EVOLUTION OF TORQUE IN THE EARTH-MOON SYSTEM AS INDICATED BY INVERTEBRATE GROWTH BANDS

Daily and annual growth bands in marine invertebrates 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 causes the transfer of angular momentum from the Earth to the Moon thus slowing Earth’s rotation as well as increasing their separation. The variables that most affect angular momentum transfer are the Earth-Moon separation and the proportion and distribution of shallow water.

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. The estimated torque averages 4.1 x 1016 Nm during the Phanerozoic and decreases exponentially over time. Deviations from the trend line are within the error bars of the age and days/year estimates but may also reflect the Phanerozoic marine transgression record. However, the required torque for the Neoproterozoic is significantly less (2.3 x 1016 Nm). Because tidal forces decrease according to an inverse cube relationship with an increasing Earth-Moon separation, the Neoproterozoic results are incompatible with the expected evolution of torque in the Earth-Moon system.

Earth’s torque history is consistent with the following scenario: High continental freeboard predominates during most of the Proterozoic due to a deficit of continental crust relative to the present. An absence of epeiric seas results in minimal tidal friction thereby reducing torque and the rate of Earth’s rotational deceleration. Progressive continental growth during the Proterozoic restricts the volume of the ocean basins thus decreasing continental freeboard. Flooding of the continental interior becomes possible when the proportion of the Earth’s surface covered by continental crust exceeds ~25%. This first occurs in the Neoproterozoic making possible the Sauk and later marine transgressions. Consequently, the present rate of rapid angular momentum transfer in the Earth-Moon system is a relatively recent development. This scenario has implications regarding the growth of continental crust and the history of epeiric seas.