Date of this Version
Well-preserved nannofossil assemblages in late Albian and Cenomanian hemipelagic sections from Ocean Drilling Program (ODP) Leg 171B preserve a record of the early history and adaptive radiation of the calcareous nannofossil genus Prediscosphaera. There was a significant rise in the abundance of the genus (from an average of 2% in the early late Albian to about 6% in the latest Albian and Cenomanian. This rise, and other shorter term pulses of increased generic abundance, corresponds to black shale deposition episodes. Twelve distinct morphotypes that were derived from the long-ranging Prediscosphera columnata and Prediscosphaera spinosa, are recognized and differentiated based on ultrastructural and morphological variations. Newly evolved morphotypes arose during four episodes of diversification that corresponded to periods of black shale deposition. These black shale intervals resulted from significant disruptions in surface water stratification, suggesting that major changes in deep water mixing and the structure of the surface water mass drove the early adaptive radiation of Prediscosphaera. Late Albian oceanic anoxic event OAE 1d and Cenomanian-Turonian oceanic anoxic event OAE 2, along with another phase of black shale accumulation in the middle Cenomanian appear to have stimulated adaptive radiation and relative abundance increase of Prediscosphaera in the mid-Cretaceous.