Tidal History of the Earth-Moon Dynamical System before Cambrian Age

Abstract

Tidal evolution of the earth-moon system was evaluated by estimating the change of dynamic responses of ocean and solid earth to the lunar and solar tidal force during the geological time. The effect of tide on the moon due to the earth was included in the evaluation. We obtained time variations of the orbital elements of the moon, the LOD (length of day) and obliquity of the spin axis of the earth. Our previous study suggested that the variation of LOD up to 600 million years before present estimated by paleontological studies is explained by temporal variation of the ocean continent distribution (Abe et al.,1997). In this study, we paid attention to discriminate how the continental distribution before 500 million years ago affects evaluation of the evolution process, using the continental distribution at 540 million years ago and 700-750 million years ago referred from Maruyama et al. (1997). After making investigations replacing the continental models used in the previous paper, it was suggested the results did not suffer so serious from ambiguity of continent models before about 1.5 billion yeas ago. This point leads us to understand that the small effect of the variation of continent models around these eras comes from the fact that the tidal frequency was remarkably far from the eigen-frequency of tidal response of the oceans. This also suggests that the earlier stage of the earth-moon system, than about 1.5 billion years ago, was possibly governed by mostly small and constant tidal response of the oceans and the comparable or larger effect of tides of the solid earth. Assuming the surface of the earth covered by whole ocean before 2.5 billion years ago, the time when earth-moon distance was very short is around 4.5 billion years ago and we can make almost clear the whole history of the earth-moon dynamical system due to tide after the moon was generated.