Length of day (LOD) varies throughout Earth history, dictated on the long term by the retreat of the Moon and growth of Earth's core. Reconstruction of LOD in deep time has been largely benefitted from the orbital cycle analyses from geological records. The Precambrian database, however, is very limited. Here we present a cyclostratigraphic study on ca. 1.1 Ga laminated argillaceous limestones of the Nanfen Formation from the North China craton. Milankovitch cycles are successfully recognized from the consistent cyclical variations of the high-resolution magnetic susceptibility data measured on the drillcores from the Nanfen Formation. Sedimentary cycles with wavelengths of 12.45 m, 10.72 m and 9.96 m in three drillcores are interpreted as 405 kyr eccentricity cycles. An obliquity period of 20.9–21.4 kyr and a precession period of 12.7–16.0 kyr are also identified. The Earth–Moon distance and LOD are thus estimated at 3.43±0.04×108 m and 18.94±0.39 h, respectively. Our new results together with the data previously published reveal an important discontinuity between 1.9 and 1.4 Ga in the evolutionary trend of the LOD. We suggest that Earth's core grew rapidly during the interval, which significantly changed the moment of inertia of the Earth–Moon system, and tidal dissipation again ruled the LOD variations after that stage.
Scientific editing by Wenjiao Xiao