Calcareous nannofossil extinction and survivorship across the Cretaceous−Paleogene boundary at Walvis Ridge (ODP Hole 1262C, South Atlantic Ocean)

Abstract

Site 1262 in the South Atlantic Ocean has provided a stratigraphically continuous deep Cretaceous/Paleogene boundary sequence. High resolution calcareous nannofossil quantitative analyses were carried out across the K/P boundary, and provided a remarkable record of the abrupt and catastrophic extinction. The calcareous nannofossil assemblages are abundant and well preserved allowing to obtain a precise bioevent sequence and to document in detail the survivors and victims and the subsequent recovery across the K/P boundary. Late Maastrichtian diversity and absolute abundance is high and no decrease towards the boundary has been observed. The relative abundance of Cretaceous species does not suffer important changes during the latest Maastrichtian. However, the increase in abundance of cool-water taxa, paralleled with a decrease of warm water taxa in the uppermost 2 cm of the Maastrichtian, revealed a pulse of surface water cooling. The K/P boundary is marked by an important decrease in calcareous nannofossil absolute abundance, the increase of Cretaceous-persistent species such as Cyclagelosphaera reinhardtii together with the dinoflagellate cysts of Thoracosphaera operculata and the appearance of the new-Paleocene taxa Cyclagelosphaera alta and Biantholithus sparsus. These events are followed by the successive relative abundance increases of other Cretaceous survivors as Zeugrhabdotus sigmoides, Markalius inversus and Biscutum recognized as r-selected taxa, adapted to eutrophic and cold water environments. In the early Danian several first occurrences of small new-Paleocene species have been observed: calcareous nannoplankton evolutionary attempts to colonize vacant niches left by the extinct Cretaceous species. Only some forms succeed and become ancestors to the Cenozoic assemblages — the other disappear in few kiloyear. The presence of a reworking/mixing interval above the K/P boundary hampered to unequivocally interpret if the few Cretaceous-vanishing taxa may have survived for a very short time after the K/P extinction.