Astronomical calibration of Oligocene-Miocene time

Lithological cyclicity was observed aboard the JOIDES RESOLUTION in sedimen t sequences recovered from the Ceara Rise during ODP Leg 154. Shipboard wor k 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 t o estimate the mean wavelength and hence the sedimentation rate of successi ve 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 Ol igocene 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 s ediment from the 14-18 Ma and 28-34 Ma intervals. Although variability is d ominated by the 41 ka cycle there is sufficient variability at the precessi on period (amplitude-modulated by eccentricity) to permit an absolute place ment of this section with reference to the calculated orbital history. Furt her work is needed to establish precisely the implications of this calibrat ion for the geological time-scale but it appears that the true ages of even ts 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 th at is of astronomical as well as geological significance.