REE PATTERNS IN SLOPE FACIES EDIACARAN CARBONATES FROM THE YANGTZE PLATFORM (SOUTH CHINA)

Marine carbonates incorporate the rare earth element (REE) signature of seawater, and may thus record temporal changes in the supply of these elements and redox evolution of basins. Analysis of REE in acetic acid digests of cap carbonates from the slope facies Panmen section near Songtao (Yangtze Platform, South China) show relatively high concentrations (0.1 to 0.5 x PAAS, a factor 10 higher than in typical Phanerozoic marine carbonate) and slight depletion of LREE and HREE relative to the MREE. All patterns display significant positive Ce anomalies (Ce/Ce* from 1.07 to 1.35, correlating inversely with Pr/Pr*). Positive Ce anomalies in seawater occur in anoxic environments due to reduction and dissolution of Fe-Mn hydroxides, which carry strongly positive Ce anomalies (e. g., De Baar et al. 1988). Since Ce/Ce* increases up section in the cap carbonates, this might be interpreted in terms of progressively reducing conditions with time during deposition of the cap carbonates. The high REE abundances are similar to levels found in cap carbonates from shallow-water sections on the Yangtze Platform (Huang et al., 2009), however, the positive Ce anomalies are not. Huang et al. (2009) report either negative or no Ce anomalies in cap carbonates from the shallow water Jiulongwan section. The difference in REE distribution between shallow platform and slope facies is most easily explained by the different extent of oxygenation of bottom waters in both settings at the time of deposition (about 635 Ma, Condon et al., 2005).

References:

De Baar, H. J.W., German, C. R., Elderfield, H., Van Gaans, P. (1988) Rare earth element distributions in anoxic waters of the Cariaco Trench. Geochimica et Cosmochimica Acta 52, 1203-1219.

Huang, J., Chu X.L., Chang, H.J., Feng, L.J. (2009) Trace element and rare earth element of cap carbonate in Ediacaran Doushantuo Formation in Yangtze Gorges

Chinese Science Bulletin 54, 3295-3302.

Condon, D., Zhu, M., Bowring S., Wang, W., Yang, A., Jin, Y. (2005) U-Pb Ages from the Neoproterozoic Doushantuo Formation, China. Science 308, 95-98.