The Variation And Implication Of Carbon Isotope From The Doushantuo Formation In The Sirenshan-dingbai Section Across The East Yangtze Gorges

The Ediacaran Doushantuo Formation of the East Yangtze Gorges covers thecritical time interval from635Ma to551Ma and exposes across a large area of theEast Yangtze Gorges. It yields rich and exquisitely preserved fossils and provides arare window for understanding the evolution of metazoan. Intensive analyses havebeen conducted for the Doushantuo Formation including biostratigraphy，chemostratigraphy, rock stratigraphy， and sequence stratigraphy. However, theresearch mainly focuses on the southern limb of the Huangling Anticline and ignoresthe northern limb. The so-called Sirenshan-Dingbai section is a composite one whichis located at the the northern limb of the Huangling Anticline. The samples of thelower30m are from the Sirenshan section which was described as Wanjiagou byprevious researchers whereas the samples of the upper77m are from the new Dingbaisection that has never been described before. The lower part includes cap carbonate,black shale, phosphorite and dolomite with banded phosphorite. And the upper partcomprises thickly-bedded dolomite, dolomite with banded and brecciated phosphorite,thickly-bedded dolomite intercalated with thinly-bedded dolomite, laminateddolomite.The13C values’ characteristics of the representative section are described fromthe bottom to the top as follows.13C values of the3m cap carbonate are similar tocharacteristic values in the cap carbonate of other sections and fluctuate around2‰.And13C values of the4m phosphorite fluctuate around+2‰. The15mthickly-bedded dolomite comprises a negative shift with13C values ranging fromabout2‰to+2‰.13C values of the35m dolomite with banded and brecciatedphosphorite change between+2‰and+6‰. The following8m dolomite records anegative shift and the most negative value is-3.3‰.13C values of the upper20mdolomite fluctuate between+4‰and+6‰. There isn’t a negative shift near theboundary between Doushantuo Formation and Dengying Formation.The upper black shale of Jiulongwan described by McFadden are not observed at Sirenshan-Dingbai section. It remains unclear that whether the lack of black shale iscaused by facies changes or erosion. The whole13C values’ variation ofSirenshan-Dingbai section are similar to the characteristics of Xiaofenghe sectionlocated at the eastern limb of the Huangling Anticline. The reason is probably thatboth sections are located at shallow water and their deposited environments aresimilar. On the one hand, there are similarities about the13C values of differentsections around the Huangling Anticline. On the other hand, there are big differencesamong the sections especially the lack of the biggest negative excursion at thenorthern and eastern limb of the Huangling Anticline. And the lack can be explainedby the stratified redox model for the Ediacaran ocean proposed by Li. And the data fillthe blank of shallow water to some extent. What’s more，it certifies the lack of thebiggest negative excursion.