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One of the most striking aspects of the Mediterranean "Messinian Salinity Crisis" as observed in landbased sections, is the basin-wide synchronicity in facies change (Krijgsman et al., 1999a). The Messinian succession of the Caltanisetta Basin on Sicily serves as the classical standard for these facies changes, which can also be recognised elsewhere in the Mediterranean, i.e. on Cyprus, Crete, northern Italy and southern Spain. It starts with an alternation of open marine marls and sapropels, passes via diatomites into evaporitic limestones, gypsum and halite of the "Lower Evaporites" (of marine origin) and, following an erosional unconformity, ends with the "Upper Evaporites" and associated fresh to brackish water deposits of the Lago Mare that are essentially of non-marine origin and contain a caspi-brackish ostracode fauna. The erosional unconformity between the "Lower and Upper Evaporites" is assumed to reflect the phase of most extreme sea level drawdown in the Mediterranean that caused significant erosion and localised channel entrenchment on the continental shelves and slopes.

A high-resolution cyclostratigraphy for the rhythmically bedded lignite‐marl sequences of the Lower Pliocene Ptolemais Formation is combined with 40 Ar= 39 Ar dating results of intercalated volcanic ash beds. Detailed field reconnaissance in three open-pit lignite mines reveals three end-member sediment types: lignites, composed primarily of organic material; grey marls, a mixture of carbonate and organic material; and beige marls, almost exclusively composed of carbonate. These lithologies are arranged in two basic types of sedimentary cycles: lignite‐grey marl and lignite‐beige marl cycles. A cyclostratigraphic composite section comprising 56 lignite‐marl cycles is constructed which combines the consistent cycle patterns from three parallel sections. The concordant positions of 20 volcanic ash beds in these sections confirm the cyclostratigraphic correlations and indicate that the lignite‐marl cycles result from regional, basin-wide forcing rather than lateral facies migrations. 40 Ar= 39 Ar ages on sanidine and biotite separates from nine volcanic ash beds were obtained by multiple total fusion and incremental-heating experiments. The 40 Ar= 39 Ar ages range between 5:00 0:05 and 4:04 0:04 Ma and are, in general, consistent with the stratigraphic order. A least-square linear regression using the measured 40 Ar= 39 Ar ages gives an average duration of 21:8 0:8 kyr per lignite‐marl cycle. Evidently, the lignite‐marl cycles in the Ptolemais Formation are linked to the precessional variation in the Earth’s orbit through its influence on Mediterranean climate. For the first time, 40 Ar= 39 Ar dating results, totally independent from any other dating and or tuning technique, confirm the astronomical theory of climate change. The 40 Ar= 39 Ar ages of the volcanic ash beds show a constant200 kyr (4.5%) age discrepancy with the astronomical ages of the same ash beds. This inconsistency remains difficult to explain. The discrepancy is unlikely to have resulted from erroneous astronomical ages, through incorrectness in the astronomical tuning, inaccuracies of the magnetostratigraphic data or the orbital time-series used, and=or errors in the APTS. The 40 Ar= 39 Ar dating results neither give clear indications for a possible source of error. From the excellent data set it is evident that neither loss of radiogenic 40 Ar, nor an underestimation of the contribution of Ca- and K-derived Ar isotopes could have caused the discrepancy. Moreover, the discrepancy is also beyond the errors in the systematic variables, like the decay constants of 40 K or the ages for the neutron-fluence monitors. © 1999 Elsevier Science B.V. All rights reserved.