B14B-01 INVITED 16:00h
Thermodynamic and Kinetic Controls on Element Incorporation into Aragonite
Much of what we know about the behavior of Earth's climate system is based on proxy records preserved in biogenic carbonates. This approach is inherently limited by uncertainty concerning the extent to which biological processes influence skeletal composition. A crucial first step to resolving this ambiguity is the establishment of a quantitative understanding of the thermodynamic and kinetic controls on the compositions of abiogenic carbonates. New experimentally determined aragonite-seawater partition coefficients, combined with the lattice strain equation of Blundy and Wood [1], indicate that the composition of biogenic aragonite are controlled primarily by the compatibility of cations in the near-surface region of a growing crystal rather than by crystal-fluid equilibrium. Experiments to determine the partitioning of the alkaline earth cations Mg$^{2+}$, Ca$^{2+}$, Sr$^{2+}$, and Ba$^{2+}$ between abiogenic aragonite and seawater were carried out over a temperature range of 15 $\deg$ to 45 $\deg$C following the protocol of Kinsman and Holland [2]. After each experiment, the seawater was analyzed for $^{25}$Mg, $^{48}$Ca, $^{86}$Sr and $^{138}$Ba using ICP-MS. Individual aragonite crystals were analyzed for $^{24}$Mg, $^{42}$Ca, $^{88}$Sr, and $^{138}$Ba by SIMS ion microprobe. Elemental ratios were converted to concentration using analyses of Ca carried out by electron microprobe. Partitioning varies systematically with cation radius and temperature. However, several lines of evidence suggest that surface equilibrium rather than crystal-fluid equilibrium is controlling element incorporation. First, the compatibilities of Sr$^{2+}$ and Ba$^{2+}$ in aragonite are equal to, or greater than, that of Ca$^{2+}$, which is the essential structural constituent and should be the most compatible cation. Second, the effective Young's moduli derived from our data (13-17 GPa) are much lower than values for aragonite (76-144 GPa). This indicates that the aragonite grains precipitated in our experiments are enriched in trace components (impurities) relative to the concentrations expected from crystal-fluid equilibrium. Our data are inconsistent with diffusive fractionations in a boundary layer adjacent to the growing crystal because compatible elements (i.e. Sr$^{2+}$; Ba$^{2+}$) are enriched rather than depleted in our experimental run products. A boundary layer effect would drive all partition coefficients toward unity, whereas the partition coefficients for Mg$^{2+}$, Sr$^{2+}$, and Ba$^{2+}$ determined in our experiments diverge from 1. Our data appear consistent with the surface enrichment model of Watson [3,4], in which the composition of the crystal reflects equilibrium element concentrations in the near-surface region of the crystal. References: [1] Blundy and Wood (1994) Nature 372:452-454. [2] Kinsman and Holland (1969) Geochim Cosmochim Acta 33:1-17. [3] Watson (1996) Geochim Cosmochim Acta 60:5013-5020. [4] Watson (2004) Geochim Cosmochim Acta 68:1473-1488.
B14B-02 INVITED 16:15h
Biological Effects in Coral Biomineralization: The Ion-Microprobe Revolution
Scleractinian corals are among the most prolific biomineralizing organisms on Earth and massive, reef-building corals are used extensively as proxies for past variations in the global climate. It is therefore of wide interest to understand the degree to which biological versus inorganic processes control the chemistry of the coral skeleton. Early workers considered aragonitic coral skeleton formation to be a purely physiochemical process. More recent studies have increasingly emphasized the role of a skeletal organic matrix, or intercalated organic macro-molecules that control the macroscopic shape and size of the growing crystals. It is now well established that organic compounds play a key role in controlling the morphology of crystals in a wide variety of calcium carbonate biomineralization processes by binding to specific sites, thereby causing direction-specific binding energies on the crystal surfaces. Macro-molecules, such as aspartic acid-rich or glutamic proteins and sulfated polysaccharides, are known to be embedded within the aragonitic skeletal components of coral. In addition, endosymbiotic algae and the layer of cells adjacent to the mineralizing surface, the calicoblastic ectoderm, are believed to play important roles in driving and controlling hermatypic coral skeletogenesis. However, until recently, further progress has been somewhat limited because it was not possible to obtain chemical analyses of the coral skeleton with sufficiently high spatial resolution and sensitivity to correlate chemical variations with the micrometer scale organization of its different structural components. The recent emergence of new ion microprobe technology is changing this situation radically. Conventional ion microprobe and laser ablation techniques have already contributed substantially to our knowledge about the micro-distribution of key trace elements such as B, Mg, Sr, Ba and U. However, with the development of the NanoSIMS, a newly designed ion microprobe capable of trace-element and isotopic analysis with a spatial resolution down to $50-100$ nanometers, it has become possible to study the intimate relationship between the chemistry and the ultra-structure of the coral skeleton. Individual structural elements, such as centers of calcification and bundles of fibrous aragonite, can be clearly resolved and their chemical and isotopic composition mapped. In this talk preliminary results of a NanoSIMS imaging study of the aragonite skeleton of {\it Pavona clavus} will be shown. {\it Pavona clavus} is a massive reef-building coral frequently used for paleo-climate reconstructions. We find that Mg and Sr are distributed very differently in this coral. In contrast to Sr, the distribution of Mg 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 Mg concentration. This could indicate a strong biological control over the Mg 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. Sub-micrometer scale chemical analysis will greatly advance our knowledge of the mechanisms that control the formation of the coral skeleton. However, in an effort to advance our understanding of biomineralization processes in general, the analytical capabilities of the NanoSIMS will be applied to a broad variety of mineralizing organisms. A consortium of researchers from Stanford University, the National Museum of Natural History in Paris, University of Paris XI-Orsay, LSCE in Gif sur Yvette, Centre Scientifique in Monaco, and Cameca are directly involved in these efforts.
B14B-03 16:30h
Elevated Levels of Magnesium in the Coral {\it Montastraea faveolata} as determined by SEM/EDS and LA ICP-MS
Using a high-resolution laser ablation inductively coupled plasma mass spectrometer (LA ICP-MS) technique, a small portion of a coral core was analyzed to determine the geochemical signatures within and among specific skeletal structures in the large framework coral Montastraea faveolata. Data reveal potential problems in geochemical analysis and interpretation. Depending upon sample size and location of sampling within the skeleton, results may differ. Vertical transects (spot/raster sampling) were conducted along three parallel skeletal structures: endothecal (septal flank), corallite wall, and exothecal (costal flank) areas. The results reveal that trace element levels vary among the three structures. The amount of magnesium (Mg) varied prominently among the adjacent structures and is most abundant within the exothecal portion of the skeleton. Using a scanning electron microscope (SEM), we found hexagonal crystals forming discs, pairs, and rosettes in several coral samples of M. faveolata. High levels of Mg within these crystals have been confirmed with energy dispersive spectrometry (EDS) and LA ICP-MS. The chemical composition is consistent with the mineral brucite (MgOH$_{2}$). The crystals, located exclusively in the exothecal area of the skeleton, are associated with green endolithic algae and are commonly associated with increased Mg levels found in the adjacent corallite walls. The excess Mg precipitated within the microenvironment of the exothecal area may be a result of photosynthetic processes. The presence and locations of high-Mg crystals found within microenvironments of the coral may explain anomalous Mg data researchers have been questioning for years.
B14B-04 INVITED 16:45h
Preliminary Observations on sea Water Utilization During Calcification in Scleractinian Corals.
Coral skeletons contain unique archive of paleo-environmental information on temperature, salinity pH, and other parameters hidden in their skeletal isotopic and trace element composition. This information is important for testing and calibrating global circulation models which predict the response of the atmosphere-ocean system to global changes such as atmospheric CO$_{2}$ increase and global warming. However, the physiological process of biomineralization in corals cause many deviations from expected thermodynamic behavior, so called "vital effects" In order to better utilize the paleo-environmental information hidden in coral skeletons it is essential to understand these processes. We have shown previously that the source of ions for calcification in the unicellular foraminifera is seawater vacuoles that transfer the ions to the site of calcification. In this study we test the possibility that seawater is the solution from which calcification proceeds also for corals. We used confocal microscopy to investigate the calcification process and the involvement of seawater in the scleractinian corals {\it Pocillopora damicornis} and {\it Stylophora pistillata} from the Red Sea. The corals are maintained in the laboratory as free microcolonies that are completely covered by tissue or as small tips which precipitate their CaCO$_{3}$ skeleton horizontally on glass slides and thus allow direct microscopic observations. We used the fluorescent probes Calcein and FITC-Dextran to trace seawater dynamics, and precipitation of CaCO$_{3}$. Natural fluorescence of the coral and its symbionts was used to trace the coral tissue physical movement. Our observations show that the corals precipitate their aragonite skeleton by sequestering seawater into the calcification space between the skeleton and the calicoblastic layer. Pulse chase experiments with membrane impermeable Calcein showed fluorescent skeleton labeling in both types of colonies, suggesting that the calicoblastic space is permeable to sea water, and crystals growth proceeds from these (probably modified) seawater. The exchange of seawater between the calcification space and the environment may be mediated by frequent tissue pulses that pump the seawater in and out. At present we investigate the pH in the calcification space trying to confirm previous microelectrode studies that reported very high (9.3) values. The implications for paleoceanographic studies are far reaching. For trace elements a Rayleigh distillation model may be applied as we suggested previously for foraminifera. For carbon isotopes there must be an important seawater component in addition to the metabolic CO$_{2}$ input, while for oxygen isotopes there may be a significant CO$_{3}^{-2}$ ion effect.
B14B-05 17:00h
The Coral Data Time Series Need To Be Revisited
Coral skeleton is formed under organism control and its geochemical properties are strongly influenced by biological effects embedding environmental signal. Geochemists have been puzzled by the diversity of geochemical responses showed by colonies grown in a same area. By revisiting the Weber and Woodhead data series (1972), gathering data from enough colonies developed in similar conditions to provide a statistical isotopic value representative of one site, we demonstrate that for Porites and Acropora, the expected isotopic thermometer is revealed when the "vital effect" is removed. On the other hand, by using Acropora cultured in controlled condition, with changing temperature on a range comprised between 23 and 29°C, the comparison of oxygen and carbon isotopic values revealed the role played by kinetic fractionation. This apparent paradox of two co-existing fractionations is explained by the isotopic analyzes of wild and cultured corals operated at micrometer size scale taking into account of microstructures of the skeleton. Two different crystals appear to be the growth units of the skeleton, each crystal corresponding to a specific deposition mechanism. Thus, the measurement performed with a conventional method is a "bulk" measurement, which depends upon two isotopic fractionations. Some investigations underlined the discrepancy of the meaning of the inter-annual and seasonal isotopic records, which could be illustrated by different isotopic calibrations assessed from seasonal or annual data. It has been also explained by micrometer analyses of Porites aragonite. A smoothing at around 400microns of isotopic measurements as well as Sr/Ca indicates that at seasonal time scale the growth unit is the month. This is in agreement with extensive studies conducted by biologists describing the mechanism governing the formation of Porites skeleton: every month is deposited a framework which is progressively filled in. By combining biologists and geochemists knowledge, we are able to improve the interpretation of the coral records in term of paleoclimatic reconstruction (cf PP15 session).
B14B-06 17:15h
Annual Variations in Aspartic Acid Content of Coral Skeleton: A new Proxy for Changes in Biological Activity of Coral
Biological or metabolic effects have often been invoked to explain abnormal changes in the annual pattern of the stable isotope record of the coral skeleton. However, it is not possible to isolate and quantify the magnitude of these effects from environmental effects controlling the stable isotopes record. Therefore, there is a need to develop a proxy which could be independently linked with the changes in biological activity of the corals. It is well known that amino acids are closely associated with biomineralization of coral skeleton. We examined variations in amino acid composition of coral skeleton by conducting high resolution micro-sampling along the coral growth axis. The samples (ca. 1 mg each) were collected at about 1 mm interval, which corresponded to about 1 month of coral skeletal growth, and hydrolyzed with 6N HCl at 110 deg.C for 22 hours. The results show that relative molar concentration of aspartic acid (Asp) shows the most pronounced annual variation, in comparison to other amino acids, over a wide range of 20 - 35 mole percent. The comparison of Asp mole content with stable oxygen isotope data shows that Asp content is the highest in summers while lowest in winters. The absence of non-protein amino acids in the samples suggests that the amino acids in the skeleton are neither degraded nor of extraneous origin, because in both the cases some amount of non-protein amino acids like beta-alanine and gama-amino butyric acid must be present in samples. Lack of correlation between Asp and stable isotope of carbon is probably due to the fact that isotope data are average values for all carbon-based compounds including carbonate carbon. In contrast, Asp relative mole content is based on only one compound and closely related with the secretion of polypeptides and amino acids by coral. Therefore, variations in Asp content is likely to reflect change in biological activity more directly than carbon isotope. Out of about 8 consecutive years record examined in this study, 5 years Asp data clearly show summer peaks while those for other years suggest some kind of abnormality in the biological activity of the coral.
B14B-07 17:30h
Stable Carbon Isotopes ($\delta^{13}$C) in Coral Skeletons: Experimental Approach and Applications for Paleoceanography
Scleractinian corals obtain fixed carbon via photosynthesis by their endosymbiotic algae (zooxanthellae) and via hetertrophy (injestion of zooplankton, $\delta^{13}$C $\approx$ -17 to -22$\permil$). Carbon dioxide (CO$_{2}$) used for photosynthesis is obtained from seawater ($\delta^{13}$C $\approx$ 0%) or from respired CO$_{2}$ within the coral host. The $\delta^{13}$C of the carbon used in the formation of the underlying coral skeleton is fractionated as a result of both of these metabolic processes. Here I have pooled evidence from several field and tank experiments on the effect of photosynthesis and heterotrophy of coral skeletal $\delta^{13}$C. In the experiments, decreases in light levels due to shading or depth resulted in a significant decrease in skeletal $\delta^{13}$C in all species studied ({\it Pavona gigantea}, {\it Pavona clavus}, {\it Porites compressa}). Decreases in photosynthesis in bleached corals also resulted in a decrease in skeletal $\delta^{13}$C compared to non-bleached corals growing under the same conditions and at the same location. Skeletal $\delta^{13}$C also decreased at higher than normal light levels most likely due to photoinhibition. Thus, decreases in photosynthesis due to reduced light levels, due to bleaching-induced decreases in chlorophyll {\it a} concentrations, or due to photodamage-induced decreases in functional cholorphyll {\it a}, results in significant $\delta^{13}$C decreases. Comprehensive interpretation of all of the data showed that changes in photosynthesis itself can drive the changes in $\delta^{13}$C. In field experiments, the addition of natural concentrations of zooplankton to the diet resulted in decreases in skeletal $\delta^{13}$C. Such a decrease was more pronounced with depth and in {\it P. gigantea} compared to {\it P. clavus}. In situ feeding experiments have since confirmed these findings. However under tank conditions with unaturally high feeding rates, enhanced nitrogen supply in the diet can disrupt the coral-algal symbiosis, stimlate zooxanthellae growth and photosynthesis, and cause an incrase in skeletal $\delta^{13}$C. It is proposed that under natural field conditions corals feed on zooplankton below this `nutrient threshold' and that increases in heterotrophy should result in decreases skeletal $\delta^{13}$C values. Overall, changes in photosynthesis and heterotrophy have significant effects on coral skeletal $\delta^{13}$C. In shallower corals, photosynthesis drives the bulk of the variation in $\delta^{13}$C. In addition, boron isotope data indicate that pH levels do not vary with changes in photosynthesis or heterotrophy suggesting that metabolically driven $\delta^{13}$C fractionation during skeletogenesis is not pH driven. Thus the skeletal $\delta^{13}$C records from shallow corals in non-upwelling regions where zooplankton concentrations are relatively constant should represent a reliable proxy of light variability. Due to the complexity associated with nutrients and heterotrophy, $\delta^{13}$C records from upwelling regions or deep corals are still difficult to resolve.
B14B-08 17:45h
Stable Oxygen ($\delta^{18}$O) and Carbon ($\delta^{13}$C) Isotopes in the Skeleton of Bleached and Recovering Corals From Hawaii
Coral skeletal stable oxygen isotopes ($\delta^{18}$O) reflect changes in seawater temperature and salinity, while stable carbon isotopes ($\delta^{13}$C) reflect a combination of both metabolic (photosynthesis and feeding) and kinetic fractionation. Together, the two isotopic signatures may be used as a proxy for past bleaching events. During bleaching, increased seawater temperatures often contribute to a decline in zooxanthellae and/or chlorophyll concentrations, resulting in a decrease in photosynthesis. We experimentally investigated the effect of bleaching and subsequent recovery on the $\delta^{13}$C and $\delta^{18}$O values of coral skeleton. Fragments from two coral species ({\it Montipora capitata} and {\it Porites compressa}) from Kaneohe Bay, Hawaii were bleached in outdoor tanks by raising the seawater temperature to 30$\deg$C. Additional fragments from the same parent colonies were maintained at ambient seawater temperatures (27$\deg$C) in separate tanks as controls. After one month in the tanks, a subset of the fragments was frozen and all remaining fragments were placed back on the reef to recover. All coral fragments were analyzed for their skeletal $\delta^{13}$C and $\delta^{18}$O compositions at five time intervals: before, immediately after, 1.5, 4, and 8 months after bleaching. In addition, rates of photosynthesis, calcification, and heterotrophy were also measured. Immediately after bleaching, $\delta^{18}$O decreased in bleached {\it M. capitata} relative to controls, reflecting their exposure to increased seawater temperatures. During recovery, $\delta^{18}$O values in the treatment {\it M. capitata} were not different from the controls. In {\it P. compressa}, $\delta^{18}$O did not significantly differ in bleached and control corals at any time during the experiment. Immediately after bleaching, $\delta^{13}$C decreased in the bleached fragments of both species relative to controls reflecting decreased photosynthetic rates. However, during recovery $\delta^{13}$C in both species was greater in bleached than control fragments despite photosynthesis remaining low. In recovering {\it M. capitata}, enriched $\delta^{13}$C values may reflect decreased calcification of bleached fragments, while in {\it P. compressa} they may reflect the combination of decreased calcification and decreased heterotrophy of bleached fragments. $\delta^{13}$C of the tissue and zooxanthellae will also be analyzed and should enhance our understanding of the relative contribution of autotrophy and heterotrophy in these species during bleaching and recovery.