SIZE AND UPWELLING CONTROL OF THE STABLE-ISOTOPE COMPOSITION OF NEOGLOBOQUADRINA-DUTERTREI (DORBIGNY), GLOBIGERINOIDES-RUBER (DORBIGNY) AND GLOBIGERINA-BULLOIDES DORBIGNY - EXAMPLES FROM THE PANAMA BASIN AND ARABIAN SEA

Significant stable isotopic variation related to size has been found in the tests of three species of planktonic foraminifera. Neogloboquadrina dutertrei, Globigerinoides ruber and Globigerina bulloides were selected to monitor the stable isotopic composition as a function of size. A Recent and last Glacial sample from piston core CD 3822 retrieved from the Panama Basin and a sample from Arabian Sea boxcore 14 were chosen. Both areas are under the influence of upwelling and we aimed to tie the isotopic behavior of the three species to variations in the upwelling intensity by using the size series. The oxygen isotopic variability of the N. dutertrei size series from all samples is much larger than in the G. ruber series, which can be related to the depth habitat of the two species. Pre-gametogenic specimens of N. dutertrei have deltaO-18 values close to the values of G. ruber, and thus these specimens lived in surface waters. In the larger sizes, deltaO-18 values are more positive, which indicates that adult specimens sank from the surface waters in the final stages of their life prior to gametogenesis or lived in deeper waters during most of their adult stage. The Arabian Sea deltaO-18 difference between pre-gametogenic G. ruber, N. dutertrei or G. bulloides at the surface and gametogenic N. dutertrei at the base of the thermocline is much smaller than the difference in the Panama Basin. Thus, the temperature contrast in the Arabian Sea surface waters is smaller than in the Panama Basin during the upwelling season, as a response to the breaking of cool ascending deeper waters at the surface reducing the temperature gradient. The carbon isotopic variations within the size series of the three species are difficult to tie to their depth habitats and associated environmental factors, and are mainly controlled by growth during ontogeny.