Geological constraints on tidal dissipation and dynamical ellipticity of the Earth over the past three million years

The evolution of the Solar System has been shown to be chaotic(1), which li mits our ability to retrace the orbital and precessional motion of the Eart h over more than 35-50 Myr (ref. 2). Moreover, the precession, obliquity an d insolation parameters(3,4) can also be influenced by secular variations i n the tidal dissipation and dynamical ellipticity of the Earth induced by g lacial cyclicity(3,5-10) and mantle convection(11). Here we determine the a verage values of these dissipative effects over the past three million year s. We have computed the optimal fit between an exceptional palaeoclimate re cord from the eastern Mediterranean Sea and a model of the astronomical and insolation history(3) by testing a number of values for the tidal dissipat ion and dynamical ellipticity parameters. We find that the combined effects of dynamical ellipticity and tidal dissipation were, on average, significa ntly lower over the past three million years, compared to their present-day values (determined from artificial satellite data and lunar ranging(3,4,12 )). This secular variation associated with the Plio-Pleistocene ice load hi story has caused an average acceleration in the Earth's rotation over the p ast 3 Myr, which needs to be considered in the construction of astronomical timescales and in research into the stationarity of phase relations in the ocean-climate system through time.