The Ediacaran period, which occurred between 635 and 538 million years ago, was a critical time in the evolution of habitability on Earth. However, the Ediacaran timeframe is not as precise as the well-established astronomical time scales for more recent periods. This hinders global stratigraphic correlations and the subdivision of the Ediacaran system, generating controversies about the tempo of deep-ocean oxygenation and eco-evolutionary dynamics. Cyclostratigraphy enables the construction of high-resolution astrochronology for deep-time sediments through a workflow that identifies geological imprints of Milanković cycles and uses specific orbital forcing cycles as a metronome for time-depth calibration.

Now, Prof. WANG Wei from the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences (NIGPAS), together with XUE Naihua, a double Ph.D. student from the Vrije Universiteit Brussel and the University of Münster, collaborated with an international team to establish the longest continuous astronomical time scale (ATS) spanning from 635.1 ± 0.6 million years ago to 568.3 ± 6.8 million years ago.

This scale is based on a drilled core near the Jiulongwan section in Yichang, South China. The provides a series of new age constraints for Ediacaran geochemical perturbations and fossil assemblages, revealing significant temporal heterogeneity of the Shuram carbon isotope excursion, as well as a coupling relationship between carbon isotope fluctuations and the second-order sea-level oscillations in the inner-shelf basin of South China. The study was published in Precambrian Research on July 15.

To construct the chronology, the researchers combined multiple geological datasets, including meter-scale δ¹³C_carb analysis, gamma ray (GR) logging with 5-cm measuring interval, and millimeter-scale X-ray fluorescence (XRF) core scanning and a sedimentation-rate-dependent meshing scheme. They also thoroughly considered the uncertainties of the absolute age anchor, the long eccentricity term, and the astronomical tuning within each segment. They considered error accumulation and propagation as well, ultimately providing an age model with a quantified uncertainty assessment.

Using the new astrochronology, the researchers established updated age constraints for major carbon isotope perturbations and fossil assemblages preserved in South China's inner-shelf basin. These include the EN3/DOUNCE event, which occurred between 584.2 ± 5.2 and 593.2 ± 4.0 million years ago and lasted between 12.2 ± 1.4 and 15.9 ± 1.7 million years; the EN2/BAINCE event, which occurred between 599.5 ± 3.2 and 599.5 ± 3.2 million years ago; and the WANCE nadir, which occurred 613.9 ± 2.0 million years ago. The chronology also constrains the temporal ranges of several important fossil assemblages, including large acanthomorphic acritarchs, the Lantian biota, the Weng'an biota, and the Miaohe biota.

Previous studies have assumed and/or emphasized the isochronism of the Shuram excursion in South China, Oman, northwestern Canada, and other locations. The newly established astrochronology highlights a novel perspective on the temporal heterogeneity of the Shuram excursion. Compared with records from open-ocean settings, the Shuram excursion in South China exhibits temporal heterogeneity in terms of a potentially earlier onset and prolonged duration. Notably, the decline from approximately 0 ‰ to the isotopic minimum occurred over 4.2 ± 0.4 million years, which is roughly 3.5 times longer than the onset reported from Oman.

The researchers suggest that future investigations into the mechanisms driving the Shuram excursion and associated biogeochemical changes should account for the spatiotemporal heterogeneity in δ¹³C variations.

To explore the potential drivers of temporal heterogeneity in Ediacaran shallow-marine carbon cycle perturbations, the researchers used lithological encoding, astronomically calibrated sedimentation rates, and time-domain ρ1 (lag-1 autocorrelation coefficient) modeling to reconstruct approximately 67-Myr relative sea-level changes in the inner-shelf basin of South China.

Their analysis revealed that carbon isotope negative excursions in the South China Sea are highly coupled with "M"-shaped secondary sea-level changes. Together, the ρ1 modeling and sedimentological and lithological evidence effectively indicate large-scale sea-level oscillations. This offers a possible dynamic mechanism for the fundamental carbon-isotope framework of the Ediacaran period. 

The sea-level dynamics of the Shuram Excursion in South China are consistent with previous sedimentological and paleogeographical hypotheses, as well as those concerning carbon isotopes, atmospheric and oceanic redox conditions, phosphorus cycling, and continental weathering.

This study was supported by the Strategic Priority Research Program of CAS, the National Key Research and Development Program of China, the National Natural Science Foundation of China, and others.

A 3D conceptual model for depicting the dynamics of the EN3/DOUNCE in South China, paced by the second-order sea-level oscillations. (Image by NIGPAS)