Late Maastrichtian to early Danian calcareous nannofossils at Elles (Northwest Tunisia). A tale of one million years across the K-T boundary

摘要

A detailed study of calcareous nannofossils of the Elles section (Tunisia) was extended over an interval of about 1 Myr including the K-T boundary. The late Maastrichtian assemblages are rich and well preserved. Pulses of surface water cooling, probably related to episodic upwelling are revealed by the increase in abundance of cool water taxa (mainly Kamptnerius magnificus, Nephrolithus frequens, Gartnerago spp.) in certain late Maastrichtian samples, paralleled by a decrease of the warm water species Watznaueria barnesae. A sensible decrease in abundance of Arkhangelskiella cymbiformis and its return to smaller specimens was also recorded at Elles, probably as the result of surface water productivity changes in the latest Maastrichtian. Some Maastrichtian samples at Elles contain Neobiscutum romeinii and Chiasmolithus? sp. These taxa, formerly though to be Tertiary coccoliths, are here considered as truly Masstrichtian occurrences. As at El Kef, the Danian of Elles is characterised by successive blooms of surviving and newly evolved species. Though the abundance of calcareous nannofossils rapidly reached the same level as in the Maastrichtian, the lower diversity still points to a mesotrophic, poorly stratified marine environment. The reworking versus survivorship of Cretaceous species found in Danian samples remains an unresolved question, but a hypothesis of survivorship is also strengthened by the presence of several coccospheres of W. barnesae and Bidiscus rotatorius well above the boundary. The effect of the marine environmental change across an expanded K-T boundary section as Elles appears complex if seen through calcareous nannofossils. From a geological point of view, the K-T boundary floral turnover at Elles seems dramatically synchronous with the signatures of an extraterrestrial impact, superimposed on long-term, background environmental and climatic changes. However, a precise succession of events based on appearance/disappearance of nannofossil species across the Cretaceous/Paleocene transition is far from being unanambiguous.