ALBERT

Description: We explored the applicability of stable isotopic ratios of nine foraminiferal species, four epifaunal (Pseudononion atlanticum, Cibicides fletcheri, Hoeglundina elegans, Hanzawaia boueana) and five infaunal (Angulogerina angulosa, Uvigerina peregrina, Buccella peruviana, Cassidulina subglobosa, Bulimina marginata), as environmental tracers on the southwest Atlantic continental shelf from samples collected along the Argentinean-Uruguayan-Brazilian Atlantic coast during austral winter 2003 and summer 2004. Linear regressions show that the stable isotopic composition of living foraminifera is associated with geographical and environmental variables (latitude, water depth, temperature, salinity, and dissolved silica). Multiple linear regressions show that the relevant variables in this study are latitude, salinity, and biogenic silica. There is a general trend of higher 18 O and lower 13 C values with increasing latitude, reflecting the influence of cooler water masses toward the south. Specifically, the 18 O data from U. peregrina, H. elegans, and P. atlanticum follow meridional temperature gradients with lower 18 O values in relatively warm Subtropical Shelf Water at the northernmost sites (27°S) and higher values in colder Sub-Antarctic Shelf Water at the southern sites (to 37°S). Angulogerina angulosa and P. atlanticum 18 O values correlate more strongly with salinity than temperature. Positive correlation of 13 C and water depth observed for some infaunal species indicates greater input of organic matter to the deeper-water benthos. In summer samples, however, this correlation between 13 C and water depth weakens, reflecting a less stratified water mass associated with upwelling. Results from this study support the application of these foraminifera to reconstructions of paleoenvironmental changes from sediment cores.

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.