Cyclostratigraphic Study In Late Proterozoic Ice-free Climate

In order to study the climate characteristics in ice-free climate,we do cyclostrati graphic analyses on Neoproterozoic Cryogenian interglacial Datangpo Formation in South China and late Mesoproterozoic to early Neoproterozoic Nanfen Formation in North China,which were located in high latitude region and middle latitude region,respectively.The research results on Datangpo Formation show transition characteristics of climate-ocean system from ice-free greenhouse climate to snowball Earth(Marinoan).Research results on Nanfen Formation constrain Earth-Moon distance and Earth’s rotation rate 1.1 Ga years ago.We propose that Milankovitch cycles also deeply affected climate changes in ice-free climate,however,the short eccentricity signal is very week.Cyclostratigraphy on magnetic susceptibility series of Datangpo Formation suggests that the duration of the interglacial period is about 9.8 million years and the Marinoan glaciation in South China initiated at ca.649 Ma.Based on the result of smoothing curve、box plot and Mann_Kendall mutation test,we divide the magnetic susceptibility series of Datangpo Formation into 4 parts(Ua-d).We find that:(1)The magnetic susceptibility of Ua is increasing,showing the increase in seafloor oxygenation;(2)Magnetic susceptibility series of Ub and Ud indicate 2 stable climate-ocean system,corresponding to 2 stable station of carbon isotope series in low latitude region.The cyclical characteristics of Ub-d indicate a climate transition like‘Mid-Pleistocene climate transition;(3)The transition period(Uc)corresponds to Tayshir carbon isotope anomaly event in low latitude regions.Combining with the result of cyclostratigraphy,the age of the anomaly is constrained in 653.13-652.42 Ma;(4)Rock magnetism shows larger size of deposited particles in Ud than Ub,the most likely reason is that the sea level in Ub is higher than in Ud.We propose the polar glacier forms after the transition period(Uc),affecting sea level and weathering rate,so Ub should be the deposition in ice-free climate.Cooling time of non-glacial period at ~653 Ma,then snowball Earth was formed at ~649 Ma.Milankovitch cycles are found in magnetic susceptibility series of greenish-gray limestone in the lower part of Nanfen Formation(1.1 Ga).The obliquity and precession periods are calculated as 20.9-21.4 kyr and 12.7-16.0 kyr,respectively.The Earth-Moon distance and Earth’s rotation rate are estimated as 3.425-3.440×108 m and 18.88-19.03 h.Based on the cyclostratigraphic result,the duration that North China craton located in polar regions is longer than 8.116 myr.Magnetic susceptibility series from ice-free climate conditions of Nanfen Formation and Datangpo Formation show weak or missing of short eccentricity cycles,so we propose that this maybe climatic characteristic of ice-free climate.