Astrochronologic calibration of the Shuram carbon isotope excursion with new data from South China.

The Shuram carbon isotope excursion (CIE) is the largest negative CIE in geological history that may have recorded dramatic changes in Earth's surface environments, yet its time, duration, and global synchroneity have been debated. In South China, the Shuram CIE has been correlated with the negative CIE of the upper Doushantuo Formation (named as EN3 or DOUNCE). It is previously suggested that the Shuram CIE in South China stratigraphically consists of the upper Member 3 and Member 4, with an ending age of ca. 551 Ma. Recent studies, however, demonstrated that the EN3 or DOUNCE shifts to near zero or positive carbon isotope (δ13C) values within carbonate rocks of the Doushantuo Member 3. To better understand the duration of this CIE and its spatial variability, we have conducted a high-resolution cyclostratigraphic study using magnetic susceptibility (MS) in two sections: the Zhengjiatang section that preserves a complete EN3 in carbonate rocks of the Doushantuo Member 3 and the Wuhe-Gaojiaxi (WG) drillcore in which the EN3 does not shift to positive δ13C values until the end of the Member 4 black shales. In both sections, power spectral analyses of the MS series of the carbonate rocks reveal periodicities that match well with those of the Milankovitch cycles at 570 Ma. The duration of the Shuram CIE is estimated to be 6.1 ± 0.2 Myr by counting the interpreted 405-kyr sedimentary cycles observed in the Zhengjiatang section. Taking the newly reported Re Os age of 567.3 ± 3.0 Ma as the approximate termination age, the Shuram CIE in the best-preserved section in South China is constrained between ca. 573.4 and 567.3 Ma. Regional correlation, however, shows a ≥ 1 Myr stratigraphic hiatus or unconformity at the top of the Doushantuo Member 3 that separates the Shuram CIE from overlying post-Shuram ẟ13C "excursions". Similar phenomena may exist in the Shuram CIE documented from stratigraphic successions in Oman, Australia, and the Death Valley region of western United States. The results invoke a reevaluation of the stratigraphic range of the Shuram CIE and suggest that the geochemical records documented from the black shales of the Doushantuo Member 4 and its globally correlative intervals may record paleoceanographic conditions millions of years after the Shuram CIE. Comparison of astrochronologically calibrated δ13C profiles reveals significant difference in the shape and amplitude of the Shuram CIE among the sections in South China and globally, which may have been caused by isotopic departure in local depositional environments and/or diagenetic overprints. • The Zhengjiatang section in South China preserves a complete Shuram excursion. • Astronomically estimated duration of 6.1 ± 0.2 Myr for the Shuram excursion. • Portion of the previously defined Shuram excursion may be post-Shuram. • The shape and amplitude of the Shuram excursion differ regionally and globally. [ABSTRACT FROM AUTHOR]