The Cretaceous/Tertiary boundary stratotype section at El Kef, Tunisia: how catastrophic was the mass extinction?

Abstract

The Cretaceous/Tertiary (K/T) boundary stratotype section at El Kef, Tunisia, represents the most complete and expanded sedimentary record across this important mass extinction horizon presently known. High resolution analysis of planktic foraminifera in two outcrops (El Kef I—stratotype and El Kef II) along with comparisons between planktic and benthic foraminifera, calcareous nannofossils, ostracods, pollen and spores, and dinoflagellates indicate that major changes across the K/T boundary are registered only in benthic and planktic foraminifera and calcareous nannofossils. Biotic changes in benthic foraminifera are unique to El Kef and similarly shallow continental shelf sections and appear to be the result of a sea-level regression in the latest Maastrichtian followed by a sea-level rise across the K/T boundary that was accompanied by expansion of the local oxygen minimum zone (OMZ). Biotic changes in planktic foraminifera appear partly related to these conditions also, but in general reflect more global oceanographic changes. For instance, species extinctions are gradual and selective as observed in K/T sections worldwide, rather than random and abrupt. Although there is a 69% decline in species richness between 25 cm below and 10 cm above the K/T boundary, only rare species disappeared. Their combined relative abundance constitute less than 20% of the total population. About 52% of these extinct taxa (8% of the population) are large, ornate, morphologically complex tropical-subtropical forms that lived at or below the thermocline. No planktic foraminifera from this depth range survived the K/T boundary event. All survivor taxa were surface dwellers living within the photic zone. Their relative abundance (∼80%) dominates both Cretaceous and early Tertiary populations. These data indicate that the K/T biotic record in the shallow continental shelf section at El Kef was significantly influenced by local conditions which, combined with the latest Maastrichtian sea-level regression and subsequent sea-level rise, resulted in shallowing of the local OMZ relative to the sea-surface. Shallowing of the local OMZ lead to the selective disappearance of benthic faunas and may have adversely affected the surviving photic zone dwellers. The selective nature of species extinctions, however, appear to be related partly to long-term global oceanographic changes which were accelerated at the K/T boundary possibly by a bolide impact.