Astronomical time scale for the middle-upper Doushantuo Formation of Ediacaran in South China: Implications for the duration of the Shuram/Wonoka negative δ13C excursion

Abstract

The Ediacaran Period (635–541 Ma) of the Neoproterozoic records the advent of multi-cellular life on Earth. The Ediacaran Doushantuo Formation (DST Fm) contains records of some phases of this bio-evolution and profound perturbations to the global carbon cycle, but the timing and the duration of these events are still unclear. The Jiulongwan (JLW) roadcut in South China is a well-studied reference section for these Ediacaran events, including the major EN3 (Shuram/Wonoka) negative carbon-isotope excursion. Spectral analysis of geochemical data through the middle-upper DST Fm indicates that the sedimentary cycles correspond to 405-kyr long-eccentricity and ~100 kyr short-eccentricity periods. Tuning the DST Fm at JLW section to the 405-kyr long-eccentricity cycles yielded a ~30 Myr-long astronomical time scale. Member DST-4 (black shale) of the uppermost DST Fm spans 7.5 Myr and the negative δ13C excursion (EN3/Shuram/Wonoka) has a duration of~20 Myr. Relative to a UPb date of 551.1 ± 0.7 Ma near the top of the DST Fm, the onset of Member DST-4 is at 557.8 ± 0.8 Ma, and the projected basal age of Member DST-3 is 579.3 ± 0.8 Ma. The onsets of positive EP2 and negative EN3 δ13C excursions were ca. 578.1 Ma and 571.1 Ma, respectively; therefore, both excursions occurred after the termination of the Gaskiers glaciation (~581 Ma). Thereby, these ages and astronomical time scale provide important new constraints on the subdivision of Ediacaran strata, and have implications for our understanding the character of the δ13C excursions. Orbital forcing may be important to the co-evolution of life, the global carbon cycle and sedimentary environments.