Evaluation of the Plio-Pleistocene astronomical timescale

An astronomically calibrated timescale has recently been established (Hilgen, 1991 a, b) for the Pliocene and earliest Pleistocene based on the correlation of dominantly pre- cession controlled sedimentary cycles (sapropels and carbonate cycles) in Mediterranean ma- rine sequences to the precession time series of the astronomical solution of Berger and Loutre ( 1991 ) (hereinafter referred to as Ber90). Here we evaluate the accuracy of this timescale by (1) comparing the sedimentary cycle patterns with 65 oN summer insolation time series of dif- ferent astronomical solutions and (2) a cross-spectral comparison between the obliquity-related components in the 65 oN summer insolation curves and high-resolution paleoclimatic records derived from the same sections used to construct the timescale. Our results show that the car- bonate cycles older than 3.5 m.y. should be calibrated to one precession cycle older than previ- ously proposed. Application of the astronomical solution of Laskar ( 1990) (hereinafter refer- red to as La90) with present-day values for the dynamical ellipticity of the Earth and tidal dis- sipation by the Sun and Moon results in the best fit with the geological record, indicating that this solution is the most accurate from a geological point of view. Application of Ber90, or La90 solutions with dynamical ellipticity values smaller or larger than the present-day value, results in a less obvious fit with the geological record. This implies that the change in the plane- tary shape of the Earth associated with ice loadine$ and unloading near the poles during the last 5.3 million years was too small to drive the precession into resonance with the perturbation term, s-gt+g 5, of Jupiter and Saturn. Our new timescale results in a slight but significant modi- fication of all ages of the sedimentary cycles, bioevents, reversal boundaries, chronostrati- graphic boundaries, and glacial cycles. Moreover, a comparison of this timescale with the as- tronomical timescales of ODP site 846 (Shackleton et al., 1995a, b) and ODP site 659 (Tiede- mann et al., 1994) indicates that all obliquity-related