Oligocene bathyal to abyssal benthic foraminifera of the Atlantic Ocean

Abstract

We document the bathymetric and geographic distributions of benthic foraminiferal biofacies in 267 upper Eocene to lowermost Miocene samples from 32 Atlantic sites ranging from ~450m to ~4700m paleowater depth, focusing on the Oligocene. We use samples from Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) sites, Gulf of Mexico Eureka boreholes, and Barbados land sections to: 1) evaluate species and biofacies changes through depth and time; 2) calibrate bathymetric distributions using independent means of calculating paleodepth so that benthic foraminifera may be used elsewhere to estimate paleobathymetry; and 3) evaluate changes in species and biofacies as potential indicators of paleoceanographic changes. Bathyal to abyssal depths in the Atlantic were characterized by Cibicidoides praemundulus , Globocassidulina subglobosa , Gyroidinoides spp., and Oridorsalis spp. in the Oligocene, with C. praemundulus more common at bathyal depths (200–2000m). A Lenticulina spp. – dominated biofacies characterized the upper to upper lower bathyal depths (200–1200m), particularly in the late Oligocene. A biofacies dominated by N. umbonifera characterized the lower abyssal zone throughout the Oligocene, and expanded to lower bathyal depths in the South Atlantic during two time periods, ~32–29 Ma and ~24–22 Ma. These expansions correspond to marine isotope zones Oi2 and Mi1, indicating that there was a faunal response to increased Southern Ocean bottom water currents and cooling associated with ice sheet expansion and sea level fall. In addition, many individual taxa show distinct and consistent paleobathymetric distributions. Others migrate or expand/contract their preferred water depth habitats through time.