Paper No. 9
Presentation Time: 3:55 PM
CARBON AND STRONTIUM ISOTOPE STRATIGRAPHY OF THE EARLIEST CAMBRIAN IN CHENGJIANG, SOUTH CHINA: A UNIQUE CONDITION FOR THE SMALL SHELLY ANIMAL DIVERSIFICATION
While the diversity of small shelly fossils (SSFs) increased dramatically in the earliest Cambrian (ca. 534-530 Ma), a large-scale phosphorite deposition and δ13C excursions occurred in shallow marine settings particularly in South China, suggesting a significant environmental change. In order to check possible causal relationships between these events, we examined chemo- (δ13C and 87Sr/86Sr) stratigraphy of the uppermost Ediacaran-lowermost Cambrian units in the Chengjiang area, eastern Yunnan. We analyzed a ~120 m-long drilled core that penetrated the Zhujiaqing and Shiyantou formations. The diversified SSF assemblage first appeared in the middle of the lowermost Cambrian Zhongyicun phosphorite (the middle member of the Zhujiaqing Fm). The sedimentology of the Zhongyicun phosphorites indicates that the SSF diversification occurred likely in extremely shallow marine embayments with uniquely phosphate-enriched seawater.
We identified two negative δ13C shifts (N1 and N2) and one positive shift (P1) in the Daibu and Zhongyicun members. These δ13C profiles confirmed that the major SSF diversification likely occurred not during the positive δ13C shift as previously regarded, but during the negative δ13C shift (N2) at least in eastern Yunnan. As this shift in carbon isotope ratio possibly suggest a link between the biological and environmental changes in the earliest Cambrian, this negative shift was named 'the Fuxian Excursion'.
The 87Sr/86Sr ratios showed values between 0.7085-0.7090 that are similar to those reported from the coeval units in South China, and agree also with those from other continents. The uniquely high value of 0.7107 detected at the horizon of the first appearance of diverse SSFs is noteworthy.