Stratigraphic gaps and durations of the carbon isotope events during the Middle-Late Ordovician in Yichang, South China

The Middle-Late Ordovician transition is one of the most important periods of the environmental changes and biotic turnovers throughout the earth's history. Here, we used magnetic susceptibility as the paleoclimate proxy to conduct cyclostratigraphic analysis on the Zhenjin and Puxihe sections in Yichang, South China, to understand the astronomical forcing in the paleoclimate changes against a refined astro-chronostratigraphy. Spectra analyses of the magnetic susceptibility sequence shows that the ratios of different sedimentary cycles in Kunuitan, Miaopo and Pagoda formations, are consistent with those of Milankovitch cycles during the Mid-Late Ordovician. A ~7.94 Myr floating astronomical time scale (ATS) is constructed based on the 405 kyr long eccentricity cycles calibration, which indicates the durations of partial middle Darriwilian isotope carbon excursion (MDICE) and Guttenberg isotope carbon excursion (GICE) are ~460 kyr and ~700 kyr, respectively. High-precision continental correlation between Yichang area in South China and the Dawangou section in Tarim reveals that the duration of hiatus on the top of the Kunuitan Formation is ~2.87 Myr. Alignment of minimum of amplitude of obliquity and 405 kyr eccentricity may trigger the MDICE.