A RESONANCE IN THE EARTH OBLIQUITY AND PRECESSION OVER THE PAST 20 MYR DRIVEN BY MANTLE CONVECTION

The motion of the Solar System is chaotic to the extent that the preci se positions of the planets are predictable for a period of only about 20 Myr (ref. 1). The Earth's precession, obliquity and insolation par ameters over this time period(1-6) can be influenced by secular variat ions in the dynamic ellipticity of the planet which are driven by long -term geophysical processes, such as post-glacial rebound(5,7-10). Her e we investigate the influence of mantle convection on these parameter s, We use viscous flow theory to compute time series of the Earth's dy namic ellipticity for the past 20 Myr and then apply these perturbatio ns to the nominal many-body orbital solution of Laskar et al.(5). We f ind that the convection-induced change in the Earth's flattening pertu rbs the main frequency of the Earth's precession into the resonance as sociated with a secular term in the orbits of Jupiter and Saturn(5), a nd thus significantly influences the Earth's obliquity. We also conclu de that updated time series of high-latitude summer solar insolation d iverge from the nominal solution for periods greater than the past sim ilar to 5 Myr. Our results have implications both for obtaining precis e solutions for precession and obliquity and for procedures that adopt astronomical calibrations to date sedimentary cycles and climatic pro xy records.