EVALUATION OF THE PLIOPLEISTOCENE ASTRONOMICAL TIMESCALE

An astronomically calibrated timescale has recently been established [ Hilgen, 1991a, b] for the Pliocene and earliest Pleistocene based on t he correlation of dominantly precession controlled sedimentary cycles (sapropels and carbonate cycles) in Mediterranean marine 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 pat terns with 65 degrees N summer insolation time series of different ast ronomical solutions and (2) a cross-spectral comparison between the ob liquity-related components in the 65 degrees N summer insolation curve s and high-resolution paleoclimatic records derived from the same sect ions used to construct the timescale. Our results show that the carbon ate cycles older than 3.5 m.y. should be calibrated to one precession cycle older than previously proposed. Application of the astronomical solution of Laskau [1990] (hereinafter referred to as La90) with prese nt-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 geologic al record, indicating that this solution is the most accurate from a g eological point of view. Application of Ber90, or La90 solutions with dynamical ellipticity values smaller or larger than the present-day va lue, results in a less obvious fit with the geological record. This im plies that the change in the planetary shape of the Earth associated w ith ice loading and unloading near the poles during the last 5.3 milli on years was too small to drive the precession into resonance with the perturbation term, s(6)-g(6)+g(5), of Jupiter and Saturn. Our new tim escale results in a slight but significant modification, of all ages o f the sedimentary cycles, bioevents, reversal boundaries, chronostrati graphic boundaries, and glacial cycles. Moreover, a comparison of this timescale with the astronomical timescales of ODP site 846 [Shackleto n et al., 1995a, b] and ODP site 659 [Tiedemann et al., 1994] indicate s that all obliquity-related glacial cycles prior to similar to 4.7 Ma in ODP sites 659 and 845 should be correlated with one obliquity cycl e older than previously proposed.