MORPHOLOGIC AND STABLE ISOTOPIC VARIABILITY WITHIN THE PLANKTIC FORAMINIFERAL GENUS NEOGLOBOQUADRINA

Dextral-coiled morphotypes of fossil planktonic foraminifera from the genus Neogloboquadrina have been examined to determine variability in isotopic composition. Morphotypes of Neogloboquadrina pachyderma and N eogloboquadrina duterfrei were examined, along with two forms of ''pac hy-duter.'' Samples were analyzed from ODP Core 769A, collected from t he Sulu Sea. This marginal basin borders the Western Tropical Pacific Ocean and is isolated during lower stands of sea level. Morphotype sel ection of specimens in the size fraction 150-350 mu m was strictly bas ed on degree of calcite encrustation (reticulate or crystalline), cham ber number and periphery morphology, Samples collected from deglacial intervals were also compared with coretop specimens from a Sulu Sea gr avity core, GGC-40, Comparison of oxygen isotopic signatures between m orphotypes from deglacial samples revealed complete separation between crystalline and reticulate forms by between 0.5 and 1.0 parts per tho usand. Separation between crystalline and reticulate delta(18)O in cor etop specimens was less than 0.3 parts per thousand, however in all sa mples, crystalline forms were consistantly enriched in oxygen-18 relat ive to reticulate forms. The carbon isotope composition of the morphot ypes examined is not grouped by texture, and appears to be influenced by mass, and, therefore, may not provide a reliable paleoceanographic signal. Three isotopically distinct morphotypes (crystalline Neogloboq uadrina pachyderma, crystalline Neogloboquadrina dutertrei and reticul ate Neogloboquadrina dutertrei) were identified for potential use with interpreting upper water column paleohydrography, Both forms of pachy -duter appear to be isotopic intergrades and should be avoided in pale oceanographic studies.