Extinction, survivorship and evolution of planktic foraminifera across the Cretaceous/Tertiary boundary at El Kef, Tunisia

Abstract

An expanded sediment record at El Kef shows that the K/T boundary extinctions of planktic foraminifera extend over an interval from 25 cm below the geochemical boundary (Ir anomaly) to 7 cm above. Species extinctions appear sequential with complex, large, ornate forms disappearing first and smaller, less ornate, forms surviving longer. The 14 species extinctions below the boundary appear unrelated to an impact event. Cretaceous species survivorship is greater than previously assumed. About 10 species survive (22%) into Subzone P1a ( Globigerina eugubina). All Cretaceous survivors are small primitive forms which are generally smaller than their ancestors in Cretaceous sediments. Species evolution after the K/T event occurs in two pulses. The first new Paleocene species evolve in the basal black clay (Zone PO) immediately after the major Cretaceous extinctions. Evolving species are small and primitive similar to Cretaceous survivors. The second pulse in species evolution occurs in the lower part of Subzone P1b with the appearance of larger more diverse species. The first major increase in carbonate sedimentation and productivity occurs at this time and signals the recoveyr of the ecosystem nearly 300,000 years after the K/T event. The species extinctions prior to the generally assumed impact event implied by the Ir anomaly, and the long recovery period of the ecosystem thereafter cannot be explained by a single impact, but suggest that multiple causes may be responsible such as climatic changes, volcanism, a sea level drop, production of warm saline bottom water and the chemical consequences associated with increased salinity.