STASIS AND SPECIES TURNOVER OF CALCAREOUS PLANKTON DURING THE LATE CRETACEOUS (TURONIAN) ULTRATHERMAL

Turonian marine sediments in southeast Tanzania have yielded some of the best preserved microfossils in the world, providing valuable new insights on species taxonomy, biostratigraphy, biodiversity, and phylogenetic relationships. This presentation highlights results of an integrated study of planktonic foraminifera and calcareous nannofossils species abundance distributions and stratigraphic ranges in shallow boreholes that were drilled during the Tanzania Drilling Project (TDP) with comparisons to a coeval deep-sea sequence drilled in the southeast Indian Ocean. Major species turnovers occur at different levels in the upper Turonian for both calcareous plankton groups. The older of these turnovers occurs at the top of the Hv. helvetica Zone, where last occurrences of four planktonic foraminiferal species are immediately followed, within several meters, by first occurrences of five foraminiferal species. There is essentially no change across this same interval among the calcareous nannofossil populations or in sediment lithology, suggesting that if a hiatus were present, it was very brief. Instead, an accompanying increase in the abundance of dwarfed planktonic forms suggests that the foraminiferal turnover may have been at least partly influenced by a shift in environmental conditions of the surface mixed layer. The subsequent late Turonian turnover is marked by extinction of three calcareous nannofossil species with no co-occurring foraminiferal extinctions and is immediately followed by first appearances of four calcareous nannofossil and two planktic foraminifer species, an abrupt and dramatic increase in the relative abundance of biserial taxa, and a sudden diagenetic shift that results in calcareous infilling of all foraminiferal tests. Comparison between the TDP planktonic foraminiferal species distributions in a mesotrophic shelf break setting with Ocean Drilling Project Hole 762C in an oligotrophic open ocean setting reveals a greater environmental influence species distributions than expected, resulting in significant differences in stratigraphic ranges and relative abundances for more environmentally sensitive species.