C- And Sr-isotope stratigraphy of the São Caetano complex, Northeastern Brazil: A contribution to the study of the Meso-Neoproterozoic seawater geochemistry

Abstract

C-isotope and 87Sr/86Sr values for five carbonate successions from the São Caetano Complex, northeastern Brazil, were used to constrain their depositional age and to determine large variations in the C- and Srisotopic composition of seawater under the framework of global tectonic events. Three C-isotope stages were identified from base to top in a composed chemostratigraphic section: (1) stage in which δ13C values vary from +2 to +3.7% o PDB and average 3% o PDB, (2) stage with δ 13C values displaying stronger oscillations (from -2% o to +3% o PDB), and (3) stage with an isotopic plateau with values around +3.7% o PDB. Constant 87Sr/86Sr values (∼ 0.70600) characterize C-isotope stage 1, whereas slightly fluctuating values (from 0.70600 to 0.70700) characterize C-isotope stage 2. Finally, 87Sr/86Sr values averaging 0.70600 characterize C-isotope stage 3. The C- and Sr- chemostratigraphic pathways permit to state: (a) the C- and Sr-isotope secular curves registered primary fluctuations of the isotope composition of seawater during late Mesoproterozoic- early Neoproterozoic transition in the Borborema Province, and (b) onset of the Cariris Velhos/Greenville cycle, widespread oceanic rifting, continental magmatic arc formation and onset of the agglutination of Rodinia supercontinent, mostly controlled the C- and Sr-isotope composition of seawater during the C-isotope stages 1, 2 and 3.