Evaluation of the Astronomically Calibrated Time-scale for the Late Pliocene and Earliest Pleistocene

An astronomically calibrated time scale was recently established (Hilgen, 1991a, b) for the Pliocene and earliest Pleistocene, based on the correlation of dominantly precession-controlled sedimentary cycles (sapropels and CaCO3 cycles) in Mediterranean deep-sea sections to astronomical solution Ber90 (Berger and Loutre, 1991). This time scale, which differed substantially from the conventional time scales then widely used, is essentially confirmed by a new generation of radiometric dates using the Ar-40/Ar-39 laser fusion technique. To evaluate this time scale, we here extract both precession- and obliquity-related components from late Pliocene - earliest Pleistocene climatic proxy records in the Mediterranean and determine coherencies and phase (time) lags between these components and the respective orbital variations. This work is found to support the new time scale, because significant coherencies are found between the astronomically related components in our proxy records and the orbital variations, not only at the main period of precession but also at the main period of obliquity and, because of the eccentricity modulation of precession, in the low-frequency eccentricity bands of the spectrum. But the calculated time lag between obliquity and 41-kyr components in the climatic proxy records shows a significant increase of 5.6 (+/- 1) kyr with respect to the late Pleistocene if the phase relation with precession is kept constant. This increase can be explained by a change in the climate response time to obliquity or to precession, by a small uncertainty in the astronomical solution, or by a combination of these factors