Eocene bathyal and abyssal benthic foraminifera from a South Atlantic transect at 20–30° S

Abstract

Eocene age (Zone P6b-P17) benthic foraminiferal faunas from 207 samples of Atlantic Ocean DSDP Sites 19, 20, 357, 363, 522 and 523 were analyzed along a transect located at approximately 30°S and spanning the depth range from upper lower bathyal to lower abyssal. There is a distinct “abyssal/bathyal” faunal boundary in the Eocene which was located above lower bathyal Site 363 in Zone P6b, dropping below that depth for the remaining early and middle Eocene, it rose again to lower bathyal depths (estimated between 1500–1800 m) at the beginning of the late Eocene. Abyssal faunas are highly dominated by spiral forms. An abyssal faunal association (1) with codominant Nuttalides truempyi and Quadrimorphina, together with Abyssamina and Clinapertina, prevailed at lower abyssal depths (Hole 20C) in the early Eocene. At upper abyssal depths (Site 19 and 523) it persisted in the middle Eocene. A N. truempyi dominant (> 25% of the total fauna) association (2) appeared at lower abyssal depths in early middle Eocene. At upper abyssal depths it first occurred in Zone P12 interchanging with association (1) in the late middle Eocene. In early Zone P15 an association with increased abundance of Nuttallides umbonifer and high abundance of Globocassidulina subglobosa replaced the N. truempyi dominant association. This replacement is observed at lower bathyal and abyssal sites (all sites except Site 357) and is the most pronounced faunal change observed in the Eocene. Bathyal faunas contain uvigerinids, bolivinids, Lenticulina, high percentages of buliminids and are characterized by the absence of highly dominant species.