Biostratigraphy and paleoceanography across the Eoccene/Oligocene boundary at Deep Sea Drilling Project Site 549.

Abstract

Drilling at Site 549, located on Goban Spur (Irish continental margin), penetrated an apparently continuous section of upper Eocene/lower Oligocene nannofossil chalks. At 123 m below sea floor (BSF), a late Eocene planktonic foraminiferal assemblage containing species of Hantkenina and the Globorotalia cerroazulensis group is replaced by a lower diversity, predominantly globigeriniform, early Oligocene assemblage. Th base of nannofossil Zone NP21, as determined by the disappearance of Discoaster saipanensis and D. barbadiensis, lies at 132 m BSF. The definition of the Eocene/Oligocene boundary and its recognition using biostratigraphic criteria remain controversial, for many authors (Berggren et al., in press; Hardenbol and Berggren 1978; Van Couvering et al., 1981) recognize the boundary using the extinctions of planktonic foraminifers (resulting in Zone NP21 sraddling the boundary) while others (Martini and Ritzkowski, 1968; Muller, 1979, this volume) correlate the boundary with the base of Zone NP21. Regardless of which boundary criteria are used at Site 549, no dramatic nannofloral or planktonic foraminiferal change occurred at the Eocene/Oligocene boundary. There was a gradual transition from a warm-water to a cool-water assemblage through the late Eocene, with maximal cooling indicated by low-diversity assemblages in lower Oligocene sediments. A bottom-water temperature drop occurred in the late Eocene through early Oligocene as indicated by an 18O enrichment in benthic foraminifers. A major benthic foraminiferal faunal abundance change, from an assemblage dominated by Nuttalides truempyi to one dominated by wide- and long-ranging taxa, occurred in the early late Eocene. No major benthic assemblage changes occurred at the Eocene/Oligocene boundary, although several taxa had first and last appearances in the latest Eocene to earliest Oligocene. -Authors