RELATIONSHIP BETWEEN SHELL SIZE, THICKNESS AND STABLE ISOTOPES IN INDIVIDUAL PLANKTONIC-FORAMINIFERA FROM 2 EQUATORIAL ATLANTIC CORES

We establish the relationship between shell oxygen (deltaO-18) and carbon (deltaC-13) 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 shells from four intervals from a western and an eastern equatorial Atlantic core corresponding to the glacial/interglacial extremes of the last 150,000 years (isotope stages 1, 2, 5e, and 6). We estimate the shell thickness of O. 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 O. universa deltaO-18 or deltaC-13 values and shell size or thickness for the size range 450-900 mum. For N. dutertrei, there are no systematic fluctuations between shell deltaO-18 and size between 450-700 mum. N. dutertrei shells larger than 500 mum show no deltaC-13:size relationship. We are confident that the stable isotopic variability contained in O. universa (450-900 mum) and N. dutertrei (>500 mum) 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.