Experimental determination of stable isotope variability in Globigerina bulloides: Implications for paleoceanographic reconstructions

We have quantified the environmental and physiological parameters responsible for stable isotopic disequilibrium in the non-symbiotic planktic foraminifera, Globigerina bulloides, via controlled experiments with living specimens. Individual test chambers secreted in the laboratory were amputated, pooled with other chambers from defined positions in the shell whorl and analyzed for their carbon and oxygen isotopic composition. When temperature, delta(18)O(water) and delta(13)C of Sigma CO2 are kept constant, the chamber delta(13)C and delta(18)O values increase 2.6 and 0.8 parts per thousand respectively between the smallest chambers (chs. 1-9, shell size approximate to 180 mu m) and final chamber (ch. 14, shell size approximate to 500 mu m) Feeding experiments with prey of different delta(13)C values show that 8-15% of the chamber delta(13)C signal is due to the incorporation of metabolic CO2. The observed ontogenetic trend is responsible for the stable isotope size-dependency in this species and may be due to a fractionation mechanism involving the incorporation of metabolic CO2 during calcification. Temperature experiments show that shell delta(18)O varies as predicted by paleotemperature equations, but is offset from equilibrium. We present correction factors that should be applied to delta(13)C and delta(18)O data from well constrained size ranges to yield either oxygen isotope equilibrium or ambient delta(13)C of seawater Sigma CO2. Our results suggest that for paleoceanographic applications, shells in the 270-320 mu m size range are optimal for paleoenvironmental reconstructions.