New carbon isotope stratigraphy of the Ediacaran-Cambrian boundary interval from SW China: implications for global correlation

The Yangtze Platform preserves relatively thick carbonate successions and excellent fossil records across the Ediacaran-Cambrian boundary interval. The intensely studied Meishucun section in East Yunnan was one of the Global Stratotype Section candidates for the Precambrian-Cambrian boundary. However, depositional breaks were Suspected in the section and the first appearance of small shelly fossils could not be verified. The Laolin section located in NE Yunnan is more Continuous and shows great potential for global correlation of carbon isotope features across the Precambrian-Cambrian boundary However, the stratigraphic framework and correlations were controversial. We Studied and systematically sampled the Laolin section and present here new carbon isotope data for this section. The Laolin section consists of, in ascending order, the Baiyanshao dolostone of the Dengying Formation, the Daibu siliceous dolostone, Zhongyicun dolomitic phosphorite, lower Dahai dolostone and upper Dahai limestone of the Zhujiaqing Formation, and the black siltstone of the Shiyantou Formation. Our data reveal a large negative delta C-13 excursion (-7.2 parts per thousand, L1') in the Daibu Member, which matches the previously published data for the Laolin section, and a large positive excursion (+3.5 parts per thousand, L4) in the Dahai Member, which was not shown in the published data. The excursion L1' correlates well with the similarly large negative excursion near the first appearance of small shelly fossils in Siberia and Mongolia. Similar magnitude, excursions are also known from Morocco and Oman, for which there are no robust fossil constraints but from where volcanic ash beds have been dated precisely at 542 Ma, thus confirming a global biogeochemical event near the Ediacaran-Cambrian boundary. Our data also indicate that deposition was more continuous at the Laolin section compared with the Meishucun section, where there are no records of a comparable negative excursion near the Ediacaran-Cambrian boundary, nor any comparable positive excursion ill the Dahai Member. Therefore, the Laolin section has proven potential to be a supplementary Global Stratotype Section for the Ediacaran-Cambrian boundary on the Yangtze Platform.