Carbon isotope fluctuations in Precambrian carbonate sequences of several localities in Brazil

Carbon isotope fluctuations in Precambrian sedimentary carbonates between 2 .8 Ga and 0.60 Ga in Brazil are examined in this study. The carbonate facie s of the BIF of the 2.8 Ga-old Carajas Formation, state of Para in northern Brazil, has rather homogeneous delta C-13 (-5 parts per thousand (PDB)), c ompatible with carbonatization of a silicate protolith by a CO2-rich fluid from mantle degassing. The Paleoproterozoic Gandarela Formation, state of M inas Gerais, displays a narrow delta C-13 variation (-1.5 to +0.5 parts per thousand) compatible with carbon isotope signatures of carbonates deposite d around 2.4 Ga worldwide. The Fecho do Funil Formation has probably record ed the Lomagundi delta C-13 positive anomaly (+6.4 to +7.1 parts per thousa nd (PDB)). The magnesite-bearing carbonates of the Oros mobile belt, state of Ceara, exhibit carbon isotope fluctuation within the range for carbonate s deposited at 1.8 Ga. The C-isotope record of the Frecheirinha Formation, northwestern state of C eara, shows negative delta C-13 values in its lower portion (-2 parts per t housand) and positive values up section (+1 to +3 parts per thousand), whic h suggests this sequence is a cap carbonate deposited after a glacial event around 0.95 Ga. The Jacoca and Acaua sedimentary carbonate Formations, sta te of Sergipe, NE Brazil, show carbon isotope fluctuations very similar to each other (average around -5 parts per thousand), compatible with a deposi tion around 0.76 Ga. The younger Olho D'Agua carbonate Formation, however, also in the state of Sergipe, displays negative delta C-13 values at the lo wer portion of the Formation, changing dramatically up section to positive values as high as +10 parts per thousand, a characteristic compatible with a Sturtian cap carbonate deposited around 0.69 Ga. On the light of the C isotope data discussed in this study, it seems that d elta C-13 fluctuations in Paleoproterozoic carbonates in Brazil are within the range found globally for metasedimentary carbonates of this age. Carbon isotope data proved to be very useful in establishing relative chronology of Neoproterozoic carbonate formations for which precise radiometric ages a re not available.