PP42A-01
On The Build-Up Of The Isotopic Signals In Coral Skeleton
The isotopic composition of reef coral's layered skeleton is extensively used to reconstruct past environmental conditions although both isotopes, carbon and oxygen, are out of isotopic equilibrium with respect to inorganic carbonate precipitates. Calibration studies produce the local correlation coefficients that determine the linear association between oxygen isotopic composition and SST. These studies are species-dependent and usually fail to produce the full range of environmental variables expected from considerations of temperature and salinity dependent fractionation. Sampling of coral skeletons for environmental-reconstruction purposes is usually vertical, along the maximal growth axis, which provides the time aspect and is the product of several calcification processes occurring within the tissue layer. To study the interrelations of the calcification processes with the isotopic composition of the skeleton, we follow the built-up of the isotopic signal at monthly resolution by a horizontal sampling of the newly added skeleton. We report the results of two in-situ underwater experiments following the growth of several Porites sp. colonies at 6 and 40-meter depths for 12 and 20 months. We show that the first, newly formed carbonate skeleton does not contain the isotopic seasonal temperature signature that is expected from the annual temperature variation of the ambient water. On the other hand, sampling along the main growth axis of the same corals reveals that the seasonal isotopic signals are well developed in skeleton materials which are older than about one year. Hence, calcification in more than one step and a strong biological control are required in order to explain these results. The weak correlation between carbon and oxygen isotopes of the newly formed skeleton also suggests that it will be inadequate to adopt a single step fractionation model to describe the isotopic compositions of shallow-water symbiont-bearing corals.
PP42A-02
Biologic Influence on Annual and Seasonal Oxygen Isotopic Ratio of Scleractinian Coral Skeleton
The oxygen isotope ratio from coral skeleton is the more commonly used seasurface temperature (SST) proxy. Thus, it is admitted that this isotopic composition is only dependent on external factors, SST and the water isotopic ratio. Even by considering ultra-structures, deep corals and tropical ones developed in constant conditions show identical crystal types, characterized by similar oxygen isotopic signature and variability. However, the calibration calculated from cultured Acropora over temperature ranging between 21 and 29°C, indicated a higher absolute slope (in isotopic deviation per 1°C) than that derived from Weber and Woodhead data (1972). The latter data series provided statistical mean annual values lacking of vital effect (isotopic variations due to biologic activity) and the obtained calibration, taking into account seawater isotopic ratio, was similar with Epstein relationship, though isotopic values are lower. Cultured Acropora submitted to light change equivalent to seasonal one showed a systematic isotopic increase. This variation was always associated with an increase of aragonite deposition and a decrease of the linear extension, illustrating the strong relationship between the algae metabolism and the skeleton formation. Temperature and light lead to opposite oxygen isotopic biology-mediated responses. Compared with the temperature, the light influence is lower but it does not mean that light effect is negligible in regard to seasonal isotopic amplitude. Is it specific to cultured Acropora? In the field, it is difficult to separate temperature and light effect, however the link between light, growth properties and oxygen isotopes has been yet demonstrated on Porites. In order to estimate the isotopic influence of a seasonal factor we compared data from literature concerning Porites colonies developed in the same site. Although influenced by identical seawater and temperature, calibrations from different species, showed high variability between colonies and also between different sampling of a single head. It can be only explained by another seasonal biologic effect. Light and temperature being the main changes recorded between winter and summer, we conclude that both temperature and light, in addition to the physical SST influence, affect the oxygen isotopic composition through metabolism. It could explain biases observed from temperature and/or salinity reconstructions using tropical corals. Based on these evidences we developed a new method using multi-proxies (see PP18).
PP42A-03
High Resolution Coral Cd Measurements Using LA-ICP-MS and ID-ICP-MS: Calibration and Interpretation
Cadmium in coral skeleton ([Cd]coral) tracks oceanic upwelling. This study assessed the Cd signal in three species of coral ( Porites lobata, Pavona gigantea, Pavona clavus) from a seasonally upwelling region (Gulf of Panamá, Pacific Ocean) using high-resolution laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Single LA tracks in all species yielded variable results, however the average of multiple paths mirrored changes in in situ seawater Cd ([Cd]sw). In addition, averaged P. clavus data from LA-ICP-MS was correlated to isotope dilution-ICP-MS data, albeit with lower concentrations using the latter method. Although the seawater and coral time series trends were similar, maximum [Cd]coral preceded [Cd]sw by approximately 1 month. When applying this 1-month offset, [Cd]coral was well correlated to [Cd]sw, providing the first direct calibration for this upwelling proxy (distribution coefficient = 1.3-1.7). A three year record of cyclic [Cd]coral demonstrated the ability of LA-ICP-MS to rapidly generate long records for paleoupwelling reconstruction. Further improvements in measurement precision would make this technique comparable to existing ID-ICP-MS methods, but with higher sample throughput and temporal resolution.
PP42A-04
Influences on the fractionation of calcium isotopes in planktonic foraminifera
Foraminiferal Ca isotope ratios have been suggested as a paleotemperature proxy, however previous work indicates a range of possible sensitivities of foraminiferal Ca isotopes to temperature. Our results from coretop and sediment trap samples add to our knowledge of potential controls on the temperature dependence of Ca isotopic fractionation in planktonic foraminifera. This data is imperative for the application of this proxy interpretation of downcore Ca isotope records. Seven species of planktonic foraminifera from coretop sediments collectively exhibited a Ca temperature dependent fractionation of 0.013 permil per degree C. This is in agreement with previously published estimates for most species of planktonic foraminifera as well as biogenic and inorganic calcite and aragonite. Four species of planktonic foraminifera collected from a sediment trap showed a considerable amount of scatter and no consistent temperature dependent fractionation. Although foraminiferal tests are one of the most commonly used carriers of paleoclimatic and paleoceanographic data, the particular pathways and processes of biomineralization in foraminifera are not well known. It is difficult to distinguish between processes that ultimately control Ca and other elemental compositions in foraminiferal calcite. In particular, the chemistry of the solution from which foraminifera precipitate their test is not well defined. A one-box model (Elderfield et al., 1996) in which Ca isotopes are allowed to fractionate by Raleigh distillation from a biomineralization reservoir (internal pool) was used to constrain the isotopic composition of the original biomineralization Ca reservoir, assuming around 85% of the Ca reservoir is precipitated and the fractionation factor during precipitation is 0.9985 + 0.00002(T). To explain the foraminiferal Ca isotope data, this model indicates that the Ca isotopic composition of the biomineralization reservoir is offset from seawater (approximately -0.8 permil).
PP42A-05
Elemental Imaging and Proxy development in Deep Sea Corals
There is an increasing interest in developing proxy records of oceanographic conditions from intermediate and deep-water environments. Elemental and isotopic ratios incorporated into the calcite skeleton of deep-sea corals have been proposed as a method for obtaining this information. Before elemental proxies can be used, rigorous testing of elemental spatial variability and calibration is needed. Using the high-spatial resolution NanoSIMS 50 we examined the spatial variability of Mg/Ca, Sr/Ca and Ba/Ca ratios (potential temperature proxies) in 10 micron square images using a ~200 nm size primary beam (O-) in thin-sections of the deep-sea corals Corallium secundum and the Isididae family corals. We found that the distribution of magnesium is not homogenous. There are many <1 micron features with Mg/Ca concentrations that ~40% higher than the surrounding area.. At this point we speculate that these features may be organic molecules rich in Mg, centers of calcification, or perhaps amorphous calcium carbonate. The Sr/Ca and Ba/Ca patterns are more homogenous; they do not show features related to the high Mg/Ca spots. The heterogeneity seen in the Mg/Ca ratio in these deep sea corals suggest that even if there is a temperature dependence, the occurrence of these high concentration Mg/Ca locations may hinder the accurate reconstruction of water temperature. The more homogenous Sr/Ca is compared to available temperature data from the region and shows good correspondence as s potential water temperature proxy.
PP42A-06
Trace element (Mg, Sr, P, Ba and Cd) variability in single foraminifera and a possible new proxy for seawater phosphate
Although the controls on the trace element composition in bulk foraminifera are reasonably well understood, elemental distributions in single foraminiferal shells are less well constrained. Here we present Mg/Ca, Sr/Ca, Ba/Ca, Cd/Ca, and P/Ca ratios from zoning profiles across the surfaces of several species of planktonic and benthic foraminifera. Ablation conditions included a 213 nm laser operating at 5 Hz, a 12 μm spot, and a fluence of ~10J/cm2; ablated material was analyzed by using a single collector, magnetic sector ICP-MS in low resolution mode. Compositions were standardized against NIST612 glass, so absolute ratios are more poorly resolved than zoning patterns. The nutrient proxies Ba, P and Cd (when detectable) commonly show considerable (5-10x) variability both between and within individual chambers; the temperature proxies Mg and Sr showed less variability (1-5x). Trace element concentrations are particularly high at chamber boundaries, suggesting that chamber boundary calicification concentrates trace elements. Covariation was significant among nutrient proxies, and between temperature proxies, but was weaker for nutrient vs. temperature proxies. We also analyzed samples at various stages of a standard trace element cleaning procedure. Bulk Mg/Ca decreased strongly with increasingly aggressive cleaning measures, suggesting the removal of Mg-rich coatings and/or primary calcite. P/Ca values, however, appear unchanged, suggesting that P is not hosted in loosely associated organic matter, but contained within the calcite itself. Relatively high Na contents suggest a straightforward substitution of Ca+PO4 for Na+CO3 in the calcite lattice. The likely occurrence of inorganic PO4 in foraminifera, the strong covariance of P/Ca with other nutrient proxies, and the ease of P measurement via ICP-MS, recommend P/Ca in foraminifera as a possible new proxy for seawater PO4 chemistry.
PP42A-07
Discovery of a Cretaceous Scleractinian Coral with a Calcitic Skeleton
It has been generally thought that scleractinian corals form purely aragonitic skeletons. We show that a well- preserved fossil coral, Coelosmilia sp. from the Upper Cretaceous (ca. 70 Ma), has preserved skeletal structural features identical to those observed in present day scleractinians. However, the skeleton of Coelosmilia sp. is entirely calcitic. Its fine-scale structure and chemistry indicate that the calcite is primary and did not from via diagenetic alteration of aragonite. This result implies that corals, like other groups of marine, calcium carbonate- producing organisms, can form skeletons of different carbonate polymorphs. Implications for coral biomineralization and evolution will be discussed.