Rock Magnetic Cyclostratigraphy of the Doushantuo Formation, South China and Its Implications for the Duration of the Shuram-Wonoka Excursion.

摘要

The Shuram-Wonoka excursion, the largest documented negative carbon isotope anomaly, occurs globally in the Ediacaran. Precise determination of the duration of the Shuram-Wonoka excursion is pivotal in understanding its controversial origin. Here, we present a detailed paleo- and rock magnetic, cyclostratigraphic and carbon isotope study of a complete record of the Shuram-Wonoka excursion in the Doushantuo Formation at the Dongdahe section in eastern Yunnan Province, South China. Although paleomagnetic results indicate a Cretaceous thermoviscous remanent magnetization related to the Yanshanian orogeny in South China, careful mineralogical examination shows that variations in the concentration of detrital magnetite can faithfully record astronomically-forced climate oscillations in the Ediacaran. Multi-taper method spectral analysis of magnetic susceptibility and anhysteretic remanent magnetization (ARM) cyclostratigraphies yields significant peaks at consistent frequencies. Based on the ratios of their wavelengths, these peaks are assigned to a suite of Milankovitch cycles (long eccentricity, short eccentricity, obliquity and precession), permitting the sediment accumulation rate of the Doushantuo Formation to be calculated as 1.03 cm/kyr. By extrapolation, a 9.4 +/- 1.0 Myr duration is suggested for the entire Shuram-Wonoka excursion in South China. This result is in good agreement with independent estimates from exposures in North America and South Australia, thus supporting a primary origin for the Shuram-Wonoka excursion. In combination with published geochronological data, we suggest that the age of the excursion's onset is possibly ca. 560 Ma, thus providing a robust chronostratigraphic framework for evaluating the relationship between the excursion and the life evolution in the Ediacaran.