HIGH STRESS ENVIRONMENT ACROSS THE CRETACEOUS/PALEOGENE BOUNDARY AT THE GALANDERUD SECTION (NORTHERN ALBORZ, IRAN)
Four distinct assemblages are recognized. Assemblage 1 is from the upper Maastrichtian and dominated by ecological specialist species. On the basis of the high diversity and percentages of k-strategy species, stable environmental conditions are inferred for this interval. Across the K/Pg boundary and in the H. holmdelensis subzone >75% of Cretaceous taxa became extinct. Opportunistic species of Guembelitria dominate the assemblages. Heterohelix and Hedbergella are regarded as generalists and are also abundant. These taxa are associated with small and poorly diversified assemblages in the earliest Danian that clearly represent the most extreme biotic stress in the aftermath of the K/Pg event. This assemblage occurs in a 30-cm thick interval in the Galanderud section. A third assemblage characterized by the appearance of new species with very small size, simple morphology, and low diversity, including species of Parvularugoglobigerina, Eoglobigerina, Globocunosa, Woodringina, and Chiloguembelina. This assemblage corresponds to the Pv. longiapertura and Pv. sabina subzone and spans a 220-cm interval. The appearance of new Danian taxa and increased diversity indicate a decrease of environmental stress. A fourth assemblage is characterized by the appearance of new species with cancellate walls and large tests, which become progressively diversified across the upper part of Pv. eugubina to Para. pseudobulloides zones. The high abundance of Parasubbotina, Subbotina, and Praemurica with complex test ultrastructure suggest a relatively stable environment.
A strong increase in the opportunistic calcareous dinoflagellate Cervisiella operculata was observed just above the K/Pg boundary in the first of three thoracosphaere-dominated chalks. We interpret these three blooms of calcareous dinoflagellates in planktic foram-poor chalk layers as highly stressed and unstable environments that punctuated recovery in the aftermath of the K/Pg mass extinction.