A VENUS ORBIT -- LUNAR ORBIT RESONANCE MODEL AS A POSSIBLE EXPLANATION OF SOME FEATURES OF THE EARLY PALEOZOIC ROCK RECORD

Decades ago both Stewart (1970, USGS-PP 620, p. 68) and Lochman-Balk (1970, GSAB, v. 81, p. 3208) concluded that there were many signatures of tidal zone deposition in rocks of Late Precambrian-Early Paleozoic age. Orbital traceback calculations suggested that the Moon was in a near circular orbit at about 52-55 earth radii (ER) during that era.

Chapman (1986, J. Roy. Astron. Soc. Can., 80, p,.336) was the first to point out that the very regular, recurrent phases of Venus were an indication of a heliocentric orbital resonance between Venus and Earth. Venus orbits the Sun ~8 times as the Earth orbit ~5 times. The perigean cycle (the prograde progression of the perigee point of the lunar orbit) at the present time is 8.85 years (60.3 ER). At 55 ER the perigean cycle is 10 earth years (16 venus years). During this 10 earth-year cycle (and for many thousands of succeeding cycles), the lunar apogee is pointing directly toward Venus during three of the six inferior conjunctions.

The main effect when moving forward in time from 52 ER to 55 ER is a slow, but unidirectional, forced eccentricity (ECC) of the lunar orbit from a nominal value of 0.055 to perhaps 0.25 or higher. In this scenario the semi-major axis (SMA) of the lunar orbit is gradually increased to the resonant value of 55 ER but the angular momentum (AM) content is that of a ~52 ER orbit. The AM is gradually transferred to the lunar orbit by the rotating Earth via the tidal friction mechanism. The maximum tidal amplitude is about two times that of a 55 ER circular orbit (i. e., the effective tidal regime is that of a 40 ER circular orbit). Tidal amplitudes then decrease over a period of several 100 Ma to those characteristic of a circular orbit at 55 ER.

The lower Paleozoic, in addition to being a time of extensive tidal sedimentation, may also be a time of vigorous plate tectonic activity (Stern, 2005, Geology, 33, p. 556). The rock tides, in this model, should aid in the plate tectonic activity (e. g., Scoppola et al., 2006, GSAB, 118, p. 199) and tidal rhythmites should be the recorders of the ocean tidal activity. The most significant predictions for tidal rhythmite sequences in this era are that from about 600 Ma until about 300 Ma the major axis of the lunar orbit, after the forced ECC, should be constant at 55 ER (~15 sidereal months per year) and the number of days per year should decrease from ~487 to ~438.