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CARBON- AND OXYGEN-ISOTOPIC BEHAVIOR OF CARBONATE ROCKS OF THE PHOSPHATIC GRAMAME FORMATION, PERNAMBUCO-PARAIBA COASTAL BASIN, NORTHEASTERN BRAZIL

In the basal section of the carbonates of the Maastrichtian Gramame Formation in the Pernambuco-Paraiba coastal basin of northeastern Brazil, major phosphate concentrations (P2O5 > 10%) were deposited in shallow-marine environments (inner shelf). In contrast, dolomite-associated phosphates are characterized by low P2O5 concentrations (<10%) and were deposited in the relatively deep water of a platform ramp, under conditions of limited oxygen availability. Calcite is the main carbonate phase remaining after the diagenesis that affected the phosphorite. A positive δ13C (up to +2%PDB), coupled with a positive MgO-δ180 correlation in the phosphate-enriched carbonates, suggests that upwelling currents were the early phosphogenic vectors during marine transgression, in contrast to warm superficial seawater that prevailed during the Maastrichtian elsewhere. The major phosphate concentrations are related to reworking and diagenesis in a shallow shelf environment during a regression pulse of sea level, followed by...

Oceanographic parameters in continental margin of the State of Ceará (northeastern Brazil) deduced from C and O isotopes in foraminifers

Specimens of Recent foraminifera of Amphistegina radiata, Peneroplis planatus and Globigerinoides ruber, from fifty samples of surface sediments of the continental margin of the State of Ceará, Brazil, have been analyzed for carbon and oxygen isotopes to investigate oceanographic parameters and determine the values of delta18O of the oceanic water. From a comparison between values of delta18O obtained for ocean water using the linear equations by (Craig and Gordon 1965) and the one by Wolff et al. (1998), it became evident that the former yielded a more reliable value (0.2 per thousand SMOW) than the latter. Lower values of delta18O for the ocean water in this continental margin resulted from continental water influence. Values of 18O (-0.3 per thousand to -1.5 per thousand PDB for benthic foraminifera and -0.6 per thousand to -2.4 per thousand PDB for planktic foraminifera), attest to a variation of temperatures of oceanic water masses, in average, between 20 to 22 degrees C in deep water and 24 to 27 degrees C, in surface water. Values of delta13C from +3.2% to -0.2 per thousand PDB (benthic foraminifera) reflect a variation in the apparent oxygen utilization (AOU) in the continental margin and indicate that the environments of bacteriological decomposition of organic matter are not continuous along the investigated area.

Ancient Coastlines in the Continental Shelf of the State of Ceará, Northeast Brazil: Evidence from Sediment Chemistry and Stable Isotopes

Continental shelf sediments from the state of Ceará, northeastern Brazil, have been studied to investigate meteoric diagenesis of marine sediments of previously defined ancient coastal lines. Major elements for 131 samples of carbonate sediments and C and O isotopes for 51 carapace samples of benthonic foraminifers of the Amphistegina radiata and Peneroplis planatus species were analyzed. Geochemical signatures of δ13C, δ18O, SiO2 (stoichiometrically calculated as quartz %), Sr and Mn, and XRD document the impact of meteoric diagenesis on relict sediments from current depths of 25-30 m, 45 m, and 80 m. Morphologies of the marine shelf at these depths, interpreted from previous bathymetric profiles, correspond to holocenic marine terraces. Despite a recent sedimentary cover, it was possible to confirm that these terraces possess relict detrital, biogenic, and continental material, that resulted from erosional reworking of ancient coastlines during the Flandrian transgression.

Caracterização química e mineralógica do material inconsolidado do fundo da Lagoa da Viração, arquipélago de Fernando de Noronha, Pernambuco

Lagoa da Viracao is a small lake on Fernando de Noronha Island, located 20 meters above sea level. Two cores were collected in order to analyze the unconsolidated material at the bottom of the lagoon. This material is made up predominantely of smectite and Fe and Ti oxides. The elements As, Ba, Br, Co, Cr, Cs, Rb, Sb, Sc, Ta, Th, U, Zn and rare earth elements (Ce, Eu, La, Lu, Nd, Sm, Tb and Yb) were measured by neutron activation analysis. The trace element signature is very close to that of the surrounding rocks. Chemical and mineralogical features indicate that this material is not sediment but an in situ weathering product of the ultramafic parent rock that formed in the lagoon in a subaquatic environment.