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 Neogloboquadrina duterfrei were examined, along with two forms of "pachy-duter." Samples were analyzed from ODP Core 769A, collected from the Sulu Sea. This marginal basin borders the Western Tropical Pacific Ocean and is isolated during lower stands of sea level. Morphotype selection of specimens in the size fraction 150-350 mu m was strictly based on degree of calcite encrustation (reticulate or crystalline), chamber number and periphery morphology, Samples collected from deglacial intervals were also compared with coretop specimens from a Sulu Sea gravity core, GGC-40, Comparison of oxygen isotopic signatures between morphotypes from deglacial samples revealed complete separation between crystalline and reticulate forms by between 0.5 and 1.0 parts per thousand. Separation between crystalline and reticulate delta(18)O in coretop specimens was less than 0.3 parts per thousand, however in all samples, crystalline forms were consistantly enriched in oxygen-18 relative to reticulate forms. The carbon isotope composition of the morphotypes 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 Neogloboquadrina pachyderma, crystalline Neogloboquadrina dutertrei and reticulate 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 paleoceanographic studies.