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

Abstract

Carbonate-dominated successions of the Meishucun section document secular variations in the C-isotopic composition of seawater. The stratigraphic trends show a decreasing δ 13C value from bed 2 to bed 5 with the transient negative δ 13C 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 δ 13C 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 δ 34S values of sulfate in apatite lattice (δ 34S p) of phosphorite and phosphate concretions provides new evidence of the S-isotopic variation pattern around the Precambrian-Cambrian boundary. An increase in δ 34S p values about + 12.7‰ from the terminal Proterozoic to early Cambrian is observed in this study.