Introduction

The Quaternary period, approximately the past two million years, is characterised by successive sequences of glacial–interglacial climatic oscillations, largely attributed to changes in the orbital parameters of the Earth's rotation around the sun and on its own axis through a variety of gravitational attractions, each varying with the temporal dynamics of the solar system (Imbrie & Imbrie, 1979; Berger, 1976, 1978). There is evidence that the orbital forcing of climate has prevailed throughout Earth history, with cyclical variations of the organic carbon and calcium carbonate components of sediments spanning much of the last 125 million years (Herbert, 1997) and, most famously, in the coal-bearing cyclothems of the Carboniferous (Pennsylvanian subperiod) (Broecker, 1997).

Orbital forcing of climate results from quasi-periodic variation in three main orbital parameters, each with a different periodicity; eccentricity, with a primary period of 100 ka, obliquity (axial tilt), with an average period of 41 ka and precession, every 19 ka. These ‘predicted’ periodicities are broadly in line with continuous ocean and ice core geochemical records (Hays et al., 1976; Barnola et al., 1987; Jouzel et al., 1987). Long oceanic records (730 ka) of the variations in oxygen isotope composition of marine foraminifera indicate rhythmic climatic variations, with strong cyclicity in the data at 106, 40–43, and 24 and 19.5 ka corresponding to the calculated return interval of each orbital parameter (Hays et al., 1976).