Astronomical calibration of Oligocene--Miocene time

Lithological cyclicity was observed aboard the JOIDES RESOLUTION in sediment sequences recovered from the Ceara Rise during ODP Leg 154. Shipboard work led to the conclusion that the Oligocene was probably characterized by ca. 41 ka cycles. Weedon and others were able to confirm this, and created a provisional time–scale for the Oligocene by assuming that the cyclicity is a response to orbital obliquity variation, and by using spectral analysis to estimate the mean wavelength and hence the sedimentation rate of successive intervals of core. We have extended this work by intercorrelating almost all the 9.5 m sediment cores from each of the four sites that recovered Oligocene sediment. We have successfully correlated all the material covering a time–interval of ca. 10 Ma from 18 Ma to 28 Ma, as well as most of the sediment from the 14 to 18 Ma and 28 to 34 Ma intervals. Although variability is dominated by the 41 ka cycle there is sufficient variability at the precession period (amplitude–modulated by eccentricity) to permit an absolute placement of this section with reference to the calculated orbital history. Further work is needed to establish precisely the implications of this calibration for the geological time–scale but it appears that the true ages of events close to the Oligocene–Miocene boundary are ca. 0.9 Ma younger than they appear on recently published time–scales. The sedimentary record preserves information concerning the amplitude modulation of the obliquity signal that is of astronomical as well as geological significance.