THE CARBON AND SULFUR ISOTOPE SIGNATURES IN THE PRECAMBRIAN-CAMBRIAN TRANSITION SERIES OF THE YANGTZE PLATFORM

Carbonate-dominated successions of the Meishucun section document secu lar variations in the C-isotopic composition of seawater. The stratigr aphic trends show a decreasing delta(13)C value from bed 2 to bed 5 wi th the transient negative delta(13)C excursion at its top, followed by a positive rise in the Dahai carbonate. It is likely that the negativ e excursion from bed 2 to bed 5 might be attributed to phosporite diag enesis. During phosphorite diagenesis, the degradation of organic matt er not only liberated soluble phosphate but also influenced the C-isot opic composition of porewaters, causing the decrease of delta(13)C val ues. 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 enhan ced productivity and high erosion rate. Enhanced primary productivity in the Precambrian-Cambrian boundary interval increased the flux of or ganic matter to the sediment. Increased erosion rate during the post-V aranger time would have supplied P and/or organic matter, particularly of organic-rich sediments to the oceans. This process may have facili tated the organic matter burial and preservation. The initial study of delta(34)S values of sulfate in apatite lattice (delta(34)S(p)) of ph osphorite and phosphate concretions provides new evidence of the S-iso topic variation pattern around the Precambrian-Cambrian boundary. An i ncrease in delta(34)S(p) values about + 12.7 parts per thousand from t he terminal Proterozoic to early Cambrian is observed in this study. ( C) 1998 Elsevier Science B.V.