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 (~650 Ma) clastic rhythmites in South Australia represent an unsurpassed palaeotidal record of ~ 60 years' duration that provides numerous palaeorotational parameters. At ~650 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 ~650 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 ~650 Ma, and indicate virtually no overall change in the Earth's moment of inertia, which militates against significant Earth expansion since ~650Ma. Early Proterozoic (~2, 500 Ma) cyclic banded iron-formation in Western Australia, that may record submarine fumarolic activity triggered by earth tides, suggests ~14.5 ±0.5 lunar months/year and a mean Earth-Moon distance of ~54.6 Earth radii at ~2, 500Ma. The combined rhythmite data suggest a mean rate of lunar retreat of ~1.27cm/year during the Proterozoic (~2, 500-650 Ma); the indicated increasing mean rate of lunar retreat since ~2, 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. © 1990, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.