Jo11ma/ of Forami11iferal Research. v. 30. no. 3. p. 202- 218. July 2000 THE CRETACEOUS/PALEOGENE (K/P) BOUNDARY AT AIN SETTARA, TUNISIA: SUDDEN CATASTROPHIC MASS EXTINCTION IN PLANKTIC FORAMINIFERA IGNACIO ARENILLAS 1 , JOSÉ A. ARZ 2 , EUSTOQUIO MOLINA 1 , AND CHRISTIAN DUPUIS.1 ABSTRACT The quantitative study and high resolution sampling of an essentially continuous and expanded Cretaceous/ Paleogene (KIP) boundary section in Tunisia allow us to test the model of extinction in planktic foraminifera. The extinction at the A'in Settara section occurred over a short period of time similar to the Tunisian sections at El Kef and Elles and the Spanish sections at Agost, Car- avaca and Zumaya. At Am Settara only 3 species dis- appeared in the latest Maastricbtian, 45 became extinct precisely at the K/P boundary and 18 disappeared in the earliest Danian. The species that became extinct at the K/P boundary constitute about 20% of the individuals in the population larger than 63 microns and 68 % of the species, which suddenly became extinct in a catastrophic event precisely coinciding with the !ayer containing evi- dence for an asteroid impact. Most of these species are large, complex and low latitude deeper to intermediate dwelling forms. This extinction event is clearly the most important catastrophic mass extinction recorded in the history of planktic foraminifera. This pattern of extinc- tion is superimposed on a controversia! gradual pattern of extinction of 21 species that apparently began in the latest Maastrichtian and ended in the early Danian. The Maastrichtian species that seem to become extinct grad- ually are generally small, cosmopolitan and simple sur- face dwellers. The catastrophic mass extinction of the 45 species coincident with the K/P boundary is compatible with the effect of the impact of a large asteroid, whereas the gradual extinction of 18 species in the basal Danian could also be attributed to the long term disruptive effect of the impact. INTRODUCTION There is a wide consensus among specialists in planktic foraminifera that the Cretaceous/Paleogene (KJP) turnover is a mass extinction because it affected more than 50% of the species. This mass extinction event has been pointed out by many specialists since the early studies of the fossil re- cord of planktic foraminifera (Berggren, 1962, 1969, 1971; Luterbacher and Premoli Silva, 1964; Bolli, 1966; Premoli Silva, 1977; Maurrasse and others, 1979; Blow, 1979). De- tailed biostratigraphic studies of the Gubbio section (Italy) allowed Luterbac her and Premoli Silva ( 1964) to describe severa) new species as well as the new Globigerina eugu- bina Biozone between the Maastrichtian and Danian, which is characterized by a planktic foraminiferal association com- ' Área de Paleontología, Departamento de Ciencias de la Tierra. Universidad de Zaragoza, E-50009. Spain. 2 Facultad de Ciencias de la Tierra, Universidad Autónoma <.le Nuevo León, Linares. MEX-67700, Mexico. 3 Laboratoire de Géologie fondamentale et appliquée. Faculté Poly- technique de Mons. B-7000, Belgium. 202 pletely different from those found in the uppermost Maas- trichtian assemblages. When Alvarez and others ( 1980) proposed the theory that the KJP boundary mass extinction was caused by a Iarge bolide impact, it was assumed that the planktic foraminifera model of extinction was very sudden and catastrophic. This theory was corroborated by Smit and Hertogen ( 1980) who documented that at the Caravaca section (Spain) the impact severely affected the planktic foraminifera and only one spe- cies survived (Guembelitria cretacea). The KJP planktic fo- raminiferal fossil record was studied in detail by Smit (1977, 1979); it yielded a new assemblage between the Maastrich- tian and Danian that led to the definition of a high resolution biozone (G. cretacea Biozone). Initially, Smit considered most of the Maastrichtian taxa found in the Iowermost Dan- ian to be reworked specimens. However, Smit subsequently (1982, 1990, I994) admitted that sorne Maastrichtian spe- cies survived and that the final extinctions may have ex- tended over a certain period of time, although he maintained that the KJP mass extinction event was sudden, catastrophic and the consequence of a large extraterrestrial bolide impact (Smit, 1990, 1997). Paleontologists have often questioned the catastrophic na- ture of the impact at the KJP boundary and they argue that the pattern of the mass extinction in the terminal Maastrich- tian is gradual (Keller, 1988, l 989a,b. 1994, 1996; Keller and others, 1993, 1995, 1998). They conclude that 2/3 of the species declined prior to becoming extinct below or at the KJP boundary and about 1/3 of the species survived well into the Danian. Consequently, Keller and others ( 1995) ar- gue that the gradual pattern of extinction cannot be attri- buted to a global biotic effect of the asteroid impact. The main reason for these two opposite hypotheses may be methodological as was pointed out by Signor and Lipps ( 1982) and Molina ( 1994, 1995, 1997). For example, it is evident that the fossil record of dinosaurs is very scarce compared with the fossil record of many microfossil groups and hence is much more affected by the Signor-Lipps effect (incomplete ranges are due to insufficient sampling). How- ever, microfossils such as planktic foraminifera have a very abundan! and continuous fossi 1 record and therefore are very suitable to confirm, or disprove, the cause and effect relation between Iarge impact and catastrophic mass extinction. In an attempt to further clarify the pattern of extinction at the KJP boundary, a blind sample test was suggested, and in 1992 a group of scientists re-sampled the El Kef section in Tunisia. Six unlabeled samples were studied by four in- vestigators and the results recently published (Lipps, 1997; Ginsburg, l 997a,b; Smit and others, I997; Canudo, 1997; Masters, 1997; Ol sson, 1997; Orue-Etxebarria, 1997) were inconclusive as the data supported neither pattem of extinc- tion (Keller, 1997; Smit and Nederbragt, 1997). The differ- ent interpretations of the blind test seemingly showed that the test failed to resolve the controversy.