FACTORS CONTROLLING THE INCREASE IN TORQUE IN THE EARTH-MOON SYSTEM AT THE NEOPROTEROZOIC – CAMBRIAN BOUNDARY

Daily and annual growth bands in marine organisms have been used to estimate the number of days/year. Days/year have progressively decreased from about 435 at 850Ma to the present. This decrease in Earth's period of rotation is attributed to tidal friction in the Earth-Moon system. Tidal friction transfers angular momentum from the Earth to the Moon thus slowing Earth's rotation as well as increasing the separation. The torque required to produce the observed variation in days/year was calculated using an Earth mass of 5.977 x 10²⁴ kg, a radius of 6378 km and a polar moment of inertia of 0.334 MR². Torque within the Phanerozoic varies from –1.03 to +85.8 Nm/s. The Earth-Moon separation was determined assuming conservation of angular momentum. Because tidal forces decrease according to an inverse cube relationship with increasing separation, torque is expected to increase going back in time. The torque residual is the difference between the torque predicted based on the separation and observed torque. Relative to the present, the residual torque ranges from –32.5 to +29.2 Nm/s. The residual torque is due to geologic factors including continental freeboard, continental fragmentation, and the proportion of continents in polar regions. The combined effect of these factors explains the range of residual torque. Whereas at different times during the Phanerozoic each of these factors is dominant, eustatic sea-level is consistently the most important factor. The average torque within the Proterozoic is two orders of magnitude smaller than what is determined for the Phanerozoic and is irrespective of the Earth-Moon separation. Consequently, the present rapid rate of angular momentum transfer in the Earth-Moon system is a relatively recent development. Factors responsible for the reduced Proterozoic torque include Neoproterozoic glaciations and the assembly of Rodinia. However, torque begins increasing only at the onset of the Sauk marine transgression. This suggests that continental freeboard was low throughout much of the Proterozoic and that epeiric seas are a relatively recent development. Consistently low freeboard throughout the Proterozoic can be explained through the progressive growth of continental crust throughout Earth history.