Tidal Rhythmites: Key to the History of the Earth's Rotation and the Lunar Orbit

Abstract

The recent recognition of cyclically laminated tidal rhythmites provides a new approach to tracing the dynamic history of the Earth-Moon system. Late Proterozoic (-5650 Ma) elastic rhythmites in South Australia represent an unsurpassed palaeotidal record of -560 years' duration that provides numerous palaeorotational parameters. At -5650 Ma there were 13.1+-0.1 lunar months/year, 400+-7 solar days/year, and 30.5+-0.5 solar days/lunar month. The lunar apsides and lunar nodal cycles were then 9.7+-0.1 years and 19.5+-0.5 years, respectively. The indicated mean Earth-Moon distance of 58.28+-0.30 Earth radii at -5650 Ma gives a mean rate of lunar retreat of 1.95+-0.29 cm/year since that time, about half the present rate of lunar retreat of 3.7+-0.2 cm/year obtained by lunar laser ranging. The rhythmite data imply a substantial obliquity of the ecliptic at -5650 Ma, and indicate virtually no overall change in the Earth's moment of inertia, which militates against significant Earth expansion since -5650 Ma. Early Proterozoic (-52, 500 Ma) cyclic banded iron-formation in Western Australia, that may record submarine fumarolic activity triggered by earth tides, suggests -514.5+-0.5 lunar months/year and a mean Earth-Moon distance of -554.6 Earth radii at -52, 500 Ma. The combined rhythmite data suggest a mean rate of lunar retreat of -51.27 cm/year during the Proterozoic (-52, 500-650 Ma); the indicated increasing mean rate of lunar retreat since -52, 500 Ma is consistent with increasing oceanic tidal dissipation as the Earth's rotation slows. A close approach of the Moon during earlier time is uncertain. Continued study of tidal rhythmites promises to further illuminate the evolving dynamics of the Earth-Moon system.