Size-related stable isotope changes in Late Cretaceous planktic foraminifera: Implications for paleoecology and photosymbiosis

The paleoecology of Cretaceous planktic foraminifera during the Late Cenomanian to Coniacian period (∼ 95–86 Ma) remains controversial since much of the tropical marine record is preserved as chalk and limestone with uncertain geochemical overprints. Here we present δ13C and δ18O data from sieve size fractions of monospecific samples of exceptionally well preserved planktic foraminifera recovered during Ocean Drilling Program Leg 207 (Demerara Rise, western tropical Atlantic). Our results suggest that all species studied (Hedbergella delrioensis, Heterohelix globulosa, Marginotruncana sinuosa, Whiteinella baltica) grew primarily in surface waters and did not change their depth habitat substantially during their life cycle. Comparison of size-related ontogenetic trends in δ13C in Cretaceous and modern foraminifera further suggests that detection of dinoflagellate photosymbiosis using δ13C is confounded by physiological effects during the early stages of foraminifer growth, raising doubts about previous interpretations of photosymbiosis in small foraminifera species. We propose that obligate photosymbiosis involving dinoflagellates may not have evolved until the Campanian or Maastrichtian since our survey of Cenomanian–Coniacian species does not find the δ18O and δ13C size-related trends observed in modern foraminifer–dinoflagellate symbioses.