The carbon and sulfur isotope signatures in the Precambrian-Cambrian transition series of the Yangtze Platform

Carbonate-dominated successions of the Meishucun section document secular variations in the C-isotopic composition of seawater. The stratigraphic trends show a decreasing delta(13)C value from bed 2 to bed 5 with the transient negative delta(13)C excursion at its top, followed by a positive rise in the Dahai carbonate. It is likely that the negative excursion from bed 2 to bed 5 might be attributed to phosporite diagenesis. During phosphorite diagenesis, the degradation of organic matter not only liberated soluble phosphate but also influenced the C-isotopic composition of porewaters, causing the decrease of delta(13)C values. The unusual burial of organic matter may have caused the positive rise of the C-isotopic excursion. We tentatively interpreted that the higher fractional burial of organic C might be correlative with enhanced productivity and high erosion rate. Enhanced primary productivity in the Precambrian-Cambrian boundary interval increased the flux of organic matter to the sediment. Increased erosion rate during the post-Varanger time would have supplied P and/or organic matter, particularly of organic-rich sediments to the oceans. This process may have facilitated the organic matter burial and preservation. The initial study of delta(34)S values of sulfate in apatite lattice (delta(34)S(p)) of phosphorite and phosphate concretions provides new evidence of the S-isotopic variation pattern around the Precambrian-Cambrian boundary. An increase in delta(34)S(p) values about + 12.7 parts per thousand from the terminal Proterozoic to early Cambrian is observed in this study. (C) 1998 Elsevier Science B.V.