New cyclostatigraphy-based estimates of tidal dissipation and dynamic ellipticity over the Cenozoic: Implications for astrochronology and global scale geophysical processes

Astronomical tuning methods have provided a means of assigning high-resolution timescales to geologic records. However, the accuracy of this type of calibration is limited by the accuracy of calculated Milankovitch cycles. In particular, there is significant uncertainty in the time history of tidal dissipation and dynamic ellipticity, both of which influence the frequency of obliquity and precession cycles (Laskar 1999). Dynamic ellipticity refers to the gravitational shape of the Earth at spherical harmonic degree 2 and order 0, and it is influenced both by thermal convection in the Earth's mantle (Forte and Mitrovica, 1997) and isostatic adjustments in response to ice age cycles (Mitrovica and Forte, 1997). Previous analyses of cyclostratigraphy have estimated that the mean dynamic ellipticity has remained unchanged over both the last 25 million years (Palike & Shackleton 2000) and the last 3 million years (Lourens et al. 2001). However, this apparent consistency leads to contradictory inferences in regard to the structure of the Earth. In particular, constant dynamic ellipticity through the Pliocene implies a weak lower mantle such that the Earth remains close to isostatic equilibrium in response to ice age cycles, while constant dynamic ellipticity over 25 Myr implies a sufficiently stiff lower mantle to ensure sluggish mantle convective flow (Morrow et al. 2012). To resolve this enigma, we re-evaluate temporal changes in tidal dissipation and dynamic ellipticity by applying new depth and time series analysis to longer geologic records. Moreover, we use updated geodynamic modeling of both mantle convection and glacial isostatic adjustment to explore the implications of the results for long term Earth system evolution.