Abstract:
Many workers are convinced of a cause and effect relationship between global mass extinction and chemical and sedimentological evidence for a bolide impact at the Cretaceous-Tertiary boundary (KTB). However, others suggest that the KTB extinctions were stepwise rather than abrupt, attributing global environmental changes during the last several million before the KTB impact as the primary cause for species turnover. Planktic foraminifera have been central to this debate since they record a mass extinction across the KTB and are widely used as monitors of environmental changes in oceanic surface waters. To test the hypothesis of pre-boundary species turnover we compared the biostratigraphy and population dynamics of well-preserved planktic foraminiferal assemblages from a depth transect of three deep-sea sites on Blake Nose in the subtropical North Atlantic Ocean. The sites include ODP sites 1049, 1050, and 1052, which currently range from 2700 to 1300 m water depth and were only slightly shallower at the end of the Cretaceous. SHE (species richness, Shannon index, and species equitability) analysis of samples spanning the late Maastrichtian Abathomphalus mayaroensis and Pseudoguembelina hariaensis zones reveals that all abundant Maastrichtian species range to the KTB impact horizon and that maximum observed species diversity (40 species) occurs within 500 kyr before the KTB. Few changes in any of the SHE indices or in planktic:benthic ratios occur within the studied interval, and those changes that do occur are not correlative among the three sites. Stable isotope analyses of planktic and benthic foraminifera from this interval also reveal no significant or correlative changes in surface water temperatures, vertical temperature gradients, or vertical carbon isotope gradients among the sites. We conclude that there is no evidence for stepwise extinction of planktic foraminifera or significant global environmental change during the last 3 m.y. of the Cretaceous and that at least 90% of the planktic foraminiferal species that existed within the last 500 kyr of the Cretaceous became extinct at the level of the KTB impact horizon.