B31D-0610
Growth patterns of an intertidal gastropod as revealed by oxygen isotope analysis
The size and morphology of mollusk shells are affected by environmental conditions. As a result, it is difficult to assess growth rate, population age structure, shell morphologies associated with ontogenetic stages, and to compare life history patterns across various environments. Oxygen isotope analysis is a useful tool for estimating minimum ages and growth rates of calcium carbonate secreting organisms. Calcite shell material from members of two northern California populations of the intertidal muricid gastropod Acanthinucella spirata was sampled for isotopic analysis. Individual shells were sampled from apex to margin, thus providing a sequential record of juvenile and adult growth. A. spirata were collected from a sheltered habitat in Tomales Bay and from an exposed reef in Bolinas. Abiotic factors, such as temperature, wave exposure, and substrate consistency, and biotic composition differ significantly between these sites, possibly resulting in local adaptations and variation in life history and growth patterns. Shell morphology of A. spirata changes with age as internal shell margin thickenings of denticle rows associated with external growth bands are irregularly accreted. It is not known when, either seasonally and/or ontogentically, these thickenings and bands form or whether inter or intra-populational variation exists. Preliminary results demonstrate the seasonal oxygen isotopic variability present at the two coastal sites, indicating 5-6 degC changes from winter to summertime temperatures; these data are consistent with local intertidal temperature records. Analysis of the seasonal patterns indicate that: 1) differences in growth rate and seasonal growth patterns at different ontogenetic stages within populations, and 2) differences in growth patterns and possibly age structure between the two A. spirata populations. These findings indicate that isotopic analyses, in addition to field observations and morphological measurements, are necessary to assess life history strategies and compare population dynamics under varying environmental conditions.
B31D-0611
Carbonate-associated sulfate in lucinid (Bivalvia) shells
Symbiosis is a fundamental driver of evolution, with examples ranging from mitochondria in eukaryotic cells to barnacle-whale commensalism. The association between sulfur-oxidizing (thiotrophic) bacteria and the lucinid bivalve clade is particularly intriguing because the inferred antiquity of the relationship (>400 m.y.) seems at odds with the relatively loose ecologic linkage of living members. Because only half of genus-level lucinid taxa are extant, and the δ13C of shell carbonate exhibits no systematic difference between symbiotic and non- symbiotic bivalves, a new morphologically-independent proxy to determine whether fossil taxa possessed thiotrophic endosymbionts is needed. The δ34S of carbonate-associated sulfate (CAS) in bivalve shells may hold promise because biogenic carbonate incorporates sulfate into its crystal structure during biomineralization. Incorporation of bacterially derived SO42- (with a more negative δ34S value due to its reduced sulfur origin) into the lucinid-shell crystal lattice would, therefore, impart a distinctly lower δ34SCAS value than that from seawater SO42-, and would be distinguishable from CAS values of co- occurring heterotrophic bivalves. We measured CAS contents, δ34SCAS and δ18OCAS values of 15 sets of lucinid and co-occurring infaunal and epifaunal heterotrophic bivalve shells collected from modern and Cenozoic shallow marine sites. The modern bivalve shells had variable CAS content, from 100 to 2600 ppm. Epifauna often had the highest concentrations relative to the other ecological groups. The δ34SCAS and δ18OCAS clustered at values corresponding to modern seawater sulfate, but with significant scatter. There was no systematic isotope- compositional difference among all bivalves in the same habitat, or among the same lucinid, infaunal, or epifaunal groups across different sites. The fossil bivalve shells tended to preserve lower CAS concentrations and the isotope compositions further deviated from seawater values. These data suggest that 1) pore-water sulfate in shallow sediments is highly heterogeneous in its concentration and isotope composition, probably due to active microbial sulfate reduction, bioturbation, and water-pumping by bivalves and other infaunal filter feeders; 2) CAS is derived from ambient porewater or pumped-in seawater for infauna or epifauna, as well as for lucinids; and 3) CAS concentration and isotope compositions are vulnerable to later diagenetic processes.
B31D-0612
Trace element ratios in bivalve shells as records of environmental conditions
Stable isotope and trace element data from the carbonate of both marine and freshwater bivalves are proving to be useful tools in studies of palaeoclimate and environmental change. However, much of the work already done has shown that the trace element ratios in bivalve shells exhibit a complex relationship with the ambient environment and caution must be exercised when attempting to use them as environmental proxies. This work examines the feasibility of using the trace element ratios Mg/Ca, Sr/Ca, Ba/Ca and Mn/Ca of the shells of a number of different species of bivalves as records of the temperature and salinity of their ambient aquatic environment. The species analysed were the estuarine oysters Saccostrea glomerata, Ostrea angasi, and Crassostrea gigas, an estuarine mussel, Mytilus galloprovincialis, and the freshwater mussel Velesunio ambiguus. The estuarine shells were taken from monitoring experiments conducted over a period of 12 months at two different field sites. Freshwater shells were collected wild, from locations close to water monitoring stations. Preliminary results show distinct variations in the Mg/Ca of O. angasi shells with an apparent seasonal pattern. V. ambiguus shells show clear patterns in Mn/Ca, linked to environmental variations.
B31D-0613
Regional Failure of Intertidal Mussel Mg/Ca as an Independent Temperature Proxy: A Cautionary Tale
Paleoceanographic reconstructions depend upon accurate estimations of mean annual temperature (MAT) and mean annual range in temperature (MART). Mollusk shells may offer an archive of seasonal to decadal data on environmental conditions. However, a suite of factors (e.g., biologic, metabolic, kinetic) may confound interpretations of mollusk records. Mytilus californianus shell chemistry was directly compared to in situ environmental data to inspect the influence of intertidal position and growth rate (ontogeny) on δ18O and Mg/Ca. M. californianus specimens from an ontogenetic spectrum were outplanted from 31 Aug 04 to 17 Sept 05 at low (-0.2 m MLLW) and high (1.1 m MLLW) intertidal positions within Cabrillo National Monument at San Diego, California. Ambient temperature was recorded in situ and water samples were collected at ~2-3 week intervals. After the outplant interval, the prismatic calcite layer of eight specimens spanning the ontogenetic spectrum at each intertidal position were serially microsampled and analyzed for stable-isotope and minor-elemental variations. Average intra-specimen Mg/Ca values for specimens show a strong and significant positive correlation with accretion rate (i.e., younger, faster-growing specimens have higher Mg/Ca). This ontogenetic effect weakens the potential of average intra-specimen Mg/Ca an accurate MAT proxy. In contrast, average intra-specimen. δ18O provided a relatively accurate temperature proxy regardless of ontogenetic stage. To assess the utility of intraskeletal δ18O and Mg/Ca variation as a MART proxy, the observed δ18O record from each specimen was ordinated in the time-domain of the outplant interval and compared to the predicted equilibrium δ18O inorganic calcite record calculated from ambient seawater temperature and δ18O. Observed specimen δ&&18O is well-correlated (r2 = 0.50, p<0.001) with predicted δ18O, but shows an ~+1.05‰ 18O-enrichment relative to inorganic calcite equilibrium precipitation. In contrast, corresponding Mg/Ca is poorly correlated with temperature (r2 = 0.05, p<0.001) due to significant positive relationships with shell accretion rate and intertidal position. Within the extrapallial fluid, pH, carbonate solution chemistry, Rayleigh fractionation and/or an undetermined source of disequilibrium may cause skeletal δ18O values to deviate from predicted ambient seawater equilibrium precipitation. Despite the consistent 18O-enrichment, δ18O M. californianus values capture instrumental MAT and 5-95 percentile MART making M. californianus a valuable proxy for paleoceanographic reconstructions.
B31D-0614
Bivalve Forensics: Sclerochronological Constraints on the Timing of a Biological Invasion in San Francisco Bay
Clams are biological chart recorders: their shells contain a record of environmental conditions in the form of periodic growth increments and geochemical variation. In most species, this archive begins with the deposition of the dissoconch following metamorphosis and ultimately ends with the death of the individual. In fossil specimens, the exact date of the beginning or end of the archive cannot be determined. Therefore, sclerochronologic and geochemical records from fossils represent short intervals of time that cannot be directly tied to absolute time—so called "floating chronologies." In contrast, the date of the end of the archive from live- collected specimens can be precisely resolved—in many cases to the day collection. Using this date as a pinning point, one can establish the timing of events throughout the remainder of the chronology relatively precisely. Here, we use this approach to constrain the timing of a biological invasion event in San Francisco Bay. The basic idea is simple. The timing of an invasion event (i.e., successful recruitment), can be resolved if the date of the initiation of shell deposition is established from the first specimens to appear in a new area. To absolutely establish the timing of an invasion, one must assume that specimens represent the initial cohort of invaders. If this assumption cannot be met, then this method provides minimum estimates for the timing of invasion events. This technique is best suited for regions where species composition is closely monitored and the likelihood of collecting the initial cohort is high. To demonstrate this technique, we examined specimens thought to represent the initial cohort of the oyster Crassostrea gigas to establish a population in South San Francisco Bay. All individuals were collected live in July or August of 2006. Sclerochronological examination suggested that the specimens were at least four years old. These age estimates were confirmed using stable oxygen isotope (δ18O) and stable carbon isotope (δ13C) variation. For each specimen, δ18O and δ13C profiles representing the complete ontogenetic history were obtained by sampling the resilifer in the left valve. In all specimens, δ18O and δ13C values were positively correlated and show strong seasonal variation. Comparison of predicted and observed δ18O variation suggests that both temperature and the δ18O of the water in which they grew—a function of Delta outflow—strongly influence the pattern of δ18O observed in the shell. Together, sclerochronological and geochemical analysis indicate that this invasion of C. gigas in South San Francisco Bay occurred in the Spring of 2002. The ability to establish the timing of a biological invasion is likely to help scientists better understand the dynamics of invasion events. Furthermore, these results may help environmental managers and policy makers manage invasive species, as well as develop strategies to prevent future invasions of C. gigas and other non- native species.
B31D-0615
Ontogenic increase of metabolic carbon in freshwater mussel shells
The carbon isotopic signature of dissolved inorganic carbon (δ13CDIC) is a powerful tool for understanding biogeochemical cycling. Biological carbonates are a potential tool to reconstruct past δ13CDIC, but are not always easy to decipher. Metabolic carbon can be incorporated in the carbonate and interfere with the environmental signal. The amount of metabolic carbon is usually considered to be low, around 10%, but up to 37% has been reported. Recently, it has been noted that the amount of metabolic carbon incorporated into marine bivalve shells is dependent on the carbon demand during calcification. When the bivalve is young and shell growth is fast they incorporate less metabolic carbon. This is also a time when the total amount of respiring tissues between the valves is small and therefore less metabolic CO2 is produced in total. When they age, shell growth slows and the amount of tissues increases, and thus total respired CO2 increases, thereby increasing the amount of metabolic carbon in the shell. We tested to see if this was also the case for freshwater bivalves. Four living individuals of Pyganodon cataracta ranging in size from 21 to 88 mm in height were collected from a 0.5 m2 area of a small stream in November 2006. Tissue δ13C and δ15N, the last year of shell carbonate δ13C and δ18O and δ13CDIC were analyzed. From these data, the percent metabolic carbon in each shell was calculated. The δ18O values were not significantly different between individuals illustrating that similar time is represented in each shell and that they utilized the same water source. δ13C on the other hand shows a clear ontogenic decrease, with the older individuals having more negative values indicating a higher metabolic carbon incorporation into the shell. This is consistent with the model described above. Interestingly, the relationship between size and percent metabolic carbon is similar between freshwater and marine bivalves.
B31D-0616
A multi-axial growth analysis of stable isotopes in the modern shell of Saxidomus gigantea: implications for sclerochronology studies
In this study we use stable-isotope ratios of two modern Saxidomus gigantea specimens from Namu, British Columbia to investigate intra- and inter-specimen isotopic variation. Seasonal stable isotope profiles (δ13Cshell, δ18Oshell) were generated along the axis of maximum growth: a standard method for sclerochronolgical investigations. The profiles show that analogous seasonal variation is recorded in δ18Oshell however, significant variability is recorded in δ13Cshell. We suggest this is caused by differences in metabolic activity between the individuals. Intra-shell variability was evaluated using a Hendy-type test and a multi-axial growth analysis. Isotopic analysis along growth horizons (Hendy-type test) produced good reproducibility for δ13Cshell, but significant variability in δ18Oshell, especially at the sinistral margin. The multi-axial growth analysis generated several profiles crossing a prominent growth band from a single specimen. Similar seasonal variations are recorded in δ18Oshell along all axes analyzed. δ13Cshell show significantly less co-variation and is possibly caused by internal metabolic activity. This study shows that δ18Oshell profiles generated from any portion of the shell are useful in evaluating seasonal fluctuations, and may be an excellent method to evaluate the types and rates of shell growth. These results have implications for stable isotope sclerochronological studies involving the use of fragmented shell material, such as that derived from archaeological sites (e.g., shell middens) or sediment cores.
B31D-0617
Phosphorus Speciation in Skeletal Aragonite of Deep Sea Corals
Phosphorus plays an important role in the world oceans as a limiting nutrient and can serve as an indicator of productivity. This link to bioactivity also relates P concentration to changes in atmospheric CO2 through biotic sequestration. The P concentration of ocean water is also connected to changes in deep sea ocean circulation that are also vehicles for global climate change. A paleoproxy for oceanic P concentration recently has been developed based on the P content in skeletal aragonite of deep-sea corals. The P-content of the septa record the ambient ocean P concentration at the time the time of deposition, which can be measured at high spatial resolution by methods such as ICP-MS. Although the correlation of P content of coral aragonite and ambient seawater suggests that phosphate is incorporated into the aragonite structure during crystal growth, the P speciation in the skeletal aragonite is unknown. We have studied P speciation in deep-sea coral aragonite collected from various localities using P-31 single pulse (SP) and cross polarization magic angle spinning (CP/MAS) nuclear magnetic resonance (NMR) spectroscopic techniques. Spectra of all samples contain a broad peak (>6 ppm full width at half maximum; FWHM) at a chemical shift of 3.0 to 3.5 ppm which is tentatively assigned to phosphate defects in the aragonite structure. Variable contact time CP/MAS NMR spectra indicates that the broad peak at 3.0 to 3.5 ppm is enhanced at short CP contact times, suggesting that H bearing species are important for accommodating phosphate in the aragonite structure. A subset of the samples gives spectra containing an additional, narrow peak (~ 1.5 ppm FWHM) at a chemical shift of 2.6 ppm. The spectral characteristics of this narrow peak, including both chemical shift and CP dynamics, are similar those of hydroxyl-containing apatite phases. On this basis and the small width of the peak it is assigned to crystalline apatite inclusions. These inclusions account for up to 34% of the phosphorus. No simple relationship of apatite content is apparent with P-content or between septa and thecal wall. These results suggest that most of the P in coral aragonite is incorporated as a defect in the crystal structure, but that some apatite may form as a surface precipitate that is incorporated during crystal growth.
B31D-0618
The Impact of Seawater Saturation State on Early Skeletal Development in Larval Corals: Insights into Scleractinian Biomineralization
Understanding the response of coral calcification to changes in seawater saturation state (ocean acidification) could provide important insights into the fundamental processes of scleractinian biomineralization. In particular, larval calcification, which involves initiation of skeletogenesis by a previously non-calcifying planktonic planula, offers a unique opportunity to examine the role and limitations of biological control over an essentially physicochemical process. Larvae of the brooding Atlantic coral Favia fragum were settled in unmodified seawater onto clay tiles within 12h of spawning, and placed into non-through flow 30 L aquaria prior to initiation of calcification. Seawater chemistry was pre-adjusted via HCl addition and continuous bubbling with laboratory air, yielding four aragonite saturation states: Omega(aragonite) = 3.71 (unmodified), 2.4, 1.04, and 0.22. The aquaria were held at 25 °C on a 12h/12h light/dark cycle, and sets of tiles harvested at 1, 5 and 8 days post-spawning. Accretion of aragonite (confirmed by Raman spectroscopy) in all treatments indicates that the settled larvae were able to elevate the saturation state of aquarium seawater sequestered within their calcifying space. However, external aqueous carbonate chemistry had a striking effect on larval mortality, on the nature and timing of basal plate formation, on skeletal growth rates (based on the length and cross-sectional area of septa), and on the structure and organization of aragonite crystals within the septa (imaged using SEM). Larval survival rates at the two lower saturation states was only 40% of that in the control and Omega = 2.35 treatments, and skeletal growth decreased by 30 % (relative to the control) in seawater with saturation state comparable to that predicted for the mid-latitude surface ocean by 2100 AD. SEM imaging of the larval skeletons revealed significant differences in the morphology of aragonite crystals accreted under different conditions. In stark contrast to the fine, closed, densely packed spherulitic bundles accreted in the control system, larvae in the lower Omega treatments produced a disorganized conglomerate of large, highly faceted crystals, consistent with slow growth under low saturation state conditions. Our results suggest that the coral calcification response to changes in seawater saturation state is linked to a physiological limitation on the organism's ability to elevate the saturation state of seawater within the calcifying space. Further, our data indicate that ocean acidification due to fossil fuel CO2 emissions will likely have a strong negative effect on the recruitment and early skeletal development of larval corals over the next several decades.
B31D-0619
Testing the Paleocene-Eocene Thermal Maximum Magnetofossil Spike Hypothesis
Ferromagnetic resonance (FMR) spectroscopy detected a magnetofossil spike in Paleocene-Eocene Thermal Maximum (PETM) kaolinitic siltstone of New Jersey's Atlantic Coastal Plain, confirmed by two independent TEM studies and consistent with (but not required by) data from conventional rock magnetic analyses [1,2]. Applying first-order reversal curve (FORC) analysis to the same sediments demonstrates for the first time that ancient magnetofossils bear a FORC signature similar to that of both cultured and environmental magnetotactic bacteria. In order to test whether the observed PETM magnetofossil enrichment was a local or global phenomenon, we compare multi-proxy enviromagnetic profiles through the Atlantic Coastal Plain clay and present new FMR and rock-magnetic stratigraphies through other Paleocene-Eocene boundary sections. Our analyses of samples from the Paleocene-Eocene GSSP at Dababiya, Egypt, indicate the presence of a positive anisotropic, medium- to-high coercivity ferromagnetic component with FMR signatures similar to transitional signatures immediately preceding and following the magnetofossil spike in New Jersey. References: [1] R. E. Kopp,T. D. Raub, D. Schumann, H. Vali, A. V. Smirnov, and J. L. Kirschvink, 2007. Paleoceanography (in press). [2] P. C. Lippert and J. C. Zachos, 2007. Paleoceanography (in press).
B31D-0620
Possible Eukaryotic Magnetite in the Paleocene-Eocene Boundary Clay, Ancora, New Jersey
We report the discovery of new forms of magnetic particles in the Paleocene-Eocene (P-E) boundary clay from the borehole at Ancora, New Jersey (Ocean Drilling Program Leg 174AX). The P-E boundary clay shows anomalous magnetic properties suggesting enrichment in single-domain (SD) magnetite. Transmission electron microscopy of the magnetic separates shows that the majority of the SD particles are magnetofossils of known crystal shape and size [1]. There are, however, larger magnetite particles having unusual morphology, including: (1) elongated prismatic magnetite of 100 nm width and up to 1 micron length, (2) leaflike magnetite particles up to 2 microns in length, and (3) elongated, conelike particles with an aspect ratio of 4:1. Lattice-fringe images and X-ray microanalysis of these particles show single-crystal structure and stochiometric magnetite composition, similar to magnetite crystals produced by magnetotactic bacteria. Although the dimensions of some of the type 2 and type 3 particles are outside that expected for single-domain behaviour as typically calculated for parallelepipeds and ellipsoids [2], electron holographic analysis reveals a SD signature. It is likely that these magnetic particles are of biogenic origin, as we were unable to find significant amounts of obviously detrital magnetite in the sediments. As these unusual magnetic particles are confined to the narrow P-E boundary layer, environmental changes along the eastern Atlantic margin of North America during the Paleocene-Eocene Thermal Maximum (PETM) may have led to enhanced growth, and perhaps diversification, of magnetite-forming microorganisms. The dimensions of the observed magnetite particles, however, exceed the size of prokaryotes; if they were, indeed, biogenic, they were likely formed by an unknown eukaryotic organism. References: [1] R. E. Kopp et al., 2007. Paleoceanography (in press). [2] R. E. Kopp and J. L. Kirschvink, 2007. Earth Science Reviews. doi:10.1016/j.earscirev.2007.08.001.
B31D-0621
Major Element Geochemistry of Biofilms in a Silica-Precipitating Hot Spring
Hydrothermal biofilm communities represent one of the best present-day representations of early microbial communities, dating back to 2.5 Ga, and possibly 3.8 Ga in the geologic record. Silica-precipitating hydrothermal springs have been thought to have great potential for biosignature preservation. The interactions of hydrothermal water, biofilms, and precipitated siliceous sinter, however, remain poorly constrained. To this end, we collected water and biofilm, as well as contextual sinter and rock samples from various hot springs in Yellowstone National Park. Here we focus on one hot spring in Sentinel Meadow (Lower Geyser Basin), with temperature and pH that vary from the source (93 C, pH 7.4) to the farthest of five collection points down channel (56 C, pH 8.2). Elemental analysis reveals that the biofilms are made up of from <1 to ~11 % dry wt. carbon and ~0.1 to 1% dry wt. nitrogen. Major element analysis via electron microprobe and complimentary x-ray fluorescence show that (excluding C and N from the total) SiO2 constitutes 86 to 94 % dry weight mass, with the rest made up of Al2O3 (3 to 8%), Na2O (1.7 to 3.7%), K2O (0.6 to 1.5%), and minor amounts of FeO, CaO, MgO, and TiO2 (<1%). Local sinter is SiO2 (97.5% dry wt.), Na2O (1.5%), and <1% Al2O3, FeO, K2O, CaO, MgO, and TiO2. In addition, sinter contains measurable amounts of carbon (1.4%) and nitrogen (0.2%). Discrepancies between the biofilm and sinter values show that the geochemical compositions of biofilms are not captured by the precipitating silica. If biofilms accumulated elements strictly from the water, then it would take as much as 440 L of water to supply 1 gram (dry wt) of biofilm with the elements contained therein, assuming complete uptake. This seems especially unlikely in the case of Al, which is quite dilute (~500 ppb), poses very little benefit nutritionally, and increases in concentration down channel. Other major element components also exhibit at least one, if not all, of these traits. A potential source of the elements found in biofilms is aeolian-deposited dust. Area country rock is dominated by siliceous volcanism, represented locally by rhyolite samples collected from Sentinel Meadow. With an average value of ~10 wt % Al2O3 for the surrounding country rock, it would take approximately 0.6 grams of the ground up rock as dust to account for the Al found in one gram of biofilm. The low Al2O3 content of the sinter indicates that the Al is not entombed from the biofilms. A hypothesis for the above discrepancies in Al (as well as other elements) is that dust deposited in the water is captured on the biofilm surfaces, and the biofilm community then breaks down the dust, utilizing any nutritionally or metabolically important elements, and either precipitating (for Si) or releasing (for Al) unnecessary elements. http://geopig.asu.edu/
B31D-0622
Biosignatures in modern sulfates: texture, composition and depositional environments of gypsum deposits at Guerrero Negro, Baja, Mexico
Gypsum (CaSO4·H2O) is an important phase in biogeochemistry and sedimentology as a mineral sink for sulfur, a paleoclimatic indicator, and an endolithic niche for phototrophic and chemotrophic bacteria. Sulfate deposits are also important targets of exploration for evidence of habitable environments and life on Mars. Gypsum deposits from a range of sedimentary environments at the Guerrero Negro crystallizer ponds and sabkha settings were investigated for microscale structure and composition to differentiate fabrics formed under microbial influence from those formed under abiogenic conditions. Sub-sedimentary gypsum forms in sabkha environments as mm to cm scale selenite discs (termed bird beak gypsum; Warren, 2006) and selenite disc aggregates. Selenite discs and other sub-sedimentary gypsum are characterized by a sinuous axial microtexture and poikilitically enclosed detrital particles. Sub-aqueous gypsum forms as cements, granules (termed gypsooids), and massive botryoidal crusts that line the sediment water interface and margins of managed crystallizer ponds and natural anchialine pools. Sub-aqueous gypsum exhibits a wide range of textures and mineral/biofilm associations that include amorphous to euhedral, tabular, needle and lensoidal morphologies. Elemental sulfur forms rinds on prismatic, growth aligned gypsum twins and reticulate magnesian carbonate is interspersed with both twinned crystals and rosette aggregates in stratified sub-aqueous environments. Intracrystalline biofilms and cell material was observed in association with nearly all sub-aqueous morphologies but only scarce evidence has been found for intercrystalline microbial communities. Columnar microbial communities living in anchialine pools were found to host precipitation of mm scale gypsum granules in their EPS matrix. Fine scale gypsum textures are unlikely to persist through diagenetic alteration, but understanding their primary associations with sulfur and carbonates is necessary for interpreting sulfates or their replacement phases in the ancient record.
B31D-0623
Calcite Growth and Dissolution in Nonstoichiometric Solutions: A Site-Specific Role for Carbonate Control in Biomineralization
Biomineralization and crystal growth/dissolution studies typically express reaction kinetics as a function of the ion activity product (IAP) in saturation state expressions. However, there is increasing evidence that changes in the cation/anion solution ratio at constant IAP can influence mineral formation. This variation may thus be important in marine environments, where Ca2+/CO32- activity ratio varies as a function of depth in the water column, as well as in biomineralizing systems, where organisms may manipulate Ca2+/CO32- ratio at sites of mineralization as a vital effect. Lastly, the ratio in surface seawater may have varied in the geologic past, and may also likely change in response to anthropogenic forcing with rising atmospheric carbon dioxide levels. Here we investigate the role of Ca2+/CO32- ratio, at constant saturation state, in determining calcite growth and dissolution using atomic force microscopy (AFM) and vertical scanning interferometry (VSI). These coupled techniques provide kinetic measurements across multiple length-scales. Our results indicate that changes in Ca2+/CO32- ratio significantly affect the overall growth and dissolution rate as well as the distribution of growth and dissolution features on the calcite surface. Kinetic measurements of both calcite growth and dissolution suggest a more critical role for the CO32- ion in governing calcite surface dynamics than previously thought. This finding broadens our understanding of the differential roles of specific species in calcite growth and dissolution, and also permits insight into the variable sensitivity of the surface with respect to trace components. Further, our results demonstrate that carbonate biomineralization cannot be understood in terms of bulk solution chemistry alone, but requires knowledge of both the structure of the biomineral surface and the specific interaction of solution species with surface sites on elementary steps. This study suggests that some of the complexity associated with understanding calcite precipitation and dissolution kinetics may reduce to site-specific interactions with carbonate ions.
B31D-0624
New Origins of the Vital Effect in Calcites: Mg-Enhancing Influence of Biomolecules
Owing to the intense interest in the compositional signatures of biominerals, the mechanistic basis for vital effects and their roles in modifying or masking impurity contents are receiving increasing scrutiny. To date, much of the effort has been focused on the influence of physical environment and inorganic chemical factors. In a recent study that investigated the effects of acidic proteins on calcite growth, our research group found that nanomolar concentratios of acidic amino acids, peptides, and full proteins accelerate the rate of mineral formation by a relationship that correlates with the acidity (hydrophilicity) of the biomolecule (Elhadj et al., 2006, PNAS). Experimental and theoretical evidence suggest that the measured rate-enhancing effect (up to 25X) arises from weak interactions of the biomolecule with the calcite surface to alter the local solvation environment. This relation suggests that the acidic macromolecules that have been isolated from diverse calcifying taxa may have yet unrecognized effects on mineralization. Because Mg has a strong hydration shell relative to Ca, we hypothesized that the presence of these rate-modifying peptides in growth solutions would also lower the barrier to incorporating Mg, and thereby increase the MgCO3 content of calcite overgrowths. To test this idea, measurements of calcite growth rate were made using Atomic Force Microscopy, and in the presence or absence of acidic, hydrophilic 27-mer peptides. The peptide increased the growth rate of obtuse flanks (42% faster, on average, and up to 92% faster) and acute flanks (17% faster on average; up to 54% faster). The calcite overgrowths from AFM experiments were then analyzed for corresponding MgCO3 compositions by Time-of-Flight Secondary Ion Mass Spectrometry. The data yield an inorganic baseline that quantifies the relation between Mg content and the solution concentration. Comparisons of the baseline Mg content to that of peptide-enriched overgrowths show the MgCO3 composition is enhanced by 50 to 70% (in acute and obtuse flanks respectively) in calcite grown in the presence of peptides. Comparisons of these measurements to the MgCO3 compositions reported by Mucci (1987, GCA) for 5-40∞C synthetic seawater show that these differences are equivalent to the offset induced by a temperature change of several degrees. Two possible explanations for the enhanced Mg content in the presence of peptide are 1) step roughening increasing the kink density and 2) partial desolvation of the hydrated Mg ion, which lowers the energy barrier to incorporation.
B31D-0625
Size Distributions and Morphologies of Synthetic and Biogenic Magnetite Nanoparticles
Magnetite crystals formed in the cells of magnetotactic bacteria have narrow size distributions and distinct, strain- specific morphologies. In order to better understand the processes that result in such specific physical properties, we studied the development of nanocrystal sizes and shapes both in cultured cells of magnetotactic bacteria and in abiotic crystal nucleation and growth experiments. Magnetite nucleation and growth was induced in resting, iron-starved cells of Magnetospirillum gryphiswaldense. Freshly induced particles have a normal size distribution and irregular morphologies. As the particles grow, their size distribution changes but remains positively skewed, in contrast to the typically negatively-skewed distribution of magnetite in continuously iron-supplemented, reference cells. The morphologies of the fast-grown magnetite crystals differ from those formed in the reference cells, indicating that the uptake rate of iron can influence the sizes and morphologies of biogenic magnetite nanoparticles. We synthesized magnetite nanoparticles by co-precipitating ferrous and ferric ions from aqueous solutions. By varying the temperature and the types and concentrations of the reagents, we controlled the mean size of the crystals from 10 to 69 nm. Adding phosphate to the solution resulted in irregular morphologies, whereas the nucleation of crystals on synthetic bacterial filaments produced a bimodal size distribution and slightly more regular morphologies. Experiments are being conducted with various organic additives, with the ultimate goal of being able to reproduce the specificity of crystal sizes and shapes observed in magnetotactic bacteria.
B31D-0627
Growth history of cultured pearl oysters based on stable oxygen isotope analysis
We investigated the oxygen isotopic ratio in shells of the pearl oyster Pinctada martensii cultivated in embayments in Mie Prefecture, central Japan, to evaluate the biomineralization of shell structures of the species and its pearls in response to environmental change. Microsamples for oxygen isotope analysis were collected from the surfaces of shells (outer, middle, and inner shell layers) and pearls. Water temperature variations were estimated from the oxygen isotope values of the carbonate. Oxygen isotope profiles of the prismatic calcite of the outer shell layer reflected seasonal variations of water temperature, whereas those of nacreous aragonites of the middle and inner shell layers and pearls recorded temperatures from April to November, June to September, and July to September, respectively. Lower temperatures in autumn and winter might slow the growth of nacreous aragonites. The oxygen isotope values are controlled by both variations of water temperature and shell structures; the prismatic calcite of this species is useful for reconstructing seasonal changes of calcification temperature.
B31D-0626 [WITHDRAWN]
Uncovering of High Strontium Content in Biogenic Calcite: New Data From a Murolith- producing Coccolithophore
Biogeochemical studies of modern coccolithophores have demonstrated a link between growth rate and the incorporation of strontium into coccolith calcite. Values for the Sr content of coccolithophores have been quoted as high as 30 mmol/mol. A paleochemical study of the Early Jurassic deposits of the Paris Basin reveals a biogeochemical anomaly. It appears indeed that a coccolithophorid genus, Crepidolithus, contains unexpected amounts of strontium (ca. 12-20% weight) in their calcite lattice that corresponds to a Sr/Ca ratio around 130 mmol/mol. These results have been obtained using a innovative protocol which enables the concentration of Crepidolithus from a polyspecific assemblage. Chemical measurements have then been performed by ICP-GAAS after a weak acid leaching. SEM spot analyses confirm the reproducibility of high values. At the same time, other calcareous particles (such as Schizosphaerella, coccoliths and euhedral monocrystals) do not show such parallel enrichments in strontium. This observation enables us to preclude diagenesis as the cause of the high Sr values. Moreover, further investigations (SEM, XRD, selective leaching) rule out an additional contaminant phase such as sulfates. Finally, in view of the good preservation of Crepidolithus in the sediment, an aragonite precursor for this coccolith genus can also be excluded. If true, these results raise the question about the mode of incorporation of strontium into the calcite lattice because these values require huge KSr (>> 0.13) and/or unrealistic Sr content in the seawater (ca. 150 ppm !). We discuss here several explanations to explain this bioincorporation of strontium into these coccoliths. 1. A mineralization of (Crepidolithus from a distinct water mass extraordinarily enriched in Sr. For that, we present the physico-chemical structure of the water column during the Early Toarcian OAE and the geographic repartition of this anomaly. 2. A differential vital effect explained by physiological pathways (cross-membrane transport). 3. A kinetic effect due to the growth mode of this murolith where vertical V-units are dominant. In order to verify that last hypothesis, we also investigate the Sr content from others same-structured coccoliths (e.g., Crucirhabdus).