Numerical ages of magnetostratigraphically calibrated biostratigraphic zones at DSDP Leg 73 Holes

Most Cenozoic nannofossil and many foraminiferal zonal boundaries have been accurately determined and magnetostratigraphically calibrated at five Leg 73 boreholes. The numerical ages of the boundaries were computed by assuming a linear seafloor spreading rate and a radiometric age of 66.5 m.y. for the Cretaceous/Tertiary boundary. Alternative magnetostratigraphic ages (given below in parentheses) were obtained by adopting a 63.5 m.y. age for the Cenozoic. Our data confirm previous determinations of the Pleistocene/Pliocene boundary at 1.8 (1.7) m.y. and of the Pliocene/ Miocene boundary at 5.1 (5.0) m.y. The Miocene/Oligocene boundary is placed within Chron C-6C and has a magnetostratigraphic age of 23.8 to 24.0 (22.7 to 22.9) m.y. The Oligocene/Eocene boundary is also very precisely located within Chron C-13-R, with a magnetostratigraphic age of 37.1 to 37.2 (35.5 to 35.6) m.y. The Eocene/Paleocene boundary should be located within an uncored interval of Chron C-24 and have a magnetostratigraphic age of 59.0 (55.4) +/- 0.2 m.y. The general accord of the magnetostratigraphic and radiometric ages supports the hypothesis that the seafloor spreading rate was linear during the Cenozoic. Two possible exceptions are noted: the middle Miocene radiometric ages are a few million years older, and the early Eocene radiometric ages are several million years younger, than the corresponding magnetostratigraphic ages