PP31C-0531
Deciphering the Geochemistry of Calcareous Pelagic Producers: Beyond Bulk Carbonate Analyses...
Despite the dominance of calcareous nannofossils in pelagic sediments, their specific geochemistry is still underexploited because of their minute dimensions. Instead, isotopic analyses are performed on bulk carbonate, which is a mixture of calcareous nannofossils and variable amounts of other calcareous components (diagenetic precipitations). The bulk thus records a mean value for different isotopic signatures, i.e. different calcareous components grown in distinct geochemical environments (different calcification depths within the photic zone, deep water masses, interstitial fluid). In order to study the geochemical signatures of each micrometric calcareous component, we propose a new protocol enabling their separation into different subsamples : calcareous nannofossils, planktic and benthic foraminifera, monocrystals. Particles are separated according to (1) size and (2) resistance to ultrasonication related to their microstructure. (1) The granulometric separation is achieved with sequential filtration steps (from 60 down to 2 microns) which allow the recovery of granulometric fractions containing few different particle types (for example mono- and oligospecific assemblages of coccoliths). (2) When various components share their dimensions but exhibit distinct microstructures (imbrication of crystal units, porosity), a strong ultrasonication is applied. During such a treatment, resistant particles remain unaltered while polycrystalline nannofossils are broken. Both particles can be further recovered by filtration. This methodology was applied to various paleoenvironments and lithologies. Oligospecific nannofossil assemblages from Neogene calcareous oozes lighten the influence of specific vital effects on the coccolith isotopic signatures. For lithified sediments, we present monospecific assemblages of coccoliths and nannoliths from three intervals/lithologies: Cretaceous-Paleogene transition clays, Early Toarcian Black Shales and Early Cretaceous marls. The comparison between the geochemical evolution of the photic-zone and the diagenetic overprint offers a better understanding of paleoceanographical changes during these three key-periods.
PP31C-0532
Inter-species, Intra-species and Intra-shell Variations in the Stable Isotope Composition of Late Ordovician Brachiopods From the Ellis Bay Formation, Anticosti Island, Quebec
Oxygen and carbon isotope results have been obtained for Late Ordovician (Hirnantian) brachiopod shells from the Ellis Bay Formation of Anticosti Island, Quebec. This location contains one of the world's most complete Ordovician-Silurian boundary successions and is renown for its abundance of well-preserved brachiopods. The site therefore offers an opportunity to explore the extent of preservation of primary oxygen and carbon isotope compositions in its ancient fossil materials. Three to four samples of each of eight species that lived contemporaneously in very similar environments were analyzed. Several analyses were performed along distinct growth lines of each shell; matrix carbonate and secondary calcite formed during diagenesis were also measured. Mean oxygen isotope values among the different species ranged from –4.1 to –3.1 per mil (VPDB) and mean carbon isotope values ranged from 0.4 to 1.8 per mil. The maximum variation among shells of the same species was 1.0 per mil for oxygen and 0.8 per mil for carbon (both for Parastrophinella). Intra-shell variation along growth lines for all brachiopod species was characterized by systematically higher carbon isotope values (up to 0.8 per mil for Vellamo) with distance from the hinge. Such variations in oxygen isotope composition were not observed. Such behaviour can be explained as a function of brachiopod metabolism. A brachiopod's shell grows radially from the central hinge structure. Young brachiopods experience faster growth rates than older brachiopods, which should produce lower carbon isotope compositions relative to those expected from equilibrium fractionation. The observed pattern in carbon isotope values suggests that original shell compositions have remained intact at this locality. The small but significant differences in shell isotopic compositions within and among shell species may also indicate preservation of local variations in environmental conditions and species-dependent metabolic behaviours. The significantly different isotopic values of secondary calcite from the interior of some shells also suggest that primary compositions were not reset during diagenesis. These data suggest that brachiopod shells from this locality can provide good proxies for the oxygen isotope composition of seawater and climatic conditions at this point in geological time.
PP31C-0533
Diel Changes in U/Ca of Cultured Orbulina universa from Laser ICP-MS Profiles
Bulk analyses of U/Ca in shells of cultured planktonic foraminifera (both symbiont-bearing Orbulina universa and symbiont-free Globigerina bulloides) showed a significant decrease with increasing pH (or carbonate ion). We explored the pH effect on U/Ca in calcite of cultured O. universa further by creating high-resolution compositional profiles of individual shells using laser ICP-MS. This approach takes advantage of the diel changes in the pH of the shell microenvironment that occur in symbiont-bearing species due to uptake and release of CO2 during photosynthesis and respiration. Samples were grown in laboratory-cultures under a 12-hour high-light (day)-12-hour dark (night) cycle, which was synchronized with transfer of living foraminifers between seawater spiked with barium (to provide a geochemical time label) and natural seawater or seawater spiked only with U. Preliminary results show that U/Ca (along with Mg/Ca) increases at night and decreases during the day. These results are consistent with previous trends observed in bulk analyses and suggest that symbiont activity (or lack thereof) has an important influence over the metal composition of calcite shells.
PP31C-0534
Effect of CO32- ions activity on the distribution of UO22+ ions between coral skeleton and seawater
In order to evaluate the paleo-ocean environmental change, trace element contents of coral skeleton have been used as proxy of temperature and salinity in seawater. For example, Sr/Ca and Mg/Ca ratios are useful parameters of past seawater temperature. During calcification, It is considered that uranium is incorporated into coral skeleton as cation UO22+ ions on Ca2+ sites. Dissolved uranium in seawater is ca.13nmol/l and present as uranyl carbonate complex ions such as UO2(CO3) 34-, as well as free UO22+ ions, where complex formation affected by pH and/or CO32- ions activity in seawater. Therefore uranium content of coral skeletons is expected to be a function of pH and/or CO32- ions activity in seawater. We investigated distribution of UO22+ ions between coral skeleton and seawater. We incubated a coral colony ( Goniastrea aspera;18×26×15 cm3) at a temperature of 40 degrees centigrade in the water tank (27×38×24 cm3) under sunlight for 8 hours and collected seawater every two hours. We analyzed uranium, calcium, alkalinity and salinity in seawater by using ICP-MS, EGTA titration, HCl titration and salinometer respectively. pH is gradually increased from 8.17 to 8.54. Uranium concentration decreased from 13.22 nmol/l to 12.89 nmol/l until pH reached to 8.45. Calcium concentration kept decreasing from 10.54 nmol/l to 10.14 nmol/l. Logarithmic distribution coefficient λ is calculated by measured uranium and calcium; λ=log(U,i/U,f)/log(Ca,i/Ca,f). Activity of carbonate ions is calculated by using activity coefficient γ\ CO32- = 0.021. Logarithmic distribution coefficient λ decreased from 2.6 to 0.6 when activity of CO32- ions decreased from 5.0 nmol/l to 4.4 nmol/l. The result of this study suggested that distribution of UO22+ ions between coral skeleton and seawater was clearly influenced by activity of carbonate or pH in seawater.
PP31C-0535
Speciation of Mg in Biologically Produced Calcium Carbonates
It is well known that Mg can exert strong influence on the crystal morphology of different carbonate phases and therefore play an important role in the biomineralization process of marine carbonate skeletons. However, very little is known about the chemical speciation of Mg in these carbonate skeletons. We have initiated an X-ray Absorption Near Edge Structure spectrocopy (XANES) study of a variety of marine carbonate skeletons and report striking differences in the chemical speciation of Mg.
PP31C-0536
High Natural Variability in Delta18O Values of the Otoliths of Young Pacific Sardines Captured in Mexican Waters
Measurements of the oxygen isotope ratios of marine fish otoliths have been used to infer patterns of habitat utilization, mixing among subpopulations and geographical population structure. Otoliths, or "ear stones", are small, metabolically inert three-dimensional structures comprised primarily of carbonate in the form of aragonite. They grow continuously throughout life by deposition of periodic rings, which has made them invaluable for estimating age and reconstructing growth histories. Oxygen isotopes incorporated into otolith carbonate are deposited at or near equilibrium with their relative abundance in seawater, and do not appear to be influenced by kinetic effects or metabolic processes. The greater discrimination against 18O during CaCO3 formation at higher temperatures results in a predictable negative relationship between temperature and otolith δ 18O values. However, biological effects, such as natural variations in otolith growth rates, can indirectly influence the isotopic composition measured in whole otoliths or in subsamples corresponding to different stages of the life cycle. Temperature estimates derived from δ 18O values thus represent an integrated measure of the conditions experienced by an individual. We measured δ 18O values in whole otoliths of young (ca. 1 yr old) Pacific sardines (Sardinops sagax caeruleus) collected throughout the Mexican Pacific to quantify natural variability, reconstruct integrated temperature histories and evaluate the suitability of isotopic measurements as natural tracers of mixing processes. Isotopic values and back-calculated temperature estimates of young sardines of similar sizes captured simultaneously were highly variable (up to 2.0 ‰ and 10 ° C). The limited variations in salinity within each location (< 0.8 psu) imply differences in thermal history among individuals and hence mixing during the first year of life. Based on the schooling behavior and vertical habitat utilization patterns of Pacific sardines, it is unlikely that differences in vertical distribution are the cause of the large variation in isotopic values. We tested the null hypothesis of limited horizontal movement of local populations by comparing (1) local δ 18O values with predicted carbonate isotopic values and (2) ‘life-to-date' sea surface temperatures (SSTs) with back-calculated temperature estimates. Some individuals exhibited values outside the range of locally predicted carbonate isotopic ratios and ‘life-to-date' SSTs, suggesting they were not permanent residents. An empirical otolith growth model was developed to examine the magnitude of the effect of growth rate variations and seasonal temperature fluctuations on δ 18O values. For a 1 yr old sardine, the percent monthly increase in otolith radius (used as a proxy for accretion rate) varied between 4-18 %\. We used the model in conjunction with average monthly SST series to calculate δ 18O values with and without weighing for monthly growth rate variations. Failing to account for monthly variations in otolith growth rates led to differences of 0.15 to 0.69 ‰ relative to unweighed estimates. Differences in otolith growth rates among individuals probably cannot fully account for the observed high variability, although they certainly influence δ18O values and hence back-calculated temperature estimates. Further research must be conducted to evaluate the potential effect of indirect biological effects such as variations in carbonate accretion rates on otolith isotopic values.
PP31C-0537
‘Clumped isotope' thermometry in benthic foraminifera and ostracods: A novel tool for reconstructing deep-ocean temperatures
The benthic oxygen isotope (δ18O) record is an archive of past deep-water temperatures and ice volume. The carbonate ‘clumped isotope' thermometer provides a new technique for reconstructing temperature. It is based on the principle that the proportion of 13C-18O bonds in carbonate minerals is temperature-dependent [1-3]. Unlike other temperature proxies, the clumping of heavy isotopes into bonds with each other is independent of the isotopic composition of the water in which the mineral precipitated. Abundances of 13C-18O bonds in carbonates are determined by measuring Δ47 values of CO2 extracted from carbonate by acid digestion; Δ47 values reflect enrichments, in per mil, of 13C18O16O in product CO2 relative to the amount expected by random chance. Paired measurements of Δ47 and δ18O can potentially yield accurate and quantitative determinations of past temperature and seawater δ18O. We have measured the Δ47 values of core-top benthic foraminifera and ostracods. Preliminary results show that multiple species with different ecologies exhibit the same temperature dependence as inorganic calcite [1]. In contrast to the benthic Mg/Ca thermometer [4], there does not appear to be a discernable influence of carbonate ion concentration on the Δ47 composition of benthic foraminifera (e.g. a carbonate ion effect). Calibration data will be shown for several benthic foraminifera, including the epifaunal taxa Planulina wuellerstorfi, Cibicidoides mundulus, Cibicidoides pachyderma, Hoeglundina elegans, Gyroidina sp. and the infaunal taxa Oridorsalis umbonatus, Melonis pompilioides, and Uvigerina sp. In addition, we will discuss results for the ostracod genus Krithe sp. and mixed ostracod species. [1] Wang, Z., Schauble, E., and Eiler, J., 2004, Equilibrium thermodynamics of multiply substituted isotopologues of molecular gases, Geochimica et Cosmochimica Acta, 68, 4779-4797l [2] Schauble, E., Ghosh, P., and Eiler, J. 2006, Preferential formation of 13C-18O bonds in carbonate minerals, estimated using first-principles lattice dynamics, Geochimica et Cosmochimica Acta, 70, 2510-2529. [3] Ghosh, P., Adkins, J., Affek, H, Balta, B., Guo, W., Schauble, E, Schrag, D. and Eiler, J., 2006, 13C–18O bonds in carbonate minerals: A new kind of paleothermometer, Geochimica et Cosmochimica Acta, 70, 1439–1456. [4] Elderfield, H., J. Yu, P. Anand, T. Kiefer, and B. Nyland, 2006, Calibrations for benthic foraminiferal Mg/Ca palaeothermometry and the carbonate ion hypothesis. Earth and Planetary Science Letters, 250, 633-649.
PP31C-0538
Coral Skeletal Records of Water Quality Change in Mesoamerica
Corals are thought to incorporate metals into their aragonitic skeletons in direct proportion to those found in the surrounding seawater. As they can live for hundreds of years, they are unique recorders of water quality over anthropogenic time scales. We utilized cores from the massive coral Montastrea faveolata from four locations across the Mesoamerican Barrier Reef, the second largest barrier reef on the planet. The sites were chosen to span an inferred gradient of runoff, from the high runoff Sapodilla Cayes and Cayos Cochinos to Utila and Turneffe Atoll, the farthest from major runoff effects. Surface samples of corals at all sites confirm that Turneffe is the least runoff-affected site. Annual samples of coral skeletal material were separated and cleaned using a multi-step leaching procedure to remove surface and interstitial contamination. 18 metals were then measured using an inductively coupled plasma mass spectrometer and normalized to calcium. Ba/Ca, a proxy for sedimentation, shows similar patterns for annual samples from the Sapodilla Cayes and Cayos Cochinos. At both sites, background Ba/Ca increases between ~1950-1970, indicating an overall increase in the amount of sediment reaching the reefs. Also, large spikes in the record may record massive runoff events from storms tracking overland, such as Hurricane Fifi in 1974. 100-150 year long records of Ba/Ca and other metals from these four sites will be compared to investigate changes in water quality over time and location on the reef.
PP31C-0539
A Cretaceous Benthic Foraminiferal Stable Isotope Compilation
We produced new stable isotope data sets of Cenomanian to Santonian benthic foraminifera from the western equatorial Atlantic (ODP Leg 207) and from the tropical Pacific Ocean (DSDP Sites 305 and 463). Together with literature data our results are compiled into a global isotope compilation, resulting in a continuous benthic d18O record from 115-65 Ma. This compilation shows four main intervals: (1) increasing temperatures before 97 Ma and (2) a subsequent super-greenhouse which are both paralleled by increasing d13C values, (3) a long-lasting cooling and decrease in carbon isotopes (90-78 Ma), and (4) globally similar d13C and d18O values after 78 Ma. Increasing sea-surface temperatures sometimes exceeding 35°C are well-known for Intervals 1 and 2. But our compilation shows, that deep-ocean temperatures were significantly warmer than today, especially in the proto- North Atlantic (20-28°C). These high temperatures are explained by a lack of cold bottom-water formation, the restricted nature of the North Atlantic, and the formation of warm saline bottom waters that sporadically were formed within epicontinental seas. The parallel positive trend in d13C is believed to reflect massive storage of Corg during Cretaceous black shale formation. Interestingly, however, d13C values of the tropical Atlantic show a similar trend but more negative values. We propose that this reflects a combination of extensive remineralization of 12C and a long residence time due to the sporadic formation of warm and saline waters. During the following interval 3, benthic d18O values of all ocean basins show similar values. This trend is interpreted to be the result of the beginning opening of the Equatorial Atlantic Gateway. This deepening and a parallel reorganization of the oceanic circulation with longitudinal water-mass and heat exchange may have favoured the observed cooling trend of interval 3. This explanation is supported by the global decrease in d13C, proposed to reflect a better connection of the former restricted North Atlantic that allows the oxidization of the organic-rich sediments formed in this basin. In contrast to the former intervals, the last 13 Ma of the Cretaceous are, on a global scale, characterized by similar values for both, oxygen and carbon isotopes. This is proposed to indicate a full connection between all ocean basins.
PP31C-0540
Variation of initial Th-230/Th-232 and limits of high precision U-Th dating of shallow-water corals
One hundred seventy eight U-Th data of 25 pristine modern and Holocene corals and 33 seawater samples were analyzed to understand the variability of coral initial 230Th/232Th (230Th/232Th0). Samples were collected from 2 continental shelf sites, Nanwan of southern Taiwan in the western Pacific and Son Tra of central Vietnam in the South China Sea; 1 open ocean site, Santo of Vanuatu in the western tropical Pacific; and 6 intertidal sites around the islands of Simeulue, Lago, North Pagai and South Pagai of Sumatra in the eastern Indian Ocean. Using site-specific 230Th/232Th0 values or isochron techniques, our study demonstrates that a coral 230Th dating method with a precision as good as 1 year is achievable for corals with an age less than 100 years. The results show that thorium is mainly attributed to the dissolved phase of seawater and the variation of 230Th/232Th0 depends on local hydrology. 230Th/232Th0 atomic ratios of 4.9 x 10-6 and 3.2 x 10-6 with a 10% variation are respectively observed in Nanwan and Son Tra with intense terrestrial material input. At the Santo site, we find a value of 5.6 x 10-6 at 4 horizons and one high value of 24 x 10-6 in 1974.6 ± 0.5 AD, attributed to the upwelling of cold water during a La Nina event from 1973-1976 AD. The natural dynamics of 230Th/232Th0 at the intertidal sites in the Sumatran islands are complicated and vary significantly from 3.0-9.4 x 10-6. Discordant ages, ranging from -23 to +4 years, are observed in 2.2% of the 139 total subsamples, accounting for detrital material with anomalous high and low 230Th/232Th values. Duplicate measurement of coeval subsamples is therefore recommended to verify the age accuracy. A high standard quality control program, with careful sampling and subsampling processes, contamination-free storage methods, and low chemistry 230Th blank levels, is required.
PP31C-0541
An Investigation of Gulf of Mexico Donax variabilis as a Potential Paleoclimate Proxy
Isotopic analysis of archaeological midden shells to examine paleoclimatic variations has been applied to geological investigations in many parts of the world. Stable isotope ratios taken from growth bands in the shell can demonstrate seasonal temperature variations of the water in which the organisms lived, as the changes in water temperature fluctuate with varying climatic conditions. However, the paleoclimate community has become increasingly aware of the importance of understanding the relationship between the biology of any particular species and the environment in which it lives prior to applying archaeological shells as a proxy for reconstructing past climate. This research focuses on the use of the mollusc Donax variabilis as a proxy in sea surface temperature reconstruction along the northern coast of the Gulf of Mexico. Donax variabilis is a short-lived species that filter-feeds in the swash zone, moving in and out with waves and tides. Since this species only lives in open coastal water, it is speculated that the salinity range for the environment in which the species lives should be narrow, thereby limiting the possible δ18Owater values when applying a paleothermometer equation to reconstruct sea surface temperature recorded in the stable oxygen isotope values of the species' carbonate shells. However, since there are several fresh water sources that empty into the Gulf of Mexico within the study area, a study of the reliability of this assumption is being conducted before the application of archaeological midden specimens from the study area can be depended on for climate reconstruction. In order to investigate the relationship of Donax variabilis to its environment, an analysis of modern specimens is being conducted. Stable oxygen isotope profiles are currently being constructed for several specimens collected from the Gulf Coast of Alabama and the westernmost part of the Florida Panhandle. Present findings show seasonal variation in the δ18Ocarbonate values recorded in the shells' ontogenies. However, for comparison between the stable oxygen isotope content of the shells with that of the water, several water samples taken from the same locations as the modern Donax variabilis specimens have been collected and are currently undergoing analysis for salinity and stable oxygen isotope content. The findings from this study may demonstrate the reliability of Donax variabilis as a paleoclimate proxy for sea surface temperature reconstruction along the northern coast of the Gulf of Mexico.
PP31C-0542
Relating Productivity Events to Holocene Bivalve Shell Growth Rates
The growth rate of a bivalve can be influenced by many environmental factors that can change during the life of the organism. In this contribution we present initial data from a millennium scale chronology to assess the relationship between ontogenetic growth in the bivalve Semele casali and paleoenvironmental conditions preserved in the shell using growth increment analysis, radiocarbon-calibrated amino acid racemization dating techniques, stable isotopes (C and O) and high spatial resolution (125-150 samples per cm of shell profile) trace element (Ba, Mn) analysis (LA-ICPMS). Time-averaged specimens of S. casali were dredged from two sites at 10 meters and 30 meters depth along the inner continental shelf at Ubatuba Bay in the Southeast Brazilian Bight, an area influenced by productivity pulses triggered by coastal runoff events and coastal upwelling. Seventy-five individual valves were dated using amino acid racemization (aspartic acid). Dates were calculated using an expanded version of a previously published relationship (Barbour Wood et al., 2006 Quaternary Research 323- 331) between aspartic acid ratios and AMS radiocarbon dates of twelve S. casali individuals from the same sampling locations. The resulting time series has complete coverage for the past three thousand years at centennial resolution. From this time series, a sub-sample of dated valves was selected for more detailed growth increment, stable isotope and high-resolution trace element (Ba/Ca and Mn/Ca) analyses. Oceanic productivity is expressed differentially in the trace element profiles of S. casali with elevated Ba/Ca and Mn/Ca ratios capturing nutrient input through coastal runoff events while elevated Ba/Ca and depressed Mn/Ca ratios represent input through coastal upwelling. Fluctuations in Ba/Ca and Mn/Ca are not correlated to fluctuations in relative growth throughout the ontogeny of an individual bivalve, nor are they expected to be as periods of increased productivity are transient events relative to the organism's lifespan. However, there are isolated intervals within valves where changes in the trace element proxies are coupled with growth rate, suggesting that nutrient availability can influence the growth rate of bivalves. While our results are preliminary, the analysis of additional valves will enable us to better understand the influence of productivity on S. casali growth rates and how the frequency of these upwelling and runoff associated productivity events changed during the Holocene. The addition of stable isotope data will enable us to determine if bivalve growth and productivity events were seasonal and how such seasonality may have changed in the Holocene.
PP31C-0543
Stable Strontium (δ88/86Sr) and U-Th isotope systematics of cold-water corals: A combined proxy for Holocene changes of the Mediterranean outflow
The application of δ18O and Sr/Ca for temperature reconstruction on cold water corals has been shown to be restricted. Hence, the development and application of alternative proxies is highly desirable. This study combines the stable strontium isotope method (δ88/86Sr, Fietzke and Eisenhauer (2006)) as a potential paleotemperature proxy with MIC-ICP-MS (multi ion counting – inductively coupled plasma – mass spectrometry) U-Th geochronolgy on cold-water corals from the central Gulf of Cadiz. The sampled reef structure in 1325 m depth on top of the Captain Arutyunov Mud Volcano (MV) consists predominantly of dead Lophelia pertusa, accompanied rarely by living solitary corals ( Dendrophyllia sp.). Potentially recorded environmental influences on these archives are significant water mass changes, e.g. variation of depth and intensity of the Mediterranean Outflow Water (MOW) due to climate changes, time intervals of marine methane emanation (surface-near gas-hydrate occurre close to the coral site) and the mud volcano activity itself. Assuming a temperature dependent strontium isotope fractionation during calcium carbonate precipitation temperatures were determined for the living solitary corals, ranging from 9 to 11.5 ° C (typical error: about 1 ° C). During sampling the bottom water temperature was 8.96 ° C and the lower MOW reached from 1075 to 1188 m depth with 10 to 10.5 ° C. First U-Th age data reflect slow growth rates of 0.13 to 0.25 mm/year for the solitary corals. However, the temperature correlation implies the stable strontium approach, which was originally deduced from reef building corals, as suitable for solitary species as well. For the fossil Lophelia pertusa colonies the actual δ88/86Sr data set indicates a range from 7.5 to 13 ° C with a distinct U-Th age distribution over the last 10 ka, closely correlating with the water depth specific record of lower MOW published by Schönfeld and Zahn (2000). The implication of a lower MOW control on reef formation is supported by coincidence of its actual elevated position with the lack of living Lophelia pertusa at the sampling site. The stable strontium isotope ratio δ88/86Sr of biogenic carbonates may serve as a new paleo-temperature proxy for reef-building and solitary deep-sea corals and thus introduce new perspectives in paleoceanography, such as changes in intermediate and deep-sea temperature and ocean circulation. Fietzke J. and Eisenhauer A. (2006), G-cubed 7 Q08009, doi:10.1029/2006GC001243. Schönfeld J. and Zahn R. (2000), Palaeogeography Palaeoclimatology, Palaeoecology 159, 85 – 111.