Early and late Neoproterozoic C, O and Sr isotope chemostratigraphy in the carbonates of West Congo and Mbuji-Mayi supergroups: A preserved marine signature?

The carbon, oxygen and strontium isotope geochemistry is the most widely applied chemostratigraphic tool in the reconstruction of paleoenvironments and indirect regional or global correlations for the Neoproterozoic times. Relatively good preserved carbonate rocks of the West Congo Supergroup, i.e. the Schisto-Calcaire Subgroup of the Democratic Republic of Congo and Gabon, and the Mbuji-Mayi Supergroup, i.e. the BII group from the Democratic Republic of Congo, make these Neoproterozoic successions unique for chemostratigraphical studies. In this paper, we propose to discuss on the fidelity of δ13Ccarb and Sr signatures in Neoproterozoic carbonates on the basis of a severe diagenetic control, using trace and major geochemistry, combined with C and O stable isotope analysis. Our result highlights that the δ13Ccarb fluctuations of the Schisto-Calcaire Subgroup reflect (i) facies variations, (ii) exchanges between isotopically light carbon in meteoric waters and carbonate during lithification and early diagenesis, and/or (iii) regional metamorphism grades rather than temporal signals of ocean chemistry, while in the unmetamorphosed carbonates of the Mbuji-Mayi Supergroup, reported to the Bitter Springs anomaly (~ 810 Ma), the use of δ13C ratios are worldwide applicable for inter-basin correlations.