The earlier discovery of high concentrations of iridium and osmium at the very base of the Tertiary in marine pelagic sequences, has led to the hypothesis that the impact of a large projectile on Earth terminated the Mesozoic era. This paper describes the distribution of other trace elements in samples closely spaced around the Cretaceous-Tertiary boundary. We found an enrichment of Co, Cr, Ni, As, Sb and Se, and a depletion of the rare earth elements (REE), coupled with the anomalously high concentration of Ir and Os. Also, in the enrichment layer, problematic sanidine spherules occur which may be spheres, ablated from the impacting projectile or microtektites. The REE depletion could not only be demonstrated in the basal millimetres of the relatively thick (∼ 10 cm) boundary marl bed of the Barranco del Gredero section, Caravaca, Spain, but also in the boundary clay of the Stevns Klint section—the “Fiskeler”—Denmark. In these boundary clay beds are also the highest concentrations of Ir recorded [44 and 86.7 ppb (1 ppb = 10−9 g/g), respectively]. This REE depletion is tentatively connected with the low REE concentration in common meteorites. In that case the basal millimetres are dominated by a meteoritic component and represent a “fall-out” layer of the impact event. This in turn would imply low terrestrial dilution at impact on Earth, suggesting that the projectile might have fallen into the ocean. Identification of the projectile remains speculative; especially the high As, Sb and Se anomalies are problematic. These latter may point to a projectile of uncommon composition, possibly a comet. An integrated scenario of a possible sequence of events is presented, chiefly based on the interpretation of micropaleontological, lithological, geochemical and stable isotope data of the studied sections. The direct and indirect effects of an impact leading to mass extinction constitute a complicated problem and may comprise a sudden temperature rise, a poisoning by cyanide or arsenic, and a few years suppression of sunlight, or preferably, a combination of these effects.

EXPLOR_STEP=$WICRI_ROOT/Wicri/Terre/explor/CobaltMaghrebV1/Data/Main/Exploration

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{{Explor lien
   |wiki=    Wicri/Terre
   |area=    CobaltMaghrebV1
   |flux=    Main
   |étape=   Exploration
   |type=    RBID
   |clé=     ISTEX:149181F715469AD765C808242D3C183A09D7F8D4
   |texte=   Trace-element patterns at the Cretaceous-Tertiary boundary—Consequences of a large impact
}}