Late Precambrian tidal rhythmites in South Australia and the history of the Earth's rotation

Sedimentary rhythmites of siltstone and fine sandstone from late Precambrian (c. 650–800 Ma) glaciogenic formations in South Australia are interpreted as distal ebb-tidal deposits that record variability in the velocity and range of palaeo-ebb tides. Variations in lamina thickness encode a full spectrum of palaeotidal cycles, including semidiurnal, diurnal, fortnightly and monthly tidal cycles as well as the lunar apsides (perigee) and nodal cycles. A half-yearly oscillation is attributable largely to a beat between the fortnightly tidal cycles of luni-solar conjunction and lunar declination; the lunar nodal cycle is discernible as an amplitude modulation of this beat oscillation. The data allow determination of the Earth's palaeorotation and the past dynamics of the Earth-Moon system with an accuracy previously unattainable for the Precambrian. The late Precambrian (c. 650 Ma) year contained 13.1 (±0.5) lunar months and c. 400 (±20) days, and the late Precambrian lunar month c. 30.5 (±1.5) days. These values suggest an average equivalent phase lag near 3^° since late Precambrian time rather than the present value of 6^°. The period of 19.5 (±0.5) years determined for the lunar nodal cycle c. 650 Ma ago indicates a lunar distance 96.9 (±1.7)% of the present distance. The low rate of lunar recession since late Precambrian time revealed by the rhythmite data militates against a close approach of the Moon during the Proterozoic. Precambrian sedimentary rhythmites may hold a key to the early history of the Earth's rotation.