THE LAST 100,000 YEARS BEFORE THE END-CRETACEOUS MASS EXTINCTION

For the past 30 years the sole cause for the end-Cretaceous (KT) mass extinction has been popularly attributed to the Chicxulub impact. With new data emerging linking this mass extinction directly to Deccan volcanism, the impact scenario is no longer supported. New biological, geochemical and mineralogical evidence indicates that the massive volcanic eruptions during the last 100 kyr of the Maastrichtian led to high stress conditions characterized by blooms of disaster opportunistic species, ocean acidification, extreme climate warming, low magnetic susceptibility and mercury anomalies, all ending with the KT mass extinction. Blooms of disaster opportunistic species in planktic foraminifera and calcareous nannofossils have long been observed before and after the mass extinction and are linked to high stress environments and climate change. Blooms of Guembelitria cretacea are documented in low and mid latitudes worldwide with highest abundance in nearshore or shallow waters, characterizing it as a nearshore dweller that responded to eutrophic waters where few other species survived.

The absence of G. cretacea in some shallow and deeper water sections has remained an enigma. Particularly puzzling is the near absence of G. cretacea at Elles, Tunisia, which is the most complete KTB section with the highest sediment record known to date. At a water depth of ~150-200 m in a middle shelf environment, G. cretacea blooms are expected to be well represented but are not. A likely reason for its absence may be an artifact of the size fraction analyzed. During high stress conditions species tend to mature early in smaller sizes. In the early Danian at El Kef, Tunisia, no G. cretacea were found in the size fraction >63 μm but smaller specimens were abundantly present in the smaller (38-63μm) size fraction. To test whether this was also the case at Elles during the latest Maastrichtian zone CF1, we analyzed the 38-63 μm size fraction. Results show Guembelitria blooms up to 60% though with variable abundance correlative with warm and cool events. This suggests that the high stress conditions that led up to the mass extinction started in zone CF1 coincident with the onset of the most massive Deccan eruptions, and that these conditions were as detrimental to marine calcifiers as during the early Danian in the aftermath of the mass extinction.