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

Significant stable isotopic variation related to size has been found i n the tests of three species of planktonic foraminifera. Neogloboquadr ina dutertrei, Globigerinoides ruber and Globigerina bulloides were se lected to monitor the stable isotopic composition as a function of siz e. 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 c hosen. Both areas are under the influence of upwelling and we aimed to tie the isotopic behavior of the three species to variations in the u pwelling intensity by using the size series. The oxygen isotopic varia bility 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 habita t of the two species. Pre-gametogenic specimens of N. dutertrei have d eltaO-18 values close to the values of G. ruber, and thus these specim ens lived in surface waters. In the larger sizes, deltaO-18 values are more positive, which indicates that adult specimens sank from the sur face waters in the final stages of their life prior to gametogenesis o r lived in deeper waters during most of their adult stage. The Arabian Sea deltaO-18 difference between pre-gametogenic G. ruber, N. dutertr ei or G. bulloides at the surface and gametogenic N. dutertrei at the base of the thermocline is much smaller than the difference in the Pan ama Basin. Thus, the temperature contrast in the Arabian Sea surface w aters 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 variati ons within the size series of the three species are difficult to tie t o their depth habitats and associated environmental factors, and are m ainly controlled by growth during ontogeny.