Tidal Rhythmites and Their Implications in Surmising Earth-Moon Dynamics

Coughenour, Christopher Lynn¹ (1) Drexel University, Philadelphia, PA

Tidal rhythmites are deposits of alternating coarse and fine sediments with foreset or laminae thicknesses that vary rhythmically and preserve tidal periodicities. Quantitative analyses of tidal rhythmites have improved and resulted in more precise elucidation of tidal periodicities encoded in the rock record. The resulting reconstruction of the history of lunar retreat from these data indicates that the earth is currently experiencing a high rate of tidal dissipation. The paucity of long-term, high resolution deposits, however, has allowed only broad trends in the history of the Earth-Moon system to be resolved. The use of more common, time-abbreviated rhythmites representing only months of deposition is likely necessary for more precise interpretations.

There is evidence that suggests Earth's moment of inertia and the total angular momentum of the Earth-Moon system have changed little through time. By assuming a perfectly spherical Earth and a Moon in circular orbit, the equations for angular momentum are simplified. The errors introduced via these simplifications are relatively small. The resultant equations for angular momentum are combined and it is assumed that the total angular momentum of the Earth-Moon couplet is conserved. Preliminary comparison to a 60-year continuum of sediments yielding precise values for the sidereal month and Earth-Moon distance suggest that the assumptions made by this method are accurate to within 10%. The results seem to affirm similar assumptions made in numerical models of lunar retreat, perhaps, providing a means of validating such models.