Nicola Allison

Factors influencing the stable carbon and oxygen isotopic composition of Porites lutea coral skeletons from Phuket, South Thailand

We determined the δ18O and δ13C composition of the same fixed growth increment in severalPorites lutea coral skeletons from Phuket, South Thailand. Skeletal growth rate and δ18O are inversely related. We explain this in terms of McConnaugheys kinetic isotopic disequilibria model. Annual trends in δ18O cannot be solely explained by observed variations in seawater temperature or salinity and may also reflect seasonal variations in calcification rate. Coral tissue chlorophylla content and δ13C of the underlying 1 mm of skeleton are positively related, suggesting that algal modification of the dissolved inorganic carbonate pool is the main control on skeletal δ13C. However, in corals that bleached during a period of exceptionally high seawater temperatures in the summer of 1991, δ13C of the outer 1 mm of skeleton and skeletal growth rate (over 9 months up to and including the bleaching event) are inversely related. Seasonal variations in °13C may reflect variations in calcification rate, zooxanthellae photosynthesis or in seawater δ13C composition. Bleached corals had reduced calcification over the 9-month period up to and including the bleaching event and over the event they deposited carbonate enriched in13C and18O compared with unaffected corals. However, calcification during the event was limited and insufficient material was deposited to influence significantly the isotopic signature of the larger seasonal profile samples. In profile, overall decreases in δ18O and δ13C were observed, supporting evidence that positive temperature anomalies caused the bleaching event and reflecting the loss of zooxanthellae photosynthesis.

Comparative determinations of trace and minor elements in coral aragonite by ion microprobe analysis, with preliminary results from Phuket, southern Thailand

The ion microprobe technique was developed for semiquantitative analysis of trace and minor elements in coralline aragonite. The size of the microprobe beam (ca. 30 μm diameter) allows an examination of the spatial geochemical heterogeneity of coral skeletons on a much finer scale than has previously been possible. Of the elements investigated magnesium, calcium, rubidium, strontium, barium, and uranium appeared suitable for analysis under the operating conditions used although it was not possible to unequivocally confirm that 85Rb and 238U were not subject to molecular interferences. A homogenous carbonate standard was only available for magnesium so some relative ion yields were calculated using a standard silicate glass. Magnesium, rubidium, strontium, and barium concentrations were in reasonable agreement with previously reported values. Uranium concentrations were considerably lower, probably reflecting a miscalibration of the ion microprobe. Reproducibility of analysis was investigated by comparing immediately adjacent pairs of analyses on coral material. Although analytical accuracy was questionable, precision was good and concentration differences between samples reflect true variations in geochemistry. High resolution time series analyses of magneisum, rubidium, strontium, barium, and uranium in Porites lutea corals indicated that the aragonite was heterogeneous on a small spatial scale (over distances attributed to less than one months skeletal growth). This heterogeneity does not appear to reflect expected variations in seawater temperature or chemistry. It may reflect the metal binding capacity of a heterogeneous organic matrix unevenly distributed throughout the coral skeleton. Individual coral colonies deposited substantially different ranges of Mg and Ba, within and between reef sites, which could lead to significantly different seawater:coral distribution coefficients being calculated between individuals of the same coral species.

Coordination of Sr and Mg in calcite and aragonite

Abstract Strontium and Mg in calcite and aragonite are widely used as proxies of temperature in palaeo-environmental reconstructions. We use X-ray absorption fine structure (XAFS) to examine Sr and Mg substitution in calcite and aragonite. We have measured the K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) of Mg and Sr-bearing calcite and aragonite, plus the carbonates: strontianite, hydromagnesite, magnesite, dolomite and a suite of calcites with differing amounts of Mg. The Sr substitutes ideally for Ca in aragonite but causes a small (2%) dilation of the site. Strontium substitutes for octahedral Ca in calcite but with a 6.5% dilation and distortion. Magnesium in the calcites studied provides a variable XANES indicating that the Mg structural state in calcite is variable. Refinement of EXAFS gives Mg-O bond distances of ~2.12 Å, which are much smaller than the Ca-O bond distance of 2.35 Å but consistent with published amounts of relaxation of the calcite structure. The XANES and EXAFS are consistent with a model whereby some calcites contain nanodomains, e.g. of dolomite and/or huntite structures. The variability in the XANES can be explained by domains of different types and/or sizes. Substitution of Mg into aragonite has 9-fold coordination but relatively short bond distances (2.08 Å) demonstrating either: (1) substantial distortion of the site; or (2) that Mg is accommodated in nanodomains of an unknown phase. Variability in the Mg structural state in calcite may be linked to the variety of temperature dependences observed, e.g. in foraminiferal calcite.

Geochemical anomalies in coral skeletons and their possible implications for palaeoenvironmental analyses

Abstract Different features are visible when thin sections and fragments of coral aragonite are examined by transmitted light and scanning electron microscopy. The geochemistry of these areas was investigated by ion microprobe analysis. Microborings and centres of calcification (the origins of crystal growth) contained significantly higher concentrations of Ba and of Ba, Sr and Mg, respectively, than the surrounding crystal areas. Geochemical anomalies in microborings may be associated with an amorphous lining observed in several of the boreholes. Centres of calcification are more porous than the surrounding material and may contain increased concentrations of the coral organic matrix which is enriched in many metals compared to the skeleton. Geochemical anomalies in these areas may significantly increase estimates of the Ba skeletal content and may effectively overwrite palaeoenvironmental signals. Mg and Sr anomalies would not significantly effect bulk determinations of these metals.

Strontium heterogeneity and speciation in coral aragonite: implications for the strontium paleothermometer

Sea surface temperatures (SSTs) have been inferred previously from the Sr/Ca ratios of coral aragonite. However, microanalytical studies have indicated that Sr in some coral skeletons is more heterogeneously distributed than expected from SST data. Strontium may exist in two skeletal phases, as Sr substituted for Ca in aragonite and as separate SrCO3 (strontianite) domains. Variations in the size, quantity, or both of these domains may account for small-scale Sr heterogeneity. Here, we use synchrotron X-ray fluorescence to map Sr/Ca variations in a Porites lobata skeleton at a 5 μm scale. Variations are large and unrelated to changes in local seawater temperature or composition. Selected area extended X-ray absorption fine structure (EXAFS) spectroscopy of low- and high-Sr areas indicates that Sr is present as a substitute ion in aragonite i.e., domains of Sr carbonate (strontianite) are absent or in minor abundance. Variations in strontianite abundance are not responsible for the Sr/Ca fluctuations observed in this sample. The Sr microdistribution is systematic and appears to correlate with the crystalline fabric of the coral skeleton, suggesting Sr heterogeneity may reflect nonequilibrium calcification processes. Nonequilibrium incorporation of Sr complicates the interpretation of Sr/Ca ratios in terms of SST, particularly in attempts to extend the temporal resolution of the technique. The micro-EXAFS technique may prove to be valuable, allowing the selection of coral microvolumes for Sr/Ca measurement where strontium is incorporated in a known structural environment.

Corals concentrate dissolved inorganic carbon to facilitate calcification

The sources of dissolved inorganic carbon (DIC) used to produce scleractinian coral skeletons are not understood. Yet this knowledge is essential for understanding coral biomineralization and assessing the potential impacts of ocean acidification on coral reefs. Here we use skeletal boron geochemistry to reconstruct the DIC chemistry of the fluid used for coral calcification. We show that corals concentrate DIC at the calcification site substantially above seawater values and that bicarbonate contributes a significant amount of the DIC pool used to build the skeleton. Corals actively increase the pH of the calcification fluid, decreasing the proportion of DIC present as CO2 and creating a diffusion gradient favouring the transport of molecular CO2 from the overlying coral tissue into the calcification site. Coupling the increases in calcification fluid pH and [DIC] yields high calcification fluid [CO3(2-)] and induces high aragonite saturation states, favourable to the precipitation of the skeleton.

Reconstruction of deglacial sea surface temperatures in the tropical Pacific from selective analysis of a fossil coral

The Sr/Ca of coral skeletons demonstrates potential as an indicator of sea surface temperatures (SSTs). However, the glacial-interglacial SST ranges predicted from Sr/Ca of fossil corals are usually higher than from other marine proxies. We observed infilling of secondary aragonite, characterized by high Sr/Ca ratios, along intraskeletal pores of a fossil coral from Papua New Guinea that grew during the penultimate deglaciation (130 {+-} 2 ka). Selective microanalysis of unaltered areas of the fossil coral indicates that SSTs at {approx}130 ka were {le} 1 C cooler than at present in contrast with bulk measurements (combining infilled and unaltered areas) which indicate a difference of 6-7 C. The analysis of unaltered areas of fossil skeletons by microprobe techniques may offer a route to more accurate reconstruction of past SSTs.

Strontium in coral aragonite: 2. Sr coordination and the long-term stability of coral environmental records

We have used X-ray Diffraction (XRD) and Sr K-edge Extended X-ray Absorption Fine Structure (EXAFS) to determine the structural state of Sr in a suite of coral aragonite samples. Our samples encompassed a selection of coral species (Porites lobata, Porites lutea, Pocillopora eydouxi, Montastrea annularis, Pavona gigantea and Pavona clavus) including some commonly used for palaeoenvironmental reconstruction. Aragonite was the only carbonate observed by XRD. We refined the isolated EXAFS against structural models for Sr in aragonite and two-phase strontianite/aragonite mixes. Our data are indistinguishable from Sr ideally substituted in aragonite and strontianite was present below detection levels (estimated at <5% of Sr present). Comparisons of recent and ancient coral aragonite show no sign of exsolution, either by spinodal decomposition or by the direct nucleation of strontianite domains. Published diffusion rates of Sr in ionic solids support the view that exsolution would occur prohibitively slowly. Coral aragonites are metastable materials with slow diffusion kinetics that have the potential to encode environments over timescales of millions of years.

Strontium in coral aragonite: 1. Characterization of Sr coordination by extended absorption X-ray fine structure

Abstract Aragonite was analyzed from Porites lobata, Pavona gigantea, Pavona clavus, and Montastrea annularis corals by Sr K-edge extended absorption X-ray fine structure (EXAFS) and compared with aragonite, strontianite, and mechanically mixed standards. Bulk analyses were performed and data compared with equivalent micro-EXAFS analyses on small (∼400 μm 3 ) analytical volumes with a microfocused X-ray beam. As a result of the architecture of the coral skeleton, the crystals within the microanalytical volume are not randomly oriented, and the microanalytical X-ray absorption spectra show orientational dependence. However, refinement of bulk and microanalytical data provided indistinguishable interatomic distances and thermal vibration parameters in the third shell (indicative of Sr speciation). The Sr K-edge EXAFS of all the coral samples refine, within error, to an ideally substituted Sr in aragonite, in contrast to previous studies, in which significant strontianite was reported. Some samples from that study were also analyzed here. Strontianite may be less widely distributed in corals than previously thought.

Particle processing by Mytilus edulis: effects on bioavailability of metals

Mytilus edulis were maintained in laboratory flow-through systems and fed estuarine permanently suspended particulate matter (PSPM), of particle diameters from <3 to 40 μm. The concentrations of Fe, Mn, Zn, and Cu in the PSPM, and in the faeces and pseudofaeces produced by the mussels, were determined by atomic absorption spectrometry, following digestion in 1 M HCl. The particulate organic carbon (POC) contents of the PSPM, faeces and pseudofaeces were also determined and used to estimate sorting efficiencies and absorption efficiencies. The concentrations of Fe, Zn, Mn and Cu in the pseudofaeces were often significantly higher than in the PSPM, indicating that mussels had rejected particles enriched in these metals. This finding was not observed in all experiments even when sorting efficiencies were high. The Cu concentrations of the faeces were always significantly lower than the PSPM, reflecting a net loss of Cu from particles in the mussel gut. The amount of Cu released during ingestion was positively related (r2=0.50) to the HCl-extractable concentration of Cu, indicating that variations in the amount of bioavailable Cu in the PSPM may have occurred. The net amount of Cu released during ingestion was also strongly inversely related (r2=0.96) to the POC ingestion rate of the mussels. Alterations in the digestive physiology of mussels (e.g. increases in the proportion of food sent to the digestive gland and increases in gut passage time) compensate for low organic ingestion rates in mussels. These alterations may increase the net release of particulate Cu and hence the bioavailability of the metal to the organism. It was not possible to quantify how much released Cu was assimilated by the mussel but the net loss of Cu:carbon from the particles was high in comparison with mussel tissue Cu concentrations, indicating that the released Cu is either not assimilated or is assimilated and quickly excreted.