MARINE CHEMISTRY OF THE TERMINAL PROTEROZOIC DOUSHANTUO FORMATION: ENVIRONMENTAL SIGNIFICANCE FOR THE PHOSPHATIZED EMBRYOS

A geochemical investigation on the terminal Proterozoic Doushantuo Formation in Weng’an county, South China reveals a dramatic paleoceanographic history that may have been linked to the fossil Lagerstätten and animal evolution before the Ediacaran radiation. The formation comprises shallow marine sediments yielding phosphatized embryos in the top unit. The d13C stratigraphic pattern exhibits covariation between carbonate and kerogen, suggesting a depositional signature. Integrated analyses of the d13C chemostratigraphy, sequence stratigraphy and biostratigraphy through coeval strata around the world indicate that the unfossiliferous unit spans between the two major ice ages of the terminal Proterozoic (c. 600-580 Ma) and that the overlying fossiliferous unit corresponds to the Ediacaran biozone IIB. Trace element abundances in the marine fraction and their interelemental relationships show marked differences between the two units, including different magnitudes of Ce anomaly (c. 0.8~1.0 vs. 0.5~0.6). Their comparison with modern seawater chemistry indicates that Black Sea-type ocean stagnation with anoxic deeper waters persisted during the unfossiliferous period, followed by a well-ventilated oxic ocean during the fossiliferous period. Additional lines of evidence suggest that mixing of two different water masses, possibly upwelling of a previous deeper water, introduced high concentrations of nutrients into surfacial environments at this geochemical transition.