Positive cerium anomaly in the Doushantuo cap carbonates from the Yangtze platform, South China: Implications for intermediate water column manganous conditions in the aftermath of the Marinoan glaciation

摘要

Oxygenation exerted an important control on the emergence and diversification of metazoans in the aftermath of the Marinoan glaciation. However, the relationship between the oceanic dissolved 02 (DO) levels and early metazoan evolution remains equivocal. To provide a temporal and spatial reconstruction of the redox conditions for this critical interval, this study investigates the early Ediacaran cap carbonate deposits within intra shelf facies at three localities in the Three Gorges area, the Yangtze platform, South China. Trace and rare earth elements are sequentially extracted from the carbonate fraction for pristine temporal seawater signals. The Member II dolostones of the Duoshantuo Formation, which overlies the cap carbonates, show a negative Ce anomaly. No Ce anomaly is observed in the lower units of the cap carbonates. In contrast, positive Ce anomalies (Ce/Ce-star 1.3) in the uppermost part of the cap carbonates are observed in all studied sections. These positive Ce anomalies are accompanied by high Mn/Fe ratios (ranging from 5.04 to 14.96) and slightly positive MREE anomalies, suggesting that a Fe-Mn-(oxyhydro) oxide coparticipation occurred during the cap carbonate deposition. The positive Ce anomalies may result from the reductive dissolution of the Ce-enriched Mn-(oxyhydro) oxides across a Mn(IV)/Mn(II) redoxcline, in a distinct manganous water wedge that was sandwiched between well-oxygenated and anoxic ferruginous deep water layers. This wedge may have exhibited low oxygen conditions, with approximately 10 mu M DO (dissolved oxygen), in comparison to the 90 mu M DO expected in an oxic setting and 0 mu M DO in an anoxic setting. Therefore, the presence of a positive Ce anomaly in the uppermost part of the cap carbonates indicates a new intermediate manganous condition and further elucidates the water column redox structure that existed following the end of the Marinoan glaciation. Early Ediacaran metazoans were possibly restricted to fully