LIFESTYLES OF THE HANTKENINIDS

Hantkenina were a distinctive tubulospined planktonic foraminifera genus that evolved in the middle Eocene and went extinct at the Eocene/Oligocene boundary. Previous oxygen isotope analyses have suggested that the hantkeninids changed their depth habitat through the middle and late Eocene from the deep thermocline to the mixed-layer. Here through carbon isotope analyses we test whether alteration of their depth habitat was accompanied by adaptations in their paleoecology, specifically the acquisition of photosymbionts. In this geochemical study of Hantknenina lifestyle, temporal trends toward increasing carbon isotopes are reported as Eocene climate transitioned from ‘greenhouse’ to ‘icehouse’ conditions. We use size separated stable isotope analysis from Ocean Drilling Program (ODP) sites 1051 and 1052 (western North Atlantic Ocean), and supplemental Gulf Coastal Plain samples spanning ~45 to 34 Ma. Samples from the early middle Eocene (~45 Ma) show no increase in carbon isotopes through ontogeny, indicating an asymbiotic life habitat, consistent with their deeper dwelling lifestyle. Samples from the uppermost Eocene (34 Ma) show an increase of carbon isotopes with test size of approximately 1 per mil, suggesting an adaptation towards a photosymbiont life style. Variations in global climate correspond to morphologic changes that appear to coincide with a modification in depth habitat, and may be related to the acquisition of algal photosymbionts in Hantkenina in the uppermost Eocene.