Perturbations in the Earth’s Rotation Induced by Internal Density Anomalies: Implications for Sea-Level Fluctuations

Abstract

The effects of internal mass anomalies on the Earth’s rotation are analyzed within the framework of linearized Liouville equations and Maxwell rheology for the mantle. Our approach is appropriate for a simplified modeling of subduction. Sea-level fluctuations induced by long-term rotational instabilities are also considered. The displacement of the Earth’s axis of rotation, called true polar wander, and associated eustatic sea-level fluctuations are very sensitive to the viscosity profile of the mantle and to the amount of chemical stratification at the 670 km seismic discontinuity. Phase-change models for the transition zone generally allow for huge amount of polar wander, except for large viscosity increase; the dominant contribution in Liouville equations comes from a secular term that reflects the viscous behaviour of the mantle. For chemically stratified models, true polar wander is drastically reduced as a consequence of complete dynamic compensation of the mass anomalies at the upper-lower mantle interface; when the source is embedded in the upper mantle in the proximity of the chemical density jump, transient rotational modes are the leading terms in Liouville equations. For all the models, internal density anomalies are important sources of true polar wander. We suggest that this long-term rotation instability must be considered as a valuable contributor to the third order cycles in the eustatic sea-level curves. Rates of sea- level fluctuations of the order of 0.05–0.1 mm/yr are obtained for displacements of the Earth’s axis of rotation compatible with paleomagnetic data