| The Cambrian is the first system of the Palaeozoic including four series（Terreneuvian,Series 2,Miaolingian and Furongian）corresponding to significant geological and biological events on Earth.The lack of a high–precision and high–resolution time scale in Cambrian hinders our understanding of these major events.The identified Milankovitch cycles in sedimentary strata could provide a continuous and high–resolution time scale for stratigraphic successions by Cyclostratigraphy.Additionally,cyclostratigraphy is also recently used to reconstruct the deep–time evolution of the Earth–Moon system during geological history through geological records.In this study,cyclostratigraphic analyses were conducted on the global stratotype section and point（GSSP）for the Jiangshangian Stage–Duibian B section,and the GSSP for the Guzhangian Stage–Luoyixi section.The main purposes of the research were to:1)reveal the depositional cycles in the Cambrian sedimentary records and their possible astronomical origins;2)improve Cambrian chronostratigraphy with astrochronology;and 3)reconstruct the deep–time evolution of the Earth–Moon system.Here,a total of 1425 magnetic susceptibility（MS）data measured by SM–30 MS meter were used to conduct cyclostratigraphic analyses on the Duibian B section.The results of the The power spectral analyses and average spectral misfit analyses（ASM）indicate that the 2.66 m,0.84–0.73 m,0.2 m,0.12–0.088 m sedimentary cycles may represent long–eccentricity（405 k.y.）,short–eccentricity（129.1 k.y.,96.2 k.y.）,obliquity（31.9 k.y.）and precession（20.5 k.y.,19.8 k.y.,17.7 k.y.,16.7 k.y.）cycles,respectively.A～5.59 m.y.floating astronomical time scale was constructed using the stable 405 k.y.eccentricity cycles to calibrate the agnostoid trilobite zones.According to the results we supposed that the duration of the Paibian stage should be more than 3m.y..The obtained fundamental obliquity period（30.7±0.7 k.y.）implies 370,180±1220 km（vs.384,000 km for present day）for the Earth–Moon distance and 21.58±0.18 h（vs.23.93 h for present day）for the length of day during the middle Cambrian（500 Ma）.4340 MS data measured by SM–30 MS meter and 170δ13C data from previous research were used to conduct cyclostratigraphic analyses on the Luoyixi section.The results of the The power spectral analyses and ASM indicate that the 25–25.53 m,6.25–6.91 m,1.90–2.03 m,and 1.08–1.19 m sedimentary cycles may represent long–eccentricity（405 k.y.）,short–eccentricity（100.9 k.y.）,obliquity（30.7 k.y.）and precession（19.2 k.y.,18.2 k.y.）cycles,respectively.A～1400 k.y.floating astronomical time scale was constructed using the stable 405 k.y.eccentricity cycles to calibrate the agnostoid trilobite zones.The obtained fundamental obliquity period（30.7±0.7 k.y.）implies 370,180±1220 km for the Earth–Moon distance and 21.58±0.18 h for the length of day during the middle Cambrian（500 Ma）. |
