The Cretaceous-Tertiary (K/T) boundary is marked by the collision of the Earth with an asteroid or a comet. This event produced high concentrations of iridium and other PGEs in K/T boundary sediments. Although delivered by a very brief event, Ir is observed over a sizeable thickness of sediments simulating a long duration phenomenon (several tens of thousands of years). In order to understand this point we have analysed rocks from the K/T section of El Kef (Tunisia), a site which offers ideal conditions (high deposition rate and low bioturbation) for a detailed stratigraphic study. The Ir distribution shows a millimetre thick pulse-like feature occurring in perfect coincidence with the beginning of the biological crisis. This feature is superimposed to a diffuse component extending over about two metres of sediment from the upper Maastrichtian to the top of the Globigerina eugubina zone in the Danian. The pulse-like feature is associated with crystals of Ni-rich spinel, a mineral derived from meteoritic material. This pulse, reminiscent of the brevity of the K/T event, results from the prompt deposition of a large amount of debris of the projectile dispersed world-wide as a result of the impact event. As far as the diffuse component is concerned, its duration agrees with the estimated residence time of Ir in sea water. This component is explained by the transit through the oceanic reservoir of the iridium associated with the finely divided fraction of the projectile.

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