The Messinian salinity crisis is widely regarded as one of the most dramatic episodes of oceanic change of the past 20 or so million years (refs 1±3). Earliest explanations were that extremely thick evaporites were deposited in a deep and desiccated Mediterranean basin that had been repeatedly isolated from the Atlantic Ocean 1,2 , but elucidation of the causes of the isolationÐwhether driven largely by glacio-eustatic or tectonic processesÐhave been hampered by the absence of an accurate time frame. Here we present an astronomically calibrated chronology for the Mediterranean Messinian age based on an integrated high-resolution stratigraphy and`tuning' of sedimentary cycle patterns to variations in the Earth's orbital parameters. We show that the onset of the Messinian salinity crisis is synchronous over the entire Mediterranean basin, dated at 5:96 6 0:02 million years ago. Isolation from the Atlantic Ocean was established between 5.59 and 5.33 million years ago, causing a large fall in Mediterranean water level followed by erosion (5.59±5.50 million years ago) and deposition (5.50±5.33 million years ago) of non-marine sediments in a large`Lago Mare' (Lake Sea) basin. Cyclic evaporite deposition is almost entirely related to circum-Mediterranean climate changes driven by changes in the Earth's precession, and not to obliquity-induced glacio-eustatic sea-level changes. We argue in favour of a dominantly tectonic origin for the Messinian salinity crisis, although its exact timing may well have been controlled by the ,400-kyr component of the Earth's eccentricity cycle.Most hypotheses about the initiation of the Messinian salinity crisis (MSC) agree that it resulted from a complex combination of tectonic and glacio-eustatic processes which progressively restricted and ®nally isolated the Mediterranean Sea from the open ocean 1±8 . The gradual modi®cation of water exchange with the Atlantic caused important palaeoceanographic changes in the Mediterranean. This is re¯ected in the classic Messinian sequence of Sicily 9 which starts at 7.24 Myr ago (ref.3) with alternations of open marine marls and sapropels, passes via diatomites into the Lower Evaporites (gypsum, evaporitic limestone and halite), and ends, above an erosional surface, with the Upper Evaporites (gypsum, marls) and fresh to brackish water deposits of Lago Mare facies.Here we de®ne the MSC as the interval of evaporite deposition and Lago Mare sedimentation in the Mediterranean before the Pliocenē ooding 5.33 Myr ago 10 . Controversies still exist, however, over the timing and duration of the MSC; these range from a synchronous event 1 (that is, onset of the MSC in all basins at the same time) to a two-step event 4 (onset of MSC ®rst in marginal basins and later in deep basins) to a completely diachronous evolution 5 (onset of MSC totally dependent on local basinal setting). Equally large controversies exist over the cause, and the effects, of the isolation of the Mediterranean; the two basic explanations are (1) a large glacio-eustatic sea-level drop, r...

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Calibration of the geological time scale is achieved by independent radioisotopic and astronomical dating, but these techniques yield discrepancies of approximately 1.0% or more, limiting our ability to reconstruct Earth history. To overcome this fundamental setback, we compared astronomical and 40Ar/39Ar ages of tephras in marine deposits in Morocco to calibrate the age of Fish Canyon sanidine, the most widely used standard in 40Ar/39Ar geochronology. This calibration results in a more precise older age of 28.201 +/- 0.046 million years ago (Ma) and reduces the 40Ar/39Ar method's absolute uncertainty from approximately 2.5 to 0.25%. In addition, this calibration provides tight constraints for the astronomical tuning of pre-Neogene successions, resulting in a mutually consistent age of approximately 65.95 Ma for the Cretaceous/Tertiary boundary.

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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.