The K/T stratotype section of El Kef (Tunisia): Events and biotic turnovers

Abstract

Abstract At El Kef (NW Tunisia), the absence of bioturbation and the high sedimentation rate preserve a high resolution record of the Cretaceous-Tertiary transition. In the upper part of the Maastrichtian (Mayaroensis zone) planktonic foraminifer assemblages show a clear marine regression while the variations of spore-pollen contents show a trend related to a climatic cooling. In addition to these gradual changes, one can see a sharp discontinuity at the top of the Mayaorensis zone. This discontinuity is marked by a thin millimeter-scale brownish goethite layer containing Ir and Ni-rich spinels, the remains of the K/T cosmic catastrophe. This event occurred in coincidence with a drastic reduction (90–95%) of calcareous microfossils (coccoliths, foraminifers, ostracods,…). In contrast, the noncalcareous algae (dinoflagellates) seem to have been insensitive to the K/T event. Moreover, spores and pollen grains from the nearby lands do not indicate such a change in the continental vegetation. The selectivity of the extinctions can be explained neither by the absorption of solar radiations by stratospheric dusts produced by the K/T impact nor by an heat flash burning everything on the Earth. All species requiring solar light for their development should have been affected. The severity of the crisis for calcareous marine species seems due to the, difficulty of microorganisms to fix carbonate. It is well known that trace elements play a fundamental role in the development of micro-organisms, particularly in the construction of their carbonate shells and tests. A significant infall of meteoritic material in the oceans could have severely disturbed the normal processes. The disappearance of Coccoliths should have produced disastrous consequences in surface oceanic warm waters, where they are the main providers of oxygen. Judging from El Kef data, the crisis was fatal to all highly specialized taxa; the others, more adaptable, could find refuge in less disturbed places and recovered when normal conditions prevailed again. In the continental vegetation, it seems that the angiosperms, newly arrived in the vegetal realm and consequently having greater adaptative ability, cross the K/T boundary without very catastrophic extinctions.