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Department of Geology, The University, Southampton SO17 1BJ, UK
A brief review is given of orbital patterns affecting the Earth which may be of use in establishing, for long or short periods, orbital forcing timescales (OFT). The metronomic variations of the Earth-Moon system and of the Earth-Sun orbital patterns produce gravitational and temperature effects which alter the physical environment on the Earth’s surface. These give an interpenetrating effect of forcing cycles ranging from twice daily tides, day-night alternations, various tidal patterns and the annual solar pattern. All of these have been used palaeontologically to give precision to short-term age determination in the past.
It is cycles of the Milankovitch band which are showing promise of enabling new practical timescales to be established for parts of geological time. These depend on changes in the Earth-Sun distance (perihelion and precession cycles of 19 and 23 ka at the present time), changes in the tilt of the Earth’s axis with respect to the Earth’s orbit round the Sun (the obliquity cycles of 41 and 54 ka), and changes in the geometry of the Earth’s orbit around the Sun (eccentricity cycles of 106 and 414 ka). Since the number of days in the year have changed through time; so have the periods of the perihelion and precession cycles. There is increasing evidence that small-scale sedimentary rhythmic couplets, often grouped into bundles, may represent the effect of some of these; often the precessional couplets are grouped into bundles of five or so within the lower eccentricity period. The disentangling of the interpenetrating cycles to produce an OFT is an exciting problem and challenge for palaeobiology and sedimentology. These should enable numerical dates to be given to biostratigraphic and chronostratigraphic timescales and eventually enable many earth processes to be analysed in real time.
26 Ma oscillations related to the Cosmic Year (c. 260 Ma) have been invoked to explain periodic mass extinctions in the fossil record. But evidence is presented to suggest such extinctions are not, in fact, periodic.
