An astronomically calibrated timescale has recently been established [Hilgen, 1991a, b] for the Pliocene and earliest Pleistocene based on the 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 patterns with 65°N summer insolation time series of different astronomical solutions and (2) a cross‐spectral comparison between the obliquity‐related components in the 65°N summer insolation curves and high‐resolution paleoclimatic records derived from the same sections used to construct the timescale. Our results show that the carbonate cycles older than 3.5 m.y. should be calibrated to one precession cycle older than previously proposed. Application of the astronomical solution of Laskar [1990] (hereinafter referred to as La90) with present‐day values for the dynamical ellipticity of the Earth and tidal dissipation 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 planetary shape of the Earth associated with ice loading 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, s6−g6+g5, of Jupiter and Saturn. Our new timescale results in a slight but significant modification of all ages of the sedimentary cycles, bioevents, reversal boundaries, chronostratigraphic boundaries, and glacial cycles. Moreover, a comparison of this timescale with the astronomical timescales of ODP site 846 [Shackleton et al., 1995a, b] and ODP site 659 [Tiedemann et al., 1994] indicates that all obliquity‐related glacial cycles prior to ∼4.7 Ma in ODP sites 659 and 846 should be correlated with one obliquity cycle older than previously proposed.

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The detailed paleomagnetic records of the upper and lower Nunivak polarity transitions have been determined from Pliocene marine marls in southern Sicily. Magnetites are the most important carrier of the remanence in both records. The transitions are recorded in two components; a low temperature and a high temperature component. The two components do not represent the geomagnetic field because the changes in these components take place at lithological boundaries. In addition, the directional changes do not completely match directional changes of the same transitions recorded in Calabria, some 250 km away. The character of the virtual geomagnetic poles paths is probably caused by smoothing of the stable directions before and after the transitions. The directional changes as well as the smoothing mechanism can be explained by the diagenetic magnetite formation model (Van Hoof et al., 1992) in which shortly after burial, the remanence carried by newly formed secondary magnetites is superposed on the initial remanence carried by primary magnetite.

Detailed paleomagnetic records of the upper and lower Thvera polarity transitions have been determined from Pliocenc marine marls in southern Sicily. The dominant magnetic mineral is fine grained magnetite. The two transitions have VGP paths following a great circle passing South America and the west coast of North America. There are strong indications that the VGP paths result from smoothing of the non-antipodal stable directions before and after the transitions. The upper Thvera transitional record is preceded by two excursions and followed by a third one. The first excursion is a sedimentary artefact caused by post-depositional migration of magnetic minerals and the third one by is caused by weathering of the sediment. The upper Thvera transition from Sicily is compared with the record from Calabria, about 250 km away. The two records show similarities as well as differences: both transitions have identical VGP paths, but in Calabria the transition is recorded lower in the sediment, the first and third excursions have different characters and the second one is not present. Apparently the registration of transitions of the Earth's magnetic field in sediments is strongly influenced by smoothing and diagenetic processes after deposition.