Carbon isotope stratigraphy of the lower to middle Cambrian on the eastern Yangtze Platform, South China

During the Cambrian Series 2 to 3, vast areas of carbonate strata were deposited on the Yangtze Platform, South China, providing excellent materials for carbon isotope stratigraphic studies. Previous studies have revealed the stratigraphic framework of the carbonate strata in the western Yangtze Platform, yet little remains known about the carbon isotope signature of the strata in the eastern Yangtze Platform. High-resolution carbon isotope curves of the Well K2 and Well WN2 drilling sections that cover the traditional Lower-Middle Cambrian boundary in the eastern Yangtze Platform are reported in this study. One positive (M1) and two negative (M2 and M3) delta C-13 excursions can be recognized in the Mufushan Formation of the Well K2 section, whereas three negative v excursions (D1, D2, and D3) are found in the Dachenling Formation of the Well WN2 section. Combined with palaeontological constraints, the M1 excursion, M2 and Dl excursions, and M3 and D3 excursions are correlated with the MICE (Mlngxinsi Carb on Isotope Excursion), AECE (Archaeocyathid Extinction Carbon isotope Excursion), and ROECE (Redlichiid-Olenellid Extinction Carbon isotope Excursion) on the generalized global delta C-13 curve through the Cambrian. The M1 excursion is accompanied by high total organic carbon contents, indicating that it was caused by high primary productivity. In contrast, the M2, Dl, and D3 excursions all coincide with lithological changes implying increased seawater depth, which supports a possible cause for the decreased delta C-13 by transgression and the introduction of delta C-13-depleted deep seawater into shallower seas. Evident differences in delta C-13 values and amplitudes of the AECE and ROECE can be recognized between the Well K2 and Well WN2 sections, revealing the existence of a shallow-to-deep ocean delta C-13 gradient during the carbon isotope excursion events. To our knowledge, this is the first report of high-resolution delta C-13 curves of the strata in the eastern Yangtze Platform, and the delta C-13 curve of the Mufushan Formation is currently the most complete delta C-13 record of Cambrian Stage 4 reported for the Yangtze Platform. Our results complement the delta C-13 dataset for the Yangtze Platform and provide a better understanding of the origin of the delta C-13 excursions during this stage. (C) 2017 Elsevier B.V. All rights reserved.