Dynamic effects from mantle phase transitions on true polar wander during ice ages

The relaxation spectrum of a stratified viscoelastic mantle with phase transitions contains a class of extremely long decaying modes with relaxation times 0 (10⁶ yr), exceeding the characteristic time scale of an individual glacial cycle. Consequently, the net motion of the rotation pole produced by the cyclic loading and unloading of ice sheets would display a nonequilibrium behaviour in the initial stages of an ice age epoch. We present here calculations of the net polar speed as a function of time for the late Cenozoic ice ages. These results show an initial transient net velocity of 0 (1° Myr^-1) which, after a few million years, decays to a steady value of 0(0.1° Myr^-1). We propose that the length of a typical ice age period around 0(10⁷ yr), is controlled by the time scale required for the steady-state polar wander to drift sufficiently far such that the necessary conditions for the Milankovitch mechanism to operate can no longer be maintained.