challenging to extract high-resolution and quantitative records of millennial-century scales in order to predict the climatic changes due to global warming. The most promising study material is a reef coral, which records temperature (and salinity) changes in its annually-laminated aragonitic skeleton. Data from coral climatology and annually-laminated polar ice cores would contribute to climate prediction although the information is geographically limited in only tropical and polar areas.For the information from temperate-subtropical climatic belts, carbonate paleoclimatology using speleothems is effective. Oxygen and carbon isotopic signatures of speleothems are now treated as possible proxies of water temperature and vegetation. Another promising material for the study on temperate-subtropical areas is a tufa, a calcitic deposit in freshwater environments of limestone areas. The tufa commonly shows annual laminations and advances to speleothems in its larger depositional rate (several millimeters per year) that allows higher analytical resolution. This study represents that rainfall events were recoded in a Japanese tufa deposit.

これからの炭酸塩古気候学

Sedimentary carbonate is the most successful material in paleoclimatological researches. Researches in this field are now challenging to extract high-resolution and quantitative records of millennial-century scales in order to predict the climatic changes due to global warming. The most promising study material is a reef coral, which records temperature (and salinity) changes in its annually-laminated aragonitic skeleton. Data from coral climatology and annually-laminated polar ice cores would contribute to climate prediction although the information is geographically limited in only tropical and polar areas.For the information from temperate-subtropical climatic belts, carbonate paleoclimatology using speleothems is effective. Oxygen and carbon isotopic signatures of speleothems are now treated as possible proxies of water temperature and vegetation. Another promising material for the study on temperate-subtropical areas is a tufa, a calcitic deposit in freshwater environments of limestone areas. The tufa commonly shows annual laminations and advances to speleothems in its larger depositional rate (several millimeters per year) that allows higher analytical resolution. This study represents that rainfall events were recoded in a Japanese tufa deposit.

How brain corals record climate: an integration of skeletal structure, growth and chemistry of Diploria labyrinthiformis from Bermuda

The aragonite skeleton of massive reef-building corals contains a record of the oceanic environment in which they grow. However, reading of the record requires understanding of how it is archived, a process complicated by the elaborate skeletal construction and seasonal growth patterns that characterize many species. In this study, we assess the utility of the massive brain coral Diploria labyrinthiformis as an archive of sea surface temperature (SST) variability in the western North Atlantic. In situ staining of live colonies combined with microscale analysis of skeletal chemistry indi- cate that D. labyrinthiformis grows throughout the year on Bermuda and records the full annual cycle of SST variability. However, skeleton accreted during the summer is overlain (thickened) by skeleton accreted during the subsequent fall and winter. As a result, conventional coarse sampling for δ 18 O enables seasonal δ 18 O cycles to be resolved but these do not capture the full amplitude of the annual SST cycle. Our data show that the shallow gradient of the δ 18 O-SST regression equation derived for D. labyrinthiformis (-0.113‰ oC -1 ) relative to the expected -0.22‰ oC -1 for marine skeletons results from dampening of the summertime peak in δ 18 O. In contrast, skeleton accreted during the winter is not thickened and wintertime δ 18 O captures the interannual wintertime SST variability at this site. Using SIMS ion microprobe to analyse strontium to calcium ratios (Sr/Ca), we avoided the thickening deposits and were able to resolve the full amplitude of the annual Sr/Ca cycle. The Sr/Ca-SST relationship obtained for D. labyrinthiformis (-0.0843 mmol/mol oC -1 ) corresponds to that derived from fast-growing tropical reef corals. X-ray intensity ratios, used as a proxy for skeletal density, reveal the expected seasonal changes associated with growth banding as well as variability on inter- annual and decadal timescales. These variations are well correlated with wintertime SST variability in the subtropical gyre and may be a valuable proxy thermometer for the North Atlantic.

High-resolution Sr/Ca records in sclerosponges calibrated to temperature in situ

Ratios of strontium to calcium have been analyzed by laser- ablation inductively coupled plasma-mass spectrometry (LA-ICP- MS) in a skeletal section of the sclerosponge Ceratoporella nichol- soni. The growth period, representative of 3 yr, was stained in the skeleton with a fluorochrome (calcein). Temperatures were record- ed at 2 h intervals within the shallow, cryptic reef enclosure that the sclerosponge inhabited on the northern coast of Jamaica, al- lowing the formulation of a direct empirical relationship between Sr/Ca and temperature. To verify this calibration, Sr/Ca ratios of two sclerosponges of the same species from depths of 67 m and 136 m in Exuma Sound, Bahamas, were analyzed by LA-ICP-MS and compared to the temperatures from these depths over a decade prior to collection. The result is an independently verified, high- resolution empirical calibration for the temperature sensitivity of Sr/Ca ratios in the aragonite skeletons of sclerosponges from Ja- maica and the Bahamas. The calibration is a first for C. nicholsoni and indicates that sclerosponges are more sensitive temperature recorders than zooxanthellate corals. It represents an important step in establishing skeletal geochemistry of sclerosponges as a proxy of temperature in the upper 250 m of the ocean.

Sr/Ca ratios and oxygen isotopes from sclerosponges: Temperature history of the Caribbean mixed layer and thermocline during the Little Ice Age

We investigate aragonitic skeletons of the Caribbean sclerosponge Ceratoporella nicholsoni from Jamaica, 20 m below sea level (mbsl), and Pedro Bank, 125 mbsl. We use δ18O and Sr/Ca ratios as temperature proxies to reconstruct the Caribbean mixed layer and thermocline temperature history since 1400 A.D. with a decadal time resolution. Our age models are based on U/Th dating and locating of the radiocarbon bomb spike. The modern temperature difference between the two sites is used to tentatively calibrate the C. nicholsoni Sr/Ca thermometer. The resulting calibration points to a temperature sensitivity of Sr/Ca in C. nicholsoni aragonite of about −0.1 mmol/mol/K. Our Sr/Ca records reveal a pronounced warming from the early 19th to the late 20th century, both at 20 and 125 mbsl. Two temperature minima in the shallow water record during the late 17th and early 19th century correspond to the Maunder and Dalton sunspot minima, respectively. Another major cooling occurred in the late 16th century and is not correlatable with a sunspot minimum. The temperature contrast between the two sites decreased from the 14th century to a minimum in the late 17th century and subsequently increased to modern values in the early 19th century. This is interpreted as a long‐term deepening and subsequent shoaling of the Caribbean thermocline. The major trends of the Sr/Ca records are reproduced in both specimens but hardly reflected in the δ18O records.

Geochemical Perspectives on Coral Mineralization

Corals open an exceptional window into many phenomena of geological, geochemical, climatic, and paleontological interest. From the Paleozoic to the present, corals provide some of the finest high-resolution archives of marine conditions. Corals are likewise exceptional for chronometric purposes, and even the terrestrial 14C timescale has now been calibrated against coral 230Th/234U. Corals also represent a testing ground for basic ideas about mineralogy and geochemistry. The shapes, sizes, and organization of skeletal crystals have been attributed to factors as diverse as mineral supersaturation levels and organic mediation of crystal growth. The coupling between calcification and photosynthesis in symbiotic corals is likewise attributed to everything from photosynthetic alkalinization of the water, to efforts by the coral to prevent photosynthetic alkalinization. Corals also leave a significant geochemical imprint on the oceans. Their aragonite skeletons accept about 10 times more strontium than does calcite, hence the proportion of marine aragonite precipitation affects the oceanic chemical balance. Biological carbonates represent the biosphere’s largest carbon reservoir, hence calcareous organisms affect the ocean’s pH, CO2 content, and ultimately global temperatures through the greenhouse gas connection. Finally, corals present some geochemical puzzles for ecology and conservation. How do symbiotic corals obtain nutrients in some of the most nutrient deficient parts of the planet? Are global geochemical changes partially responsible for the widespread declines in coral reefs during recent decades? We will address many of these issues, but will concentrate on coral skeletal structure and calcification mechanism. These topics bear most directly on the biomineralization process and generally affect the choice of skeletal materials and analytical techniques used in geochemical investigations. The coral reef is probably the planet’s most spectacular biomineralization product. These grand and complex ecosystems build on the accumulated skeletal debris of countless generations of organisms, especially calcareous algae and symbiotic …

Analysis of the mitochondrial 12S rRNA gene supports a two-clade hypothesis of the evolutionary history of scleractinian corals

Scleractinian corals have long been assumed to be a monophyletic group characterized by the possession of an aragonite skeleton. Analyses of skeletal morphology and molecular data have shown conflicting patterns of suborder and family relationships of scleractinian corals, because molecular data suggest that the scleractinian skeleton could have evolved as many as four times. Here we describe patterns of molecular evolution in a segment of the mitochondrial (mt) 12S ribosomal RNA gene from 28 species of scleractinian corals and use this gene to infer the evolutionary history of scleractinians. We show that the sequences obtained fall into two distinct clades, defined by PCR product length. Base composition among taxa did not differ significantly when the two clades were considered separately or as a single group. Overall, transition substitutions accumulated more quickly relative to transversion substitutions within both clades. Spatial patterns of substitutions along the 12S rRNA gene and likelihood ratio tests of divergence rates both indicate that the 12S rRNA gene of each clade evolved under different constraints. Phylogenetic analyses using mt 12S rRNA gene data do not support the current view of scleractinian phylogeny based upon skeletal morphology and fossil records. Rather, the two-clade hypothesis derived from the mt 16S ribosomal gene is supported.

Valences of iron and copper in coral skeleton: X-ray absorption spectroscopy analysis

Synchrotron-based X-ray absorption spectroscopy (XAS) indicates that both iron and copper are present in their normal fully oxidized states, Fe(III) and Cu(II), in the aragonite (CaCO 3, orthorhombic) skeletons of scleractinian reef corals. The Fe(III) may substitute for Ca(II) at its structural sites; alternatively, it may be present in clay or other detritus incorporated in the coral skeleton. The XAS-determined Cu(II) valence is consistent with copper substitution for calcium; Cu(II) is known to coprecipitate preferentially with aragonite. Independent evidence from stepwise dissolution of coral aragonite also suggests substitution of Cu(II) for Ca(II) at the latter's structural sites. The low concentrations of both Fe and Cu in the corals (<100 ppm) prevents more detailed structural analysis. Nonetheless, in situ XAS analysis has proved valuable in constraining the general nature of incorporation of these elements into coral.

Changes in the D- and L-content of aspartic acid, glutamic acid, and alanine in a scleractinian coral over the last 300 years

The D- and L-contents of aspartic acid (Asp), glutamic acid (Glu), and alanine (Ala) together with absolute and relative concentrations of 17 amino acids (AAs) were determined in samples of aragonite skeletons of a scleractinian coral from the Caribbean through the last 308 years. Regular patterns of increasing D/L ratios with increasing age are seen for the last 300 years. High and linear racemization rates occur through the first 250 years in Asp, the first 150 years in Glu, and the first 250 years in Ala. An evaluation of the utility of the D/L ratios of these amino acids as a chronological tool through the last 250 years yields standard errors for individual age estimates ranging between 1.7 and 6.4 years. Thereafter the racemizations slow down and in the 308-years old terminal sample the D/L values are discordant. A rapid decrease in the total amino acid concentration of 33% over approximately the last hundred years is identified. The rapid rate of decrease in Asp and Glu may be responsible for this decrease in the total AA concentration. The relative concentrations of Asp, Glu, and Ala (g AA/100 g protein) show a general pattern of decrease, although not significant, through the time interval analyzed. A regularity of the decrease in L-contents and an irregularity of the decrease in D-contents are observed. This may imply that, in addition to possible diagenetic alterations and/or leaching of amino acids, contamination of D-amino acids explains the discordant D/L values. The D-amino acids could originate from peptidoglycans in bacterial cell walls.

Comparative studies of skeletal soluble matrices from some Scleractinian corals and Molluscs

The soluble organic matrices extracted from aragonitic skeletons of five Scleractinia and two Molluscs were analyzed using FTIR spectrometry, HPLC and 2-D gel electrophoresis. All these methods show that scleractinian and molluscan matrices are different. Both matrices are acidic with poorly separated molecular weights. The scleractinian matrices are highly glycosylated, whereas the molluscan matrices are not as shown by stains in 2D-electrophoresis. These results were in accordance with earlier data: high S contents in Scleractinia skeletons and low S contents in Mollusca shells. These results confirm the control of the morphology and the chemical composition of aragonite biocrystals in various taxa.

Shallow burial diagenesis of skeletal carbonates: selective loss of aragonite shell material (Miocene to Recent, Queensland Plateau and Queensland Trough, NE Australia) — implications for shallow cool-water carbonates

In burial environments, carbonate sediments undergo mineralogical stabilization and increasing lithification with depth. As yet, however, little knowledge exists with respect to the corresponding effects on fossil preservation and taphonomic modification of the original sediment composition. Countings of particles (>63 μm) in Miocene to Recent periplatform sediments (ODP Leg 133, NE Australia) exhibit a clear trend of reduction of skeletal aragonite downcore. Low- and high-Mg-calcite grains occur in a continuous order of magnitude over the studied interval (<600 m sub-bottom depth). Original microtextures are retained in high-Mg-calcite biota, although converted to low-Mg-calcite. Thus, the conversion to low-Mg-calcite appears to occur without introducing a significant quantitative bias. Aragonite skeletons (pelagic gastropods), however, exhibit a successive exposure of deeper crystal layers and a chalky preservation with burial depth, which we interpret to result from dissolution. Hints for the originally more numerous existence of aragonite biota exist in soft sediments and chalks by the presence of internal moulds, shells replaced by microspar, and mouldic porosity in early cemented hardgrounds. In deep sections barren of aragonite, the number of casts and replaced shells remains unchanged and is insignificant as compared to aragonitic biota probably originally present within the sediment (<2% in ooze/chalk vs. 30–50% of grains in modern periplatform sediments). Therefore, palaeontological information must be significantly biased through selective removal of aragonite. Rates of preservation and destruction depend on external factors during sediment accretion (sedimentation rates, clay content, total organic carbon content) and rates of fluid flow within the sediment. These observations are relevant with respect to the diagenetic potential and patterns of fossil preservation in little cemented calcitic cool-water carbonates, because they may originally have contained more aragonite biota as important constituents of an ecosystem than is commonly suspected, and calcite/aragonite ratios in ancient carbonate sediments may not necessarily reflect original input signals (climate or sea level).

Sclerosponges as a new potential recorder of environmental changes: Lead in Ceratoporella nicholsoni

Lead concentrations have been analyzed on a 223 yr profile through the aragonitic skeleton of the reef-building Caribbean sclerosponge Ceratoporella nicholsoni by using laser-ablation inductively coupled plasma‐mass spectrometry. A parallel study of the δ 13 C distribution in the skeleton validates the previously established mean annual growth rate of 230 µm/yr, at least for long-term important environmental changes. The Pb trend in the specimen displays a general increase from 0.30 ppm ca. A.D. 1760 to 2.15 ppm ca. A.D. 1984; a major threefold increase occurred after 1930. This Pb profile is analogous to results acquired from ice or coral cores and clearly highlights the potential of sclerosponges as a new proxy of environmental changes for time series extending over several centuries.

Diagenetic comparisons between non-tropical Cenozoic limestones of New Zealand and tropical Mississippian limestones from Indiana, USA: Is the non-tropical model better than the tropical model?

Mississippian limestones exposed in Indiana, U.S.A., were deposited in a shallow tropical ocean. However, many properties of these limestones are more like those of modern and Cenozoic non-tropical limestones such as those found in New Zealand. The dominant skeletal grains in the Indiana limestones are calcitic echinoderms, bryozoans, and brachiopods. The dominant skeletal grains in most Cenozoic limestones of New Zealand are calcitic bryozoans, echinoderms, bivalve molluscs, and foraminifera. In contrast, modern and Cenozoic tropical limestones contain an abundance of aragonitic green algae, corals, and molluscs. Early in diagenesis the metastable aragonite dissolves and reprecipitates as calcite, causing early cementation of the sediments. Originally aragonitic fossils that have dissolved can be identified as molds that are commonly filled with secondary calcite. Because they contained little aragonite, most of the Indiana and New Zealand limestones did not have an abundant source of early cement. Except for local cases in which grains were cemented in contact with carbonate supersaturated seawater, grainstones were relatively deeply buried with little cement between the grains. This resulted in mechanical and chemical compaction of skeletal grains, producing a `fitted fabric' with greatly reduced pore space, either open or filled with cement between the grains. Cement in these aragonite-poor grainstones comes largely from pressure dissolution between grains and along stylolitic seams in the rock, features that are common only after burial beyond a few hundred meters. The final product of deeply buried (up to 2000 m) Cenozoic New Zealand grainstones is similar to the Mississippian grainstones of Indiana. In the Indiana limestones we have only the final product of this extensive burial diagenesis. However, the New Zealand sediments and rocks reveal all steps of formation of the final deeply buried limestone. The reason for the scarcity of originally aragonitic fossil grains in Paleozoic rocks worldwide is unknown. Organisms with aragonitic skeletons such as some molluscan groups and calcareous green algae were present, but seldom in much abundance. The aragonitic scleractinian corals had not yet evolved. Previous researchers have noted that non-skeletal precipitates such as ooids and cements have at times during the Paleozoic been predominantly aragonite and at other times calcite. They have attributed this difference to secular variation in seawater chemistry (icehouse vs. greenhouse seas). Abundance of aragonitic and calcitic skeletal grains does not follow this pattern.

Skeletal carbonate mineralogy of New Zealand bryozoans

On many cool-water carbonate shelves, both modern and ancient, bryozoans are the dominant sediment contributor. Because skeletal carbonate mineralogy can be related to both taxonomic and environmental controls, and has important implications for understanding the geochemistry and diagenesis in carbonate deposits, it is highly relevant to develop a comprehensive database for bryozoan skeletal mineralogy. This X-ray diffraction study of modern New Zealand bryozoans more than doubles the published data available on bryozoan mineralogy and allows for statistical analysis of variability in mineralogy, particularly the hitherto unknown background variation within individuals and within populations. Such a database allows evaluation of differences among populations, taxa, and growth forms. The 412 analyses of 49 bryozoan species fall into four mineralogical categories: (a) 61% are single calcite, which ranges widely from low to high wt% MgCO 3 varieties; (b) 6% (in the genera Cellaria and Macropora) comprise dual calcites, a dominant one low in Mg ( X=1.8 wt% MgCO 3) and the other much higher ( X=9.0 wt% MgCO 3); (c) 27% are mainly calcite with some aragonite; and (d) 6% are mainly aragonite with some calcite. The bimineralic calcite–aragonite skeletons can arise from different processes, including biologically mediated changes in precipitation and diagenetic alteration following death. There is small but significant variation in Mg content in calcite both within colonies (Mg increasing with age) and within populations, for a total of within-species variance of 0.526. We found, however, more variation among species in both calcite/aragonite ratio and Mg content than that within species. Whereas some genera and families are reasonably consistent mineralogically (e.g., low-Mg calcite cyclostomes such as Telopora buski), others are highly variable (e.g., Chaperia acanthina). Among the higher taxa, cyclostomes tend to be entirely or mainly calcite with a low to moderate range of MgCO 3 ( X=2.1 wt%), whereas cheilostomes are far more variable, with anascans ( X=4.1 wt%) having a lower MgCO 3 content than ascophorans ( X=5.6 wt%). All dual-calcite species are anascans in the Calloporoidea. The proportions of aragonite and of Mg in calcite are greater in evolutionarily younger taxa, at least at the higher levels. In relation to growth forms, erect rigid varieties are calcitic except for several robust-branching genera such as Adeonellopsis. Encrusting forms, particularly unilaminar sheets, are highly variable and contain most of the bimineralic calcite–aragonite species. Erect flexible forms are either single or dual calcite, and extremely variable in Mg content. Free-living or vagrant forms are usually aragonitic. In relation to latitude and temperature variation, few species show any consistent or significant mineralogical trends, although Schizoporella unicornis deposits increasing amounts of aragonite at lower (warmer) latitudes. Mecynoecia purpurascens includes more (but still low) Mg in its calcite skeleton with increasing latitude, a reversal of the Mg–temperature relationship noted in several other skeletal carbonate producers.

Middle Triassic cnidarians from the New Pass Range, Central Nevada

We describe three scleractinian corals and one species of hydrozoan from the New Pass Range, central Nevada, which together constitute the oldest Triassic cnidarian assemblage from North America. They occur in carbonate rocks tentatively correlated with the Augusta Mountain Formation, Star Peak Group. At generic and higher levels, these cnidarians seem representative of early Mesozoic Tethyan faunas and carbonate lithofacies, but they indicate some endemism. Although the original aragonitic skeletons and microstructure are destroyed by recrystallization, the corals still yield important details allowing their correct taxonomic assignment. They contain the minitrabecular cerioid coral,Ceriostella variabilisnew genus and species, the thick-trabecular, thamnasteroid coralMesomorpha newpassensisnew species, and an undeterminable cuifastreid coral tentatively assigned toCuifastraea.The discovery ofMesomorphamarks the first occurrence of this genus outside the Jurassic and Cretaceous seas. Also discovered is a remarkably corallike hydrozoan,Cassianastraea reussi(Laube), already known from the Carnian stage of the western Tethys. This is the first occurrence of this species outside the western Tethys.

Partitioning of Divalent Elements between Calcium Carbonates and Water: A Solid Solution Model and Implications for Palaeoenvironmental and Palaeoecological Reconstructions from Biogenic Carbonates

Divalent element incorporation in biogenic calcium carbonates depends on various factors such as the crystal structure (calcite versus aragonite), water temperature, water composition, biological activity (group and genus of the organism, skeleton growth rate)... Thus they are potential tracers of palaeotemperatures, palaeoenvironments, secular seawater composition variations. For instance, the fractionation of Sr between coral aragonitic skeletons and seawater has been extensively used as a proxy to surface seawater temperature and empirically calibrated on living organismes (Weber, 1973). More recently, it has been shown that other divalent elements such as Mg display an even greater temperature dependence of their fractionation between coral aragonite and seawater (Mitsuguchi et al, 1996). In more complex organisms such as bivalves, seasonal variations are observed but are more difficult to relate to temperature variations because of the strong influence of the organism metabolism on the shell compositions. Finally, the mechanisms of incorporation the trace or minor elements determine the fractionation in several ways. Trace metals may be trapped in organism skeletons as adsorbed species on the calcium carbonate surface, in substitution of calcium on crystallographic sites, or in fluid or organic matrix inclusions in the more or less porous matrix. These different sites also determine the resistance of the original geochemical signature of the biogenic carbonate to alteration through geological times. Thermodynamic modelling of the equilibrium partitioning between calcium carbonates and water can be attempted and compared with empirical calibrations in order assess the factors governing the actual composition of biogenic shells. We test here a simple elastic model of trace element substitution energies which has been successfully applied to the description of trace element partioning between minerals and liquids at high temperature (Blundy and Wood, 1994) to estimate the equilibrium constants for divalent metals between calcite or aragonite and water and compare it with available experimental data and empirical calibrations on various biogenic carbonates in order to decipher the crystal-chemical and biological sensu lato controls.

Composition of soluble mineralizing matrices in zooxanthellate and non-zooxanthellate scleractinian corals: Biochemical assessment of photosynthetic metabolism through the study of a skeletal feature

Soluble organic matrices extracted from aragonitic skeletons produced by recent zooxanthellate and non-zooxanthellate scleractinian corals were studied after suitable hydrolyses, by HPLC chromatographies allowing characterization of their amino acid and monasaccharide compositions. Clear compositional differences can be correlated with the symbiotic or non-symbiotic character in both proteic (via Asp, Glu, Ala and Ser) and glucidic phases of soluble matrices (via GalN, GlcN and Gal), providing new criteria to assess the impact of photosynthetic metabolism on skeletal features of scleractinian corals.