ABSOLUTE TIME-RECKONING USING TIDAL RHYTHMITES: AN IMPROVED METHOD FOR STUDYING THE HISTORY OF THE EARTH-MOON SYSTEM

In the past, researchers have interpreted tidal rhythmite data based on a measure of the sidereal month in terms of days. Because day length changes through geological time as the Earth's rotation slows, these studies yielded results which are not reliable for calculating Earth-Moon distances. The exception to this scenario was the study of the Elatina Formation (~620 Ma) tidal rhythmites in South Australia (Williams, 2000), which contain an extraordinary 60+ year continuous record. The vast majority of tidal rhythmites, however, record intervals well under a year and are more difficult to interpret. Determining the absolute time represented by these sequences would allow for a more accurate determination of historical Earth-Moon distances.

We propose a new method for interpreting tidal rhythmite data which yields more accurate computations of ancient Earth-Moon distance and tidal cycles in absolute time. This method is based on the assumptions of fixed angular momentum for the Earth-Moon system, a fixed moment of inertia for Earth, and a fixed number of seconds per solar year. With the number of solar days per sidereal month, obtained through Fourier analysis of tidal rhythmite records, Earth-Moon distance can be calculated using Kepler's 3rd Law and the sidereal period in seconds (i.e. absolute time) can be determined.

Because the results of this technique are independent of Earth's changing rate of rotation, they provide a more accurate method for determining Earth-Moon distance. By standardizing time for tidal rhythmite data sets of disparate ages, the resolution of the record of lunar recession from Earth can be improved. Continued study may further lead to insights into lunar formation and the processes associated with tidal friction.

Williams, G. E., 2000, Geological constraints on the Precambrian history of Earth's rotation and the Moon's orbit, Reviews of Geophysics, v. 38, p. 37-60.