During the late Maastrichtian (66.8-65.5 Ma) the Asyut Basin in central Egypt experienced a breakdown of the surface to bottom gradient of the ¹³C/¹²C ratio with planktic N¹³C values 0.2-0.8x lighter than benthic values. Planktic foraminiferal species diversity was reduced by more than 50%, with assemblages dominated (60-90%) by the opportunistic blooms of the disaster species Guembelitria cretacea, which mimic the early Danian. The prolonged breakdown in productivity occurred during a time of tectonic activity and increased terrestrial runoff that may have resulted in highly eutrophic waters, coupled with a sea-level regression (65.5 Ma) that led to restricted circulation. Increased productivity during the short climate warming between 65.4 and 65.2 Ma is associated with increased species diversity, abundant rugoglobigerinids and common heterohelicids. At the end of the Maastrichtian, decreased productivity coincided with the K/T impact and mass extinction, followed by characteristically early Danian low diversity assemblages. The similarity of the late Maastrichtian and post-K/T impact Guembelitria-dominated assemblages reveals that the planktic foraminiferal response to the K/T catastrophe was not unique, but followed a predictable pattern of response to severe environmental perturbations.  PDF