The evolution and paleobiogeography of abyssal agglutinated foraminifera since the Early Cretaceous: A tale of four faunas

The diversity record of abyssal agglutinated foraminifera reveals specific intervals during the past 150 million years when the diversity of species and ecological partitioning was high, interspersed by periods of lower diversity. We distinguish herein three faunas recovered from abyssal red claystones that represent periods of widespread oligotrophic conditions with low rates of sea-floor organic carbon flux. These are termed: the Tithonian to Barremian "Argo Fauna", the Turonian-Maastrichtian "Krashenninikov fauna", and the Eocene to early Oligocene "Iberia-Celebes Fauna". A fourth fauna (the mid-Cretaceous "Hatteras Fauna" and the "Biofacies B Faunas") is observed during intervals characterised by eutrophic conditions that result in high organic flux and poorly oxygenated conditions at the sea floor. The taxonomic composition, diversity, and microhabitat preferences of agglutinated foraminiferal faunas in Cretaceous to Cainozoic deep sea sediments are related to a combination of environmental factors, including depth of the calcite compensation depth, ventilation of the bottom waters, and the amount of sea floor organic flux. These environmental conditions have changed in different ocean basins at different times and consequently the paleobiogeographical patterns of "Deep water agglutinated foraminifera" (DWAF) reflect general circulation patterns and resulting trophic conditions. We believe that the ecological structuring of agglutinated foraminifera within habitats may have been one of the most important driving forces in the evolution of this group. We conclude that the diversification of distinctive abyssal assemblages at specific times in ocean history depends upon the establishment of oligotrophic conditions in the deep sea following perturbations to the trophic structure caused by periods of increased organic carbon flux. Our comparison of Cretaceous to Recent DWAF reveals that evolutionary changes mainly took place among forms adapted to the oligotrophic end of the trophic resource continuum (i. e. passive deposit feeders, suspension feeders and some infaunal forms), whereas stasis prevailed among species adapted to eutrophic conditions (active epifaunal and shallow infaunal deposit feeders such as ammodiscids). Although the Neogene record of DWAF faunas is very poor, we observe that the modern sub-CCD fauna of the oligotrophic abyssal plains had its origins in the abyssal faunas of the Cretaceous and Paleogene.