Astrochronology and Heterogeneous Oceanic Oxygenation in the Ediacaran Period

The Ediacaran period (635–538 Ma) is a pivotal interval in Earth's history, marked by the largest negative carbon isotope excursion (EN3/DOUNCE/Shuram excursion) and the emergence of complex macroscopic life. However, the current temporal framework of the Ediacaran Period remains fragmentary, thereby limiting advances in understanding the co-evolution processes between ocean oxygenation and multicellular eukaryotes. This study proposes a new analytical strategy for cyclostratigraphy, utilizing multiproxy analyses and a sedimentation-rate-dependent meshing scheme. It provides a scalable/propagable approach to address the challenge that the resolution and continuity of proxies cannot be achieved simultaneously, thereby establishing continuous astrochronology across long temporal spans. Based on this method, a continuous ~66.8-Myr astrochronology (from 635.11 ± 0.57 Ma to 568.34 ± 6.83 Ma) is established with quantified uncertainty evaluation by decoding geological imprints of Milanković cycles in the Wuhe drill core of South China, and using the Venus–Jupiter-dominated secular frequency term as metronome for time-depth calibration. These approaches ensure our results to be coordinated effectively with existing chronological constraints. Based on newly established astronomical time scale and carbon isotopic profile throughout the Doushantuo Formation of South China, the EN3/DOUNCE of South China exhibits a pronounced temporal heterogeneity contrasting with the Shuram excursion recorded elsewhere, marked by its earlier onset (no later than 584.21 ± 0.57 Ma), prolonged duration (exceeding 12.2 Myr), and a more gradual triggering process (reaching the nadir form 0‰ around 4.23 Myr), suggesting a temporally heterogeneous patterns of oceanic oxygenation during the Ediacaran. Moreover, this study reconstructed the relative sea level oscillations in South China, based on the lag-1 autocorrelation coefficient model, astronomically calibrated sedimentation rate, and sedimentological description. The prominent carbon cycle perturbations and coeval variations in the oceanic phosphorus concentration during the Ediacaran Period in South China were paced by "M-shaped" second-order sea-level oscillations, providing a plausible dynamic mechanism for the fundamental carbon-isotope framework of the Ediacaran Period. In conclusion, the newly established ~66.8-Myr high-resolution astrochronology significantly refines the current understanding of the evolutionary tempo and dynamics of Ediacaran macroscopic organisms and oceanic environment. The global oceanic environment throughout the Ediacaran Period is probably influenced by global/local sea-level oscillations and by differential diffusion of oxidants in water bodies, and appears more intricate than previously recognized. This study further suggests that comparing and improving the high-resolution time framework of the Ediacaran period, fully considering the possibility of globally spatiotemporal heterogeneity in Ediacaran δ13C variations, and multi-basin reconstructions of Ediacaran sea-level changes constitute key steps towards the subdivision of the Ediacaran System into meaningful stages and series.