B13E-01 13:40h
Microscopic and Spectroscopic Characterization of Calcified Microorganisms at the Nanometer-Scale in Experimental and Field Samples.
Calcium phosphates and calcium carbonates are the most prevalent minerals involved in microbial fossilization. Structural characterization of both the organic and mineral components in such samples is, however, usually difficult at the appropriate spatial resolution, i.e., at the submicrometer scale. We have used a combination of Scanning Transmission X-ray microscopy (STXM), a synchrotron-based technique, and High-Resolution Transmission Electron Microscopy (HRTEM) to characterize both the Ca-containing biominerals and the functional groups present in the organic components associated with them (STXM). These data, in turn, provide a better understanding of the mechanisms, products, and biomolecules involved in microbial calcification. We have studied the experimental biomineralization of the model strain Caulobacter crescentus by calcium phosphates, and the calcification of natural biofilms by aragonite in an alkaline lake in Turkey. The precipitation of calcium phosphate and calcium carbonate by microorganisms likely involves different mechanisms. The resulting biominerals were found to have unique features with dimensions in the nanometer-range, preferential crystallographic orientations or unusual morphologies, which provide potential biosignatures. By using C K-edge NEXAFS spectroscopy at a submicrometer scale, we were also able to document the evolution of the organic molecules during the fossilization process and to characterize those involved as templates in the formation of calcium phosphate and carbonate minerals.
B13E-02 13:55h
Foraminiferal Inter-species and Intra-individual Crystal Structure Variability Characterized by Micro-Raman Spectroscopy
Raman spectroscopy was used to investigate inter-species and intra-individual differences in crystal lattice structure in cultured benthic foraminiferal tests. This {\it non-destructive} technique can be used to map differences in crystal structure associated with different mechanisms of biomineralization. Inter-species differences in {\it Bulimina aculeata} and {\it Rosalina vilardeboana} produced significantly different Raman spectra (bands: {\it B. aculeata} 281, 482, 711, 1002, 1086 cm$^{-1}$; {\it R. vilardeboana} 224, 620, 798, 1001, 1032, 1156 cm$^{-1}$). Intra-individual variability of Raman peak shifts and peak widths, which indicate variation in crystalline structure, was observed in localized regions of {\it B. aculeata} tests. The intra-individual variability was consistent with significant differences in minor element signatures measured by ICP-MS, and correlate with variation in biomineralization processes during the foraminifer's lifespan. These variations were observed in oxidatively-cleaned and photobleached foraminifers. Subsequent reductive cleaning of the tests and reanalysis produced significantly less spectral variability, potentially due to partial dissolution of the less-ordered calcite. Further work is planned to elucidate the source of Raman spectral variation by identifying structurally varying regions, isolating them by high-resolution laser microsectioning, and then analyzing them directly for minor element composition by ICP-MS.
B13E-03 INVITED 14:10h
Has the Classical Thermodynamic Concept of Solubility Lost Its Meaning for Carbonate Minerals in Complex Solutions
For well over half a century the application of classical chemical thermodynamic concepts to mineral solution-interactions has served well the advancement of aquatic geochemistry, not only for abiotic processes, but also in advancing our understanding of biomineralization. Probably no group of minerals has received more attention in this regard than carbonate minerals and in particular, calcite and aragonite. As a result, serious questions have arisen about the limitations of equilibrium thermodynamics as a useful predictive theoretical framework to describe carbonate mineral-solution interactions that have a direct bearing on biomineralization processes. One of the central questions is whether or not it is useful to apply equilibrium thermodynamics to very non-equilibrium mineral-solution interactions. The classical example of this problem is that pure calcite can't be in equilibrium with a multicomponent solution such as seawater; attempts to "solve" this problem through introduction of stoichiometric solubility constants arbitrarily fix compositional relations and offer neither insight nor solution. Simply put, if dissolution and precipitation reactions aren't the same you can't have thermodynamic equilibrium. This problem further manifests itself when solubility is predicted by extrapolating reaction rates to some apparent "kinetic" solubility that differs from that predicted thermodynamically and is a function of reaction inhibitor concentration. It thus becomes rather meaningless to formulate reaction kinetics in terms of thermodynamic free energy "distance" from equilibrium. Many other macroscopic examples could be given including problems with rate dependence of distribution coefficients, heterogeneity, nucleation and growth of a metastable phase on a stable phase, for example aragonite on calcite and in structural ordering involving double carbonates. However, the most recent and disturbing problems have arisen from the many rapidly evolving techniques for observing processes occurring in the near mineral-solution interfacial region. These observations reveal complex processes, often of a highly heterogeneous nature, on a nanoscale size range. The great challenge is how to integrate these into observations into a useful macroscale predictive formulation. It is our opinion that a potential path for doing so has been shown in chemistry where statistical mechanics have been used to provide molecular-based mathematical models capable of predicting macroscale processes in integrated form. This approach has particular relevance for advancing understanding of biomineralization where organisms employ strategies involving molecular level manipulation to "escape" what would be expected from equilibrium thermodynamics.
B13E-04 14:25h
Determinism and Chaos: The Role and Importance of Nonlinear Interactions Involving Elementary Surface Mechanisms in the Dissolution Of Crystalline Materials
Much of the work involving the reaction kinetics of crystalline materials implicitly assumes that reaction rates are constant for a given set of externally imposed physical and chemical conditions, an assumption often justified by observations of exponential approach of an experimental rate to a (quasi)steady state value. In crystal dissolution experiments, we formalize this result by assigning rate {\it{constants}} to reactions that are presumably free of composition and surface area terms. In a general way, we also assume that crystal dissolution far from equilibrium is free of feedback effects that often characterize dissipative systems during crystal growth, in which nonlinear relationships involving reaction and mass transport give rise to self-organized phenomena such as oscillatory zoning and other spatial distributions. However, there is no {\it{a priori}} reason to expect that feedback effects cannot also characterize dissolution reactions, or that such interactions may give rise to outcomes that are uniquely predictable. We now have the opportunity to observe such phenomena in nano-scale processes involving the evolution of surface topography undergoing dissolution, both with and without microbial intervention. We test this hypothesis through an examination of AFM and vertical scanning interferometry results, kinetic Monte Carlo modeling, and theoretical considerations, and discuss how crystal surfaces may yield dynamical examples of deterministic chaos.
B13E-05 14:40h
Preliminary experimental results for trace element uptake in carbonates: Pb2+ in calcite and U6+ in aragonite at various growth rates and temperatures
The surface of a crystal in equilibrium with surrounding fluid can have a composition and structure that differs from the bulk crystal. If the growth rate is fast relative to the diffusive equilibrium time, then the crystal surface composition may be partially "captured" by the newly-formed lattice. The extent of this growth entrapment increases with crystal growth rate and is suppressed by diffusive equilibration between the surface and the bulk lattice of the crystal. Partitioning of Sr into calcite is probably affected by this entrapment mechanism. To explore the relevance of this idea to other elements and other carbonates we conducted experiments on Pb$^{2+}$ in calcite and U$^{6+}$ in aragonite, for which relatively few data exist. There are, however, some intriguing indications in the published data that show the concentration of Pb$^{2+}$ is higher at the calcite surface relative to the bulk lattice; U$^{6+}$ partitioning into aragonite decreases with increasing growth rate. Both these previous studies involved the use of bulk analytical techniques; we, in contrast, performed in situ measurements on single crystals of known growth rate. The carbonate crystals were grown by two different mechanisms: 1) precipitation of calcite on a plate (pre-coated with calcite), using a steady flow of CaCl$_{2}$ - PbCl$_{2}$ and Na$_{2}$CO$_{3}$ solutions mixed just before passage through a tube and dripped onto a plate (cave-type experiments, I=0.02); 2) growth of calcite and aragonite from a CaCl$_{2}$ - NH$_{4}$Cl (or NaCl) - PbCl$_{2}$ (or UO$_{2}$[NO$_{3}$]$_{2}$) solution by diffusion of CO$_{2}$ from an ammonium carbonate source (drift experiments, I=0.52-0.58). The growth rate of individual crystals was determined by comparison of final size with duration of the experiment. The precipitated crystals show significant variation in size even when the runs have the same input rate of CaCO$_{3}$ components. The cave-type runs at 25$\mp$2, 40$\mp$1, and 50$\mp$1$\deg$C yielded 15-40 $\mu$m calcites, but in the drift experiments the aragonite crystals attained sizes up to 500 $\mu$m. Electron microprobe analysis across the large crystals shows that the concentration of U is higher in the center and decreases toward the edge. This is probably due to the cube root dependence of linear growth on volume change of the growing crystals. We also observed that increasing aragonite growth rate (V, nm/s) enhances U partitioning between the bulk crystal and the liquid, K$_{d}$$^{bulk/liquid}$=(U/Ca)$_{bulk}$/(U/Ca)$_{liquid}$ (in contrast to the previous study). K$_{d}$$^{bulk/liquid}$ = 0.65 to 1.91$\mp$0.08 when log(V)=-0.73 to 0.31 at 53$\mp$3$\deg$C in the drift-type run (I=0.58). This evidence supports the idea that U is enriched at the calcite surface relative to the bulk crystal during crystal growth. The Pb partition coefficient varies from 0.01 to 1.26. At present time we are characterizing the parameter that responsible for this broad interval of K$_{d}$ variation. Uranium partitioning into aragonite decreases with increasing temperature. Our preliminary data can be described by log(K$_{d}$) versus 10$^{4}$/T(K) plot with the slope equal to 0.064, corresponding to $\Delta$H$^{0}$ of -12.29 (kJ/mol).
B13E-06 14:55h
Skeletal Structure and Chemistry of a Bermudan Gorgonian Plexaurella dichotoma
Long-lived Gorgonian corals are distributed across the world's oceans from the surface to great depths. They accrete annually banded skeletons of both organic gorgonin and mineralised components and thus provide the opportunity to access previously unavailable records of climate change. We used SIMS ion microprobe to analyse Mg/Ca and Sr/Ca ratios of high-Mg calcite loculi within the skeleton of a Bermudan gorgonian, Plexaurella dichotoma collected from an inner lagoon reef site in October 2001. Loculi are 10 - 140 microns in diameter and 3-5 mm in length and are composed of radiating bundles of needle crystals. Both loculi and gorgonin are arranged in concentric rings about the axial core; loculi density decreases towards the outermost bands. Discrete sample spots, 20 microns diameter, were sputtered from individual loculi from the axial core to the outermost band, a distance of 4.75 mm. 38 annual bands were counted along the sample path, indicating an age of 38 years (1963 -2001). Both Mg/Ca and Sr/Ca ratios track the actual interannual sea surface temperature variability recorded at Hydrostation S for the corresponding time period. Mg/Ca ratios of the calcite loculi are positively correlated with temperature whereas the Sr/Ca ratios are inversely correlated with temperature, in agreement with observations for other calcite skeletons and laboratory-grown inorganic calcites. At our site, the strongest correlation amongst Mg/Ca and Sr/Ca, and temperature occurs in the autumn (October-December) indicating that maximum growth of the loculi occurs within this period, after the reproductive season has ended. The sensitivity of Mg/Ca to temperature is higher in Plexaurella dichotoma than is observed for inorganic calcite. Mg/Ca and Sr/Ca ratios within individual loculi exhibited little variability, further supporting our hypothesis that growth of these structures may be restricted to a short period of the year. These hypotheses are currently being tested with field-staining techniques. Our study demonstrates the enormous potential of gorgonian corals as archives of past ocean conditions, from the surface to the deep sea, from the equator to the subpolar regions.
B13E-07 15:10h
The Interpretation of Mg/Ca in Ostracode Valves: Biokinetic vs. Thermodynamic Controls
The geochemistry of the calcite valves of ostracodes (a group of micro-crustacean) is often used to reconstruct the history of aqueous environments in both marine and fresh-water settings. These benthic animals can be very abundant in lakes and ponds and their low-Mg calcite valves are easily recovered from sediment cores. Many studies have used minor-element ratios (Mg/Ca and Sr/Ca) as indicators of temperature and/or salinity change through time and numerous calibration studies have been undertaken. There is considerable disagreement on the interpretation of both historical data and calibration studies because of differing views on what controls elemental ratios in ostracode valves. Here we focus on Mg/Ca ratios and critique the dominant assumption that Mg/Ca ratios in ostracode calcite are interpretable as a temperature-dependant distribution (or partition) coefficient. The use of a distribution coefficient, usually defined as a ratio of shell-to-water Mg/Ca ratios, assumes that the ratio in the water plays a significant role in the resultant ratio in the shell. Ostracode biomineralization is most commonly viewed as equivalent to inorganic precipitation of low-Mg calcite from solution, a system in which distribution coefficients are probably valid models. However, a re-examination of published studies shows that in many cases Mg/Ca(water) has no statistically demonstrable affect on the Mg/Ca ratio of ostracode valve calcite. The valve Mg/Ca ratio is most often a function of ambient temperature. In a number of studies the importance of the water's Mg/Ca ratio cannot be determined due to auto-correlation with other environmental factors. This implies that there is considerable biological control on the minor element chemistry of the ostracode valve. This is supported by a number of observations: valve calcification is rapid and initiated by the animal; Mg/Ca ratios within the valve vary greatly on a microscopic scale; the earliest carbonate formed during calcification approaches magnesite; other crustaceans accumulate Mg prior to molting. Finally, we note that ostracode valves typically fall into the 0.003 to 0.04 range of molar Mg/Ca ratios, and that ratios greater than 0.05 are extremely rare in the valves of fully-calcified adult ostracodes of any species. Yet these animals thrive in waters where the range of Mg/Ca ratio covers more than 3 orders of magnitude. A distribution coefficient model incorrectly predicts a similar 3 order of magnitude range in shell chemistry. Alternatively a bio-kinetic model for valve chemistry yields very promising results for the use of Mg/Ca ratios in ostracode valves as a paleothermometer.
B13E-08 15:25h
Trace element proxies (Sr/Ca, Ba/Ca and Pb/Ca) in Bivalve shells: environmental signals or not?
Coral and sclerosponge skeletons have both been used as recorders of their environment. Sr/Ca, Ba/Ca and Pb/Ca have all shown to be useful in these substrates, giving insight into the past environment in which the skeleton grew (e.g., Lea et al., 1989, Nature 340, 373-376; Beck et al., 1992, Science 257, 644-647; Lazareth et al., 2000, Geology 28, 515-518; Rosenheim et al., 2004, Geology 32, 145-148). Although bivalves have not been studied as extensively as corals, these proxies are apparently not as reliable in bivalves (e.g., Vander Putten et al., 2000, GCA 64, 997-1011). We therefore investigate Sr/Ca and Pb/Ca in two species of aragonitic clams (Mercenaria mercenaria and Saxidomus giganteus) and Ba/Ca in the calcite layer of the mussel Mytilus edulis. Results indicate that Sr/Ca is primarily controlled by growth rate in S. giganteus whereas there was no relationship between these parameters in M. mercenaria. Pb/Ca is somewhat reproducible between specimens of S. giganteus, however long-term Pb/Ca records (1949-2003) in the shell of M. mercenaria did not show the expected curve of anthropogenically introduced lead, indicating that they are not recording environmental Pb concentrations. Therefore, Sr/Ca and Pb/Ca incorporation seem to be regulated by biological processes and not directly by environmental parameters. Ba/Ca in M. edulis shells on the other hand, does seem to be directly linked to the environment. Shells grown under laboratory and natural conditions both show the same linear relationship between dissolved Ba/Ca and shell Ba/Ca. Experiments involving manipulations of dissolved and particulate (i.e. food) Ba/Ca, suggest that the dominant pathway of barium into the shell is from the dissolved phase via the hemolymph. We were unable to explain the large peaks noted in the Ba/Ca profiles, however, they did not seem linked to phytoplankton blooms as has been previously suggested (Stecher et al., 1996, GCA 60, 3445-3456; Vander Putten et al., 2000; Lazareth et al., 2003, Est. Coast. Shelf Sci. 17, 1461-1470).