Eventoestratigrafía del límite Cretácico/Terciario en Aïn Settara, Tunicia: ¿disminución de la productividad y/o de la oxigenación oceánicas?

Abstract

The qualitative and quantitative study on benthic foraminifera from Aïn Setara (Tunisia) allows us to recognize qualitative and quantitative changes just at the Cretaceous/Tertiary boundary (K/T). Benthic foraminiferal extinction rate at Aïn Settara (21.5% of the species) is lower than planktic foraminiferal rate. Nevertheless, there is an important turnover in benthic foraminiferal assemblages across the K/T boundary, and more than 50% of the species are Lazarus taxa, which disappeared just at the boundary and reappeared along Guembelitria cretacea and Parvularugoglobigerina eugubina Biozones.

Aerobic morphogroups dominated calcareous benthic foraminiferal assemblages at Aïn Settara during the uppermost Maastrichtian. Just above the K/T boundary, aerobic morphogroups increased from 50% up to 75%, whereas anaerobic morphogroups decreased 18%. These morphogroups allow us to calculate the Oxygen Index (OI), whose values increased from Abathomphalus mayaroensis to upper Plummerita hantkeninoides Biozones, and reflect a good oxygenation of the bottom-floor water during the uppermost Maastrichtian.

Changes in epifaunal and infaunal assemblages, as well as in aerobic and anaerobic morphogroups, suggest that a sudden event took place just at the K/T boundary. Just above the K/T boundary infaunal groups drastically decreased, suggesting a decrease in primary productivity. This benthic foraminiferal turnover coincides with a catastrophic mass extinction in planktic foraminifera, and several impact evidences, such as an anomalous content in Ir or Ni-rich spinels. All these changes recorded in Aïn Settara are very compatible with a meteoritic impact in Aïn Settara at the K/T boundary.