Astronomic calibration of the late Oligocene through early Miocene geomagnetic polarity time scale

At Ocean Drilling Program (ODP) Site 1090 (subantarctic South Atlantic), benthic foraminiferal stable isotope data (from Cibicidoides and Oridorsalis) span the late Oligocene through early Miocene ( similar to 24-16 Ma) at a temporal resolution of similar to 5 ky. Over the same interval, a magnetic polarity stratigraphy can be unequivocally correlated to the geomagnetic polarity time scale (GPTS), thereby providing direct correlation of the isotope record to the GPTS. In an initial age model, we use the newly derived age of the Oligocene/Miocene (O/M) boundary of 23.0 Ma of Shackleton et al. [Geology 28 (2000) 447], revised to the new astronomical calculation (La-2003) of Laskar et al. [Icarus (in press)] to recalculate the spline ages of Cande and Kent [J. Geophys. Res. 100 (1995) 6093]. We then tune the Site 1090 6180 record to obliquity using La-2003. In this manner, we are able to refine the ages of polarity chrons C7n through C5Cn. In. The new age model is consistent, within one obliquity cycle, with previously tuned ages for polarity chrons C7n through C6Bn from Shackleton et al. [Geology 28 447-450 (2000)] when rescaled to La-2003. The results from Site 1090 provide independent evidence for the revised age of the Oligocene/Miocene boundary of 23.0 Ma. For early Miocene polarity chrons C6AAr through C5Cn, our obliquity-scale age model is the first to allow a direct calibration to the GPTS. The new ages are generally within one obliquity cycle of those obtained by rescaling the Cande and Kent [J. Geophys. Res. 100 (1995) 6093] interpolation using the new age of the O/M boundary (23.0 Ma) and the same middle Miocene control point (14.8 Ma) used by Cande and Kent [J. Geophys. Res. 100 (1995) 6093]. (C) 2004 Elsevier B.V. All rights reserved.