The DOUNCE event at the top of the Ediacaran Doushantuo Formation, South China: Broad stratigraphic occurrence and non-diagenetic origin

The DOUNCE (Doushantuo negative carbon isotope excursion), characterized by a large shift in delta C-13(carb) from +5 parts per thousand down to -12 parts per thousand in the upper part of the Ediacaran Doushantuo Formation, was previously known only from a few sections mainly in the Yangtze Gorges area, western Hubei, South China. As a consequence, it remains uncertain whether the DOUNCE is of diagenetic origin, of local significance only, or corresponds to the worldwide mid-Ediacaran Shuram-Wonoka anomaly. Here we report the widespread occurrence of the DOUNCE in nine sections from various depositional settings ranging from coastal shallow basin, offshore intra-shelf basin to deepwater slope on the Yangtze platform. The results indicate that the DOUNCE starts in the upper part of Doushantuo Sequence 2 and ends at the base of the Dengying Formation. The variable appearance of the DOUNCE is facies dependent and any absence of the DOUNCE is shown to be the result of stratigraphic hiatus. The fall and recovery of delta C-13(carb) values that bracket the DOUNCE occur over a relatively short stratigraphic interval and were possibly of shorter duration than the nadir of delta C-13(carb) values which remain stable for a much longer interval from the transgressive systems tract to the condensed portion of the sedimentary sequence. Both the fall and the recovery of delta C-13(carb) values are gradual and independent of lithology, suggesting a non-diagenetic origin for the DOUNCE which is further supported by the lack of any systematic diagenetic positive covariation between delta C-13(carb) and delta O-18(carb) data, and systematic differences in absolute isotope values between dolostone and limestone units. Field investigations and sedimentary analysis suggests the occurrence of a major sequence boundary at the basal part of the DOUNCE and reveals that previous reports of an interval of positive delta C-13 values within the DOUNCE resulted from either post-depositional sliding or faulting. Similarities between the evolutionary pattern and stratigraphic occurrence of the DOUNCE and global Shuram-Wonoka anomalies demonstrate that the DOUNCE represents a global change in seawater chemistry during the mid-Ediacaran which was closely related to the evolution of macroscopic metazoans. (C) 2011 Elsevier B.V. All rights reserved.