TEN MILLION YEAR HISTORIES OF OBLIQUITY AND PRECESSION: THE INFLUENCE OF THE ICE-AGE CYCLE

Abstract

Redistribution of mass both on and within the Earth due to the direct and indirect effects of the late Pleistocene glaciation-deglaciation process changes the principal moments of inertia of the planet, and consequently causes variations in its dynamical ellipticity, H. As demonstrated in Laskar et al. (1993), the numerical calculation of obliquity and precessional parameters of the Earth over a long time scale is extremely sensitive to changes in H. It is, therefore, of considerable interest to investigate the influence of such changes when these are determined by a realistic model of the ice-age cycle of the late Pleistocene epoch of Earth history.Our analyses show that the effect due to physically realistic changes in H(t) upon the obliquity fluctuations is less than 0.1o over a time scale of 10 Myr assuming that the dynamical ellipticity H(t) were equal to the present day value prior to 780,000 yr ago. This impact upon the obliquity is only one third of that due to tidal dissipation. If we assume that H(t) prior to 780,000 yr ago were equal to the average of H(t) over the time interval from 780,000 yr ago to the present, the impact upon the obliquity fluctuations is about 0.2o during the most recent 2 Myr, which constitute the entire Pleistocene epoch. The impact upon the minimum, average and maximum values of the obliquity oscillation is less than 0.03o for all of the models of ΔH(t)H0 considered. The induced changes in the precession rate of the spin axis are found to be less than 0.1 s/yr. These results appear to settle what has become an extremely important issue in paleoclimatology, since it has been suggested that the influence of the glaciation cycle upon H could be such as to modify significantly the precession period from its present day value and, thus significantly compromise the integrity of the calculations of orbital variations in effective solar insolation that have previously been performed. Only if the glaciation-deglaciation history could be shown to differ significantly from the ICE-4G model of Peltier (1994) would it be possible to continue to seriously entertain this idea.