Radiolarian faunal turnover and paleoceanographic change around Eocene/Oligocene boundary in the central equatorial Pacific, ODP Leg 199, Holes 1218A, 1219A, and 1220A

Abstract

Continuous Eocene through Oligocene sedimentary sedimentary sections were obtained at Holes 1218A, 1219A, and 1220A drilled during Leg 199 of the Ocean Drilling Program in the central equatorial Pacific. Quantitative analysis of radiolarian assemblages between the upper Eocene through lower Oligocene sedimentary successions revealed that major faunal changes of radiolarians occurred in the late Eocene (35.5–35.7 Ma; Subchron C16n1), around the Eocene/Oligocene boundary (33.7–33.8 Ma; upper Subchron C13r) and in the early Oligocene (30.9–31.3 Ma; uppermost Subchron C12r). The faunal change in the late Eocene is characterized by a stepwise decrease of tropical Eocene species and an increase in cosmopolitan, probably cool-water-tolerant, radiolarians; however, without a significant change in species diversity and radiolarian accumulation rates. The stratigraphic level of this late Eocene turnover came after the late Eocene impact event, and may have been more related to global cooling and associated oceanographic changes than to asteroid impact. The turnover around the Eocene/Oligocene boundary, corresponding to the Oi-1 event, is the most critical during the Eocene–Oligocene transition. Many biotic events such as a sudden drop of species diversity, an increase in the cosmopolitan radiolarian group, a large number of species turnovers, and a drop in radiolarian accumulation rates occurred at or just above this level. Together these indicators suggest a major restructuring of radiolarian assemblages from the high diversity Eocene fauna to a low diversity Oligocene one. The third turnover event within early Oligocene is represented by a replacement of dominant species with a sudden decrease in radiolarian accumulation rates. However, this Oligocene turnover is not consistent with other major δ 18O isotope events, such as Oi-1b and Oi-2.