Stable isotope data and the ontogeny of Paleocene planktonic foraminifera

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Abstract
We have made oxygen and carbon isotope measurements in up to nine different size fractions of planktonic foraminifera from well-preserved sediments at DSDP Site 577. In the Morozovella and Acarinina species there is a systematic trend towards more positive 13C values, and an insignificantly small trend in 18O values, with increasing size. Within this group, isotope values vary more with size than with species. In Subbotina spp., similar but much less marked trends are observed. All the data indicate that the species with globorotaliid morphology (Morozovella spp., Acarinina spp.) occupied the photic zone, and that the species with globigerinid morphology (Subbotina spp.) occupied a distinctly deeper habitat, in the region of low dissolved oxygen concentration. This may be contrasted with the situation today, and through the Neogene, where species with globorotaliid morphology (Globorotalia spp.) inhabit the deeper parts of the water column.