Alessandro M. Forte

A resonance in the Earth's 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 precise positions of the planets are predictable for a period of only about 20 Myr (ref. 1). The Earths precession, obliquity and insolation parameters over this time period can be influenced by secular variations in the dynamic ellipticity of the planet which are driven by long-term geophysical processes, such as post-glacial rebound,. Here we investigate the influence of mantle convection on these parameters. We use viscous flow theory to compute time series of the Earths dynamic ellipticity for the past 20 Myr and then apply these perturbations to the nominal many-body orbital solution of Laskar et al.. We find that the convection-induced change in the Earths flattening perturbs the main frequency of the Earths precession into the resonance associated with a secular term in the orbits of Jupiter and Saturn, and thus significantly influences the Earths obliquity. We also conclude that updated time series of high-latitude summer solar insolation diverge from the nominal solution for periods greater than the past ∼5 Myr. Our results have implications both for obtaining precise solutions for precession and obliquity and for procedures that adopt astronomical calibrations to date sedimentary cycles and climatic proxy records.

Mantle convection and core-mantle boundary topography: explanations and implications

Abstract Recent seismic inferences of the topography on the core-mantle boundary (CMB) constitute a new and potentially important constraint on the dynamics of the thermal convective circulation in the Earths mantle. We employ viscous flow models for the mantle to predict the flow-induced deflections of the CMB that are expected on the basis of seismic tomographic inferences of internal mantle density heterogeneity. The good agreement between our theoretical predictions and the seismically inferred CMB deflections of Morelli and Dziewonski (1987) suggests that the latter may indeed be good approximations to the dynamically induced CMB topography. The amplitude of the seismically inferred CMB topography may be construed to provide a direct constraint on the temperature derivative of seismic P-wave velocity in the lower mantle and we determine a value for this parameter, in the course of fitting the data, that differs significantly from the value determined by laboratory measurements as appropriate for upper-mantle phases. The seismically observed topography on the CMB is also shown to provide evidence favouring the whole-mantle convection hypothesis and in this connection we argue that deep penetration of subducted lithospheric slabs into the lower mantle is likely to be responsible for controlling many of the features of this topography.