Effects of the astronomical orbital cycle on organic matter accumulation during the Late Ordovician-early Silurian in the Upper Yangtze area, South China

The Upper Ordovician - lower Silurian marine shales in the Upper Yangtze area have a continuous sedimentary record of high-frequency sea level fluctuations associated with ice sheet expansion and shrinkage. To decipher the effects of the astronomical orbit cycles on organic matter accumulation during the Hirnantian-Rhuddanian in the Upper Yangtze area, we carried out a high-resolution cyclostratigraphic study of the gamma-ray (GR) series and associated geochemical analysis of the Upper Ordovician Wufeng and lower Silurian Longmaxi organic-rich shales. Both units have recorded Milankovitch cycles of -405 kyr long eccentricity and -95 kyr short ec-centricity. We also identified the long-period modulation cycles of -1.2 Myr obliquity, which drive the eustatic changes in the Upper Yangtze area by controlling the climate conversion between greenhouse and icehouse. The floating astronomical time scale (ATS) was constructed on the X1 Well in the Hirnantian-early Aeronian, which was calibrated using the stable deposition of 405 kyr long eccentricity cycle. The ATS was used for estimating the durations of the graptolite zones, and gave durations of 1.49 +/- 0.40 Ma for the Hirnantian Stage and 1.73 +/- 0.40 Ma for the Rhuddanian Stage. Milankovich cycles drive organic matter accumulation by controlling pale-oclimate and eustatic changes. Finally, the results show that the organic matter accumulation of marine shales during the Hirnantian Stage was comprehensively influenced by terrigenous influx, paleoredox conditions and paleoproductivity changes driven by 405 kyr long eccentricity. However, in the early Rhuddanian Stage, the change of long-period modulation cycles of-1.2 Myr obliquity resulted in the intensified thermohaline current and the increasing paleoproductivity, while leads to a preservation-controlled organic matter accumulation model.