Sea-land oxygen isotopic relationships from planktonic foraminifera and speleothems in the Eastern Mediterranean region and their implication for paleorainfall during interglacial intervals

An astronomically calibrated timescale has recently been established (Hilgen, 1991 a, b) for the Pliocene and earliest Pleistocene based on the correlation of dominantly pre- cession controlled sedimentary cycles (sapropels and carbonate cycles) in Mediterranean ma- rine 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 oN summer insolation time series of dif- ferent astronomical solutions and (2) a cross-spectral comparison between the obliquity-related components in the 65 oN summer insolation curves and high-resolution paleoclimatic records derived from the same sections used to construct the timescale. Our results show that the car- bonate cycles older than 3.5 m.y. should be calibrated to one precession cycle older than previ- ously proposed. Application of the astronomical solution of Laskar ( 1990) (hereinafter refer- red to as La90) with present-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 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 plane- tary shape of the Earth associated with ice loadine$ 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, s-gt+g 5, of Jupiter and Saturn. Our new timescale results in a slight but significant modi- fication of all ages of the sedimentary cycles, bioevents, reversal boundaries, chronostrati- graphic boundaries, and glacial cycles. Moreover, a comparison of this timescale with the as- tronomical timescales of ODP site 846 (Shackleton et al., 1995a, b) and ODP site 659 (Tiede- mann et al., 1994) indicates that all obliquity-related

Evaluation of the Plio‐Pleistocene astronomical timescale

Astronomical calibration of Gauss to Matuyama sapropels in the Mediterranean and implication for the Geomagnetic Polarity Time Scale

The Croll–Milankovitch astronomical theory of climate1–3 has received strong support from the evidence of a linear climatic forcing by obliquity and precession, although nonlinearity had to be assumed for eccentricity4,5. Moreover, interglacials have appeared to be controlled by the orbital insolation6 although a phase shift of 6,000–5,000 yr is seen between an astronomical climate index and terrestrial climate indicators, dominated by the northern ice-sheet dynamics. Climate proxy data of low latitudes are less directly dependent on the ice sheets. Here it is shown that during the past 464,000 yr, African monsoons signalled by the East Mediterranean sapropels were heaviest always and only when a northern summer monsoon index, computed from the orbital variation of insolation, reached maximum values. This occurred during all the interglacials, but also twice during glacial periods. Thus tropical aridity is not the single climatic pattern of glacial periods. An immediate terrestrial climate response to orbital variations of insolation is established.

Colette Vergnaud-Grazzini

Papers

Stable isotopic fractionation, climate change and episodic stagnation in the eastern Mediterranean during the late Quaternary