Sequence of Events around the K/T Boundary at El Kef (NW Tunisia)

摘要

The section at El Kef (NW Tunisia) has been selected as the stratotype of the K/T boundary. It offers exceptional conditions for a very accurate study of the biological crisis. Three events can be identified from geochimical and paleontological studies. Two of them developed over long periods of time: a marine regression and a climatic cooling. The third one is a cosmic catastrophe characterized by the presence of cosmic material (Iridium, Ni-rich spinels...) in a thin millimetric brown-reddish layer (goethite). This layer is now considered as the K/T boundary. The marine regression produced gradual changes in the planktonic assemblages and favoured the development of taxa adapted for shallow waters. The climatic change modified the composition of terrestrial supply (spores and pollen grains); tropical taxa gradually decrease in number till total extinction and are replaced by taxa with more European affinities. The cosmic event produced the most dramatic effects: immediately above the goethite layer, 90 to 95 % of marine populations with carbonate tests are missing. The remaining 5 to 10 % get progressively extinct in the above 10 cm marly sediments. Ante-crisis conditions are recovered about two meters above the boundary. On the contrary, dinoflagellates, algae with a chitinous cyst, seem much less affected by the K-T event. Continental palynoflora are also seriously disturbed by the K-T boundary: in the first centimeters above the boundary their rate of extinction is about hundred times higher than observed in the upper Maastrichtian, However, many taxa survived. The most widely accepted hypothesis is that the large quantity of dust produced by the cosmic collision resulted in the absorption of solar radiations and the reduction of photosynthesis. However, this very attractive hypothesis does not fit El Kef observations: the absorption of solar light should have also dramatically affected dinoflagellates. This is not what we observe. Data suggest that, in addition to the reduction of photosynthesis, the cosmic event produced chemical conditions in the ocean which disturbed the formation process of calcareous tests.