Description: We establish the relationship between shell oxygen(6le0) and carbon (S13C) isotopic composition and size and thickness for the planktonic foraminifers Orbulina universa and Neogloboquadrina dutertrei (size only). The degree to which the stable isotopic composition of individual shells correlates with shell size limits their applicability to paleoenvironmental reconstructions and must therefore be examined carefully. We selected shelis from four intervals from a western and an eastern equatorial Atlantic core corresponding to the glacialhnterglacial extremes of the last 150,000 years (isotope stages 1, 2, 5e, and 6). We estimate the shell thickness of 0, universa using a mathematical relationship between the geometry and physical properties of a calcite sphere. Thickness measurements on selected shells of this species confirm that the calculated thickness, once it has been corrected for shell porosity, is equivalent to the measured thickness. There are no consistent trends between 0. universa 6l80 or 613C values and shell size or thickness for the size range 450-900 pm. For N. dutertrei, there are no systematic fluctuations between shell 6l80 and size between 450-700 pm. N. dutertrei shells larger than 500 pm show no 613C : size relationship. We are confident that the stable isotopic variability contained in 0. universa (450-900 pm) and N. dutertrei (〉500 pm) assemblages analyzed does not fluctuate with differences in shell morphology. The stable isotope data from these size ranges can thus be confidently used for paleoenvironmental reconstructions.

Description: Living specimens of Globigerinoides sacculifer (Brady) were subjected to varying degrees of damage in 2 series of experiments. In the first 3-group experiment, one, two or all chambers of the shells were crushed, whereas in the second 3-group experiment, successive chambers were amputated from the shells. The purpose of both sets of experiments was to determine the relative rate of recovery of and mode of shell regeneration by the injured organisms. The great majority in 5 of the 6 groups recovered and continued shell growth by regenerating spines, cementing broken parts, adding new chambers and undergoing gametogenesis. The shell shapes which resulted from the crushed specimens were generally bizarre, whereas the amputated specimens regrew chambers of normal shape. The nucleus in adult G. sacculifer shells appears to be located in the F-3 or earlier chamber. Our experiments indicate that in spite of severe injury to the shell, these tenacious marine protozoans can generally restore their biological functions of calcification, predation, symbiosis with algae, and gametogenesis. In nature, abnormal shells which show evidence of physical damage are most likely caused by predators; thus their frequency reflects a relative predation pressure on planktonic foraminifera.

Description: Culture experiments were carried out with the planktonic foraminifer Orbulina universa under high and low light levels in order to determine the influence of symbiont photosynthetic activity on the boron isotopic composition of shell calcite. Under low light (reduced photosynthetic rates) the boron isotopic composition of the tests is 1.5‰ lower compared to shells grown under high light (elevated photosynthetic rates). In terms of inferred pH, the lower boron isotope values correspond to a reduction in pH of approximately 0.2 units. The boron isotopic composition of Orbulina universa from plankton tows is similar to that of shells grown under low light conditions in the laboratory. These data are consistent with reduced symbiont concentrations in recently secreted shells. In addition to laboratory and field grown O. universa, we present the first data for a symbiont-barren foraminifer, Globigerina bulloides. Data obtained for G. bulloides fall ∼1.4‰ below those of the field grown O. universa. Although the plankton tow results are preliminary, they support the hypothesis that respiration and photosynthesis are the key physiological parameters responsible for species-specific vital effects.

Description: Here, we present a new technique for the direct measurement of 44Ca/40Ca isotope ratios on a Multicollector Inductively Coupled Plasma Mass Spectrometer (MC–ICP–MS, AXIOM) using the “cool plasma” technique. By reducing the plasma energy to about 400 W, the isobaric effect resulting from 40Ar+ can be significantly reduced, enabling the simultaneous and precise measurement of 44Ca and 40Ca beam intensities in different Faraday cups. In contrast to the TIMS technique requiring a 43Ca/48Ca double spike, the isotope measurements on MC–ICP–MS can be performed by bracketing standards. We express the calcium isotope variation relative to NIST SRM 915a (δ44/40Ca [‰]=[((44Ca/40Ca)sample/(44Ca/40Ca)NIST SRM 915a)−1]*1000). Isobaric effects of 24Mg16O+ and 23Na16OH+ interfering with 40Ca and 26Mg16OH2+ with 44Ca can be neglected by measuring calcium isotopes near the low-mass edge of the peaks. No influence of 87Sr2+ monitored on 43.5 atomic mass units (amu) was found. Repeated measurements of two Johnson Matthey CaCO3 standards (lot No. 4064 and lot No. 9912) revealed values of about −11.29 (‰ SRM 915a) and 0.57 (‰ SRM 915a). These values are in accordance with previous values published by Russell et al. [Geochim. Cosmochim. Acta 42 (1978) 1075], Heuser et al. [Int. J. Mass Spectrom. 220 (2002) 385], Hippler et al. [Geostand. Newsl. 27 (2003) 267] and Schmitt et al. [Geochim. Cosmochim. Acta 67 (2003) 2607]. Repeated measurement of the NIST SRM 915a CaCO3 standard showed that the variance of a single δ44/40Ca measurement is about 0.14‰ RSD being comparable with TIMS. MC–ICP–MS-based δ44/40Ca values measured on inorganically precipitated aragonite samples are indistinguishable from earlier measurements based on TIMS, confirming the positive correlation of δ44/40Ca and temperature. MC–ICP–MS-based δ44/40Ca measurements on cultured Orbulina universa showed a slope of about 0.026‰/°C being similar to the TIMS-based δ44/40Ca measurements showing a slope of about 0.019‰/°C. The large offset of about 5‰ between the two techniques is shown to be caused by a “matrix” effect, indicating that any δ44/40Ca measurements on MC–ICP–MS are sensitively controlled by the Ca concentration and the acidity of the solution. Our study demonstrates the possibility to measure the whole dispersion of calcium isotopes with MC–ICP–MS, showing that 40Ca can be used for normalization of 44Ca.