Radiolarian stratigraphy across the Oligocene/Miocene transition in low latitudes

Recently the International Union of Geological Sciences (Commission on Stratigraphy, Working Group on the Paleogene/Neogene Boundary) proposed that the Oligocene/Miocene boundary be placed at the base of Chron C6Cn2n at 23.8 Ma on the Cande and Kent (1992) magnetic time scale, where it is approximated by planktic foraminifera at the first occurrence of Globorotulia kugleri, and by calcareous nannofossils at the last occurrence of Sphenolithus ciperoensis and the first and last occurrences of Sphenolithus delphix and S. capricornutus. Herein we show that, in terms of radiolarians, the base of Chron C6Cn2n can be correlated with the upper part of the Lychnocanoma elongata Zone between the last occurrence of Artophormis gracilis (23.94 Ma) and the first occurrence of Cyrtocapsella tetrapera (23.69 Ma). Since the proposed stratotype at Lemme-Carrosio (Italy) does not contain radiolarians at the boundary, we re-examined 13 DSDP sites and established the stratigraphic sequence of 29 first and last radiolarian occurrences and one evolutionary transition across the boundary. Nine of these sites contain both calcareous and siliceous microfossils and thus allow for an integrated biostratigraphy. Paleomagnetic stratigraphy is not available for any of the DSDP cores examined. However, use of Hodell and Woodruff's (1994) strontium isotope curve from DSDP Site 289 has permitted calibration of several low latitude microfossil datum levels against the geomagnetic polarity scale. Two new species, Lychnocanoma apodora and Eucyrtidium plesiodiaphanes, are described.